Astrophysical false positive in direct imaging: a white...

Astronomy & Astrophysics manuscript no. papero c© ESO 2012December 10, 2012

Astrophysical false positive in direct imaging: a white dwarf closeto a rejuvenated star. ?

A. Zurlo1,2, A. Vigan1, J. Hagelberg3, S. Desidera2, G. Chauvin4,K. Biazzo5, M. Bonnefoy6, E. Covino5, P. Delorme4, R. Gratton2, D. Mesa2, S. Messina7, C. Moutou1, V. D’Orazi8, M.

Turatto2, D. Segransan3, S. Udry3, F. Wildi3

1Aix Marseille Universite, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388, Marseille, France2INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova, ITALY, 35122-I3Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland4Institut de Planetologie et d’Astrophysique de Grenoble, UJF, CNRS, 414 rue de la piscine, 38400 Saint Martin d’Heres, France5INAF-Osservatorio Astronomico di Capodimonte, salita Moiariello 16, Napoli, 80131, Italy6Max Planck Institute for Astronomie, Konigsthul 17, Heidelberg, Germany7INAF-Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy8Department of Physics & Astronomy, Macquarie University, Balaclava Rd, North Ryde, Sydney, NSW 2109, Australia

Received / accepted

ABSTRACT

Context. As the other techniques involved in the research of exoplanets the direct imaging has to take into account the probabilityof the so-called astrophysical false positives. The latter are phenomena that mimic the signature of the object we are looking for.Sometimes those are difficult to be distinguished from the target.Aims. In this work we aim to present a case of false positive found during a direct imaging survey conducted on VLT/NACO. Apromising candidate was detected around the star HD 8049 in July 2010. Its contrast of ∆H = 7.35 at 1.56 arcsec allowed us to guessthe presence of a 35 MJ companion at 50 AU distance for the nominal age and distance.Methods. As a follow-up observation was needed, the star was observed for the second time one year later and it turned out that theprobability of the close body to be a background object was very low. We also collected radial velocity points of the host star in abaseline of ∼ 30 yr and analyzed the stellar spectrum and measured several age indicators.Results. The discrepancy between the age indicators suggested that we are not facing up with a bona-fide young star and that thecompanion is a white dwarf. Also the significant radial velocity trend indicated an object likely more massive than a brown dwarf.After evaluations on the age and mass of HD 8049 and the U-band photometry of the candidate and some properties of the systemwe concluded that the object is a white dwarf companion. The moderately large level of chromospheric activity and fast rotation,mimicking the properties of a young star might be induced by the exchange of mass with the progenitor of the WD. This example tellus another time how is important in astronomy to check the age and the distance of the stars with all available methods. We shouldconclude on the probability of white dwarf companions as false positives.

Key words. Instrumentation: spectrographs - imagers - Methods: data analysis - Techniques: radial velocities - direct imaging - Stars:planetary systems - white dwarfs - Stars: individual: HD 8049

1. Introduction

In the research for extrasolar planets we have lots of examplesof objects or physical phenomena with a similar signal signa-ture of a substellar object, a so-called astrophysical false posi-tives. The techniques used in the research of exoplanets are theradial velocities, differential photometry during transits, timing,microlensing and direct imaging. A large fraction of the planetsdiscovered so far has been detected by the radial velocity tech-nique. Rotational modulations of star spots and magnetic activ-ity cycles might produce spourious radial velocity periodic sig-nals that could mimic a planet signature (see e.g. Queloz et al.2001; Lovis et al. 2011). A number of controversial cases havebeen reported in the literature (Setiawan et al. 2008; Huelamoet al. 2008; Hernan-Obispo et al. 2010; Figueira et al. 2010).Simultaneous monitoring of activity indicators, spectral line pro-

? Based on observations collected at La Silla and ParanalObservatory, ESO (Chile): Programs 184.C-0567 (NAOS-CONICA),184.D-1151 (EFOSC), 60.A-9036 (HARPS), 089.C-0665 (SINFONI).Based on the All Sky Automated Survey (ASAS) photometric data.

file changes and photometric variations allows to recognize thetrue origin of the radial velocity variations and at least partiallyto correct for them (see e.g. Boisse et al. 2011; Lanza et al. 2010;Dumusque et al. 2012). Transit searches are even more heav-ily plagued by the issue of various kind of eclipsing binariesmimiking a transiting planet photometric dim, that can outnum-ber the true planets (Brown 2003). The issue of blended eclips-ing binaries is the most severe as described in O’Donovan et al.(2006). Various observational diagnostics, based on the photo-metric light curves or on additional spectroscopic or photometricobservations, have been developed. Statistical tools can also beexploited to explore all possible scenarios of binary systems andthe probability of their occurrence (e.g. BLENDER code Torreset al. 2011, PASTIS code, Diaz et al., in preparation). Timingvariations in eclipsing binaries might be due to circumbinaryplanets but alternative explanations cannot firmly ruled out yet(Zorotovic & Schreiber 2012). Finally, a case of possible mi-crolensing planet detection that revealed to be a false positivedue to the variability of the lensed star is described in Gouldet al. (2012).

