Characterizing small planets transiting small stars with SPIRou
A. Santerne, J.-F. Donati, R. Doyon, X. Delfosse, É. Artigau, I. Boisse, X. Bonfils, F. Bouchy, G. Hébrard, C. Moutou, S. Udry, SPIRou science team
SSF2A 2013
L. Cambr´esy, F. Martins, E. Nuss and A. Palacios (eds)
CHARACTERIZING SMALL PLANETS TRANSITING SMALL STARS WITH SPIROU
A. Santerne , J.-F. Donati , R. Doyon , X. Delfosse , E. Artigau , I. Boisse , X. Bonfils ,F. Bouchy , G. H´ebrard , , C. Moutou , , S. Udry and the SPIRou science team Abstract.
SPIRou, a near infrared spectropolarimeter, is a project of new instrument to be mounted at theCanada France Hawaii Telescope in 2017. One of the main objectives of SPIRou is to reach a radial velocityaccuracy better than 1 m.s − in the YJHK bands. SPIRou will make a cornerstone into the characterizationof Earth-like planets, where the exoplanet statistics is very low. This is even more true for planets transitingM dwarfs, since only 3 low-mass planets have been secured so far to transit such stars. We present here allthe synergies that SPIRou will provide to and benefit from photometric transit-search programs from theground or from space ( Kepler , CHEOPS , TESS , PLATO 2.0 ). We also discuss the impact of SPIRou forthe characterization of planets orbiting actives stars.Keywords: transit; exoplanet; photometry; radial velocity, infrared spectroscopy
As they pass in front of their host star, transiting exoplanets are providing us numerous key information tounderstand the diversity of planets in the galaxy. For these planets, it is possible to determine their mass andradius and thus their bulk density, needed to determine their nature (rocky, gazeus, water-world, etc. . . ) andmodel their internal structure as well as their orbital configuration (orbital period, eccentricity, obliquity). Thisprovides strong constraints to planet’s formation, migration and evolution theories. Last but not least, transit-ing exoplanets are today the only targets to explore atmospheric composition from transmission spectroscopyduring the transit.Nearly 300 transiting planets have been confirmed so far with only a handful of terrestrial or very-low massplanets having an accurate determination of mass and radius (see Figure 1). To explore this regime of low-massplanets, small stars like M dwarfs are the most favorable for detection since they allow larger signals in bothphotometry (the depth of the transit scales as 1/R (cid:63) ) and radial velocity (hereafter RV; the amplitude of theRV variation scales as 1/M / (cid:63) ). For example, for a planet in the habitable zone, the RV signal is seven timeslarger around a M dwarf than around a solar-type star (two effects combined: the habitable zone is closer tothe stars and the mass of the star is lighter).Since atmospheric characterization primarily requires deep transits on the one hand, and bright stars onthe other hand (in the nIR, where absorption from atmospheric molecules mostly concentrates), M dwarfs are Centro de Astrof´ısica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal UPS-Toulouse / CNRS-INSU, Institut de Recherche en Astrophysique et Plan´etologie (IRAP) UMR 5277, Toulouse, F31400France D´epartement de physique and Observatoire du Mont M´egantic, Universit´e de Montr´eal, C.P. 6128, Succursale Centre-Ville,Montr´eal, QC H3C 3J7, Canada UJF-Grenoble 1/CNRS-INSU, Institut de Plan´etologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble,France Aix Marseille Universit´e, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France Institut dAstrophysique de Paris, CNRS, Universit´e Pierre et Marie Curie, 98bis Bd. Arago, 75014 Paris, France Observatoire de Haute-Provence, CNRS/OAMP, 04870 Saint-Michel-l’Observatoire, France CNRS, Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Hwy., Kamuela, HI 96743, USA Observatoire de Gen`eve, Universit´e de Gen`eve, 51 chemin des Maillettes, Sauverny 1290, Switzerlandc (cid:13)
Soci´et´e Francaise d’Astronomie et d’Astrophysique (SF2A) 2013 a r X i v : . [ a s t r o - ph . E P ] O c t
