C. Sneden

University of Notre Dame, South Bend, Indiana, United States

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Publications (409)1373.39 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: New red and violet system line lists for the CN isotopologues 13C14N and 12C15N have been generated. These new transition data are combined with those previously derived for 12C14N, and applied to the determination of CNO abundances in the solar photosphere and in four red giant stars: Arcturus, the bright, very low-metallicity star HD 122563, and the carbon-enhanced metal-poor stars HD 196944 and HD 201626. When both red and violet system lines are detectable in a star, their derived N abundances are in good agreement. The mean N abundances determined in this work are also generally in accord with published values.
    The Astrophysical Journal Supplement Series 10/2014; 214(2):26. · 16.24 Impact Factor
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    ABSTRACT: Galactic open and globular clusters (OCs, GCs) appear to inhabit separate regions of the age-mass plane. However, the transition between them is not easily defined because there is some overlap between high-mass, old OCs and low-mass, young GCs. We are exploring the possibility of a clear-cut separation between OCs and GCs using an abundance feature that has been found so far only in GCs: (anti)correlations between light elements. Among the coupled abundance trends, the Na-O anticorrelation is the most widely studied. These anticorrelations are the signature of self-enrichment, i.e., of a formation mechanism that implies multiple generations of stars. Here we concentrate on the old, massive, metal-rich OC NGC 6791. We analyzed archival Keck/HIRES spectra of 15 NGC 6791 main sequence turn-off and evolved stars, concentrating on the derivation of C, N, O, and Na abundances. We also used WIYN/Hydra spectra of 21 evolved stars (one is in common). Given the spectral complexity of the very metal-rich NGC 6791 stars, we employed spectrum synthesis to measure most of the abundances. We confirmed the cluster super-solar metallicity and abundances of Ca and Ni that have been derived in past studies. More importantly, we did not detect any significant star-to-star abundance dispersion in C, N, O and Na. Based on the absence of a clear Na-O anticorrelation, NGC 6791 can still be considered a true OC, hosting a single generation of stars, and not a low-mass GC.
    09/2014;
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    ABSTRACT: New experimental absolute atomic transition probabilities are reported for 203 lines of V II. Branching fractions are measured from spectra recorded using a Fourier transform spectrometer and an echelle spectrometer. The branching fractions are normalized with radiative lifetime measurements to determine the new transition probabilities. Generally good agreement is found between this work and previously reported V II transition probabilities. Use of two spectrometers, independent radiometric calibration methods, and independent data analysis routines enables a reduction in systematic uncertainties, in particular those due to optical depth errors. In addition, new hyperfine structure constants are measured for selected levels by least squares fitting line profiles in the FTS spectra. The new V II data are applied to high resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to determine new, more accurate V abundances. Lines covering a range of wavelength and excitation potential are used to search for non-LTE effects. Very good agreement is found between our new solar photospheric V abundance, log {\epsilon}(V) = 3.95 from 15 V II lines, and the solar-system meteoritic value. In HD 84937, we derive [V/H] = -2.08 from 68 lines, leading to a value of [V/Fe] = 0.24.
    The Astrophysical Journal Supplement Series 08/2014; 214(2). · 16.24 Impact Factor
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    ABSTRACT: New red and violet system line lists for the CN isotopologues 13C14N and 12C15N have been generated. These new transition data are combined with those previously derived for 12C14N, and applied to the determination of CNO abundances in the solar photosphere and in four red giant stars: Arcturus, the bright very low-metallicity star HD 122563, and carbon-enhanced metal-poor stars HD 196944 and HD 201626. When lines of both red and violet system lines are detectable in a star, their derived N abundances are in good agreement. The mean N abundances determined in this work generally are also in accord with published values.
