Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...437..270s&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 437, no. 1, p. 270-280
Astronomy and Astrophysics
Astrophysics
62
Cool Stars, Cosmic Dust, Interstellar Extinction, Photons, Stellar Color, Stellar Envelopes, Stellar Temperature, Temperature Effects, Interstellar Chemistry, Interstellar Matter, Photodissociation, Photoelectric Effect, Polycyclic Aromatic Hydrocarbons, Radiation Distribution
Scientific paper
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 (PAHs) and very small grains for radiation fields characterized by different effective temperatures. We show that the efficiency for cooler (Teff approximately = 6000-10,000 K) stars is at most an order of magnitude smaller than that for hotter (Teff approximately = 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 (n0 approximately = 103, G0 approximately = 103). For cooler radiation fields, the H/H2 and C(+)/C/CO transition layers shift toward the surface of the PDR, because fewer photons are available to photodissociate H2 and CO and to ionize C. The dominant cooling lines are the (C II) 158 micron and the (O I) 63 micron 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.
Bakes Emma L. O.
Spaans Marco
Tielens Alexander G. G. M.
van Dishoeck Ewine F.
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