38 SF2A 2013therefore optimal targets for this quest. Detailed simulations show that atmospheric components of Earth-likeextrasolar planets will produce a detectable signal for planets around M dwarfs planets using JWST/NASAand/or ELTs, but not for similar planets around Sun-like stars (e.g. Rauer et al. 2011; Tinetti et al. 2012).Today, only a handful of very-bright transiting systems have been discovered. A prime goal of astronomy isto discover other Earths and super-Earths whose atmosphere can be scrutinized and characterized with spacemissions (such as JWST) in the next decade, including the search for biomarkers.Dedicated searches for planets around M dwarfs in radial velocity have been performed for years (e.g. Delfosseet al. 1998; Marcy et al. 1998, 2001, with the HiReS and HARPS spectrographs) unveiling ∼
10 very low-massplanets (e.g. Rivera et al. 2005), including super-earths in the habitable zone of Gl 581(Mayor et al. 2009),Gl 667C (Delfosse et al. 2013; Feroz & Hobson 2013) and Gl 163 (Bonfils et al. 2013b). Very importantly,these surveys also revealed that small planets are very common around small stars, and that, 41% +54% − of theM dwarfs harbor a super-earth planet in the habitable zone (Bonfils et al. 2013a). Among the planets foundaround M dwarfs in RV surveys, only two have been found to transit their host star: GJ 436 b (Butler et al.2004; Gillon et al. 2007) and GJ 3470 b (Bonfils et al. 2012). The Mearth ground-based photometric surveydedicated to M dwarfs reported a third super-Earth with measured mass and radius: GJ 1214 b (Charbonneauet al. 2009). All of them present a bulk density close to Neptune’s density (see Fig. 1). A giant planet transitinga M dwarf has been characterized among the Kepler candidates by Johnson et al. (2012), totalizing only fourfully-characterized planets known to transit M dwarfs.To improve the statistics on this population of planets and to discover the prime targets for mission dedicatedto extra-solar planet’s atmospheric characterization, it is very important to characterize many more planetstransiting nearby M dwarfs. Since M dwarfs are quite faint in the optical but luminous in the infrared, suchstudies are much more efficient with infrared facilities.
Fig. 1.
Mass-radius diagram of transiting exoplanets discovered so far. The colors display the effective temperature ofthe host stars. The inset is a zoom to the transiting super-Earths. Only a few small planets have been characterizedaround small stars (here displayed in red). The solid, dashed and dotted lines display the density of the Earth, Neptuneand of 0.2 g.cm − (respectively). The SPIRou spectrograph (SpectroPolarim`etre Infra-Rouge) is a project for a near infrared spectropolarimeterthat will be mounted at the 3.6-m Canada France Hawaii Telescope (CFHT) in 2017. The main goals of thisunique spectropolarimeter will be both to search for habitable exo-Earths orbiting low-mass & very-low massstars using high-accuracy RV (better than 1 m.s − ) and to explore the impact of magnetic fields on star &planet formation, by detecting magnetic fields of various types of young stellar objects and by characterizingtheir large-scale topologies (Artigau et al. 2011). This spectropolarimeter will be a high-resolution (R ∼ µ m and 2.35 µ m (i.e. YJHK photometricbands) with a large throughput (up to 15%). Thanks to its large throughput, SPIRou will be able to providespectrum with a signal-to-noise ratio (SNR) of ∼
110 per pixel in one hour on a star of magnitude J = 12 or K= 11. SPIRou is expected to reach a photon noise of 1m.s − with a SNR of 160. Therefore SPIRou will be thebest instrument for RV studies of M dwarfs, especially in the context of transiting planet surveys. Current and future (optical or infrared) photometric surveys are targeting M dwarfs to find new transitingplanets. Ground-based nIR spectroscopy is essential in this context: spectroscopy is indeed mandatory toestablish the planetary nature of all transiting objects detected around low-mass dwarfs through photometricmonitoring (and discard false detections, e.g., caused by background eclipsing binaries ; Santerne et al. 2012)and to measure their mass from the RV amplitude of their host dwarfs orbital motion. SPIRou will also detectplanets in a dedicated M-dwarf RV survey, that will be after search for transits by ground- or space-basedobservatories.
