Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating

Abstract

We model the photoelectric emission from and charging of interstellar dust and obtain photoelectric gas heating efficiencies as a function of grain size and the relevant ambient conditions. We employ improved estimates for photoelectric thresholds, yields, and electron capture rates. Using realistic grain size distributions, we evaluate the net gas heating rate for various interstellar environments and Ðnd less heating for dense regions characterized by than for di †use regions with We provide R V \ 5.5 R V \ 3.1. Ðtting functions that reproduce our numerical results for photoelectric heating and recombination cooling for a wide range of interstellar conditions. Finally, we investigate the potential importance of photoelectric heating in H II regions, including the warm ionized medium. We Ðnd that photoelectric heating could be comparable to or exceed heating due to photoionization of H for high ratios of the radiation intensity to the gas density. We also Ðnd that photoelectric heating by dust can account for the observed variation of temperature with distance from the Galactic midplane in the warm ionized medium.

PDRs and XDRs: Theory and Observations

“…The depth at which the C + /C/CO transition occurs depends on the C + recombination efficiency. Because radiative recombination is slow, processes such as mutual neutralization of C + ions with negative PAH − ions (Lepp & Dalgarno 1988;Bakes & Tielens 1998), and "grain assisted" recombination (Weingartner & Draine 2001) are important (slides 13 and 14).…”

“…The primary gas heating mechanim in PDRs is photoelectric emission from dust grains, first considered by Spitzer (1948) and investigated in many subsequent studies (Draine 1978;Bakes & Tielens 1994;Weingartner & Draine 2001). In this process energetic electrons are ejected from the grains at a rate that depends on the FUV intensity and the grain charge, set by the balance between photoemission and recombination (slide 8).…”

“…This argument predicts cooling emission lines with intensities of order 1% of the thermal IR dust continua, consistent with observations. Photoelectric heating is probably dominated by the smallest (< 15Å) grains or PAHs for typical grain size distributions (Bakes & Tielens 1994;Weingartner & Draine 2001). The fine-structure line intensties are, therefore, probes of the incident FUV fluxes.…”

Unbiased Large Spectroscopic Surveys of Galaxies Selected by SPICA Using Dust Bands

“…Their emission features are detected not only from many nearby galaxies (e.g., Smith et al 2007) but also from distant galaxies up to redshift z ∼ 4 (Yan et al 2007;Sajina et al 2012;Riechers et al 2014;Kirkpatrick et al 2015). PAHs are believed to be the most important heating agents of gas in PDRs (e.g., Weingartner & Draine 2001), and thus their emissions are crucial probes to study the interstellar media associated with star-formation activity. In particular, PAH spectral features at 3.…”

Star formation in damped Ly$\alpha$ systems

“…Several authors have computed and show it to depend on the parameter J ν √ T /n e where T and n e are gas temperature and electron density (e.g. Weingartner & Draine 2001;Bakes & Tielens 1994). Because κ can be computed from depletion patterns in the absorbing gas (WPG03), we can determineψ * from the heating rate provided the thermal phase of the DLA gas is specified.…”