Max Planck Institute for Astronomy - Annual Report 2005

24 II. Highlights
II.3 The First Heidelberg Extrasolar Planet
Since 1995, more than 180 extrasolar planets have been
discovered. The majority of them orbit solar-like stars.
Until now, only very few discoveries of substellar companions
around giant stars are known , for instance. An
international team of astronomers under the leadership
of the MPIA has detected a planet around a red giant
star. The mass of the star is about two solar masses. The
star HD 11977A has a planetary companion of at least
6.5 Jupiter masses at a distance of just under 2 AU. This
finding provides an important contribution to theories on
the formation and evolution of planetary systems.
Large extrasolar planet search programs so far concentrated
on solar-like stars. For these kind of stars, the
precise stellar radial velocity method has been established.
It is of great interest to investigate whether other
stars which are different than the sun also have planets.
Among them are stars of different mass ranges and evolutionary
stages.
Main-sequence stars which are more massive than the
sun are not suitable for the precise radial velocity method.
These stars are hotter than the sun, thus their stellar
spectra contain fewer absorption lines. Usually they also
rotate faster than solar type stars. This leads to strong
Radial velocity [km/s]
o–c [m/s]
–17.7
–17.8
–17.9
–18.0
–18.1
–18.2
–18.3
100
50
0
–50
–100
FERROS @ 1.5 m LSO
FERROS @ 2.2–m MPG/ESO
spectral line broadening so that the radial velocities cannot
be measured accurately. When main-sequence stars
of intermediate (1.5 – 4 M � ) or even higher masses evolve,
the atmosphere becomes cooler so that there are more
spectral lines. While the star is moving towards the red
giant branch, it rotates slower and the atmosphere cools
because of the expansion of the stellar envelope. This
makes the accurate measurement of the radial velocities
easier. However, there are other effects to be considered.
Red giants possess higher stellar activity compared to
solar-like stars, for example: large star spots or stellar
oscillations. Nevertheless, these phenomena can be
identified from the spectroscopy and distinguished from
variation in the radial velocity induced by the presence
of unseen companions.
Since 1999, the international team of astronomers had
already monitored the red giant HD 11977, which is 216
light years away from Earth within a study of the physics
of evolved stars. The target sample consists of about 200
giant stars from spectral type G and K. The observations
Fig. II.3.1: Variations of the radial velocity in HD 11977, measured
with FeroS from October 1999 to November 2004.
�(o–c) = 29.1 m/s
�2 � = 1.01
1200 1600 2000 2400 2800 3200 3600
Time (JD 2450000) [days]