Nuclear stellar disk-like nature in the kinematics of SiO maser stars around Sagittarius A*

Abstract We present a detailed analysis of the kinematics of SiO maser stars around the center of the Milky Way, Sagittarius A$^\ast$ (Sgr A$^\ast$). We used the archive data in the SiO $v=1$, J = 2–1 emission line obtained by the Atacama Large Millimeter/Submillimeter Array (ALMA) in 2017 and 2021 (#2016.1.00940.S, PI J. Darling and #2019.1.00292.S, PI J. Paine). We detected 37 SiO maser stars in the channel maps and derived their angular offsets relative to Sgr A$^\ast$ and LSR radial velocities. We derived the proper motions of 35 stars by comparing their angular offsets in the two epochs. The proper motions of Wolf–Rayet and O star in the nuclear star cluster are reported to be rather random, except for the co-moving clusters IRS13E and IRS13N (Tsuboi et al. 2022, PASJ, 74, 738). However, the derived proper motions of SiO maser stars do not look completely random. The proper motions of the SiO maser stars show a tendency to lie along the Galactic plane. The proper motion amplitudes of SiO maser stars are larger than the local standard of rest (LSR) velocity amplitudes. We estimated the 3D motions from the proper motions and LSR velocities. Many 3D velocities are near to or larger than the upper limit velocities for Kepler orbits around Sgr A$^\ast$, whose mass is assumed to be $4\times 10^6$ M$_{\odot }$. These indicate that the SiO maser stars around Sgr A$^\ast$ are members of the nuclear stellar disk rather than the nuclear star cluster.