C. Sneden

University of Texas at Austin, Austin, Texas, United States

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Publications (279)617.5 Total impact

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    ABSTRACT: In this paper, we present a detailed high-resolution spectroscopic study of post main sequence stars in the Globular Cluster M68. Our sample, which covers a range of 4000 K in $T_{eff}$, and 3.5 dex in $log(g)$, is comprised of members from the red giant, red horizontal, and blue horizontal branch, making this the first high-resolution globular cluster study covering such a large evolutionary and parameter space. Initially, atmospheric parameters were determined using photometric as well as spectroscopic methods, both of which resulted in unphysical and unexpected $T_{eff}$, $log(g)$, $\xi_{t}$, and [Fe/H] combinations. We therefore developed a hybrid approach that addresses most of these problems, and yields atmospheric parameters that agree well with other measurements in the literature. Furthermore, our derived stellar metallicities are consistent across all evolutionary stages, with $\langle$[Fe/H]$\rangle$ = $-$2.42 ($\sigma$ = 0.14) from 25 stars. Chemical abundances obtained using our methodology also agree with previous studies and bear all the hallmarks of globular clusters, such as a Na-O anti-correlation, constant Ca abundances, and mild $r$-process enrichment.
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    ABSTRACT: We present detailed chemical compositions of 10 red giant star members of the Galactic (open) cluster NGC 752, derived from high-resolution (R $\approx$ 60,000), high signal-to-noise ($S/N$ $\geq$ 140) spectra. We confirmed cluster memberships by measuring the stellar radial velocities, and by deriving model atmosphere parameters ($T_{eff}$, logg, [Fe/H] and $\xi_{t}$) from equivalent widths of Fe I, Fe II, Ti I, and Ti II lines. The metallicity we obtained for NGC 752 ([Fe/H]=$-$0.02$\pm$0.05) is in good agreement with previous studies. We derived abundances of $\alpha$ (Mg, Si, Ca), light odd-Z (Na, Al), Fe-group (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), $n$-capture (Y, La, Nd, Eu), and $p$-capture (Li, C, N, O) species for each star. Furthermore, we also derived abundances of the LiCNO $p$-capture element group and carbon isotopic ratios, using synthetic spectrum analyses of the Li I 6707~\AA\ resonance doublet, the [O I] line at 6300~\AA, the CH G-band features near 4311 and 4325 \AA, the C$_{2}$ bandheads at 5160 and 5631 \AA, and $^{12,13}$CN red system lines in the 7995$-$8040~\AA\ region. By applying recent isochrones to NGC~752 photometry, and comparing the color-magnitude diagram information to our Li abundances and \ciso\ ratios, we suggest that the 10 observed red giants can be separated into three first-ascent, six red-clump and one red horizontal branch star.
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    ABSTRACT: We report the discovery of nine metal-poor stars with high levels of r-process enhancement (+0.81<[Eu/Fe]<+1.13), including six subgiants and three stars on the red horizontal branch. We also analyze four previously-known r-process-enhanced metal-poor red giants. From this sample of 13 stars, we draw the following conclusions. (1) High levels of r-process enhancement are found in a broad range of stellar evolutionary states, reaffirming that this phenomenon is not associated with a chemical peculiarity of red giant atmospheres. (2) Only 1 of 10 stars observed at multiple epochs shows radial velocity variations, reaffirming that stars with high levels of r-process enhancement are not preferentially found among binaries. (3) Only 2 of the 13 stars are highly-enhanced in C and N, indicating that there is no connection between high levels of r-process enhancement and high levels of C and N. (4) The dispersions in [Sr/Ba] and [Sr/Eu] are larger than the dispersions in [Ba/Eu] and [Yb/Eu], suggesting that the elements below the second r-process peak do not always scale with those in the rare earth domain, even within the class of highly-r-process-enhanced stars. (5) The light-element (12<Z<30) abundances of highly-r-process-enhanced stars are indistinguishable from those with normal levels of r-process material at the limit of our data, 3.5 per cent (0.015 dex) on average. The nucleosynthetic sites responsible for the large r-process enhancements did not produce any detectable light-element abundance signatures distinct from normal core-collapse supernovae.
    Monthly Notices of the Royal Astronomical Society 09/2014; 445(3). DOI:10.1093/mnras/stu1977 · 5.23 Impact Factor
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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). DOI:10.1088/0067-0049/214/2/18 · 14.14 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). DOI:10.1088/0067-0049/208/2/27 · 14.14 Impact Factor
