Context. The formation of water on the dust grains in the interstellar medium may proceed with hydrogen peroxide (H2O2) as an intermediate. Recently gas-phase H2O2 has been detected in ρ Oph A with an abundance of ~1E-10 relative to H2.
Aims. We aim to reproduce the observed abundance of H2O2 and other species detected in ρ Oph A quantitatively.
Methods. We make use of a chemical network which includes gas phase reactions as well as processes on the grains; desorption from the grain surface through chemical reaction is also included. We run the model for a range of physical parameters.
Results. The abundance of H2O2 can be best reproduced at ~6E5 yr, which is close to the dynamical age of ρ Oph A. The abundances of other species such as H2CO, CH3OH, and O2 can be reasonably reproduced also at this time. In the early time the gas-phase abundance of H2O2 can be much higher than the current detected value. We predict a gas phase abundance of O2H at the same order of magnitude as H2O2, and an abundance of the order 1E-8 for gas phase water in ρ Oph A. A few other species of interest are also discussed.
Conclusions. We demonstrate that H2O2 can be produced on the dust grains and released into the gas phase through non-thermal desorption via surface exothermic reactions. The H2O2 molecule on the grain is an important intermediate in the formation of water. The fact that H2O2 is over-produced in the gas phase for a range of physical conditions suggests that its destruction channel in the current gas phase network may be incomplete.
Context. Evaporative (sublimation) cooling of icy interstellar grains occurs when the grains have been suddenly heated by a cosmic-ray (CR) particle or other process. It results in thermal desorption of icy species, affecting the chemical composition of interstellar clouds. Aims. We investigate details on sublimation cooling, obtaining necessary knowledge before this process is considered in astrochemical models. Methods. We employed a numerical code that describes the sublimation of molecules from an icy grain, layer by layer, also considering a limited diffusion of bulk-ice molecules toward …
Observations of gaseous complex organic molecules (COMs) in cold starless and prestellar cloud cores require efficient desorption of the COMs and their parent species from icy mantles on interstellar grains. With a simple astrochemical model, we investigate if mechanical removal of ice fragments in oblique collisions between grains in two size bins (0.01 and 0.1 micron) can substantially affect COM abundances. Two grain collision velocities were considered - 10 and 50 meters per second, corresponding to realistic grain relative speeds arising from ambipolar diffusion and turbulence, respective…
With the presence of evermore complex S-bearing molecules being detected lately, studies on their chemical formation routes need to keep up the pace to rationalize observations, suggest new candidates for detection, and provide input for chemical evolution models. In this paper, we theoretically characterize the hydrogenation channels of OCS on top of amorphous solid water as an interstellar dust grain analog in molecular clouds. Our results show that the significant reaction outcome is trans-HC(O)SH, a recently detected prebiotic molecule toward G+0.693. The reaction is diastereoselective, ex…
The work is devoted to the adaptation of the results of laboratory studies of the laser-induced dissociation of molecules of benzene adsorbed on a quartz substrate to the conditions of the interstellar medium. Adsorption was performed under conditions of low temperature and deep vacuum. The difference between the photolysis of adsorbed molecules and molecules in the gas phase is identified. Significance of process of photolytic desorption in the interstellar conditions is analyzed, in particular, in the conditions of photodissociation regions. It is shown that the efficiency and dissociation c…
Cosmic-ray (CR) induced heating of whole interstellar grains is an important desorption mechanism for grain surface molecules in interstellar molecular clouds. This study aims to provide a detailed temperature spectra for such CR-induced heating. For this, olivine grains with radius of 0.05, 0.1 and 0.2 microns shielded by interstellar gas with isotropic column densities characteristic to dark cores were considered. The accumulation of an ice mantle of increasing thickness was taken into account. The CR energy spectra was obtained for these column densities for 32 cosmic-ray constituents. We c…
Clustering and dynamics of nano-sized particles (nano dust) is investigated using high-resolution ($1024^3$) simulations of compressible isothermal hydrodynamic turbulence, intended to mimic the conditions inside cold molecular clouds in the interstellar medium. Nano-sized grains may cluster in a turbulent flow (small-scale clustering), which increases the local grain density significantly. Together with the increased interaction rate due to turbulent motions, aggregation of interstellar nano-dust may be plausible.
