High- and Low-$alpha$ Disk Stars Separate Dynamically at all Ages. (arXiv:1903.04030v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gandhi_S/0/1/0/all/0/1">Suroor S Gandhi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ness_M/0/1/0/all/0/1">Melissa K Ness</a>
There is a dichotomy in the Milky Way in the $[alpha/$Fe]-[Fe/H] plane, in
which stars fall into high-$alpha$, and low-$alpha$ sequences. The
high-$alpha$ sequence comprises mostly old stars, and the low-$alpha$
sequence comprises primarily young stars. The origin of this dichotomy is
uncertain. To better understand how the high- and low-$alpha$ stars are
affiliated, we examine if the high- and low-$alpha$ sequences have distinct
orbits at all ages, or if age sets the orbital properties of stars irrespective
of their $alpha$-enhancement. Orbital actions $J_R$, $J_z$, and $J_phi$ (or
$L_z$) are our labels of stellar dynamics. We use ages for 58,278 LAMOST stars
(measured to a precision of 40%) within $leq$2kpc of the Sun and we calculate
orbital actions from proper motions and parallaxes given by Gaia’s DR2. We find
that emph{at all ages}, the high- and low-$alpha$ sequences are dynamically
distinct. This implies separate formation and evolutionary histories for the
two sequences; a star’s membership in the high- or low-$alpha$ sequence
indicates its dynamical properties at a given time. We use action space to make
an efficient selection of halo stars and subsequently report a group of old,
low-$alpha$ stars in the halo, which may be a discrete population from an
infall event.
There is a dichotomy in the Milky Way in the $[alpha/$Fe]-[Fe/H] plane, in
which stars fall into high-$alpha$, and low-$alpha$ sequences. The
high-$alpha$ sequence comprises mostly old stars, and the low-$alpha$
sequence comprises primarily young stars. The origin of this dichotomy is
uncertain. To better understand how the high- and low-$alpha$ stars are
affiliated, we examine if the high- and low-$alpha$ sequences have distinct
orbits at all ages, or if age sets the orbital properties of stars irrespective
of their $alpha$-enhancement. Orbital actions $J_R$, $J_z$, and $J_phi$ (or
$L_z$) are our labels of stellar dynamics. We use ages for 58,278 LAMOST stars
(measured to a precision of 40%) within $leq$2kpc of the Sun and we calculate
orbital actions from proper motions and parallaxes given by Gaia’s DR2. We find
that emph{at all ages}, the high- and low-$alpha$ sequences are dynamically
distinct. This implies separate formation and evolutionary histories for the
two sequences; a star’s membership in the high- or low-$alpha$ sequence
indicates its dynamical properties at a given time. We use action space to make
an efficient selection of halo stars and subsequently report a group of old,
low-$alpha$ stars in the halo, which may be a discrete population from an
infall event.
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