Determination of the Spiral Pattern Speed in the Milky Way from Young Open Star Clusters

Abstract

We have estimated the spiral pattern speed in the Galaxy $\Omega_p$ from a large sample of young open star clusters (OSCs). For this purpose, we have used 2494 OSCs younger than 50 Myr. Their mean proper motions, line-of-sight velocities, and distances were calculated by Hunt and Reffert (2023) based on data from the Gaia~DR3 catalogue. Three methods have been applied to estimate $\Omega_p$. They all are based on the linear Lin-Shu spiral density wave theory. We have obtained an estimate of $\Omega_p=24.26\pm0.52$~km s$^{-1}$ kpc$^{-1}$ by the first method, which is most reliable in our view, using the velocity perturbations $f_R$ and $f_\theta$ found through a spectral analysis of the radial, $V_R$, and residual rotation, $\Delta V_{\rm circ},$ velocities. Using the second method, we have found the velocity perturbations $f_R$ and $f_\theta$ by solving the basic kinematic equations together with the Galactic rotation parameters and obtained an estimate of $\Omega_p= 23.45\pm0.53$~km s$^{-1}$ kpc$^{-1}$. We have found $\Omega_p= 28.9\pm2.8$~km s$^{-1}$ kpc$^{-1}$ by the third method based on an analysis of the position angles of OSCs at their birth time $\theta_{\rm birth}$.