Deep SDSS optical spectroscopy of distant halo stars

II. Iron, calcium, and magnesium abundances

E. Fernández-Alvar, C. Allende Prieto, K. J. Schlesinger, T. C. Beers, A. C. Robin, Donald P. Schneider, Y. S. Lee, D. Bizyaev, G. Ebelke, E. Malanushenko, V. Malanushenko, D. Oravetz, K. Pan, A. Simmons

Research output: Contribution to journalArticle

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Abstract

Aims. We analyze a sample of 3944 low-resolution (R ∼ 2000) optical spectra from the Sloan Digital Sky Survey (SDSS), focusing on stars with effective temperatures 5800 ≤ Teff ≤ 6300 K, and distances from the Milky Way plane in excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. Methods. We followed the same methodology as in the previous paper in this series, deriving atmospheric parameters by X2 minimization, but this time we obtained the abundances of individual elements by fitting their associated spectral lines. Distances were calculated from absolute magnitudes obtained by a statistical comparison of our stellar parameters with stellar-evolution models. Results. The observations reveal a decrease in the abundances of iron, calcium, and magnesium at large distances from the Galactic center. The median abundances for the halo stars analyzed are fairly constant up to a Galactocentric distance r ∼ 20 kpc, rapidly decrease between r ∼ 20 and r ∼ 40 kpc, and flatten out to significantly lower values at larger distances, consistent with previous studies. In addition, we examine [Ca/Fe] and [Mg/Fe] as a function of [Fe/H] and Galactocentric distance. Our results show that the most distant parts of the halo show a steeper variation of [Ca/Fe] and [Mg/Fe] with iron. We found that at the range <1.6 < [Fe/H] < -0.4, [Ca/Fe] decreases with distance, in agreement with earlier results based on local stars. However, the opposite trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the halo are more metal-poor than the inner regions, based on in situ observations of distant stars, agrees with recent results based on inferences from the kinematics of more local stars, and with predictions of recent galaxy formation simulations for galaxies similar to the Milky Way.

Original languageEnglish (US)
Article numberA81
JournalAstronomy and Astrophysics
Volume577
DOIs
StatePublished - May 1 2015

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magnesium
calcium
halos
spectroscopy
iron
stars
kinematics
stellar evolution
galactic evolution
methodology
inference
metal
prediction
line spectra
optical spectrum
simulation
galaxies
temperature
trends
optimization

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Fernández-Alvar, E., Allende Prieto, C., Schlesinger, K. J., Beers, T. C., Robin, A. C., Schneider, D. P., ... Simmons, A. (2015). Deep SDSS optical spectroscopy of distant halo stars: II. Iron, calcium, and magnesium abundances. Astronomy and Astrophysics, 577, [A81]. https://doi.org/10.1051/0004-6361/201425455
Fernández-Alvar, E. ; Allende Prieto, C. ; Schlesinger, K. J. ; Beers, T. C. ; Robin, A. C. ; Schneider, Donald P. ; Lee, Y. S. ; Bizyaev, D. ; Ebelke, G. ; Malanushenko, E. ; Malanushenko, V. ; Oravetz, D. ; Pan, K. ; Simmons, A. / Deep SDSS optical spectroscopy of distant halo stars : II. Iron, calcium, and magnesium abundances. In: Astronomy and Astrophysics. 2015 ; Vol. 577.
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abstract = "Aims. We analyze a sample of 3944 low-resolution (R ∼ 2000) optical spectra from the Sloan Digital Sky Survey (SDSS), focusing on stars with effective temperatures 5800 ≤ Teff ≤ 6300 K, and distances from the Milky Way plane in excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. Methods. We followed the same methodology as in the previous paper in this series, deriving atmospheric parameters by X2 minimization, but this time we obtained the abundances of individual elements by fitting their associated spectral lines. Distances were calculated from absolute magnitudes obtained by a statistical comparison of our stellar parameters with stellar-evolution models. Results. The observations reveal a decrease in the abundances of iron, calcium, and magnesium at large distances from the Galactic center. The median abundances for the halo stars analyzed are fairly constant up to a Galactocentric distance r ∼ 20 kpc, rapidly decrease between r ∼ 20 and r ∼ 40 kpc, and flatten out to significantly lower values at larger distances, consistent with previous studies. In addition, we examine [Ca/Fe] and [Mg/Fe] as a function of [Fe/H] and Galactocentric distance. Our results show that the most distant parts of the halo show a steeper variation of [Ca/Fe] and [Mg/Fe] with iron. We found that at the range <1.6 < [Fe/H] < -0.4, [Ca/Fe] decreases with distance, in agreement with earlier results based on local stars. However, the opposite trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the halo are more metal-poor than the inner regions, based on in situ observations of distant stars, agrees with recent results based on inferences from the kinematics of more local stars, and with predictions of recent galaxy formation simulations for galaxies similar to the Milky Way.",
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Fernández-Alvar, E, Allende Prieto, C, Schlesinger, KJ, Beers, TC, Robin, AC, Schneider, DP, Lee, YS, Bizyaev, D, Ebelke, G, Malanushenko, E, Malanushenko, V, Oravetz, D, Pan, K & Simmons, A 2015, 'Deep SDSS optical spectroscopy of distant halo stars: II. Iron, calcium, and magnesium abundances', Astronomy and Astrophysics, vol. 577, A81. https://doi.org/10.1051/0004-6361/201425455

