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Astronomy & Astrophysics manuscript no. 39032corr-bis c©ESO 2020September 3, 2020

The CARMENES search for exoplanets around M dwarfs

Rubidium abundances in nearby cool stars

C. Abia1, H. M. Tabernero2, 4, S. A. Korotin3, D. Montes4, E. Marfil4, J. A. Caballero5, O. Straniero6, N. Prantzos7,I. Ribas8, 9, A. Reiners10, A. Quirrenbach11 , P. J. Amado12, V. J. S. Béjar13, 14, M. Cortés-Contreras8 , S. Dreizler9,Th. Henning15, S. V. Jeffers10, A. Kaminski11, M. Kürster15, M. Lafarga8, 9, Á. López-Gallifa4 , J. C. Morales8, 9,

E. Nagel15, V. M. Passegger17, 18, S. Pedraz19, C. Rodríguez López12, A. Schweitzer17, M. Zechmeister10

1 Departamento de Física Teórica y del Cosmos, Universidad de Granada, E-18071 Granada, Spaine-mail: [email protected]

2 Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal3 Crimean Astrophysical Observatory, Nauchny 298409, Republic of Crimea4 Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM),

Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain5 Centro de Astrobiología (CSIC-INTA), ESAC, Camino bajo del castillo s/n, E-28691 Villanueva de la Cañada, Madrid, Spain6 Istituto Nazionale di Astrofisica - Osservatorio Astronomico d’Abruzzo, Via Maggini snc, I-64100, Teramo, Italy7 Institut d’Astrophysique de Paris, UMR7095 CNRS, Univ. P. & M. Curie, 98bis Bd. Arago, F-75104 Paris, France8 Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, c/ de Can Magrans s/n, E-08193 Bellaterra, Barcelona, Spain9 Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain

10 Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany11 Landessternwarte, Zentrum für Astronomie der Universtät Heidelberg, Königstuhl 12, D-69117 Heidelberg, Germany12 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, E-18008 Granada, Spain13 Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain14 Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain15 Thüringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany16 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany17 Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany18 Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 West Brooks Street, Norman, OK-73019

Oklahoma, United States of America19 Centro Astronómico Hispano-Alemán (CSIC-MPG), Observatorio de Calar Alto, Sierra de los Filabres, E-04550 Gérgal, Spain

Received dd July 2020 / Accepted dd Month 2020

ABSTRACT

Due to their ubiquity and very long main-sequence lifetimes, abundance determinations in M dwarfs provide a powerful and alternativetool to GK dwarfs to study the formation and chemical enrichment history of our Galaxy. In this study, abundances of the neutron-capture elements Rb, Sr, and Zr are derived, for the first time, in a sample of nearby M dwarfs. We focus on stars in the metallicityrange −0.5 . [Fe/H] . +0.3, an interval poorly explored for Rb abundances in previous analyses. To do this we use high-resolution,high-signal-to-noise-ratio, optical and near-infrared spectra of 57 M dwarfs observed with CARMENES. The resulting [Sr/Fe] and[Zr/Fe] ratios for most M dwarfs are almost constant at about the solar value, and are identical to those found in GK dwarfs of thesame metallicity. However, for Rb we find systematic underabundances ([Rb/Fe]< 0.0) by a factor two on average. Furthermore, atendency is found for Rb—but not for other heavy elements (Sr, Zr)—to increase with increasing metallicity such that [Rb/Fe]& 0.0is attained at metallicities higher than solar. These are surprising results, never seen for any other heavy element, and are difficult tounderstand within the formulation of the s- and r-processes, both contributing sources to the Galactic Rb abundance. We discuss thereliability of these findings for Rb in terms of non-LTE (local thermodynamic equilibrium) effects, stellar activity, or an anomalous Rbabundance in the Solar System, but no explanation is found. We then interpret the full observed [Rb/Fe] versus [Fe/H] trend within theframework of theoretical predictions from state-of-the-art chemical evolution models for heavy elements, but a simple interpretationis not found either. In particular, the possible secondary behaviour of the [Rb/Fe] ratio at super-solar metallicities would require amuch larger production of Rb than currently predicted in AGB stars through the s-process without overproducing Sr and Zr.

Key words. nuclear reactions, nucleosynthesis, abundances – stars: abundances – stars: late type

1. Introduction

Analysis of the rubidium abundance in the Solar System showsthat the neutron capture s- and r-processes are about equallyresponsible for the synthesis of this element (e.g. Sneden et al.2008; Prantzos et al. 2020). Rubidium is present in two iso-

topic forms: 85Rb, which is stable, and 87Rb, which with ahalf-life of 5 × 1010 yr may be considered stable from an as-trophysical point of view. Astronomical detection of Rb relieson the Rb i lines, which do not permit measurement of the rel-ative isotopic Rb abundances from stellar spectra. The main s-process, which manufactures elements with atomic mass in the

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range 90 < A < 209, is identified with the He-burning shellin low- and intermediate-mass stars (LIMS; M . 8 M⊙) dur-ing the asymptotic giant branch (AGB) phase (e.g. Busso et al.1999; Käppeler et al. 2011), where neutrons are mainly pro-vided by the 13C(α,n)16O reaction. The weak s-process, respon-sible for a major contribution to the s-process nuclides up toA ∼ 90, has been recognised as the result of neutron capturesynthesis mainly during core He- and shell C-burning phasesof massive stars (M & 10 M⊙; Arnett & Thielemann 1985;Prantzos et al. 1990; Raiteri et al. 1991; Pignatari et al. 2010;Limongi & Chieffi 2018) with the reaction 22Ne(α,n)25Mg beingthe major neutron source.

The situation concerning the astrophysical site of the r-process is still far from clear. After more than 50 years ofresearch into its astrophysical origin(s), the identification ofa fully convincing site remains elusive. In the recent past,the r-process was usually associated to the explosion of mas-sive stars, but nowadays the neutron-star merging scenariois gaining support, bolstered by the recent joint detection ofelectromagnetic and gravitational signals from the γ-ray burstGW 170817/GRB 170817A (see Pian et al. 2017, and referencestherein). Nevertheless, the behaviour of the abundance ratioswith respect to Fe of genuine r-process elements, such as Eu,implies that core collapse supernova may also play an importantrole in r-process nucleosynthesis. In any case, to date, no nu-merical simulation in the proposed scenarios is able to fully re-produce the observed distribution of the r-process elemental andisotopic abundances in the Solar System. Excellent reviews onthis topic can be found in Thielemann et al. (2017), Cowan et al.(2019), and Arnould & Goriely (2020).

There is substantial observational evidence showing thatRb is produced through the main s-process in AGBstars. Abundance analyses of many Galactic and extra-galactic AGB stars both of C- and O-rich types showconsiderable Rb enhancements, namely [Rb/Fe] > 0.01

(Lambert et al. 1995; Abia & Wallerstein 1998; Abia et al.2001; García-Hernández et al. 2006; de Laverny et al. 2006;García-Hernández et al. 2009; Pérez-Mesa et al. 2017). Gen-erally, these enhancements can be explained by the currentnucleosynthesis models of AGB stars (Gallino et al. 1998;Cristallo et al. 2011; van Raai et al. 2012; Karakas & Lattanzio2014; Cristallo et al. 2015). Two are the possible neutron sourcesin AGB stars: the 13C(α,n)16O reaction, which is the main neu-tron source in low-mass AGB stars, and the 22Ne(α,n)25Mg re-action, which becomes important in massive AGBs. Theoreticalmodels predict [Rb/Sr,Y,Zr] ratios lower or larger than solar de-pending on whether the 13C(α,n)16O or the 22Ne(α,n)25Mg reac-tion is the main neutron source, respectively. The difference isin the neutron density, which is rather low for the 13C(α,n)16Oreaction (Nn ∼ 107 cm−3), but much larger in the case of the22Ne(α,n)25Mg reaction (Nn > 1011 cm−3). Abundance ratios[Rb/Sr,Y,Zr] < 0.0 are indeed found in most carbon-rich AGBstars, which indicates their low-mass AGB origin (M ∼ 1.3 −3.0 M⊙), while positive ratios are found in very luminous O-richAGB stars with masses M & 4 M⊙, in which the 22Ne neu-tron source is dominant (Lambert et al. 1995; Abia et al. 2001;Pérez-Mesa et al. 2017).

The evolution of the Galaxy’s s- and r-process products onthe other hand is directly observed from the stellar abundancesof elements that are predominantly attributable to either the s-

1 Here we follow the standard abundance notation, [X/H] =log (X/H)⋆ − log (X/H)⊙, where X/H is the abundance by number ofthe element X, and log ǫ(X) ≡ log (X/H) + 12.

or the r-process (see e.g. Jofré et al. 2019). Traditional tracersinclude Sr, Ba, and La for the s-process and Eu as the gen-uine representative element of the r-process2. Due to the nearlyequal s- and r-process production of Rb, the study of the evo-lution of the abundance of this element with metallicity is par-ticularly suitable for elucidating the timescales on which bothneutron capture processes contribute to the chemical history ofthe Galaxy. However, the unfavourable electronic structure of Rbprovides only a few lines in stellar spectra. The most used linesare the Rb i resonance lines at λλ7800.3 and 7947.6 Å. Due tothe low cosmic abundance of Rb, these lines are usually weak,and in G dwarfs they are also heavily blended. Because of theirweakness, the blend issue is aggravated in stars with near solarmetallicity (or higher). To date, only two Rb abundance studiesin these stars exist, namely those of Gratton & Sneden (1994)and Tomkin & Lambert (1999). These authors derived Rb abun-dances in a sample of metal-poor disc and halo G dwarfs andconcluded that the [Rb/Fe] versus [Fe/H] relationship behaves atlow metallicity ([Fe/H] < −1) as does [Eu/Fe], that is, showingan approximately constant [Rb/Fe] ratio as a typical r-processelement. However, the behaviour at higher metallicities ([Fe/H]& −0.5) is poorly understood. It is precisely at those metallic-ities that the s-process contribution to the Galactic Rb abun-dance is expected to peak (e.g. Busso et al. 1999; Cristallo et al.2015; Bisterzo et al. 2017; Prantzos et al. 2018). Detection viathe Rb i resonance lines at the expected low Rb abundances ismore favourable for cool dwarfs and giants. Due to the lowereffective temperature and higher gravity of M dwarfs, the Rbresonance lines show a moderate intensity in these stars and canbe relatively easily identified in their spectra for a wide range ofmetallicities, in particular in stars with metallicity close to solarusing high spectral resolution. However, there is a caveat to us-ing these lines: although prominent, they are contaminated withthe presence of a plethora of TiO and VO absorption lines andas a consequence the spectral continuum is usually difficult tolocate.

