„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)

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?

Motivational background

Yes we can!

Do planets exist in external galaxies? Probably, but can we find out?

Observe nearby halo stream stars!

Motivational background

Yes we can!

Target selectionThe Helmi stream (Helmi et al. 1999)

Target selectionThe Helmi stream (Helmi et al. 1999)

SErGiO

SErGiOStar of Extra-Galactic Origin

Stellar parameters Literature & our results

Spectral typeBrightness (V)DistanceEffective temperatureRadiusSurface gravity (log g)Stellar massMetallicity [Fe/H]Projected rotational velocityConfirmed membershipEvolutionary stage

F29.84 mag700 pc6025 K6.7 RϿ

2.690.82 MϿ

-2.0910.3 km/sHelmi streamRHB

Radial Velocity

FEROSSept. 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

Starspots

Line-profile asymmetry (bisector, BVS)

Ca II variations (K & H, IR triplet)

Other spectroscopic activity indicator

(Hα, Teff variation, …)

Photometric variations

If the stellar activity can be excluded

Rotational modulation

Stellar pulsations

Rotational modulation

Stellar pulsations

Companion

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

PeriodAmplitudeEccentricitySemimajor axisMinimum mass (m sin i)

16.2 days120 m/s0.280.116 AU1.3 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.