[tel-00726959, v1] Caractériser le milieu interstellaire ... - HAL - INRIA

A&A 518, A45 (2010)DOI: 10.1051/0004-6361/201014510c○ ESO 2010Astronomy&AstrophysicsThe CO luminosity and CO-H 2 conversion factor of diffuse ISM:does CO emission trace dense molecular gas? ⋆H. S. Liszt 1 ,J.Pety 2,3 , and R. Lucas 41 National Radio Astronomy Observatory, 520 Edgemont Road, VA 22903-2475 Charlottesville, USAe-mail: hliszt@nrao.edu2 Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France3 Obs. de Paris, 61 av. de l’Observatoire, 75014 Paris, France4 Al-MA, Avda. Apoquindo 3846 Piso 19, Edificio Alsacia, Las Condes, Santiago, ChileReceived 26 March 2010 / Accepted 8 May 2010ABSTRACTtel-00726959, version 1 - 31 Aug 2012Aims. We wish to separate and quantify the CO luminosity and CO-H 2 conversion factor applicable to diffuse but partially-molecularISM when H 2 and CO are present but C + is the dominant form of gas-phase carbon.Methods. We discuss galactic lines of sight observed in Hi, HCO + and CO where CO emission is present but the intervening cloudsare diffuse (locally A V< ∼ 1 mag) with relatively small CO column densities N CO< ∼ 2 × 10 16 cm −2 . We separate the atomic and molecularfractions statistically using E B−V as a gauge of the total gas column density and compare N H2 to the observed CO brightness.Results. Although there are H 2 -bearing regions where CO emission is too faint to be detected, the mean ratio of integrated CO brightnessto N H2 for diffuse ISM does not differ from the usual value of 1 K km s −1 of integrated CO brightness per 2 × 10 20 H 2 cm −2 .Moreover, the luminosity of diffuse CO viewed perpendicular to the galactic plane is 2/3 that seen at the Solar galactic radius insurveys of CO emission near the galactic plane.Conclusions. Commonality of the CO-H 2 conversion factors in diffuse and dark clouds can be understood from considerations ofradiative transfer and CO chemistry. There is unavoidable confusion between CO emission from diffuse and dark gas and misattributionof CO emission from diffuse to dark or giant molecular clouds. The character of the ISM is different from what has beenbelieved if CO and H 2 that have been attributed to molecular clouds on the verge of star formation are actually in more tenuous,gravitationally-unbound diffuse gas.Key words. ISM: molecules – ISM: clouds1. IntroductionIt is a truism that sky maps of CO emission are understoodas uniquely tracing the Galaxy’s molecular clouds, dense, coldstrongly-shielded regions where the hydrogen is predominantlymolecular and the dominant form of gas phase carbon is CO.Moreover, CO emission plays an especially important role inISM studies as the tracer of cold molecular hydrogen throughthe use of the so-called CO-H 2 conversion factor which directlyscales the integrated 12 CO J = 1–0 brightness W CO to H 2 columndensity N H2 . This deceptively simple conversion is critically importantto determining molecular and total gas column densitiesand so to understanding the most basic properties of star formation(Leroy et al. 2008; Bigiel et al. 2008; Bothwell et al. 2009),the origins of galactic dust emission (Draine et al. 2007), andother such fundamentals.Yet, it is increasingly recognized that CO emission is presentalong lines of sight lacking high extinction or large molecularcolumn densities (Liszt & Lucas 1998). It is also the case thatsome very opaque lines of sight showing CO emission are comprisedentirely of diffuse material and H 2 -bearing diffuse clouds(McCall et al. 2002): a discussion of such a line of sight fromour own work is described in Appendix A here. Even in canonicaldark clouds like Taurus, detailed high-resolution mappingof the CO emission (Goldsmith et al. 2008) reveals that much⋆ Appendix E is only available in electronic form athttp://www.aanda.orgof it originates in relatively weakly-shielded gas where 13 CO isstrongly enhanced through isotopic fractionation, implying thatthe dominant form of gas phase carbon must be C + (Watson et al.1976).Conversely, it is also the case that molecular gas is detectedin the local ISM even when CO emission is not. Lines of sightwith N CO> ∼ 10 12 cm −2 , N H2> ∼ 10 19 cm −2 have long been detectablein surveys of uv absorption (Sonnentrucker et al. 2007;Burgh et al. 2007; Sheffer et al. 2007, 2008), with expected integratedCO brightnesses as low as W CO = 0.001 K km s −1 (Liszt2007b). And, as discussed here, mm-wave HCO + and CO absorptionfrom clouds with N H2> ∼ 10 20 cm −2 are also more commonthan CO emission along the same lines of sight (see Lucas& Liszt 1996; Liszt & Lucas 2000, and Appendix A).Thus we are led to ask two questions that are of particularimportance to the use of CO emission as a molecular gas tracer.First, where and how does the observed local CO luminosity reallyoriginate? Second, how completely is the molecular materialrepresented by CO emission? The approach we take to addressthese issues is based on radiofrequency surveys of Hi, HCO +and CO absorption and emission along lines of sight through theGalaxy toward extragalactic background sources. By combining1) measurements of extinction (constraining the total gas columndensity); 2) measurements of Hi absorption (to determinethe gas column of atomic hydrogen); 3) the strength of HCO + absorption(tracing H 2 directly) and 4) the integrated CO J = 1–0brightness W CO , we relate W CO to N H2 along sightlines where weArticle published by EDP Sciences Page 1 of 10