The Double-lined Spectrum of LBV 1806-20

Despite much theoretical and observational progress, there is no known firm upper limit to the masses of stars. Our understanding of the interplay between the immense radiation pressure produced by massive stars in formation and the opacity of infalling material is subject to theoretical uncertainties, and many observational claims of ``the most massive star'' have failed the singularity test. LBV 1806-20 is a particularly luminous object, L~10⁶ L_solar, for which some have claimed very high mass estimates (M_initial>200 M_solar), based in part on its similarity to the Pistol star. We present high-resolution near-infrared spectroscopy of LBV 1806-20, showing that it is possibly a binary system with components separated in velocity by ~70 km s^-1. If correct, then this system is not the most massive star known, yet it is a massive binary system. We argue that a binary, or merged, system is more consistent with the ages of nearby stars in the LBV 1806-20 cluster. In addition, we find that the velocity of V_LSR=36 km s^-1 is consistent with a distance of 11.8 kpc, a luminosity of 10^6.3 L_solar, and a system mass of ~130 M_solar.

Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The observatory was made possible by the generous financial support of the W. M. Keck Foundation.