In the Galactic disk, stellar [Fe/H] and age predict orbits and precise
  [X/Fe]
  We explore the structure of the element abundance--age--orbit distribution of
the stars in the Milky Way's low-$\alpha$ disk, by (re-)deriving precise
[Fe/H], [X/Fe] and ages, along with orbits, for red clump stars from the APOGEE
survey. There has been a long-standing theoretical expectation and
observational evidence that metallicity ([Fe/H]) and age are informative about
a star's orbit, e.g. about its angular momentum and the corresponding mean
Galactocentric distance or its vertical motion. Indeed, our analysis of the
APOGEE data confirms that [Fe/H] or age alone can predict the stars' orbits far
less well than the combination of the two. Remarkably, we find and show
explicitly, that for known [Fe/H] and age, the other abundances [X/Fe] of
Galactic disk stars can be predicted well (on average to 0.02 dex) across a
wide range of Galactocentric radii, and therefore provide little additional
information, e.g. for predicting their orbit. While the age-abundance space for
metal poor stars and potentially for stars near the Galactic center is rich or
complex, for the bulk of the Galaxy's low-$\alpha$ disk it is simple: [Fe/H]
and age contain most information, unless [X/Fe] can be measured to 0.02, or
better. Consequently, we do not have the precision with current (and likely
near-future) data to assign stars to their individual (coeval) birth clusters,
from which the disk is presumably formed. We can, however, place strong
constraints on future models of galactic evolution, chemical enrichment and
mixing.
