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„We are not talking about cosmology...“ (A. Sozzetti). The visitor from an ancient galaxy A planetary companion around an old, metal-poor Red Horizontal Branch star. R. J. Klement, J. Setiawan, Th. Henning, H.-W. Rix, B. Rochau, T. Schulze-Hartung, J. Rodman (ESA/ESTEC,NL).
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„We are not talking about cosmology...“ • (A. Sozzetti)
The visitor from an ancient galaxyA planetary companion around an old, metal-poor Red Horizontal Branch star • R. J. Klement, J. Setiawan, Th. Henning, H.-W. Rix, B. Rochau, • T. Schulze-Hartung, J. Rodman (ESA/ESTEC,NL)
Motivational background • Do planets exist in external galaxies? Probably, but can we find out?
Motivational background • Do planets exist in external galaxies? Probably, but can we find out? Yes we can!
Motivational background • Do planets exist in external galaxies? Probably, but can we find out? • Observe nearby halo stream stars! Yes we can!
Target selection The Helmi stream (Helmi et al. 1999)
Target selection The Helmi stream (Helmi et al. 1999) SErGiO
SErGiOStar of Extra-Galactic Origin Stellar parameters Literature & our results • Spectral type • Brightness (V) • Distance • Effective temperature • Radius • Surface gravity (log g) • Stellar mass • Metallicity [Fe/H] • Projected rotational velocity • Confirmed membership • Evolutionary stage • F2 • 9.84 mag • 700 pc • 6025 K • 6.7 RϿ • 2.69 • 0.82 MϿ • -2.09 • 10.3 km/s • Helmi stream • RHB
Radial Velocity FEROS Sept. 2009 – July 2010 36 RV measurements 550 spectral lines for RV computations semi-amplitude 120 m/s uncertainty ~ 50 m/s FAP = 5.5 x 10e-6
Sources of RV variation • Rotational modulation • Stellar pulsations • Rotational modulation • Stellar pulsations • Companion Starspots Line-profile asymmetry (bisector, BVS) Ca IIvariations (K & H, IR triplet) Other spectroscopic activity indicator (Hα, Teff variation, …) Photometric variations If the stellar activity can be excluded
Sources of RV variation periodic variations of - the BVS ~ 5 days - CaII λ8498 ~ 6 days → rotational modulation → Vrot ~60 km/s SuperWASP photometry (~3600 data points) → stellar pulsations
Planet: SerGiO jr.Setiawan, Klement et al., submitted Parameter Value • Period • Amplitude • Eccentricity • Semimajor axis • Minimum mass (m sin i) • 16.2 days • 120 m/s • 0.28 • 0.116 AU • 1.3 M(Jup) The estimated minimum mass places the companion in the substellar regime.
What is interesting? • Sergio has an extra-galactic origin • Sergio has the lowest metallicity among known planet host stars, [Fe/H] = -2.09 ± 0.25 dex
What is interesting? • The planet has survived the RGB phase during the stellar evolution. • Probably, at the beginning the planet had a larger orbit, but then spiraled in to a closer orbit • The star is a fast rotator. Sign that the inner planets have been engulfed already(?)
Conclusions • Planets can indeed form around metal-poor stars as low as [Fe/H] ~ - 2 • Under certain circumtances planets can survive the red giant phase. The orbits may be changed during the stellar evolution. • SErGiO jr is the first planet detected in a tidal stellar stream and probably the first known exoplanetary system with an extragalactic origin.