Stephen Lepp

Thermal Balance in Dense Molecular Clouds: Radiative Cooling Rates and Emission-Line Luminosities

We consider the radiative cooling of fully shielded molecular astrophysical gas over a wide range of temperatures ( 10 K < T < 2500 K) and H2 densities ( 10(exp 3)/cc < n(H2) < 10(exp 10)/cc). Our model for the radiative cooling of molecular gas includes a detailed treatment of the interstellar chemistry that determines the abundances of important coolant molecules, and a detailed treatment of the excitation of the species H2, CO, H2O, HCl, O2, C, O, and their isotopic variants where important. We present results for the total radiative cooling rate and for the cooling rate due to individual coolant species, as a function of the gas temperature, density, and optical depth. We have also computed the individual millimeter, submillimeter, and far-infrared line strengths that contribute to the total radiative cooling rate, and we have obtained example spectra for the submillimeter emission expected from molecular cloud cores. Many of the important cooling lines will be detectable using the Infrared Space Observatory and the Submillimeter Wave Astronomy Satellite.

Atomic and molecular processes in the early Universe

Most of the information about the environment of the early Universe comes to us from radiation emitted from atoms and molecules. An understanding of the relevant atomic and molecular processes is needed to correctly interpret this radiation. Atomic and molecular process also control the evolution of the early Universe. In this paper, we review the atomic and molecular processes that are important in the early Universe.

Molecules in the early universe

We present calculations of the formation of astrophysically interesting molecules (H/sub 2/, HD, LiH, and HeH/sup +/) by gas-phase reactions during the postrecombination epoch (redshifts z = 300-30). In standard Friedmann cosmological models, H/sub 2//Hroughly-equal10/sup -6/, HD/H/sub 2/roughly-equal10/sup -4.5/, and LiH/H/sub 2/roughly-equal10/sup -6.5/. These molecules may dominate the cooling and trigger the collapse of primordial gas clouds. The dipole rotational transitions of HD and LiH are particularly important at high density and low temperature. Additional molecules form during spherical collapse of these clouds, their rotational cooling keeps the gas temperature between 400 and 1500 K over 12 decades of density increase until the H/sub 2/ lines become optically thick. The existence of molecular coolants at high redshift has significant implications for the first generation of stars and for thermal instabilities in intergalactic matter.

The Deuterium Chemistry of the Early Universe

The chemistry of deuterium, as well as that of hydrogen and helium, in the postrecombination era of the expanding early universe is presented. A thorough survey of all potentially important gas-phase reactions involving the primordial elements produced in the Big Bang, with a particular emphasis on deuterium, is given. The reaction set, consisting of 144 processes, is used in a nonequilibrium chemistry model to follow the production of primordial molecules in the postrecombination era. It is found that significant deuterium fractionation occurs for HD+, HD, and H2D+, while the abundance of D+ is reduced compared to the proton abundance. Even with the enhanced fractionation of H2D+, its abundance is predicted to be too small to cause any interesting cosmological consequences, such as possible attenuation of spatial anisotropies in the cosmic background radiation field, detections of the epochs of reionization and reheating, or constraints on the primordial deuterium abundance. HD, being the second most abundant primordial molecule after H2, may play a role in subsequent structure formation because of its cooling radiation.

Cosmic-ray-induced photodissociation and photoionization rates of interstellar molecules

In the Prasad-Tarafdar mechanism, ultraviolet photons are created in the interior of dense interstellar clouds by the impact excitation of molecular hydrogen by secondary electrons generated by cosmic-ray ionization. Detailed calculations of the emission spectrum are described, and the resulting photodissociation and photoionization rates of a wide range of interstellar molecules are calculated. 84 refs.

Polycyclic aromatic hydrocarbons in interstellar chemistry

Interstellar chemistry modifications resulting form the presence of large molecules such as polycyclic aromatic hydrocarbons (PAHs) are investigated. For abundances of PAH relative to hydrogen of greater than 10 to the -8th, free electrons attach to PAH molecules to yield PAH(-) ions, and qualitative interstellar chemistry changes are shown to result as atomic and molecular ions undergo nondestructive mutual neutralization reactions with these negative ions. An increase in the steady state abundances of carbon-bearing molecules is also noted. For a PAH abundance ratio relative to hydrogen of 10 to the -7th, the equilibrium densities of C3H2 and neutral atomic C are found to be enhanced by two orders of magnitude. 18 references.

Deuterium fractionation mechanisms in interstellar clouds

The theory of the fractionation of deuterated molecules is extended to include reactions with atomic deuterium. With the recognition that dissociative recombination of H/sup +//sub 3/ is not rapid, observational data can be used in conjunction with the theory to derive upper and lower bounds to the cosmic deuterium-hydrogen abundance ratio. We find that (D)/(H) is at least 3.4 x 10/sup -6/ and at most 4.0 x 10/sup -5/ with a probable value of 1 x 10/sup -5/. Because of the reaction HCO/sup +/+D..-->..DCO/sup +/+H, upper limits can be derived for the fractional ionization which depend only weakly on the cosmic ray flux, zeta. In four clouds, the upper limits to the fractional ionization lie between 1.1 x 10/sup -6/ and 1.5 x 10/sup -6/ if zeta = 10/sup -7/ s/sup -1/ and between 3.1 x 10/sup -6/ and 1.8 x 10/sup -6/ if zeta = 10/sup -16/ s/sup -1/.

Large molecules in diffuse interstellar clouds

The effects of the presence of a substantial component of large molecules on the chemistry of diffuse molecular clouds are explored, and detailed models of the zeta Persei and zeta Ophiuchi clouds are constructed. The major consequence is a reduction in the abundances of singly charged atomic species. The long-standing discrepancy between cloud densities inferred from rotational and fine-structure level populations and from the ionization balance can be resolved by postulating a fractional abundance of large molecules of 1 x 10 to the -7th for zeta Persei and 6 x 10 to the -7th for zeta Ophiuchi. If the large molecules are polycyclic aromatic hydrocarbons (PAH) containing about 50 carbon atoms, they contain 1 percent of the carbon in zeta Persei and 7 percent in zeta Ophiuchi. Other consequences of the possible presence of PAH molecules are discussed.

X-ray sources in molecular clouds

Models are calculated for the structure and infrared line emission from a dense interstellar gas cloud containing a compact X-ray source. For constant gas pressure models, the resulting structure consists of nested spherical shells containing, respectively, coronal gas at T>10/sup 6/ K, an H II region with Tapprox.10/sup 4/ K, and H I region with Tapprox.8000 K, and finally an H/sub 2/ region with T<5000 K. Scaling laws are given for the locations of the transitions. Approximately 10% of the X-ray luminosity absorbed in the H/sub 2/ region is converted into infrared emission lines that may be observable. Line ratios are predicted.

Cosmic-ray-induced photodestruction of interstellar molecules in dense clouds

The ultraviolet spectrum of radiation generated by cosmic rays inside dense molecular clouds is presented, and the resulting rates of photodissociation for a variety of interstellar molecules are estimated. The effects of this radiation on the chemistry of dense molecular clouds are discussed, and it is argued that the cosmic-ray-induced photons will significantly inhibit the production of complex molecular species. 30 references.