Extrasolar Planet Research at the LSW

Research of the LSW Radial Velocity Group

We are regularly monitoring the radial velocities of a sample of about 360 G and K giant stars at Lick Observatory, using the 60cm CAT Telescope and the Hamilton High Resolution Echelle Spectrograph in conjunction with an iodine cell. With this setup, we reach a relative radial velocity accuracy between 5 and 8 m/s for most of our giant stars.

The giants have been selected from the Hipparcos Catalogue for brightness (brighter than 6 mag) and for mass; we intend to study the effect of primary mass on substellar companion properties.

The interpretation of the observed radial velocities of giant stars is somewhat more complicated than those of main-sequence stars, because several phenomena can mimic the existence of a substellar companion. The most notable of such phenomena are non-radial pulsations. Non-radial pulsations would, in contrast to an orbiting substellar companion, cause changes in the shapes of the spectral lines with the same period as the radial velocities. A detailed spectral analysis demonstrating the absence of such periodic changes in the spectral lines is thus a pre-requisite for the interpretation of the observed radial velocity changes in terms of an orbiting substellar companion.

Discoveries

iota Draconis

The first substellar companion known around a giant star was announced by our group in 2001 and published in 2002. The companion orbits the K giant iota Draconis in an eccentric orbit with a period of about 1.5 years and has a minimum mass of 8.8 Jupiter masses; depending on the unknown inclination, it might be either a massive planet or a lightweight brown dwarf. Due to the non-detection of the companion in the Hipparcos Catalogue, the object must be of substellar nature.

The original press release and some updated information about the orbital parameters of iota Draconis b can be found on our iota Dra webpage.

Pollux

It has long been speculated that Pollux harbors a planet. Our radial velocities for that star, along with many older ones, helped to confirm that this hypothesis is correct. The period is about 590 days, and the minimum mass of the companion is about 1.9 Jupiter masses; it is thus most likely a planet and not a brown dwarf, although the inclination of the orbit is unknown. The planet orbits in an almost circular orbit.

Radial Velocities of Giant Stars in General

Our group has also published some papers characterizing the radial velocities of giant stars in a statistical sense. We demonstrated for example that there is trend for the redder K giants to display larger intrinsic radial velocity jitter than the early K giants or late G giants. Also, we showed that for the early K giants, the typical intrinsic radial velocity jitter is of order 20 m/s, allowing for the detection of the more massive planetary companions around these stars.

See the publication page of our group for more details!

A statistical analysis of the binary star properties of K giants is ongoing.

last updated: November 5, 2006, by Sabine Reffert