Rebecca Bernstein

University of Michigan, Ann Arbor, MI, United States

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Publications (9)25.86 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.
    No preview · Chapter · Nov 2008
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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.
    Preview · Article · Dec 2007 · The Astronomical Journal
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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
    Full-text · Article · Mar 2007 · The Astrophysical Journal Supplement Series
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    ABSTRACT: A new large-area Washington M, T2+DDO51 filter survey of more than 10 deg2 around the Carina dSph galaxy reveals a spectroscopically confirmed power-law radial density "break" population of Carina giant stars extending several degrees beyond the central King profile. Magellan telescope MIKE spectroscopy establishes the existence of Carina stars to at least 4.5 times its central King limiting radius, rlim, and primarily along Carina's major axis. To keep these stars bound to the dSph would require a global Carina mass-to-light ratio of M/L ≥ 6300 (M/L)☉. The MIKE velocities, supplemented with ~950 additional Carina field velocities from archived VLT+GIRAFFE spectra with r rlim, demonstrate a nearly constant Carina velocity dispersion (σv) to just beyond r = rlim and both a rising σv and a velocity shear at still larger radii. Together, the observational evidence suggests that the discovered extended Carina population represents tidal debris from the dSph. Of 65 giant candidates at large angular radii from the Carina center for which MIKE spectra have been obtained, 94% are associated with either Carina or a second, newly discovered diffuse, but strongly radial velocity-coherent (σv = 9.8 km s-1), foreground halo system. The 15 stars in this second, retrograde velocity population have (1) a mean metallicity ~1 dex higher than that of Carina and (2) colors and magnitudes consistent with the red clump of the LMC. Additional spectroscopy of giant star candidates in fields linking Carina and the LMC shows a smooth velocity gradient between the LMC and the retrograde Carina moving group. We conclude that we have found Magellanic stars almost twice as far (22°) from the LMC center than previously known.
    Full-text · Article · Sep 2006 · The Astrophysical Journal
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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.
    Preview · Article · Aug 2006 · The Astronomical Journal
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    ABSTRACT: The Magellan Telescopes are a collaboration between the Observatories of the Carnegie Institution of Washington (OCIW), University of Arizona, Harvard University, University of Michigan, and Massachusetts Institute of Technology (MIT) consisting of two 6.5 meter telescopes located at Las Campanas Observatory, in the Chilean Andes. The Walter Baade telescope achieved first light in September 2000 and the Landon Clay telescope started science operations in September 2002. In addition to two modified spectroscopic instruments, the Boller and Chivens Spectrograph and the Low Dispersion Survey Spectrograph (LDSS-2), four first generation instruments are now deployed at the Magellan Telescopes. Here we briefly describe the operations and performance of MagIC - a direct imaging CCD camera, MIKE - a double echelle spectrograph, PANIC - a near-IR imager, and IMACS - a multi-purpose, multi-object imaging spectrograph.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    No preview · Article · Sep 2004
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    ABSTRACT: The Giant Magellan Telescope (GMT) is a joint project of a consortium of universities and research institutions to build and operate a 21.5-m equivalent aperture astronomical telescope for use at visible and IR wavelengths. This paper briefly summarizes the science goals for the project and provides an overview of the preliminary telescope and enclosure concepts and site test program. The telescope is a Gregorian design with a fast, f/0.7, primary mirror that allows a compact and stiff mount structure. The 25.3-meter diameter primary mirror consists of six off-axis 8.4-meter circular mirrors arranged in a hexagon around a center 8.4-meter mirror. The Gregorian secondary mirror is adaptive allowing two-mirror, wide-field adaptive optics. Several corrector designs have been studied for wide-field applications and one such design is shown. Instruments being considered for GMT provide a wide range of scientific capabilities. Instruments mount below the primary mirror on an instrument platform. Instrument mounting and servicing provisions are summarized.
    No preview · Article · Sep 2004 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: The Magellan Telescopes are a collaboration between the Observatories of the Carnegie Institution of Washington (OCIW), University of Arizona, Harvard University, University of Michigan, and Massachusetts Institute of Technology (MIT) consisting of two 6.5 meter telescopes located at Las Campanas Observatory, in the Chilean Andes. The Walter Baade telescope achieved first light in September 2000 and the Landon Clay telescope started science operations in September 2002. In addition to two modified spectroscopic instruments, the Boller and Chivens Spectrograph and the Low Dispersion Survey Spectrograph (LDSS-2), four first generation instruments are now deployed at the Magellan Telescopes. Here we briefly describe the operations and performance of MagIC - a direct imaging CCD camera, MIKE - a double echelle spectrograph, PANIC - a near-IR imager, and IMACS - a multi-purpose, multi-object imaging spectrograph.
    No preview · Article · Sep 2004 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: The Magellan Inamori Kyocera Echelle (MIKE) is a double echelle spectrograph designed for use at the Magellan Telescopes at Las Campanas Observatory in Chile. It is currently in the final stages of construction and is scheduled for commissioning in the last quarter of 2002. In standard observing mode, the blue (320-480 nm) and red (440-1000 nm) channels are used simultaneously to obtain spectra over the full wavelength range with only a few gaps in wavelength coverage at the reddest orders. Both channels contain a three-group set of all-spherical, standard optical glass and calcium fluoride lenses which function as both camera and collimator in a double pass configuration. A single, standard echelle grating is used on each side and is illuminated close to true Littrow. Prism cross-dispersers are also used double- pass, and provide a minimum separation between orders of 6 arcsec. Spectral resolution is 19,000 and 25,000 on the red and blue sides, respectively, with a 1 arcsec slit. Typical rms image diameter is less than 0.2 arcsec, so that resolution increases linearly with decreasing slit width. The standard observing mode will use a slit up to 5" long, however a fiber- fed mode will also be available using blocking filters to select the desired orders for up to 256 objects at a time. In this paper, we describe the optical and mechanical design of the instrument.
    Preview · Article · Mar 2003 · Proceedings of SPIE - The International Society for Optical Engineering