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    ABSTRACT: Recent studies suggest that metal-poor stars enhanced in carbon but containing low levels of neutron-capture elements may have been among the first to incorporate the nucleosynthesis products of the first generation of stars. We have observed 16 stars with enhanced carbon or nitrogen using the MIKE Spectrograph on the Magellan Telescopes at Las Campanas Observatory and the Tull Spectrograph on the Smith Telescope at McDonald Observatory. We present radial velocities, stellar parameters, and detailed abundance patterns for these stars. Strontium, yttrium, zirconium, barium, europium, ytterbium, and other heavy elements are detected. In four stars, these heavy elements appear to have originated in some form of r-process nucleosynthesis. In one star, a partial s-process origin is possible. The origin of the heavy elements in the rest of the sample cannot be determined unambiguously. The presence of elements heavier than the iron group offers further evidence that zero-metallicity rapidly-rotating massive stars and pair instability supernovae did not contribute substantial amounts of neutron-capture elements to the regions where the stars in our sample formed. If the carbon- or nitrogen-enhanced metal-poor stars with low levels of neutron-capture elements were enriched by products of zero-metallicity supernovae only, then the presence of these heavy elements indicates that at least one form of neutron-capture reaction operated in some of the first stars.
    The Astrophysical Journal 02/2014; 784(2). DOI:10.1088/0004-637X/784/2/158 · 6.28 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.
    Proceedings of the International Astronomical Union 12/2013; 9(S298). DOI:10.1017/S1743921313006522
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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).
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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).
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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. DOI:10.1088/0004-637X/769/1/40 · 6.28 Impact Factor
  • M. Afşar, C. Sneden, B.-Q. For
    The European Physical Journal Conferences 04/2013; 43:02006-. DOI:10.1051/epjconf/20134302006
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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. DOI:10.1088/0067-0049/205/2/11 · 14.14 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.
    The European Physical Journal Conferences 03/2013; 43:02003-. DOI:10.1051/epjconf/20134302003
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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 off er 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.
    Astronomy and Astrophysics 02/2013; 550:122-. DOI:10.1051/0004-6361/201219949 · 4.48 Impact Factor
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    ABSTRACT: We obtained six exposures of Be 39 with the multi-object spectrograph VLT/FLAMES, as in our previous studies on the Na-O anti-correlations in clusters. High resolution UVES and GIRAFFE spectra have been acquired with FLAMES@VLT. We measured elemental abundances and present here the EWs, separately for the UVES and GIRAFFE stars. Based on observations obtained under ESO Programme 386.B-0009. (5 data files).
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    ABSTRACT: It is now commonly accepted that globular clusters (GCs) have undergone a complex formation and that they host at least two stellar generations. This is a recent paradigm and is founded on both photometric and spectroscopic evidence. We concentrate on results based on high-resolution spectroscopy and on how we moved from single to multiple stellar populations concept for GCs. We underline that the peculiar chemical composition of GC stars is fundamental in establishing the multiple populations scenario and briefly outline what can be learned from observations. Finally, recent observational results on large samples of stars in different evolutionary phases are discussed.
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    ABSTRACT: It is now commonly accepted that globular clusters (GCs) have undergone a complex formation and that they host at least two stellar generations. This is a recent paradigm and is founded on both photometric and spectroscopic evidence. We concentrate on results based on high-resolution spectroscopy and on how we moved from single to multiple stellar populations concept for GCs. We underline that the peculiar chemical composition of GC stars is fundamental in establishing the multiple populations scenario and briefly outline what can be learned from observations. Finally, recent observational results on large samples of stars in different evolutionary phases are discussed.
    01/2013;
