J/A+A/622/A26ivo://CDS.VizieR/J/A+A/622/A26doi:10.26093/cds/vizier.36220026CDSGerin M.Liszt H.Neufeld D.Godard B.Sonnentrucker P.Pety J.,Roueff E.2019-02-04T08:46:31Z2019-01-24T09:18:35ZCDS support team

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

cds-question@unistra.frinterferometryinterstellar-mediumdiffuse-molecular-cloudsh-ii-regionsradio-spectroscopyThe transition between atomic and molecular hydrogen is associated with important changes in the structure of interstellar clouds, and marks the beginning of interstellar chemistry. Most molecular ions are rapidly formed (in ion-molecule reactions) and destroyed (by dissociative recombination) in the diffuse ISM. Because of the relatively simple networks controlling their abundances, molecular ions are usually good probes of the underlying physical conditions including for instance the fraction of gas in molecular form or the fractional ionization. In this paper we focus on three possible probes of the molecular hydrogen column density, HCO^+^, HOC^+^, and CF^+^. We presented high sensitivity ALMA absorption data toward a sample of compact HII regions and bright QSOs with prominent foreground absorption, in the ground state transitions of the molecular ions HCO^+^, HOC^+^, and CF^+^ and the neutral species HCN and HNC, and from the excited state transitions of C_3_H^+^(4-3) and ^13^CS(2-1). These data are compared with Herschel absorption spectra of the ground state transition of HF and p-H_2_O. We show that the HCO^+^, HOC^+^, and CF^+^ column densities are well correlated with each other. HCO^+^ and HOC^+^ are tightly correlated with p-H_2_O, while they exhibit a different correlation pattern with HF depending on whether the absorbing matter is located in the Galactic disk or in the central molecular zone. We report new detections of C_3_H^+^ confirming that this ion is ubiquitous in the diffuse matter, with an abundance relative to H_2_ of ~7x10^-11^. We confirm that the CF^+^ abundance is lower than predicted by simple chemical models and propose that the rate of the main formation reaction is lower by a factor of about 3 than usually assumed. In the absence of CH or HF data, we recommend to use the ground state transitions of HCO^+^, CCH, and HOC^+^ to trace diffuse molecular hydrogen, with mean abundances relative to H_2_ of 3x10^-9^, 4x10^-8^ and 4x10^-11^ respectively, leading to sensitivity N(H_2_)/{int}{tau}dv of 4x10^20^, 1.5x10^21^, and 6x10^22^cm^-2^/km/s, respectively.2019A&A...622A..26Ghttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/622/A26CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/622/A26https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/622/A26http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/622/A26https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/622/A26https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/622/A26http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/622/A26GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/622/A26/sources (List of spectra)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/622/A26/sources?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/622/A26/sources?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/622/A26/sources?GETtext/xml+votable180.050000true263.2613055417-13.08041666670.0055555555555555566/28818 28822 28830 28912 29091 29570https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/622/A26?format=asciiMillimeterdefaultJ/A+A/622/A26/sourcesList of spectraMolObserved moleculemeta.notespShow the spectrummeta.ref.urlcharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSourceSource designationmeta.idRAJ2000Right ascension (J2000)pos.eq.ra;meta.mainDEJ2000Declination (J2000)pos.eq.dec;meta.mainGLONGalactic longitudedegpos.galactic.lonGLATGalactic longitudedegpos.galactic.latdoubleTCContinuum fluxJyphot.flux.density;spect.continuumdoubleFreqDoppler-shifted rest Frequency of the line observed in the first channelMHzem.freq;spect.linedoubleFileNameName of the file with spectrum in subdirectory spmeta.id;meta.filecharNameName of the QSO or HII regionmeta.id;meta.maincharSimbadSimbad column added by the CDSmeta.ref.urlchar