Red Supergiants in the Inner Galaxy: Stellar Properties

Red supergiants (RSGs) are luminous cool stars detectable in disks of distant spirals. About a thousand are known in the Galaxy. Here, we analyze a sample of late-type stars recently observed by Messineo et al. in the inner Galaxy (${10}^{\circ }<l<{60}^{\circ }$). A total of 58 targets (25% of previously known RSGs) were reported as supergiants based on their large equivalent widths of the CO band-head at 2.293 μm (EWs > 45 Å, ≳M0I). In this work, luminosities are estimated with infrared measurements and distance moduli for 47 spectroscopically classified RSGs; they range from $3.2\times {10}^4$ to $1.3\times {10}^5$ $L_{\odot }$. Six other RSGs with smaller EW(CO)s are classified according to their luminosities. Using a prescription based on $K_s\text{--}\left[W4\right]$, moderate mass-loss rates from 10⁻⁸ to ${10}^{-6.5}$ $M_{\odot }$ yr⁻¹ are inferred. In addition, we report on H and K spectra of 26 stars at R = 1500–2200. EWs of the CO at 2.293 and 1.620 μm, and of atomic lines, are consistent with those of nearby RSGs, within uncertainties. Mg i appears to be a useful diagnostic to confirm RSGs at R = 1500–2200. RSG #66 is a member of the cluster Alicante 7. Star #92 is projected onto the overdensity #495 of Camargo et al., but our analysis suggests they are unrelated. Remaining targets are isolated from other known RSGs within ≈3′.