Ages and kinematics of chemically selected, accreted Milky Way halo stars. (arXiv:1903.09320v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Das_P/0/1/0/all/0/1">Payel Das</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hawkins_K/0/1/0/all/0/1">Keith Hawkins</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jofre_P/0/1/0/all/0/1">Paula Jofre</a>
We exploit the [Mg/Mn]-[Al/Fe] chemical abundance plane to help identify
nearby halo stars in the 14th data release from the APOGEE survey that have
been accreted on to the Milky Way. Applying a Gaussian Mixture Model, we find a
`blob’ of 856 likely accreted stars, with a low disc contamination rate of ~7%.
Cross-matching the sample with the second data release from Gaia gives us
access to parallaxes and apparent magnitudes, which place constraints on
distances and intrinsic luminosities. Using a Bayesian isochrone pipeline, this
enables us to estimate new ages for the accreted stars, with typical
uncertainties of ~20%. Our new catalogue is further supplemented with estimates
of orbital parameters.
The blob stars span a metallicities between -0.5 to -2.5, and [Mg/Fe] between
-0.1 to 0.5. They constitute ~30% of the metal-poor ([Fe/H] < -0.8) halo at
metallicities of ~-1.4. Our new ages are mainly range between 8 to 13 Gyr, with
the oldest stars the metal-poorest, and with the highest [Mg/Fe] abundance. If
the blob stars are assumed to belong to a single progenitor, the ages imply
that the system merged with our Milky Way around 8 Gyr ago and that star
formation proceeded for ~5 Gyr. Dynamical arguments suggest that such a single
progenitor would have a total mass of ~1011Msun, similar to that found by other
authors using chemical evolution models and simulations. Comparing the scatter
in the [Mg/Fe]-[Fe/H] plane of the blob stars to that measured for stars
belonging to the Large Magellanic Cloud suggests that the blob does indeed
contain stars from only one progenitor.
We exploit the [Mg/Mn]-[Al/Fe] chemical abundance plane to help identify
nearby halo stars in the 14th data release from the APOGEE survey that have
been accreted on to the Milky Way. Applying a Gaussian Mixture Model, we find a
`blob’ of 856 likely accreted stars, with a low disc contamination rate of ~7%.
Cross-matching the sample with the second data release from Gaia gives us
access to parallaxes and apparent magnitudes, which place constraints on
distances and intrinsic luminosities. Using a Bayesian isochrone pipeline, this
enables us to estimate new ages for the accreted stars, with typical
uncertainties of ~20%. Our new catalogue is further supplemented with estimates
of orbital parameters.
The blob stars span a metallicities between -0.5 to -2.5, and [Mg/Fe] between
-0.1 to 0.5. They constitute ~30% of the metal-poor ([Fe/H] < -0.8) halo at
metallicities of ~-1.4. Our new ages are mainly range between 8 to 13 Gyr, with
the oldest stars the metal-poorest, and with the highest [Mg/Fe] abundance. If
the blob stars are assumed to belong to a single progenitor, the ages imply
that the system merged with our Milky Way around 8 Gyr ago and that star
formation proceeded for ~5 Gyr. Dynamical arguments suggest that such a single
progenitor would have a total mass of ~1011Msun, similar to that found by other
authors using chemical evolution models and simulations. Comparing the scatter
in the [Mg/Fe]-[Fe/H] plane of the blob stars to that measured for stars
belonging to the Large Magellanic Cloud suggests that the blob does indeed
contain stars from only one progenitor.
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