In this study we derive Rb abundances from high-resolutionspectra in a sample of nearby, bright, single M dwarfs withmetallicities close to solar. Simultaneously, we also determinedtheir Sr and Zr abundances, which are two elements with a pre-dominantly main s-process origin. For that, we use the highsignal-to-noise template spectra of 57 M dwarfs observed underthe CARMENES exoplanet survey guaranteed time observations(GTOs; Quirrenbach et al. 2018; Reiners et al. 2018). The de-rived abundance ratios are compared with state-of-the-art chemi-cal evolution model predictions for the solar neighbourhood.Ouraim is to better identify the onset of the significant s-processcontribution in the chemical history of the Galaxy and put con-straints on the role played by the s- and r-processes in the Galac-tic Rb budget. The structure of this paper is as follows: the obser-vational material and analysis are presented in Section 2, wherethe data acquisition and reduction procedures are briefly de-scribed; we also discuss the atmospheric parameters used in thisstudy, the line lists, and the derivation of the abundances fromthe spectra, together with an evaluation of the observational andanalysis uncertainties. In Section 3 the main results are discussedand compared with the current nucleosynthesis models througha chemical evolution model for the solar neighbourhood. Finally,Section 4 summarises the main conclusions of this study.

2 Prantzos et al. 2020 derived an s-process contribution in the SolarSystem abundances of 91 %, 88 %, and 80 % for Sr, Ba, and La, respec-tively, while the r-process contribution for Eu is 95 %.


C. Abia et al.: Rubidium abundances in CARMENES M dwarfs

2. Stellar sample and analysis

2.1. Targets, observations, and reduction

Installed at the 3.5 m telescope at Calar Alto Observatory inAlmería, Spain, CARMENES is a new-generation spectrographdesigned to detect Earth-mass planets around M dwarfs bymeans of the radial-velocity technique (Quirrenbach et al. 2014,2018). CARMENES consists of two optical and near-infrared(NIR) channels that extend over 5200–9600Å in the visual (VIS)and 9600–17100Å in the NIR regions with resolving powers ofR = 94, 600 and R = 80, 400, respectively. The CARMENES ex-oplanet radial-velocity survey comprises about 350 M dwarfsthat cover the full M sequence from M0.0 V to M9.0 V spec-tral types (and a couple of K7 V stars; Reiners et al. 2018). TheCARMENES GTO sample contains a set of carefully selectedM dwarfs in the immediate solar neighbourhood that are thebrightest stars in each spectral subtype without physical com-panions at less than 5 arcsec (Caballero et al. 2016a). Most ofthe target stars should have close to solar composition, althoughthere are some known exceptions (Alonso-Floriano et al. 2015;Passegger et al. 2018, 2019). Almost all the stars studied hereare located within a few tens of parsecs around the Sun. How-ever, there are members in all the different Galactic kinematicpopulations except for the halo, that is, the young thin, thin-to-thick transition, and thick discs (Cortes-Contreras 2016), whichensures a broad investigated metallicity range.

All investigated M dwarfs were spectroscopically observedwith CARMENES at a number of epochs since January 2016(Reiners et al. 2018). The median number of observations perstar in the VIS channel is 24. Their spectra were reduced ac-cording to the standard CARMENES GTO data flow and thuswere processed and wavelength calibrated with the caracalpipeline (Caballero et al. 2016b), which computes a single co-added template spectrum per star and instrument channel (seealso Zechmeister et al. 2018). The wavelength calibration isprovided by a combination of hollow cathode lamps and twotemperature- and pressure-stabilised Fabry-Pérot units that even-tually provide median uncertainties of the level of 1 m s−1 inthe VIS channel (Trifonov et al. 2018; Zechmeister et al. 2019),which covers the investigated Rb i, Sr ii, and Zr i lines. In ad-dition, we added a step in the data flow, namely correcting theindividual spectra from telluric correction before merging themin the template spectra. This correction provided a further im-provement in the S/N of the eventually used spectra, up to about300. Further details on the removal of the telluric features can befound in Nagel et al. (2020, submitted).

We selected a sample of 57 M dwarfs from the CARMENESGTO programme in a wide metallicity range (−0.5 ≤ [Fe/H]≤ +0.3), imposing Teff > 3400 K to minimise the impactof molecular absorption, according to the values reported byPassegger et al. (2019). This metallicity interval was poorly ex-plored for Rb abundances in the literature.

2.2. Stellar parameter refinement

A number of studies focused on improving the precision of Mdwarf stellar parameters, namely Teff, surface gravity (log g),and metallicity, can be found in the recent literature. For in-stance, different ways to determine the first two parameterswere discussed by Veyette et al. (2017), Schweitzer et al. (2019),and Souto et al. (2020), while Birky et al. (2020) discussed thederivation of the metallicity. In contrast to FGK dwarfs, the spec-troscopic method to derive these parameters is, in general, not

recommended for M dwarfs because of their much more com-plex spectra. Due to their relatively low effective temperatures(Teff . 4000 K), the spectra of M dwarfs show a plethora ofmolecular absorption lines that leave almost no atomic line freeof blends. Furthermore, the C/O ratio is revealed as an additionalrelevant parameter in the analysis. The envelopes of M dwarfsare not expected to be affected by the nuclear burning in thestellar interior, and therefore they should show a C/O identicalto that of the interstellar medium (ISM) from which they wereborn, that is, lower than unity. This means that almost all theavailable carbon in the atmospheres of M dwarfs will be lockedinto forming CO, while the remaining available oxygen wouldform metallic oxides, water, and other, less abundant O-bearingmolecules. In fact, the VIS spectra of M dwarfs are dominated byTiO and VO absorption bands (and some CaH; Kirkpatrick et al.1991; Alonso-Floriano et al. 2015), whereas CO, OH, and H2Oprevail in the NIR region. Therefore, to determine the stellar pa-rameters of M dwarfs, a full spectral synthesis analysis is rec-ommended instead of studying individual spectral lines. To dothis, an estimation of C/O is vital as the intensity of the above-mentioned O-bearing molecules mainly depends on this ratio.As a consequence, this combined molecular absorption system-atically depresses the spectral continuum, which is usually notwell defined (e.g. Tsuji & Nakajima 2014).

In a first step we adopted the stellar parameters derived byPassegger et al. (2019), who fitted an updated version grid of thePHOENIX synthetic spectra presented by (Husser et al. 2013) tothe selected high-resolution CARMENES spectra in the VIS andNIR spectral ranges using a χ2 method. We refer to their workfor further details on the specific method followed, for a detaileddiscussion on the evaluation of the uncertainties, and for com-parison with the most recent literature. In general, the agreementwith other studies was good within the uncertainties, althoughfor some stars with metallicity higher than solar, Passegger et al.(2019) derived systematically higher metallicities.

With the stellar parameters estimated as mentioned above, amodel atmosphere was interpolated within the grid of MARCSatmosphere models (Gustafsson et al. 2008) for the specific Teff ,log g, and [M/H] (i.e. average metallicity) derived for each star.A microturbulence parameter of ξ = 1 km s−1 was adopted forall the stars. Certainly, other microturbulence parameters mightbe possible for M dwarfs, but the severe blending preventedus for deriving this parameter spectroscopically. However, un-certainties of 0.5 km s−1 in the microturbulent velocity resultin very small changes in the derived abundances (see below).The atomic line lists were taken from the VALD3 databaseadopting the corrections performed within the Gaia-ESO survey(Heiter et al. 2015) in the VIS wavelength range, while in theNIR range we used the atomic line list from the APOGEE sur-vey (Hasselquist et al. 2016; Cunha et al. 2017; Majewski et al.2017; Smith et al. in prep.). We refer to these surveys for a de-tailed description of these line lists. Molecular line lists wereprovided by B. Plez3 and they include several C- and O-bearingmolecules (CO, CH, CN, C2, HCN, TiO, VO, H2O) and a fewmetallic hydrides (FeH, MgH, CaH). TiO and H2O determine,in particular, the continuum level at VIS and NIR wavelengths,respectively.

Oscillator strengths for the Rb i resonance lines at λλ7800and 7947 Å are known with good accuracy (Morton 2000), butthey have multiple components due to the hyperfine structure

3 These molecular line lists are publicly available athttps://nextcloud.lupm.in2p3.fr/s/r8pXijD39YLzw5T ,where detailed bibliographic sources can be also found.


https://nextcloud.lupm.in2p3.fr/s/r8pXijD39YLzw5T


Table 1. Spectroscopic data of the used lines and hyperfine structure ofRb i.

