Browsing by Subject "chemical abundances"

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Enhanced Abundances In Spiral Galaxies Of The Pegasus I Cluster
(2012-03) Robertson, Paul; Shields, Gregory A.; Blanc, Guillermo A.; Robertson, Paul; Shields, Gregory A.; Blanc, Guillermo A.
We study the influence of cluster environment on the chemical evolution of spiral galaxies in the Pegasus I cluster. We determine the gas-phase heavy element abundances of six galaxies in Pegasus derived from H II region spectra obtained from integral-field spectroscopy. These abundances are analyzed in the context of Virgo, whose spirals are known to show increasing interstellar metallicity as a function of H I deficiency. The galaxies in the Pegasus cluster, despite its lower density and velocity dispersion, also display gas loss due to interstellar-medium-intracluster-medium interaction, albeit to a lesser degree. Based on the abundances of three H I deficient spirals and two H I normal spirals, we observe a heavy element abundance offset of +0.13 +/- 0.07 dex for the H I deficient galaxies. This abundance differential is consistent with the differential observed in Virgo for galaxies with a similar H I deficiency, and we observe a correlation between log(O/H) and the H I deficiency parameter DEF for the two clusters analyzed together. Our results suggest that similar environmental mechanisms are driving the heavy element enhancement in both clusters.
Multi-Element Abundance Measurements From Medium-Resolution Spectra. I. The Sculptor Dwarf Spheroidal Galaxy
(2009-11) Kirby, Evan N.; Guhathakurta, Puragra; Bolte, Michael; Sneden, Christopher; Geha, Marla C.; Sneden, Christopher
We present measurements of Fe, Mg, Si, Ca, and Ti abundances for 388 radial velocity member stars in the Sculptor dwarf spheroidal galaxy (dSph), a satellite of the Milky Way (MW). This is the largest sample of individual a element (Mg, Si, Ca, and Ti) abundance measurements in any single dSph. The measurements are made from Keck/Deep Imaging Multi-Object Spectrometer medium-resolution spectra (6400-9000 angstrom, R similar to 6500). Based on comparisons to published high-resolution (R greater than or similar to 20,000) spectroscopic measurements, our measurements have uncertainties of sigma[Fe/H] = 0.14 and sigma[alpha/Fe] = 0.13. The Sculptor [Fe/H] distribution has a mean <[Fe/H]> = -1.58 and is asymmetric with a long, metal-poor tail, indicative of a history of extended star formation. Sculptor has a larger fraction of stars with [Fe/H] < -2 than the MW halo. We have discovered one star with [Fe/H] = -3.80 +/- 0.28, which is the most metal-poor star known anywhere except the MW halo, but high-resolution spectroscopy is needed to measure this star's detailed abundances. As has been previously reported based on high-resolution spectroscopy, [alpha/Fe] in Sculptor falls as [Fe/H] increases. The metal-rich stars ([Fe/H] similar to -1.5) have lower [alpha/Fe] than Galactic halo field stars of comparable metallicity. This indicates that star formation proceeded more gradually in Sculptor than in the Galactic halo. We also observe radial abundance gradients of -0.030 +/- 0.003 dex arcmin(-1) in [Fe/H] and +0.013 +/- 0.003 dex arcmin(-1) in [alpha/Fe] out to 11 arcmin (275 pc). Together, these measurements cast Sculptor and possibly other surviving dSphs as representative of the dwarf galaxies from which the metal-poor tail of the Galactic halo formed.
New H-Band Stellar Spectral Libraries For The SDSS-III/APOGEE Survey
(2015-06) Zamora, O.; Garcia-Hernandez, D. A.; Prieto, C. A.; Carrera, R.; Koesterke, L.; Edvardsson, B.; Castelli, F.; Plez, B.; Bizyaev, D.; Cunha, K.; Perez, A. E. G.; Gustafsson, B.; Holtzman, J. A.; Lawler, J. E.; Majewski, S. R.; Manchado, A.; Meszaros, S.; Shane, N.; Shetrone, M.; Smith, V. V.; Zasowski, G.; Koesterke, L.
