Rebecca Bernstein

University of Michigan, Ann Arbor, MI, United States

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Publications (4)22.24 Total impact

  • Rebecca Bernstein, Andrew McWilliam
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    ABSTRACT: We are developing a technique for measuring the detailed chemical composition of unresolved, extragalactic globular clusters (GCs) from echelle spectra of their integrated light. To do this, we are using a “training set” of spatially resolved clusters. We scan these clusters to obtain integrated light spectra, and also take spectra of individual stars in these clusters to obtain “fiducial” abundances by the usual analysis methods. We briefly describe here the importance of obtaining detailed abundances, the technique we are developing to analyze integrated light spectra, and the accuracies that can be obtained with our technique.
    11/2008: pages 11-15;
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    ABSTRACT: We present new radial velocity results for 176 stars in the Fornax dwarf spheroidal galaxy, of which at least 156 are probable Fornax members. We combine with previously published data to obtain a radial velocity sample with 206 stars, of which at least 176 are probable Fornax members. We detect the hint of rotation about an axis near Fornax's morphological minor axis, although the significance of the rotation signal in the galactic rest frame is sensitive to the adopted value of Fornax's proper motion. Regardless, the observed stellar kinematics is dominated by random motions, and we do not find kinematic evidence of tidal disruption. The projected velocity dispersion profile of the binned data set remains flat over the sampled region, which reaches a maximum angular radius of 65'. Single-component King models in which mass follows light fail to reproduce the observed flatness of the velocity dispersion profile. Two-component (luminous plus dark matter) models can reproduce the data, provided that the dark component extends sufficiently beyond the luminous component and the central dark matter density is of the same order as the central luminous density. These requirements suggest a more massive, darker Fornax than standard core-fitting analyses have previously concluded, with M/LV over the sampled region reaching 10-40 times the M/LV of the luminous component. We also apply a nonparametric mass estimation technique, introduced in a companion paper. Although it is designed to operate on data sets containing velocities for >1000 stars, the estimation yields preliminary results suggesting M/LV ~ 15 inside r < 1.5 kpc.
    The Astronomical Journal 12/2007; 131(4):2114. DOI:10.1086/500193 · 4.05 Impact Factor
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    ABSTRACT: We introduce a stellar velocity survey of dwarf spheroidal galaxies, undertaken using the Michigan/MIKE Fiber System (MMFS) at the Magellan/Clay 6.5 m telescope at Las Campanas Observatory. As of 2006 November we have used MMFS to collect 6415 high-resolution (R= 20000-25000) spectra from 5180 stars in four dwarf spheroidal galaxies: Carina, Fornax, Sculptor and Sextans. Spectra sample the range 5140-5180 Angstroms, which includes the prominent magnesium triplet absorption feature. We measure radial velocity (RV) to a median precision of 2.0 km/s for stars as faint as V ~ 20.5. From the spectra we also are able to measure the strength of iron and magnesium absorption features using spectral indices that correlate with effective temperature, surface gravity and chemical abundance. Measurement of line strength allows us to identify interloping foreground stars independently of velocity, and to examine the metallicity distribution among dSph members. Here we present detailed descriptions of MMFS, our target selection and spectroscopic observations, the data reduction procedure, and error analysis. We compare our RV results to previously published measurements for individual stars. In some cases we find evidence for a mild, velocity-dependent offset between the RVs we measure using the magnesium triplet and previously published RV measurements derived from the infrared calcium triplet. In companion papers we will present the complete data sets and kinematic analyses of these new observations. Comment: Accepted for publication in the Astrophysical Journal Supplemement Series. 30 pages, 24 figures (6 color). Dowload pdf including high-resolution figures and complete Table 1 from http://www.astro.lsa.umich.edu/~mgwalker/mmfsfigs/ms.pdf
    The Astrophysical Journal Supplement Series 03/2007; 171(2). DOI:10.1086/517886 · 14.14 Impact Factor
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    ABSTRACT: On publication of this work it was discovered that repeat observations of four stars were listed in Table 1 under unique stellar IDs. The stars F1-5, F1-13, F1-15, and F1-24 have repeat measurements listed under the names F22-1, F7-4, F7-2, and F2-9, respectively. The affected rows in Table 3, giving the weighted mean radial velocity, formal uncertainty, chi2, and p(chi2) after combining these repeat measurements, are shown below. Based on the repeat measurements, two of these stars, F1-13 and F1-24, are binary star candidates. The three levels of Fornax membership rejection considered in this work now correspond to samples of size N=172, 178, and 182. We have verified that our scientific conclusions are unchanged. The global velocity dispersions estimated from these three samples are 11.2+/-0.7, 12.5+/-0.7, and 13.5+/-0.7 km s-1, respectively; these differ from the reported values by less than 0.2 km s-1. For velocity dispersion profiles, values calculated within all bins differ from plotted values by less than 0.3 km s-1, and most are identical. Apparent rotation signals are not affected. We thank Ewa Lokas for bringing this matter to our attention.
    The Astronomical Journal 08/2006; 132(2):968-968. DOI:10.1086/505303 · 4.05 Impact Factor