2 Zurlo et al.: Astrophysical false positive in the NACO-LP

Table 1. Resuming table of observation of HD 8049.

Instrument Date Type of DataCORAVEL June 14 UT, 1983 RV, spectra

December 31 UT, 1983 RV, spectraNovember 30 UT, 1985 RV, spectra

CORALIE(98) from December 26 UT, 2000 RVto January 21 UT, 2007 RV

HARPS September 15 UT, 2004 RV, spectraCORALIE(07) from July 3 UT, 2007 RV

to July 8 UT, 2012 RVNACO July 30 UT, 2010 Imaging

July 28 UT, 2011 ImagingEFOSC December 21 UT, 2011 Imaging, spectraSINFONI August 28 UT, 2012 IR spectra

Concerning the direct imaging technique, first we have to as-sure that the companion we detect is gravitationally bound withthe host star. Detection of such unbound objects is frequent sincemost direct imaging data search for planets far from the host starto permit a high contrast. If the follow-up observation is closein time to the first epoch and the displacement of the star on thesky due to proper motion is small, we cannot determine with ahigh probability that the object is bound. One example is givenby Metchev & Hillenbrand (2009) where an object that resultedcompatible with being bound from the proper motion test wasfound to be a M background star thanks to spectroscopy. Again,the case of the false positive around IM Lupi (Mawet et al. 2012)teaches that when the distance determination is tricky one cannotpredict univocally the proper motion and consequently it is pos-sible to confuse a background object with a bound one. As theluminosity of substellar objects strongly depends on the age ofthe system (younger age corresponds to a brighter magnitude ofthe companion, see e.g. Baraffe et al. 2003; Madhusudhan et al.2011), spourious young stars might also provide badly classi-fied companions. Second, even if a target has a strong proba-bility of being a bound object we can jump into the case of abinary system. In this paper we present the case of an astrophys-ical false positive found in the NACO Large Program (NACO-LP1; Chauvin 2010). The first result of the survey is the study atunprecedented resolution of the components of the debris diskaround HD61005, known as “The Moth” (Buenzli et al. 2010).The object of this study, HD 8049, was observed in July 2010.The companion found from direct imaging observations couldhave a substellar mass according to its H band magnitude andnominal age of the central star. However it turned out to be awhite dwarf. The case is then similar to that of the object orbit-ing Gliese86 (see Mugrauer & Neuhauser 2005), initially con-sidered a brown dwarf by Els et al. (2001). Other recent casesof close white dwarf companions around HD 147513 and HD27442 are described in Porto de Mello & da Silva (1997) andChauvin et al. (2007) respectively.

This work is structured as follows: in Section 2 we presentall observations we made. In Section 3 the companion character-istics and in Section 4 we resume the stellar properties and howthey are influenced by the companion.

2. Observations and data reduction

HD 8049 was observed with several instruments during a periodof about 30 years. All data we collected are listed in Table 1.

1 ESO program 184.C-0567, PI J.-L. Beuzit, “Probing theOccurrence of Exoplanets and Brown Dwarfs at Wide Orbits”.

2.1. NACO observations

HD 8049 was observed as part of the NACO Large Programimaging survey. We used VLT/NaCo (Lenzen et al. 2003;Rousset et al. 2003) high-contrast adaptive optics system withthe Ks-band (2.00–2.35 µm) and the S13 camera, which providesa spatial sampling of ∼13 mas/pixel and a field of view (FoV)of 14′′ × 14′′. The target was observed in pupil-tracking modeto allow proper implementation of angular differential imaging(ADI, Marois et al. 2006) data analysis techniques. The ob-serving sequence consisted of a series of unsaturated images ofthe PSF, which served as reference for differential photometryand astrometry, and a series of deep saturated exposures opti-mized for the detection of faint companions. Data reduction forsaturated and unsaturated sequences followed standard proce-dures (flat field, sky subtraction, bad pixels correction). The sat-urated sequence was then analyzed with two methods: (1) theLOCI algorithm (Lafreniere et al. 2007) with a separation cri-terion Nδ = 0.75 FWHM, and (2) a spatial filtering of all im-ages in a 5×5 FWHM box followed by derotation and median-combination. Differential astrometry was performed using fittingof a 2D gaussian profile on the primary star and on the pointsources detected in the reduced images.

The target was first observed on July 30 UT, 2010, anda faint point source (herefafter HD 8049 B) at a separation of1.560 ± 0.006 ′′ and a position angle of 118.4 ± 0.2 deg wasdetected with a magnitude difference ∆H = 7.4 mag, snapshot inFigure 1. Follow-up observations on July 28 UT, 2011 revealedthat the point source is comoving with the primary. Thanks to thehigh proper motion of the central star (µα = 65.99± 1.18 mas/yrand µδ = −240.99 ± 0.98 mas/yr), χ2 probability test on ∆α and∆δ with respect to the star at two epochs rejected the possibilityof the companion to be a background source with a probabilityhigher than 99%.

