scholarly journals Ice mantles on dust grains: dramatic variation of thickness with grain size

2021 ◽  
Vol 507 (4) ◽  
pp. 6205-6214
Author(s):  
Kedron Silsbee ◽  
Paola Caselli ◽  
Alexei V Ivlev
Keyword(s):  
Grain Size ◽  
Gas Phase ◽  
Strong Dependence ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Desorption Rate ◽  
Dust Grains ◽  
Grain Sizes ◽  
Standard Values ◽  
Dramatic Variation

ABSTRACT We compute the desorption rate of icy mantles on dust grains as a function of the size and composition of both the grain and the mantle. We combine existing models of cosmic ray (CR)-related desorption phenomena with a model of CR transport to accurately calculate the desorption rates in dark regions of molecular clouds. We show that different desorption mechanisms dominate for grains of different sizes and in different regions of the cloud. We then use these calculations to investigate a simple model of the growth of mantles, given a distribution of grain sizes. We find that modest variations of the desorption rate with grain size lead to a strong dependence of mantle thickness on grain size. Furthermore, we show that freeze-out is almost complete in the absence of an external ultraviolet (UV) field, even when photodesorption from CR-produced UV is taken into consideration. Even at gas densities of $10^4\, {\rm cm^{-3}}$, less than 30 per cent of the CO remain in the gas phase after 3 × 105 yr for standard values of the CR ionization rate.

scholarly journals Nautilus multi-grain model: Importance of cosmic-ray-induced desorption in determining the chemical abundances in the ISM

2018 ◽  
Vol 615 ◽  
pp. A20 ◽  
Author(s):  
Wasim Iqbal ◽  
Valentine Wakelam
Keyword(s):  
Grain Size ◽  
Surface Temperature ◽  
Gas Phase ◽  
Numerical Models ◽  
Cosmic Ray ◽  
Grain Sizes ◽  
Small Grains ◽  
Grain Model ◽  
Grain Surface ◽  
The Impact

Context. Species abundances in the interstellar medium (ISM) strongly depend on the chemistry occurring at the surfaces of the dust grains. To describe the complexity of the chemistry, various numerical models have been constructed. In most of these models, the grains are described by a single size of 0.1 μm. Aims. We study the impact on the abundances of many species observed in the cold cores by considering several grain sizes in the Nautilus multi-grain model. Methods. We used grain sizes with radii in the range of 0.005 μm to 0.25 μm. We sampled this range in many bins. We used the previously published, MRN and WD grain size distributions to calculate the number density of grains in each bin. Other parameters such as the grain surface temperature or the cosmic-ray-induced desorption rates also vary with grain sizes. Results. We present the abundances of various molecules in the gas phase and also on the dust surface at different time intervals during the simulation. We present a comparative study of results obtained using the single grain and the multi-grain models. We also compare our results with the observed abundances in TMC-1 and L134N clouds. Conclusions. We show that the grain size, the grain size dependent surface temperature and the peak surface temperature induced by cosmic ray collisions, play key roles in determining the ice and the gas phase abundances of various molecules. We also show that the differences between the MRN and the WD models are crucial for better fitting the observed abundances in different regions in the ISM. We show that the small grains play a very important role in the enrichment of the gas phase with the species which are mainly formed on the grain surface, as non-thermal desorption induced by collisions of cosmic ray particles is very efficient on the small grains.

scholarly journals Rapid elimination of small dust grains in molecular clouds

2020 ◽  
Vol 641 ◽  
pp. A39 ◽  
Author(s):  
Kedron Silsbee ◽  
Alexei V. Ivlev ◽  
Olli Sipilä ◽  
Paola Caselli ◽  
Bo Zhao
Keyword(s):  
Size Distribution ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Microwave Emission ◽  
Dust Grains ◽  
Gas Temperature ◽  
Neutral Gas ◽  
Cloud Cores ◽  
The Impact ◽  
Small Dust

We argue that impact velocities between dust grains with sizes of less than ∼0.1 μm in molecular cloud cores are dominated by drift arising from ambipolar diffusion. This effect is due to the size dependence of the dust coupling to the magnetic field and the neutral gas. Assuming perfect sticking in collisions up to ≈50 m s−1, we show that this effect causes rapid depletion of small grains, consistent with starlight extinction and IR and microwave emission measurements, both in the core center (n ∼ 106 cm−3) and envelope (n ∼ 104 cm−3). The upper end of the size distribution does not change significantly if only velocities arising from this effect are considered. We consider the impact of an evolved grain-size distribution on the gas temperature, and argue that if the depletion of small dust grains occurs as expected from our model, then the cosmic ray ionization rate must be well below 10−16 s−1 at a number density of 105 cm−3.

scholarly journals The Determination of Electron Abundances in Interstellar Clouds

1980 ◽  
Vol 87 ◽  
pp. 339-340
Author(s):  
Alwyn Wootten ◽  
Ronald Snell ◽  
A. E. Glassgold
Keyword(s):  
Gas Phase ◽  
Molecular Clouds ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Interstellar Clouds ◽  
Ion Molecule Reactions ◽  
Upper Limits ◽  
Gas Phase Ion ◽  
Existing Data

A new method for estimating electron fractions in shielded molecular clouds is proposed on the basis of gas phase ion-molecule reactions which involves measuring the quantity . Applied to existing data, it yields upper limits to Xe in the range from 10−8 to 10−7 for a variety of clouds, warm as well as cool. An upper bound to the cosmic ray ionization rate is also obtained.