Deep SDSS optical spectroscopy of distant halo stars : II. Iron, calcium, and magnesium abundances. / Fernández-Alvar, E.; Allende Prieto, C.; Schlesinger, K. J.; Beers, T. C.; Robin, A. C.; Schneider, Donald P.; Lee, Y. S.; Bizyaev, D.; Ebelke, G.; Malanushenko, E.; Malanushenko, V.; Oravetz, D.; Pan, K.; Simmons, A.

In: Astronomy and Astrophysics, Vol. 577, A81, 01.05.2015.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Deep SDSS optical spectroscopy of distant halo stars

T2 - II. Iron, calcium, and magnesium abundances

AU - Fernández-Alvar, E.

AU - Allende Prieto, C.

AU - Schlesinger, K. J.

AU - Beers, T. C.

AU - Robin, A. C.

AU - Schneider, Donald P.

AU - Lee, Y. S.

AU - Bizyaev, D.

AU - Ebelke, G.

AU - Malanushenko, E.

AU - Malanushenko, V.

AU - Oravetz, D.

AU - Pan, K.

AU - Simmons, A.

PY - 2015/5/1

Y1 - 2015/5/1

N2 - Aims. We analyze a sample of 3944 low-resolution (R ∼ 2000) optical spectra from the Sloan Digital Sky Survey (SDSS), focusing on stars with effective temperatures 5800 ≤ Teff ≤ 6300 K, and distances from the Milky Way plane in excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. Methods. We followed the same methodology as in the previous paper in this series, deriving atmospheric parameters by X2 minimization, but this time we obtained the abundances of individual elements by fitting their associated spectral lines. Distances were calculated from absolute magnitudes obtained by a statistical comparison of our stellar parameters with stellar-evolution models. Results. The observations reveal a decrease in the abundances of iron, calcium, and magnesium at large distances from the Galactic center. The median abundances for the halo stars analyzed are fairly constant up to a Galactocentric distance r ∼ 20 kpc, rapidly decrease between r ∼ 20 and r ∼ 40 kpc, and flatten out to significantly lower values at larger distances, consistent with previous studies. In addition, we examine [Ca/Fe] and [Mg/Fe] as a function of [Fe/H] and Galactocentric distance. Our results show that the most distant parts of the halo show a steeper variation of [Ca/Fe] and [Mg/Fe] with iron. We found that at the range <1.6 < [Fe/H] < -0.4, [Ca/Fe] decreases with distance, in agreement with earlier results based on local stars. However, the opposite trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the halo are more metal-poor than the inner regions, based on in situ observations of distant stars, agrees with recent results based on inferences from the kinematics of more local stars, and with predictions of recent galaxy formation simulations for galaxies similar to the Milky Way.

AB - Aims. We analyze a sample of 3944 low-resolution (R ∼ 2000) optical spectra from the Sloan Digital Sky Survey (SDSS), focusing on stars with effective temperatures 5800 ≤ Teff ≤ 6300 K, and distances from the Milky Way plane in excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. Methods. We followed the same methodology as in the previous paper in this series, deriving atmospheric parameters by X2 minimization, but this time we obtained the abundances of individual elements by fitting their associated spectral lines. Distances were calculated from absolute magnitudes obtained by a statistical comparison of our stellar parameters with stellar-evolution models. Results. The observations reveal a decrease in the abundances of iron, calcium, and magnesium at large distances from the Galactic center. The median abundances for the halo stars analyzed are fairly constant up to a Galactocentric distance r ∼ 20 kpc, rapidly decrease between r ∼ 20 and r ∼ 40 kpc, and flatten out to significantly lower values at larger distances, consistent with previous studies. In addition, we examine [Ca/Fe] and [Mg/Fe] as a function of [Fe/H] and Galactocentric distance. Our results show that the most distant parts of the halo show a steeper variation of [Ca/Fe] and [Mg/Fe] with iron. We found that at the range <1.6 < [Fe/H] < -0.4, [Ca/Fe] decreases with distance, in agreement with earlier results based on local stars. However, the opposite trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the halo are more metal-poor than the inner regions, based on in situ observations of distant stars, agrees with recent results based on inferences from the kinematics of more local stars, and with predictions of recent galaxy formation simulations for galaxies similar to the Milky Way.

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