Radial motions in disk stars: ellipticity or secular flows?
  Average stellar orbits of the Galactic disk may have some small intrinsic
ellipticity which breaks the exact axisymmetry and there may also be some
migration of stars inwards or outwards. Both phenomena can be detected through
kinematic analyses. We use the red clump stars selected spectroscopically from
APOGEE (APO Galactic Evolution Experiment), with known distances and radial
velocities, to measure the radial component of the Galactocentric velocities
within 5 kpc$<R<$16 kpc, $|b|<5^\circ$ and within 20 degrees from the
Sun-Galactic center line. The average Galactocentric radial velocity is
$V_R=(1.48\pm 0.35)[R({\rm kpc})-(8.8\pm 2.7)]$ km/s outwards in the explored
range, with a higher contribution from stars below the Galactic plane. Two
possible explanations can be given for this result: i) the mean orbit of the
disk stars is intrinsically elliptical with a Galactocentric radial gradient of
eccentricity around 0.01 kpc$^{-1}$; or ii) there is a net secular expansion of
the disk, in which stars within $R\approx 9-11$ kpc are migrating to the region
$R\gtrsim 11$ kpc at the rate of $\sim 2$ M$_\odot $/yr, and stars with
$R\lesssim 9$ kpc are falling toward the center of the Galaxy. This migration
ratio would be unattainable for a long time and it should decelerate, otherwise
the Galaxy would fade away in around 1 Gyr. At present, both hypotheses are
speculative and one would need data on the Galactocentric radial velocities for
other azimuths different to the center or anticenter in order to confirm one of
the scenarios.