E

N1"

Fig. 1. First detection in the NACO-LP, image of the companionafter spatial filtering in a 5 × 5λ/D box, derotation and mediancombination of the images.

Zurlo et al.: Astrophysical false positive in the NACO-LP 3

2.2. CORAVEL, CORALIE and HARPS high resolutionspectroscopy

HD 8049 has been observed as part of a long term extra-solarplanet search program started in June 1998 with the CORALIEhigh-resolution fiber-fed echelle spectrograph mounted on the1.2 m Euler Swiss telescope at La Silla (ESO,Chile). In addi-tion to that we have one spectrum taken with HARPS, and weextracted 3 RV observations with a 100 m/s precision from theCORAVEL database (Baranne et al. 1996). This enables us tohave a total time-span of nearly 29 years. For these three in-struments radial velocities are computed by cross-correlating themeasured stellar spectra with a binary mask, whose non-zerozones correspond to the theoretical positions and widths of stel-lar absorption lines at zero velocity. For HARPS and CORALIEthe instrumental velocity drifts are monitored and correctedusing the “simultaneous thorium referencing technique” withdual fibers (more details in Baranne et al. 1996), whereas forCORAVEL a standard radial velocity star (HD168454) was usedfor calibration. Radial velocities points are shown in Figure 2.The trend in the radial velocity points is likely produced by theinfluence of a more massive object than a brown dwarf. More ob-servations are needed to better explore the scenario. Additionalactivity indicators (S index, log RHK) and the projected rotationalvelocity were also measured on CORALIE and HARPS spec-trum.

Fig. 2. Radial velocity trend of the star. Different colors corre-spond to the instruments used: CORAVEL (green), CORALIE98(red), CORALIE07 (blue) and HARPS (black). The plot showsa trend on the radial velocities induced by the white dwarf to theprimary star. Such a trend could be more probably induced by amassive object rather than a brown dwarf.

2.3. NTT/EFOSC

We obtained 10 images of HD 8049 on December 21.10 UT,2011, with EFOSC2 at NTT during the program 184.D-1151.Each frame was taken with the Johnson filter U, a 0.2005 s ex-posure to avoid saturation, and 2×2 binning, seeing of 0.8′′. We

reduced the U-band images with IRAF2 task and performed theaperture photometry with the GAIA utility3. The star is resolvedas a binary system with a difference of 1.67 mag. The differencewas calculated taking into account the correction for the U-Bcolor index of the two stars using equation 9 of the EFOSC2manual 4. From Cousins (1983) we derived the U magnitude ofthe unresolved system and we found the values of U = 10.31 forHD 8049 A and U = 11.98 for the HD 8049 B.

Fig. 3. U-band EFOSC image of the binary system. HD 8049 B,in the right lower side, is just 1.67 mag fainter.

During the same night we performed spectroscopy of thesystem. Grism n.11 and 1′′ slit were used, providing a resolutionR ∼ 360 over the range 3500 to 7400 Å. The slit was orientedalong the two stars (i.e. PA = 121 deg). The spectral reductionwas done using standard IRAF tasks. The raw data were biassubtracted and flat-field corrected. The spectra were extractedwith the IRAF task apall, wavelength calibrated using arc lampexposures and cross-correlated with the sky lines. The relativeflux calibration was done by deriving the sensitivity curve forthe instrument set-up from the spectroscopic standard star L745-46A (Hamuy et al. 1994). The result is that in the blue part ofthe spectrum a low S/N signature of HD 8049 B is present, nev-ertheless the flux contribution of the white dwarf is completelyblended with the one of the primary star.

2.4. VLT/SINFONI

The companion was then characterized on August 28 UT, 2012using the Spectrograph for INtegral Field Observations in theNear Infrared (SINFONI, see Eisenhauer et al. 2003; Bonnetet al. 2004b,a) mounted at the VLT/UT4. SINFONI providesadaptive optics assisted integral field spectroscopy in the near-infrared. It is composed by a SPectrograph for Infrared FaintField Imaging (SPIFFI) fed by a modified version of the Multi-Applications Curvature Adaptive Optics system (MACAO, seeBonnet et al. 2003). We used the gratings J and H+K to coverthe 1.1-1.4 and 1.45-2.45 µm ranges at a spectral resolutionof ∼2000 and ∼1500 respectively and the smallest platescale

2 IRAF is distributed by the National Optical AstronomyObservatory, which is operated by the Association of Universitiesfor Research in Astronomy (AURA) under cooperative agreement withthe National Science Foundation.

3 GAIA is a derivative of the Skycat catalogue and image displaytool, developed as part of the VLT project at ESO. Skycat and GAIAare free software under the terms of the GNU copyright.

