The Abundance Pattern of $alpha$ elements in the Triangulum-Andromeda Overdensity. (arXiv:2007.15764v1 [astro-ph.GA])

The Abundance Pattern of $alpha$ elements in the Triangulum-Andromeda Overdensity. (arXiv:2007.15764v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Silva_J/0/1/0/all/0/1">J. V. Sales Silva</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cunha_K/0/1/0/all/0/1">K. Cunha</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Perottoni_H/0/1/0/all/0/1">H. D. Perottoni</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rocha_Pinto_H/0/1/0/all/0/1">H. J. Rocha-Pinto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Daflon_S/0/1/0/all/0/1">S. Daflon</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Almeida_Fernandes_F/0/1/0/all/0/1">F. Almeida-Fernandes</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Souto_D/0/1/0/all/0/1">Diogo Souto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Majewski_S/0/1/0/all/0/1">S. R. Majewski</a>

The close relationship between the nature of the Triangulum-Andromeda
(TriAnd) overdensity and the Galactic disk has become increasingly evident in
recent years. However, the chemical pattern of this overdensity (R$_{GC}$ = 20
– 30 kpc) is unique and differs from what we know of the local disk. In this
study, we analyze the chemical abundances of five $alpha$ elements (Mg, O, Si,
Ca, and Ti) in a sample of stars belonging to the TriAnd overdensity, including
stars with [Fe/H] $<$ $-$1.2, to investigate the evolution of the $alpha$
elements with metallicity. High-resolution spectra from Gemini North with
GRACES were analyzed. Overall, the TriAnd population presents an
$alpha$-element pattern that differs from that of the local disk; the TriAnd
stars fall in between the local disk and the dwarf galaxies in the [X/Fe] vs.
[Fe/H] plane. The high [Mg/Fe] ratios obtained for the lower metallicity TriAnd
stars may indicate a roughly parallel sequence to the Milky Way local disk at
lower values of [Fe/H], revealing a ‘knee’ shifted towards lower metallicities
for the TriAnd population. Similar behavior is also exhibited in the [Ca/Fe]
and [Si/Fe] ratios. However, for O and Ti the behavior of the [X/Fe] ratios
shows a slight decay with decreasing metallicity. Our results reinforce the
TriAnd overdensity as a unique stellar population of the Milky Way, with an
abundance pattern that is different from all stellar populations studied to
date. The complete understanding of the complex TriAnd population will require
high-resolution spectroscopic observations of a larger sample of TriAnd stars.

The close relationship between the nature of the Triangulum-Andromeda
(TriAnd) overdensity and the Galactic disk has become increasingly evident in
recent years. However, the chemical pattern of this overdensity (R$_{GC}$ = 20
– 30 kpc) is unique and differs from what we know of the local disk. In this
study, we analyze the chemical abundances of five $alpha$ elements (Mg, O, Si,
Ca, and Ti) in a sample of stars belonging to the TriAnd overdensity, including
stars with [Fe/H] $<$ $-$1.2, to investigate the evolution of the $alpha$
elements with metallicity. High-resolution spectra from Gemini North with
GRACES were analyzed. Overall, the TriAnd population presents an
$alpha$-element pattern that differs from that of the local disk; the TriAnd
stars fall in between the local disk and the dwarf galaxies in the [X/Fe] vs.
[Fe/H] plane. The high [Mg/Fe] ratios obtained for the lower metallicity TriAnd
stars may indicate a roughly parallel sequence to the Milky Way local disk at
lower values of [Fe/H], revealing a ‘knee’ shifted towards lower metallicities
for the TriAnd population. Similar behavior is also exhibited in the [Ca/Fe]
and [Si/Fe] ratios. However, for O and Ti the behavior of the [X/Fe] ratios
shows a slight decay with decreasing metallicity. Our results reinforce the
TriAnd overdensity as a unique stellar population of the Milky Way, with an
abundance pattern that is different from all stellar populations studied to
date. The complete understanding of the complex TriAnd population will require
high-resolution spectroscopic observations of a larger sample of TriAnd stars.

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