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- Persson, Magnus V., 1983, et al.
(författare)
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The ALMA-PILS Survey: Formaldehyde deuteration in warm gas on small scales toward IRAS 16293-2422 B
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 610
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Tidskriftsartikel (refereegranskat)abstract
- Context. The enhanced degrees of deuterium fractionation observed in envelopes around protostars demonstrate the importance of chemistry at low temperatures, relevant in pre- and protostellar cores. Formaldehyde is an important species in the formation of methanol and more complex molecules. Aims. Here, we aim to present the first study of formaldehyde deuteration on small scales around the prototypical low-mass protostar IRAS 16293-2422 using high spatial and spectral resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations. We determine the excitation temperature, abundances and fractionation level of several formaldehyde isotopologues, including its deuterated forms. Methods. Excitation temperature and column densities of formaldehyde in the gas close to one of the components of the binary were constrained through modeling of optically thin lines assuming local thermodynamical equilibrium. The abundance ratios were compared to results from previous single dish observations, astrochemical models and local ISM values. Results. Numerous isotopologues of formaldehyde are detected, among them H 2 C 17 O, and D 2 13 CO for the first time in the ISM. The large range of upper energy levels covered by the HDCO lines help constrain the excitation temperature to 106 ± 13 K. Using the derived column densities, formaldehyde shows a deuterium fractionation of HDCO/H 2 CO = 6.5 ± 1%, D 2 CO/HDCO = 12.8 -4.1 +3.3 %, and D 2 CO/H 2 CO = 0.6(4) ± 0.1%. The isotopic ratios derived are 16 O/ 18 O = 805 -79 +43 , 18 O/ 17 O = 3.2 -0.3 +0.2 , and 12 C/ 13 C = 56 -11 +8 . Conclusions. The HDCO/H 2 CO ratio is lower than that found in previous studies, highlighting the uncertainties involved in interpreting single dish observations of the inner warm regions. The D 2 CO/HDCO ratio is only slightly larger than the HDCO/H 2 CO ratio. This is consistent with formaldehyde forming in the ice as soon as CO has frozen onto the grains, with most of the deuteration happening toward the end of the prestellar core phase. A comparison with available time-dependent chemical models indicates that the source is in the early Class 0 stage.
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| 2. |
- Manigand, S., et al.
(författare)
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The ALMA-PILS survey: The first detection of doubly-deuterated methyl formate (CHD2OCHO) in the ISM
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746.
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Tidskriftsartikel (refereegranskat)abstract
- Studies of deuterated isotopologues of complex organic molecules can provide important constraints on their origin in regions of star formation. In particular, the abundances of deuterated species are very sensitive to the physical conditions in the environment where they form. Due to the low temperatures in regions of star formation, these isotopologues are enhanced to significant levels, making detections of multiply-deuterated species possible. However, for complex organic species, only the multiply-deuterated variants of methanol and methyl cyanide have been reported so far. The aim of this paper is to initiate the characterisation of multiply-deuterated variants of complex organic species with the first detection of doubly-deuterated methyl formate, CHD2OCHO. We use ALMA observations from the Protostellar Interferometric Line Survey (PILS) of the protostellar binary IRAS 16293-2422, in the spectral range of 329.1 GHz to 362.9 GHz. We report the first detection of doubly-deuterated methyl formate CHD2OCHO in the ISM. The D/H ratio of CHD2OCHO is found to be 2-3 times higher than the D/H ratio of CH2DOCHO for both sources, similar to the results for formaldehyde from the same dataset. The observations are compared to a gas-grain chemical network coupled to a dynamical physical model, tracing the evolution of a molecular cloud until the end of the Class 0 protostellar stage. The overall D/H ratio enhancements found in the observations are of the same order of magnitude as the predictions from the model for the early stages of Class 0 protostars. However, the higher D/H ratio of CHD2OCHO compared to the D/H ratio of CH2DOCHO is still not predicted by the model. This suggests that a mechanism is enhancing the D/H ratio of singly- and doubly-deuterated methyl formate that is not in the model, e.g. mechanisms for H-D substitutions.
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