B. Marsteller

Publications (10)36.28 Total impact

• Article: Two Stellar Components in the Halo of the Milky Way

  D. Carollo, T. C. Beers, Y. S. Lee, M. Chiba, J. E. Norris, R. Wilhelm, T. Sivarani, B. Marsteller, J. A. Munn, C. A. L. Bailer-Jones, P. Re Fiorentin, D. G. York
  [show abstract] [hide abstract]
  ABSTRACT: The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, which 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.
  Nature 07/2007; · 36.28 Impact Factor
• Article: The Frequency of Carbon-Enhanced Metal-Poor Stars Based on SDSS Spectroscopy

  B. Marsteller, T.C. Beers, T. Sivarani, S. Rossi, J. Knapp, B. Plez, J. Johnson
  [show abstract] [hide abstract]
  ABSTRACT: The publicly available stellar database from SDSS contains many hundreds of metal-poor stars with large enhancements of carbon. The Galactic extension of SDSS, SEGUE, will identify several thousand more. Many of these Carbon-Enhanced Metal-Poor (CEMP) stars are likely to be enhanced in s-process elements created by AGB companions and dumped to the surviving member of a binary pair through either Roche-Lobe mass transfer or the operation of a stellar wind (CEMP-s stars). Based on previous high-resolution investigation of CEMP stars, an interesting subset of this sample is expected to show little or no s-process enhancement (CEMP-no stars).Utilizing a novel technique of automatically fitting [Fe/H] and [C/Fe] for a large number of stars from SDSS through spectral synthesis methods, we are able to derive a new estimate of the frequency of CEMP stars as a function of metallicity for the largest sample of stars to date. Using this approach, we can also measure abundances (or limits) for species such as Sr and Ba, which can be used to roughly separate CEMP stars into the CEMP-s and CEMP-no classes.
  Proceedings of the International Astronomical Union 11/2006; 2:252 - 253.
• Source

  Available from: arxiv.org

  Article: The Hamburg/ESO R-process Enhanced Star survey (HERES) II. Spectroscopic analysis of the survey sample

  P. S. Barklem, N. Christlieb, T. C. Beers, V. Hill, M. S. Bessell, J. Holmberg, B. Marsteller, S. Rossi, F. -J. Zickgraf, D. Reimers
  [show abstract] [hide abstract]
  ABSTRACT: We present the results of analysis of ``snapshot'' spectra of 253 metal-poor halo stars -3.8 < [Fe/H] < -1.5 obtained in the HERES survey. The spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy codes of Valenti & Piskunov. Elemental abundances of moderate precision have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where detectable. Among the sample of 253 stars, we find 8 r-II stars and 35 r-I stars. We also find three stars with strong enhancements of Eu which are s-process rich. A significant number of new very metal-poor stars are confirmed: 49 stars with [Fe/H] < -3 and 181 stars with -3 < [Fe/H] < -2. We find one star with [Fe/H] < -3.5. We find the scatter in the abundance ratios of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to be similar to the estimated relative errors and thus the cosmic scatter to be small, perhaps even non-existent. The elements C, Sr, Y, Ba and Eu, and perhaps Zr, show scatter at [Fe/H] < -2.5 significantly larger than can be explained from the errors in the analysis, implying scatter which is cosmic in origin. Significant scatter is observed in abundance ratios between light and heavy neutron-capture elements at low metallicity and low levels of r-process enrichment. (*** abridged ***) Comment: Accepted for A&A; 31 pages, 2 electronic tables presently available at http://www.astro.uu.se/~barklem/papers/heres_tables.tar.gz ; Minor corrections added
  05/2005;
• Article: The Hamburg/ESO R-process Enhanced Star survey (HERES): Project Overview, and New r-II Stars