High Radiation Pressure on Interstellar Dust Computed by Light-Scattering Simulation on Fluffy Agglomerates of Magnesium-silicate Grains with Metallic-iron Inclusions
Interstellar dust spans a wide range in size distribution, ranging from ultrasmall grains of a few Angstroms to micrometer-size grains. While the presence of nanometer-size dust grains in the Galactic interstellar medium was speculated six decades ago and was previously suggested based on early infrared observations, systematic and direct analysis of their properties over a wide range of environments has been lacking. Here we report the detection of nanometer-size dust grains that appear to be universally present in a wide variety of astronomical environments, from Galactic high-latitude cloud…
The properties of interstellar grains, such as grain size distribution and grain porosity, are affected by interstellar processing, in particular, coagulation and shattering, which take place in the dense and diffuse interstellar medium (ISM), respectively. In this paper, we formulate and calculate the evolution of grain size distribution and grain porosity through shattering and coagulation. For coagulation, we treat the grain evolution depending on the collision energy. Shattering is treated as a mechanism of forming small compact fragments. The balance between these processes are determined…
Interstellar space hosts nanometre- to micron-sized dust grains. The carbonaceous-rich component of these grain populations emits in infrared bands, observed remotely for decades with telescopes and satellites. They are a key ingredient of astrochemical dust evolution. The precise carriers for most of these bands are still unknown and not well reproduced in the laboratory. In this work, we show the high-energy mechanochemical synthesis of disordered aromatic and aliphatic analogues provides interstellar relevant dust particles. The mechanochemical milling of carbon-based solids under a hydroge…
Molecular hydrogen is the most abundant molecule in the universe. It is the first one to form and survive photo-dissociation in tenuous environments. Its formation involves catalytic reactions on the surface of interstellar grains. The micro-physics of the formation process has been investigated intensively in the last 20 years, in parallel of new astrophysical observational and modeling progresses. In the perspectives of the probable revolution brought by the future satellite JWST, this article has been written to present what we think we know about the H$_2$ formation in a variety of interst…
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 val…
We present a new method for using the observed starlight polarization and polarized submm emission to constrain the shapes and porosities of interstellar grains. We present the modified picket fence approximation (MPFA), and verify that it is sufficiently accurate for modeling starlight polarization. We introduce the starlight polarization integral $Π_{\rm obs}$ as a measure of overall strength of the observed polarization of starlight, and the starlight polarization efficiency integral $Φ$ to characterize the effectiveness of different grain types for producing polarization of starlight. The …
Interstellar dust grains are non-spherical and, in some environments, partially aligned along the direction of the interstellar magnetic field. Numerous alignment theories have been proposed, all of which examine the grain rotational dynamics. In 1999, Lazarian & Draine introduced the important concept of thermal flipping, in which internal relaxation processes induce the grain body to flip while its angular momentum remains fixed. Through detailed numerical simulations, we study the role of thermal flipping on the grain dynamics during periods of relatively slow rotation, known as `crossovers…
Whether ice in cold cosmic environments is physically separated from the silicate dust or mixed with individual silicate moieties is not known. However, different grain models give very different compositions and temperatures of grains. The aim of the present study is a comparison of the mid-IR spectra of laboratory silicate-grains/water-ice mixtures with astronomical observations to evaluate the presence of dust/ice mixtures in interstellar and circumstellar environments. The laboratory data can explain the observations assuming reasonable mass-averaged temperatures for the protostellar envel…
In this paper, we present a new gas-grain chemical code for interstellar clouds written in pure Python (GGCHEMPY). By combining with the high-performance Python compiler Numba, GGCHEMPY is as efficient as the Fortran-based version. With the Python features, flexible computational workflows and extensions become possible. As a showcase, GGCHEMPY is applied to study the general effects of three-dimensional projection on molecular distributions using a two-core system which can be easily extended for more complex cases. By comparing the molecular distribution differences between two overlapping c…
Grain growth by accretion of gas-phase metals is a common assumption in models of dust evolution, but in dense gas, where the timescale is short enough for accretion to be effective, material is accreted in the form of ice mantles rather than adding to the refractory grain mass. It has been suggested that negatively-charged small grains in the diffuse interstellar medium (ISM) can accrete efficiently due to the Coulomb attraction of positively-charged ions, avoiding this issue. We show that this inevitably results in the growth of the small-grain radii until they become positively charged, at …
In the interstellar medium (ISM), the formation of complex organic molecules (COMs) is largely facilitated by surface reactions. However, in cold dark clouds, thermal desorption of COMs is inefficient because of the lack of thermal energy to overcome binding energies to the grain surface. Non-thermal desorption methods are therefore important explanations for the gas-phase detection of many COMs that are primarily formed on grains. Here we present a new non-thermal desorption process: cosmic ray sputtering of grain ice surfaces based on water, carbon dioxide, and a simple mixed ice. Our model …
Angle-dependent radiative grain alignment; Confirmation of a magnetic field - radiation anisotropy angle dependence on the efficiency of interstellar grain alignment
Within the framework of The Heterogeneous dust Evolution Model at the IaS (THEMIS) this work investigates in detail the surface processes and chemistry relating to core/mantle interstellar and cometary grain structures and its influence on the nature of these fascinating particles. It appears that a realistic consideration of the nature and chemical reactivity of interstellar grain surfaces could self-consistently and within a coherent framework explain: the anomalous oxygen depletion, the nature of the CO dark gas, the formation of 'polar ice' mantles, the red wing on the 3 um water ice band,…
The binding energies (BE) of molecules on the interstellar grains are crucial in the chemical evolution of the interstellar medium (ISM). Both temperature programmed desorption (TPD) laboratory experiments and quantum chemistry computations have often provided, so far, only single values of the BE for each molecule. This is a severe limitation, as the ices enveloping the grain mantles are structurally amorphous, giving rise to a manifold of possible adsorption sites, each with different BEs. However, the ice amorphous nature prevents the knowledge of structural details, hindering the developme…
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 gravita…
