Yuji Norimoto

A PLANETARY COMPANION TO THE G-TYPE GIANT STAR HD 104985

We report the detection of a planetary-mass companion to the G9 III giant star HD 104985 from precise Doppler velocity measurements made using the High Dispersion Echelle Spectrograph (HIDES) at Okayama Astrophysical Observatory. The radial velocity variability of this star is best explained by an orbital motion with a period of 198.2 ± 0.3 days, a velocity semiamplitude of 161 ± 2 m s-1, and an eccentricity of 0.03 ± 0.02. Assuming a stellar mass of 1.6 M☉, we obtained a minimum mass and a semimajor axis of 6.3MJ and 0.78 AU, respectively, for the companion. A probable upper limit to the stellar mass of 3 M☉ yielded m2 sin i = 9.6MJ, which falls in the planetary-mass regime. This is the first discovery of a planetary companion orbiting a G-type giant star.

Photospheric Abundances of Volatile and Refractory Elements in Planet-Harboring Stars

By using the high-dispersion spectra of 14 bright planet-harboring stars (along with 4 reference stars) observed with the new coude echelle spectrograph at Okayama Astrophysical Observatory, we investigated the abundances of volatile elements (C, N, O, S, Zn; low condensation temperature Tc) in order to examine whether these show any significant difference compared to the abundances of other refractory elements (Si, Ti, V, Fe, Co, Ni; high Tc) which are known to be generally overabundant in those stars with planets, since a Tc-dependence is expected if the cause of such a metal-richness is due to the accretion of solid planetesimals onto the host star. We found, however, that all elements we studied behave themselves quite similarly to Fe (i.e., [X/Fe]~0) even for the case of volatile elements, which may suggest that the enhanced metallicity in those planet-bearing stars is not so much an acquired character (by accretion of rocky material) as rather primordial.

Spiral Structure in WZ Sagittae around the 2001 Outburst Maximum

Intermediate-resolution phase-resolved spectra of WZ Sge were obtained on five consecutive nights (July 23–27) covering the initial stage of the 2001 superoutburst. Double-peaked emission lines of He II at 4686 u A, which were absent on July 23, emerged on July 24 together with emission lines of C III/N III Bowen blend. Analyses of the He II emission lines using the Doppler tomography revealed an asymmetric spiral structure on the accretion disk. This finding demonstrates that spiral shocks with a very short orbital period can arise during the initial stage of an outburst and may be present in all SU UMa stars.