Anna Frebel

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

Bright Metal-poor Stars from the Hamburg/ESO Survey. I. Selection and Follow-up Observations from 329 Fields

We present a sample of 1777 bright (9 1.0) metal-poor ([Fe/H] 20%) and higher values with increasing distance from the Galactic plane. Although the numbers of stars at low metallicity are falling rapidly at the lowest metallicities, there is evidence that the fraction of carbon-enhanced metal-poor stars is increasing rapidly as a function of declining metallicity. For ~60 objects, high-resolution data have already been obtained; one of these, HE 1327-2326, is the new record holder for the most iron-deficient star known.

The detailed optical light curve of GRB 030329

We present densely sampled BVRI light curves of the optical transient associated with the gamma-ray burst (GRB) 030329, the result of a coordinated observing campaign conducted at five observatories. Augmented with published observations of this GRB, the compiled optical data set contains 2687 photometric measurements, obtained between 78 minutes and 79 days after the burst. This data set allows us to follow the photometric evolution of the transient with unprecedented detail. We use the data to constrain the light curve of the underlying supernova (SN) 2003dh and show that it evolved faster than and was probably somewhat fainter than the Type Ic SN 1998bw, associated with GRB 980425. We find that our data can be described by a broken power-law decay perturbed by a complex variable component. The early- and late-time decay slopes are determined to be alpha(1) approximate to 1.1 and alpha(2) approximate to 2. Assuming this single-break power-law model, we constrain the break to lie between similar to3 and similar to8 days after the burst. This simple, singly broken power-law model, derived only from the analysis of our optical observations, may also account for available multiband data, provided that the break happened similar to8 days after the burst. The more complex double-jet model of Berger et al. provides a comparable fit to the optical, X-ray, millimeter, and radio observations of this event. The unique early coverage available for this event allows us to trace the color evolution of the afterglow during the first hours after the burst. We detect a significant change in optical colors during the first day. Our color analysis is consistent with a cooling-break frequency sweeping through the optical band during the first day. The light curves of GRB 030329 reveal a rich array of variations, superposed over the mean power-law decay. We find that the early variations (less than or similar to8 days after the burst) are asymmetric, with a steep rise followed by a relatively slower ( by a factor of about 2) decline. The variations maintain a similar timescale during the first 4 days and then get significantly longer. The structure of these variations is similar to those previously detected in the afterglows of several GRBs.

Chemical Abundance Analysis of the Extremely Metal-Poor Star HE 1300+0157

We present a detailed chemical abundance analysis of HE 1300+0157, a subgiant with Fe/H = -3.9. Froma high-resolution, high-S/N Subaru HDS spectrum we find the star to be enriched in C ([C/Fe](1D) ...

The Oxygen Abundance of HE 1327-2326

From a newly obtained VLT/UVES spectrum, we have determined the oxygen abundance of HE 1327-2326, the most iron-poor star known to date. Ultraviolet OH lines yield one-dimensional LTE abundances of [O/Fe]OH = 3.7 (subgiant case) and [O/Fe]OH = 3.4 (dwarf case). Using a correction of -1.0 dex to account for three-dimensional (3D) effects on OH line formation, the abundances are lowered to [O/Fe] = 2.8 and [O/Fe] = 2.5, respectively, which we adopt. Without 3D corrections, the UV OH-based abundance would be in disagreement with the upper limits derived from the O I triplet lines: [O/Fe]trip < 2.8 (subgiant) and [O/Fe]trip < 3.0 (dwarf). We also correct the previously determined carbon and nitrogen abundances for 3D effects. Knowledge of the O abundance of HE 1327-2326 has implications for the interpretation of its abundance pattern. A large O abundance is in accordance with HE 1327-2326 being an early Population II star that formed from material chemically enriched by a first-generation supernova. Our derived abundances, however, do not exclude other possibilities such as a Population III scenario.

Broadband UBVRCIC photometry of horizontal-branch and metal-poor candidates from the HK and Hamburg/ESO surveys. I.

We report broadband UBV and/or BVRCIC CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/ A, N = 576), or candidate low-metallicity stars (N = 1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/ A candidates they are used to accurately separate the lower gravity FHB stars from various higher gravity A-type stars, a subsample that includes the so-called blue metal poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists.

The new record holder for the most iron-poor star: HE 1327 2326, a dwarf or subgiant with (Fe/H) = 5.4

We describe the discovery of HE 1327−2326, a dwarf or subgiant with (Fe/H) = −5.4. The star was found in a sample of bright metal-poor stars selected from the Hamburg/ESO survey. Its abundance pattern is characterized by very high C and N abundances. The detection of Sr which is overabundant by a factor of 10 as compared to iron and the Sun, suggests that neutron-capture elements had already been produced in the very early Galaxy. A puzzling Li depletion is observed in this unevolved star which contradicts the value of the primordial Li derived from WMAP and other Li studies. Possible scenarios for the origin of the abundance pattern (Pop. II or Pop. III) are presented as well as an outlook on future observations.

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...