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Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

Koch, A.; Feltzing, Sofia LU ; Adén, Daniel LU and Matteucci, F. (2013) In Astronomy & Astrophysics 554.
Abstract
We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 angstrom absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log epsilon(Ba) less than or similar to 0.7 dex (3 sigma). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 angstrom-line, but most of those... (More)
We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 angstrom absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log epsilon(Ba) less than or similar to 0.7 dex (3 sigma). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 angstrom-line, but most of those would only lead to even lower abundances. A better match with metal-poor halo and dSph stars can only be reached by including a large uncertainty in the continuum placement. This contrasts with the high dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the latter spreads are typical of the very low luminosity, dark-matter dominated dSphs, a high level of depletion in heavy elements suggests that chemical enrichment in Hercules was governed by very massive stars, coupled with a very low star formation efficiency. While very low abundances of some heavy elements are also found in individual stars of other dwarf galaxies, this is the first time that a very low Ba abundance is found within an entire dSph over a broad metallicity range. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
galaxies: evolution, abundances, nucleosynthesis, nuclear reactions, stars: abundances, stars: Population II, galaxies: dwarf, galaxies: individual: Hercules
in
Astronomy & Astrophysics
volume
554
publisher
EDP Sciences
external identifiers
  • wos:000320444200005
  • scopus:84878310426
ISSN
0004-6361
DOI
10.1051/0004-6361/201220742
language
English
LU publication?
yes
id
524d2da5-bb2f-473e-a5e8-dd490e7aefba (old id 3979130)
date added to LUP
2013-08-23 12:26:31
date last changed
2019-01-27 04:28:16
@article{524d2da5-bb2f-473e-a5e8-dd490e7aefba,
  abstract     = {We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 angstrom absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log epsilon(Ba) less than or similar to 0.7 dex (3 sigma). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 angstrom-line, but most of those would only lead to even lower abundances. A better match with metal-poor halo and dSph stars can only be reached by including a large uncertainty in the continuum placement. This contrasts with the high dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the latter spreads are typical of the very low luminosity, dark-matter dominated dSphs, a high level of depletion in heavy elements suggests that chemical enrichment in Hercules was governed by very massive stars, coupled with a very low star formation efficiency. While very low abundances of some heavy elements are also found in individual stars of other dwarf galaxies, this is the first time that a very low Ba abundance is found within an entire dSph over a broad metallicity range.},
  articleno    = {A5},
  author       = {Koch, A. and Feltzing, Sofia and Adén, Daniel and Matteucci, F.},
  issn         = {0004-6361},
  keyword      = {galaxies: evolution,abundances,nucleosynthesis,nuclear reactions,stars: abundances,stars: Population II,galaxies: dwarf,galaxies: individual: Hercules},
  language     = {eng},
  publisher    = {EDP Sciences},
  series       = {Astronomy & Astrophysics},
  title        = {Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy},
  url          = {http://dx.doi.org/10.1051/0004-6361/201220742},
  volume       = {554},
  year         = {2013},
}