  T.C.  Beers , P.S.  Barklem , M.S.  Bessell , V.  Hill , B.  Marsteller , F.-J.  Zickgraf 
  [show abstract] [hide abstract]
  ABSTRACT: We report on a dedicated effort to identify and study metal-poor stars that are strongly enhanced in r-process elements ($\mbox{[r/Fe]}>+1.0$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 ($R\sim 20,000$; $S/N \sim 50/1$ 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 $-3.2<\mbox{[Fe/H]}<-2.6$. Future searches for r-II stars should therefore focus on stars in this [Fe/H] range. Moderately r-process enhanced stars (i.e., $+0.3\,\mbox{dex}<\mbox{[r/Fe]}<+1.0\,\mbox{dex}$; r-I stars) were found at metallicities as high as $\mbox{[Fe/H]} = -1.5$. 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).
  Proceedings of the International Astronomical Union 04/2005; 1:439 - 443.
• Source

  Available from: G. J. Wasserburg

  Article: The Hamburg/ESO R-process Enhanced Star survey (HERES). I. Project description, and discovery of two stars with strong enhancements of neutron-capture elements

  N. Christlieb, T. C. Beers, P. S. Barklem, M. Bessell, V. Hill, J. Holmberg, A. J. Korn, B. Marsteller, L. Mashonkina, Y. -Z. Qian, S. Rossi, G. J. Wasserburg, F. -J. Zickgraf, K. -L. Kratz, B. Nordstr{ö}m, B. Pfeiffer, J. Rhee, S. G. Ryan
  [show abstract] [hide abstract]
  ABSTRACT: We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ([r/Fe] > 1 dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). Moderate-resolution (~2A) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey (HES) as well as the HK survey to identify sharp-lined stars with [Fe/H] < -2.5dex. For several hundred confirmed metal-poor giants brighter than B~16.5mag (most of them from the HES), ``snapshot'' spectra (R~20,000; S/N~30 per pixel) are being obtained with VLT/UVES, with the main aim of finding the 2-3% r-II stars expected to be among them. These are studied in detail by means of higher resolution and higher S/N spectra. In this paper we describe a pilot study based on a set of 35 stars, including 23 from the HK survey, 8 from the HES, and 4 comparison stars. We discovered two new r-II stars, CS29497-004 ([Eu/Fe] = 1.64 +/- 0.22) and CS29491-069 ([Eu/Fe] = 1.08 +/- 0.23). A first abundance analysis of CS29497-004 yields that its abundances of Ba to Dy are on average enhanced by 1.5dex with respect to iron and the Sun and match a scaled solar r-process pattern well, while Th is underabundant relative to that pattern by 0.3dex, which we attribute to radioactive decay. That is, CS29497-004 seems not to belong to the class of r-process enhanced stars displaying an ``actinide boost'', like CS31082-001 (Hill et al. 2002), or CS30306-132 (Honda et al. 2004b). The abundance pattern agrees well with predictions of the phenomenological model of Qian & Wasserburg. Comment: Accepted for publication in A&A
  08/2004;
• Source

  Available from: arxiv.org

  Article: Carbon-Enhanced Metal-Poor Stars in the Early Galaxy

  B. Marsteller, T. C. Beers, S. Rossi, N. Christlieb, M. Bessell, J. Rhee
  [show abstract] [hide abstract]
  ABSTRACT: Very metal-deficient stars that exhibit enhancements of their carbon abundances are of crucial importance for understanding a number of issues -- the nature of stellar evolution among the first generations of stars, the shape of the Initial Mass Function, and the relationship between carbon enhancement and neutron-capture processes, in particular the astrophysical s-process. One recent discovery from objective-prism surveys dedicated to the discovery of metal-deficient stars is that the frequency of Carbon-Enhanced Metal-Poor (CEMP) stars increases with declining metallicity, reaching roughly 25% for [Fe/H] < -2.5. In order to explore this phenomenon in greater detail we have obtained medium-resolution (2 A) spectroscopy for about 350 of the 413 objects in the Christlieb et al. catalog of carbon-rich stars, selected from the Hamburg/ESO objective prism survey on the basis of their carbon-enhancement, rather than metal deficiency. Based on these spectra, and near-IR JHK photometry from the 2MASS Point Source Catalog, we obtain estimates of [Fe/H] and [C/Fe] for most of the stars in this sample, along with reasonably accurate determinations of their radial velocities. Of particular importance, we find that the upper envelope of carbon enhancement observed for these stars is nearly constant, at [C/H] ~ -1.0, over the metallicity range -4.0 < [Fe/H] < -2.0; this same level of [C/H] applies to the most iron-deficent star yet discovered, HE 0107-5240, at [Fe/H] = -5.3. Comment: Contributed paper to The Eigth Nuclei in the Cosmos conference, to appear (in refereed form) in Nuclear Physics A
  08/2004;
• Source

