伊藤 洋一

Context. To reveal details of the internal structure, the relationship between chromospheric activity and the Rossby number has been extensively examined for main-sequence stars. For active pre-main-sequence (PMS) stars, it is suggested that the level of activity be assessed using optically thin emission lines, such as Mg I. Aims. We aim to detect Mg I chromospheric emission lines from PMS stars and to determine whether the chromosphere is activated by the dynamo process or by mass accretion from protoplanetary disks. Methods. We analyzed high-resolution optical spectra of 64 PMS stars obtained with the Very Large Telescope (VLT)/X-shooter and UVES and examined the infrared Ca II (8542 Å) and Mg I (8807 Å) emission lines. To detect the weak chromospheric emission lines, we determined the atmospheric parameters (T_eff and log 𝑔) and the degree of veiling of the PMS stars by comparing the observed spectra with photospheric model spectra. Results. After subtracting the photospheric model spectrum from the PMS spectrum, we detected Ca II and Mg I as emission lines. The strengths of the Mg I emission lines in PMS stars with no veiling are comparable to those in zero-age main-sequence (ZAMS) stars if both types of stars have similar Rossby numbers. The Mg I emission lines in these PMS stars are thought to be formed by a dynamo process similar to that in ZAMS stars. In contrast, the Mg I emission lines in PMS stars with veiling are stronger than those in ZAMS stars. These objects are believed to have protoplanetary disks, where mass accretion generates shocks near the photosphere, heating the chromosphere. Conclusions. The chromosphere of PMS stars is activated not only by the dynamo process but also by mass accretion.

We present the discoveries of two giant plants orbiting the red-giant-branch star HD 112570 and the red-clump star HD 154391, based on the radial-velocity (RV) measurements from the Xinglong station and Okayama Astrophysical Observatory. Spectroscopic and asteroseismic analyses suggest that HD 112570 has a mass of 1.15 +/- 0.12 M-circle dot, a radius of 9.85 +/- 0.23 R-circle dot, a metallicity [Fe/H] of -0.46 +/- 0.1, and logg of 2.47 +/- 0.1. With the joint analysis of RV and Hipparcos-Gaia astrometry, we obtain a dynamical mass of = Mp 3.42(-0.84)(+1.4) M-Jup Jup, a period of P = 2615 (+85)(-77) days, and a moderate eccentricity of = e = 0.20(-0.14)(+0.16) for the Jovian planet HD 112570 b. For HD 154391, it has a mass of 2.07 +/- 0.03 M-circle dot, a radius of 8.56 +/- 0.05 R-circle dot, a metallicity [Fe/H] of 0.07 +/- 0.1, and logg of 2.86 +/- 0.1. The super Jupiter HD 154391 b has a mass of = M-p 9.1(-1.9)(+2.8) M-Jup, a period of = P 5163(-57)(+60) days, and an eccentricity of e = 0.20(-.0.04)(+0.04). We found that HD 154391 b has one of the longest orbital periods among those ever discovered orbiting evolved stars, which may provide a valuable case in our understanding of planetary formation at wider orbits. Moreover, while a mass gap at 4 M-Jup seems to be present in the population of giant stars, there appear to be no significant differences in the distribution of metallicity among giant planets with masses above or below this threshold. Finally, the origin of the abnormal accumulation near 2 au for planets around large evolved stars (R-star > 21 R-circle dot), remains unclear.

We investigated chromospheric activities of pre-main-sequence (PMS) stars. First, we studied the Ca II infrared triplet emission lines with Subaru/HDS and other spectroscopic instruments. Most PMS stars have narrow Ca II lines whose intensities are as large as the maximum of the zero-age main-sequence (ZAMS) stars. The chromosphere of PMS stars is suggested to be filled by the Ca II emitting region. Second, we found many faint chromospheric emission lines such as Mg I and Fe I for more than half of the ZAMS stars. Third, we searched the periodic light variation caused by a starspot for the 26 PMS stars. Their TESS light variations and Ca II emission line strengths show the positive correlation, and are located on the extensions of the superflare stars. In summary, PMS stars have very active chromosphere driven by strong dynamo process due to the fast rotation and the long convection timescale.