The Sloan Digital Sky Survey-III (SDSS-III) Apache Point Observatory Galactic Evolution Experiment (APOGEE) has obtained high-resolution (R similar to 22,500), high signal-to-noise ratio (>100) spectra in the H-band (similar to 1.5-1.7 mu m) for about 146,000 stars in the Milky Way galaxy. We have computed spectral libraries with effective temperature (T-eff) ranging from 3500 to 8000 K for the automated chemical analysis of the survey data. The libraries, used to derive stellar parameters and abundances from the APOGEE spectra in the SDSS-III data release 12 (DR12), are based on ATLAS9 model atmospheres and the ASS epsilon T spectral synthesis code. We present a second set of libraries based on MARCS model atmospheres and the spectral synthesis code Turbospectrum. The ATLAS9/ASS epsilon T (T-eff = 3500-8000 K) and MARCS/Turbospectrum (T-eff = 3500-5500 K) grids cover a wide range of metallicity (-2.5 <= [M/H] <= + 0.5 dex), surface gravity (0 <= log g <= 5 dex), microturbulence (0.5 <= xi <= 8 km s(-1)), carbon (-1 <= [C/M] <= + 1 dex), nitrogen (-1 <= [N/M] <= + 1 dex), and alpha-element (-1 <= [alpha/M] <= + 1 dex) variations, having thus seven dimensions. We compare the ATLAS9/ASS.T and MARCS/Turbospectrum libraries and apply both of them to the analysis of the observed H-band spectra of the Sun and the K2 giant Arcturus, as well as to a selected sample of well-known giant stars observed at very high resolution. The new APOGEE libraries are publicly available and can be employed for chemical studies in the H-band using other high-resolution spectrographs.
Oxygen Abundances In Low- And High-Alpha Field Halo Stars And The Discovery Of Two Field Stars Born In Globular Clusters
(2012-10) Ramirez, Ivan; Melendez, Jorge; Chaname, J.; Ramirez, Ivan
Oxygen abundances of 67 dwarf stars in the metallicity range -1.6 < [Fe/H] < -0.4 are derived from a non-LTE analysis of the 777 nm O I triplet lines. These stars have precise atmospheric parameters measured by Nissen and Schuster, who find that they separate into three groups based on their kinematics and alpha-element (Mg, Si, Ca, Ti) abundances: thick disk, high-alpha halo, and low-alpha halo. We find the oxygen abundance trends of thick-disk and high-alpha halo stars very similar. The low-alpha stars show a larger star-to-star scatter in [O/Fe] at a given [Fe/H] and have systematically lower oxygen abundances compared to the other two groups. Thus, we find the behavior of oxygen abundances in these groups of stars similar to that of the a elements. We use previously published oxygen abundance data of disk and very metal-poor halo stars to present an overall view (-2.3 < [Fe/H] < +0.3) of oxygen abundance trends of stars in the solar neighborhood. Two field halo dwarf stars stand out in their O and Na abundances. Both G53-41 and G150-40 have very low oxygen and very high sodium abundances, which are key signatures of the abundance anomalies observed in globular cluster (GC) stars. Therefore, they are likely field halo stars born in GCs. If true, we estimate that at least 3% +/- 2% of the local field metal-poor star population was born in GCs.
