The connection between W31, SGR 1806–20, and LBV 1806–20: Distance, extinction, and structure

Abstract

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 HII complex W31. Based on observations of CO emission lines and NH_3 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$^{+1.8}_{-1.3}$ 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 $\pm$ 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$^{+1.8}_{-1.3}$ kpc, implying that W31 could be decomposed into two components along the line of sight.