arXiv subjects

Juris Kalvans

Publications and source records attributed to Juris Kalvans.

15 recordsLinked to original sources

arXiv2025

The Emergence of Prebiotic Chemistry in the ISM

Contrary to popular belief, the interstellar medium (ISM) is not empty; it is filled with atoms, dust particles, and molecules. Some of these molecules may have been the very building blocks of life that, delivered to Earth via comets and meteorites, could have given rise to Life itself. A large-area single-dish telescope with superb sensitivity, field-of-view and multi-band instruments will allow us to explore the limits of chemical complexity in the interstellar medium, across our Galaxy and in external galaxies, determining whether amino acids, sugars, or RNA/DNA nucleobases can form in space.

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arXiv2024

Effects of grain temperature distribution on organic protostellar envelope chemistry

Context. Dust grains in circumstellar envelopes are likely to have a spread-out temperature distribution. Aims. To investigate how trends in temperature distribution between small and large grains affect the hot corino chemistry of complex organic molecules (COMs) and warm carbon-chain chemistry (WCCC). Methods. A multi-grain multi-layer astrochemical code with an up-to-date treatment of surface chemistry was used with three grain temperature trends: grain temperature proportional to grain radius to the power -1/6 (Model M-1/6), to 0 (M0), and to 1/6 (M1/6). The cases of hot corino and WCCC chemistry were investigated, for a total of six models. The essence of these changes is for the main ice reservoir - small grains - having higher (M-1/6) or lower (M1/6) temperature than the surrounding gas. Results. The chemistry of COMs shows better agreement with observations in models M-1/6 and M1/6 than in Model M0. Model M-1/6 shows best agreement for WCCC because earlier mass-evaporation of methane ice from small grains induces the WCCC phenomenon at lower temperatures. Conclusions. Models considering several grain populations with different temperatures can more precisely reproduce circumstellar chemistry.

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arXiv2022

Temperature Spectra of Interstellar Dust Grains Heated by Cosmic Rays. III. Mixed Composition Grains

Icy grains in the interstellar medium and star-formation regions consist of a variety of materials. Such composite grains interact differently with cosmic-ray (CR) particles compared to simple single-material grains. We aim to calculate the spectra of energies and temperatures of mixed-composition grains undergoing whole-grain heating by CRs. The grains were assumed to consist of a mixture of carbon and olivine, covered by ices consisting of carbon oxides and water. The energy and temperature spectra for grains with radii 0.05; 0.1, and 0.2 microns impacted by CRs were calculated for eight values of column density, relevant to molecular clouds and star-forming cores. The approach takes into account changes in ice thickness and composition with increasing column density. These detailed data for CR interaction with interstellar grains are intended for applications in astrochemical models. The main finding is that the a more accurate approach on grain heat capacity and other factors prevent a frequent heating of 0.1 micron or larger icy grains to high temperatures.

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arXiv2021

The connection between warm carbon chain chemistry and interstellar irradiation of star-forming cores

Some observations of warm carbon chain chemistry (WCCC) cores indicate that they are often located near the edges of molecular clouds. This finding may suggest that WCCC is promoted in star-forming cores exposed to radiation from the interstellar medium. We aim to investigate the chemistry of carbon chains in such a core. A chemical simulation of a gas parcel in a low-mass star-forming core with a full level of irradiation by interstellar photons and cosmic rays was compared to a simulation of a core receiving only one-tenth of such irradiation. In the full irradiation model, the abundances of carbon chains were found to be higher by a factor of few to few hundred, compared to the model with low irradiation. Higher carbon-chain abundances in the prestellar stage and, presumably, in the extended circumstellar envelope, arise because of irradiation of gas and dust by interstellar photons and cosmic rays. A full standard rate of cosmic-ray induced ionization is essential for a high carbon-chain abundance peak to occur in the circumstellar envelope, which is heated by the protostar (the "true" WCCC phenomenon). The full irradiation model has lower abundances of complex organic molecules than the low-irradiation model. We conclude that WCCC can be caused by exposure of a star-forming core to interstellar radiation, or even just to cosmic rays. The Appendix describes an updated accurate approach for calculating the rate of cosmic-ray induced desorption.

