Infrared Observations of the Candidate LBV 1806‐20 and Nearby Cluster Stars

Abstract

We report near-infrared photometry, spectroscopy, and speckle imaging of the hot, luminous star we identify as candidate LBV 1806-20. We also present photometry and spectroscopy of three nearby stars, which are members of the same star cluster containing LBV 1806-20 and SGR 1806-20. The spectroscopy and photometry show that LBV 1806-20 is similar in many respects to the luminous "Pistol star," albeit with some important differences. They also provide estimates of the effective temperature and reddening of LBV 1806-20 and confirm distance estimates, leading to a best estimate for the luminosity of this star of greater than 5 × 106 L☉. The nearby cluster stars have spectral types and inferred absolute magnitudes that confirm the distance (and thus luminosity) estimate for LBV 1806-20. If we drop kinematic measurements of the distance (15.1 kpc), we have a lower limit on the distance of greater than 9.5 kpc and on the luminosity of greater than 2 × 106 L☉, based on the cluster stars. If we drop both the kinematic and cluster star indicators for distance, an ammonia absorption feature sets yet another lower limit to the distance of greater than 5.7 kpc, with a corresponding luminosity estimate of greater than 7 × 105 L☉ for the candidate LBV 1806-20. Furthermore, on the absis of very high angular resolution speckle images, we determine that LBV 1806-20 is not a cluster of stars but is rather a single star or binary system. Simple arguments based on the Eddington luminosity lead to an estimate of the total mass of LBV 1806-20 (single or binary) exceeding 190 M☉. We discuss the possible uncertainties in these results and their implications for the star formation history of this cluster.