4 EFOSC2 manual, version 3.6. ESO user facilities.


1.1 1.2 1.3 1.4Wavelength (µm)

0.0

0.5

1.0

1.5

2.0

Fλ

norm

aliz

ed

HD8049 B

HeI linesHeII lines

Paβ

1.4 1.6 1.8 2.0 2.2 2.4Wavelength (µm)

0.0

0.5

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Fλ

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HD8049 B

HeI linesPaα

Brackett lines

Fig. 4. SINFONI spectrum of HD 8049B in J band (left) and HK band (right).

(12.5 mas) to Nyquist sample the PSF. The primary served as areference for the AO wavefront sensor and was kept outside theinstrument field of view (0.8 ′′ × 0.8 ′′). Each sequence is com-posed by 12 acquisitions on the source with small dithering andone acquisition on the sky. Finally, we observed the standard starHIP 17280 (B5V) following a AB pattern right after our sciencetargets to correct our spectra from telluric features. HD 8049 andthe standard were both observed under poor atmospheric condi-tions with a seeing of 1.3 − 1.5 ′′, although the AO loop remainsstable, and airmass lower than 1.1.

We reduced homogeneously our dataset with the ESO datareduction pipeline version 1.9.8 (Abuter et al. 2006). Thepipeline successfully carried out bad pixels detection, flat field-ing, distortion coefficient computation, and wavelength calibra-tion using calibration frames acquired the day after the ob-servations. Individual datacubes were reconstructed from sky-subtracted object frames and merged into a final mozaicked dat-acube. Datacubes of telluric standard stars (STD) were obtainedin a similar way. Finally, we integrated the source flux in eachfinal datacube over selected apertures to optimize the signal tonoise ratio without introducing differential flux losses. The spec-tra of the standard star were corrected from their intrinsic fea-tures and divided by a black body at the appropriate temperature(Pickles 1998).

3. Nature and properties of HD 8049 B

3.1. Early discovery

HD 8049 was previously identified as binary system by van denBos (1929) in 1928. At epoch 1928.86, a projected separationρ = 3.2′′ and position angle θ = 153.5 deg were measured.The magnitude resulting from the visual observations is 13.5(rounded to 0.5 mag) and a bluish color is reported. This is theonly available measurement of the pair before our observations,as listed in WDS (Mason et al. 2001). This position is compatiblewith the one measured in our EFOSC U-band imaging with a rel-ative position of ∆α = 1373±24 mas and ∆δ = -734±24 mas, alsocompatible with NACO results. Table 3.1 summarizes the avail-able astrometry, including the two epochs from NACO. valuesof CCDM are approximated, to be recalculated with the originalvalues.

Errors of 1928 observation are not given in the original pa-per, they should be no less than 100 mas. From NACO ob-

servations over 1 yr, we derive a motion of 2.7 mas/yr in αand 30.4 mas/yr in δ. Doing the same for 1928 vs 2010 ob-servations we get a motion of 0.4 mas/yr in α and 26 mas/yrin δ. These determinations are obviously much smaller thanthe proper motion of the star (µα = 65.99 ± 1.18 mas/yr andµδ = −240.99 ± 0.98 mas/yr), supporting the physical associ-ation of the faint companion. On the other hand, the motionswith respect to the central star derived between 1928 to 2010and 2010 to 2011 are consistent within their errors, making vi-able the hypothesis that the companion detected in 1928 in theoptical is the same we found with NACO and that we are seeingthe orbital motion of HD 8049 B around HD 8049 A. The char-acterization of the orbital properties of the binary is discussed insection 5.1.

Table 2. Astrometry of candidate(s) around HD 8049

Epoch ∆α ∆δ Referencemas mas

1928.86 1400±?? −2876±?? van den Bos (1929)2010.57 1372 ± 7 −743 ± 7 NACO-LP2011.57 1376 ± 10 −701 ± 10 NACO-LP

3.2. HD 8049 B, a new nearby white dwarf

Considering the combined optical and NIR photometry ofHD 8049 B (V, U and H-bands, see Table 4) from EFOSC andNaCo observations, the hypothesis of a substellar companionis unambiguously excluded. The combined visible and NIRphotometry indicate more probably a white dwarf nature forHD 8049 B. The SINFONI near-infrared spectrum of the objectis shown in Figure 4. The latter indeed confirm this assump-tion as the observed spectral energy distribution shows a hot anddense atmosphere that can be associated to the one of a degener-ated white dwarf. The NIR spectrum is rather featurless as we donot detect any Hydrogen or Helium lines over the all 1.0–2.5 µmspectral range. Despite the low-SNR, it could indicate a feature-less Helium-rich nature (DB white dwarf) or a Hydrogen-richwhite dwarf (DA) with a relatively hot atmosphere, thereforeless contrasted Hydrogen lines (see Dobbie et al. 2005). Visible


spectroscopy should enable a better characteriztion of the whitedwarf nature of HD 8049 B.

