Observations of [Cii] 158 Micron Line and Far‐Infrared Continuum Emission toward the High‐Latitude Molecular Clouds in Ursa Major

We report the results of a rocket-borne observation of [C II] 158\micron lineand far-infrared continuum emission at 152.5\micron toward the high latitudemolecular clouds in Ursa Major. We also present the results of a follow-upobservation of the millimeter ^{12}CO J=1-0 line over a selected regionobserved by the rocket-borne experiment. We have discovered three small COcloudlets from the follow-up ^{12}CO observations. We show that these molecularcloudlets, as well as the MBM clouds(MBM 27/28/29/30), are not gravitationallybound. Magnetic pressure and turbulent pressure dominate the dynamic balance ofthe clouds. After removing the HI-correlated and background contributions, wefind that the [C II] emission peak is displaced from the 152.5\micron and COpeaks, while the 152.5\micron continuum emission is spatially correlated withthe CO emission. We interpret this behavior by attributing the origin of [C II]emission to the photodissociation regions around the molecular cloudsilluminated by the local UV radiation field. We also find that the ratio of themolecular hydrogen column density to velocity-integrated CO intensity is1.19+-0.29x10^{20} cm^{-2} (K kms^{-1})^{-1} from the FIR continuum and the COdata. The average [C II] /FIR intensity ratio over the MBM clouds is 0.0071,which is close to the all sky average of 0.0082 reported by the FIRAS on theCOBE satellite. The average [C II]/CO ratio over the same regions is 420, whichis significantly lower than that of molecular clouds in the Galactic plane.Comment: 15 pages, LaTeX (aaspp4.sty) + 2 tables(apjpt4.sty) + 6 postscript figures; accepted for publication in the Astrophysical Journal; Astrophys. J. in press (Vol. 490, December 1, 1997 issue