A. J. B. Steinhauer

HE 1327?2326, an Unevolved Star with [Fe/H] < ?5.0. I. A Comprehensive Abundance Analysis

HE 1327-2326, an Unevolved Star with [Fe/H] < -5.0. : I. A Comprehensive Abundance Analysis

Low-Mass Stars and Brown Dwarfs in NGC 2024: Constraints on the Substellar Mass Function

We present results from a near-infrared spectroscopic study of candidate brown dwarfs and low-mass stars in the young cluster NGC 2024. Using FLAMINGOS on the KPNO 2.1 m and 4 m telescopes, we have obtained spectra of ~70 new members of the cluster and classified them via the prominent J- and H-band water absorption features. Derived spectral types range from ~M1 to later than M8 with typical classification errors of 0.5-1 subclasses. By combining these spectral types with JHK photometry, we place these objects on the H-R diagram and use pre-main-sequence evolutionary models to infer masses and ages. The mean age for this low-mass population of NGC 2024 is 0.5 Myr, and derived masses range from ~0.7 to 0.02 M☉ with 23 objects falling below the hydrogen-burning limit. The logarithmic mass function rises to a peak at ~0.2 M☉ before turning over and declining into the substellar regime. There is a possible secondary peak at ~0.035 M☉; however, the errors are also consistent with a flat IMF in this region. The ratio of brown dwarfs to stars is similar to that found in the Trapezium but roughly twice the ratio found in IC 348, leading us to conclude that the substellar IMF in young clusters may be dependent on the local star-forming environment.

Boron Depletion in F and G Dwarf Stars and the Beryllium-Boron Correlation*

Boron provides a special probe below the stellar surface since it survives to greater depths than do Li and Be. To search for B depletions we have observed B in 13 F and G dwarfs with large Be depletions; for comparison we have also obtained spectra of five stars that are undepleted in Li and Be. We have used HST with STIS to obtain spectra of the B I resonance line at 2497 A. The spectral resolution is 30,000 or 114,000, and the median signal-to-noise ratio is 70 pixel-1. New Be and Li spectra have been obtained at Keck I with HIRES of four of the five standard stars at ~48,000 resolution. Abundances have been determined by the spectrum synthesis method with MOOG. A comparison between the standard stars and those with severe Be depletions shows a distinct difference in the B abundances between the two groups of 0.22 dex. We have discovered a correlation between the Be and B abundances. The slope between A(Be) and A(B)NLTE is 0.22 ± 0.05 [where A(element) = log N(element)/N(H) + 12.00], which, as expected, is shallower than the slope between A(Li) and A(Be) of 0.38. We have normalized the light-element abundances to account for the observation that the initial abundances are somewhat lower in lower metallicity stars by employing recently published empirical relations between Be and [Fe/H] and between B and [Fe/H]. The correlation between the normalized A(Be) and A(B)NLTE has a slope of 0.18 ± 0.06. The star with the largest Be depletion, HR 107, a main-sequence Ba star, also has the largest B depletion, with the B abundance lower by a factor of 3.5 relative to the standard stars.

An abundance study of the most iron‐poor star HE1327‐2326 with Subaru/HDS

We present an elemental abundance analysis of HE 1327‐2326, the most iron‐deficient star known, based on a comprehensive investigation of spectra obtained with the Subaru Telescope. HE 1327‐2326 is either in its main sequence or subgiant phase of evolution, hence it is essentially unevolved. The chemical abundances of this star have the following properties, which provide new constraints on models of nucleosynthesis processes that occurred in first‐generation objects:(1)The iron abundance (NLTE) is [Fe/H]= −5.45. This value is 0.2 dex lower than that of HE 0107‐5240, the previously most iron‐poor object known. No object having [Fe/H]= −5 ∼ −4 is known to date.(2)This star, as well as HE 0107‐5240, exhibits extremely large overabundances of carbon relative to solar ratios ([C/Fe]∼ +4).(3)HE 1327‐2326 exhibits remarkable overabundances of the light elements (N, Na, Mg and Al), while HE 0107‐5240 shows only relatively small excesses of N and Na.(4)A large overabundance of Sr is found in HE 1327‐2326 as compa...