R. A. Bernstein

University of California, Santa Cruz, Santa Cruz, California, United States

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Publications (11)33.28 Total impact

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    ABSTRACT: We present [Fe/H], ages, and Ca abundances for an initial sample of 10 globular clusters in NGC 5128 obtained from high resolution, high signal-to-noise ratio echelle spectra of their integrated light. All abundances and ages are obtained using our original technique for high resolution integrated light abundance analysis of globular clusters. The clusters have a range in [Fe/H] between -1.6 to -0.2. In this sample, the average [Ca/Fe] for clusters with [Fe/H]<-0.4 is +0.37$\pm$0.07, while the average [Ca/Fe] in our MW and M31 GC samples is +0.29 $\pm$0.09 and +0.24 $\pm$0.10, respectively. This may imply a more rapid chemical enrichment history for NGC 5128 than for either the Milky Way or M31.This sample provides the first quantitative picture of the chemical history of NGC 5128 that is directly comparable to what is available for the Milky Way. Data presented here were obtained with the MIKE echelle spectrograph on the Magellan Clay Telescope.
    The Astrophysical Journal Letters 07/2013; 773(2). · 6.35 Impact Factor
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    ABSTRACT: We present a comparison of high-resolution, integrated-light, detailed chemical abundances for Galactic and extragalactic globular clusters in both massive galaxies and dwarf galaxies. We include measurements of Fe, Ca, Si, Na, and Al for globular cluster samples in the Milky Way, M31, Large Magellanic Cloud, and NGC 5128. These and other recent results from our group on M31 and NGC 5128 are the first chemical abundances derived from discrete absorption features in old stars beyond the Milky Way and its nearest neighbors. These abundances can provide both galaxy enrichment histories and constraints on globular cluster formation and evolution.
    Memorie della Societa Astronomica Italiana. 02/2013;
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    J. E. Colucci, R. A. Bernstein, J. Cohen
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    ABSTRACT: We present detailed chemical abundances for $>$20 elements in $\sim$30 globular clusters in M31. These results have been obtained using high resolution ($\lambda/\Delta\lambda\sim$24,000) spectra of their integrated light and analyzed using our original method. The globular clusters have galactocentric radii between 2.5 kpc and 117 kpc, and therefore provide abundance patterns for different phases of galaxy formation recorded in the inner and outer halo of M31. We find that the clusters in our survey have a range in metallicity of $-2.2<$[Fe/H]$<-0.11$. The inner halo clusters cover this full range, while the outer halo globular clusters at R$>$20 kpc have a small range in abundance of [Fe/H]$=-1.6 \pm 0.10$. We also measure abundances of alpha, r- and s-process elements. These results constitute the first abundance pattern constraints for old populations in M31 that are comparable to those known for the Milky Way halo.
    10/2012;
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    J. E. Colucci, R. A. Bernstein
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    ABSTRACT: As part of an ongoing program to measure detailed chemical abundances in nearby galaxies, we use a sample of young to intermediate age clusters in the Large Magellanic Cloud with ages of 10 Myr to 2 Gyr to evaluate the effect of isochrone parameters, specifically core convective overshooting, on Fe abundance results from high resolution, integrated light spectroscopy. In this work we also obtain fiducial Fe abundances from high resolution spectroscopy of the cluster individual member stars. We compare the Fe abundance results for the individual stars to the results from isochrones and integrated light spectroscopy to determine whether isochrones with convective overshooting should be used in our integrated light analysis of young to intermediate age (10 Myr -3 Gyr) star clusters. We find that when using the isochrones from the Teramo group, we obtain more accurate results for young and intermediate age clusters over the entire age range when using isochrones without convective overshooting. While convective overshooting is not the only uncertain aspect of stellar evolution, it is one of the most readily parametrized ingredients in stellar evolution models, and thus important to evaluate for the specific models used in our integrated light analysis. This work demonstrates that our method for integrated light spectroscopy of star clusters can provide unique tests for future constraints on stellar evolution models of young and intermediate age clusters.
    The Astrophysical Journal 02/2012; 749(2). · 6.73 Impact Factor
  • Janet E. Colucci, R. A. Bernstein
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    ABSTRACT: We present chemical abundances and ages of stellar populations in M31, the nearest massive spiral galaxy. We have measured detailed chemical abundances for over 20 elements in a sample of 30 M31 globular clusters using high resolution spectra of their integrated light. The GCs are dispersed throughout the inner and outer halo, with galactocentric radii between 2.5 kpc and 117 kpc. We find a range of [Fe/H] within 20 kpc of the center of M31, and a constant [Fe/H] -1.6 for the outer halo GCs. We also derive cluster ages using Fe lines and present the first accurate age-metallicity relationship for the M31 GC system, with the result that the more metal-rich clusters in the sample are also the youngest. We find consistently high α-element abundances for a large range in [Fe/H] of -2.2 to -0.26. This implies that these M31 GCs formed in an environment with a high star formation rate, perhaps higher and more sustained than the early star formation rate of the Milky Way. We also measure the first abundances of 8 heavy neutron-capture elements, which are the first abundances of their kind in old stellar populations found more than 150 kpc from the Milky Way. These results provide the most comprehensive abundance history ever obtained for any other massive galaxy besides the Milky Way.
