Hiroko Ito

We have conducted follow-up high-resolution spectroscopy with the Subaru Telescope High Dispersion Spectrograph (HDS) of extremely metal-poor (EMP) star candidates discovered by the Sloan Digital Sky Survey (SDSS), in particular the sub-survey SEGUE, in order to determine accurate metallicities and elemental abundance ratios. Here we report first results for 115 objects, among which 70 stars have [Fe/H]<-3, doubling the known samples of such stars. We clearly confirm the rapid decrease of the number of stars below [Fe/H]=-3.5, although the lowmetallicity cutoff is not as sharp as that found by previous studies based on medium-resolution spectra. We identified two cool (5000 K) main-sequence stars with [Fe/H]<-3, the first example of this class of stars studied based on high-resolution spectra. One of them exhibits an extremely large enhancement of r-process elements (e.g., [Eu/Fe]= +1.9). The metallicity of this object ([Fe/H] =-3.4) is the lowest yet found for such r-process-enhanced stars.

We performed a 1D LTE chemical abundance analysis of an extremely metal-poor star BD+44°493 ([Fe/H]= −3.7), and set a very low upper limit for its Be abundance: A(Be) < −2.0. It may indicate that the decreasing trend of Be abundances with lower [Fe/H] still holds at [Fe/H] < −3.5, and demonstrate that high C and O abundances do not necessarily imply high Be abundances. However, since the star is a subgiant with Teff ~ 5500K, Be may be depleted.

We performed a chemical abundance analysis of the very bright (V = 9.1) carbon-enhanced metal-poor (CEMP) star BD+44°493, which is the first star found with metallicity [Fe/H] < −3.5 and an apparent magnitude V < 12. The star is classified as a CEMP-no” subgiant, and its abundance pattern implies that a first-generation faint supernova is the most likely origin of its carbon excess. We set an very low upper limit on this star's beryllium abundance, which demonstrates that high C and O abundances do not necessarily imply high Be abundances.

We present a 1D LTE chemical abundance analysis of the very bright (V=9.1) Carbon-Enhanced Metal-Poor (CEMP) star BD +44 493, based on high-resolution, high signal-to-noise spectra obtained with Subaru/HDS. The star is shown to be a subgiant with an extremely low iron abundance ([Fe/H]=-3.7), while it is rich in C ([C/Fe]=+1.3) and O ([O/Fe]=+1.6). Although astronomers have been searching for extremely metal-poor stars for decades, this is the first star found with [Fe/H]<-3.5 and an apparent magnitude V<12. Based on its low abundances of neutron-capture elements (e.g., [Ba/Fe]=-0.59), BD +44 493 is classified as a "CEMP-no" star. Its abundance pattern implies that a first-generation faint supernova is the most likely origin of its carbon excess, while scenarios related to mass loss from rapidly-rotating massive stars or mass transfer from an AGB companion star are not favored. From a high-quality spectrum in the near-UV region, we set an very low upper limit on this star's beryllium abundance (A(Be)=log(Be/H)+12<-2.0), which indicates that the decreasing trend of Be abundances with lower [Fe/H] still holds at [Fe/H]<-3.5. This is the first attempt to measure a Be abundance for a CEMP star, and demonstrates that high C and O abundances do not necessarily imply high Be abundances.

A non-LTE analysis of K I resonance lines at 7664.91 and 7698.97 A was carried out for 15 red giants belonging to three globular clusters of different metallicity (M 4, M 13, and M 15) along with two reference early-K giants (rho Boo and alpha Boo), in order to check whether the K abundances are uniform within a cluster and to investigate the behavior of [K/Fe] ratio at the relevant metallicity range of -2.5 <[Fe/H] < -1. We confirmed that [K/H] (as well as [Fe/H]) is almost homogeneous within each cluster to a precision of < ~0.1 dex, though dubiously large deviations are exceptionally seen for two peculiar stars showing signs of considerably increased turbulence in the upper atmosphere. The resulting [K/Fe] ratios are mildly supersolar by a few tenths of dex for three clusters, tending to gradually increase from ~+0.1-0.2 at [Fe/H] ~-1 to ~+0.3 at [Fe/H] ~ -2.5. This result connects reasonably well with the [K/Fe] trend of disk stars (-1 < [Fe/H]) and that of extremely metal-poor stars (-4 <[Fe/H] < -2.5). That is, [K/Fe] appears to continue a gradual increase from [Fe/H]~0 toward a lower metallicity regime down to [Fe/H]~-3, where a broad maximum of [K/Fe]~+0.3-0.4 is attained, possibly followed by a slight downturn at [Fe/H]<~-3.