arXiv subjects

S. Honda

Publications and source records attributed to S. Honda.

24 records · Page 2Linked to original sources

arXiv2004

Spectroscopic Studies of Extremely Metal-Poor Stars with Subaru/HDS:I. Observational Data

We have obtained high-resolution (R $\simeq$ 50,000 or 90,000), high-quality (S/N $\ga$ 100) spectra of 22 very metal-poor stars ([Fe/H] $\la$ -2.5) with the High Dispersion Spectrograph fabricated for the 8.2m Subaru Telescope. The spectra cover the wavelength range from 3500 to 5100 Å; equivalent widths are measured for isolated lines of numerous elemental species, including the $α$ elements, the iron-peak elements, and the light and heavy neutron-capture elements. Errors in the measurements and comparisons with previous studies are discussed. These data will be used to perform detailed abundance analyses in the following papers of this series. Radial velocities are also reported, and are compared with previous studies. At least one moderately r-process-enhanced metal-poor star, HD 186478, exhibits evidence of a small-amplitude radial velocity variation, confirming the binary status noted previously. During the course of this initial program, we have discovered a new moderately r-process-enhanced, very metal-poor star, CS 30306-132 ([Fe/H] $= -2.4$; [Eu/Fe] $= +0.85$), which is discussed in detail in the companion paper.

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arXiv2004

Spectroscopic Studies of Extremly Metal-Poor Stars with Subaru/HDS:II.The r-process Elements, Including Thorium

We present the abundance analyses for the neutron-capture elements, and discuss the observed abundance distributions in very metal-poor stars with excesses of r-process elements. As has been found by previous abundance studies, the star-to-star scatter in the abundances of neutron-capture elements are very large. The abundance patterns of the heavy neutron-capture elements (56 $\leq$ Z $\leq$ 70) in seven objects with moderate to large excesses of the neutron-capture elements are similar to that of the solar system r-process component. These results strongly suggest that the heavy neutron-capture elements in these objects are primarily synthesized by the r-process. On the other hand, the abundance ratios of the light neutron-capture elements (38 $\leq$ Z $\leq$ 46) exhibit a rather large dispersion. Our inspection of the correlation between Sr and Ba abundances in very metal-poor stars reveals that the dispersion of the Sr abundances clearly decreases with increasing Ba abundance. This results support previous suggestions that the light neutron-capture elements are likely to have been produced in different astrophysical sites from those associated with the production of the heavier ones. The Th/Eu abundance ratios (log(Th/Eu)) measured for the seven r-process-enhanced stars range from -0.10 to -0.59. Since these very metal-poor stars are believed to be formed in the early Galaxy, this result means that a small dispersion appears in the abundance ratios between Th and rare-earth elements, such as Eu, in very metal-poor stars. In order to apply the Th/Eu ratios to estimates of stellar ages, further understanding for the Th production by the r-process nucleosynthesis is required.

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arXiv2002

Measurement of the Europium Isotope Ratio for the Extremely Metal-Poor, r-Process-Enhanced Star CS31082-001

We report the first measurement of the isotope fraction of europium (151Eu and 153Eu) for the extremely metal-poor, r-process-enhanced star CS31082-001, based on high-resolution spectra obtained with the Subaru Telescope High Dispersion Spectrograph. We have also obtained new measurements of this ratio for two similar stars with previous europium isotope measurements, CS22892-052 and HD115444. The measurements were made using observations of the Eu lines in these spectra that are most significantly affected by isotope shifts and hyperfine splitting. The fractions of 151Eu derived for CS31082-001, CS22892-052, and HD115444 are 0.44, 0.51, and 0.46, respectively, with uncertainties of about +/-0.1. CS31082-001, the first star with a meaningful measurement of U outside of the solar system, is known to exhibit peculiar abundance ratios between the actinide and rare-earth elements (e.g., Th/Eu), ratios that are significantly different from those for other stars with large excesses of r-process elements, such as our two comparison objects. Nevertheless, our analysis indicates that the Eu isotope ratio of CS31082-001 agrees, within the errors, with those of other r-process-enhanced objects, and with that of solar-system material.

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arXiv2002

Chemical composition of carbon-rich, very metal-poor subgiant LP625-44 observed with the Subaru/HDS

We have obtained a high resolution (R~90,000) spectrum of the carbon- and s-process-element-rich, very metal-poor ([Fe/H]=-2.7) subgiant LP625-44, as well as that of HD140283 (a metal-poor subgiant with normal abundance ratio) for comparison, with the High Dispersion Spectrograph (HDS) for the Subaru Telescope for detailed abundance study. The oxygen abundance derived from the O I triplet around 7770A is uncertain, but the excess of oxygen in LP625-44 seems remarkable (perhaps by nearly a factor 10), in comparison with that of HD140283 derived from the same lines. The Na enhancement in LP625-44 is by about a factor 50, suggesting hydrogen burning in the 22Ne-rich layer in an asymptotic giant branch star which produces the abundance pattern of this object. In our new spectrum of LP625-44, the Pb I lambda 3683A line has been detected, as well as the Pb I lambda 4057A line which has already been studied, confirming the Pb abundance (log epsilon(Pb)~1.9) derived by the previous work. The abundance ratio of s-process elements at the second peak (e.g., La, Ce and Nd) to that at the third peak (Pb) in LP 625-44 is significantly higher (by a factor 5) than that in other three s-process element-rich objects recently studied by van Eck et al.. Recent theoretical works have modeled the s-process nucleosynthesis in the radiative layer of asymptotic giant branch stars in the inter-pulse phase, and the above results means that these processes produced a large scatter in the abundance ratios. Another possiblity is that different processes (e.g., s-process nucleosynthesis during thermal pulses) have contributed to heavy elements in the early Galaxy.

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arXiv2002

Behavior of Sulfur Abundances in Metal-Poor Giants and Dwarfs

LTE and NLTE abundances of sulfur in 6 metal-poor giants and 61 dwarfs (62 dwarfs, including the Sun) were explored in the range of -3 \lsim [Fe/H] \lsim $+0.5$ using high-resolution, high signal-to-noise ratio spectra of the \SI 8693.9 Åand 8694.6 Ålines observed by us and measured by \Francois (1987, 1988) and Clegg et al. (1981). NLTE effects in S abundances are found to be small and practically negligible. The behavior of [S/Fe] vs. [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.

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arXiv2001

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.

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