$R$-process enhancements of Gaia-Enceladus in GALAH DR3. (arXiv:2101.07791v1 [astro-ph.GA])

$R$-process enhancements of Gaia-Enceladus in GALAH DR3. (arXiv:2101.07791v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Matsuno_T/0/1/0/all/0/1">Tadafumi Matsuno</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hirai_Y/0/1/0/all/0/1">Yutaka Hirai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tarumi_Y/0/1/0/all/0/1">Yuta Tarumi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hotokezaka_K/0/1/0/all/0/1">Kenta Hotokezaka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tanaka_M/0/1/0/all/0/1">Masaomi Tanaka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Helmi_A/0/1/0/all/0/1">Amina Helmi</a>

The dominant site of production of $r$-process elements remains unclear
despite recent observations of a neutron star merger. Observational constraints
on the properties of the sites can be obtained by comparing $r$-process
abundances in different environments. The recent Gaia data releases and large
samples from high-resolution optical spectroscopic surveys are enabling us to
compare $r$-process element abundances between stars formed in an accreted
dwarf galaxy, Gaia-Enceladus, and those formed in the Milky Way. We aim to
understand the origin of $r$-process elements in Gaia-Enceladus. We first
construct a sample of stars to study Eu abundances without being affected by
the detection limit. We then kinematically select 71 Gaia-Enceladus stars and
93 in-situ stars from the Galactic Archaeology with HERMES (GALAH) DR3, of
which 50 and 75 stars can be used to study Eu reliably. Gaia-Enceladus stars
clearly show higher ratios of [{Eu}/{Mg}] than in-situ stars. High [{Eu}/{Mg}]
along with low [{Mg}/{Fe}] are also seen in relatively massive satellite
galaxies such as the LMC, Fornax, and Sagittarius dwarfs. On the other hand,
unlike these galaxies, Gaia-Enceladus does not show enhanced [{Ba}/{Eu}] or
[{La}/{Eu}] ratios suggesting a lack of significant $s$-process contribution.
From comparisons with simple chemical evolution models, we show that the high
[{Eu}/{Mg}] of Gaia-Enceladus can naturally be explained by considering
$r$-process enrichment by neutron-star mergers with delay time distribution
that follows a similar power-law as type~Ia supernovae but with a shorter
minimum delay time.

The dominant site of production of $r$-process elements remains unclear
despite recent observations of a neutron star merger. Observational constraints
on the properties of the sites can be obtained by comparing $r$-process
abundances in different environments. The recent Gaia data releases and large
samples from high-resolution optical spectroscopic surveys are enabling us to
compare $r$-process element abundances between stars formed in an accreted
dwarf galaxy, Gaia-Enceladus, and those formed in the Milky Way. We aim to
understand the origin of $r$-process elements in Gaia-Enceladus. We first
construct a sample of stars to study Eu abundances without being affected by
the detection limit. We then kinematically select 71 Gaia-Enceladus stars and
93 in-situ stars from the Galactic Archaeology with HERMES (GALAH) DR3, of
which 50 and 75 stars can be used to study Eu reliably. Gaia-Enceladus stars
clearly show higher ratios of [{Eu}/{Mg}] than in-situ stars. High [{Eu}/{Mg}]
along with low [{Mg}/{Fe}] are also seen in relatively massive satellite
galaxies such as the LMC, Fornax, and Sagittarius dwarfs. On the other hand,
unlike these galaxies, Gaia-Enceladus does not show enhanced [{Ba}/{Eu}] or
[{La}/{Eu}] ratios suggesting a lack of significant $s$-process contribution.
From comparisons with simple chemical evolution models, we show that the high
[{Eu}/{Mg}] of Gaia-Enceladus can naturally be explained by considering
$r$-process enrichment by neutron-star mergers with delay time distribution
that follows a similar power-law as type~Ia supernovae but with a shorter
minimum delay time.

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