ACCESSTRansiting Exoplanet Atmosphere Transmission Spectra

Dániel Apai, Ben Rackham, Alex Bixel [1] - Mercedes López-Morales, Chima McGruder, Ian Weaver [2] - Andrés Jordan [3], Néstor Espinoza [3, 5] - David Osip [4]

+ Jonathan Fortney (UCSC), Nicole Lewis (STScI), Jonathan Fraine (STScI), Florian Rodler (ESO)

[1] [2] [3] [4] [5]

One of the most exciting possibilities enabled by transiting exoplanets is to measure their atmospheric properties through the technique of transmission spectroscopy. The optical window, in particular, is rich in atmospheric information which is capable of disentangling several degeneracies in atmospheric modeling. Initiated in late 2013, ACCESS is a multi-institutional collaboration aimed at responding to this opportunity by generating the first uniform observational set of ~ 0.4–0.9µm transmission spectra of a meaningful sample (~20) exoplanet atmospheres, spanning those from hot Jupiters to super-Earths. We present the survey methodology and lessons learned to date following over 40 transits observed using the IMACS multi-object spectrograph on the 6.5-m Magellan Baade Telescope at Las Campanas Observatory (LCO). ACCESS minimizes systematics caused by different instruments and data analysis techniques allowing us to develop approaches to separate and correct for instrumental, environmental, and astrophysical systematics to improve the diagnostic power of transit spectroscopy. Our observations are providing not only insight into the range of properties of exoplanet atmospheres, they also elucidate the properties of the host stellar photospheres. We highlight some of our recent submitted/published TRansiting Exoplanet Atmosphere Transmission Spectra including several other presentations at this conference. Finally, we elucidate where we see the strongest synergies and brightest future prospects in the era of space missions such as TESS and JWST.

Our survey targets cover a wide range of exoplanet physical parameters. The figures above demonstrate the survey targets for which we have already obtained successful transit observations and are under analysis.

In the following TREATS we highlight recently submitted and published results …

Rackham et al., 2017

Espinoza et al., 2018 (cf. Poster 176)

Interpretation of ground based transmission spectroscopy must keep in mind that there are at least 3 atmospheres involved… 1.) Our local atmosphere is treated in de-trending systematics with the use of multiple comparison star spectra varying with airmass and environmental conditions. • However, strong atmospheric absorption features in particular should be treated carefully when interpreting suspected signals in these regions. 2.) The target transiting exoplanet atmosphere that we seek to investigate 3.) The transited host star photosphere - of course for differential transmission spectra there are the photospheres of the other comparison stars that also come into play. • Stellar photosphere is not a clean constant light source during the observed transit event nor from one event to another …

Heterogenous stellar photospheres are a much more realistic view of our background illumination source as opposed to a simple constant illumination disc. Photometric monitoring from the same source as the original transit candidate detection observations or alternate survey or fixed observations clearly distinguish variability attributed to rotation of a non uniform stellar surface. In addition, many high precision transit light curves provide additional evidence of stellar spot crossings [cf. central plots in image above]. • Modeling of heterogeneous photospheres even in a simplified form can constrain spot or faculae covering fractions and temperature differentials between cooler or warmer regions.

In addition to colour and brightness information, variability monitoring is also important to selection of comparison stars . This is important for timescales within the duration of a transit as well as between observed transits. ASAS-SN (https://asas-sn.osu.edu) and other such surveys have proven to be able tools in advanced assessment of targets. At LCO the currently developing HATPI (cf. Bakos - Poster 174) may very well become our de-facto monitoring tool.

Wide field imaging simultaneous with our spectral observations is also being regularly carried out for vetting of overall sky conditions and stability of target and comparison stars. ACCESS has adopted defocussed imaging with the 25’ FOV CCD at LCO's Swope 1m. telescope for more than a dozen events to date. CHAT (Jordan - Poster 94) introduces a new dedicated facility at LCO for just such transit photometry.

References: Jordan et al., 2013, ApJ, 778,18 ; Carter and Winn 2009. ApJ 454, 22 ; Dressler et al., 2011, PASP, 123, 288 ; Espinoza et al., 2018, MNRAS submitted ; Rackham et al., 2017, ApJ, 834, 151 ; Rackham, Apai, Giampapa, 2018, ApJ, 853, 122 ; Huitson, C. M., Désert, J. M., Bean, J. L., et al. 2017, AJ, 154, 95

Lessons Learned & Future Prospects

WASP-31b

(cf. Poster 49 by Weaver et al.)

(cf. Poster 128 by McGruder et al.)

Analysis of a pair of transits observed of WASP-31b in 2013 in the earliest stages of ACCESS is not yet able to relieve the tension between the reported detection of potassium in 2015 by Sing et al. and the absence of any such feature in 2017 by Gibson et al. Additional data from a more standard ACCESS setup in 2014 are under analysis to improve our understanding.

WASP-4b Bixel et al., 2018 in preparation

Left: Joint atmosphere and photosphere models for the transmission spectrum of WASP-4b, showing tentative evidence for potassium absorption at 768 nm. This compliments a possible detection of sodium absorption at 589 nm from Gemini/GMOS [Huitson en al. 2017], suggesting that a narrow region of atomic absorption may exist above the optically thick layer. Right: A model for the flux contrast ratio of a star spot occulted during one of the transits. The data are best fit by a single, large spot with a temperature contrast of 100 K.

