Bd+44493: a Ninth Magnitude Messenger from the Early Universe; Carbon Enhanced and Beryllium Poor

We present a 1D LTE chemical abundance analysis of the very bright (V = 9.1) Carbon-Enhanced Metal-Poor (CEMP) star BD+44493, 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+44493 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. Subject headings: Galaxy: abundances — stars: abundances — stars: individual(BD+44493) — stars: Population II