Yoichi Takeda

CENTRAL ROTATION CURVES OF SPIRAL GALAXIES

We present high-resolution central-to-outer rotation curves for Sb, SBb, Sc, and SBc galaxies. We discuss their general characteristics, particularly their central behavior, as well as dependencies on morphological types, activity, and peculiarity. The rotation curves generally show a steep nuclear rise and high-velocity central rotation, followed by a broad maximum in the disk and then a flat rotation due to the massive halo. Since the central high velocity and steep rise are common to all massive galaxies, they cannot be due to noncircular motions. Disk rotation curves of barred galaxies show larger dispersion than those of normal galaxies, probably because of noncircular motions. Interacting galaxies often show perturbed outer rotation curves, while their central rotation shows no particular peculiarity. In addition, central activities, such as starbursts and active galactic nuclei, appear to show no particular correlation with the property of rotation curves. This would suggest that the central activities are triggered by a more local effect than the global dynamical property.

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.

Stellar Parameters and Elemental Abundances of Late-G Giants

The properties of 322 intermediate-mass late-G giants (comprising 10 planet-host stars) selected as the targets of the Okayama Planet Search Program, many of which are red-clump giants, were comprehensively investigated by establishing their various stellar parameters (atmospheric parameters, including turbulent velocity fields, metallicity, luminosity, mass, age, projected rotational velocity, etc.), and their photospheric chemical abundances for 17 elements, in order to study their mutual dependence, connection with the existence of planets, and possible evolution-related characteristics. The metallicity distribution of planet-host giants was found to be almost the same as that of non-planet-host giants, making marked contrast to the case of planet-host dwarfs tending to be metal-rich. Generally, the metallicities of these comparatively young (typical age of � 10 9 yr) giants tend to be somewhat lower than those of dwarfs at the same age, and super-metal-rich ([Fe=H] > 0.2) giants appear to be lacking. Apparent correlations were found between the abundances of C, O, and Na, suggesting that the surface compositions of these elements have undergone appreciable changes due to dredge-up of H-burning products by evolution-induced deep

Oxygen line formation in late-F through early-K disk/halo stars - Infrared O I triplet and [O I] lines

In order to investigate the formation of O uf769 7771-5 and (O uf769) 6300/6363 lines, extensive non-LTE calculations for neutral atomic oxygen were carried out for wide ranges of model atmosphere parameters, which are applicable to early-K through late-F halo/disk stars of various evolutionary stages. The formation of the triplet O uf769 lines was found to be well described by the classical two-level-atom scattering model, and the non-LTE correction is practically determined by the parameters of the line-transition itself without any significant relevance to the details of the oxygen atomic model. This simplifies the problem in the sense that the non-LTE abundance correction is essentially determined only by the line-strength (Wλ), if the atmospheric parameters of Teff, log g, and ξ are given, without any explicit dependence of the metallicity; thus allowing a useful analytical formula with tabulated numerical coefficients. On the other hand, our calculations lead to the robust conclusion that LTE is totally valid for the forbidden (O uf769) lines. An extensive reanalysis of published equivalent-width data of O uf769 7771-5 and (O uf769) 6300/6363 taken from various literature resulted in the conclusion that, while a reasonable consistency of O uf769 and (O uf769) abundances was observed for disk stars (−1 < (Fe/H) < 0), the existence of a systematic abundance discrepancy was confirmed between O uf769 and (O uf769) lines in conspicuously metal-poor halo stars (−3 < (Fe/H) < −1) without being removed by our non-LTE corrections, i.e., the former being larger by ∼0. 3d ex at−3 < (Fe/H) < −2. An inspection of the parameter-dependence of this discordance indicates that the extent of the discrepancy tends to be com- paratively lessened for higher Teff/ log g stars, suggesting the preference of dwarf (or subgiant) stars for studying the oxygen abundances of metal-poor stars.

Behavior of Sulfur Abundances in Metal-poor Giants and Dwarfs

LTE and non-LTE (NLTE) abundances of sulfur in six metal-poor giants and 61 dwarfs (62 dwarfs including the Sun) were explored in the range of -3 [Fe/H] +0.5 using high-resolution, high signal-to-noise ratio spectra of the S I 8693.9 and 8694.6 A lines observed by us and measured by Francois and Clegg, Lambert, & Tomkin. NLTE effects in S abundances are found to be small and practically negligible. The behavior of [S/Fe] versus [Fe/H] exhibits a linear increasing trend without plateau with decreasing [Fe/H]. Combining our results with those available in the literature, we find that the slope of the increasing trend is -0.25 in the NLTE behavior of [S/Fe], which is comparable to that observed in [O/Fe]. The observed behavior of S may require chemical evolution models of the Galaxy, in which scenarios of hypernovae nucleosynthesis and/or time-delayed deposition into interstellar medium are incorporated.

