Stellar Physics Revealed by Planetary Transits

1. Stellar Physics Revealed by Planetary Transits Willie Torres Harvard-Smithsonian Center for Astrophysics IAU General Assembly, Special Session 13 High Precision Tests of Physics from High-Precision Photometry Beijing, 29 August 2012 IAU SS13, Beijing

2. Selected Topics • Accurate mass and radius determinations for low-mass stars • Known disagreements between observations and stellar evolution theory • Eclipsing binaries are by-products of transit surveys: many new light curves available • Circumbinary transiting planets • Spin-orbit alignment for planetary systems • Spots on the host stars of transiting planets: spot properties and distribution IAU SS13, Beijing

3. Low-Mass Stars and the Disagreements with Models • Many low-mass stars are both larger and cooler than predicted by stellar evolution theory • Evidence has been accumulating for many years, mostly from double-lined eclipsing binaries (Lacy 1977, Popper 1997, Clausen 1999, Torres & Ribas 2002, and many others) • The vast majority of these binary systems have short orbital periods (mostly < 3 days) • Stellar activity has long been suspected as the underlying cause (tidal synchronization  rapid rotation  activity) • Magnetic fields inhibit convective energy transport • Spot coverage reduces radiating surface area IAU SS13, Beijing

4. CM Dra compared with models (Dotter et al. 2008) • Age and metallicity not well known • Population II star? • Solar-metallicity models do not fit • Age is not totally irrelevant • Detailed studies • Feiden et al. 2011 • Spada & Demarque 2012 • MacDonald & Mullan 2012 IAU SS13, Beijing

5. CM Dra compared with models (Dotter et al. 2008) • Age and metallicity not well known • Population II star? • Solar-metallicity models do not fit • Age is not totally irrelevant • Detailed studies • Feiden et al. 2011 • Spada & Demarque 2012 • MacDonald & Mullan 2012 • [Fe/H] = +0.50 fits, but binary is unlikely to be that metal-rich IAU SS13, Beijing

6. If short-period binaries show disagreements with theory, should long-period binaries behave better? • Tidal forces should be much weaker • In principle the binary components should rotate more slowly, and should be relatively inactive • However, such long-period systems are rare among eclipsing binaries (difficult to study) • Two long-period eclipsing binaries recently found, both as by-products of transit surveys: • LSPM J1112+7626 (MEarth; Irwin et al. 2011) • Kepler-16 (Doyle et al. 2011, Winn et al. 2011), a system with a circumbinary transiting planet IAU SS13, Beijing

7. Circumbinary planet K5V LSPM J1112+7626 (Irwin et al. 2011) P = 41.032 days M4V P = 41.079 days [Fe/H] = 0.30 Kepler-16 (Doyle et al. 2011, Winn et al. 2011) Dartmouth models (Dotter et al. 2008) Age and metallicity unknown; secondary may still be active IAU SS13, Beijing

8. Feiden et al. 2011 KOI-126 B This same model fits KOI-126 A KOI-126 C Age = 4.1 Gyr Current models do agree with the measurements of at least one low-mass system: KOI-126 BC Triple System found by Kepler Pair of M dwarfs with P = 1.77 days in a 33.9-day orbit around a G dwarf [Fe/H] = +0.15 (Carter et al. 2011) Photo-dynamical modeling of the Kepler light curve IAU SS13, Beijing

9. Models are able to explain the larger radii and cooler temperatures of late-type stars, but add free parameters that must be tuned to each case •  (magnetic inhibition parameter; Mullan & MacDonald 2001) • ML , and spot filling factor  (Chabrier et al. 2007) • Systematic effects play an important role in measuring masses and radii of low-mass stars (e.g., spots change with time, and can affect the results) • High-precision photometry and continuous coverage (e.g., Kepler, CoRoT) is an advantage • Photo-dynamical modeling in multiple systems can alleviate some of the problems caused by spots IAU SS13, Beijing

10. Spin-Orbit Alignment • Transiting planet observations can provide information on the orientation of the stellar spin axis relative to the planetary orbit • Rossiter-McLaughlin effect • Observation of spot anomalies • Obliquity measurements can tell us about the efficiency of tidal interactions, energy dissipation, and have a bearing on planet migration theories IAU SS13, Beijing