The
Kepler space telescope (Borucki et al. 2009) detected 95 planet candidates (the so-called Kepler Objects ofInterest, i.e. KOIs) transiting M dwarfs (Dressing & Charbonneau 2013) with more than 2000 planet candidates(Batalha et al. 2013) transiting FGK stars. These KOIs present radii in the range 0.5 R ⊕ – 17 R ⊕ and orbitalperiods in the range 0.5 – 82 days. Most of them have a radius similar to the one of the Earth. Assumingbulk densities from the solar system planets (Neptune’s density for planet candidates larger than 2.5 R ⊕ andthe Earth density for those smaller than 2.5 R ⊕ ), it is possible to estimate their RV semi-amplitude. Figure 2displays this expected RV amplitude, as function of the J magnitude of the host star. The majority of these KOIsare expected to present a RV semi-amplitude larger than 1 m.s − . Radial velocity follow-up have been initiatedwith optical facilities (e.g. with SOPHIE, HiReS, HARPS-N) but no low-mass planet has been characterizedyet. Observations with an infrared spectrograph, like SPIRou, would be much more efficient. If SPIRou wasalready built, it would already be able to characterize new small planets transiting the smallest Kepler stars.However, since the
Kepler targets are faint, this would required a lot of telescope time. . . . . . . J magnitude − E x pe c t ed R ad i a l V e l o c i t ys e m i - a m p li t ude [ m . s − ] SPIRou exp. time O r b i t a l pe r i od [ d ] Fig. 2.
Expected radial velocity semi-amplitude of the 95 KOIs from Dressing & Charbonneau (2013) as function of theJ magnitude of their host star. The color of the mark indicates the orbital period of the candidate, while their shapeindicates the multiplicity of the system: circles for single-planet candidates and squares for multiple-planetary systems.The black solid, dashed and dot-dashed lines indicate the RV accuracy that SPIRou will reach in 15 minutes, 1 hour and2 hours (respectively) of exposure time.
40 SF2A 2013
Several ground-based photometric surveys are only targeting M dwarfs to detect small transiting planets. Forexample, the Mearth observatory was used to discover the transiting mini-Neptune GJ 1214 b (Charbonneau etal. 2009). Other similar facilities are in preparation, such as ExTrA, Apache and Speculoos. For example, theExTrA (Exoplanets in Transit and their Atmospheres) infrared observatory that will start observations in 2015and is expected to discover nearly 50 new small planets (down to 0.5 R ⊕ ) transiting bright M dwarfs (Bonfils,private communication). The infrared spectrograph SPIRou will be able to characterize these new transitingplanets with a better efficiency than other optical facilities. Moreover, those ground-based observatory will beable to perform a photometric follow-up of the new planets that SPIRou will detect as part of the RV survey. TESS (Transiting Exoplanet Survey Satellite) is a all-sky space-based photometric survey of all stars brighterthan V magnitude of 12 (Ricker et al. 2010) and M-dwarfs until V=13 (Charbonneau, private communication)that will be launched in 2017. This survey will therefore include numerous M dwarfs.
TESS is expected todetect more than 300 earth and super-earth transiting bright stars ∗ . The vast majority of them will orbit aroundM dwarfs due to the deeper signal for smaller stars. Since (i) most Earths and super-Earths detected with TESSwill orbit around M dwarfs, and (ii) less than ∼
30% of them will be accessible to optical RV follow-up (Deminget al. 2009), SPIRou will be the best RV instrument to monitor in the near infrared the ∼
150 best candidatesvisible from CFHT (assuming ∼
60 visits per star and spectra SNR of ∼
160 per visit, this observational effortwill represent a total of 150 CFHT nights).
PLATO 2.0 (PLAnetary Transit and Oscillations of stars; Rauer et al., 2013) is a M3 mission candidate tothe ESA Cosmic Vision program. If selected,
PLATO 2.0 will operate from 2024 to 2031+. Its main objectiveis to detect transiting planets out to the habitable zone of bright Sun-like stars and to characterize the hoststar simultaneously through asteroseismology. Since they orbit bright stars, those planets will be more easilycharacterizable with ground-based spectrographs than the much fainter
Kepler targets. With
PLATO 2.0 , itwill be possible de measure the density of planets with an unprecedented accuracy. Like
TESS , PLATO 2.0 willobserve thousands of bright M dwarfs but will deeper probe their planet population, towards smaller planetsand longer orbital periods (even longer than the habitable zone of early-M dwarfs). To characterize these uniqueplanets that only
PLATO 2.0 will be able to find, infrared spectrographs like SPIRou are needed.
