| Description |
Luminous stars have a profound influence on their environment as their far-UV (6-13.6 eV) photons dissociate and ionize surrounding gas. The gas in these so-called PhotoDissociation Regions (PDRs) is heated by the photo-electric effect on polycyclic aromatic hydrocarbon (PAH) molecules and very small grains (VSG) and cools through emission in atomic fine-structure (OI, CII, SiII) and molecular rotational (H2, CO) lines. PDR models are known to have intrinsic problems and have moreover only been tested using observations with very large beams on complex regions. Herschels high sensitivity, high spatial resolution, and wide wavelength coverage allows for the first time a study of the far-IR spectra of spatially resolved PDRs.We propose to map the key diagnostic, far-IR and sub-mm, atomic (CII 158m, OI 63, 145m) and molecular (CO) lines using PACS and SPIRE in a sample of edge-on, spatially resolved Galactic PDRs. The sources in this sample are all well-studied over a wide-wavelength range and have all been mapped by IRS/Spitzer in the rotational H2 lines, the SiII 34m line, the PAH features and the VSG continuum. The goals of our combined Herschel/Spitzer study are to determine the density and temperature structure of the region, to quantify the gas energetics (cooling/heating efficiency), and to compare the gas (heating) characteristics with the emission characteristics of the (neutral & cationic) PAHs and VSGs. This will provide deep insight in the photo-electric heating of atomic gas which is central to the structure of PDRs, the phases of the ISM, and the structure of protoplanetary disks. In addition, these well-known Galactic PDRs provide natural laboratories for studies of the interaction of massive stars with their environment and a semi-empirical way of calibrating the infrared characteristics of regions of massive star formation and, thus, the observational characteristics of galaxies out to the early Universe. |
