Exoplanet Host Stars

The Hertzsprung-Russel (HR)Diagram

The Hertzsprung-Russel (HR)Diagram Standard Doppler Surveys

The Hertzsprung-Russel (HR)Diagram Direct Imaging detections Standard Doppler Surveys

The Dependence of Planet Formation on Stellar Mass Don’t forget observational selection effects! Are trends real?

Exoplanets around massive stars Difficult with the Doppler method because more massive stars have higher effective temperatures and thus fewer spectral lines. They also rotate faster. Result: Only a few planets around early-type, more massive main sequence stars, and these are mostly around F-type stars (~ 1.4 solar masses) A way around this is to look for planets around evolved (subgiant and giant) stars.

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

The Hertzsprung-Russel (HR)Diagram Direct Imaging detections Standard Doppler Surveys

The Hertzsprung-Russel (HR)Diagram Sub-/Giants feasible with Doppler Direct Imaging detections Standard Doppler Surveys

Careful! Rotation periods of giants ~ hundreds of days… Early Evidence for Planets around Giant stars (Hatzes & Cochran 1993) Careful! Rotation periods of giants ~ hundreds of days…

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

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

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)

The Hertzsprung-Russel (HR)Diagram Sub-/Giants feasible with Doppler Direct Imaging detections Standard Doppler Surveys Doppler 8-10m Microlensing

M Dwarfs in a Nutshell: complex spectra (high opacity due to TiO, VO and other molecules) Mass = 0.6 – 0.1 MSun MV = 7.5 – 20 (absolute magnitude, at 10pc) L ~ 0.2 – 5 x 10-4 LSun (Luminosity) Teff ~ 3800 – 2100 K Radii = 0.5 – 0.1 RSun slow rotators, still exhibit activity related phenomena (flares, spots, etc) most numerous stars

Exoplanets around low mass stars Microlensing statistics point toward lower frequency of giant planets around M dwarfs (Gaudi et al. 2002) Doppler programs (past & on-going): ESO UVES program (Kürster, Endl et al.): 40 stars HET Program (Endl, Cochran et al.) : 100 stars Keck Program (Marcy, Butler et al.): 200 stars HARPS Program (Mayor et al.):~200 stars Results from Doppler method of nearby M dwarfs: Giant planets within 1-2 AU are rarer around M dwarfs than around F,G,K stars (Endl et al. 2006). Confirmed by Keck & HARPS results BUT: Hot neptunes & Superearths more frequent (HARPS).

Proxima Cen [M5Ve] 0.12 MSun Endl et al. (2007):

Mstar ~ 1.4 Msun Mstar ~ 1 Msun Mstar ~ 0.4 Msun Exoplanets around main sequence stars of different masses Don’t forget obs. selection effects!

Preliminary conclusions: more massive stars have more massive planets with higher frequency. Less massive stars have less massive planets → planet formation is a sensitive function of the stellar mass.

Planets and the Properties of the Host Stars: The Star-Metallicity Connection Astronomer‘s Metals More Metals ! Even more Metals !!

The „Bracket“ [Fe/H] Take the abundance of heavy elements (Fe for instance) Ratio it to the solar value Take the logarithm e.g. [Fe/H] = –1 → 1/10 the iron abundance of the sun

The Planet-Metallicity Connection: These are stars with metallicity of 0.3 to 3.16 x solar 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. This is often used as evidence in favor of the core accretion theory of giant planet formation

Cochran, Endl et al. 2007: HD 155358 two giant planets and.. …[Fe/H] = –0.68. This certainly muddles the metallicity-planet connection

Summary: Giant planet frequency is a strong function of stellar mass Giant planets are more frequent around metal-rich stars