J/A+A/612/A37ivo://CDS.VizieR/J/A+A/612/A37doi:10.26093/cds/vizier.36120037CDSWiesemeyer H.Guesten R.Menten K.M.Duran C.A.Csengeri T.,Jacob A.M.Simon R.Stutzki J.Wyrowski F.2019-11-04T14:38:30Z2018-04-17T07:12:40ZCDS support team

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

cds-question@unistra.frmolecular-cloudsspectroscopyThe methylidyne radical CH is commonly used as a proxy for molecular hydrogen in the cold, neutral phase of the interstellar medium. The optical spectroscopy of CH is limited by interstellar extinction, whereas far-infrared observations provide an integral view through the Galaxy. While the HF ground state absorption, another H_2_ proxy in diffuse gas, frequently suffers from saturation, CH remains transparent both in spiral-arm crossings and high-mass star forming regions, turning this light hydride into a universal surrogate for H_2_. However, in slow shocks and in regions dissipating turbulence its abundance is expected to be enhanced by an endothermic production path, and the idea of a "canonical" CH abundance needs to be addressed. The N=2<-1 ground state transition of CH at {lambda}149um has become accessible to high-resolution spectroscopy thanks to GREAT, the German Receiver for Astronomy at Terahertz Frequencies aboard the Stratospheric Observatory for Infrared Astronomy, SOFIA. Its unsaturated absorption and the absence of emission from the star forming regions makes it an ideal candidate for the determination of column densities with a minimum of assumptions. Here we present an analysis of four sightlines towards distant Galactic star forming regions, whose hot cores emit a strong far-infrared dust continuum serving as background signal. Moreover, if combined with the sub-millimeter line of CH at {lambda}560um, environments forming massive stars can be analyzed. For this we present a case study on the "proto-Trapezium" cluster W3 IRS5. While we confirm the global correlation between the column densities of HF and those of CH, both in arm and interarm regions, clear signposts of an over-abundance of CH are observed towards lower densities. However, a significant correlation between the column densities of CH and HF remains. A characterization of the hot cores in the W3 IRS5 proto-cluster and its envelope demonstrates that the sub-millimeter/far-infrared lines of CH reliably trace not only diffuse but also dense, molecular gas. In diffuse gas, at lower densities a quiescent ion-neutral chemistry alone cannot account for the observed abundance of CH. Unlike the production of HF, for CH^+^ and CH, vortices forming in turbulent, diffuse gas may be the setting for an enhanced production path. However, CH remains a valuable tracer for molecular gas in environments reaching from diffuse clouds to sites of high-mass star formation.2018A&A...612A..37Whttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/612/A37CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toJ/A+A/490/213 : 1.4 and 3.4mm interferometry of W3 IRS5 (Rodon+, 2008)J/A+A/567/L5 : W3(OH) high angular resolution 7mm images (Dzib+, 2014)J/ApJ/647/418 : Ground-state OH masers in W3(OH) study (Fish+, 2006)J/ApJ/668/331 : OH masers in W3(OH) (Fish+, 2007)J/ApJ/393/149 : W49N H2O maser outflow: distance and kinematics (Winn+, 1992)J/ApJ/429/253 : W49N H2O masers (Gwinn, 1994)J/A+A/582/A64 : W51/e2 and G34.3+0.2 IRAM spectra (Lykke+, 2015)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/612/A37https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/612/A37http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/612/A37https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/612/A37https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/612/A37http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/612/A37GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/612/A37/list (List of fits files)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/612/A37/list?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/612/A37/list?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/612/A37/list?GETtext/xml+votable180.050000true36.41962.097020.0055555555555555566/3649 30618 30694https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/612/A37?format=asciidefaultJ/A+A/612/A37/listList of fits filesSimbadNameSimbad column added by the CDSmeta.idcharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right Ascension of center (J2000)degpos.eq.ra;meta.mainDEJ2000Declination of center (J2000)degpos.eq.dec;meta.mainNxNumber of pixels along X-axismeta.numberintObs.DateObservation date (YYYY-MM-DDThh:mm:ss)time.epoch;obscharbFreqLower value of frequency intervalGHzem.freq;stat.mindoubleBFreqUpper value of frequency intervalGHzem.freq;stat.maxdoubledFreqFrequency resolutionHzspect.resolutiondoublesizeSize of FITS fileKibytephys.size;meta.fitsintspShow the spectrummeta.ref.urlcharFileNameName of FITS file, in subdirectory fitsmeta.id;meta.fitscharTitleTitle of the FITS filemeta.title;meta.fitscharJ/A+A/612/A37/table2Synopsis of velocity-integrated column densities of CH, HF, and OH on the sightline to W49 NrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameSpiral arm designationmeta.id;src.morph.paramcharn_Name[abc] Note on Name (1)meta.notecharVlowLower value of velocity intervalkm/sphys.veloc;time.interval;stat.minfloatVupUpper value of velocity intervalkm/sphys.veloc;pos.heliocentricfloatN(CH)CH velocity-integrated column density1e+13cm**-2phys.columnDensityfloatE_N(CH)Error on N(CH) (upper value) (4)1e+13cm**-2stat.error;stat.maxfloate_N(CH)Error on N(CH) (lower value) (4)1e+13cm**-2stat.errorfloatN(HF)HF velocity-integrated column density1e+13cm**-2phys.columnDensityfloatE_N(HF)Error on N(HF) (upper value) (4)1e+13cm**-2stat.error;stat.maxfloate_N(HF)Error on N(HF) (lower value) (4)1e+13cm**-2stat.errorfloatN(OH)OH velocity-integrated column density1e+13cm**-2phys.columnDensityfloatE_N(OH)Error on N(OH) (upper value) (4)1e+13cm**-2stat.error;stat.maxfloate_N(OH)Error on N(OH) (lower value) (4)1e+13cm**-2stat.errorfloatfNH2Molecular hydrogen fraction, 2N(H_2_)/(N(HI)+2N(H_2_)) (2)phys.mass;arith.ratiofloatE_fNH2Error on fNH2 (upper value) (4)stat.error;stat.maxfloate_fNH2Error on fNH2 (lower value) (4)stat.errorfloatN(HF)/N(CH)Molecular ratio, N(HF)/N(CH) (3)phys.abundfloatE_N(HF)/N(CH)Error on N(HF)/N(CH) (upper value) (4)stat.error;stat.maxfloate_N(HF)/N(CH)Error on N(HF)/N(CH) (lower value) (4)stat.errorfloatN(OH)/N(CH)Molecular ratio, N(OH)/N(CH) (3)phys.abundfloatE_N(OH)/N(CH)Error on N(OH)/N(CH) (upper value) (4)stat.error;stat.maxfloate_N(OH)/N(CH)Error on N(OH)/N(CH) (lower value) (4)stat.errorfloat