CO TO H2 RATIO OF INTERSTELLAR MOLECULAR CLOUDS IN THE DIRECTIONS OF EARLY TYPE STARS

We present measurements of interstellar CO absorption lines in the spectra of 7 early-type stars that were observed with the FUSE(Far Ultraviolet Spectroscopic Explorer). Among 54 early-type target stars in the Galactic disk and halo observed with the BEFS(Berkeley Extreme and Far-ultraviolet Spectrometer), we choose 7 program stars (HD 37903, HD 97991, HD 149881, HD 156110, HD 164794, HD 214080 and HD 219188) which have only a single velocity component in the high-resolution optical measurements, in order to avoid line blending. To analyze the CO molecule, we select the E-X (0-0) band at 1076 Å, which has a large oscillator strength and is not blended with other interstellar absorption lines. We detect the CO absorption lines in three (HD 37903, HD 164794, and HD 214080) out of seven targets, and derive CO column densities for those targets. We also estimated the CO to H2 ratios toward the three stars, based on the previously estimated H2 column densities.