The Connection between W 31 , SGR 1806 - 20 , & LBV 1806 - 20 : Distance , Extinction , and Structure

We present new millimeter and infrared spectroscopic observations towards the radio nebula G10.0−0.3, which is powered by the wind of the Luminous Blue Variable star LBV 1806−20 also closely associated with the soft gamma-ray repeater SGR 1806−20, and believed to be located in the giant Galactic H ii complex W31. Based on observations of CO emission lines and NH3 absorption features from molecular clouds along the line of sight to G10.0−0.3, as well as the radial velocity and optical extinction of the star powering the nebula, we determine its distance to be 15.1 −1.3 kpc in agreement with Corbel et al. (1997). In addition, this strengthens the association of SGR 1806−20 with a massive molecular cloud at the same distance. All soft gamma-ray repeaters with precise location are now found to be associated with a site of massive star formation or molecular cloud. We also show that W31 consists of at least two distinct components along the line of sight. We suggest that G10.2−0.3 and G10.6−0.4 are located on the −30 km s spiral arm at a distance from the Sun of 4.5 ± 0.6 kpc and that G10.3−0.1 may be associated with a massive molecular cloud at the same distance as the LBV star, i.e. 15.1 −1.3 kpc, implying that W31 could be decomposed into two components along the line of sight.