    08/2014;
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    ABSTRACT: We use the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope to obtain new high-quality spectra covering the 1900 to 2360 Angstrom wavelength range for two metal-poor stars, HD 108317 and HD 128279. We derive abundances of Cu II, Zn II, As I, Se I, Mo II, and Cd II, which have not been detected previously in either star. Abundances derived for Ge I, Te I, Os II, and Pt I confirm those derived from lines at longer wavelengths. We also derive upper limits from the non-detection of W II, Hg II, Pb II, and Bi I. The mean [As/Fe] ratio derived from these two stars and five others in the literature is unchanged over the metallicity range -2.8 < [Fe/H] < -0.6, <[As/Fe]> = +0.28 +/- 0.14 (std. dev. = 0.36 dex). The mean [Se/Fe] ratio derived from these two stars and six others in the literature is also constant, <[Se/Fe]> = +0.16 +/- 0.09 (std. dev. = 0.26 dex). The As and Se abundances are enhanced relative to a simple extrapolation of the iron-peak abundances to higher masses, suggesting that this mass region (75 < A < 82) may be the point at which a different nucleosynthetic mechanism begins to dominate the quasi-equilibrium alpha-rich freezeout of the iron peak. <[CuII/CuI]> = +0.56 +/- 0.23 in HD 108317 and HD 128279, and we infer that lines of Cu I may not be formed in local thermodynamic equilibrium in these stars. The [Zn/Fe], [Mo/Fe], [Cd/Fe], and [Os/Fe] ratios are also derived from neutral and ionized species, and each ratio pair agrees within the mutual uncertainties, which range from 0.15 to 0.52 dex.
    The Astrophysical Journal 06/2014; 791(1). · 6.73 Impact Factor
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    ABSTRACT: We present an elemental-abundance analysis, in the near-ultraviolet (NUV) spectral range, for the extremely metal-poor star BD+44 493, a 9th magnitude sub-giant with [Fe/H] = -3.8 and enhanced carbon, based on data acquired with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. This star is the brightest example of a class of objects that, unlike the great majority of carbon-enhanced metal-poor (CEMP) stars, does not exhibit over-abundances of heavy neutron-capture elements (CEMP-no). In this paper, we validate the abundance determinations for a number of species that were previously studied in the optical region, and obtain strong upper limits for beryllium and boron, as well as for neutron-capture elements from zirconium to platinum, many of which are not accessible from ground-based spectra. The boron upper limit we obtain for BD+44 493, logeps(B) < -0.70, the first such measurement for a CEMP star, is the lowest yet found for very and extremely metal-poor stars. In addition, we obtain even lower upper limits on the abundances of beryllium, logeps(Be) < -2.3, and lead, logeps(Pb) < -0.23 ([Pb/Fe] < +1.90), than those reported by previous analyses in the optical range. Taken together with the previously measured low abundance of lithium, the very low upper limits on Be and B suggest that BD+44 493 was formed at a very early time, and that it could well be a bona-fide second-generation star. Finally, the Pb upper limit strengthens the argument for non-s-process production of the heavy-element abundance patterns in CEMP-no stars.
    The Astrophysical Journal 06/2014; 790(1). · 6.73 Impact Factor
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    ABSTRACT: We present radial velocities, equivalent widths, model atmosphere parameters, and abundances or upper limits for 53 species of 48 elements derived from high resolution optical spectroscopy of 313 metal-poor stars. A majority of these stars were selected from the metal-poor candidates of the HK Survey of Beers, Preston, and Shectman. We derive detailed abundances for 61% of these stars for the first time. Spectra were obtained during a 10-year observing campaign using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert G. Tull Coude Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory, and the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We perform a standard LTE abundance analysis using MARCS model atmospheres, and we apply line-by-line statistical corrections to minimize systematic abundance differences arising when different sets of lines are available for analysis. We identify several abundance correlations with effective temperature. A comparison with previous abundance analyses reveals significant differences in stellar parameters, which we investigate in detail. Our metallicities are, on average, lower by approx. 0.25 dex for red giants and approx. 0.04 dex for subgiants. Our sample contains 19 stars with [Fe/H] < -3.5, 84 stars with [Fe/H] < -3.0, and 210 stars with [Fe/H] < -2.5. Detailed abundances are presented here or elsewhere for 91% of the 209 stars with [Fe/H] < -2.5 as estimated from medium resolution spectroscopy by Beers, Preston, and Shectman. We will discuss the interpretation of these abundances in subsequent papers.
    The Astronomical Journal 03/2014; 147(6). · 4.97 Impact Factor
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    ABSTRACT: Atomic transition probability measurements for 371 Ni I lines in the UV through near IR are reported. Branching fractions from data recorded using a Fourier transform spectrometer and a new echelle spectrograph are combined with published radiative lifetimes to determine these transition probabilities. Generally good agreement is found in comparisons to previously reported Ni I transition probability measurements. Use of the new echelle spectrograph, independent radiometric calibration methods, and independent data analysis routines enable a reduction of systematic errors and overall improvement in transition probability uncertainty over previous measurements. The new Ni I data are applied to high resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to derive new, more accurate Ni abundances. Lines covering a wide range of wavelength and excitation potential are used to search for non-LTE effects.
