Apparent Stellar Wobble by a Planet in a Circumstellar Disk: Limitations on Planet Detection by Astrometry

Astrometric detection of a stellar wobble on the plane of the sky willprovide us a next breakthrough in searching extrasolar planets. The SpaceInterferometry Mission (SIM) is expected to achieve a high-precision astrometryas accurate as 1 micro-as, which is precise enough to discover a new-bornJupiter mass planet around a pre-main-sequence (PMS) star in the Taurus-Aurigastar forming region. PMS stars, however, have circum-stellar disks that may beobstacles to the precise measurement of the stellar position. We presentresults on disk influences to the stellar wobble. The density waves excited bya planet move both of the disk's mass center and the photo-center. The motionof the disk mass center induces an additional wobble of the stellar position,and the motion of the disk photo-center causes a contamination in themeasurement of the stellar position. We show that the additional stellar motiondynamically caused by the disk's gravity is always negligible, but that thecontamination of the disk light can interfere with the precise measurement ofthe stellar position, if the planet's mass is smaller than ~10 Jupiter mass.The motion of the disk photo-center is sensitive to a slight change in the wavepattern and the disk properties. Measurements by interferometers are generallyinsensitive to extended sources such as disks. Because of this property SIMwill not suffer significant contaminations of the disk light, even if theplanet's mass is as small as 1 Jupiter mass.Comment: 22 pages, 14 figures, accepted by Ap