Research

Ongoing Doppler planet searches among the globe have discovered more than 250 extrasolar planets mainly around solar-like stars. On the otherhand, little is known about planetary systems around other types of stars. Planetary systems around more massive stars the Sun are particularly important for constructing planet formation theory in the point that their properties can be used to constrain key factors of planet formation, such as timescale, role of radiation of central stars, dependence on disk mass, etc. However, precise radial velocity measurements are difficult when they are on the main sequence (A- or B-type dwarfs) due to the lack of appropriate absorption lines in their spectra.

Since 2001, we have been carrying out Doppler planet searches targeting "G- and K-type giants", which are intermediate-mass (1.5-5 solar mass) stars in evolve stages with many sharp absorption lines in their spectra suitable for precise raidal velocity measurements. So far, we have discovered several extrasolar planets around GK giants using 1.88m telescope at Okayama Astrophysical Observatory of Japan. We are now trying to further extend our project in the framework of international collaboration with Chinese and Korean astronomers ("East Asian Planet Search Network").

Reference;
Sato, B., Izumiura, H., Toyota, E. et al. 2008, "Planetary Companions around Three Intermediate-Mass G and K Giants: 18 Del, ξAql, and HD 81688", PASJ, in press (astro-ph/0802.2590)

"Hot Jupiters" are giant planets in short-period oribts represented by the first extrasolar planet, 51 Pegasi b. They provide information not only on formation and evolution of planetary systems but also on planets themselves such as internal structure, atmosphere, etc. Thanks to the short-periods, hot Jupiters are high-probability "transit" candidates, which can provide information on planet radii, densities, and hence internal structure of planets combined with masses obtained by Doppler observations.

Since 2004, we have been carrying out an extensive search for hot Jupiters using 8.2m Subaru telescope (Hawaii) in collaboration with US astronomers ("N2K consortium"). So far, we have discovered two peculiar transiting planets; one has an extraordinary huge solid core up to 70 Earth mass and the other resides in a fairly eccentric orbit despite of its short period. Both of them give deep insights into understanding of formation mechanism of hot Jupiters.

Reference;
Sato, B., et al. 2005, "The N2K Consortium. II. A Transiting Hot Saturn around HD 149026 with a Large Dense Core", ApJ, 633, 465-473

To find planetary companions, we detect a tiny wobble of a star in radial velocity due to the reflex motion caused by the gravitational pull of unseen planets. In the case of our solar system, the velocity amplitude of the Sun caused by the Jupiter, the most massive planet in the solar system, is about 13 m/s. The velocity variation is quite small compared to typical observational error of a few hundred m/s when we use conventional spectroscopic technique.

Iodine absorption cells are now widely used among the globe for the purpose of "precise" radial velocity measurements. In this technique, stellar spectra are taken through a cell enclosing iodine molecular gas put in front of the entrance slit of the spectrograph and then the cell directly superposes thousands of sharp iodine abosorption lines onto the stellar spectra. Because the iodine lines and the stellar lines experience the same instrumental shifts and distortions of the telescope and the spectrograph, we can use the iodine lines both as a wavelength standard and as a reference to correct for the instrumental profile of the spectrograph.

The best precision we can now achieve using 1.88m telescope of Okayama Astrophysical Observatory is 2m/s. With the precision, we can detect Neptune-mass planets if they resides close to the central stars. We are now trying to improve the precision down to less than 1m/s.

Reference;
Kambe, E., Ando, H., Sato, B., et al. 2008, "Development of Iodine Cells for Subaru HDS and Okayama HIDES. III. An Improvement on Radial Velocity Measurement Technique", PASJ, in press

Stellar evolution, pulsation, activity, etc. (under construction)