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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