Observational tests for grain chemistry: posterior isotopic labelling

Charnley, Steven B.; Ehrenfreund, P.; Millar, T. J.; Boogert, A. C. A.; Markwick, A. J.; Butner, H. M.; Ruiterkamp, R.; Rodgers, S. D.

doi:10.1111/j.1365-2966.2004.07188.x

Cited by 52 publications

(44 citation statements)

References 39 publications

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“…Following the suggestion of Charnley et al (2004), we have also made comparisons of the gas-phase methanol to the 12 C/ 13 C ratios in CO 2 ices, here as observed by ISO (Boogert et al 2000;Gibb et al 2004). These ratios are given in the rightmost column of Table 3.…”

Section: Discussionmentioning

confidence: 99%

“…One such test, isotope labelling "a posteriori", was suggested by Charnley et al (2004). It is based on the observed effect of 13 C fractionation into CO at low temperatures due to the reaction 13 C + + 12 CO −→ 13 CO + 12 C + + ΔE,…”

Section: Introductionmentioning

confidence: 99%

“…If this selective fractionation remains unaltered by the processes of adsorption and desorption, then the 12 C/ 13 C ratio in various molecules could be used to distinguish between formation from CO on cold grains and gas-phase formation. Therefore Charnley et al (2004) propose that the 12 C/ 13 C ratio of hot-core methanol A&A 533, A24 (2011) should be compared to the corresponding isotopic ratio in CO 2 ices, since CO on cold grains has been shown to react also with atomic oxygen to form solid CO 2 (Roser et al 2001). Ices are not observed towards "regular" hot cores, but some embedded young stellar objects (YSOs), being at an earlier stage of evolution and harbouring ices in their envelopes, show characteristics implying a small hot core closest to the central object.…”

Section: Introductionmentioning

confidence: 99%

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Observational tests of interstellar methanol formation

Wirström

Geppert

Hjalmarson

et al. 2011

A&A

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Context. It has been established that the classical gas-phase production of interstellar methanol (CH 3 OH) cannot explain observed abundances. Instead it is now generally thought that the main formation path has to be by successive hydrogenation of solid CO on interstellar grain surfaces. Aims. While theoretical models and laboratory experiments show that methanol is efficiently formed from CO on cold grains, our aim is to test this scenario by astronomical observations of gas associated with young stellar objects (YSOs). Methods. We have observed the rotational transition quartets J = 2 K -1 K of 12 CH 3 OH and 13 CH 3 OH at 96.7 and 94.4 GHz, respectively, towards a sample of massive YSOs in different stages of evolution. In addition, the J = 1−0 transitions of 12 C 18 O and 13 C 18 O were observed towards some of these sources. We use the 12 C/ 13 C ratio to discriminate between gas-phase and grain surface origin: If methanol is formed from CO on grains, the ratios should be similar in CH 3 OH and CO. If not, the ratio should be higher in CH 3 OH due to 13 C fractionation in cold CO gas. We also estimate the abundance ratios between the nuclear spin types of methanol (E and A). If methanol is formed on grains, this ratio is likely to have been thermalized at the low physical temperature of the grain, and therefore show a relative over-abundance of A-methanol. Results. We show that the 12 C/ 13 C isotopic ratio is very similar in gas-phase CH 3 OH and C 18 O, on the spatial scale of about 40 , towards four YSOs. For two of our sources we find an overabundance of A-methanol as compared to E-methanol, corresponding to nuclear spin temperatures of 10 and 16 K. For the remaining five sources, the methanol E/A ratio is less than unity. Conclusions. While the 12 C/ 13 C ratio test is consistent with methanol formation from hydrogenation of CO on grain surfaces, the result of the E/A ratio test is inconclusive.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Observational tests of interstellar methanol formation

Wirström

Geppert

Hjalmarson

et al. 2011

A&A

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“…These high abundances indicate that solid CH 3 OH must have been evaporated from the icy grain mantles into the gas phase. In the grain-surface chemistry scheme, CH 3 OH is formed through successive hydrogenation of CO onto dust grains (CO → HCO → H 2 CO → CH 2 OH → CH 3 OH), and subsequent grain mantle evaporation is then predicted to cause the gas-phase abundance of CH 3 OH to be >10 −8 (e.g., van der Tak et al 2000;Charnley et al 2004;Garrod et al 2008). In addition to grain heating, methanol can be released from grain mantles by moderate shocks (e.g., Bachiller & Perez Gutierrez 1997, and references therein).…”

