The Effects of Grain Size and Grain Growth on the Chemical Evolution of Cold Dense Clouds

Acharyya, Kinsuk; Hassel, G. E.; Herbst, Eric

doi:10.1088/0004-637x/732/2/73

Cited by 47 publications

(59 citation statements)

References 36 publications

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“…Wickramasinghe 1965). More complicated modelling supports this in the sense that the smallest grains grow relatively faster than large grains (Acharyya et al 2011). The effect would therefore be modest in terms of relative growth for micron-sized grains and is inefficient in itself to build the largest grains from 0.1 µm or smaller grains that can be created through coagulation.…”

Section: Grain Growth and Water-ice Abundancementioning

confidence: 87%

A common column density threshold for scattering at 3.6μm and water-ice in molecular clouds

Andersen

Steinacker

Tothill

2014

A&A

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Context. Observations of scattered light in the 1−5 µm range have revealed dust grains in molecular cores with sizes larger than commonly inferred for the diffuse interstellar medium. It is currently unclear whether these grains are grown within the molecular cores or are an ubiquitous component of the interstellar medium. Aims. We investigate whether the large grains necessary for efficient scattering at 1−5 µm are associated with the abundance of waterice within molecular clouds and cores. Methods. We combined water-ice abundance measurements for sight lines through the Lupus IV molecular cloud complex with measurements of the scattered light at 3.6 µm for the same sight lines. Results. We find that there is a similar threshold for the cores in emission in scattered light at 3.6 µm (τ 9.7 = 0.15 ± 0.05, A K = 0.4 ± 0.2) as water-ice (τ 9.7 = 0.11 ± 0.01, A K = 0.19 ± 0.04) and that the scattering efficiency increases as the relative water-ice abundance increases. The ice layer increases the average grain size, which again strongly increases the albedo. Conclusions. The higher scattering efficiency is partly due to layering of ice on the dust grains. Although the layer can be relatively thin, it can enhance the scattering substantially.

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Section: Grain Growth and Water-ice Abundancementioning

confidence: 87%

A common column density threshold for scattering at 3.6μm and water-ice in molecular clouds

Andersen

Steinacker

Tothill

2014

A&A

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“…The adsorption rate of gaseous species onto grain surfaces and grain-surface reaction rates (e.g., if the rate is limited by the accretion of gaseous particles) depend on the size distribution of grains. Ideally, the grain size distribution should be taken into account (Acharyya et al 2011). Most chemical models of disks and molecular clouds, however, assume a single size of 0.1 μm, for simplicity, which we follow in the present work.…”

Section: Chemical Model: Full Networkmentioning

confidence: 99%

ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N₂H⁺RING IN PROTOPLANETARY DISKS

Aikawa

Furuya

Nomura

et al. 2015

ApJ

101

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We investigate the chemistry of ion molecules in protoplanetary disks, motivated by the detection of the N 2 H + ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N 2 H + is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N 2 . Using the full gas-grain network model, we reproduced the N 2 H + ring in a disk model with millimeter grains. The chemical conversion of CO and N 2 to less volatile species (sink effect hereinafter) is found to affect the N 2 H + distribution. Since the efficiency of the sink depends on various parameters such as activation barriers of grain-surface reactions, which are not well constrained, we also constructed the no-sink model; the total (gas and ice) CO and N 2 abundances are set constant, and their gaseous abundances are given by the balance between adsorption and desorption. Abundances of molecular ions in the no-sink model are calculated by analytical formulae, which are derived by analyzing the full-network model. The N 2 H + ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N 2 H + , however, is different among the full-network model and no-sink model. The column density of N 2 H + in the no-sink model depends sensitively on the desorption rate of CO and N 2 and the cosmic-ray flux. We also found that N 2 H + abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N 2 are the same.

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“…Katz et al (1999) found this ratio to be ∼0.8 for hydrogen by fitting laboratory data. Due to the lack of laboratory data for other species, several values are used in the literature: 0.3 (Hasegawa et al 1992), 0.5 (Garrod & Herbst 2006;Garrod et al 2008;Acharyya et al 2011), and a timedependent value (Garrod & Pauly 2011). Our adopted diffusion-to-binding energy ratio for this calculation is 0.5.…”

Section: Chemical Network and Modelmentioning

confidence: 99%

Simulations of the Chemistry in the Small Magellanic Cloud

Acharyya

Herbst

2016

ApJ

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The Large (LMC) and Small (SMC) Magellanic Clouds are irregular satellite galaxies of the Milky Way. Both are metal-and dust-poor, although the SMC is significantly poorer in both. We have recently simulated the chemistry in cold dense regions of the LMC and found that a rich chemistry exists in the gas-phase. In this paper, we report a companion study of the chemistry of dense regions of the SMC, confining our attention to cold regions of dense clouds with a variety of densities, visual extinctions, and grain temperatures, and a fixed gas-phase temperature. With a gas-to-dust ratio and elemental abundances based on observations and scaling, we found that for molecules like CO and N 2 , which are predominantly formed in the gas phase, their abundances are consistent with the reduced elemental abundances of their constituent elements above 25 K; however, for species that are produced fully (e.g., CH 3 OH) or partially on the grain surface (e.g., H 2 CO, NH 3 ), the dependence on metallicity can be complex. Most of the major gas-phase species observed in our Galaxy are produced in the SMC although in lower quantities. With our simulations, we are able to explain observed gas-phase abundances reasonably well in the dense sources N27 and LIRS 36. We have also compared our calculated abundances of selected ices with limited observations in dense regions in front of young stellar objects.

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The Effects of Grain Size and Grain Growth on the Chemical Evolution of Cold Dense Clouds

Cited by 47 publications

References 36 publications

A common column density threshold for scattering at 3.6μm and water-ice in molecular clouds

A common column density threshold for scattering at 3.6μm and water-ice in molecular clouds

ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N₂H⁺RING IN PROTOPLANETARY DISKS

Simulations of the Chemistry in the Small Magellanic Cloud

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The Effects of Grain Size and Grain Growth on the Chemical Evolution of Cold Dense Clouds

Cited by 47 publications

References 36 publications

A common column density threshold for scattering at 3.6μm and water-ice in molecular clouds

A common column density threshold for scattering at 3.6μm and water-ice in molecular clouds

ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N2H+RING IN PROTOPLANETARY DISKS

Simulations of the Chemistry in the Small Magellanic Cloud

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ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N₂H⁺RING IN PROTOPLANETARY DISKS