The Flare and Warp of the Young Stellar Disk traced with LAMOST DR5 OB-type stars

TL;DR

This study analyzes the spatial density and flaring of the young OB stellar disk in the Milky Way using LAMOST DR5 data, revealing symmetrical flaring, a shorter radial scale length than the gas disk, and slight vertical asymmetries.

Contribution

It provides detailed measurements of the flaring, scale height, and mid-plane displacements of the young OB stellar disk, highlighting differences from the gas disk and implications for disk evolution.

Findings

Flaring is symmetrical on both sides of the Galactic plane.

The OB stellar disk has a shorter radial scale length than the gas disk.

The scale height of OB stars is slightly thicker than the gas disk by 33 and 9 pc.

Abstract

We present analysis of the spatial density structure for the outer disk from 8$-$14 \,kpc with the LAMOST DR5 13534 OB-type stars and observe similar flaring on north and south sides of the disk implying that the flaring structure is symmetrical about the Galactic plane, for which the scale height at different Galactocentric distance is from 0.14 to 0.5 \,kpc. By using the average slope to characterize the flaring strength we find that the thickness of the OB stellar disk is similar but flaring is slightly stronger compared to the thin disk as traced by red giant branch stars, possibly implying that secular evolution is not the main contributor to the flaring but perturbation scenarios such as interactions with passing dwarf galaxies should be more possible. When comparing the scale height of OB stellar disk of the north and south sides with the gas disk, the former one is slightly thicker than the later one by $\approx$ 33 and 9 \,pc, meaning that one could tentatively use young OB-type stars to trace the gas properties. Meanwhile, we unravel that the radial scale length of the young OB stellar disk is 1.17 $\pm$ 0.05 \,kpc, which is shorter than that of the gas disk, confirming that the gas disk is more extended than stellar disk. What is more, by considering the mid-plane displacements ($Z_{0}$) in our density model we find that almost all of $Z_{0}$ are within 100 \,pc with the increasing trend as Galactocentric distance increases.