The Effect of Selective Desorption Mechanisms During Interstellar Ice Formation

Monthly Notices of the Royal Astronomical Society

2018

Processing by interstellar photons affects the composition of the icy mantles on interstellar grains. The rate of photodissociation in solids differs from that of molecules in the gas phase. The aim of this work was to determine an average, general ratio between photodissociation coefficients for molecules in ice and gas. A 1D astrochemical model was utilized to simulate the chemical composition for a line of sight through a collapsing interstellar cloud core, whose interstellar extinction changes with time. At different extinctions, the calculated column densities of icy carbon oxides and ammonia (relative to water ice) were compared to observations. The latter were taken from literature data of background stars sampling ices in molecular clouds. The bestfit value for the solid/gas photodissociation coefficient ratio was found to be ≈ 0.3. In other words, gas-phase photodissociation rate coefficients have to be reduced by a factor of 0.3 before applying them to icy species. A crucial part of the model is a proper inclusion of cosmic-ray induced desorption. Observations sampling gas with total extinctions in excess of ≈ 22 mag were found to be uncorrelated to modelling results, possibly because of grains being covered with non-polar molecules.

Section: Methodsmentioning

confidence: 99%

The efficiency of photodissociation for molecules in interstellar ices

Monthly Notices of the Royal Astronomical Society

2018

“…The astrochemical model employed in this study is the latest iteration of the code Alchemic-Venta (Kalvāns 2015b). An updated approach of WGH frequencies and detailed chemical processes in ices on the heated grains were added to the code.…”

Section: Methodsmentioning

confidence: 99%

“…This corresponds to 2.4 × 10 −4 erg cm −2 s −1 . Shielding from the ISRF of H2, CO and N2 molecules was included with the help of tabulated data (Kalvāns 2015b Figure 1). The model considers grains with radius a = 0.1 µm and density 3 g cm −3 that constitute 1 per cent of cloud mass.…”

Section: Macrophysical Modelmentioning

confidence: 99%

“…The surface density of adsorption sites was taken to be 1.5 × 10 15 cm −2 , consistent with an olivine grain (Hasegawa et al 1992). The mantle was described as consisting of four layers -the surface, ≈ 1 monolayer (3.5 × 10 −8 cm) thick, and three subsurface bulk-ice layers (Kalvāns 2015b). As the MLs accumulate, the surface area of the grain increases accordingly.…”

Section: Macrophysical Modelmentioning

confidence: 99%

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Chemical significance of different temperature regimes for cosmic-ray-induced heating of whole interstellar grains

Monthly Notices of the Royal Astronomical Society

Kalnin

2019

Self Cite

Cosmic-ray-induced whole-grain heating induces evaporation and other processes that affect the chemistry of interstellar clouds. With recent data on grain heating frequencies as an input for a modified rate-equation astrochemical model, this study examines, which whole-grain heating temperature regime is the most efficient at altering the chemical composition of gas and ices. Such a question arises because low-temperature heating, albeit less effective at inducing evaporation of adsorbed species, happens much more often than high-temperature grain heating. The model considers a delayed gravitational collapse of a Bonnor-Ebert sphere, followed by a quiescent cloud core stage. It was found that the whole-grain heating regimes can be divided in three classes, depending on their induced physico-chemical effects. Heating to low-temperature thresholds of 27 and 30 K induce desorption of the most volatile of species -N 2 and O 2 ices, and adsorbed atoms. The medium-temperature thresholds 40, 50, and 60 K allow effective evaporation of CO and CH 4 , delaying their accumulation in ices. We find that the 40 K threshold is the most effective cosmic-ray induced whole-grain heating regime because its induced evaporation of CO promotes major abundance changes also for other compounds. An important role in grain cooling may be played by molecular nitrogen as the most volatile of the abundant species in the icy mantles. Whole-grain heating determines the sequence of accretion for different molecules on to grain surface, which plays a key role in the synthesis of complex organic molecules.

“…Photodesorption yield Y ph was taken to be 3×10 −4 for interstellar photons (Arasa et al 2015;Furuya et al 2015) and a 1.5 times lower value for cosmic-ray-induced photons (Kalvāns 2015c). Eq.…”

Section: Chemical Modelmentioning

confidence: 99%

Chemical fractionation of deuterium in the protosolar nebula

Mon. Not. R. Astron. Soc.

Shmeld

Kalnin

et al. 2017

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 chemistry. 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., superdeuteration, 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 D 2 O/HDO abundance ratio was calculated to be higher than the HDO/H 2 O ratio owing to gas-phase chemistry. Species that undergo evaporation from ices have high molecular D/H ratio and a high gas-phase abundance.