To constrain the physical properties of HD 8049 B, we con-sider the emitted fluxes in the photometric bands FUV, NUV,U, V and H. In the ultraviolet we applied the correction for thechromospheric emission of the primary as described in Sect. 4.3.Values of apparent magnitudes for HD 8049B are shown in Table4. We assumed that the error on the V band photometry of theobservations in 1928 is at least 0.5 mag. The compilation of coolWD by Bergeron et al. (2001) is not optimal to our goal, be-cause it does not include objects as hot as our candidate. Thetheoretical sequences for hydrogen WD by Vennes et al. (2011)(their App. A) instead extend to range of parameters appropri-ate for the candidate. As HD 8049B seemed to be a peculiarobject in respect to the models, we collected also empirical se-quences to check the possibility of a scatter from expected pointsin color-magnitude diagrams and to evaluate the cooling ageof HD8094B. We used the catalog of nearby white dwarf byGiammichele et al. (2012), that we cross-matched with avail-able GALEX magnitudes and H, J and K magnitudes of 2MASSwhere the latter values were missing. The final sample consistedin 107 nearby (≤ 51 pc) white dwarfs, 22 of which with FUVmagnitude, 18 with NUV magnitude and 84 with H, J, K magni-tudes calibrated by Giammichele et al. (2012).

We choose the mass of 0.6 M, close to the peak of massdistribution of WDs and most probable value taking into accountthe discrepancies in temperature and colors for a more massiveobject. From the empirical sequences fit we derived a tempera-ture of 14000 K for the ultraviolet bands and 19000 K for V andH. The ultraviolet bands are neither in a good agreement withmodels nor with empirical points; on the other hand, a smallerror on the magnitude for the V and H band would strongly af-fect the determination of the temperature. The cooling time is of∼ 70 Myr, but there is a small scatter between the four bands.

4. HD 8049 A

We exploited the spectroscopic data described in Sec. 2.2 andadditional data from public archives With the aim of improvingthe age determination and the other stellar parameters of the pri-mary.

4.1. Rotation period

HD 8049 was observed in the All Sky Automated Survey(ASAS5 Variable star catalog; Pojmanski 2002). Archive ob-servations span the range from November 20 UT, 2000 untilOctober 28 UT, 2009. After outlier removal and averaging obser-vations collected within short time interval (less than 2 hr), wewere left with 583 data point for rotation period search. The av-erage photometric accuracy is 0.02 mag. The brightest V magni-tude is V = 8.68 ± 0.02. To minimize the effects of active regiongrowth and decay on the periodogram results, we sectioned thedata timeseries into 14 light curves. The light curve amplitudeis variable and with values up to ∆V = 0.045 mag. The Fourieranalysis was carried out on the complete timeseried data as wellas on each of 14 light curves. We found a rotation period of P =8.3±0.1 days in the complete timeseries data, and the same pe-riod in 4 out of 14 time intervals (with FAP ≤ 0.1%). The ratiobetween the average residuals from the sinusoidal fit with the ro-tation period and the light curve amplitude was found to be lessthan 1. in 13 out of 14 time segments. According to the criteria

5 http://www.astrouw.edu.pl/asas/

adopted in Messina et al. (2011) to assign a confidence level tothe rotation period determination, the period of HD 8049 can beconsidered likely. The P = 8.3 days rotation period and projectedrotational velocity (3.5±1.2 km/s, see Torres et al. 2006a) are inagreement to each other for any stellar radius R≥ 0.6 R.

Fig. 7. Analysis of ASAS V band time series to determine therotation period of the star.

4.2. Age indicators

Table 3 summarizes the measurement and corresponding agesas derived from different indicators. From our analysis of theHARPS spectrum, we confirm that no lithium is detectable,as found by Torres et al. (2006b). Two sets of log RHK mea-surements are available: those from our CORALIE spectra andthat by Gray et al. (2006). They give basically the same value(log RHK = −4.25). log LX/Lbol was derived from ROSAT All-Sky Bright Source Catalog (Voges et al. 1999) assuming thesource 1RXS J011915.1-433803 is the X-ray counterpart ofHD 8049. The X-ray emission should be dominated by the ac-tive K dwarf primary as the WD is cooler than the WDs showingsignificant X-ray emission (Te f f > 25000 K, Vennes 1999). TheU,V, W space velocities were obtained adopting the trigonomet-ric parallax of 29.79 ± 1.21 mas (van Leeuwen 2007). The un-certainty in the center of mass velocity due to orbital motion isof ∼ 1 Km/s (to be quantified, min value of 200 m/s, max value 3Km/s from the simulations.)


Fig. 5. Magnitude versus effective temperature diagrams with models of Vennes et al. (2011) (black lines) and sample of nearbydwarf collected by Giammichele et al. (2012) (diamond points for object with all the magnitude available and squared points for theothers). Colors indicate the mass from the lowest (in blu) to the highest (in red). Our object value of magnitude is rapresented bythe blue horizontal line.

Table 3. Age indicators of HD 8049. The 2nd column lists theadopted value for the various parameters, the 3rd column thecorresponding reference; the 4th column the resulting age, thelast column the adopted calibration for age determination.

