Carbon and oxygen in metal-poor halo stars. (arXiv:1901.03592v1 [astro-ph.SR])

Carbon and oxygen in metal-poor halo stars. (arXiv:1901.03592v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Amarsi_A/0/1/0/all/0/1">A. M. Amarsi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nissen_P/0/1/0/all/0/1">P. E. Nissen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Asplund_M/0/1/0/all/0/1">M. Asplund</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lind_K/0/1/0/all/0/1">K. Lind</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Barklem_P/0/1/0/all/0/1">P. S. Barklem</a>

Carbon and oxygen are key tracers of the Galactic chemical evolution; in
particular, a reported upturn in [C/O] towards decreasing [O/H] in metal-poor
halo stars could be a signature of nucleosynthesis by massive Population III
stars. We reanalyse carbon, oxygen, and iron abundances in thirty-nine
metal-poor turn-off stars. For the first time, we take into account
three-dimensional (3D) hydrodynamic effects together with departures from local
thermodynamic equilibrium (LTE) when determining both the stellar parameters
and the elemental abundances, by deriving effective temperatures from 3D
non-LTE H$beta$ profiles, surface gravities from Gaia parallaxes, iron
abundances from 3D LTE Feii equivalent widths, and carbon and oxygen abundances
from 3D non-LTE Ci and Oi equivalent widths. We find that [C/Fe] stays flat
with [Fe/H], whereas [O/Fe] increases linearly up to $0.75$ dex with decreasing
[Fe/H] down to $-3.0$ dex. As such [C/O] monotonically decreases towards
decreasing [O/H], in contrast to previous findings, mainly by virtue of less
severe non-LTE effects for Oi at low [Fe/H] with our improved calculations.

Carbon and oxygen are key tracers of the Galactic chemical evolution; in
particular, a reported upturn in [C/O] towards decreasing [O/H] in metal-poor
halo stars could be a signature of nucleosynthesis by massive Population III
stars. We reanalyse carbon, oxygen, and iron abundances in thirty-nine
metal-poor turn-off stars. For the first time, we take into account
three-dimensional (3D) hydrodynamic effects together with departures from local
thermodynamic equilibrium (LTE) when determining both the stellar parameters
and the elemental abundances, by deriving effective temperatures from 3D
non-LTE H$beta$ profiles, surface gravities from Gaia parallaxes, iron
abundances from 3D LTE Feii equivalent widths, and carbon and oxygen abundances
from 3D non-LTE Ci and Oi equivalent widths. We find that [C/Fe] stays flat
with [Fe/H], whereas [O/Fe] increases linearly up to $0.75$ dex with decreasing
[Fe/H] down to $-3.0$ dex. As such [C/O] monotonically decreases towards
decreasing [O/H], in contrast to previous findings, mainly by virtue of less
severe non-LTE effects for Oi at low [Fe/H] with our improved calculations.

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