Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Actions are weak stellar age indicators in the Milky Way disk   The orbital properties of stars in the disk are signatures of their
formation, but they are also expected to change over time due to the dynamical
evolution of the Galaxy. Stellar orbits can be quantified by three dynamical
actions, J_r, L_z, and J_z, which provide measures of the orbital eccentricity,
guiding radius, and non-planarity, respectively. Changes in these dynamical
actions over time reflect the strength and efficiency of the evolutionary
processes that drive stellar redistributions. We examine how dynamical actions
of stars are correlated with their age using two samples of stars with
well-determined ages: 78 solar twin stars (with ages to ~5%) and 4376 stars
from the APOKASC2 sample (~20%). We compute actions using spectroscopic radial
velocities from previous surveys and parallax and proper motion measurements
from Gaia DR2. We find weak gradients in all actions with stellar age, of (7.51
+/- 0.52, -29.0 +/- 1.83, 1.54 +/- 0.18) kpc km/s/Gyr for J_r, L_z, and J_z,
respectively. There is, however, significant scatter in the action-age
relation. We caution that our results will be affected by the restricted
spatial extent of our sample, particularly in the case of J_z. Nevertheless,
these action-age gradients and their associated variances provide strong
constraints on the efficiency of the mechanisms that drive the redistribution
of stellar orbits over time and demonstrate that actions are informative as to
stellar age. The shallow action-age gradients combined with the large
dispersion in each action at a given age, however, renders the prospect of age
inference from orbits of individual stars bleak. Using the precision
measurements of [Fe/H] and [$\alpha$/Fe] for our stars we investigate the
abundance-action relationship and find weak correlations. Similar to our
stellar age results, dynamical actions afford little discriminating power
between low- and high-$\alpha$ stars.

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The categories of this paper are: solv-int
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The categories of this paper are: cmp-lg
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The categories of this paper are: nucl-th
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The categories of this paper are: funct-an
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The categories of this paper are: gr-qc
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The categories of this paper are: cs
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The categories of this paper are: q-fin
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The categories of this paper are: physics
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The categories of this paper are: chao-dyn
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The categories of this paper are: dg-ga
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The categories of this paper are: math-ph
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The categories of this paper are: q-bio
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The categories of this paper are: nucl-ex
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The categories of this paper are: chem-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: math