Planets around Stars Beyond the Main Sequence (Evolved Stars)

1. Planets around Stars Beyond the Main Sequence (Evolved Stars) • RV measurements of Giant Stars • Timing Variations • Pulsar Planets • Planets around oscillating stars • Planets in eclipsing binaries

3. Exoplanets around Giant stars Mass on main sequence Difficult on the main sequence, easier (in principle) for evolved stars

4. One way to detect planets around more massive stars with the RV method: A 1.9 Mסּ main sequence star A 1.9 Mסּ K giant star

5. Early Evidence for Planets around Giant stars (Hatzes & Cochran 1993)

6. P = 1.5 yrs M = 9 MJ Frink et al. 2002 Planet around the giant star Iota Dra (M ~ 2.2 MSun)

7. The Planet around Pollux McDonald 2.1m CFHT TLS McDonald 2.7m The RV variations of b Gem taken with 4 telescopes over a time span of 26 years. The solid line represents an orbital solution with Period = 590 days, m sin i = 2.3 MJup. Mass of star = 1.9 solar masses

8. The Planet around Aldebaran CFHT McDonald 2.1m DAO TLS McDonald 2.7m The RV variations of a Tau taken. The solid line represents an orbital solution with Period = 633 days, m sin i =5.3 MJup. Mass of star = 1.06 solar masses

9. The first Tautenburg Planet: HD 13189 P = 471 d Msini = 14 MJ M* = 3.5 Msun

10. HD 13189 : Short Term Variations All Giant stars show stellar oscillations with periods of hours to days and amplitudes of 10-50 m/s

11. HD 13189 HD 13189 b

12. From Michaela Döllinger‘s Ph.D thesis P = 517 d Msini = 10.6 MJ e = 0.09 M* = 1.84 Mסּ P = 272 d Msini = 6.6 MJ e = 0.53 M* = 1.2 Mסּ P = 657 d Msini = 10.6 MJ e = 0.60 M* = 1.2 Mסּ P = 159 d Msini = 3 MJ e = 0.03 M* = 1.15 Mסּ P = 1011 d Msini = 9 MJ e = 0.08 M* = 1.3 Mסּ P = 477 d Msini = 3.8 MJ e = 0.37 M* = 1.0 Mסּ M sin i = 3.5 – 10 MJupiter

13. Stellar Mass Distribution: Tautenburg Sample N M (Mסּ) Mean = 1.4 Mסּ ~20% of the intermediate mass stars have giant planets Median = 1.3 Mסּ

14. Johnson et al. (2010): Planets around „retired“ A stars Johnson et al. also estimate that ~25% of stars with mass > 1.5 Msun have giant planets

15. Eccentricity versus Period Blue points: Giant stars with planets Open points: Main sequence stars with planets

16. Planet Mass Distribution for Solar-type main sequence stars with P> 100 d Planet Mass Distribution for Giant and Main Sequence stars with M > 1.1 Mסּ N More massive stars tend to have more massive planets and at a higher frequency M sin i (Mjupiter)

17. The Planet-Metallicity Connection Revisitied Valenti & Fischer There is believed to be a connection between metallicity and planet formation. Stars with higher metalicity tend to have a higher frequency of planets.

18. Planet-Metallicity Effect in Giant stars? Percent [Fe/H] Giant stars show no metallicity effect

19. Maybe pollution can explain the metallicity-planet connection Figure 1: Metal distribution for planet-hosting (P-H) giants (full line), P-H dwarfs with periods larger than 180 days (dashed line) and all P-H dwarfs (dotted). The giants show a distribution shifted to lower metallicity by about 0.2-0.3 dex with respect to the dwarfs Giant hosting planet stars do not show a metallicity enhancement such as the planet hosting stars on the main sequence. Pasquini et al. (2007) hypothesize that the high metal content is due to pollution by planets. When the stars evolve to giants they have deeper convection zones which mixes the chemicals.

20. Pollution hypothesis: During planet formation the giant planets migrate in and collide with the star. They have a higher content of metals and thus pollute the outer layers of the stellar atmosphere. Because the convection zone for main sequence stars is not as deep, the „polluted“ layers survive for some time. For giant stars that have a deep convection zone, this polluted layer gets mixed and one does not see a higher metal content.