Indicator Measure Ref Age Ref. cal.Li EW (mA) 0.0 1,2 >550 3log RHK -4.25 1,4 90 5log LX/Lbol 1 182 5Prot (d) 8.3 ± 0.1 1 360 5Prot (d) 380 ± 30 6Kinematic 18,-47,-28 old (few Gyr) 1(U, V, W), km/s

References. 1: this paper; 2: Torres et al. (2006b); 3: SPHERE prep.; 4:Gray et al. (2006); 5: Mamajek & Hillenbrand (2008); 6: Delorme et al.(2011)

The results of these various age indicators are somewhatpuzzling. Chromospheric HK emission, coronal emission, androtation period indicate an age between 100-400 Myr. On theother hand, the lack of lithium indicates an age older than about500 Myr while the kinematic parameters are quite far from thetypical locus of young stars (Zuckerman & Song 2004). Thegalactic orbit as given in Nordstrom et al. (2004) (last revisionof catalog in Vizier) has e = 0.19 and zmax = 0.33 kpc. Theseproperties are more typical of stars with an age comparable tothe Sun.

These discrepancies strongly suggest that HD 8049 is nota bona-fide young star but a rather peculiar object. We iden-tify the source of these anomalies in the presence of the whitedwarf companion detected through direct imaging. While itsnon-negligible flux in the UV might somewhat alter the log RHKmeasurement, the contribution to the X-ray flux should be neg-ligible, as the WD is cooler than the WDs showing significantX-ray emission (Te f f > 25000 K, Vennes 1999). Obviouslythe determination of the rotation period is not affected by thepresence of the companion. We then conclude that the youngrotation-activity age is not spurious. The star should have beenrejuvenated by the accretion of some amount of mass and angu-lar momentum by the central star at the time of mass loss fromfrom the WD progenitor. This scenario will be better investigatedin Sec. 3.

4.3. Broad band photometry

Table 4 lists the available broad band photometry.

Table 4 lists the available broad band photometry. A verybright FUV magnitude is listed in the GALEX catalog (Martinet al. 2005). The offset between optical and GALEX position of1.67′′ along SE direction indicates that the FUV source is bettercoincident with the WD rather than the K2 star.

Such a bright FUV magnitude is not expected from the pri-mary, even taking into account the high level of chromosphericactivity of the star (see e.g. Fig. 6 in Smith & Redenbaugh2010). Corrections for chromospheric contribution of the pri-


Fig. 6. Magnitude versus cooling age diagrams with the empiric sequences made by the sample of nearby dwarf collected byGiammichele et al. (2012) (diamond points for object with all the magnitude available and squared points for the others). Colorsindicate the mass from the lowest (in blu) to the highest (in red). Our object value of magnitude is rapresented by the black horizontalline.

mary have been calculated using equations 2 and 4 of Findeisenet al. (2011). We obtained FUV = 10.01 and NUV = 11.09 us-ing the value of the color B-V of the integrated system (assum-ing that the contribution of the white dwarf is negligible) andlog RHK = -4.25.

4.4. Abundance analysis

In order to better characterize the primary and to explore possi-ble signatures of accretion of material processed by the WD pro-genitor, we derived the stellar spectroscopic parameters and wemeasured abundance of several elements. To this aim, we usedthe HARPS high resolution spectrum of HD 8049 (Sec. 2.2) andadopted the prescriptions of Biazzo et al. (2012). We refer to thatpaper for details on the adopted line list.

The iron abundance analysis of our target was performed dif-ferentially with respect to the Sun. To this purpose, we analyzeda Ganymede spectrum acquired with HARPS at high S/N, ob-taining log n(FeI) = log n(FeII) = 7.53±0.05 for the Sun. In theend, we found the following stellar parameters and iron abun-dance for HD 8049: effective temperature Teff = 5050 ± 50 K,surface gravity log g = 4.50 ± 0.10 dex, microturbulence ve-locity ξ = 1.42 ± 0.05 km/s, [FeI/H] = −0.10 ± 0.07 dex, and[FeII/H] = −0.10 ± 0.11 dex. to be compared with previous lit-erature results With the goal of better constraining the possibleaccretion from the WD progenitor and its original mass, we de-rived abundances of the s-elements Y, Ba, La, and Cu, which areproduced in the atmospheres of AGB stars more massive than1.3 − 1.5 M (Busso et al. 1999). The abundances of Y, Ba, and

La were derived as in D’Orazi et al. (2012). The Cu abundancewas derived adopting the hyperfine splitting from Steffen (1985)and the isotopic mixture from Simmerer et al. (2003) (69% for63Cu and 31% for 65Cu We found [Y/Fe] = −0.15±0.10 [Ba/Fe]= −0.10 ± 0.10 [La/Fe] = −0.12 ± 0.10 [Cu/Fe] = 0.00 ± 0.07.Errors are those derived from spectral synthesis errors from at-mospheric parameters to be added quadratically We also mea-sured the abundances of Na and Al, as enhancements are ex-pected in the ejecta of massive AGB stars (Ventura et al. 2001).We found [Na/Fe]=0.05± 0.09 and [Al/Fe]=0.14± 0.14 indicat-ing no significant pollution from this kind of stars.

