The age-velocity dispersion relation of the Galactic discs from LAMOST-Gaia data

TL;DR

This study analyzes the age-velocity dispersion relation of stars in the solar neighborhood using LAMOST and Gaia data, revealing differences between thin and thick disc populations and insights into disc heating mechanisms.

Contribution

It provides a detailed 3D AVR for different stellar populations, highlighting the impact of age, metallicity, and vertical position on stellar kinematics.

Findings

Thick disc stars show an abrupt increase in velocity dispersion at >7 Gyr.

Thin disc stars exhibit a power-law AVR with indices around 0.3-0.5.

Older thin disc stars have a rounder velocity ellipsoid, indicating vertical heating.

Abstract

We present the age-velocity dispersion relation (AVR) in three dimensions in the solar neighbourhood using 3,564 commonly observed sub-giant/red-giant branch stars selected from LAMOST, which gives the age and radial velocity, and \emph{Gaia}, which measures the distance and proper motion. The stars are separated into metal-poor (${\rm [Fe/H]<-0.2}$\,dex and metal-rich (${\rm [Fe/H]>-0.2}$\,dex) groups, so that the metal-rich stars are mostly $\alpha$-poor, while the metal-poor group are mostly contributed by $\alpha$-enhanced stars. Thus, the old and metal-poor stars likely belong to the chemically defined thick disc population, while the metal-rich sample is dominated by the thin disc. The AVR for the metal-poor sample shows an abrupt increase at $\gtrsim7$\,Gyr, which is contributed by the thick disc component. On the other hand, most of the thin disc stars with ${\rm [Fe/H]>-0.2}$\,dex display a power-law like AVR with indices of about 0.3--0.4 and 0.5 for the in-plane and vertical dispersions, respectively. This is consistent with the scenario that the disc is gradually heated by the spiral arms and/or the giant molecular clouds. Moreover, the older thin disc stars ($>7$\,Gyr) have a rounder velocity ellipsoid, i.e. $\sigma_\phi/\sigma_{\rm z}$ is close to 1.0, probably due to the more efficient heating in vertical direction. Particularly for the old metal-poor sample located with $|z|>270$\,pc, the vertical dispersion is even larger than its azimuthal counterpart. Finally, the vertex deviations and the tilt angles are plausibly around zero with large uncertainties.