Detection of ionized gas toward an OH/IR star: A measure of the UV radiation field near the galactic center

The nature of the ionizing UV radiation field in the inner Galaxy is investigated by searching for radio continuum emission from the outer envelope of the population of OH/IR stars toward the Galactic center region. Radio continuum observations of a sample of OH/IR stars have been made at 3.6 cm using the VLA in its A, B, and DnC configurations. A highly scattered OH/IR star has been detected with a flux density of ∼1 mJy, which suggests that the star is photoionized by an external radiation field. The UV flux responsible for ionization is estimated to be 2.5 × 10⁹(Ṁ_-5)^-4 photons cm^-2 s^-1, where Ṁ_-5 is the mass-loss rate in units of 10^-5 M_⊙ yr^-1. We note that the continuum emission should become optically thick below a few hundred megahertz and that measurements at these frequencies will be able to determine both the UV flux and Ṁ.