Article

Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. I. Observational Data

Komazawa University, Edo, Tōkyō, Japan
The Astrophysical Journal Supplement Series (Impact Factor: 14.14). 05/2004; 152(1). DOI: 10.1086/383201
Source: OAI

ABSTRACT We have obtained high-resolution (R 50,000 or 90,000), high-quality (S/N 100) spectra of 22 very metal-poor stars ([Fe/H] -2.5) with the High Dispersion Spectrograph fabricated for the 8.2 m 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.

Download full-text

Full-text

Available from: Taka Kajino, Jun 17, 2014
0 Followers
 · 
83 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ([r/Fe] > 1 dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). Moderate-resolution (~2A) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey (HES) as well as the HK survey to identify sharp-lined stars with [Fe/H] < -2.5dex. For several hundred confirmed metal-poor giants brighter than B~16.5mag (most of them from the HES), ``snapshot'' spectra (R~20,000; S/N~30 per pixel) are being obtained with VLT/UVES, with the main aim of finding the 2-3% r-II stars expected to be among them. These are studied in detail by means of higher resolution and higher S/N spectra. In this paper we describe a pilot study based on a set of 35 stars, including 23 from the HK survey, 8 from the HES, and 4 comparison stars. We discovered two new r-II stars, CS29497-004 ([Eu/Fe] = 1.64 +/- 0.22) and CS29491-069 ([Eu/Fe] = 1.08 +/- 0.23). A first abundance analysis of CS29497-004 yields that its abundances of Ba to Dy are on average enhanced by 1.5dex with respect to iron and the Sun and match a scaled solar r-process pattern well, while Th is underabundant relative to that pattern by 0.3dex, which we attribute to radioactive decay. That is, CS29497-004 seems not to belong to the class of r-process enhanced stars displaying an ``actinide boost'', like CS31082-001 (Hill et al. 2002), or CS30306-132 (Honda et al. 2004b). The abundance pattern agrees well with predictions of the phenomenological model of Qian & Wasserburg. Comment: Accepted for publication in A&A
    Astronomy and Astrophysics 08/2004; 428(3). DOI:10.1051/0004-6361:20041536 · 4.48 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We obtained high resolution spectra of 40 very metal-poor stars, and measured the abundances of heavy elements. The abundance pattern of the heavy neutron-capture elements (56 <= Z <= 70) in r-process-enhanced, metal-poor stars are quite similar to that of the r-process component in solar-system material. In contrast, the abundance ratios of the light neutron-capture elements (38 <= Z <= 40) to heavier ones show a large dispersion. We investigated the correlation between Sr (Z = 38) and Ba (Z = 56) abundances, and obtained two clear results: (1) Ba-enhanced stars also show large excess of Sr (there is no object which is Ba-rich and Sr-poor); (2) stars with low Ba abundance show large scatter in Sr abundance. This trend is naturally explained by hypothesizing the existence of two processes, one Chat produces Sr without Ba and the other that produces Sr and Ba in similar proportions.
    Nuclear Physics A 07/2005; 758:296-299. DOI:10.1016/j.nuclphysa.2005.05.052 · 2.50 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have derived abundances of 33 elements and upper limits for 6 additional elements for the metal-poor ([Fe/H] = -2.42) turn-off star HE 0338-3945 from high-quality VLT-UVES spectra. The star is heavily enriched, by about a factor of 100 relative to iron and the Sun, in the heavy s-elements (Ba, La, ..). It is also heavily enriched in Eu, which is generally considered an r-element, and in other similar elements. It is less enriched, by about a factor of 10, in the lighter s-elements (Sr, Y and Zr). C is also strongly enhanced and, to a somewhat lesser degree, N and O. These abundance estimates are subject to severe uncertainties due to NLTE and thermal inhomogeneities which are not taken into detailed consideration. However, an interesting result, which is most probably robust in spite of these uncertainties, emerges: the abundances derived for this star are very similar to those of other stars with an overall enhancement of all elements beyond the iron peak. We have defined criteria for this class of stars, r+s stars, and discuss nine different scenarios to explain their origin. None of these explanations is found to be entirely convincing. The most plausible hypotheses involve a binary system in which the primary component goes through its giant branch and asymptotic giant branch phases and produces CNO and s-elements which are dumped onto the observed star. Whether the r-element Eu is produced by supernovae before the star was formed (perhaps triggering the formation of a low-mass binary), by a companion as it explodes as a supernova (possibly triggered by mass transfer), or whether it is possibly produced in a high-neutron-density version of the s-process is still unclear. Several suggestions are made on how to clarify this situation. Comment: Accepted for A&A; 22 pages, 9 figures, 2 tables. Table 2 is in electronic form and available at http://www.astro.uu.se/~karin/table2.dat with description at http://www.astro.uu.se/~karin/jonsellReadMe)
    Astronomy and Astrophysics 01/2006; 451(2). DOI:10.1051/0004-6361:20054470 · 4.48 Impact Factor