On the nearest molecular clouds. III - MBM 40, 53, 54, and 55

Abstract

In an attempt to determine the distances to four high-latitude molecular clouds (HLCs), echelle spectra near the Na I D lines, accurate MK spectral types, and photoelectric photometry for 25 nearby stars have been obtained. Fairly firm distance limits may be placed on MBM 40 (d smaller than or equal to 140 pc) and MBM 53 (d greater than or equal to 110 pc and less than or equal to 155 pc), based on the presence or absence of strong interstellar Na I absorption towards stars projected on or near those HLCs. Weak interstellar absorption lines observed toward many of the stars located near MBM 54 and 55 make the distances to those clouds less certain (about 265 pc for both). Interstellar CH absorption at 4300 A was detected in the spectrum of HD 218662, located behind MBM 53 with a CH column density of 2.1 x 10 to the 13th per sq cm, thus implying a CH abundance comparable to that observed in other molecular clouds. Morphological and velocity agreement among CO emission, the Na I absorption, the 100 micron infrared cirrus emission, and the 21 cm H I emission near these HLCs suggest a close association of the interstellar material responsible for those phenomena.