The Pistol Star

We present new near-infrared data and analysis, which indicate that the Pistol Star is one of the most luminous stars known, adding another test point for massive star formation and stellar evolution theories. We estimate an extinction of A_K = 3.2 +/- 0.5 using the near-infrared colors of the star and of surrounding stars in the young Quintuplet cluster. Using our wind/atmosphere code, we find two families of models that fit the spectral energy distribution and detailed line profiles. The lower luminosity models give L = 10^6.6+/-0.2 L_⊙ and T_eff = 10^4.15+/-0.01 K, while the higher luminosity models give L = 10^7.2+/-0.2 L_⊙ and T_eff = 10^4.33+/-0.01 K; the error in luminosity assumes an uncertainty of +/-0.5 in A_K, while the error in T_eff is constrained by detailed line modeling. The models also reveal a helium enriched surface. As previously existing stellar evolution models do not extend to such high luminosities, we employ new evolutionary tracks for very massive stars to determine the initial mass and age of the Pistol Star, and estimate M_initial = 200-250 M_⊙ and an age of 1.7-2.1 Myr. The inferred luminosity and temperature place the star in a sparsely populated zone in the H-R diagram where luminous blue variables (LBVs) are often found. This is consistent with our evolutionary models, which predict that the star is in an unstable evolutionary stage. We interpret the star and its surrounding nebula as an LBV that has recently ejected large amounts of material. Our K-band speckle-imaging data reveal the star to be single down to a projected separation of 110 AU.