    01/2012;
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    ABSTRACT: We present detailed chemical abundances in 8 clusters in the Large Magellanic Cloud (LMC). We measure abundances of 22 elements for clusters spanning a range in age of 0.05 to 12 Gyr, providing a comprehensive picture of the chemical enrichment and star formation history of the LMC. The abundances were obtained from individual absorption lines using a new method for analysis of high resolution ($R\sim$25,000) integrated light spectra of star clusters. This method was developed and presented in Papers I, II, and III of this series. In this paper, we develop an additional integrated light $\chi^2$-minimization spectral synthesis technique to facilitate measurement of weak ($\sim$15 m\AA) spectral lines and abundances in low signal-to-noise ratio data (S/N$\sim$30). Additionally, we supplement the integrated light abundance measurements with detailed abundances that we measure for individual stars in the youngest clusters (Age$<$2 Gyr) in our sample. In both the integrated light and stellar abundances we find evolution of [$\alpha$/Fe] with [Fe/H] and age. Fe-peak abundance ratios are similar to those in the Milky Way, with the exception of [Cu/Fe] and [Mn/Fe], which are sub-solar at high metallicities. The heavy elements Ba, La, Nd, Sm, and Eu are significantly enhanced in the youngest clusters. Also, the heavy to light s-process ratio is elevated relative to the Milky Way ([Ba/Y]$>+0.5$) and increases with decreasing age, indicating a strong contribution of low-metallicity AGB star ejecta to the interstellar medium throughout the later history of the LMC. We also find a correlation of integrated light Na and Al abundances with cluster mass, in the sense that more massive, older clusters are enriched in the light elements Na and Al with respect to Fe, which implies that these clusters harbor star-to-star abundance variations as is common in the Milky Way.
    The Astrophysical Journal 11/2011; 746(1). · 6.73 Impact Factor
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    ABSTRACT: In this paper we refine our method for the abundance analysis of high resolution spectroscopy of the integrated light of unresolved globular clusters (GCs). This method was previously demonstrated for the analysis of old ($>$10 Gyr) Milky Way GCs. Here we extend the technique to young clusters using a training set of 9 GCs in the Large Magellanic Cloud (LMC). Depending on the signal-to-noise ratio of the data, we use 20-100 Fe lines per cluster to successfully constrain the ages of old clusters to within a $\sim$5 Gyr range, the ages of $\sim$2 Gyr clusters to a 1-2 Gyr range, and the ages of the youngest clusters (0.05-1 Gyr) to a $\sim$200 Myr range. We also demonstrate that we can measure [Fe/H] in clusters with any age less than 12 Gyrs with similar or only slightly larger uncertainties (0.1-0.25 dex) than those obtained for old Milky Way GCs (0.1 dex); the slightly larger uncertainties are due to the rapid evolution in stellar populations at these ages. In this paper, we present only Fe abundances and ages. In the next paper in this series, we present our complete analysis of the $\sim 20$ elements for which we are able to measure abundances. For several of the clusters in this sample, there are no high resolution abundances in the literature from individual member stars; our results are the first detailed chemical abundances available. The spectra used in this paper were obtained at Las Campanas with the echelle on the du Pont Telescope and with the MIKE spectrograph on the Magellan Clay Telescope.
    The Astrophysical Journal 04/2011; 735. · 6.73 Impact Factor
  • Janet E. Colucci, R. A. Bernstein, A. McWilliam
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    ABSTRACT: We present detailed chemical abundances of 21 elements in 8 clusters in the Large Magellanic Cloud (LMC). These abundances were obtained from individual absorption lines using our new method of analyzing high resolution (R 25,000) integrated-light spectra of globular clusters. We have previously used old (>10 Gyr) Milky Way clusters to demonstrate that we obtain accuracies comparable to those obtained from analysis of individual RGB stars. Here we use the LMC, which contains the necessary sample of clusters over a wide age range, to demonstrate for the first time that this abundance analysis method can be successfully applied to clusters of any age (50 Myrs to 13 Gyrs). Uniformly obtaining chemical abundances of clusters over a wide range in age allows us to investigate the chemical evolution history of the LMC in great detail. We find a spread in [alpha/Fe] at constant [Fe/H], as well as evolution of [alpha/Fe] with [Fe/H] and age. In older clusters, the light elements Na, Al, and Mg appear to be strongly affected by inter-cluster abundance variations; intermediate age and young clusters have light element abundances that are more consistent with LMC field stars. The heavy elements Ba, La, Nd, Sm, and Eu are enhanced in the youngest clusters. The heavy to light s-process ratio is elevated, ([Ba/Y] >+0.5), and increases with decreasing age, indicating a strong contribution of low-metallicity AGB star ejecta throughout the later history of the LMC. These results are consistent with prolonged, low-efficiency star formation between short bursts of more rapid star formation that resulted in massive, surviving clusters. Detailed chemical abundances measured using our new method can be obtained for galaxies within 4 Mpc using current telescopes. These abundances can be used to constrain the star formation history of galaxies in and beyond the Local Group in unprecedented detail.
