A HIGH-RESOLUTION 21 CENTIMETER LINE STUDY OF INFRARED CIRRUS

New high-resolution (1' x 1' x 0.4 km s-1) H I observations of an IR cirrus are presented. The H I emission presents a complex morphology of interwoven filaments and clumps whose aspect changes significantly from one velocity channel to the next. The velocity spread is 12 km s-1 (FWHM), the cloud size is 2 pc, and its mass 10 M.. The velocity field does not appear fully stochastic. Some filaments possess systematic velocity gradients of the order of 0.15 km s-1 arcmin-1. At the resolution scale, emission features have velocity widths of the order of 3-5 km s-1 (FWHM). The turbulent and magnetic energy are comparable and larger than the gravitational energy by three orders of magnitude. The turbulent and magnetic pressures are one order of magnitude larger than the interstellar medium pressure. Within the cirrus there is a good correlation between the integrated H I emission and the 100-mu-m brightness except in two localized regions which show excess IR emission. This excess indicates the presence of either molecular hydrogen or cold atomic gas (T less-than-or-equal-to 30 K). In the infrared maps the H I features are spatially connected to a molecular cloud. All belong to a large loop of gas, 20-degrees in size, known as the north celestial loop. The turbulent kinetic energy per unit mass of the atomic cloud is one order of magnitude higher than that of the neighboring molecular cloud. The difference in ps composition and kinetic energy between the two sections of the infrared cirrus may be related to the formation of the loop. The energy source for the large internal motions seen in the atomic gas, which is necessarily external to the cloud, could be related to the formation of the loop.