    The Astrophysical Journal Supplement Series 02/2014; 208(2). · 16.24 Impact Factor
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    ABSTRACT: We present new, accurate predictions for rotational line positions, excitation energies, and transition probabilities of the 12C13C isotopologue Swan d3Π-a3Π system 0-0, 0–1, 0–2, 1–0, 1–1, 1–2, 2–0, 2–1, and 2–2 vibrational bands. The line positions and energy levels were predicted through new analyses of published laboratory data for the 12C13C lines. Transition probabilities were derived from recent computations of transition dipole moments and related quantities. The 12C13C line data were combined with similar data for 12C2, reported in a companion paper, and applied to produce synthetic spectra of carbon-rich metal-poor stars that have strong C2 Swan bands. The matches between synthesized and observed spectra were used to estimate band head positions for a few of the 12C13C vibrational bands and to verify that the new computed line data match observed spectra. The much weaker C2 lines of the bright red giant Arcturus were also synthesized in the band head regions.
    The Astrophysical Journal Supplement Series 02/2014; 211(1):5. · 16.24 Impact Factor
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    ABSTRACT: HERMES is a new high-resolution multi-object spectrograph on the Anglo Australian Telescope. The primary science driver for HERMES is the GALAH survey, GALactic Archaeology with HERMES. We are planning a spectroscopic survey of about a million stars, aimed at using chemical tagging techniques to reconstruct the star-forming aggregates that built up the disk, the bulge and halo of the Galaxy. This project will benefit greatly from the stellar distances and transverse motions from the Gaia mission.
    12/2013;
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    ABSTRACT: We report new metallicities for stars of Galactic globular cluster M4 using the largest number of stars ever observed at high spectral resolution in any cluster. We analyzed 7250 spectra for 2771 cluster stars gathered with the VLT FLAMES+GIRAFFE spectrograph at VLT. These medium resolution spectra cover by a small wavelength range, and often have very low signal-to-noise ratios. We attacked this dataset by reconsidering the whole method of abundance analysis of large stellar samples from beginning to end. We developed a new algorithm that automatically determines the atmospheric parameters of a star. Nearly all data preparation steps for spectroscopic analyses are processed on the syntheses, not the observed spectra. For 322 Red Giant Branch stars with $V \leq 14.7$ we obtain a nearly constant metallicity, $<[{\rm Fe}/{\rm H}]> = -1.07$ ($\sigma$ = 0.02). No difference in the metallicity at the level of $0.01 ~\textrm{dex}$ is observed between the two RGB sequences identified by \cite{Monelli:2013us}. For 1869 Subgiant and Main Sequence Stars $V > 14.7$ we obtain $<[{\rm Fe}/{\rm H}]> = -1.16$ ($\sigma$ = 0.09) after fixing the microturbulent velocity. These values are consistent with previous studies that have performed detailed analyses of brighter RGB stars at higher spectroscopic resolution and wavelength coverage. It is not clear if the small mean metallicity difference between brighter and fainter M4 members is real or is the result of the low signal-to-noise characteristics of the fainter stars. The strength of our approach is shown by recovering a metallicity close to a single value for more than two thousand stars, using a dataset that is non-optimal for atmospheric analyses. This technique is particularly suitable for noisy data taken in difficult observing conditions.
    The Astronomical Journal 11/2013; 147(2). · 4.97 Impact Factor
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    Jose Govea, Thomas Gomez, George W. Preston, Christopher Sneden
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    ABSTRACT: We report the first extensive model atmosphere and detailed chemical abundance study of eight RR Lyrae variable stars of c subclass throughout their pulsation cycles. Atmospheric parameters effective temperature, surface gravity, microturbulent velocity, and metallicity have been derived. Spectra for this abundace analysis have been obtained with the echelle spectrograph of 100-inch du Pont telescope at Las Campanas Observatory. We have found metallicities and element abundance ratios to be constant within observational uncertainties at all phases of all stars. Moreover, the $\alpha$-element and Fe-group abundance ratios with respect to iron are consistent with other horizontal-branch members (RRab, blue and red non-variables). The [Fe/H] values of these eight RRc stars have been used to anchor the metallicity scale of a much larger sample of RRc stars obtained with low S/N "snapshot" spectra.
