J/A+A/710/A301ivo://CDS.VizieR/J/A+A/710/A301CDSKim W.-J.Jacob A.M.Neufeld D.A.Schilke P.Wiesemeyer H.Gerin M.,Wolfire M.G.Ossenkopf-Okada V.Valdivia V.Falgarone E.Lis D.C.,Bialy S.Rugel M.R.Sanchez-Monge A.Busch M.Moeller T.Wyrowski F.,Seifried D.Menten K.M.Saintonge A.2026-06-24T08:25:32Z2026-06-24T09:24:19ZCDS support team

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

cds-question@unistra.frinterstellar-mediummolecular-cloudsspectroscopyinfrared-astronomyThe HyGAL Stratospheric Observatory for Infrared Astronomy (SOFIA) legacy program aims at characterizing the interstellar medium in the Milky Way using hydrides, [CII], and [OI] absorption lines with the 2.7m SOFIA telescope toward twenty-five submillimeter-bright Galactic star-forming regions. As part of HyGAL, we investigated correlations among the known H_2_ tracers- CH and OH from SOFIA observations, and HCO^+^ and CCH from ancillary absorption line data from ground-based telescopes. We also examined the abundance variation of neutral atomic oxygen, [OI], observed in absorption. CH, OH, HCO^+^, and CCH all exhibit strong mutual correlations. OH in particular shows tight correlations with HCO^+^ and CCH, reflecting their linked chemical and physical pathways. Column density ratios among these H_2_ tracers are consistent with previous measurements in local diffuse clouds and remain uniform across Galactic environments and velocity intervals. The gas phase oxygen abundance relative to total hydrogen, <X(O)>=N(O)/N(Htotal), is (3.09+/-0.64)x10^-4^, slightly below the elemental solar value but consistent with the previous observations measuring gas-phase abundances. We also find that N(HI) decreases toward the regions where the molecular fraction exceeds f^N^_H2_~0.5, marking the onset of the molecular phase. While the atomic oxygen abundance remains roughly constant, the abundances of OH, HCO^+^, and CCH increase with the molecular fraction. Gas traced by the HCO^+^ absorption corresponds to higher molecular fractions than that traced by HI and hydride ions, highlighting density variations in the diffuse-to-translucent ISM along different lines of sight.2026A&A...710A.301Khttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/710/A301CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/710/A301https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/710/A301http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/710/A301https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/710/A301https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/710/A301http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/710/A301GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/710/A301/tablea1 (The summary of HyGAL source information)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/710/A301/tablea1?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/710/A301/tablea1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/710/A301/tablea1?GETtext/xml+votable180.050000true155.067083-58.0652780.0055555555555555566/3649 14317 14704 28813 28829 29096-29097 29510 30259 30262 30269 30312 30622 37343 37924 38024 41812 42159 42169 42224 42381 42456 42498 43013https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/710/A301?format=asciiInfrareddefaultJ/A+A/710/A301/tablea1The summary of HyGAL source informationrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameSource namemeta.id;meta.maincharRAJ2000Right Ascension (J2000)pos.eq.ra;meta.mainDEJ2000Declination (J2000)pos.eq.dec;meta.mainGLONGalactic longitudedegpos.galactic.lonGLATGalactic latitudedegpos.galactic.latfloatvLSRLSR velocitykm/sphys.veloc;pos.lsrfloatdDistance or lower value of distance intervalkpcpos.distancefloatd2? Upper value of distance intervalkpcpos.distancefloatnullableRgalGalactodentric distance or lower value of Galactodentric radius intervalkpcpos.distance;pos.galactocentricfloatRgal2? Upper value of Galactodentric distance intervalkpcpos.distance;pos.galactocentricfloatnullablesp1Show the spectrummeta.ref.urlcharspCCHName of the CCH spectrum file in subdirectory