SiO Masers in the Galactic Bulge and Disk: Kinematics from the BAaDE Survey

TL;DR

This paper reports initial findings from the BAaDE survey, revealing two distinct kinematic populations of SiO maser stars in the Galactic bulge and disk, with implications for understanding stellar dynamics and populations.

Contribution

First large radio kinematic survey of SiO masers toward the Galactic bulge and plane, identifying two distinct kinematic stellar populations.

Findings

Identified a kinematically cold disk population with $\sigma \\sim 50$ km/sec.

Detected a kinematically hot bulge/bar population with $\sigma \\sim 100$ km/sec.

Most sources are luminous AGB stars with $M_{bol} \\sim -5$.

Abstract

We present the first results from the BAaDE (Bulge Asymmetries and Dynamic Evolution) survey. Though only a subset of the complete survey ($\sim$2700 out of $\sim$20000 final sources), our data comprise the largest radio kinematic survey to date of stellar SiO masers observed toward the Galactic bulge and plane from $-15^\circ < l < +12^\circ$ and $-6^\circ < b < +6^\circ$. Our sources include a substantial number of line-of-sight (LoS) velocities in high extinction regions within $\pm 1^\circ$ of the Galactic plane. When matched with 2MASS photometry, our radio-detected sample lies significantly brighter than and red-ward of the first red giant branch tip, reaching extremes of $(J-Ks)_0 > 8$, colors consistent with Mira variables and mass losing AGB stars. We see a clean division into two kinematic populations: a kinematically cold ($\sigma \sim 50\rm\ km\ sec^{-1}$) population that we propose is in the foreground disk, consisting of giants with 2MASS $Ks<5.5$, and a kinematically hot ($\sigma \sim 100\rm\ km\ sec^{-1}$) candidate bulge/bar population for most giants with $Ks>5.5$. Only the kinematically hot giants with $Ks>5.5$ include the reddest stars. Adopting 8.3 kpc to the Galactic Center, and correcting for foreground extinction, we find that most of the sources have $M_{bol} \sim -5$, consistent with their being luminous, and possibly intermediate age, AGB stars. We note some tension between the possibly intermediate age of the kinematically hot population, and its high velocity dispersion compared to the disk.