Section: Ch 3 Ohmentioning

confidence: 99%

A molecular line study of the filamentary infrared dark cloud G304.74+01.32

Miettinen

2012

A&A

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Context. Infrared dark clouds (IRDCs) are promising sites to study the earliest formation stages of stellar clusters and high-mass stars, and the physics of molecular-cloud formation and fragmentation. Aims. We attempt to improve our understanding of the physical and chemical properties of the filamentary IRDC G304.74+01.32 (hereafter, G304.74). In particular, we investigate the kinematical and dynamical state of the cloud and clumps within it, and the amount of CO depletion. Methods. All of the submillimetre peak positions in the cloud identified from our previous LABOCA 870-μm map were observed in C 17 O(2−1) with APEX. These are the first line observations along the whole filament that have been made so far. Selected positions were also observed in the 13 CO(2−1), SiO(5−4), and CH 3 OH(5 k −4 k ) transitions at ∼1 mm. Results. The C 17 O lines were detected towards all target positions at similar radial velocities. CO does not appear to be significantly depleted in the clumps, the largest depletion factors being only about 2. Two to three methanol 5 k −4 k lines near ∼241.8 GHz were detected towards all selected positions, whereas SiO(5−4) was seen in only one of these positions, namely SMM 3. In the band covering SiO(5−4), we also detected the DCN(3−2) line towards SMM 3. The 13 CO(2−1) lines display blue asymmetric profiles, which are indicative of large-scale infall motions. The clumps show transonic to supersonic non-thermal motions, and a virial-parameter analysis suggests that most of them are gravitationally bound. The external pressure may also play a non-negligible role in the dynamics. Our analysis suggests that the fragmentation of the filament into clumps is caused by a "sausage"-type instability, in agreement with results from other IRDCs. Conclusions. The uniform C 17 O radial velocities along the G304.74 cloud shows that it is a coherent filamentary structure. Although the clumps appear to be gravitationally bound, the ambient turbulent ram pressure may be an important factor in the cloud dynamics. This is qualitatively consistent with our earlier suggestion that the filament was formed by converging supersonic turbulent flows. The poloidal magnetic field could resist the radial cloud collapse, which conforms to the low infall velocites that we derived. The cloud may be unable to form high-mass stars based on the mass-size threshold. The star-formation activity in the cloud, such as outflows, is likely responsible for the release of CO from the icy grain mantles back into the gas phase. Shocks related to outflows may also have injected CH 3 OH, SiO, and DCN into the gas-phase in SMM 3.

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“…The OCS molecules form on grain surfaces through the addition of S atom to CO, or via the O atom addition to CS (e.g., Charnley et al 2004). Solid-state OCS has been detected towards high-mass star-forming regions by, e.g, Gibb et al (2004).…”

Section: Appendix A: Other Detected Transitionsmentioning

confidence: 99%

Deuterium fractionation and the degree of ionisation in massive clumps within infrared dark clouds

Miettinen

Hennemann

Linz

2011

A&A

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Context. Massive clumps associated with infrared dark clouds (IRDCs) are promising targets for studying the earliest stages of highmass star and cluster formation. Aims. We aim to determine the degrees of CO depletion, deuterium fractionation, and ionisation in a sample of seven massive clumps associated with IRDCs. Methods. The APEX telescope was used to observe the C 17 O(2−1), H 13 CO + (3−2), DCO + (3−2), N 2 H + (3−2), and N 2 D + (3−2) transitions towards the clumps. The spectral line data were used in conjunction with the previously published and/or archival (sub)millimetre dust continuum observations of the sources. The data were used to derive the molecular column densities and fractional abundances for the analysis of deuterium fractionation and ionisation. Results. The CO molecules do not appear to be significantly depleted in the observed clumps. The DCO + /HCO + and N 2 D + /N 2 H + column density ratios are about 0.0002-0.014 and 0.002-0.028, respectively. The former ratio is found to decrease as a function of gas kinetic temperature. A simple chemical analysis suggests that the lower limit to the ionisation degree is in the range x(e) ∼ 10 −8 −10 −7 , whereas the estimated upper limits range from a few 10 −6 up to ∼10 −4 . Lower limits to x(e) imply that the cosmic-ray ionisation rate of H 2 lies between ζ H 2 ∼ 10 −17 −10 −15 s −1 . These are the first estimates of x(e) and ζ H 2 towards massive IRDCs reported so far. Some additional molecular transitions, mostly around 216 and 231 GHz, were detected towards all sources. In particular, IRDC 18102-1800 MM1 and IRDC 18151-1208 MM2 show relatively line-rich spectra. Some of these transitions might be assigned to complex organic molecules, although the line blending hampers the identification. The C 18 O(2−1) transition is frequently seen in the image band. Conclusions. The finding that CO is not depleted in the observed sources conforms to the fact that they show evidence of star formation activity, which is believed to release CO from the icy grain mantles back into the gas phase. The observed degree of deuteration is lower than in low-mass starless cores and protostellar envelopes. Decreasing deuteration with increasing temperature is likely to reflect the clump evolution. On the other hand, the association with young high-mass stars could enhance ζ H 2 and x(e) above the levels usually found in low-mass star-forming regions. On the scale probed by our observations, ambipolar diffusion cannot be a main driver of clump evolution unless it occurs on timescales 10 6 yr.

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Observational tests for grain chemistry: posterior isotopic labelling

Cited by 52 publications

References 39 publications

Observational tests of interstellar methanol formation

Observational tests of interstellar methanol formation

A molecular line study of the filamentary infrared dark cloud G304.74+01.32

Deuterium fractionation and the degree of ionisation in massive clumps within infrared dark clouds

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