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

Abstract

Abstract We present the first results from the Bulge Asymmetries and Dynamic Evolution (BAaDE) survey. Though only a subset of the complete survey (∼2700 out of ∼20,000 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° < l < +12° and −6° < b < +6°. Our sources include a substantial number of line-of-sight velocities in high extinction regions within ±1° of the Galactic plane. When matched with 2MASS photometry, our radio-detected sample lies significantly brighter than and redward 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 (σ ∼ 50 km s−1) population that we propose is in the foreground disk, consisting of giants with 2MASS Ks < 5.5, and a kinematically hot (σ ∼ 100 km s−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 ∼ −5, consistent with their being luminous and possibly intermediate-age AGB stars. We note some tension between the possible intermediate age of the kinematically hot population, and its high-velocity dispersion compared to the disk.