5. System properties

5.1. Orbital properties

To constrain the possible orbit of the object we performed aMonte Carlo simulation following the example of Desidera et al.(2011). The simulation generates each time random orbital ele-ments and rejects all the orbits that do not fit both the RV trendand the astrometric data. We found correlations between orbitalelements, showed in Figure 8. Distribution of each element isshown in histograms of Figure 9. The distribution of the incli-nations is bimodal and have its peaks on the values of 5 deg and65 deg. The argument of periastron ω and the longitude of thenode Ω have the higher probability around values of 200 deg and250 deg respectively. The most probable period is around 300 yr,corresponding to a semi-major axis of ∼ 50 AU. There is a lackof edge-on inclinations in the possible configuration of the orbit


Table 4. Broad band photometry of HD 8049

Band Mag ReferenceA+B

V 8.734 1B-V +0.892 1V-I 0.95 2U-B +0.472 1J 7.077±0.027 3H 6.649±0.059 3K 6.523±0.031 3V-H +2.04 1,3V-K +2.17 1,3

AV 8.747 4U 10.29 4

BV 13.5 5U 12.08 4H 14.00 4FUV 12.64 6NUV 13.46 6FUV-NUV -0.82 6

References. 1: Cousins (1983); 2: Perryman et al. (1997); 3: 2MASS(Skrutskie et al. 2006) ; 4: this paper; 5: CCDM (Dommanget & Nys1995); 6: GALEX (Martin et al. 2005)

that produce the similar cut on the pattern in the correlation plotsof ω and Ω.

5.2. System history

As mentioned in Sec. 4.2, the discrepant results of various ageindicators for HD 8049 A might be qualitatively explained by theaccretion of some amounts of mass and angular momentum fromits WD companion HD 8049 B.

In the past, comparable cases were already discovered.Jeffries & Stevens (1996) showed that the accretion of slow windfrom the AGB progenitor of the WD can transfer sufficient an-gular momentum to speed up their companions even for orbitalseparation up to 100 AU and cases of pairs formed by a WD anda fast rotating, barium-rich stars have been reported (Jeffries &Smalley 1996). Another case is represented by the binary GL86that is known to host giant planet and a white dwarf companion(GL86B; Els et al. 2001; Mugrauer & Neuhauser 2005). GL86also shows anomalies in the age indicators that are similar evenif less extreme than those we found for HD 8049 with a youngerage from X ray and H&K than from kinematic (see Desidera &Barbieri 2007, their App. B). Another more direct indication ofwind accretion on a companion during the AGB phase is rep-resented by the accretion disk around Mira B (Karovska et al.1997; Ireland et al. 2007), which lies at a projected separationof about 70 AU, which is slighly larger than the most probablevalue for HD 8049.

Following these works we can speculate that part of the ma-terial lost through stellar wind6 by the WD progenitor was ac-creted by HD 8049, increasing its rotation rate and then mim-icking a young age for what concerns the indicators that are di-rectly or indirectly tied to stellar rotation (rotation itself, X rayand H&K). In this case, we expect that the activity and rotation

6 A separation of 50 AU is clearly much larger than that needed for adirect mass exchange between the components.

age of the system is related to the time elapsed since the ac-cretion event, i.e. the cooling age of the WD7. In this scenario,increasing the mass of the progenitor of the WD allows for largeramounts of accreted material, both because of large amounts ofmass loss due to stellar wind at the end of AGB phase and ofthe larger widening of the binary orbits due to mass loss. Moremassive progenitor implies also a younger age for the system.On the other hand, the lack of significant abundance anomalieson HD 8049A (Sect. 4.4) indicates that either the s-process en-hancement of the AGB progenitor is small, which may arise onlyfor masses lower than 1.3 − 1.5 M, or that the amounts of ac-creted material is small with respect to the heavy elements con-tent of the convective zone.

To explore the possible original configuration of the system,we used the BSE package (Hurley et al. 2002), evolving binarysystems with different primary masses and orbital parameters.We use the default parameters to take into account mass lossand wind accretion. These elements have significant uncertaintyto the results are intended for a qualitative view of the possibleconfiguration. A selection of results is reported in Table 5.

A system with a 1.5−2.0 M star initially at 30 AU will pro-duce a WD of roughly the correct mass with a semimajor axisclose to the current projected separation and within the range ofparameters for current orbit presented in Sect. 5.1. The result-ing age of system (1.5-3 Gyr) is compatible with the kinematicproperties of HD 8049. About 0.01 M are accreted by the othercomponent in this case, that is about one tenth of the mass of thestellar convective zone as derived following the prescriptions byMurray et al. (2001).

try to better quantify the whole scenario, including the ex-pected amounts of Ba enhancement

6. Conclusions

Ongoing work, still missing:-characterization of the WD (Gael?)-statistic on the possibility of finding this type of FP (any-

one?)A NACO-LP promising candidate turned out to be a white

dwarf companion rather than a brown dwarf, as initially sus-pected from the H band magnitude difference. With direct imag-ing of the object we assured that we were dealing with a boundobject and the second step to avoid a false positive was to deter-mine its mass through luminosity.

