THE DISTANCE TO AN X-RAY SHADOWING MOLECULAR CLOUD IN URSA MAJOR

摘要

We have obtained high-resolution optical spectra toward nine stars in the direction of a high-latitude, intermediate-velocity neutral cloud in Ursa Major in order to ascertain the distance to this complex. This cloud is of interest for several reasons. It is an infrared cirrus cloud, shows a distinct X-ray shadow, and turns out to be one of only three molecular clouds known to be well above the plane of the Galaxy. Interstellar Na I absorption is detected in four of the nine stars, but only the most distant star in our sample (BD + 63°985) shows absorption at the velocity of the cloud as determined by the 21 cm and CO observations of Heiles, Reach, & Koo. We use several Fe I and Fe II stellar absorption features to determine the spectral type and luminpsity class of the three most distant stars. Using the spectral type-absolute magnitude relationship from Schmidt-Kaler, and making no correction for extinction, we derive a distance to the cloud of d = 355 ± 95 pc, which corresponds to z = 285 ± 75 pc. Estimating the effects of extinction, we find that the true value could be as low as d = 240 pc. This distance puts the cloud beyond the expected extent of the Local Bubble of hot (T ≈ 10~6 K) gas, showing that the X-ray emission behind this cloud arises in the Galactic halo. The cloud has dimensions ～15 x 50 pc, with a total estimated atomic mass of ～ 1600 solar mass. The molecular mass of the cloud core, G135.3 ± 54.5, is ～0.1 solar mass.