Non-LTE Analysis of the Sodium Abundance of Metal- Poor Stars in the Galactic Disk and Halo

We performed an extensive non-LTE analysis of the neutral sodium lines of Na i 5683/5688, 5890/5896, 6154/6161, and 8183/8195 in disk/halo stars of types F-K covering a wide metallicity range ( 4 < (Fe/H) < +0.4), using our own data as well as data collected from the literature. For comparatively metal- rich disk stars ( 1 < (Fe/H) < +0.4) where the weaker 6154/6161 lines are the best abundance indicators, we confirmed (Na/Fe) 0 with an upturn (i.e., a shallow/broad dip around 0.5 < (Fe/H) < 0) as already reported in previous studies. For the metal-deficient halo stars, where the much stronger 5890/5896 or 8183/8195 lines subject to considerable (negative) non-LTE corrections amounting to 0.5 dex have to be used, our analysis suggests mildly subsolar (Na/Fe) values down to 0.4 (with a somewhat large scatter of ± 0.2 dex) on the average at the typical halo metallicity of (Fe/H) 2, followed by a rise again to a near-solar ratio of (Na/Fe) 0 at the very metal-poor regime (Fe/H) 3 to 4. These results are discussed in comparison with the previous observational studies along with the theoretical predictions from the available chemical evolution models.

Behavior of Li abundances in solar-analog stars - II. Evidence of the connection with rotation and stellar activity

Context. We previously attempted to ascertain why the Li i 6708 line-strengths of Sun-like stars differ so significantly despite the superficial similarities of stellar parameters. We carried out a comprehensive analysis of 118 solar analogs and reported that a close connection exists between the Li abundance (ALi) and the line-broadening width (vr+m; mainly contributed by rotational effect), which led us to conclude that stellar rotation may be the primary control of the surface Li content. Aims. To examine our claim in more detail, we study whether the degree of stellar activity exhibits a similar correlation with the Li abundance, which is expected because of the widely believed close connection between rotation and activity. Methods. We measured the residual flux at the line center of the strong Ca ii 8542 line, r0(8542), known to be a useful index of stellar activity, for all sample stars using newly acquired spectra in this near-IR region. The projected rotational velocity (ve sini )w as estimated by subtracting the macroturbulence contribution from vr+m that we had already established. Results. A remarkable (positive) correlation was found in the ALi versus (vs.) r0(8542) diagram as well as in both the r0(8542) vs. ve sini and ALi vs. ve sini diagrams, as had been expected. With the confirmation of rotation-dependent stellar activity, this clearly shows that the surface Li abundances of these solar analogs progressively decrease as the rotation rate decreases. Conclusions. Given this observational evidence, we conclude that the depletion of surface Li in solar-type stars, probably caused by effective envelope mixing, operates more efficiently as stellar rotation decelerates. It may be promising to attribute the low-Li tendency of planet-host G dwarfs to their different nature in the stellar angular momentum.

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.

On the Abundance of Potassium in Metal-Poor Stars

Based on extensive statistical-equilibrium calculations, we performed a non-LTE analysis of the K I 7699 equivalent-width data of metal-deficient stars for the purpose of clarifying the behavior of the photospheric potassium abundance in disk/halo stars. While the resulting non-LTE abundance corrections turned out to be considerably large, amounting to 0.2–0.7dex, their effect on the [K/Fe] vs. [Fe/H] relation is not very important, since these corrections do not show anysignificant dependenceon the metallicity. Hence, we again confirmed the results of previous LTE studies, that [K/Fe] shows a gradual systematic increase toward a lowered metallicity up to [K/Fe] ∼0.3–0. 5a t [Fe/H] ∼− 1t o−2, such as in the case of α elements.

Lithium, Carbon, and Oxygen Abundances of Hyades F–G Type Stars

In an attempt to carry out a systematic study on the behavior of the photospheric abundances of Li, C, and O (along with Fe) for Hyades main-sequence stars in the T_eff range of ~5000-7000K, we conducted an extensive spectrum-synthesis analysis applied to four spectral regions (comprising lines of Fe-group elements, Li I 6708 line, C I 7111-7119 lines, and O I 6156-8 lines) based on the high-dispersion spectra of 68 selected F-G type stars belonging to this cluster. The abundances of C and O turned out to be fairly uniform in a marginally supersolar level such like the case of Fe: = +0.15 (sigma = 0.08), = +0.22 (sigma = 0.14), and = +0.11(sigma = 0.08), suggesting that the primordial abundances are almost retained for these elements. Strictly, however, they show a slightly increasing trend with a decrease in T_eff (typically on the order of ~10^(-4) dex/K; while this might be due to an improper choice of atmospheric parameters, we found it hard to give a quantitatively reasonable explanation. Regarding Li, we confirmed the well-known T_eff-dependent trend in the Li abundance reported so far (a conspicuous Li-trough at 6300K <T_eff< 6700K and a progressive decrease toward a lower T_eff at T_eff < 6000K), which means that the surface Li of Hyades stars is essentially controlled only by T_eff and other parameters such as the rotational velocity are almost irrelevant.