CHEOPS (CHaracterising ExOPlanets Satellite Broeg et al. 2013) is the first S-class mission selected by ESAin its Cosmic Vision Program. The objective of this space mission is to detect new transiting planets aroundbright stars by performing a photometric follow-up of known planets detected by RV surveys. In operationbetween 2017 and 2021,
CHEOPS will be able to observe photometrically the first planets detected by SPIRouas part of its RV survey.
Stellar activity is one of the main limitation to the characterization of transiting planets (e.g. Hartman etal. 2011). The case of CoRoT-7 (L´eger et al. 2009) is a good illustration of this limitation in the context ofthe characterization of small planets around active stars (for a complete view of the saga about the mass ofCoRoT-7 b, see: Queloz et al. 2009; Lanza et al. 2010; Hatzes et al. 2010; Pont et al. 2011; Boisse et al. 2011;Ferraz-Mello et al. 2011; Hatzes et al. 2011). To better disentangle stellar activity from the planetary signal inCoRoT-7 b, new data from
CoRoT and HARPS have been obtained simultaneously and will be discussed inHaywood et al. (submitted), Barros et al. (in prep.), Hatzes et al. (in prep.) and Lanza et al. (in prep.). Asimilar configuration have been discussed in the case of Alpha Cen Bb where the stellar activity signal is larger ∗ http://science.nasa.gov/media/medialibrary/2013/04/22/secure-RICKER-TESS_NASA_APS_17Apr2013_NoVideo_v4-1.pdf PIRou characterizing small planets transiting small stars 241than the planetary one (Dumusque et al. 2012; Hatzes 2013).By observing in the infrared, SPIRou is expected to be less sensitive to stellar activity since the contrastbetween the photosphere and the spots is smaller in the infrared than in the optical (Mart´ın et al. 2006; Pratoet al. 2008). A few observations with SPIRou will therefore help to better model stellar activity and betterconstrain planetary masses detected with current spectrographs (e.g. HiReS, HARPS, HARPS-N, SOPHIE) orfuture ones (e.g. ESPRESSO, APF). Furthermore, since SPIRou is designed as a spectropolarimeter to studythe magnetic fields of stars, it will be a powerful facility to understand stellar activity of stars (e.g. Morin et al.2008, 2010, 2011) and to constrain exoplanetary masses orbiting actives FGKM type stars.
SPIRou will be a pioneer high-resolution spectropolarimeter to reach a radial velocity accuracy better than1 m.s − in the infrared (YJHK bands). This will be a cornerstone for the studies of extrasolar planets aroundM dwarfs (especially for those in transit) in a domain where the statistics is very low: only ∼
10 low-massplanets known to orbit M dwarfs, including 3 in transit. The new SPIRou instrument will have strong synergieswith current and future photometric ground-based observatories (e.g. Mearth, ExTrA) and space missions (
Ke-pler , CHEOPS , TESS , PLATO 2.0 ), being the most efficient spectrograph to characterize the mass of planetsorbiting M dwarfs. The observations that SPIRou will perform will be extremely useful for planet formation,migration and evolution theories, as well as to provide fully-characterized planets as key targets for future at-mospheric characterization with, e.g.
JWST , ECHO from space and the E-ELT from the ground.In a more general context (i.e. not limited to M dwarfs studies), SPIRou will also support the characterizationof new planets orbiting actives stars that will be in the scope of spectrographs like SOPHIE, HARPS, HARPS-N,ESPRESSO, APF, etc. . .
AS acknowledges the support by the European Research Council/European Community under the FP7 through Starting Grantagreement number 239953. AS is also grateful to the administrative council of SF2A for providing him a grant to attend the 2013’sannual meeting.
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