Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface

Whittet, D. C. B.; Gerakines, Perry A.; Shenoy, S.

doi:10.1086/318421

Cited by 268 publications

(357 citation statements)

References 51 publications

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“…This component is thought to be formed when the cloud density increases and the gaseous atomic H/CO ratio decreases (Tielens & Hagen 1982), and thus occurs at a later stage or deeper in the cloud than the water ice formation. This is consistent with the higher observed extinction threshold for CO ice formation compared with that of H 2 O (Whittet et al 2001). In the highest density Article published by EDP Sciences regions (>few ×10 5 cm −3 ), the timescales for collisions of CO with the grains become so short that most of the gaseous CO is removed from the gas.…”

Section: Introductionsupporting

confidence: 88%

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Cuppen

Dishoeck

Herbst

et al. 2009

A&A

164

203

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Context. Methanol and its precursor formaldehyde are among the most studied organic molecules in the interstellar medium and are abundant in the gaseous and solid phases. We recently developed a model to simulate CO hydrogenation via H atoms on interstellar ice surfaces, the most important interstellar route to H 2 CO and CH 3 OH, under laboratory conditions. Aims. We extend this model to simulate the formation of both organic species under interstellar conditions, including freeze-out from the gas and hydrogenation on surfaces. Our aim is to compare calculated abundance ratios with observed values and with the results of prior models. Methods. Our model utilises the continuous-time, random-walk Monte Carlo method, which -unlike other approaches -is able to simulate microscopic grain-surface chemistry over the long timescales in interstellar space, including the layering of ices during freeze-out. Results. Simulations under different conditions, including density and temperature, have been performed. We find that H 2 CO and CH 3 OH form efficiently in cold dense cores or the cold outer envelopes of young stellar objects. The grain mantle is found to have a layered structure with CH 3 OH on top. The species CO and H 2 CO are found to exist predominantly in the lower layers of ice mantles where they are not available for hydrogenation at late times. This finding is in contrast with previous gas-grain models, which do not take into account the layering of the ice. Some of our results can be reproduced by a simple quasi-steady-state analytical model that focuses on the outer layer. Conclusions. Observational solid H 2 CO/CH 3 OH and CO/CH 3 OH abundance ratios in the outer envelopes of an assortment of young stellar objects agree reasonably well with our model results, which also suggest that the large range in CH 3 OH/H 2 O observed abundance ratios is due to variations in the evolutionary stages. Finally, we conclude that the limited chemical network used here for surface reactions apparently does not alter the overall conclusions.

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Section: Introductionsupporting

confidence: 88%

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Cuppen

Dishoeck

Herbst

et al. 2009

A&A

164

203

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“…Considering the evolution of grain surface mantles, it can indeed be easily explained why CO is in a CH 3 OH rather than a H 2 O environment. Observational studies show that H 2 O freezes out at much lower extinctions (Whittet et al 2001;Chiar et al 2011;Boogert et al 2013;Whittet et al 2013) than CO, which needs a higher density to freeze out (Chiar et al 1994). Methanol forms at even higher extinction as shown by ice observations (Boogert et al 2011;Chiar et al 2011;Whittet et al 2011), which is consistent with the picture that a relatively high density is needed to efficiently convert CO into CH 3 OH Boogert, Gerakines & Whittet 2015).…”

Section: Discussion a N D C O N C L U D I N G R E M A R K Ssupporting

confidence: 72%

Spectroscopic constraints on CH₃OH formation: CO mixed with CH₃OH ices towards young stellar objects

Penteado

Boogert

Pontoppidan

et al. 2015

Mon. Not. R. Astron. Soc.

