1996A&A...310...93L

Allen & Lequeux (1993) have reported the detection of faint CO emission from two dust clouds (D268 and D478, Hodge 1980a) in the inner disk of M 31. During the same observing run, several other positions in the inner disk of M 31 were observed, and we report here the results of these observations. Two dust clouds located less than a few arcminutes from the center were observed; one of them was reported earlier by Sofue & Yoshida (1993) to be a CO(1-0) source. We failed to detect emission in either cloud, although our sensitivity was much better than that of the previous authors. The lack of detection for these clouds indicates that the physical state of the gas associated with the dust very close to the center of M 31 is different from that of the other dust clouds in the inner disk of this galaxy. This difference may be related to the intense UV field present in the nucleus of M 31. Farther from the center, at R≤4kpc (but still inside the bright ring at R∼10kpc), 4 other isolated positions associated with dust clouds were observed. Faint CO(1-0) and CO(2-1) emission was detected for all of them. As a check on the relative calibration of the CO(1-0) line, we observed as a reference position the brightest CO feature in the small region mapped by Casoli, Combes & Stark (1987) in the star-forming ring; here, we find a more common value of 0.63±0.23 for the line ratio. Finally, several positions on another dust cloud (D192) were observed. This cloud is located in the star-forming ring, but in a region of locally diminished radio-continuum radiation. The CO emission is again quite faint and well associated with the dust itself. The ratio of the integrated CO(2-1) to CO(1-0) line intensities for D192 is 0.50±0.15, intermediate between the value found for D478 or D268 and those found for our reference position.