Heating of Interstellar H i Clouds by Ultraviolet Photoelectron Emission from Grains

Abstract

The physical processes and consequences of photoemission from interstellar grains in H i regions are investigated. For likely grain materials except ice, the photoemission is expected from laboratory data to be relatively efficient for photon energies of ^ 10-13.6 eV. It causes these interstellar grains to be positively charged in H i regions if the galactic ultraviolet flux is that indicated by recent observations. The ejected photoelectrons represent an energy input into interstellar H i clouds that is comparable to that from proposed fluxes of low energy cosmic rays or X-rays. Photoemission alone can maintain gas temperatures near 50° K in "standard" H i clouds. The exact value of the temperature depends on the grain density in the gas and the possible underabundance of carbon.

A STUDY OF HEATING AND COOLING OF THE ISM IN NGC 1097 WITHHERSCHEL-PACS ANDSPITZER-IRS

“…Using IRAS and ISO observations of the [C ii]158 μm, [O i]63 μm, and H 2 lines, Habart et al (2004) quantified the photoelectric efficiencies attributed to the PAH, very small grains (VSGs), and big grains (BGs) populations in the ρ Oph cloud, finding PAH = 3%, VSG = 1%, and BG = 0.1%, respectively. This suggests that PAHs are the most efficient gas heating channel in starforming galaxies and therefore drive the heating of the neutral gas regions (Watson 1972;Hollenbach & Tielens 1999). Indeed, Helou et al (2001) showed that the PAH contribution to the gas heating appears constant in galaxies with increasing F (60 μm)/F (100 μm), and dominant over the contribution of large grains, based on the relatively constant [C ii]158 μm/PAH against a rapidly falling [C ii]158 μm/FIR.…”

“…Observationally, this is often approximated as the ratio of [C ii]158 μm or [C ii]158 μm+[O i]63 μm to the infrared emission from dust, assuming that [C ii]158 μm and [O i]63 μm account for most of the cooling of the heated gas, and total infrared emission accounts for most of the UV absorbed by grains. However, photoelectric heating is dominated by the smallest grains in the ISM (Watson 1972;Jura 1976), whereas the total infrared emanates primarily from the larger grains. To avoid this complication, Helou et al (2001) used [C ii]158 μm/F (5-10 μm) as an estimate of efficiency limited to PAH, and showed that it varies considerably less than [C ii]158 μm/FIR, thus demonstrating that the smallest grains, and more specifically PAHs, do indeed dominate photoelectric heating.…”

“…The most important heating process in the neutral interstellar medium (ISM) is expected to be the photoelectric effect working on polycyclic aromatic hydrocarbons (PAHs) and small dust grains (Watson 1972;Tielens & Hollenbach 1985;Wolfire et al 1995;Hollenbach & Tielens 1999). Incident far-ultraviolet * Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.…”

Models of Chemistry, Thermal Balance, and Infrared Spectra from Intermediate‐Aged Disks around G and K Stars

“…Lowering a min tends to raise the gas temperature for two other reasons. Smaller grains have higher efficiencies of heating the gas through the grain photoelectric heating mechanism (see Watson 1972;Weingartner & Draine 2001), which may dominate the heating once a min ≤ 10 −2 µm. More importantly for this paper, smaller grains are hotter than larger grains due to the heating by stellar radiation and the inefficient radiative cooling of small grains.…”

Nanoparticle destruction by X-ray absorption

“…B. A. Mitchell, e-mail: mitchell@univ-rennes1.fr (Watson 1972;Verstraete et al 1990;Dwek & Smith 1996) as it may represent a heating mechanism for interstellar clouds. A recent synchrotron radiation experiment bears direct relevance to this problem and may have identified a new mechanism for the release of molecular species from grain surfaces into the interstellar medium.…”