– 43 –

F

1 � F � D�

Occulted Photospheric Component

Unocculted Photospheric Component

D�

Fig. 10.— Schematic of composite photosphere and atmospheric transmission

(CPAT) model. In this model, the exoplanet blocks a wavelength-dependent fraction of

the stellar disk D�. The transit chord probes a region with a characteristic stellar spectrum

(the “occulted” spectrum), while a fraction of the stellar disk F , either continuous or not,

is described by another (the “unocculted” spectrum). During the transit, the di↵erence

between the stellar spectra is imprinted in the observed transmission spectrum.

Left: Schematic of composite photosphere and atmosphere transmission (CPAT) model.

Right: Best fitting model spectra considering only atmospheric absorption through the exoplanet limb followed further right by the best-fitting transmission spectra using CPAT-absorber model for a heterogeneous stellar photosphere.

We have developed our own suite of custom python tools for pipeline reduction of time resolved differential spectroscopy. These pipeline tools are capable of adaptation to other multi object spectrographs and we are currently vetting for LDSS3 at LCO and Binospec at MMT. The most complete details are provided in Espinoza et al., 2018.

Standard reduction procedures: bias removal, flat fielding, wavelength correction and extraction into desired wavelength bins.

De-trending: Use multiple comparison star spectral variations to model and remove systematics due to instrumental or local atmosphere from the target spectra via a principal component analysis (Jordan, Espinoza, el al., 2013) and/or polynomial fitting (Rackham et al., 2017). Nominally, this leaves only the signal due wavelength dependent opacity variation in the atmosphere of the transiting exoplanet.

Transit models are then fit in desired spectral bins (eg. Carter and Winn 2009). The transit depth and thus the planetary radius estimated at each wavelength interval is then analyzed as a transmission spectrum. Atmospheric modeling is adopted to investigate the physical nature of the observed opacity sources. Caveat - there is still that other atmosphere present…

The objective of transmission spectroscopy (measurement of the change in transit depth - providing an apparent planetary radius - as a function of wavelength due to opacity in the exoplanetary atmosphere) is to explore the composition of the exoplanet atmosphere probed by the host stellar spectra passing through the planetary limb during transit.

Differential spectroscopy across wide fields is challenged by systematic effects (instrumental, environmental, and astrophysical). With the goal of eventually characterizing the atmospheres of Earth-like planets, tools and techniques need to be defined and refine

Band-integrated (i.e. “white-light”) light curves for our IMACS observations of WASP-19b with the model-averaged systematics removed (black points with error bars), along with corresponding residuals (in parts-per-million, ppm)

Thorough analysis of 5 high precision independent transits observed of WASP-19b spanning 2014-2017 are all consistent with a transmission spectrum without any significant spectral features across the 400-900nm wavelength window.

Transmission spectrum for a uniform wavelength bin size of 10 nm sampled over 4 transits by WASP-43b in 2015-2018. Reference Na I-D, Hα, K I, and Na I-8200 marked by vertical dashed lines. Each individual spectrum and the combined version is consistent with a flat and featureless atmosphere.

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WASP-43b

ACCESS : Arizona - CfA - Catolica - Carnegie Exoplanet Spectroscopy Survey

Tools & Techniques

WASP-19b

Our WASP-19b observations highlight the importance of (1) measurement repeatability to confirm/reject tentative detections of atomic/molecule signatures in transmission spectra and (2) proper modelling of both the star and the planet when interpreting transmission spectra.

Want to see the details? Check out Poster 176 by Néstor Espinoza!

GJ1214b

Stellar Atmospheres count… see “The Fault in our Stars…” by Ben Rackham on Thursday at 11:50

Observational Methodology Observatory: LCO (Las Campanas Observatory, Chile ) Telescope: Magellan I, Baade Instrument: IMACS FOV: 24’ unvignetted Disperser: Grism 300_17.5 blazed at 670nm Blocking filters: GG455 Masks: layout for wide 10” long 90” slits; independent calibration masks with 1" wide slits Sampling cadence ~1 minute Duration: transit event plus equivalent baseline outside transit ~4-8hr

The preferred observational setup with IMACS [Dressler et el., 2011] has evolved over time with initial observations adopting the f/4 camera covering a 15’ FOV. Moving to the f/2 camera with 27’ FOV was preferred for securing better comparison stars and has the advantage that the CCD array is oriented such that the observed spectra cross only one CCD gap - or fit within a single detector. Dispersing elements and blocking filter combinations have been experimented with leading us to prefer the 300 line grism blazed with a a central wavelength of 6700A yet offering full coverage of the uncontaminated first order spectrum. The optimal setup is documented above. However, we note that a newly installed 400 line grism blazed at 4730A offering even better blue transmission is likely to be favored going forward.

1. There are multiple atmospheres involved and one is cautioned to not favour a particular atmosphere when interpreting the resultant spectra (especially from single events).

2. ACCESS observations are providing not only insight into the range of properties of exoplanet atmospheres from super earths to hot Jupiters, they also elucidate the properties of the stellar host photospheres

3. Indeed, for this reason, we strongly suggest multiple event observations with the same instrumental setup as fundamental to clearly associate features with the target exoplanet atmosphere. Host star and comparison star variability complicate the proper reconciliation of multiple event transmission spectra.

4. Observational setup favors wide field, wide slits, multiple reference stars, order-blocking filter, stable telescope track.

Looking to TESS - our high precision white light-curves [demonstrated in the plots at left] provide proof of ability to obtain ~200ppm precision photometry that will be required for quality followup, discrimination, and investigations of additional system members.

Looking to JWST - our wavelength range provides complimentary coverage and extends well below the blue cutoff at 600nm to more thoroughly analyze and apply retrieval modeling to the exoplanet atmosphere opacity sources as well as inform the nature of the heterogenous photosphere of its host star.