Microindentation of Coarse-Grained Polycrystalline α-Brass

2002 ◽  
Vol 750 ◽  
Author(s):  
Nian Zhang ◽  
Changjin Xie ◽  
Wei Tong
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Indentation Depth ◽  
Strong Dependence ◽  
Weak Dependence ◽  
Coarse Grained ◽  
Grain Sizes ◽  
Vickers Indenter ◽  
Indent Depth ◽  
Crystallographic Orientations

ABSTRACTEffects of the indent depth, the distance between the indent and the grain boundary, grain sizes, and crystallographic orientations on the microhardness of annealed coarse-grained polycrystalline α-brass were investigated using a Vickers indenter with indents much smaller than the α-brass grains (ranging from 80 μm to 550 μm). It is found that the microhardness of α-brass crystals shows a strong dependence on the indentation depth when it is smaller than 5 μm and a very weak dependence on both the distance between the indent and the grain boundary and the grain size when the indentation depth is about 5 μm and more. No significant dependence of the microhardness on the crystallographic orientations was observed in the fourteen grains of different orientations studied in this investigation.

scholarly journals Chemistry in Protoplanetary Disks

2005 ◽  
Vol 13 ◽  
pp. 515-517
Author(s):  
Yuri Aikawa
Keyword(s):  
Molecular Evolution ◽  
Gas Phase ◽  
Thermal Desorption ◽  
Molecular Clouds ◽  
Dominant Species ◽  
Protoplanetary Disks ◽  
Ionization Rate ◽  
Desorption Rate ◽  
Volatile Species ◽  
Grain Surface

AbstractMolecular evolution is calculated in migrating fluid parcels in a protoplanetary disk considering gas-phase and grain-surface reactions. Radial distribution of molecules is obtained, which depends on ionization rate, temperature and/or desorption rate in the disk. If the temperature is high enough (> 20 K) or non-thermal desorption is efficient enough to sublimate dominant species such as CO in the comet forming regions, the desorbed species are transformed to less volatile species and again incorporated into ice mantles. Chemistry in these outer disks is similar to that in molecular clouds; it produces both oxidized and reduced species with a high D/H ratio.

scholarly journals The role of radiolysis in the modelling of C2H4O2 isomers and dimethyl ether in cold dark clouds

2020 ◽  
Vol 500 (3) ◽  
pp. 3414-3424
Author(s):  
Alec Paulive ◽  
Christopher N Shingledecker ◽  
Eric Herbst
Keyword(s):  
Gas Phase ◽  
Dimethyl Ether ◽  
Recent Observation ◽  
Cosmic Ray ◽  
Thermal Method ◽  
Dark Clouds ◽  
Ice Surface ◽  
Dust Grain ◽  
Complex Organic

ABSTRACT Complex organic molecules (COMs) have been detected in a variety of interstellar sources. The abundances of these COMs in warming sources can be explained by syntheses linked to increasing temperatures and densities, allowing quasi-thermal chemical reactions to occur rapidly enough to produce observable amounts of COMs, both in the gas phase, and upon dust grain ice mantles. The COMs produced on grains then become gaseous as the temperature increases sufficiently to allow their thermal desorption. The recent observation of gaseous COMs in cold sources has not been fully explained by these gas-phase and dust grain production routes. Radiolysis chemistry is a possible non-thermal method of producing COMs in cold dark clouds. This new method greatly increases the modelled abundance of selected COMs upon the ice surface and within the ice mantle due to excitation and ionization events from cosmic ray bombardment. We examine the effect of radiolysis on three C2H4O2 isomers – methyl formate (HCOOCH3), glycolaldehyde (HCOCH2OH), and acetic acid (CH3COOH) – and a chemically similar molecule, dimethyl ether (CH3OCH3), in cold dark clouds. We then compare our modelled gaseous abundances with observed abundances in TMC-1, L1689B, and B1-b.

scholarly journals Investigation of Movement of Nourishment Sand of Different Grain Sizes Using Model for Predicting Bathymetric and Grain Size Changes

2009 ◽  
Vol 65 (1) ◽  
pp. 691-695
Author(s):  
Yasuhito NOSHI ◽  
Akio KOBAYASHI ◽  
Takaaki UDA ◽  
Masumi SERIZAWA ◽  
Takayuki KUMADA
Keyword(s):  
Grain Size ◽  
Grain Sizes

The Method of Eliminating Banded Structure and the Effects of Banded Structure on the Transverse Properties in 20G Steel

2013 ◽  
Vol 347-350 ◽  
pp. 1171-1175 ◽  
Author(s):  
Bin Wang ◽  
Hong Mei Hu ◽  
Cui Zhou
Keyword(s):  
Heat Treatment ◽  
Grain Size ◽  
Stress Concentration ◽  
Base Metal ◽  
Metallic Inclusion ◽  
Banded Structure ◽  
Grain Sizes ◽  
Fine Grain ◽  
Treatment Processes ◽  
Metallic Inclusions

The transverse properties were inferior to the longitudinal properties for the existence of banded structure in 20G steel. In order to eliminate the banded structure and improve the transverse performance of 20G steel, different heat treatment processes were adopted. The results showed that conventional normalizing could reduce the banded structure and refine the grain sizes. When 20G was heated with 10°C/min heating rated and then held at 920°C for 2h, the banded structure in the steel was almost eliminated and the microstructure was homogeneous with fine grain size, the strength increased by 14%. The non-metallic inclusion and carbide in the microstructure leaded to stress concentration and separation with the base metal. To some extent, heat treatment can improve the distribution and form of non-metallic inclusions.

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