    01/2011;
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    J. E. Colucci, R. A. Bernstein, A. McWilliam
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    ABSTRACT: We present detailed chemical abundances of Fe, Ca and Ba for 17 globular clusters (GCs) in 5 Local Group dwarf galaxies: NGC 205, NGC 6822, WLM, the SMC and LMC. These abundances are part of a larger sample of over 20 individual elements measured in GCs in these galaxies using a new analysis method for high resolution, integrated light spectra. Our analysis also provides age and stellar population constraints. The existence of GCs in dwarf galaxies with a range of ages implies that there were episodes of rapid star formation throughout the history of these galaxies; the abundance ratios of these clusters suggest that the duration of these burst varied considerably from galaxy to galaxy. We find evolution of Fe, Ca, and Ba with age in the LMC, SMC, and NGC 6822 that is consistent with extended, lower-efficiency SF between bursts, with an increasing contribution of low-metallicity AGB ejecta at late times. Our sample of GCs in NGC 205 and WLM are predominantly old and metal-poor with high [Ca/Fe] ratios, implying that the early history of these galaxies was marked by consistently high SF rates. Comment: 2 pages, To appear in the proceedings of the conference "A Universe of Dwarf Galaxies" (Lyon, June 14-18, 2010)
    EAS Publications Series 09/2010;
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    ABSTRACT: We present the first detailed chemical abundances of extragalactic globular clusters in the Local Group using a new method for analyzing high-resolution (lambdaDeltalambda 25,000) integrated light spectra. Here we present new detailed abundances for 20 alpha, Fe-peak, and neutron capture elements obtained from individual spectral lines for clusters in M31, the dwarf irregular NGC 6822, and the LMC. Our new analysis method (derived from standard RGB star analysis methods and as accurate) also provides age constraints for unresolved clusters. As we demonstrate on LMC clusters, our analysis methods let us measure accurate abudances and ages for clusters of all ages. The high S/N, high resolution spectra needed for detailed abundance analysis can be obtained for unresolved clusters in galaxies within 4 Mpc with current telescopes. Application of this new method to large samples of globular clusters in a range in environments and of a variety of ages will help to constrain the star formation histories of galaxies of different masses, as well as put constraints on merger history and hierarchical buildup of massive galaxies. As globular clusters form during major episodes of star formation, they are important tracers of the assembly history of galaxies, especially at distances where detailed spectroscopic studies of individual stars are not possible. The detailed abundances presented here for globular clusters in M31 are the first such measurements for old stars in a large galaxy other than the Milky Way.
    01/2010;
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    ABSTRACT: We report the first detailed chemical abundances for 5 globular clusters (GCs) in M31 from high-resolution (R ~ 25,000) spectroscopy of their integrated light. These GCs are the first in a larger set of clusters observed as part of an ongoing project to study the formation history of M31 and its globular cluster population. The data presented here were obtained with the HIRES echelle spectrograph on the Keck I telescope, and are analyzed using a new integrated light spectra analysis method that we have developed. In these clusters, we measure abundances for Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Y, and Ba, ages >10 Gyrs, and a range in [Fe/H] of -0.9 to -2.2. As is typical of Milky Way GCs, we find these M31 GCs to be enhanced in the alpha-elements Ca, Si, and Ti relative to Fe. We also find [Mg/Fe] to be low relative to other [alpha/Fe], and [Al/Fe] to be enhanced in the integrated light abundances. These results imply that abundances of Mg, Al (and likely O, Na) recovered from integrated light do display the inter- and intra-cluster abundance variations seen in individual Milky Way GC stars, and that special care should be taken in the future in interpreting low or high resolution integrated light abundances of globular clusters that are based on Mg-dominated absorption features. Fe-peak and the neutron-capture elements Ba and Y also follow Milky Way abundance trends. We also present high-precision velocity dispersion measurements for all 5 M31 GCs, as well as independent constraints on the reddening toward the clusters from our analysis. Comment: 32 pages, 34 Figures, Accepted for publication in ApJ
    The Astrophysical Journal 08/2009; · 6.73 Impact Factor

Publication Stats

60 Citations
33.28 Total Impact Points

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Institutions

  • 2011–2013
    • University of California, Santa Cruz
      • Department of Astronomy and Astrophysics
      Santa Cruz, California, United States
  • 2011–2012
    • University of California Observatories
      Santa Cruz, California, United States
  • 2009
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States