J/A+A/587/A130ivo://CDS.VizieR/J/A+A/587/A130doi:10.26093/cds/vizier.35870130CDSBacmann A.Faure A.2018-01-30T07:18:08Z2016-03-02T08:13:20ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frinterstellar-mediumspectroscopyThe recent unexpected detection of terrestrial complex organic molecules in the cold (~10K) gas has cast doubts on the commonly accepted formation mechanisms of these species. Standard gas-phase mechanisms are inefficient and tend to underproduce these molecules, and many of the key reactions involved are unconstrained. Grain-surface mechanisms, which were presented as a viable alternative, suffer from the fact that they rely on grain surface diffusion of heavy radicals, which is not possible thermally at very low temperatures. One of the simplest terrestrial complex organic molecules, methanol is believed to form on cold grain surfaces following from successive H atom additions on CO. Unlike heavier species, H atoms are very mobile on grain surfaces even at 10K. Intermediate species involved in grain surface methanol formation by CO hydrogenation are the radicals HCO and CH_3_O, as well as the stable species formaldehyde H_2_CO. These radicals are thought to be precursors of complex organic molecules on grain surfaces. We present new observations of the HCO and CH_3_O radicals in a sample of prestellar cores and carry out an analysis of the abundances of the species HCO, H_2_CO, CH_3_O, and CH_3_OH, which represent the various stages of grain- surface hydrogenation of CO to CH_3_OH. The abundance ratios between the various intermediate species in the hydrogenation reaction of CO on grains are similar in all sources of our sample, HCO:H_2_CO:CH_3_O:CH_3_OH~10:100:1:100. We argue that these ratios may not be representative of the primordial abundances on the grains but, rather, suggest that the radicals HCO and CH_3_O are gas-phase products of the precursors H_2_CO and CH_3_OH, respectively. Various gas-phase pathways are considered, including neutral-neutral and ion-molecule reactions, and simple estimates of HCO and CH_3_O abundances are compared to the observations. Critical reaction rate constants, branching ratios, and intermediate species are finally identified.2016A&A...587A.130Bhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/587/A130CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/587/A130https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/587/A130http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/587/A130https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/587/A130https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/587/A130http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/587/A130GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/587/A130/table1 (Observed sources and their properties)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/587/A130/table1?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/587/A130/table1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/587/A130/table1?GETtext/xml+votable180.050000true248.70125-24.63444440.0055555555555555566/1050 1075 1114 1168 1290 29560 42970 42993https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/587/A130?format=asciiOpticaldefaultJ/A+A/587/A130/table3Line parameters for HCO transitionsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSourceSource namemeta.id;meta.maincharFreqFrequency of the transitionMHzem.freq;phys.atmol.transitiondoublermsrms noise in spectrummKstat.stdev;instr.det.noisefloatTmb? Line peak intensity, T_mb_mKspect.line.intensityintnullabledv? Linewidthkm/sspect.line.widthfloatnullablel_IntLimit flag on intensitymeta.code.errorcharIntIntegrated intensityK.km/sphot.flux.density;em.radiofloate_Int? Error on integrated intensityK.km/sstat.errorfloatnullablespShow the spectrummeta.ref.urlcharFileNameName of corresponding spectrum file in subdirectory sp (1)meta.id;meta.filecharJ/A+A/587/A130/table1Observed sources and their propertiesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSourceSource namemeta.id;meta.maincharprimaryRAJ2000Right ascension (J2000)pos.eq.ra;meta.mainDEJ2000Declination (J2000)pos.eq.dec;meta.mainnc(H2)H_2_ column densitycm**-2phys.columnDensityfloatRegionRegionmeta.notecharr_nc(H2)References for nc(H2) (1)meta.ref;pos.framecharJ/A+A/587/A130/table4Line parameters for CH_3_O transitionsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSourceSource namemeta.id;meta.maincharFreqFrequency of the transitionMHzem.freq;phys.atmol.transitiondoublermsrms noise in spectrummKstat.stdev;instr.det.noisefloatTmb? Line peak intensity, T_mb_mKspect.line.intensityintnullabledv? Linewidthkm/sspect.line.widthfloatnullablel_IntLimit flag on intensitymeta.code.errorcharIntIntegrated intensityK.km/sphot.flux.density;em.radiofloate_Int? Error on integrated intensityK.km/sstat.errorfloatnullablespShow the spectrummeta.ref.urlcharFileNameName of corresponding spectrum file in subdirectory sp (1)meta.id;meta.filecharJ/A+A/587/A130/table5Line parameters for CH_3_OH transitionsspShow the spectrummeta.ref.urlcharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSourceSource namemeta.id;meta.maincharFreqFrequency of the transitionMHzem.freq;phys.atmol.transitiondoublermsrms noise in spectrummKstat.stdev;instr.det.noisefloatTmb? Line peak intensity, T_mb_mKspect.line.intensityintnullabledv? Linewidthkm/sspect.line.widthfloatnullablel_IntLimit flag on intensitymeta.code.errorcharIntIntegrated intensityK.km/sphot.flux.density;em.radiofloate_Int? Error on integrated intensityK.km/sstat.errorfloatnullableFileNameName of corresponding spectrum file in subdirectory sp (1)meta.id;meta.filechar