Actions Are Weak Stellar Age Indicators in the Milky Way Disk

Abstract

Abstract The orbital properties of stars in the Milky Way 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 the 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 precise 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 with significant scatter for all actions as a function of stellar age. These 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. However, the shallow action–age gradients combined with the large dispersion in each action at a given age render the prospect of age inference from orbits of individual stars bleak. Using the precision measurements of [Fe/H] and [α/Fe] we find that, similarly to our stellar age results, the dynamical actions afford little discriminating power between individual low- and high-α stars.