    10/2013;
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    ABSTRACT: The 14 1.0m FTS (Fourier transform spectrometer) spectra (1989 Feb 28 to 1992 Jul 29) used in this TiII branching fraction analysis are available in the NSO electronic archives. In addition to the 14 FTS spectra, 48 charge-coupled device (CCD) frames of Ti HCD spectra (from 2011 Jul 27 to 2012 Feb 23), recorded with the echelle spectrometer, are part of this TiII branching fraction study. See section 3 for further details. (4 data files).
    VizieR Online Data Catalog. 10/2013;
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    ABSTRACT: Rotationally resolved lines within the A2{Pi}-X2{ transition of the 25MgH and 26MgH isotopologues were measured from spectra recorded by J. Black, P. F. Bernath, C. R. Brazier, and R. Hubbard in 1984 (National Solar Observatory (NSO) archive reference 1984/03/13/#2) with the 1m Fourier transform spectrometer associated with the McMath-Pierce Solar Telescope of the NSO at Kitt Peak. (6 data files).
    VizieR Online Data Catalog. 09/2013;
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    ABSTRACT: Using laboratory hollow cathode spectra we have identified lines of the less common magnesium isotopologues of MgH, 25MgH and 26MgH, in the A 2Π-X 2Σ+ system. Based on the previous analysis of 24MgH, molecular lines have been measured and molecular constants derived for 25MgH and 26MgH. Term values and linelists, in both wavenumber and wavelength units, are presented. The A 2Π-X 2Σ+ system of MgH is important for measuring the magnesium isotope ratios in stars. Examples of analysis using the new linelists to derive the Mg isotope ratio in a metal poor dwarf and giant are shown.
    The Astrophysical Journal Supplement Series 07/2013; 207(2):26. · 16.24 Impact Factor
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    ABSTRACT: Two independent studies recently uncovered two distinct populations among giants in the distant, massive globular cluster (GC) NGC 2419. One of these populations has normal magnesium (Mg) and potassium (K) abundances for halo stars: enhanced Mg and roughly solar K. The other population has extremely depleted Mg and very enhanced K. To better anchor the peculiar NGC 2419 chemical composition, we have investigated the behavior of K in a few red giant branch stars in NGC 6752, NGC 6121, NGC 1904, and ω Cen. To verify that the high K abundances are intrinsic and not due to some atmospheric features in giants, we also derived K abundances in less evolved turn-off and subgiant stars of clusters 47 Tuc, NGC 6752, NGC 6397, and NGC 7099. We normalized the K abundance as a function of the cluster metallicity using 21 field stars analyzed in a homogeneous manner. For all GCs of our sample, the stars lie in the K-Mg abundance plane on the same locus occupied by the Mg-normal population in NGC 2419 and by field stars. This holds for both giants and less-evolved stars. At present, NGC 2419 seems unique among GCs.
    The Astrophysical Journal 05/2013; 769(1):40. · 6.73 Impact Factor
  • M. Afşar, C. Sneden, B.-Q. For
    04/2013;
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    ABSTRACT: New atomic transition probability measurements for 948 lines of Ti I are reported. Branching fractions from Fourier transform spectra and from spectra recorded using a 3 m echelle spectrometer are combined with published radiative lifetimes from laser-induced fluorescence measurements to determine these transition probabilities. Generally good agreement is found in comparisons to the NIST Atomic Spectra Database. The new Ti I data are applied to re-determine the Ti abundance in the photospheres of the Sun and metal-poor star HD 84937 using many lines covering a range of wavelength and excitation potential to explore possible non-local thermal equilibrium effects. The variation of relative Ti/Fe abundance with metallicity in metal-poor stars observed in earlier studies is supported in this study.