11. Orbital axis Stellar spin axis Transiting planet The obliquity (or spin-orbit angle ) is the angle between the spin axis of the host star and the axis of the orbit of the planet. Typically we can only measure its projection on the plane of the sky, λ. IAU SS13, Beijing

12. The Rossiter-McLaughlin Effect Queloz et al. (2000), Ohta, Taruya, & Suto (2005), Gaudi & Winn (2007)

13. A  (Rp/R*)2v sin i R-M observations are relatively easy for hot Jupiters transiting bright and rapid rotators IAU SS13, Beijing

14. WASP-17 HAT-P-2 WASP-15 WASP-8 HAT-P-7 HAT-P-1 IAU SS13, Beijing

15. Migration and Stellar Properties • Two broadly different migration mechanisms proposed for hot Jupiters • Interactions with a flat circumstellar disk  Low obliquities • Dynamical processes (e.g., planet-planet scattering)  High obliquities • Winn et al. (2010) first noticed that hot Jupiters orbiting early-type stars tend to be misaligned, while those around cool stars are not • Initial obliquities were nearly random (scattering), and low obliquities result from subsequent tidal interactions • Albrecht et al. (2012) provided additional support for the scattering process IAU SS13, Beijing

16. Obliquities measured via the R-M effect For hot Jupiters, systems with high obliquities tend to be associated with hotter stars Albrecht et al. (2012) IAU SS13, Beijing

17. Spots on the Host Stars of Transiting Planets • A nuisance: they interfere with the determination of planet properties • They cause variations in transit depth, biasing the radius • They produce chromatic effects that can be mistaken for atmospheric absorption • They cause anomalies in individual light curves • They can bias transit timing measurements • An opportunity to learn about the planet, its orbit, and the parent star • Stellar rotation period (Silva-Valio 2008) • Spot distribution (Lanza et al. 2009; Désert et al. 2011) • Spin-orbit alignment (Nutzman et al. 2011, Deming et al. 2011; Sanchis-Ojeda et al. 2011, 2012) IAU SS13, Beijing

18. 1 2 3 Time Aligned axes Porb/Prot = 0.1 IAU SS13, Beijing

19. 1 2 3 Time Misaligned axes IAU SS13, Beijing

20. Starspots, spin-orbit alignment, and active latitudes in the HAT-P-11 exoplanetary system (Sanchis-Ojeda et al. 2011) Out-of-transit variability from Kepler Prot 30.5 days K4V star with a “super-Neptune” Orbital period = 4.9 days Rp = 4.7 R Mp = 26 M Known to be misaligned (λ = 103º) from R-M measurements Spot anomalies seem to occur at two specific phases of the transit IAU SS13, Beijing

21. Latitude = ±19.7º is = 80º Latitude = 67º is = 168º Sanchis-Ojeda et al. (2011) Spot distribution on HAT-P-11 Two preferred phases  Two preferred latitudes? Planetary transits of active stars allow us to constrain the three-dimensional stellar obliquity  (not just λ) based on the observed pattern of spot anomalies and a simple geometrical model IAU SS13, Beijing

22. A Butterfly Diagram for HAT-P-11? Spot distribution as a function of time HAT-P-11 If the active latitudes change with time analogously to the “butterfly diagram” of the Sun’s activity, future Kepler observations should reveal changes in the preferred phases of spot-crossing anomalies Sanchis-Ojeda et al. (2011) The Sun IAU SS13, Beijing

23. Summary Spot anomalies detected with high-precision photometry are now a common and useful tool for measuring rotation periods and obliquities in transiting systems (complementary to the R-M effect). They can also serve to characterize the spot distribution. R. Sanchis-Ojeda KOI-126 Carter et al. (2011) Accurate measurements of stellar properties (masses, radii) are enabled by the many new light curves resulting from transit surveys, and photo-dynamical modeling in special configurations (triples, circumbinary transiting planets) IAU SS13, Beijing