The M/L relation for exoplanets strongly depends on theage of the system (see e.g. models from Baraffe et al. 2003;Madhusudhan et al. 2011) so a misunderstanding on the propri-ety of the host star leads to an erroneous calculation of the massof the companion. That was the case of HD 8049, a star likely re-juvenated by the progenitor of a white dwarf. With the nominalage and separation the target was confused with a 35 MJ browndwarf. From radial velocity analysis we found a trend that madeus think about a more massive object. Indeed, a reanalysis of thespectrum of the star made apparent some incongruences on theage indicators.

Final clue that univocally confirms our hypothesis was theobservation of the system in U-band. A bright object is observedat the same astrometric position of the NACO candidate. Themagnitude of this object is just 1.7 mag fainter than the host star.We found photometric values for the system and its components.

7 However, the activity-rotation age might be larger than the coolingage of the WD if the primary was not spun-up to the fast rotationalvelocity typical of a very young star


We made a Monte Carlo simulation, following the exampleof Desidera et al. (2011), to constrain the orbital parameters. Wefound that the white dwarf moves on a orbit with period around300 yr (∼ 50 AU), the most probable values for the argumentof periastron ω and the longitude of node Ω are 200 deg and250 deg respectively. We did not find any prevalent value of ec-centricity while the distribution of the inclinations is bimodalwith peaks in 5 deg and 65 deg.

In our scenario, the existence of the WD companion (withmag in H-band compatible with a substellar object) and the spou-rious young age of the star as derived from the rotational period,the log RHK and the X emission are intrinsically linked.

As nearby young stars are prime targets for direct imagingplanet searches and new instrumentation optimized to this sci-ence goal is about to start operations, one might ask which isthe expected frequency of the kind of false alarm we found inNACO-LP. to be investigated, of course this depends sensitivelyon the properties of the input sample and the ways on which theages were estimated

As discussed in this paper, a way to identify the spuriousyoung stars that were affected by wind accretion is to comparethe ages resulting from several age indicators, with discrepanciesbewteen rotation-activity ages and lithium and kinematic ages.In case the sample is focused on young stars, a white dwarf falsealarm is related to a WD with a short cooling age, to explaina young rotation-activity age for its companion. Such objectsare most easily identified in UV bandpass, as done in our studyexploiting archive GALEX observations.

Resolved systems with dwarf companions close enough tohave harboured some accretion phenomena are interesting tar-gets to further constrain the wind accretion occurring in moder-ately wide binaries during the AGB phase of the WD progenitorand to investigate the maximum binary separation at which bar-ium stars can be observed. Finally, our result confirms that thereis a population of white dwarf companions awaiting discoveryeven at small distance from the Sun, as suspected by Holberg(2009), and that AO imaging can play a relevant role for unveil-ing it.

Acknowledgements. We are grateful to all people involved during the observa-tions at VLT, NTT and 3.6m telescope at La Silla. We also thank the TASTEgroup of Padua for the precious help for EFOSC photometry. A.Z., S.D, andD.M. ackowledges partial support from PRIN INAF 2010 “Planetary systems atyoung ages”.

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Fig. 8. Correlations between orbital parameters derived with a Monte Carlo simulation following the example of Desidera et al.(2011). Colors underline different levels of χ-squared (decreasing from black to blue).

Table 5. Results of simulations made using BSE package (Hurley et al. 2002) to investigate possible original configuration of thesystem. The subscript start refer to the initial conditions; the subscript end to the parameters after the mass loss phase by the originalprimary. Tmassloss is the age at which the primary evolves into a WD.

M1start M2start astart estart Tmassloss M1end M2end aend eend(M) (M) (AU) (Myr) (M) (M) (AU)

2.000 0.900 20.00 0.50 1494 0.637 0.924 34.49 0.482.000 0.900 30.00 0.50 1494 0.637 0.912 54.17 0.492.000 0.900 50.00 0.50 1494 0.637 0.905 93.06 0.502.000 0.900 30.00 0.80 1494 0.637 0.923 40.47 0.73

1.000 0.900 30.00 0.50 12463 0.520 0.903 39.70 0.501.500 0.900 30.00 0.50 3098 0.577 0.909 47.35 0.501.750 0.900 30.00 0.50 1944 0.607 0.911 50.92 0.492.000 0.900 30.00 0.50 1494 0.637 0.912 54.17 0.492.250 0.900 30.00 0.50 1076 0.644 0.915 58.18 0.492.500 0.900 30.00 0.50 801 0.690 0.916 60.73 0.493.000 0.900 30.00 0.50 477 0.748 0.919 66.54 0.49


Fig. 9. Histograms of orbital parameters derived with a Monte Carlo simulation following the example of Desidera et al. (2011).

Astrophysical false positive in direct imaging: a white...

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