G25.5 + 0.2: a new luminous blue variable in the Galaxy?

Abstract

Using the Australia Telescope, the Galactic source G25.5 + 0.2 has been detected in broad H105α and H106α radio recombination lines. Subsequent observations with the Very Large Array detected the H92α transition. The width of the recombination line is about 200 km s^–1, attributed to large-scale velocity structure. Infrared imaging and spectroscopy with the Anglo–Australian Telescope reveals a luminous central star with an absolute magnitude of M_k ∼ –7 that is at least as hot as an O7 main-sequence star. The object is not a supernova remnant as was previously suggested. We interpret G25.5 + 0.2 as an unusually energetic ejection associated with a post-main-sequence star of very high mass. The exceptional thermal bremsstrahlung emission from the nebula is due to photoionization by the high Lyman continuum flux of the central star.