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arXiv2020

Evaporative cooling of icy interstellar grains. I. Basic characterization

Context. While radiative cooling of interstellar grains is a well-known process, little detail is known about the cooling of grains with an icy mantle that contains volatile adsorbed molecules. Aims. We explore basic details for the cooling process of an icy grain with properties relevant to dark interstellar clouds. Methods. Grain cooling was described with a numerical code considering a grain with an icy mantle that is structured in monolayers and containing several volatile species in proportions consistent with interstellar ice. Evaporation was treated as first-order decay. Diffusion and subsequent thermal desorption of bulk-ice species was included. Temperature decrease from initial temperatures of 100, 90, 80, 70, 60, 50, 40, 30, and 20K was studied, and we also followed the composition of ice and evaporated matter. Results. We find that grain cooling occurs by partially successive and partially overlapping evaporation of different species. The most volatile molecules (N2) first evaporate at the greatest rate and are most rapidly depleted from the outer ice monolayers. The most important coolant is CO, but evaporation of more refractory species, such as CH4 and even CO2, is possible when the former volatiles are not available. Cooling of high-temperature grains takes longer because volatile molecules are depleted faster and the grain has to switch to slow radiative cooling at a higher temperature. For grain temperatures above 40K, most of the thermal energy is carried away by evaporation. Evaporation of the nonpolar volatile species induces a complete change of the ice surface, as the refractory polar molecules (H2O) are left behind. Conclusions. The effectiveness of thermal desorption from heated icy grains (e.g., the yield of cosmic-ray-induced desorption) is primarily controlled by the thermal energy content of the grain and the number and availability of volatile molecules.

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arXiv2019

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

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 - N2 and O2 ices, and adsorbed atoms. The medium-temperature thresholds 40, 50, and 60 K allow effective evaporation of CO and CH4, 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.

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arXiv2018

The efficiency of photodissociation for molecules in interstellar ices

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 best-fit 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.

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arXiv2018

The Effects of Subsurface Chemistry in the Grain Mantles on the Deuterium Chemistry in Molecular Clouds

The deuterium enrichment in molecules in dark molecular cloud cores and starforming regions is usually attributed to gas-phase chemistry. Here we examine the effects of surface and mantle chemical reactions on the deuteration of species. We use a simple kinetic chemistry model that includes gas, surface and mantle pore phase reactions of deuterated species. The mantle is assumed to be partially reactive due to pores with sufficient surface area for chemical reactions, that are continuously transformed by cosmic-rays. Calculation results show that surface reactions generally enhance the deuteration for at least several molecules. However, once they are buried and become mantle molecules, they lose their deuteration over a timescale of 10 million years due to processes in the mantle. If deuterated species in young star-forming regions come from grain mantles, a cautious conclusion is that the freeze-out of molecules, perhaps, should not occur more than 10 Myr before the mantle evaporates to the gas phase.

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arXiv2015

Ice chemistry in starless molecular cores

Starless molecular cores are natural laboratories for interstellar molecular chemistry research. The chemistry of ices in such objects was investigated with a three-phase (gas, surface, and mantle) model. We considered the center part of five starless cores, with their physical conditions derived from observations. The ice chemistry of oxygen, nitrogen, sulfur, and complex organic molecules (COMs) was analyzed. We found that an ice-depth dimension, measured, e.g., in monolayers, is essential for modeling of chemistry in interstellar ices. Particularly, the H2O:CO:CO2:N2:NH3 ice abundance ratio regulates the production and destruction of minor species. It is suggested that photodesorption during core collapse period is responsible for high abundance of interstellar H2O2 and O2H, and other species synthesized on the surface. The calculated abundances of COMs in ice were compared to observed gas-phase values. Smaller activation barriers for CO and H2CO hydrogenation may help explain the production of a number of COMs. The observed abundance of methyl formate HCOOCH3 could be reproduced with a 1kyr, 20K temperature spike. Possible desorption mechanisms, relevant for COMs, are gas turbulence (ice exposure to interstellar photons) or a weak shock within the cloud core (grain collisions). To reproduce the observed COM abundances with the present 0D model, 1-10% of ice mass needs to be sublimated. We estimate that the lifetime for starless cores likely does not exceed 1Myr. Taurus cores are likely to be younger than their counterparts in most other clouds.