21. time time Change in arrival time = apmpsini M*c ap, mp = semimajor axis, mass of planet Timing Variations: Pulsars Due to the orbital motion the distance the Earth changes. This causes differences in the light travel time

22. The Progenitors to Pulsars: Exploding Massive stars The burning stages of massive stars Main sequence lifetime ~ 10 million years Helium burning ~ 1 million years Carbon burning ~ 300 years Oxygen burning ~ 2/3 year Silicon burning ~ 2 days After Si burning the core collapses resulting in a supernova explosion. What is left behind is a neutron star. These Type II supernova

23. Type Ia: Exploding White Dwarf that has accreted matter to send it over the Chandrasekhar Limit of ~ 1.4 Msun

24. Energy output 1049– 1051 ergs

25. Properties of Neutron Stars (Pulsars) Progenitor Mass: 8-20 Msun Remnant mass: < 3 Msun (otherwise it becomes a black hole) Pressure support: Neutron degeneracy pressure Radius: ~10 km Density: 200 million tons/cm3 Magnetic field strength: ~ 1013 Gauss Periods: 1.5 millisecs to 8.5 s Rotation period of B1937+21: P = 0.0015578064924327 ±0.0000000000000004 secs These are very stable clocks!

26. The „Lighthouse“ Beacons of Pulsars

27. PSR B0329+54 P = 0.7s Vela P = 0.089 s Crab P = 0.033 s

31. Why do they think it is a planet? • Checked the barycentric correction of the Earth: Ok • 300 other pulsars observed and no 6 month periodicity was found. If it is due to the wrong barycentric why was it not seen in the other pulsars? • Possible problems • Pulsars have a rotational instability. Unlikely, especially since it is periodic • The barycentric correction is in fact wrong….hmmmmmm…

32. initial position of the pulsar used in the barycentric motion of the Earth was off by 7 arcmin • They detected the ellipticity of the Earth

34. 98 d orbit removed, 66 d orbit remains 66 d orbit removed, 98 d orbit remains

37. The “radiation from the star” is due to the rotational energy loss from the star: dE/dt = I w dw/dt w = 2p/P dw/dt = rate of rotational change dE/dt ~ 4 x 107 ergs cm–2 s–1 Solar radiation at Earth = 1.42 x 106 ergs cm–2 s–1 This is about 30 times the flux at the Earth, so the temperature of the planets should be ~670 K, comparable to Mercury

38. Pulsar with a 0.3 Msun mass companion in a 191 d orbit • After removing the timing variations of the stellar companion there are additional variations in the residuals

39. Phinney 1993: Period variations due to planet 14-400 Mearth with P > 15 yrs. • Thorsett et al. 1993 : Variations are consistent either with a planet at ~10 AU, or a star at ~50 AU orbit This planet is uncertain. Currently there is only one pulsar with planetary companions

40. Origin of the Pulsar Planets • First Generation Planets: These „rocks“ are remnants of planets (maybe giant planets) that survived the supernova explosion • Second Generation Planets: Planets that formed in the debris disk left behind after the supernova explosion Unfortunately, we only have one example of a pulsar with planets, until we find more such systems the nature of pulsar planets will be unknown.

41. One can also use stellar oscillations as „clocks“ for timing variations Searching for Planets Around Oscillating White Dwarfs Optical Light Curves of ZZ Ceti Stars Mullally et al. (2008, ApJ, 676, 573) looked at a sample of 15 pulsating white dwarfs

42. One, GD 66 looks promising: But the amplitude of the mode shows variations Arrival time variations consistent with a ~2 MJup companion in a 4.5 year orbit…but one has to be careful: • Evolutionary changes can cause period changes • Unstable modes can cause period changes • Beating of modes can cause period changes (WD stars tend to be multiperiodic pulsators).

43. Subdwarf B Stars (sdB) • sdB stars are believed to be core He-burning stars of 0.5 M on the extended horizontal branch that have lost their envelope • Teff ~ 22.000 – 40.000 K • Periods 100 – 250 secs V391 Peg

44. O-C for two pulsation frequencies look the same

45. Prototype sdB pulsating star

46. Sub-stellar Objects in Strange Places: The Sub-stellar companion to the sdB star HD 149382 found with traditional radial velocity variations A 8-20 MJup mass object in a 2.9 d orbital period…so why is this interesting?

47. sdB stars P = 2.9 days → a = 0.05 AU (assuming a 2 solar mass star) = 10 solar radii. On giant branch: Stellar radius 10-50 Rsun At one point this companion was in the envelope of the star!

48. Planets around the cataclysmic eclipsing binary NN Ser Orbital Period 3.12 hours Mass transfer White Dwarf: Mass: 0.535 Solar masses Temperature = 37000 K M4 Dwarf companion: Mass: 0.11 Solar masses Temperature ~ 3000 K NN Ser is an eclipsing system. If there are additional companions around one or both stars this will change the expected time of the eclipse.

49. One planet fit to the variations in the eclipse timing: Two planet fit: 2:1 resonance P1 = 15.5 years M1 = 6.9 MJup P2 = 7.7 years M2 ~ 2 MJup

50. These planets have to be circumbinary planets: X Formation scenarios: First or Second Generation planets First Generation: Planets formed with stars, but these would have to have survived the supernova explosion Second Generation: Planets formed after the common envelope phase