spmeta.id;meta.filecharsp2Show the spectrummeta.ref.urlcharspCHName of the CH spectrum file in subdirectory spmeta.id;meta.filecharsp3Show the spectrummeta.ref.urlcharspHCO+Name of the HCO+ spectrum file in subdirectory spmeta.id;meta.filecharsp4Show the spectrummeta.ref.urlcharSimbadNameSimbad column added by the CDSmeta.idcharnullablesp5Show the spectrummeta.ref.urlcharspOHName of the OH spectrum file in subdirectory spmeta.id;meta.filecharspOIName of the OI spectrum file in subdirectory spmeta.id;meta.filecharJ/A+A/710/A301/tablec1Column densities over VI(A)e_N(HCO+)? N(HCO+) lower limit sigmacm**-2stat.errordoublenullablerecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameSource namemeta.id;meta.maincharVminMinimum velocitykm/sphys.velocfloatVmaxMaximum velocitykm/sphys.velocfloatRgalmin? Minimum Galactocentric distancekpcpos.distance;stat.minfloatnullableRgalmax? Maximum Galactocentric distancekpcpos.distance;stat.maxfloatnullableN(CH)? CH column densitycm**-2phys.columnDensitydoublenullablee_N(CH)? N(CH) lower limit sigmacm**-2stat.errordoublenullableE_N(CH)? N(CH) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(OH)? OH column densitycm**-2phys.columnDensitydoublenullablee_N(OH)? N(OH) lower limit sigmacm**-2stat.errordoublenullableE_N(OH)? N(OH) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(HCO+)? HCO+ column densitycm**-2phys.columnDensitydoublenullableE_N(HCO+)? N(HCO+) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(CCH)? CCH column densitycm**-2phys.columnDensitydoublenullablee_N(CCH)? N(CCH) lower limit sigmacm**-2stat.errordoublenullableE_N(CCH)? N(CCH) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(O)? Oxygen column densitycm**-2phys.columnDensitydoublenullablee_N(O)? N(O) lower limit sigmacm**-2stat.errordoublenullableE_N(O)? N(O) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(H2)? H2 column densitycm**-2phys.columnDensitydoublenullablee_N(H2)? N(H2) lower limit sigmacm**-2stat.errordoublenullableE_N(H2)? N(H2) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(HI)? HI column densitycm**-2phys.columnDensitydoublenullablee_N(HI)? N(HI) lower limit sigmacm**-2stat.errordoublenullableE_N(HI)? N(HI) upper limit sigmacm**-2stat.error;stat.maxdoublenullablefH2? Molecular gas fraction (fH2)meta.numberfloatnullableJ/A+A/710/A301/tablec2Column densities over VI(M)recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameSource namemeta.id;meta.maincharVminMinimum velocitykm/sphys.velocfloatVmaxMaximum velocitykm/sphys.velocfloatRgalmin? Minimum Galactocentric distancekpcpos.distance;stat.minfloatnullableRgalmax? Maximum Galactocentric distancekpcpos.distance;stat.maxfloatnullableN(CH)? CH column densitycm**-2phys.columnDensitydoublenullablee_N(CH)? N(CH) lower limit sigmacm**-2stat.errordoublenullableE_N(CH)? N(CH) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(OH)? OH column densitycm**-2phys.columnDensitydoublenullablee_N(OH)? N(OH) lower limit sigmacm**-2stat.errordoublenullableE_N(OH)? N(OH) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(HCO+)? HCO+ column densitycm**-2phys.columnDensitydoublenullablee_N(HCO+)? N(HCO+) lower limit sigmacm**-2stat.errordoublenullableE_N(HCO+)? N(HCO+) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(CCH)? CCH column densitycm**-2phys.columnDensitydoublenullablee_N(CCH)? N(CCH) lower limit sigmacm**-2stat.errordoublenullableE_N(CCH)? N(CCH) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(O)? Oxygen column densitycm**-2phys.columnDensitydoublenullablee_N(O)? N(O) lower limit sigmacm**-2stat.errordoublenullableE_N(O)? N(O) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(H2)? H2 column densitycm**-2phys.columnDensitydoublenullablee_N(H2)? N(H2) lower limit sigmacm**-2stat.errordoublenullableE_N(H2)? N(H2) upper limit sigmacm**-2stat.error;stat.maxdoublenullableN(HI)? HI column densitycm**-2phys.columnDensitydoublenullablee_N(HI)? N(HI) lower limit sigmacm**-2stat.errordoublenullableE_N(HI)? N(HI) upper limit sigmacm**-2stat.error;stat.maxdoublenullablefH2? Molecular gas fraction (fH2)meta.numberfloatnullable