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arXiv2015

The effect of selective desorption mechanisms during interstellar ice formation

Major components of ices on interstellar grains in molecular clouds - water and carbon oxides - occur at various optical depths. This implies that selective desorption mechanisms are at work. An astrochemical model of a contracting low-mass molecular cloud core is presented. Ice was treated as consisting of the surface and three subsurface layers (sublayers). Photodesorption, reactive desorption, and indirect reactive desorption were investigated. The latter manifests itself through desorption from H+H reaction on grains. Desorption of shallow subsurface species was included. Modeling results suggest the existence of a "photon-dominated ice" during the early phases of core contraction. Subsurface ice is chemically processed by interstellar photons, which produces complex organic molecules. Desorption from the subsurface layer results in high COM gas-phase abundances at Av = 2.4...10mag. This may contribute towards an explanation for COM observations in dark cores. It was found that photodesorption mostly governs the onset of ice accumulation onto grains. Reaction-specific reactive desorption is efficient for small molecules that form via highly exothermic atom-addition reactions. Higher reactive desorption efficiency results in lower gas-phase abundances of COMs. Indirect reactive desorption allows to closely reproduce the observed H2O:CO:CO2 ratio towards a number of background stars. Presumably this can be done by any mechanism whose efficiency fits with the sequence CO > CO2 >> H2O. After the freeze-out has ended, the three sublayers represent chemically distinct parts of the mantle. 8...10.5mag is the likely AV threshold for the appearance of CO ice. The lower value is supported by observations.

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arXiv2014

Cosmic-ray-induced dissociation and reactions in warm interstellar ices

Context. Cosmic ray particles that hit interstellar grains in dark molecular cores may induce whole-grain heating. The high temperature of a CR-heated grain allows energy barriers for bulk diffusion and reactions to be overcome. Additionally, ice molecules are destroyed by direct cosmic-ray induced dissociation. Aims. We provide a justified estimate of the significance of cosmic-ray induced surface-mantle diffusion, chemical reactions in ice, and dissociation of ice species in a star-forming interstellar cloud core. Methods. We considered a gas clump in a collapsing low-mass prestellar core and during the initial stages of protostellar envelope heating with a three-phase chemical kinetics model. The model includes a proper treatment of the stochastic aspect of whole-grain heating and new experimental data for dissociation. Results. We found that the cosmic-ray-induced effects are mostly limited to an increase in abundance for carbon-chain species. The effect on major species abundances is a few percentage points at most. The HNC to HCN ice abundance ratio in ice is increased. Conclusions. Among the processes considered in the model, dissociation is probably the most significant, while diffusion and reactions on warm grains are less important. All three processes facilitate the synthesis of complex molecules, including organic species.

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arXiv2014

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.

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arXiv2013

Modeling of subsurface ice mantle on interstellar dust grains with astrochemical code ALCHEMIC (research note)

Interstellar ices are layers of molecules deposited on !ne dust grains in dark and dense molecular cloud cores. Subsurface ice has been considered in a few astrochemical models, which have shown that it can be of great importance. The aim of this work is to introduce an established subsurface ice description into the state-of-the-art astrochemical model ALCHEMIC. The model has been developed by the Heidelberg astrochemistry group. The result is an up-to-date model for interstellar molecular cloud research with possible application for protoplanetary disks.

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arXiv2013

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.

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arXiv2012

Subsurface chemistry of mantles of interstellar dust grains in dark molecular cores

Context. The abundances of many observed compounds in interstellar molecular clouds still lack an explanation, despite extensive research that includes both gas and solid (dust-grain surface) phase reactions. Aims. We aim to qualitatively prove the idea that a hydrogen-poor subsurface chemistry on interstellar grains is responsible for at least some of these chemical "anomalies". This chemistry develops in the icy mantles when photodissociation reactions in the mantle release free hydrogen, which escapes the mantle via diffusion. This results in serious alterations of the chemical composition of the mantle because pores in the mantle provide surfaces for reactions in the new, hydrogen-poor environment. Methods. We present a simple kinetic model, using existing astrochemical reaction databases. Gas phase, surface and subsurface pore reactions are included, as are physical transformations of molecules. Results. Our model produces significantly higher abundances for various oxidized species than most other models. We also obtain quite good results for some individual species that have adequate reaction network. Thus, we consider that the hydrogen-poor mantle chemistry may indeed play a role in the chemical evolution of molecular clouds. Conclusions. The significance of outward hydrogen diffusion has to be proved by further research. A huge number of solid phase reactions between many oxidized species is essential to obtain good, quantitative modeling results for a comparison with observations. We speculate that a variety of unobservable hydrogen-poor sulfur oxoacid derivatives may be responsible for the "disappearance" of sulfur in dark cloud cores.

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