Cosmic-ray induced diffusion in interstellar ices

Cosmic rays are able to heat interstellar dust grains. This may enhance molecule mobility in icy mantles that have accumulated on the grains in dark cloud cores. A three-phase astrochemical model was used to investigate the molecule mobility in interstellar ices. Specifically, diffusion through pores in ice between the subsurface mantle and outer surface, assisted by whole-grain heating, was considered. It was found that the pores can serve as an efficient transport route for light species. The diffusion of chemical radicals from the mantle to the outer surface are most effective. These species accumulate in the mantle because of photodissociation by the cosmic-ray induced photons. The faster diffusion of hydrogen within the warm ice enhances the hydrogenation of radicals on pore surfaces. The overall result of the whole grain heating-induced radial diffusion in ice are higher abundances of the ice species whose synthesis involve light radicals. Examples of stable species synthesized this way include the complex organic molecules, OCS, H2O2 and cyanoplyynes

The Effect of an Inert Solid Reservoir on Molecular Abundances in Dense Interstellar Clouds

The question, what is the role of freeze-out of chemical species in determining the molecular abundances in the interstellar gas is a matter of debate. We investigate a theoretical case of a dense interstellar molecular cloud core by time-dependent modelling of chemical kinetics, where grain surface reactions deliberately are not included. That means, the gas-phase and solid-phase abundances are influenced by gas reactions, accretion on grains, and desorption, only. We compare the results to a reference model, where no accretion occurs and only gas-phase reactions are included. We can trace that the purely physical processes of molecule accretion and desorption have major chemical consequences on the gas-phase chemistry. The main effect of introduction of the gas-grain interaction is long-term molecule abundance changes that come nowhere near an equilibrium in during the typical lifetime of a prestellar core.Comment: 8 pages, 6 figures, 1 table. Material initially presented in conference BAASP 2012 at Ventspils University College, May 201

Nonlinear magneto-optical resonances for systems with J~100 observed in K2 molecules

We present the results of an experimental as well as theoretical study of nonlinear magneto-optical resonances in diatomic potassium molecules in the electronic ground state with large values of the angular momentum quantum number J~100. At zero magnetic field, the absorption transitions are suppressed because of population trapping in the ground state due to Zeeman coherences between magnetic sublevels of this state along with depopulation pumping. The destruction of such coherences in an external magnetic field was used to study the resonances in this work. K2 molecules were formed in a glass cell filled with potassium metal at a temperature above 150C. The cell was placed in an oven and was located in a homogeneous magnetic field B, which was scanned from zero to 0.7 T. Q-type and R-type transitions were excited with a tunable, single-mode diode laser with central wavelength of 660 nm. Well pronounced nonlinear Hanle effect signals were observed in the intensities of the linearly polarized components of the laser-induced fluorescence (LIF) detected in the direction parallel to the (B)-field with polarization vectors parallel (I_par) and perpendicular (I_per) to the polarization vector of the exciting laser radiation, which was orthogonal to (B). The intensities of the LIF components were detected for different experimental parameters, such as laser power density and vapor temperature, in order to compare them with numerical simulations that were based on the optical Bloch equations for the density matrix. We report good agreement of our measurements with numerical simulations. Narrow, subnatural line width dark resonances in I_per(B) were detected and explained

Chemical fractionation of deuterium in the protosolar nebula

Understanding gas-grain chemistry of deuterium in star-forming objects may help to explain their history and present state. We aim to clarify how processes in ices affect the deuterium fractionation. In this regard, we investigate a Solar-mass protostellar envelope using an astrochemical rate-equation model that considers bulk-ice chem- istry. The results show a general agreement with the molecular D/H abundance ratios observed in low-mass protostars. The simultaneous processes of ice accumulation and rapid synthesis of HD on grain surfaces in the prestellar core hampers the deuteration of icy species. The observed very high D/H ratios exceeding 10 per cent, i.e., super- deuteration, are reproduced for formaldehyde and dimethyl ether, but not for other species in the protostellar envelope phase. Chemical transformations in bulk ice lower D/H ratios of icy species and do not help explaining the super-deuteration. In the protostellar phase, the D2O/HDO abundance ratio was calculated to be higher than the HDO/H2O ratio owing to gas-phase chemistry. Species that undergo evaporation from ices have high molecular D/H ratio and a high gas-phase abundance.Comment: 11 pages, 4 tables, 6 figures; +3 figures in appendix. Accepted for publication in MNRA

Detailed studies of non-linear magneto-optical resonances at D1 excitation of Rb-85 and Rb-87 for partially resolved hyperfine F-levels

Experimental signals of non-linear magneto-optical resonances at D1 excitation of natural rubidium in a vapor cell have been obtained and described with experimental accuracy by a detailed theoretical model based on the optical Bloch equations. The D1 transition of rubidium is a challenging system to analyze theoretically because it contains transitions that are only partially resolved under Doppler broadening. The theoretical model took into account all nearby transitions, the coherence properties of the exciting laser radiation, and the mixing of magnetic sublevels in an external magnetic field and also included averaging over the Doppler profile. Great care was taken to obtain accurate experimental signals and avoid systematic errors. The experimental signals were reproduced very well at each hyperfine transition and over a wide range of laser power densities, beam diameters, and laser detunings from the exact transition frequency. The bright resonance expected at the F_g=1 --> F_e=2 transition of Rb-87 has been observed. A bright resonance was observed at the F_g=2 --> F_e=3 transition of Rb-85, but displaced from the exact position of the transition due to the influence of the nearby F_g=2 --> F_e=2 transition, which is a dark resonance whose contrast is almost two orders of magnitude larger than the contrast of the bright resonance at the F_g=2 --> F_e=3 transition. Even in this very delicate situation, the theoretical model described in detail the experimental signals at different laser detunings.Comment: 11 pages, 9 figure