Abstract
This work presents high spectral resolution observations of the [CII] line at 158 mu m, one of the major cooling lines of the interstellar medium, taken with the HIFI heterodyne spectrometer on the Herschel satellite. In BCLMP 691, an HII region far north (3.3 kpc) in the disk of M 33, the [CII] and CO line profiles show similar velocities within 0.5 kms(-1), while the Hi line velocities are systematically shifted towards lower rotation velocities by similar to 5 kms(-1). Observed at the same 12" angular resolution, the [CII] lines are broader than those of CO by about 50% but narrower than the Hi lines. The [CII] line intensities also follow those of CO much better than those of Hi. A weak shoulder on the [CII] line suggests a marginal detection of the [(CII)-C-13] line, insufficient to constrain the [CII] optical depth. The velocity coincidence of the CO and [CII] lines and the morphology at optical/UV wavelengths indicate that the emission is coming from a molecular cloud behind the HII region. The relative strength of [CII] with respect to the FIR continuum emission is comparable to that observed in the Magellanic Clouds on similar linear scales but the CO emission relative to [CII] is stronger in M 33. The [CII] line to far-infrared continuum ratio suggests a photoelectric heating efficiency of 1.1%. The data, together with published models indicate a UV field G(0) similar to 100 in units of the solar neighborhood value, a gas density n(H) similar to 1000 cm(-3), and a gas temperature T similar to 200 K. Adopting these values, we estimate the C+ column density to be NC+ approximate to 1.3 x 10(17) cm(-2). The [CII] emission comes predominantly from the warm neutral region between the HII region and the cool molecular cloud behind it. From published abundances, the inferred C+ column corresponds to a hydrogen column density of N-H similar to 2 x 10(21) cm(-2). The CO observations suggest that N-H = 2N(H2) similar to 3.2 x 10(21) cm(-2) and 21 cm measurements, also at 12" resolution, yield N-HI approximate to 1.2 x 10(21) cm(-2) within the [CII] velocity range. Thus, some H-2 not detected in CO must be present, in agreement with earlier findings based on the SPIRE 250-500 mu m emission.