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The prominent infrared absorption band of solid CO -commonly observed towards young stellar objects (YSOs) -consists of three empirically determined components. The broad 'red component' (2136 cm −1 , 4.681 μm) is generally attributed to solid CO mixed in a hydrogenbonded environment. Usually, CO embedded in the abundantly present water is considered. However, CO:H 2 O mixtures cannot reproduce the width and position of the observed red component without producing a shoulder at 2152 cm −1 , which is not observed in astronomical spectra. Cuppen et al. showed that CO:CH 3 OH mixtures do not suffer from this problem. Here, this proposition is expanded by comparing literature laboratory spectra of different CO-containing ice mixtures to high-resolution (R = λ/ λ = 25 000) spectra of the massive YSO AFGL 7009S and of the low-mass YSO L1489 IRS. The previously unpublished spectrum of AFGL 7009S shows a wide band of solid 13 CO, the first detection of 13 CO ice in the polar phase. In this source, both the 12 CO and 13 CO ice bands are well fitted with CO:CH 3 OH mixtures, while respecting the profiles and depths of the methanol bands at other wavelengths, whereas mixtures with H 2 O cannot. The presence of a gradient in the CO:CH 3 OH mixing ratio in the grain mantles is also suggested. Towards L1489 IRS, the profile of the 12 CO band is also better fitted with CH 3 OH-containing ices, although the CH 3 OH abundance needed is a factor of 2.4 above previous measurements. Overall, however, the results are reasonably consistent with models and experiments about formation of CH 3 OH by the hydrogenation of CO ices.

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“…The initial radius of the clump at time t 1 is assumed to be 10 14 cm. Whittet et al (2001) show that the detection threshold for water ice in the Taurus Dark Clouds is A V = 3.2. Assuming this number is also valid for clumps in AGB winds we find for the shielding time scale…”

Section: A Clumpy Agb Windmentioning

confidence: 91%

Water ice growth around evolved stars

Dijkstra

Dominik

Bouwman

et al. 2006

A&A

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We present combined radiative transfer and H 2 O ice formation calculations for the dusty envelopes of oxygen-rich evolved stars. We study the effects of various (circum-)stellar parameters on the spectral energy distribution of these stars, their infrared spectral water ice features at 3 µm and in the 30−100 µm region, and the properties of (water ice on) the grains in their envelopes. We also study the ice formation process as a function of stellar evolution for a star with an initial mass of 5 M , which is followed during the AGB, post-AGB and planetary nebula (PN) phase. We find that its water ice features probe its evolution. Both crystalline and amorphous water ice form in our models. The 43 and 62 µm crystalline water ice features are most prominent during the post-AGB phase, and only modestly or not present during the AGB and PN phase, in agreement with observations. The strength of the 3, 43 and 62 µm water ice features decreases with decreasing initial mass of the star. The total amount of ice predicted (a few percent of the total dust mass) also agrees with observations, but the crystalline ice mass fraction is consistently about two orders of magnitude lower. This is mainly due to efficient amorphization by interstellar UV photons, and leads to weaker 43 and 62 µm crystalline water ice features than observed. The intensity of the interstellar UV radiation field strongly influences the strength of these features. We discuss several means to increase the crystalline water ice mass, and hence their strength. The strength of the features increases dramatically when the mass-loss rate over luminosity ratio of the star,Ṁ/L, is large in the AGB phase. In case of the post-AGB star HD 161796 we demonstrate that this indeed leads to the correct crystalline ice mass fraction and feature strengths. Also, the formation of clumps in the AGB wind provides high densities to stimulate the formation of (crystalline) ice. For stars with high initial masses, it additionally provides sufficient shielding from interstellar UV radiation to keep ice crystalline during the post-AGB and PN phase. Axisymmetric mass loss on the AGB provides favorable conditions for the formation and preservation of water ice, and crystalline water ice in particular, as well. In contrast we find that post-AGB crystallization of AGB produced amorphous ice is unimportant for increasing the crystalline water ice mass around 5 M stars.

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Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface

Cited by 268 publications

References 51 publications

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Spectroscopic constraints on CH₃OH formation: CO mixed with CH₃OH ices towards young stellar objects

Water ice growth around evolved stars

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Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface

Cited by 268 publications

References 51 publications

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Spectroscopic constraints on CH3OH formation: CO mixed with CH3OH ices towards young stellar objects

Water ice growth around evolved stars

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Spectroscopic constraints on CH₃OH formation: CO mixed with CH₃OH ices towards young stellar objects