Element/ λ log g f

isotope (Å)

87Rb7800.183 –0.6637800.186 –0.6637800.188 –1.0617800.317 –0.2167800.322 –0.6637800.325 –1.3627947.507 –0.6697947.524 –1.3687947.651 –0.6697947.668 –0.669

85Rb7800.233 –0.7447800.234 –0.6477800.235 –0.7607800.292 –0.2827800.295 –0.6477800.296 –1.1917947.563 –0.6537947.570 –1.1977947.626 –0.7507947.634 –0.653

Sr ii10327.311 –0.353

Zr i8063.090 –1.6208070.080 –0.790

of the atomic levels. The individual hyperfine structure com-ponents and their respective oscillator strengths are given inTable 1. We adopted the meteoritic 85Rb/87Rb = 2.43 ratio(Lodders 2019). Unfortunately, the isotopic splitting is tiny anddoes not allow the derivation of the 85Rb/87Rb ratio from ourspectra. Concerning Sr, this element has only a few lines use-ful for abundance determinations. In fact, the available stron-tium lines (usually Sr ii) in the blue spectral region of coolstars with near-solar metallicity are usually strongly blended.An alternative are the three Sr ii lines in the NIR at λλ10036,10327, and 10914 Å, which are almost free of blends and telluriccontamination. However, as shown in Belyakova & Mashonkina(1997), Andrievsky et al. (2011), and Bergemann et al. (2012),these lines are instead strongly affected by deviations from lo-cal thermodynamical equilibrium (LTE; see below). Of theseSr lines, we could use only that at λ10327 Å because that atλ10036 Å is very weak and blended and located at the edge ofone of the CARMENES orders, and the other, at λ10914 Å, is inone of the spectral gaps of the instrument4. For the 10327 Å line,we used the oscillator strength given by Warner (1968) and theVan der Waals parameters provided by the Anstee, Barklem, andO’Mara theory (see Barklem et al. 2000). This Sr ii λ10327 Åline is apparently free of atomic and molecular blends in ourstars. In fact, in this spectral region the true spectral continuumcan be placed more easily. Finally, the only useful zirconiumlines detected in our spectra are the Zr i lines at λλ8063.105 and8070.115 Å. These lines are rather weak but not particularly af-

4 https://carmenes.caha.es/ext/instrument/

fected by TiO blends in our stars. Oscillator strengths for theselines were taken directly from the VALD3 database. Unfortu-nately, no yttrium line useful for abundance analysis was de-tected in the available spectral range.

2.3. Spectral synthesis

We firstly tested our line list by fitting the observed spec-tra of the Sun and Arcturus (Reiners et al. 2016; Hinkle et al.1995) in the spectral ranges of interest. In these stars, the Rb iλ7800 Å line is heavily blended with a Si i line, for which weadopted log g f = −0.75 from VALD3. Synthetic spectra inLTE were computed with the corresponding model atmosphereusing the code Turbospectrum v19.15 (Plez 2012). For theSun, we used a MARCS atmosphere model with parametersTeff/ log g/ξ = 5770/4.44/1.1, and for Arcturus a model atmo-sphere with parameters according to Ryde et al. (2009). Theo-retical spectra were convolved with a Gaussian kernel accord-ing to the resolving power of the observed spectra and to mimicthe macroturbulence parameter. For the Sun, we obtained anLTE Rb abundance of log ǫ(Rb) = 2.47 ± 0.05 (average valueof the two lines). This nicely agrees with the value derivedby Grevesse et al. (2015). For Arcturus, the Rb abundance de-rived was 2.01 ± 0.01, which is also in very good agreementwith the abundance obtained by Tomkin & Lambert (1999) andYong et al. (2006) of namely 2.02 ± 0.02. A similar test wasmade for Zr from the selected Zr i lines. Also, nice agreementin the abundance of Zr was found with the values derived byGrevesse et al. (2015) and Peterson et al. (1993) in the Sun andArcturus, respectively, within 0.05 dex. In the case of Sr, the LTEabundance derived from the Sr ii λ10327 Å is significantly largerthan that usually accepted in the Sun and Arcturus, suggestingdeviations from LTE in the formation of this line. Eventually,we adopted the solar LTE abundances recommended by Lodders(2019) for Rb (2.47) and Zr (2.58) which nicely agree with thevalues obtained here (for Sr, see below). We also used the solarphotospheric abundances recommended by this author for therest of the elements.

The spectral synthesis method for abundance analysis shouldbe applied preferentially when the true continuum can be de-fined. For this reason, it may be difficult to apply to M dwarfs,for which the true continuum cannot be defined in general. Inthese stars, the presence of strong TiO absorption and a veil ofnumerous weak lines of H2O in the VIS and NIR spectral ranges,respectively, depress the continuum level by some parts per hun-dred. However, in our case an upper envelope of the spectrumcan be well defined by connecting the highest flux points in thespectrum in all the stars. This pseudo-continuum can then berelocated with the help of the synthetic spectrum itself. To placethis pseudo-continuum, the estimation of C/O is critical, becausethis ratio determines the intensity of TiO and H2O lines for agiven set of stellar parameters. We followed the iterative methoddescribed below.

(a) First, a synthetic spectrum was computed for each star withthe carbon and oxygen abundances scaled to the metallic-ity of the model atmosphere (see above and Passegger et al.2019). However, for the star J13450+176 (BD+18 2776;[M/H] = −0.5), we considered an oxygen enhancement inthe model atmosphere, namely, [O/Fe] = +0.4, a value typ-ically found in G dwarfs of similar metallicity. We alwayskept the C abundance constant. Unfortunately, the CO and

5 https://github.com/bertrandplez/Turbospectrum2019/


https://carmenes.caha.es/ext/instrument/

https://github.com/bertrandplez/Turbospectrum2019/


Fig. 1. Comparison of the observed (black dots) and synthetic (solid red line) spectra for the M3.0 V star J00570+450 (G 172–30) in the spectralregion around λλ15750 Å. Some OH lines, from which the O abundance was derived, and less intense Mg i lines near λλλ15740, 15749, and15765 Å are marked and labelled. In this region the pseudo-continuum is reduced mainly due to the contribution of an H2O veil.

CN lines in the covered spectral ranges are too weak to deter-mine the C abundance. Nevertheless, we checked that varia-tions of the C abundance (within ∼ 0.25 dex) have almostno effect on the synthetic spectrum in the spectral rangesof interest. To compute this initial theoretical spectrum, wedid not include the atomic and OH lines in the synthesis.In a similar way, theoretical spectra were convolved with aGaussian function according to the spectral resolution andto mimic the macroturbulence parameter. The stars KarmnJ11201–104 (LP 733–099) and J15218+209 (OT Ser), whichshow broader lines, were left out of this convolution. Forthese stars, the observed spectrum was better fitted with a ro-tational profile. Indeed, Reiners et al. (2018) quoted v sin i >2 km s−1 for these stars. For them, we used a rotational profilewith FWMH in the range 4–6 km s−1. The profiles of the Rblines in these stars and in J18174+483 (TYC 3529–1437–1)may also be affected by the magnetic field (see below).

(b) We then compared the location of the pseudo-continuum inthe NIR region with that inferred from the theoretical spec-trum itself (mainly determined by the H2O veil). In gen-eral, corrections smaller than about 3 % were needed. Wethen included the atomic and OH lines in the synthesis andestimated the O abundance by fitting selected OH lines, in

particular those in the region between 1.57 and 1.59 µm(see Fig. 1). The O abundance derived from these selectedOH lines should be compatible with the metallicity obtainedfrom the nearby metallic lines (within its uncertainty of±0.19 dex according to Passegger et al. (2019); see Fig. 1)6.This agreement also served as an additional test for the reli-ability of the pseudo-continuum location.

(c) With the O abundance derived in this way, we then com-pared the observed and calculated intensity of TiO bands inthe VIS region, in particular the band at λ > 7000 Å. Ad-justments to the O abundance adopted in the synthetic spec-trum were made when needed; in any case, they never exceed∼ 0.20 dex.

(d) Finally, with this new O abundance, we fitted again the OHlines in the NIR region and repeated the procedure untilwe achieved convergence (within ∼ 0.1 dex) between theO abundances estimated from both spectral ranges. Table 2shows the final C/O derived in our stars. As expected, mostof them are very close to the solar ratio (0.575, accordingto Lodders (2019)), but a few stars show significantly lowerC/O or higher than this value.

6 For unevolved, near-solar metallicity stars, [O/Fe] ≈ 0.0.



Once the C/O ratio was determined, the next step in theanalysis was the determination of the stellar metallicity in amore accurate way. As mentioned above, the initial metallic-itiy was taken from Passegger et al. (2019). However, thesemetallicities were derived from metallic lines that have mod-erate (or large) intensity in the atmospheres of M dwarfs andtherefore might be affected by saturation effects. A better de-termination of the metallicity was therefore desirable. To dothat, we used a number of weak metallic lines available in thespectral range close to the Rb, Sr, and Zr lines. The specificlines were the Ca i line at λ10343.819Å; the Ti i lines at λλ7791.349, 7949.152, 8068.239, and 8069.799 Å; the Fe i linesat λλ7802.473, 7941.088, 7945.846, and 10340.885Å; and theNi i lines at λλ7788.930 and 7797.580 Å. The weakness of theselines (except that of Ca i) should minimise saturation effects andpossible deviations from LTE. In addition, their proximity tothe heavy element (Rb, Sr, and Zr) lines may reduce system-atic effects introduced by the uncertain location of the pseudo-continuum when deriving the elemental ratios with respect to theaverage metallicity ([X/M]), since this uncertainty should cancelout. Figures 3 and 4 show synthetic fits to some of these metalliclines. In most of the stars we found good agreement between themetallicity derived from these metallic species, with a dispersionof about ±0.1 dex. Also, in general our metallicities agree withinthe uncertainty with those derived by Passegger et al. (2019), ini-tially adopted here. Nevertheless, when a difference larger than0.15 dex was found, we recalculated a model atmosphere withthe new metallicity and repeated the derivation of the metallicityuntil convergence was reached. The largest differences with re-spect to the metallicities by Passegger et al. (2019) are found instars with super-solar metalliciy ([M/H] > 0.0). For these stars,we found systematically lower metallicities. This difference maybe due, as mentioned above, to the fact that in metal-rich starsthe lines selected by these latter authors are very strong andpossibly more affected by saturation effects. Systematic differ-ences in metallicity are found between Passegger et al. (2019)and similar studies in M dwarfs, although they could also berelated to differences in the stellar parameters (see the discus-sion in Passegger et al. 2019 for details). Table 2 shows the fi-nal average metallicity ([M/H]) derived for our stars. The over-whelming majority have metallicity close to solar except for thestar J13450+176 (BD+18 2776), which is a mildly metal-poorstar. This classification agrees with the metallicity determinedby other studies for this star (e.g. Gaidos et al. 2014; Mann et al.2015). Since for near-solar metallicity stars, it holds that [Ni/Fe]≈ [Ti/Fe] ≈ [Ca/Fe] ≈ 0.0, in the following we refer indistinctlyto [M/H] or [Fe/H] as the stellar metallicity.

