| Description |
The 1.9 THz CII observations provide a powerful probe of warm diffuse clouds, because they can observe them in emission and are useful as a tracer of their molecular H2 not directly traced by CO or other means. HIFI observations of CII provide a high resolution of 12 arcsec, better than that for single dish CO (> 30 arcsec) maps, and much better than HI (>30 arcsec). Thus with HIFI we have an opportunity probe the small scale structures in diffuse clouds in the inner Galaxy at distances > 3 kpc. To study the structure of diffuse ISM gas at small scales we propose HIFI maps of 1.9 THz (158 micron) CII line emission in a selection of 16 lines of sight (LOSs) towards the inner Galaxy, which are also being observed as part of the GOT C+ survey of CII in the Galactic plane. GOT C+ provides mainly single point spectra without any spatial data. Maps of CII will constrain better the cloud properties and models when combining CII and HI data. The proposed OTF X map will be along the longitude and latitude centered on 18 selected GOT C+ LOS over a length of 3 arcmin in each direction, which is adequate enough to provide sufficient spatial information on the small scale structures at larger distances (>3 kpc) and to characterize the CII filling factor in the larger beams of the ancillary (HI, CO, and CI data). The CI 609 & 370micron and the 12CO(7-6) (which lies within the CI band) are excellent diagnostics of the physical conditions of transition clouds and PDRs. We will use the ratio of the CI lines to constrain the kinetic temperature and volume density of the CII/CI/CO transition zones in molecular clouds using radiative transfer codes. We also propose OTF X maps in both the CI lines for all CII target LOSs. We anticipate fully resolved structural data in CII on at least 300 velocity resolved clouds along with their CI emissions. We request a total of 33.2 hrs of HIFI observing time. |
