Kinematics of Hot Subdwarfs from the Gaia DR2 Catalogue

Abstract

We have studied the kinematic properties of the candidates for hot subdwarfs (HSDs) selected by Geier et al. from theGaiaDR2 catalogue. We have used a total of 12 515 stars with relative trigonometric parallax errors less than 30%. The HSDs are shown to have different kinematics, depending on their positions on the celestial sphere. For example, the sample of low-latitude (|b| < 20°) HSDs rotates around the Galactic center with a linear velocity V0 = 221 ± 5 km s⁻¹. This suggests that they belong to the Galactic thin disk. At the same time, they lag behind the local standard of rest by ΔV_☉ ~ 16 km s⁻¹ due to the asymmetric drift. The high-latitude (|b| ≥ 20°) HSDs rotate with a considerably lower velocity, V_☉ = 168 ^± 6 km s⁻¹. Their lagging behind the local standard of rest is already ΔV_☉ ∼ 40 km s⁻¹. Based on the entire sample of 12 515 HSDs, we have found a positive rotation around the x axis significantly differing from zero with an angular velocity ω₁ = 1.36±0.24 km s⁻¹ kpc⁻¹. We have studied the samples of HSDs that are complete within r < 1.5 kpc. Based on them, we have traced the evolution of the parameters of the residual velocity ellipsoid as a function of both latitude |b| and coordinate |z|. The following vertical disk scale heights have been found: h = 180 ± 6 and 290 ± 10 pc from the low- and high-latitude HSDs, respectively. A new estimate of the local stellar density Σ_out = 53 ± 4 M☉ kpc⁻² has been obtained for z_out = 0.56 kpc from the high-latitude HSDs.