  Available from: adsabs.harvard.edu

  Article: The Mass of the Galaxy from Large Samples of Field Horizontal-Branch Stars in the SDSS Early Data Release

  T. C. Beers, M. Chiba, T. Sakamoto, R. Wilhelm, C. Allende Prieto, J. Sommer-Larsen, H. J. Newberg, B. Yanny, B. Marsteller, J. R. Pier
  [show abstract] [hide abstract]
  ABSTRACT: We present a new estimate of the mass of the Milky Way, making use of a large sample of 955 field horizontal-branch (FHB) stars from the Early Data Release of the Sloan Digital Sky Survey. This sample of stars has been classified on the basis of an automated analysis approach, in combination with other methods, in order to obtain estimates of the physical parameters of the stars, i.e., T_eff, log g, [Fe/H], and should be relatively free of contamination from halo blue stragglers. The stars all have measured radial velocities and photometric distance estimates, and the sample includes objects as distant as ˜ 75 kpc from the Galactic center. Application of a Bayesian likelihood method, for a specific model of the Galaxy, indicates that the total mass of the Galaxy lies in the range 1.5-4.0 x 1012 Msun. Our sample appears to reveal a clear signature of a dual halo population of FHB stars, with the boundary between the inner and outer halo around 20 kpc, and the possibility of rather striking differences in the rotational properties of the Galaxy at low metallicity.
  Proceedings of the International Astronomical Union 06/2004; 220:195.
• Article: The Hamburg/ESO R-process enhanced star survey (HERES)

  P. S. Barklem, N. Christlieb, T. C. Beers, V. Hill, M. S. Bessell, J. Holmberg, B. Marsteller, S. Rossi, F. -J. Zickgraf, D. Reimers
  [show abstract] [hide abstract]
  ABSTRACT: We present the results of analysis of “snapshot” spectra of 253 metal-poor halo stars $-3.8\leq {\rm [Fe/H]} \leq -1.5$ obtained in the HERES survey. The snapshot spectra have been obtained with VLT/UVES and have typically $S/N\sim 54$ per pixel (ranging from 17 to 308), $R\sim20\, 000$, $\lambda = 3760$–4980 Å. This sample represents the major part of the complete HERES sample of 373 stars; however, the CH strong content of the sample is not dealt with here. 
The spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy (SME) codes of Valenti & Piskunov. Elemental abundances of moderate precision (absolute rms errors of order 0.25 dex, relative rms errors of order 0.15 dex) have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where detectable. Of these elements, 14 are usually detectable at the 3$\sigma$ confidence level for our typical spectra. The remainder can be detected in the least metal-poor stars of the sample, spectra with higher than average $S/N$, or when the abundance is enhanced. 
Among the sample of 253 stars, disregarding four previously known comparison stars, we find 8 r-II stars and 35 r-I stars. The r-II stars, including the two previously known examples CS 22892-052 and CS 31082-001, are centred on a metallicity of ${\rm [Fe/H]} = -2.81$, with a very small scatter, on the order of 0.16 dex. The r-I stars are found across practically the entire metallicity range of our sample. We also find three stars with strong enhancements of Eu which are $s$-process rich. A significant number of new very metal-poor stars are confirmed: 49 stars with ${\rm [Fe/H]}<-3$ and 181 stars with $-3<{\rm [Fe/H]}<-2$. We find one star with ${\rm [Fe/H]}<-3.5$.
We find the scatter in the abundance ratios of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to be similar to the estimated relative errors and thus the cosmic scatter to be small, perhaps even non-existent. The elements C, Sr, Y, Ba and Eu, and perhaps Zr, show scatter at ${\rm [Fe/H]} \la -2.5$ significantly larger than can be explained from the errors in the analysis, implying scatter which is cosmic in origin. Significant scatter is observed in abundance ratios between light and heavy neutron-capture elements at low metallicity and low levels of $r$-process enrichment.