Once C/O and metallicity were determined, the abundancesof Rb, Sr, and Zr were also determined by spectral synthesis fitsto the corresponding spectral features. Figure 2 shows an exam-ple of a theoretical fit (red lines) to the spectral regions of theRb lines in a representative star of the sample. The depressionof the continuum mainly due to TiO is particularly apparent inthe region of the Rb i λ7800 Å line. Fits to some of the metal-lic lines used for the determination of the average metallicity arealso shown. Similarly, Fig. 3 shows fits to the Zr i λ8070 Å (leftpanel) and Sr ii λ10327 Å (right panel) lines, respectively. Thespectral region of the strontium line is apparently not affected bythe veil of molecular absorption and therefore the real continuumcan be more easily traced.

The two main sources of error in the abundances are mea-surement and analysis error caused by errors in the adoptedmodel atmosphere parameters. The scatter of the abundances

provided by individual lines of the same species is a good guideto measurement error. We found excellent agreement betweenthe Rb abundances derived from both lines, with a dispersionof less than 0.05 dex (see Table 2). This agreement contrastswith the differences (∼ 0.1 dex) found by Tomkin & Lambert(1999) and Yong et al. (2006), which led them to exclude the Rb iλ7947 Å line from their analyses, which appeared to be morehighly blended. For zirconium, we also found excellent agree-ment between the abundances derived from both lines (see againTable 2). The error caused by uncertainties in the adopted stellarparameters can be estimated by modifying the stellar parametersby the quoted errors in the analysis of a typical star in the sam-ple and checking the effect on the abundance derived for eachspecies, namely: ±60 K in Teff, ±0.06 dex in log g, ±0.5 km s−1

in ξ, ±5 % in C/O, and ±0.15 dex in [Fe/H]. For a typical Mdwarf with parameters Teff/ log g/[Fe/H] = 3750/4.7/0.0, wefound that the abundances derived are mostly affected by the un-certainty in Teff: ±0.10, ±0.10, and ±0.08 dex for Rb, Sr, and Zr,respectively. The uncertainties in gravity and microturbulencehave a similar, although minimal impact on the abundances de-rived for the three elements (≤ 0.03 dex). The uncertainty inthe metallicity and C/O has a slightly larger effect and is typ-ically 0.05 dex for the three species. Adding these uncertaintiestogether quadratically with the dispersion around the mean abun-dance value and the continuum uncertainty (about 1–2 % in thecase of Rb and Zr), we estimated a total uncertainty in [X/H] of±0.12 dex for Rb and Sr, and ±0.10 dex for Zr. For these heavyelements, the abundance of the element relative to average metal-licity, [X/Fe], holds the most interest. This ratio is more or lesssensitive to the uncertainties in the atmospheric parameters de-pending on whether changes in the stellar parameters affect theheavy element abundance and metallicity in the same or oppo-site sense. In our case, we estimated total uncertainties of ±0.12,±0.14, and ±0.13 dex for [Rb/Fe], [Sr/Fe], and [Zr/Fe], respec-tively.

2.4. Non-LTE effects in Rb i and Sr ii lines

It is well known that the resonant lines of other alkali elements,such as Na or K, are strongly influenced by deviations from LTE(Bruls et al. 1992). The structure of the energy levels of Rb i isvery similar to those of K i and Na i and, by analogy, some non-LTE effects in the formation of Rb lines are expected. As far aswe know, no previous study exists on non-LTE effects in Rb iin the literature. Below we briefly describe the method followedto evaluate these effects in M dwarfs. A detailed discussion ina wider range of stellar parameters will be presented elsewhere(Korotin 2020).

Our rubidium atom model includes 29 levels of Rb i and theground level of Rb ii. Radiative transitions between higher levelsare very weak and should not affect the accuracy of the non-LTEcalculation. Fine structure was considered only for the 5p 2P0

level, since it is closely connected to the most important transi-tions including the resonance lines. In the atmospheres of cooldwarfs, Rb exists almost exclusively in the form of Rb ii, since itsfirst ionisation potential is only 4.18 eV. In turn, all excited Rb iilevels are separated from the main one by more than 16.5 eV.Nevertheless, 180 supplementary levels of Rb i and 15 levels ofRb ii were included in the model to assure the conservation ofthe particle number. In total, 160 radiative transitions were takeninto account for the calculation of the population levels. We in-cluded collisional transition rates between the first six levels ofRb i according to Vainshtein et al. (1979). For other collisional



Fig. 2. Same as Fig. 1 but for the regions around Rb i λ7800 Å (top panel) and Rb i λ7947 Å (bottom panel) for the M1.5 V star J22565+165(HD 216899). Dash-dotted, solid, and dotted curves show theoretical spectra computed with log ǫ(Rb) = 2.8, 2.55 (best fit), and no Rb, respectively.Some Ti i, Ni i, and Fe i lines are marked and labelled. In these regions, the pseudo-continuum is reduced mainly due to the contribution of TiO.

transitions between levels with energies below 3.9 eV, calcula-tions from Park (1971) were used. For the rest of the bound–bound collisional transitions, we adopted the formulae fromvan Regemorter (1962) for permitted transitions and from Allen(1973) for the forbidden ones. Collisional ionisation rates werecalculated using the formula from Seaton (1962). Collisions withhydrogen for the first nine levels were described using the de-tailed quantum mechanical calculations from Yakovleva et al.(2018). For other levels, the classical Drawin formula was usedin the form suggested by Steenbock & Holweger (1984) with thecorrection factor S H = 0.05. The population of the atomic levels

for Rb i was determined using the code MULTI (Carlsson 1986)with some modifications following Korotin et al. (1999).

The non-LTE corrections lead to a strengthening of the res-onance lines in cool dwarfs (log g > 4.0). This strengtheningis due to the overpopulation of the ground level because of theso-called super-recombination processes. These processes arecommon in other alkali metals and were described in detail inBruls et al. (1992). We first tested our non-LTE calculations forthe Rb lines with the Sun. The correction was −0.12 dex, andwe obtained a solar non-LTE abundance of Rb of 2.35 ± 0.05.It is noteworthy that this value is, for the first time, in very



Fig. 3. Same as Fig. 1 but for the spectral regions of Zr i λ8070 Å (left panel) and Sr i λ10327 Å (right panel) for the same star. Dash-dotted,solid, and dotted curves show theoretical spectra computed with log ǫ(Zr) = 3.0, 2.75 (best fit), and no Zr, respectively, in the left panel, and withlog ǫ(Sr) = 3.5, 3.2 (best fit in LTE), and no Sr, respectively, in the right panel.

good agreement with the recommended meteoritic abundance(2.36 ± 0.03, Lodders 2019). We then calculated a grid of non-LTE corrections for the stellar parameters typical of our stel-lar sample, namely Teff from 3500 to 4500 K, log g from 4.0 to5.0 dex, [Fe/H] from −0.5 to +0.5 dex, and [Rb/H] from −0.4to +0.4 dex (see Table 2). In general, the non-LTE correctiondecreases with increasing temperature since the hydrogen colli-sions become more important. The value of the correction is alsodetermined by changes in the distribution of the average intensitywith wavelength and depth in the stellar atmosphere, also affect-ing the efficacy of the super-recombination processes. However,

the fraction of Rb i below ∼ 4500 K increases and the intensityof the resonance lines increases sharply. The resonance lines areformed mainly in the highest layers of the photosphere where thedensity is the lowest and the collision rates cannot compensatethe imbalance introduced by radiative processes, and thereforethe non-LTE correction increases. This effect is enhanced withincreasing [Rb/Fe] ratio (see Fig. 4). Non-LTE abundance cor-rections range from −0.25 to −0.03 dex, being ∼ −0.15 dex onaverage for the typical atmosphere parameters of our stars. Fig-ure 4 shows an example of the non-LTE abundance correctionsto the Rb i λ7800 Å line for specific atmosphere parameters and



Fig. 4. Difference between the LTE and non-LTE Rb abundances de-rived from the Rb i λ7800 Å line for typical Teff and log g (colour coded)in our sample stars. Solid and dashed lines correspond to [Rb/Fe] ratiosof 0.0 and −0.4, respectively (see text for details).

derived [Rb/Fe] ratios. As expected, the non-LTE correction de-creases with increasing Teff and decreasing log g. However, forthe Rb i λ7947 Å line, corrections are slightly lower for a givenset of atmosphere parameters and Rb abundance because thisline forms slightly deeper in the atmosphere. Again, a more de-tailed discussion will be presented in Korotin (2020) includingnon-LTE corrections in giant stars. In passing, we note that ournon-LTE correction for Rb in Arcturus is almost zero.