    The Astrophysical Journal Supplement Series 03/2013; 205(2):11. · 16.24 Impact Factor
  • M. Afşar, C. Sneden, B.-Q. Fo
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    ABSTRACT: A large sample survey of Galactic red horizontal-branch (RHB) stars was conducted to investigate their atmospheric parameters and elemental abundances. High-resolution spectra of 76 Galactic field stars were obtained with the 2.7 m Smith Telescope at McDonald Observatory. Only the color and the parallax were considered during the selection of the field stars. Equivalent width or synthetic spectrum analyses were used in order to determine the relative abundances of the following elements: proton-capture elements C, N, O and Li, alpha-elements Ca and Si, and neutron-capture elements Eu and La. Additionally, 12C/13C isotopic ratios were derived by using the CN features mainly located in the 7995 - 8040 Å spectral region. The evaluation of effective temperatures, surface gravities and 12C/13C isotopic ratios together with evolutionary stages of the candidates revealed that 18 out of 76 stars in our sample are probable RHBs. Including both kinematic and evolutionary status information, we conclude that we have five thick disk and 13 thin disk RHB stars in our sample. Although RHB stars have been regarded as thick disk members of the Galaxy, the low-velocity RHBs with a solar metallicity in our sample suggests the existence of a large number of thin disk RHBs, which cannot be easily explained by standard stellar evolutionary models.
    03/2013;
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    ABSTRACT: Context. The origin and site(s) of the r-process nucleosynthesis is(are) still not known with certainty, but complete, detailed r-element abundances offer our best clues. The few extremely metal-poor (EMP) stars with large r-element excesses allow us to study the r-process signatures in great detail, with minimal interference from later stages of Galactic evolution. CS 31082-001 is an outstanding example of the information that can be gathered from these exceptional stars. Aims: Here we aim to complement our previous abundance determinations for third-peak r-process elements with new and improved results for elements of the first and second r-process peaks from near-UV HST/STIS and optical UVES spectra. These results should provide new insight into the nucleosynthesis of the elements beyond iron. Methods: The spectra were analyzed by a consistent approach based on an OSMARCS LTE model atmosphere and the Turbospectrum spectrum synthesis code to derive abundances of heavy elements in CS 31082-001, and using updated oscillator strengths from the recent literature. Synthetic spectra were computed for all lines of the elements of interest to check for proper line intensities and possible blends in these crowded spectra. Our new abundances were combined with the best previous results to provide reliable mean abundances for the first and second-peak r-process elements. Results: We present new abundances for 23 neutron-capture elements, 6 of which - Ge, Mo, Lu, Ta, W, and Re - have not been reported before. This makes CS 31082-001 the most completely studied r-II star, with abundances for a total of 37 neutron-capture elements. We also present the first NLTE+3D abundance of lead in this star, further constraining the nature of the r-process. Based on observations made with the NASA/ESA Hubble Space Telescope (HST) through the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555; and with the ESO Very Large Telescope at Paranal Observatory, Chile; Progr. ID 165.N-0276.Appendix A is available in electronic form at http://www.aanda.org
    Astronomy and Astrophysics 02/2013; · 5.08 Impact Factor

Publication Stats

6k Citations
1,373.39 Total Impact Points

Institutions

  • 2014
    • University of Notre Dame
      • Department of Physics
      South Bend, Indiana, United States
    • National Optical Astronomy Observatory
      Tucson, Arizona, United States
  • 2013–2014
    • Old Dominion University
      • Department of Chemistry and Biochemistry
      Norfolk, Virginia, United States
  • 2–2014
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 2010–2013
    • The Astronomical Observatory of Brera
      Merate, Lombardy, Italy
  • 2005–2012
    • Michigan State University
      • Department of Physics and Astronomy
      East Lansing, Michigan, United States
  • 2008
    • Carnegie Institution for Science
      • Department of Terrestrial Magnetism
      Washington, West Virginia, United States
  • 1995–2006
    • University of Oklahoma
      • Homer L. Dodge Department of Physics and Astronomy
      Oklahoma City, OK, United States
  • 2004
    • Johannes Gutenberg-Universität Mainz
      • Institute for Nuclear Chemistry
      Mainz, Rhineland-Palatinate, Germany
  • 2002
    • University of Wisconsin, Madison
      • Department of Physics
      Madison, MS, United States
  • 1994–2001
    • University of California, Santa Cruz
      • Department of Astronomy and Astrophysics
      Santa Cruz, California, United States
  • 1996
    • Università degli Studi di Trieste
      Trst, Friuli Venezia Giulia, Italy
  • 1992
    • Harvard-Smithsonian Center for Astrophysics
      Cambridge, Massachusetts, United States
  • 1991
    • University of North Carolina at Chapel Hill
      North Carolina, United States
  • 1988
    • University of Cape Town
      Kaapstad, Western Cape, South Africa
  • 1978
    • University of Wyoming
      Laramie, Wyoming, United States