G25.5+0.2 - A RING NEBULA AROUND A LUMINOUS BLUE STAR

We have obtained infrared images and infrared spectroscopy of the Galactic radio source G25.5+0.2, previously suggested to be a young supernova remnant. The 2.2 mum image exhibits a similar nebular structure to that seen in the radio. Spectroscopic measurements at 2.17 mum show a Brgamma line that has the line-to-continuum ratio expected from ionized hydrogen at a temperature near 10,000 K. The line is resolved and has a total extent of 300 km s-1. Ten micron photometry clearly establishes a physical connection between G25.5+0.2 and the IRAS source 18344-0632. Most important, the infrared image reveals a point source in the center of the nebula that has properties of a blue supergiant star that could excite, an ionized ring nebula with the observed radio properties. G25.5 + 0.2 is almost certainly a ring nebula around a mass losing luminous blue star approximately 13.5 kpc distant and reddened by 20 mag of visual extinction.