To evaluate the non-LTE effects in the formation of theSr ii line in the NIR, we used the atom model proposed byAndrievsky et al. (2011). We refer to this author for a more de-tailed description of the atomic levels used and non-LTE cal-culations. Briefly, collisional transitions caused by inelastic col-lisions with H i were considered according to the formulae ofDrawin (1968) as compiled by Steenbock & Holweger (1984)with a correction factor of S H = 0.01. This correction factorwas obtained by fitting the line profile of different multipletsin the solar spectrum, and was confirmed by the analysis ofBergemann et al. (2012). Atomic level populations were also de-termined using the code MULTI with modifications according toKorotin et al. (1999). The atomic model used here describes cor-rectly the profiles of the Sr lines in the blue and NIR spectrumof the Sun, giving an identical Sr abundance (2.92; this is thevalue adopted in this study as the solar Sr abundance). For Arc-turus, the non-LTE analysis gave a very similar Sr abundance,namely 2.23 ± 0.04 dex from the three NIR lines, which is sig-nificantly lower than the LTE value of 2.40 from Peterson et al.(1993). It should be noted that non-LTE effects in Sr lines dif-fer significantly from line to line. For instance, in the spec-

Fig. 5. [Rb,Sr,Zr/Fe] vs. [Fe/H] diagrams, from top to bottom. Inthe three panels, blue filled circles are our program stars. In the toppanel, grey dots with error bars are the [Rb/Fe] ratios derived inhalo and disc giant and dwarf stars by Gratton & Sneden (1994) andTomkin & Lambert (1999). In the middle and bottom panels, grey dotsare the [Sr/Fe] ratios derived by Mishenina et al. (2019), and [Zr/Fe]ratios by Battistini & Bensby (2016) and Delgado Mena et al. (2017),both in thin disc dwarf stars. A t ypical error bar in the [X/Fe] ratiosfor our M dwarfs is shown in the bottom left corner of each panel.For dwarf and giant stars (grey dots) the error bars are slightly smaller,which we have omitted for clarity. The red open circles in all the panelsare the stars in which the Rb lines may be affected by magnetic field.Continuous lines are theoretical GCE predictions by Prantzos et al.(2018, 2020). Orange lines include the contributions from low- andintermediate-mass stars (LIMS), rotating massive stars (MSR), and ther-proces. Magenta lines include only LIMS.

trum of the Sun the resonance lines (λλ4077.71 and 4215.52 Å)form almost in LTE, the two subordinate blue lines (λλ4161.79and 4305.44 Å) show weak non-LTE corrections (∼ +0.05 dex),while for the NIR lines non-LTE corrections are severe (from−0.20 to −0.36 dex). Unfortunately, of these three lines, onlythat at λ10327 Å is covered by CARMENES. Similarly to Rb,we calculated a grid of non-LTE abundance corrections withinthe range of the atmospheric parameters of our stars and Sr abun-dances in the range −0.3 ≤ [Sr/H] ≤ +0.3. Non-LTE abundancecorrections are also negative and range from −0.28 to −0.13dex, with the correction decreasing with increasing temperatureand gravity. The correction is larger for moderately metal-poorstars due to the smaller contribution of collisions in the statisticalequilibrium. The non-LTE abundances of Sr derived are shownin Table 2 (we prefer not to include the non-LTE Rb abundancesin this table for succinctness. The effect is abundances typicallylower by ∼ 0.15 dex.

3. Results and Discussion

Table 2 shows the Rb, Sr, and Zr abundances determined in oursample of stars. As is customary, the abundances in Table 2 are



plotted as [X/Fe] against [Fe/H] in Fig. 5 to reveal trends in therelative abundance of element X and the metallicity. Two pointsare immediately apparent:

(1) The [Sr,Zr/Fe] ratios derived in M dwarfs (blue dots) nicelyagree with those derived in FGK dwarfs (grey dots) for themetallicity range studied here. Data in FGK dwarf stars weretaken from Battistini & Bensby (2016), Delgado Mena et al.(2017), and Mishenina et al. (2019), which we considered asrepresentative studies in the recent literature based on veryhigh S/N spectra (formal errors in [Fe/H] and [Sr,Zr/Fe] inthese studies were typically 0.06 dex and ∼ 0.10 dex, respec-tively). For the range of metallicity studied, the [Sr/Fe] ra-tio clusters around the solar value, the average ratio being−0.06 ± 0.10 dex. The [Zr/Fe] ratio shows a small trend ofincreasing ratio for decreasing metallicity, identical to thatfound in FGK dwarfs (grey dots), although observations in-dicated that this trend flattens in metal-poor stars ([Fe/H] <−1.5 (see e.g. Brewer & Carney 2006; Reddy et al. 2006)7.For [Zr/Fe] the average ratio is 0.06±0.09 dex. In both cases(Sr and Zr), the dispersion around the mean is lower than theexpected observational errors in the [X/Fe] ratios.

(2) The [Rb/Fe] ratios derived in M dwarfs are systematicallybelow the solar value. This deficiency is on average [Rb/Fe]= −0.26 ± 0.13 dex in the metallicity range studied, andwould remain if non-LTE Rb abundances were used insteadbecause the systematic correction by ∼ 0.15 dex to lowerabundances would be almost compensated by a lower solarRb abundance (2.35; see above) when deriving the [Rb/Fe]ratio. Interestingly, Fig. 5 (top panel) shows that the few[Rb/Fe] ratios derived in FGK dwarfs in the similar metal-licity range also show negative values. This behaviour is incontrast with that of its associated elements Sr and Zr. How-ever, the average [Rb/Fe] ratio shows a significant disper-sion, slightly larger than the expected observational errors.Furthermore, a hint of an increasing trend in the [Rb/Fe] ra-tio with metallicity can be seen for [Fe/H] & −0.3, althoughthis trend is weak (correlation coefficient of r = 0.43).

The Rb underabundances found in our M dwarfs are unex-pected and require further discussion. We cannot exclude sys-tematic errors in the analysis, such as the existence of unknownblends in the Rb i lines, although there is no evidence of this. Inparticular, in our program stars, the strengthening of the line withdecreasing Teff is consistent with the behaviour of a resonanceline of a heavy-element neutral species. The very well knownblend at λ ∼ 7800 Å with a Si i line is not expected to play anyrole here because of its high excitation energy (χ = 6.18 eV) andthe low Teff of our stars. Another possibility is incorrect place-ment of the spectral continuum. We explored this possibility bychanging its position up (down) by a given fraction. However,this led systematically to unrealistically large (small) Ti or/andO abundances from fits to the TiO lines in this region. On theother hand, underestimation of Teff would lead systematicallyto the derived low Rb abundances, but differences of at leastas large as ∼ 150 K would be required to explain this Rb un-derabundance. Although such a systematic error in Teff cannotbe fully excluded, this would also imply a significant change inthe derived Sr and Zr abundances, which would be translatedinto a disagreement between the [Sr,Zr/Fe] ratios in M and FGKdwarfs (see Fig. 5). Systematic errors in the other stellar param-eters at the level of the expected uncertainties have only a minor

7 It is beyond the scope of this study to discuss the abundance trendsin metal-poor stars.

impact on the derived Rb abundance. In this respect, the mostimportant parameter is C/O. However, we verified that unrealis-tically low O and C abundances would be required to remove theobserved Rb deficiency (which would also affect the fits of theTiO lines in the Rb spectral region).

We also explored possible correlations of the Rb abundancewith the adopted Teff and log g. While there is no correla-tion with Teff , we found a weak correlation with log g, namelylarger Rb abundances for decreasing log g (correlation coef-ficient r = 0.44). This correlation, although weak, could berelated with the stellar age. Evolutionary tracks show that Mdwarfs of Z ∼ 0.0 and log g . 4.6 dex may have youngages because they are still contracting quasi-hydrostaticallytowards the main sequence (Bressan et al. 2012; Tang et al.2014; Baraffe et al. 2015). Effects of low gravity in spectraof very young M (and L) dwarfs have been observed in stel-lar kinematic groups and the 125 Ma-old Pleiades open cluster(Mohanty & Basri 2003; Cruz et al. 2009; Shkolnik et al. 2009;Zapatero Osorio et al. 2014; Alonso-Floriano et al. 2015). Inter-estingly, the overwhelming majority of the stars in our samplewith log g < 4.7 dex have [Fe/H] > +0.15 dex and thereforemay be relatively young. However, stellar chromospheric activ-ity is observationally known to decline steeply with stellar agefor solar-type field dwarfs that are younger than the Sun, andthereafter continues to decrease very slowly as stars get signifi-cantly older (Soderblom 2010; Ramírez et al. 2014). This seemsto also be the case for M dwarfs (see e.g. Delfosse et al. 1998;Pizzolato et al. 2003; Astudillo-Defru et al. 2017) and, since theresonance Rb lines form in the upper layers of the atmosphere,the formation of these lines could be affected by the stellar ac-tivity and thus lead to incorrect estimation of the Rb abundance.

Related to the stellar ages above, another possible effect totake into account for very active stars with strong magnetic fieldsis the Zeeman broadening of the spectral lines with large Landéfactors. The affected lines are shallower when compared to thoseof a non-active star, and this can affect the abundance deter-mination by spectral synthesis if the magnetic field is not in-cluded. We identified three stars in our sample, namely J11201–104 (LP 733–099), J15218+209 (OT Ser), and J18174+483(TYC 3529–1437–1), with strong activity levels based on theirHα emission from the CARMENES activity indicator analysis(Schöfer et al. 2019). By comparison with non-active stars of thesame spectral type, we confirmed that in the three cases both Rblines are indeed affected by Zeeman broadening (Fig. 6). Forthese three stars, the Rb abundances are amongst the lowest de-rived in the sample, and are consequentially highly uncertain(see Table 2).We plot them with red open circles in Figs. 5 and8. Nevertheless, even excluding these active stars and those withlog g . 4.7 dex from our analysis, the [Rb/Fe] deficiency and thepositive correlation with [Fe/H] still remains (showing a slightlyhigher correlation, r ∼ 0.47). On the other hand, no correlationis found between the derived Sr and Zr abundances with Teff andlog g.

There could be other possibilities. Indeed, Lambert et al.(1995) found a similar problem when deriving Rb abundancesin evolved cool M giants. These latter authors obtained system-atic Ti underabundances from TiO lines in the region of the Rblines, which is at odds with the metallicity derived from atomiclines. They discussed different possibilities to explain this ef-fect, but could only solve this problem by adding an extra quasi-continuous opacity κ(TiO) in the spectral synthesis. They alsoshowed that the Rb abundance expressed as [Rb/Fe] was almostinsensitive to the chosen combination of quasi-continuous opac-ity and Ti abundances as long as [Fe/H] is derived from metal-



Fig. 6. Normalised spectra around the Rb i λ7947 Å line of stars withstrong (J15218+209, OT Ser, M1.5 V, top, black line) and faint mag-netic field (J04376–110, BD-11 916, M1.5 V, bottom, black line), incomparison with the spectra of a CARMENES GTO, slowly rotatinginactive star of the same spectral type (J16254+543, GJ 625, M1.5 V,red line) broadened to the corresponding target star v sin i.

lic lines around the Rb i lines (see Section 2). However, we didnot find this issue here, as the metallicty inferred from the TiOlines around the Rb spectral region is compatible (within theuncertainty) with that derived from individual metallic lines inthe other spectral ranges. Therefore, other possibilities have tobe explored. For instance, it is well known that the Rb abun-dance in the Sun is rather controversial (Lodders 2019). Ini-tial determinations by Withbroe (1971), Ross & Aller (1976),Anders & Grevesse (1989), and Palme & Jones (2003), amongothers, obtained 2.60 with very little dispersion. This value issignificantly higher than the meteoritic value derived from CIchondrites (2.36 ± 0.03; e.g. Lodders & Palme 2009). Later,Grevesse et al. (2015) reduced this discrepancy and determined2.47 ± 0.07 dex using a 3D atmosphere model for the Sun, butstill ∼ 0.1 dex larger than the meteoritic value. While our non-LTE abundance (2.35± 0.02 dex; see Section 2) is in nice agree-ment with the meteoritic value, it seems that the issue may nolonger be with the Rb in the Sun, but perhaps with the Sun inrespect to the nearby stars.

