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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 M סּmain sequence star A 1.9 M סּK 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 Early Evidence for Planets around Giant stars (Hatzes & Cochran 1993) Careful! Rotation periods of giants ~ hundreds of days… P = 1.5 yrs Frink et al. 2002 M = 9 MJ 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 StarMetallicity 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: 1. Giant planet frequency is a strong function of stellar mass 2. Giant planets are more frequent around metal-rich stars