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    ABSTRACT: The abundance ratios of the magnesium isotopes are a clue to the neutron source responsible for the s-process. Detailed analysis of the MgH A2Π - X2Σ+ transition is necessary to extend existing knowledge of the 25Mg/24Mg and 26Mg/24Mg isotopic ratios. However, the required laboratory spectroscopy of the 25MgH and 26MgH isotopologues is sparse. We have analyzed existing laboratory hollow cathode spectra from the archives of the McMath-Pierce FTS. Lines from 24MgH, 25MgH and 26MgH were identified and the frequencies measured. Hamiltonians were fit to the 25MgH and 26MgH line frequencies. Results for the more abundant 24MgH variant have been taken from the literature. A line list for the three isotopologues has been produced.
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    ABSTRACT: New atomic transition probability measurements for 364 lines of Ti II in the UV through near IR are reported. These results are used to determine the Ti abundance of the Sun and a very metal-poor main-sequence turnoff dwarf star over a range of wavelength and E. P. values to search for non-LTE effects. Branching fractions from data recorded using a Fourier transform spectrometer (FTS) and a 3 m echelle spectrometer are combined with published radiative lifetimes from laser induced fluorescence measurements to determine these new transition probabilities. The new results are in generally good agreement with previously reported FTS measurements and the NIST Atomic Spectra Database. The use of a new echelle spectrometer, new radiometric calibration methods, and independent data analysis routines allows for a reduction of systematic errors and overall improvement in transition probability uncertainties over previous measurements. This work represents the largest and most complete single source of Ti II transition probabilities to date in this wavelength range. The new Ti II data are applied to high resolution visible and UV spectra of the Sun and HD 84937 to derive new, more accurate titanium abundances. We use 36 lines from the solar photosphere to derive log(ɛ(Ti))=4.98 (σ=0.03), in good agreement with the recommended Ti abundance from previous solar abundance reviews and in very good agreement with our solar abundance derived using Ti I. For HD 84937, a preliminary analysis with a subset of available Ti II lines yields log(ɛ(Ti))=3.12 (σ=0.04, 19 lines), or [Ti/H]=-1.86, also in very good agreement with our abundance derived using Ti I. This work is supported by NASA grant NNX10AN93G and NSF grant AST-1211055 (JEL) and NSF grants AST-0908978 and AST-1211585 (CS).
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    ABSTRACT: The Galactic globular cluster NGC 1851 has raised much interest since Hubble Space Telescope photometry revealed that it hosts a double subgiant branch. Here we report on our homogeneous study into the cyanogen (CN) band strengths in the red giant branch (RGB) population (17 stars) and asymptotic giant branch (AGB) population (21 stars) using AAOmega/2dF spectra with R {approx} 3000. We discover that NGC 1851 hosts a quadrimodal distribution of CN band strengths in its RGB and AGB populations. This result supports the merger formation scenario proposed for this cluster, such that the CN quadrimodality could be explained by the superposition of two 'normal' bimodal populations. A small sample overlap with an abundance catalog allowed us to tentatively explore the relationship between our CN populations and a range of elemental abundances. We found a striking correlation between CN and [O/Na]. We also found that the four CN peaks may be paired-the two CN-weaker populations being associated with low Ba and the two CN-stronger populations with high Ba. If true, then s-process abundances would be a good diagnostic for disentangling the two original clusters in the merger scenario. More observations are needed to confirm the quadrimodality and also the relationship between the subpopulations. We also report CN results for NGC 288 as a comparison. Our relatively large samples of AGB stars show that both clusters have a bias toward CN-weak AGB populations.
    The Astrophysical Journal Letters 12/2012; 761(1). DOI:10.1088/2041-8205/761/1/L2 · 5.60 Impact Factor

Publication Stats

4k Citations
617.50 Total Impact Points

Institutions

  • 2–2014
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 1990–2013
    • The Astronomical Observatory of Brera
      Merate, Lombardy, Italy
  • 2003
    • Università di Pisa
      Pisa, Tuscany, Italy
  • 2002
    • Indiana University Bloomington
      Bloomington, Indiana, United States
  • 1999
    • National Optical Astronomy Observatory
      Tucson, Arizona, United States
    • Johannes Gutenberg-Universität Mainz
      Mayence, Rheinland-Pfalz, Germany
  • 1991
    • University of North Carolina at Chapel Hill
      North Carolina, United States
  • 1978
    • University of Wyoming
      Laramie, Wyoming, United States
    • Princeton University
      Princeton, New Jersey, United States