  http://dx.doi.org/10.1051/0004-6361:20052967.
• Article: The Hamburg/ESO R-process Enhanced Star survey (HERES)

  N. Christlieb, T. C. Beers, P. S. Barklem, M. Bessell, V. Hill, J. Holmberg, A. J. Korn, B. Marsteller, L. Mashonkina, Y. -Z. Qian, S. Rossi, G. J. Wasserburg, F. -J. Zickgraf, K. -L. Kratz, B. Nordström, B. Pfeiffer, J. Rhee, S. G. Ryan
  [show abstract] [hide abstract]
  ABSTRACT: We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in $r$-process elements ($\mbox{[r/Fe]}>1$ dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). Moderate-resolution (~$2$ Å) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey (HES) as well as the HK survey to identify sharp-lined stars with $\mbox{[Fe/H]}<-2.5$ dex. For several hundred confirmed metal-poor giants brighter than $B\sim 16.5$ mag (most of them from the HES), “snapshot” spectra ($R\sim 20\,000$; $S/N \sim 30$ per pixel) are being obtained with VLT/UVES, with the main aim of finding the $2{-}3$% r-II stars expected to be among them. These are studied in detail by means of higher resolution and higher $S/N$ spectra. In this paper we describe a pilot study based on a set of 35 stars, including 23 from the HK survey, eight from the HES, and four comparison stars. We discovered two new r-II stars, CS 29497-004 ($\mbox{[Eu/Fe]}=1.64\pm 0.22$) and CS 29491-069 ($\mbox{[Eu/Fe]}=1.08\pm 0.23$). A first abundance analysis of CS 29497-004 yields that its abundances of Ba to Dy are on average enhanced by $1.5$ dex with respect to iron and the Sun and match a scaled solar $r$-process pattern well, while Th is underabundant relative to that pattern by 0.3 dex, which we attribute to radioactive decay. That is, CS 29497-004 seems not to belong to the class of $r$-process enhanced stars displaying an “actinide boost”, like CS 31082-001 (Hill et al. 2002), or CS 30306-132 (Honda et al. 2004b). The abundance pattern agrees well with predictions of the phenomenological model of Qian & Wasserburg.
  http://dx.doi.org/10.1051/0004-6361:20041536.
• Article: Carbon-Enhanced Metal-Poor Stars in the Early Galaxy

  B. Marsteller, T.C. Beers, S. Rossi, N. Christlieb, M. Bessell, J. Rhee
  [show abstract] [hide abstract]
  ABSTRACT: Very metal-deficient stars that exhibit enhancements of their carbon abundances are of crucial importance for understanding a number of issues – the nature of stellar evolution among the first generations of stars, the shape of the Initial Mass Function, and the relationship between carbon enhancement and neutron-capture processes, in particular the astrophysical s-process. One recent discovery from objective-prism surveys dedicated to the discovery of metal-deficient stars is that the frequency of Carbon-Enhanced Metal-Poor (CEMP) stars increases with declining metallicity, reaching roughly 25% for [Fe/H] < −2.5.In order to explore this phenomenon in greater detail we have obtained medium-resolution (2 Å) spectroscopy for about 350 of the 413 objects in the Christlieb et al. catalog of carbon-rich stars, selected from the Hamburg/ESO objective prism survey on the basis of their carbon-enhancement, rather than metal deficiency. Based on these spectra, and near-IR JHK photometry from the 2MASS Point Source Catalog, we obtain estimates of [Fe/H] and [C/Fe] for most of the stars in this sample, along with reasonably accurate determinations of their radial velocities. Of particular importance, we find that the upper envelope of carbon enhancement observed for these stars is nearly constant, at [C/H] ∼ −1.0, over the metallicity range −4.0 < [Fe/H] < −2.0; this same level of [C/H] applies to the most iron-deficent star yet discovered, HE 0107-5240, at [Fe/H] = -5.3.
  Nuclear Physics A.