There is some observational evidence that Rb in the SolarSystem could be overabundant with respect to the local ISM.Neutral Rb is expected to follow the spatial trend of neutral K(another alkali element) in interstellar clouds because these lat-ter have similar ionisation potentials and chemical properties andtherefore a determination of the interstellar elemental Rb/K ratiomay provide insight into this issue. While the alkali ratios Li/Kand Na/K measured in the local ISM (Welty & Hobbs 2001) areconsistent with those in the Solar System, this is not the casefor Rb/K. Walker et al. (2009) studied Rb/K toward seven dif-ferent ISM directions from the Sun and obtained values ∼ 34 %lower than the meteoritic (solar) value, except perhaps towardsρOph A. The condensation temperature for Rb is lower than thatfor K. As a result, if depletion onto grains was the only decid-

ing factor then one would expect a higher Rb/K, contrary to theobservational results. Depletion patterns in the local ISM are notalways found for elements with similar condensation tempera-tures to that of Rb. For example, Cartledge et al. (2006) found adepletion by ∼ 25 % in Ge with respect to the meteoritic valuebut no evidence for Cd and Sn. This result led Walker et al.(2009) to suggest that neutron capture elements synthesised bythe weak s-process and the r-process in massive stars (as Rb)appear to show significantly lower interstellar abundances thanseen in the Solar System when compared with those synthesisedmainly in low-mass stars. However, Ritchey et al. (2018), in amore comprehensive study on the depletion pattern of severalneutron-capture elements (Ge, Ga, As, Kr, Cd, etc.) in the localISM, concluded that a simple dichotomy between the productionof heavy elements by low- and high-mass stars cannot accountfor the unexpectedly low interstellar abundance of Rb and otherneutron capture elements. The value found here for M dwarfs,if confirmed, may open this discussion again. To elucidate this,K abundance determinations are suggested in M dwarfs. Unfor-tunately, most K i lines identified in the CARMENES spectra ofM dwarfs are very strong and are affected by magnetic activity(Fuhrmeister et al., in prep.). Accepting the [Rb/Fe] discussedabove as genuine, in the following section we compare our ob-servational results with a Galactic chemical evolution (GCE)model for neutron-capture elements in order to extract informa-tion about possible nucleosynthetic implications.

3.1. Comparison with GCE models

Figure 5 compares the observed [Rb,Sr,Zr/Fe]-versus-[Fe/H]trends with the theoretical predictions according to the GCEmodel from Prantzos et al. (2018, 2020) for the solar neigh-bourhood. This model is a state-of-the-art one-zone GCE modelas far as the neutron capture elements are concerned, whichuses recent yields from LIMS (Cristallo et al. 2015), as wellas of massive rotating stars (MSRs; Limongi & Chieffi 2018).The former are the main contributors to neutron-capture ele-ments formed through the main component of the s-process,while the latter contribute through the weak s-process, for whichthe inclusion of stellar rotation becomes critical. The r-processis assumed in this GCE model to be associated to the evolu-tion of massive stars. For Rb, at the time of the Solar Sys-tem formation, this GCE model predicts s- and r-fractions of51 % and 49 % (33 %, 67 % for 85Rb; 96 %, 4 % for 87Rb),and 91.2 %, 73 %, 82 % (s-fraction) and 8.3 %, 22 %, 18 % (r-fraction) for the light heavy elements Sr, Y, and Zr, respectively8.We note that the model accounts for the observed Solar Systemabundances of most of the heavy elements (Z> 30) within 10 %.A detailed description of this GCE model, as well as of the stellaryields used, can be found in Prantzos et al. (2018, 2020).

We compare the model predictions with observations in twoillustrative cases in Fig. 5: the orange line shows the predictedrelationships when all the nucleosynthetic sources are included(LIMS, MSR, and r-process), which we refer to as the ‘baseline’model, while the magenta line shows the result when only theLIMS contribution is considered (i.e. the main s-process contri-bution). The baseline model considerably better reproduces theaverage observed [Rb/Fe]-versus-[Fe/H] trend at low metallicity([Fe/H]. −1.0; top panel of Fig. 5), where the [Rb/Fe] ratio isalmost flat (∼ +0.3 dex; i.e. it has a primary behaviour), sim-ilar to that observed for a genuine r-process element (e.g. Eu;Mashonkina et al. 2003; Zhao et al. 2016; Delgado Mena et al.

8 There is a 0.5 % fraction contribution for Sr due to the p-process.



2017). We conclude that at low metallicity the r-process is themain source of Rb, although with a non-negligible contribu-tion from the weak s-process occurring in MRS. The Rb pro-duction from LIMS has a secondary behaviour (magenta line)and only becomes significant at [Fe/H] & −0.3 (see Fig. 5, toppanel). At [Fe/H] & −1.0, the observed [Rb/Fe] ratio declinesin a similar way as other primary elements (or elements witha main r-process origin) due to the increasing contribution toFe from type Ia supernovae. However, contrary to what is typ-ically observed for these elements, the [Rb/Fe] ratio reaches∼ 0.0 at a lower metallicity and is below the solar ratio at[Fe/H]≈ 0.0 according to our observational results. We note thatGratton & Sneden (1994) and Tomkin & Lambert (1999) alsoderived [Rb/Fe] < 0.0 in a few stars with [Fe/H] > −1.09. Thisis an unexpected result and has not been observed in any otherheavy element. Typically most of the heavy elements show av-erage solar ratios ([X/Fe]) at [Fe/H] ∼ 0, but not systemati-cally below solar, as can be seen, for instance, in the [Sr,Zr/Fe]-versus-[Fe/H] relationships (middle and lower panels of Fig. 5).The observed behaviour of the [Sr,Zr/Fe] ratios is reasonablywell reproduced by our baseline model. However, this baselinemodel cannot reproduce the observed [Rb/Fe] ratios at [Fe/H]& −1.0. This fact would imply at least two things: (a) the Rbproduction in massive stars is overestimated in the GCE modelat these metallicities; and (b) because of the time delay of theAGB stars producing cosmic Rb, their contribution is theoreti-cally predicted to start contributing at metallicities higher than[Fe H]∼ −0.7 (Bisterzo et al. 2014; Prantzos et al. 2018). How-ever, as seen in Fig. 5, the contribution from AGB stars is notenough to explain the observed [Rb/Fe] ratios at nearly solarmetallicity. We note that Rb yields from different AGB stellarmodels (e.g. Karakas & Lattanzio 2014) may be systematicallylarger with respect to those by Cristallo et al. (2015) adoptedin Prantzos et al. (2018). These differences are due to differentprescriptions used for mixing and mass-loss rates. However, inthis case the yields of other heavy elements produced by the s-process, such as Sr, Y and Zr in particular, are also enhanced,resulting in a probable overproduction of these elements, whichis not required according to our GCE model (see Fig. 5, middleand bottom panels).

As mentioned before, for metallicities above solar, we foundan increasing [Rb/Fe] trend, which would imply a secondary be-haviour with metallicity. This trend is surprising, because ac-cording to GCE models, the Fe contribution from type Ia su-pernovae should nominally decrease the trend. The secondarynature of this trend excludes a standard r-process contribution,while it could favour a s-process scenario. The Rb s-process pro-duction is indeed secondary in both non-rotating massive starsand AGB stars. This production is related to the activation of the22Ne(α, n)25Mg neutron source, whose efficiency increases withmetallicity because of the larger amount of 22Ne left by the burn-ing of CNO 10. In AGB stars in particular, this neutron sourceis active during the thermal pulses when the temperature in theHe-shell attains or exceeds ∼ 300 MK. However, in low-massAGB stars, which are the galactic polluters responsible for the production of the s-process main component, the 22Ne(α, n)25Mg

9 We have scaled the [Rb/Fe] ratios derived by these authors to thesolar Rb abundance (2.47) adopted here.10 At the end of the H burning, the initial amount of C+N+O is mostlytransformed into 14N. Later on, when the He burning starts, 22Ne is pro-duced by the chain 14N(α, γ)18F(β)18O(α, γ)22Ne. In practice, the initialamount of C+N+O is transformed into 22Ne. This scenario is altered ifrotation-induced mixing brings fresh C, as synthesised by the He burn-ing, into the H-burning shell.

Fig. 7. Scheme illustrating the operation of the two branches at 85Kr(ground state) and 86Rb that determine the Rb production by the s-process. Black filled nuclei are stable, while the empty ones are unsta-ble. Arrows show possible nuclear reactions, particularly neutron cap-tures or β-decays. In case of low neutron density (Nn < 109 cm−3),both branches are closed and the s-process follows the chain 84Kr →85

Kr →85 Rb →86 Rb →86 Sr. Owing to the large neutron-capture crosssection of 85Kr compared to those of the neutron-magic nuclei (N=50)88Sr, 89Y, and 90Zr, a low amount of Rb is produced. On the contrary, athigher neutron density, the two branches are open and the s-process se-quence deviates toward 86Kr and 87Rb, both of which are neutron magicnuclei, and therefore both present very low neutron-capture cross sec-tions. In the latter case, large overabundances of Kr and Rb are found.

is only marginally activated because of the low temperature de-veloped during the thermal pulses. In contrast, the main neu-tron source, namely 13C(α, n)16O, is active during the interpulsephase, when the temperature is ∼ 90 MK, but the resulting neu-tron density is much lower than the threshold needed to openthe 85Kr (ground state) and 86Rb branches. This occurrence isillustrated in Figure 7. As a result, low [Rb/Sr,Y,Zr] ratios areexpected.

