PHOTON-DOMINATED REGIONS AROUND COOL STARS - THE EFFECTS OF THE COLOR TEMPERATURE OF THE RADIATION-FIELD

We have investigated the influence of the color temperature of the illuminating radiation field on the chemical and thermal structure of photon-dominated regions (PDRs). We present the results of a study of the photoelectric efficiency of heating by large molecules such as polycyclic aromatic hydrocarbons and very small grains for radiation fields characterized by different effective temperatures. We show that the efficiency for cooler (T-eff approximate to 6000-10,000 K) stars is at most an order of magnitude smaller than that for hotter (T-eff approximate to 20,000-30,000 K) stars. While cooler radiation fields result in less ultraviolet photons capable of heating, the efficiency per absorbed photon is higher, because the grains become less positively charged. We also present detailed calculations of the chemistry and thermal balance for generic PDRs (n(0) approximate to 10(3), G(0) approximate to 10(3)). For cooler radiation fields, the H/H-2 and C+/C/CO transition layers shift toward the surface of the PDR, because fewer photons are available to photodissociate H-2 and CO and to ionize C. The dominant cooling lines are the [C II] 158 mu m and the [O I] 63 mu m lines for the hotter radiation fields, but cooling by CO becomes dominant for a color temperature of 6000 K or lower. The [C II]/CO and [O I]/CO ratios are found to be very good diagnostics for the color temperature of the radiation field.