Brian Elliott Marsteller

Two stellar components in the halo of the Milky Way

The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, and this information can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components—an inner and an outer halo—that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.

The Frequency of Carbon-enhanced Metal-poor Stars in the Galaxy from the HERES Sample

We estimate the frequency of carbon-enhanced metal-poor (CEMP) stars among very metal-poor stars, based on an analysis of 349 stars with available high-resolution spectra observed as part of the Hamburg/ESO R-process Enhanced Star (HERES) survey. We obtain that a lower limit of 21% ± 2% of stars with [Fe/H] ≤ -2.0 exhibit [C/Fe] ≥ +1.0. These fractions are higher than those that have been reported by recent examinations of this question, based on substantially smaller samples of stars. We discuss the source of this difference and suggest that in order to take into account effects that result in a decrease of surface carbon abundance with advancing evolution, a definition of CEMP stars based on a [C/Fe] cutoff that varies as a function of luminosity is more appropriate. We discuss the likely occurrence of dilution and mixing for many CEMP stars, which, if properly accounted for, would increase this fraction still further.

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.

Carbon-enhanced metal-poor stars. III. main-sequence turnoff stars from the SDSS SEGUE sample

The original article can be found at: http://www.iop.org/EJ/journal/apj Copyright American Astronomical Society DOI: 10.1086/533517 [Full text of this article is not available in the UHRA]

A Catalog of Candidate Field Horizontal-Branch and A-Type Stars. III. A 2MASS-Cleaned Version

We present coordinates and available photometric information (either from previous or recent broadband UBV observations, and near-infrared photometry from the 2MASS Point Source Catalog) for 12,056 stars (11,516 of which are unique) identified in the HK Survey of Beers and colleagues as candidate field horizontal-branch or A-type stars. These stars, in the apparent magnitude range 10 ≤ B ≤ 16.0, were selected using an objective-prism/interference-filter survey technique. The availability of 2MASS information permits assembly of a cleaned version of this catalog, comprising likely blue horizontal-branch (BHB) stars or blue stragglers in the color interval [-0.2 ≤ (B - V)0 ≤ +0.2], which are of particular interest for investigation of the structure, kinematics, and dynamics of the thick disk and inner halo of the Milky Way, the total mass and mass profile of the Galaxy, and as potential foreground/background objects in efforts to bracket distances to high-velocity clouds of H I. A comparison of the stars classified as high-likelihood BHB candidates with previous classifications based on UBV photometry and medium-resolution spectroscopy indicates that this class contains 78% correct identifications.

The frequency of Carbon-enhanced stars in HERES and SDSS

Recent large surveys of metal-poor stars in the Galaxy have r e aled that a surprising fraction are enhanced in their carbon-to-iron ratios by factors of fr om 10-10,000 relative to the solar ratio. Although most of the stars in the metallicity interval −2.7 ≤ [Fe/H] ≤ −2.0 are likely to have arisen from Asymptotic Giant Branch processing (and subseq uent dumping via mass transfer to a surviving companion), there exist many stars with [Fe/H] < −3.0 (including the two lowest [Fe/H] stars known, with [Fe/H] < −5.0) that cannot be accounted for by this process. Rather, primordial (or nearly primordial) progenitors are implica ted.

Refined Estimates of Carbon Abundances for Carbon‐Enhanced Metal‐Poor Stars

We present results from a refined set of procedures for estimation of the metallicities ([Fe/H]) and carbon abundance ratios ([C/Fe]) based on a much larger sample of calibration objects (on the order of 500 stars) then were available to Rossi et al. (2005), due to a dramatic increase in the number of stars with measurements obtained from high‐resolution analyses in the past few years. We compare results obtained from a new calibration of the KP and GP indices with that obtained from a custom set of spectral synthesis based on MOOG. In cases where the GP index approaches saturation, it is clear that only spectral synthesis achieve reliable results.

New Results from Bright Metal‐Poor Stars in the Hamburg/ESO Survey

We present an abundance analysis of HE 1327–2326, currently the most iron‐poor star known, based on a newly acquired VLT spectrum. The 1D abundance pattern is corrected for 3D effects. The 3D iron abundance is [Fe/H] = −5.9, while the CNO elements of the star are extremely overabundant [CNO/Fe] ∼3 to 4). The cosmologically important element Li is still not detected; the new upper limit is A(Li)<0.6. A new analysis of the medium‐resolution data of the sample of bright metal‐poor stars from the Hamburg/ESO Survey (HES) was carried out. We are using this sample to obtain clues to the chemical nature of the early Universe by investigating the kinematic properties of the sample. Based on estimated [Fe/H] and [C/Fe], we are also able to use the sample to test a formation mechanism for low‐mass metal‐poor stars.

Revised Parameter Estimates For The Most Metal‐Poor Candidates In SDSS‐I And SEGUE

There are several hundred thousand R = 2000 stellar spectra reported in the final public release of the Sloan Digital Sky Survey (SDSS‐I) and the continuing project SEGUE (Sloan Extension for Galactic Exploration and Understanding), which has completed roughly half of its scheduled set of observations to date.The stars in this sample were targeted for a wide variety of reasons, and hence do not represent a sample from which an unbiased metallicity distribution function (MDF) of stars in the halo or thick‐disk populations may be drawn. However, there exist over 6500 stars with estimated metallicities [Fe/H]<−2.0 and effective temperatures in the range 4500 K<Teff<7000 K among this sample, based on application of the SDSS/SEGUE spectroscopic parameter analysis pipeline. We have continued to refine estimates of the stellar parameters for these stars, using an automated synthetic spectrum approach, autoMOOG. This technique produces estimates of [Fe/H] as well as [C/Fe] (or upper limits on these quantities) ba...

The Hamburg/ESO R-process Enhanced Star survey (HERES): Project Overview, and New r-II Stars

We report on a dedicated effort to identify and study metal-poor stars that are strongly enhanced in r-process elements ([r/Fe] > +1.0 dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). In a sample of 253 confirmed metal-poor stars for which “snapshot” spectra (R ∼ 20, 000; S/N ∼ 50/1 per pixel) were obtained with VLT/UVES, and abundances were determined in an automated fashion using the methods of Barklem et al. (2005), we identified eight new r-II stars. They are now being studied in detail by means of higher resolution and S/N spectroscopy. The new r-II stars have metallicities in the range −3.2 < [Fe/H] < −2.6. Future searches for r-II stars should therefore focus on stars in this [Fe/H] range. Moderately r-process enhanced stars (i.e., +0.3 dex < [r/Fe] < +1.0 dex; r-I stars) were found at metallicities as high as [Fe/H] = −1.5. The [Fe/H] ranges in which r-I and r-II stars can be found may provide an important constraint for the identification of the site(s) of the r-process(es).