the SDSS-III APOGEE Spectral Line List for H-Band Spectroscopy
(2015-12) Shetrone, Matthew; Bizyaev, D.; Lawler, James E.; Prieto, Carlos Allende; Johnson, J. A.; Smith, V. V.; Cunha, K.; Holtzman, J.; Perez, A. E. G.; Meszaros, S.; Sobeck, J.; Zamora, O.; Garcia-Hernandez, D. A.; Souto, D.; Chojnowski, D.; Koesterke, L.; Majewski, S.; Zasowski, G.; Shetrone, Matthew
We present the H-band spectral line lists adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). The APOGEE line lists comprise astrophysical, theoretical, and laboratory sources from the literature, as well as newly evaluated astrophysical oscillator strengths and damping parameters. We discuss the construction of the APOGEE line list, which is one of the critical inputs for the APOGEE Stellar Parameters and Chemical Abundances Pipeline, and present three different versions that have been used at various stages of the project. The methodology for the newly calculated astrophysical line lists is reviewed. The largest of these three line lists contains 134,457 molecular and atomic transitions. In addition to the format adopted to store the data, the line lists are available in MOOG, Synspec, and Turbospectrum formats. The limitations of the line lists along with guidance for its use on different spectral types are discussed. We also present a list of H-band spectral features that are either poorly represented or completely missing in our line list. This list is based on the average of a large number of spectral fit residuals for APOGEE observations spanning a wide range of stellar parameters.
Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultrafaint Dwarf Spheroidal Satellite Galaxies
(2008-09) Kirby, Evan N.; Simon, Joshua D.; Geha, Marla; Guhathakurta, Puragra; Frebel, Anna; Frebel, Anna
We present new metallicity measurements for 298 individual red giant branch stars in eight of the least luminous dwarf spheroidal galaxies (dSphs) in the Milky Way ( MW) system. Our technique is based on medium-resolution Keck DEIMOS spectroscopy coupled with spectral synthesis. We present the first spectroscopic metallicities at of stars in a dwarf galaxy, with individual stellar metallicities as low as. Because [Fe/H] < -3.0 [Fe/H] = -3.3 our [Fe/H] measurements are not tied to empirical metallicity calibrators and are sensitive to arbitrarily low metallicities, we are able to probe this extremely metal-poor regime accurately. The metallicity distribution of stars in these dSphs is similar to the MW halo at the metal-poor end. We also demonstrate that the luminosity-metallicity relation previously seen in more luminous dSph galaxies (M(v) = -13.4 to -8.8) extends smoothly down to an absolute magnitude of M(v) = -3.7. The discovery of extremely metal-poor stars in dSphs lends support to the Lambda CDM galaxy assembly paradigm wherein dwarf galaxies dissolve to form the stellar halo of the MW.
An Upper Limit On The Sulphur Abundance In HE 1327-2326
(2012-08) Bonifacio, P.; Caffau, E.; Venn, K. A.; Lambert, D. L.; Lambert, D. L.
Context. Star HE 1327-2326 is a unique object, with the lowest measured iron abundance ([Fe/H] similar to -6) and a peculiar chemical composition that includes large overabundances of C, N, and O with respect to iron. One important question is whether the chemical abundances in this star reflect the chemical composition of the gas cloud from which it was formed or if they have been severely affected by other processes, such as dust-gas winnowing. Aims. We measure or provide an upper limit to the abundance of the volatile element sulphur, which can help to discriminate between the two scenarios. Methods. We observed HE 1327-2326 with the high resolution infra-red spectrograph CRIRES at the VLT to observe the S i lines of Multiplet 3 at 1045 nm. Results. We do not detect the S i line. A 3 sigma upper limit on the equivalent width (EW) of any line in our spectrum is EW < 0.66 pm. Using either one-dimensional static or three-dimensional hydrodynamical model-atmospheres, this translates into a robust upper limit of [S/H] < -2.6. Conclusions. This upper limit does not provide conclusive evidence for or against dust-gas winnowing, and the evidence coming from other elements (e. g., Na and Ti) is also inconclusive or contradictory. The formation of dust in the atmosphere versus an origin of the metals in a metal-poor supernova with extensive >fall-back> are not mutually exclusive. It is possible that dust formation distorts the peculiar abundance pattern created by a supernova with fall-back, thus the abundance ratios in HE 1327-2326 may be used to constrain the properties of the supernova(e) that produced its metals, but with some caution.