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Observational tests of interstellar methanol formation
Stockholm University, Faculty of Science, Department of Physics.
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2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 533, A24- p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2011. Vol. 533, A24- p.
Keyword [en]
ISM: molecules, astrochemistry, radio lines: ISM
National Category
Natural Sciences
URN: urn:nbn:se:su:diva-67293DOI: 10.1051/0004-6361/201116525ISI: 000295168100024OAI: diva2:469931
authorCount :9Available from: 2011-12-27 Created: 2011-12-27 Last updated: 2011-12-27Bibliographically approved
In thesis
1. Dissociative Recombination of Astrochemically Interesting Ions
Open this publication in new window or tab >>Dissociative Recombination of Astrochemically Interesting Ions
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis the major work described concerns experimental determination of the dissociative recombination (DR) reaction for several molecular ions of astrochemical interest. DR is the process where an electron recombines with a molecular ion to form an excited neutral that disintegrates into two or more neutral fragments to release the gained excess energy. It is very efficient under cold conditions and therefore ubiquitously occurring in interstellar environments such as dark clouds and plays an important role in aeronomical plasmae, lightnings and in man-made plasmas such as in combustion engines and fusion reactors. Although DR reactions are crucial processes in all these environments, product branching fractions of DR reactions have proven to be very unpredictable and present one of the great remaining challenges for theoreticians. The experimental work includes determination of reaction rates and product distribution of DR of complex ions such as protonated alcohols and ethers. The following species have been investigated and are discussed in this thesis:

CH3OH2+ (protonated methanol), CD3OD2+ (deuteronated methanol), CD3OCD2+ (methoxymethyl cation), CD3CDOD+ (deuteronated acetaldehyde), CH3CH2OH2+ (protonated ethanol) and (CD3)2OD+ (deuteronated dimethyl ether).

The results of these measurements are used in astrochemical model calculations in which the rates used hitherto greatly have been based on educated guesses. Employing the outcome of the DR investigations of the CH3OH2+ and CD3OD2+ ions have shown a great impact on such models. The DR investigations have been followed up by astronomical observations. Theoretical models and laboratory experiments show that methanol should be formed from CO on cold grains. This scenario was tested by astronomical observations of gas associated with young stellar objects (YSOs). Two independent tests were showing consistency with methanol formation on grain surfaces.

Abstract [sv]

I den här avhandlingen redovisas mitt arbete som till stor del baseras på experimentell bestämning av dissociativa rekombinations (DR) processer för molekylära joner av astrokemiskt intresse. DR är en process där en elektron rekombinerar med en molekylär jon som splittras up i två eller fler neutrala fragment för att göra sig av med den extra energi som erhållits. Processen är väldigt effektiv i kalla miljöer varför den är allestädes återkommande i omgivningar som interstellära moln och kometkoman och spelar en betydande roll i aeronomiska plasman, blixturladdningar men även i mänskligt skapade plasman såsom de i förbränningsmotorer och fusionsreaktorer. Det har dock visat sig att produkt distributionsförhållandena från DR reaktioner är mycket oförutsägbara och kvarstår som en av de stora återstående utmaningarna för teoretiker. Det experimentella arbetet består av bestämning av reaktionshastigheter samt produktdistribution för DR av komplexa joner som protonerade alkoholer och etrar. De följande jonerna har blivit undersökta och diskuteras i denna avhandling:

CH3OH2+ (protonerad metanol), CD3OD2+ (deuteronerad metanol), CD3OCD2+ (metoxymetyl katjon), CD3CDOD+ (deuteronerad acetaldehyd), CH3CH2OH2+ (protonerad etanol) och (CD3)2OD+ (deuteronerad dimetyleter).

Resultaten av mätningarna används i astrokemiska modelberäkningar i vilka reaktionshastigheterna som hittills använts till stor del baserats på kvalificerade gissningar. Insättning av resultaten av CH3OH2+ och CD3OD2+ jonerna har visat sig ha en stor effekt på sådana modeller. DR undersökningarna har följts upp av astronomiska observationer. Teoretiska modeller och laboratorieundersökningar visar att metanol borde kunna formas från CO på kalla iskornsytor, detta scenario har testats med astronomiska observationer av gas som associeras med unga stjärnor. Två oberoende undersökningar visade på förenlighet med metanolformation på kornytor.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2010. 79 p.
Dissociative Recombination, Astrochemistry
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
urn:nbn:se:su:diva-38833 (URN)978-91-7447-089-5 (ISBN)
Public defence
2010-05-28, Svedbergssalen, sal FD5, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
At the time of the doctoral defense, the following papers were unpublished  and had a status as follows: Paper 1: Manuscript. Paper 2: In press. Paper 3: Manuscript. Paper 5: Manuscript.Available from: 2010-05-06 Created: 2010-04-29 Last updated: 2011-12-27Bibliographically approved

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Geppert, Wolf D.Vigren, Erik
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