THE HIGH-LATITUDE CLOUD MBM-7-I-HI AND CO OBSERVATIONS

The high-latitude cloud (HLC) MBM 7 has been observed in the 21 cm H I line and the (CO)-C-12(1-0) and (CO)-C-13(1-0) lines with similar spa tial resolutions. The data reveal a total mass similar to 30 M. for MB M 7 and a complex morphology. The cloud consists of a cold dense core of 5 M. surrounded by atomic and molecular gas with about 25 M., which is embedded in hotter and more diffuse H I gas. We derive a total col umn density N(H I + 2H(2)) of 1 x 10(21) cm(-2) toward the center and 1 x 10(20) cm(-3) toward the envelope of MBM 7. The CO line indicates the existence of dense cores [n(H-2) greater than or equal to 2000 cm( -3)] of size (FWHM) similar to 0.5 pc. The morphology suggests shock c ompression from the southwest direction, which can form molecular core s along the direction perpendicular to the H I distribution. The H I c loud extends to the northeast, and the velocity gradient appears to be about 2.8 km s(-1) pc(-1) in this direction, which indicates a system atic outward motion which will disrupt the cloud in similar to 10(6) y r. The observed large line widths of similar to 2 km s(-1) for CO sugg est that turbulent motions exist in the cloud, and hydrodynamical turb ulence may dominate the line broadening. Considering the energy and pr essure of MBM 7, the dense cores appear not to be bound by gravity, an d the whole cloud including the dense cores seem to be expanding. The distance to HLCs suggest that they belong to the galactic plane, since the scale height of the cloud is less than or similar to 100 pc. Comp ared to the more familiar dense dark clouds, HLCs may differ only in t heir small mass and low density, with their proximity reducing the fil ling factor and enhancing the contrast of the core and envelope struct ure.