An interesting deviation from the standard AGB scenario il-lustrated so-far was firstly described by Cristallo et al. (2009)(see also Karakas 2010; Cristallo et al. 2018). At nearly solarmetallicity, the 13C is not completely burnt during the interpulseand part of it is engulfed in the convective shell generated by thefollowing thermal pulse and burned at higher temperature (∼ 200MK). The resulting neutron density is high enough to open the85Kr and 86Rb branches, allowing the production of the neutronmagic rubidium isotope: 87Rb. In this case, owing to the lowneutron-capture cross section of this magic nucleus, an overpro-duction of Rb is expected, while Sr, Y, and Zr, which are mainlyproduced by the interpulse 13C burning, are not affected. Theoccurrence of this additional 13C neutron burst requires particu-larly low interpulse temperatures, and in turn low core masses.For this reason, it may only occur in AGB stars with low masses(M. 3 M⊙) and high metallicities ([Fe/H]> −0.1). An impor-tant ingredient of the theoretical stellar models that hampers adefinite validation of this hypothesis is our poor knowledge ofthe 13C(α, n)16O reaction (Cristallo et al. 2018). In this context,a lower rate at T ∼ 90 MK would greatly enhance the contri-bution to the galactic Rb of low-mass AGB stars at nearly solar



Fig. 8. Same as Fig. 5 but for [Rb/<Sr+Zr>] vs. [Fe/H]. Blue dots are Mdwarfs in this study. Grey dots corresponds to the giants and dwarfs starsanalysed in common by Gratton & Sneden (1994), Tomkin & Lambert(1999), and Mishenina et al. (2019). Red open circles are stars in whichRb lines may be affected by the magnetic field. Coloured solid lines arethe GCE predictions as in Fig. 5. The black solid line shows the GCEprediction when the weak s-process is inhibited in massive stars (i.e. norotation).

and super-solar metallicity. It is possible that the observed high-metallicity trend of galactic Rb shown in Fig. 5 (top panel) is ahint in favour of this scenario.

A similar trend of increasing [Ba/Fe] with increasingmetallicity was reported at young ages in solar twins, openclusters, and local associations (e.g. D’Orazi et al. 2012;Reddy & Lambert 2015; Nissen 2016; Tabernero et al. 2017).However, this trend is not seen for other heavy elements of sim-ilar s-process origin (e.g. La, Nd). To account for this Ba trend,Maiorca et al. (2012) suggested that the observed enhancementcan be produced by nucleosynthesis in AGB stars of low mass(M < 1.5 M⊙) if they release neutrons from the 13C(α,n)16O re-action in reservoirs larger by a factor of four than assumed inmore massive AGB stars (M > 1.5 M⊙). Mishenina et al. (2015)instead reintroduced the intermediate neutron-capture i-processdiscussed earlier by Cowan & Rose (1977) to explain this fact, inwhich the neutron density operating the process is approximately1015 cm−3 . Also, recently (Ryde et al. 2020) found a secondarybehaviour of [F/Fe] for supersolar metallicities in K giants. Wenote that F has a significant cosmic contribution from AGBstars (Jorissen et al. 1992; Karakas & Lattanzio 2014; Abia et al.2019). Although F is produced in AGB stars from an alternativenuclear channel to the s-elements, its production needs neutronsand is therefore linked to the s-process (see e.g. Cristallo et al.2014; Abia et al. 2019). Nevertheless, as far as the [Ba/Fe] ra-tio is concerned this trend has recently been shown to be corre-lated with the stellar activity of young stars and to not be nucle-osynthetic in origin (Reddy & Lambert 2017). Some of the starsstudied here were flagged as active by Passegger et al. (2019),but except for the three stars mentioned above, we checked thatthe rest of the stars flagged as active are distributed uniformly

within the range of [Rb/Fe] ratios derived here. In any case, a de-tailed study of possible connection between stellar activity andRb abundances is suggested in M dwarfs without showing anypeculiar abundance pattern.

Another valuable piece of information on the productionsources of Rb and its evolution with metallicity comes from itsabundance ratio with respect to the neighbouring s-elements Sr,Y, and Zr. As mentioned in Section 1 this abundance ratio pro-vides relevant information on the physical conditions prevailingat the s-process site, in particular about the neutron density. Thismay allow, for instance, the characteristic stellar mass of the pro-ducing objects to be inferred. Figure 8 shows the [Rb/<Sr+Zr>]-versus-[Fe/H] ratios derived in our stars (blue dots), as well asin the studies by Gratton & Sneden (1994), Tomkin & Lambert(1999), and Mishenina et al. (2019) in FGK dwarfs (grey dots).<Sr+Zr> is the average ratio between [Sr/H] and [Zr/H] in thecorresponding star. As in Fig. 5, the orange line is the theoret-ical prediction according to the GCE model by Prantzos et al.(2018), which includes LIMS, MRS, and the r-process contribu-tions, while the magenta line is the prediction when only yieldsfrom LIMS are included. Figure 8 also shows the predictionin the case where rotation is excluded in massive stars (blackline), that is, the weak s-process contribution is strongly inhib-ited. We note that the observed trend does not change signifi-cantly if non-LTE Rb abundances are used. At low metallicity([Fe/H] < −1.0) the observed [Rb/<Sr+Zr>] ratio is above thesolar value and again is better reproduced when all the possibleproduction sources of Rb are included (orange line). At thesemetallicities, the r-process dominates the production of Rb, Sr,and Zr but, as mentioned before, there is a non-negligible contri-bution from the weak s-process in massive stars as can be ap-preciated by comparison with the prediction with no weak s-process (black line). At the typical neutron densities of the weaks-process (Nn ∼ 1011−13 cm−3), some branchings, closed in thecase of the 13C(α,n)16O neutron source, are opened (see Fig. 7).This allows alternative paths, in particular that at the 85Kr, whichleads to an overproduction of Rb with respect to Sr and Zr, giv-ing [Rb/<Sr+Zr>]≥ 0.0. We note that the 22Ne(α,n)25Mg sourcebecomes relevant even for stellar masses as low as ∼ 2.5 M⊙ andZ. 0.01 Z⊙ (Straniero et al. 2014). Figure 8 shows that as metal-licity increases, the [Rb/<Sr+Zr>] diminishes and is below solaron average for [Fe/H]∼ 0.0, although with a significant disper-sion. This dispersion is considerably larger than expected fromthe uncertainties in the analysis and may therefore be real. Thedecrease in the [Rb/<Sr+Zr>] ratio for increasing metallicity isclearly due to the increasing relevance of the contribution of low-mass stars in the production of Rb, Sr, and Zr through the mains-process, for which the 13C(α,n)16O is the main neutron source.When this neutron source is at work, [Rb/<Sr+Zr>]< 0.0 is ex-pected, as shown by GCE model (magenta line). Nevertheless, inFig. 8 there are a number of stars showing [Rb/<Sr+Zr>] & 0.0.If the Rb abundances in these stars were correct (see the discus-sion above), these ratios would be better explained if massive (in-termediate) AGB stars contribute significantly at [Fe/H]& 0.0. Insummary, with the available yields for Rb from different sources,the observed [Rb/<Sr+Zr>] ratios at near-solar metallicity seemto require a mix of these contributing sources. It is obvious thatdeterminations of Rb abundances in FGK dwarf stars, the anal-ysis of which is subject to less uncertainties, are needed to shedlight on this Rb puzzle.



4. Summary

We derived, for the first time, abundances of Rb and its associ-ated elements Sr and Zr in a sample of M dwarfs in the closesolar neighbourhood and in the metallicity range −0.5 . [Fe/H]. +0.3. These metallicity and spectral type ranges were poorlyexplored in previous studies for Rb abundances. We used high-resolution and high-S/N spectra in the VIS and NIR acquiredwith the CARMENES spectrograph. The main novel result ofour study is that, in the explored metalliciy range and relative tothe metallicity, rubidium is systematically underabundant withrespect to the Sun. This underabundance is of almost a factortwo on average, although with significant dispersion. This re-sult is in contrast with the figure found for Sr and Zr ratios withrespect to the metallicity, which are very close to the solar ratio,and is identical to that found in unevolved FGK dwarfs of similarmetallicity. Furthermore, we find a possible positive correlationof the [Rb/Fe] ratio with increasing metallicity. We discuss thereliability of these results for Rb, never found for other neutron-capture element in the metallicity range studied here, in termsof non-LTE effects, systematic errors in the analysis, anomalousSolar System Rb abundance, and stellar activity, but no plausibleexplanation is found.

Under the assumption that the abundance trend is real,we interpret the [Rb/Fe]-versus-[Fe/H] relationship in the ob-served full range of metallicity by comparing it with theo-retical predictions from state-of-the-art one-dimensional GCEmodels for neutron-capture elements in the solar neighbour-hood (Prantzos et al. 2018). The observed Rb overabundancesat [Fe/H] . −1.0 can be explained if the r-process is the mainsource of the Galactic Rb budget at these metallicities, althoughwith a significant contribution from the weak s-process occur-ring in rotating massive stars. However, the [Rb/Fe] underabun-dances observed at higher metallicities cannot be reproduced bythis model, nor can the possible secondary behaviour of [Rb/Fe]with metallicity. This secondary behaviour would require a muchhigher Rb yield from AGB stars through the main s-process thanis currently obtained in s-process nucleosynthesis calculationswithout overproducing Sr, Y, and Zr. A possible nucleosynthe-sis scenario occurring in low-mass and high-metallicity AGBstars is suggested. In addition, the negative [Rb/<Sr+Zr>] ra-tios observed at near solar and higher metallicities, when com-pared with the GCE predictions, indicate that AGB stars are in-deed the main producers of Rb at these metallicities, althoughthe large scatter observed suggests that there are other contribut-ing sources.

Overall, our abundance analysis showcases the value ofabundance determinations in M dwarfs for Galactic chemicalevolution studies. Additional Rb abundance measurements inFGK dwarfs of near solar metallicity, as well as an evaluationof the impact of stellar activity on abundance determinations inM dwarfs, are urgently needed to confirm or disprove the mainfindings of this study.

