Infrared Observations of the Candidate LBV 1806-20 & 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 3 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 $>5\times 10^6{L}_{\odot }$. The nearby cluster stars have spectral types and inferred absolute magnitudes which confirm the distance (and thus luminosity) estimate for LBV 1806-20. If we drop kinematic measurements of the distance ($15.1_{-1.3}^{+1.8}$ kpc), we have a lower limit on the distance of $>9.5$ kpc, and on the luminosity of $>2\times 10^6{L}_{\odot }$, 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 $>5.7$ kpc, with a corresponding luminosity estimate of $>7\times 10^5{L}_{\odot }$ for the candidate LBV 1806-20. Furthermore, based on 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 $190M_{\odot }$. We discuss the possible uncertainties in these results, and their implications for the star formation history of this cluster.

36 pages, including 8 figures (Figures 1 and 7 in JPG format due to space); Accepted for publication in ApJ

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