Chemo-dynamics of outer halo dwarf stars, including textit{Gaia}-Sausage and textit{Gaia}-Sequoia candidates. (arXiv:1909.11969v3 [astro-ph.GA] UPDATED)

Chemo-dynamics of outer halo dwarf stars, including textit{Gaia}-Sausage and textit{Gaia}-Sequoia candidates. (arXiv:1909.11969v3 [astro-ph.GA] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Monty_S/0/1/0/all/0/1">Stephanie Monty</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Venn_K/0/1/0/all/0/1">Kim A. Venn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lane_J/0/1/0/all/0/1">James M. M. Lane</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lokhorst_D/0/1/0/all/0/1">Deborah Lokhorst</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yong_D/0/1/0/all/0/1">David Yong</a>

The low-metallicity, kinematically interesting dwarf stars studied by
Stephens & Boesgaard (2002, SB02) are re-examined using Gaia DR2 astrometry,
and updated model atmospheres and atomic line data. New stellar parameters are
determined based on the Gaia DR2 parallactic distances and Dartmouth Stellar
Evolution Database isochrones. These are in excellent agreement with
spectroscopically determined stellar parameters for stars with [Fe/H]$>-2$;
however, large disagreements are found for stars with [Fe/H]$le-2$, with
offsets as large as $Delta$T$_{rm eff}sim+500$ K and
$Delta$log,$gsim+1.0$. A subset of six stars (test cases) are analysed ab
initio using high resolution spectra with Keck HIRES and Gemini GRACES. This
sub-sample is found to include two $alpha$-challenged dwarf stars, suggestive
of origins in a low mass, accreted dwarf galaxy. The orbital parameters for the
entire SB02 sample are re-determined using textit{Gaia} DR2 data. We find 11
stars that are dynamically coincident with the textit{Gaia}-Sausage accretion
event and another 17 with the textit{Gaia}-Sequoia event in action space. Both
associations include low-mass, metal-poor stars with isochrone ages older than
10 Gyr. Two dynamical subsets are identified within textit{Gaia}-Sequoia. When
these subsets are examined separately, a common knee in [$alpha$/Fe] is found
for the textit{Gaia}-Sausage and low orbital energy textit{Gaia}-Sequoia
stars. A lower metallicity knee is tentatively identified in the
textit{Gaia}-Sequoia high orbital energy stars. If the metal-poor dwarf stars
in these samples are true members of the textit{Gaia}-Sausage and
textit{Gaia}-Sequoia events, then they present a unique opportunity to probe
the earlier, more pristine, star formation histories of these systems.

The low-metallicity, kinematically interesting dwarf stars studied by
Stephens & Boesgaard (2002, SB02) are re-examined using Gaia DR2 astrometry,
and updated model atmospheres and atomic line data. New stellar parameters are
determined based on the Gaia DR2 parallactic distances and Dartmouth Stellar
Evolution Database isochrones. These are in excellent agreement with
spectroscopically determined stellar parameters for stars with [Fe/H]$>-2$;
however, large disagreements are found for stars with [Fe/H]$le-2$, with
offsets as large as $Delta$T$_{rm eff}sim+500$ K and
$Delta$log,$gsim+1.0$. A subset of six stars (test cases) are analysed ab
initio using high resolution spectra with Keck HIRES and Gemini GRACES. This
sub-sample is found to include two $alpha$-challenged dwarf stars, suggestive
of origins in a low mass, accreted dwarf galaxy. The orbital parameters for the
entire SB02 sample are re-determined using textit{Gaia} DR2 data. We find 11
stars that are dynamically coincident with the textit{Gaia}-Sausage accretion
event and another 17 with the textit{Gaia}-Sequoia event in action space. Both
associations include low-mass, metal-poor stars with isochrone ages older than
10 Gyr. Two dynamical subsets are identified within textit{Gaia}-Sequoia. When
these subsets are examined separately, a common knee in [$alpha$/Fe] is found
for the textit{Gaia}-Sausage and low orbital energy textit{Gaia}-Sequoia
stars. A lower metallicity knee is tentatively identified in the
textit{Gaia}-Sequoia high orbital energy stars. If the metal-poor dwarf stars
in these samples are true members of the textit{Gaia}-Sausage and
textit{Gaia}-Sequoia events, then they present a unique opportunity to probe
the earlier, more pristine, star formation histories of these systems.

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