Acknowledgements. CARMENES is an instrument for the Centro AstronómicoHispano-Alemán (CAHA) at Calar Alto (Almería, Spain), operated jointlyby the Junta de Andalucía and the Instituto de Astrofísica de Andalucía(CSIC). CARMENES was funded by the Max-Planck-Gesellschaft (MPG),the Consejo Superior de Investigaciones Científicas (CSIC), the Ministerio deEconomía y Competitividad (MINECO) and the European Regional Develop-ment Fund (ERDF) through projects FICTS-2011-02, ICTS-2017-07-CAHA-4, and CAHA16-CE-3978, and the members of the CARMENES Consortium(Max-Planck-Institut für Astronomie, Instituto de Astrofísica de Andalucía, Lan-dessternwarte Königstuhl, Institut de Ciències de l’Espai, Institut für Astro-physik Göttingen, Universidad Complutense de Madrid, Thüringer Landesstern-warte Tautenburg, Instituto de Astrofísica de Canarias, Hamburger Sternwarte,Centro de Astrobiología and Centro Astronómico Hispano-Alemán), with ad-

ditional contributions by the MINECO, the Deutsche Forschungsgemeinschaftthrough the Major Research Instrumentation Programme and Research UnitFOR2544 “Blue Planets around Red Stars”, the Klaus Tschira Stiftung, the statesof Baden-Württemberg and Niedersachsen, and by the Junta de Andalucía. Weacknowledge financial support from the Agencia Estatal de Investigación of theMinisterio de Ciencia e Innovación, the Universidad Complutense de Madrid,the Fundação para a Ciência e a Tecnologia, the Generalitat de Catalunya,ERDF, and NASA through projects PGC2018-095317-B-C21, PID2019-109522GB-C51/2/3/4, PGC2018-098153-B-C33, AYA2016-79425-C3-[1,2,3]-P, FPU15/01476, UID[B,P]/04434/2020, PTDC/FIS-AST/28953/2017, POCI-01-0145-FEDER-028953, SEV-2015-0548, SEV-2017-0709, MDM-2017-0737,NNX17AG24G, and the CERCA programme. Finally, we thank to Verne Smith,Katia Cunha, and the APOGEE/ASPCAP team for the APOGEE line list, and toK. Lodders for her useful comments on the manuscript.

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Table 2. Stellar parameters and abundances derived in the sample of M dwarfsa.

Karmn Name Teff (K) log g C/O [M/H] log ǫ(Rb) log ǫ(Sr) log ǫ(Zr)

J00051+457 GJ 2 3772 4.68 0.59 0.10 2.32 ± 0.02 2.91 2.70J00183+440 GX And 3606 4.77 0.61 −0.14 2.12 ± 0.05 2.58 2.50J00389+306 G 60–10 3491 4.78 0.59 −0.10 2.22 ± 0.01 2.73 2.55J00570+450 G 172–30 3397 4.85 0.50 −0.10 2.13 ± 0.01 2.90 2.80J01013+613 GJ 47 3496 4.80 ... −0.11 2.00 ± 0.02 2.80 2.55 ± 0.02J01025+716 GJ 48 3432 4.78 0.47 0.00 2.19 ± 0.01 2.85 2.60J01026+623 BD+61 195 3810 4.67 0.54 0.06 2.45 2.98 2.63J01433+043 G 3–14 3485 4.80 0.50 −0.10 1.99 ± 0.03 2.70 2.60 ± 0.01J01518+644 G 244–37 3666 4.64 0.58 0.20 2.42 ± 0.02 2.96 2.72 ± 0.02J02015+637 G 244–47 3452 4.79 0.59 0.03 2.44 ± 0.01 2.90 2.61 ± 0.01J02123+035 BD+02 348 3605 4.90 0.58 −0.35 1.80 ± 0.02 2.57 2.45 ± 0.03J02222+478 BD+47 612 3916 4.64 0.58 0.15 2.43 ± 0.02 2.95 2.70 ± 0.01J02358+202 BD+19 381 3726 4.63 0.59 0.18 2.48 ± 0.03 2.93 2.60 ± 0.01J02442+255 G 36–31 3434 4.83 0.58 0.00 2.12 ± 0.02 2.98 2.63 ± 0.01J02565+554W G 174–19 3889 4.64 0.54 0.30 2.54 ± 0.02 2.98 2.73 ± 0.01J03181+382 BD+37 748 3854 4.64 0.58 0.20 2.72 ± 0.01 3.20 2.83 ± 0.02J03213+799 GJ 133 3533 4.78 0.59 −0.10 2.05 ± 0.05 ... 2.50 ± 0.01J03217−066 G 77–46 3502 4.85 0.54 −0.10 1.99 ± 0.04 2.98 2.65 ± 0.02J03463+262 HD 23453 4001 4.66 0.54 0.20 2.40 ± 0.05 3.11 2.78 ± 0.02J03531+625 Ross 567 3454 4.79 0.59 −0.05 2.00 ± 0.02 2.65 2.48 ± 0.01J04225+105 LSPM J0422+1031 3415 4.80 ... 0.05 2.55 ± 0.01 2.89 2.73J04290+219 HD 28343 4187 4.63 0.54 0.30 2.65 ± 0.01 3.17 2.85 ± 0.05J04376−110 BD−11 916 3611 4.74 0.59 0.05 2.12 ± 0.05 2.93 2.70 ± 0.01J04376+528 HD 232979 4010 4.64 0.58 0.00 2.10 ± 0.05 2.92 2.63 ± 0.01J04429+189 HD 285968 3689 4.66 0.58 0.12 2.54 ± 0.01 2.85 2.60 ± 0.01J04538−177 L 736-30 3543 4.76 0.58 −0.10 2.06 ± 0.01 2.60 2.50 ± 0.01J04588+498 BD+49 1280 4004 4.64 0.52 0.15 2.36 ± 0.03 3.00 2.79 ± 0.04J05127+196 GJ 192 3570 4.75 0.54 −0.01 2.19 ± 0.02 2.87 2.78 ± 0.02J05314−036 HD 36395 3700 4.95 0.67 0.25 2.70 ± 0.02 3.40 2.97 ± 0.02J05365+113 HD 245409 4062 4.66 0.43 0.05 2.24 ± 0.03 3.05 2.79 ± 0.01J05415+534 HD 233153 3869 4.63 0.63 0.15 2.38 ± 0.02 3.00 2.82 ± 0.02J06103+821 GJ 226 3500 4.90 0.57 −0.05 2.07 ± 0.02 2.85 2.65 ± 0.05J06105−218 HD 42581 3842 4.68 0.54 0.10 2.43 ± 0.02 2.90 2.66 ± 0.03J06371+175 Ross 85 3732 4.77 0.63 −0.25 1.83 ± 0.02 2.40 2.37J06548+332 Wolf 294 3410 4.85 0.57 0.00 2.28 ± 0.05 2.87 2.62 ± 0.01J07044+682 GJ 258 3421 4.81 0.57 0.05 2.32 ± 0.01 2.90 2.65 ± 0.01J07287−032 GJ 1097 3420 4.81 0.58 0.01 2.30 ± 0.02 2.83 2.63 ± 0.02J07353+548 GJ 3452 3513 4.81 0.59 −0.05 2.08 ± 0.01 2.70 2.63 ± 0.01J07361−031 BD−02 2198 3790 4.70 ... −0.04 2.05 ± 0.02 2.60 2.59 ± 0.02J08161+013 G 113–0 3534 4.77 0.57 −0.04 2.15 ± 0.01 2.60 2.60 ± 0.01J08293+039 2M J08292191+0355092 3685 4.64 0.54 0.22 2.52 ± 0.01 2.88 2.63 ± 0.01J08358+680 G 234–37 3434 4.82 0.57 −0.06 2.05 ± 0.01 2.85 2.64 ± 0.03J09133+688 G 234–57 3494 4.86 ... −0.05 2.00 ± 0.05 2.98 2.70 ± 0.05J09140+196 LP 427–16 3591 4.65 0.58 0.11 2.20 ± 0.05 2.95 2.59 ± 0.01J09143+526 HD 79210 4007 4.67 0.58 0.07 2.17 ± 0.02 2.97 2.68 ± 0.01J09144+526 HD 79211 3980 4.67 0.49 0.10 2.17 ± 0.01 2.95 2.71 ± 0.02J09163−186 L 749–34 3524 4.85 0.57 −0.12 1.97 ± 0.01 2.89 2.72 ± 0.02J09411+132 Ross 85 3654 4.72 0.57 0.03 2.19 ± 0.02 2.90 2.65 ± 0.01J09425+700 GJ 360 3718 4.67 0.41 0.10 2.55 ± 0.02 2.80 2.80J11054+435 BD+44 2051A 3617 4.79 0.43 −0.20 2.05 ± 0.02 2.80 2.53J11201−104b LP 733–099 3469 4.91 0.58 −0.20 1.78 ± 0.02 2.57 2.50 ± 0.01J12111−199 LTT 4562 3440 4.83 0.59 −0.10 2.18 ± 0.02 2.75 2.70J13450+176 BD+18 2776 3900 4.80 0.57 −0.50 1.75 ± 0.05 2.30 2.20J13457+148 HD 119850 3610 4.73 0.47 −0.10 2.12 ± 0.02 2.58 2.65J15218+209b OT Ser 3500 4.90 0.58 0.10 2.05 2.71 2.70 ± 0.02J18174+483b TYC 3529-1437-1 3435 4.80 0.59 −0.08 1.85 ± 0.02 2.88 2.58 ± 0.01J22565+165 HD 216899 3606 4.77 0.59 0.15 2.55 ± 0.01 3.02 2.75

Notes. (a) The C/O ratio in the Sun is 0.575 (Lodders 2019). Abundances of Rb, Sr, and Zr are given on the scale log N(H) ≡ 12. The Sr abundancesshown are corrected for non-LTE effects (see text). For Rb and Zr abundances, the abundance dispersion is also tabulated. (b) The resonance Rblines in these stars may be affected by magnetic field. Their abundance ratios are indicated as red open circles in Figs. 5 and 7.



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