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

A&A 518, A45 (2010)1001010⌠⌡ τ(H I)dv [km s -1 ]10⌠⌡ τ(HCO + )dv [km s -1 ]1W CO [K km s -1 ]1f H2 =1/31w/COw/HCO +No HCO + ,CO,CO0.1HCO + only, no H I0.1HCO + onlyH I onlyHI&HCO +0.1 1E B-V [mag]0.1 1E B-V [mag]0.1 1E B-V [mag]tel-00726959, version 1 - 31 Aug 2012Fig. 1. Atomic and molecular absorption and emission vs. total reddening. Left:IntegratedVLAHi optical depth from Dickey et al. (1983) and thiswork. Middle: integrated PdBI HCO + optical depth from Lucas & Liszt (1996) and this work. Right: integrated ARO12m CO J = 1–0 brightnessat 1 ′ resolution. In each case the horizontal axis is the total line of sight reddening E B−V (Schlegel et al. 1998). For explanation of the symbols usedin the plots, see Sect. 3.have previously shown that the intervening gas is diffuse, neitherdark nor dense, and the CO column densities are relatively small.The results are somewhat surprising: although there is muchvariability, the mean CO brightness per H 2 -molecule W CO /N H2 ,i.e. the CO-H 2 conversion factor, does not differ between diffuseand fully molecular clouds. Although this was phenomenologicallyinferred long ago, the physical basis for it is now betterunderstood in terms of the radiative transfer and chemistry ofH 2 - and CO-bearing diffuse and dark gas.The plan of the present work is as follows. Section 2 describesthe observational material that is used here, some ofwhich is new. Section 3 derives the CO-H 2 conversion factor indiffuse gas. Section 4 discusses the fraction of the local galacticCO luminosity (viewed perpendicular to the galactic plane) thatcan be attributed to diffuse gas. Section 5 discusses the physicalprocesses at play to set the ratio of CO brightness to H 2 columndensity and explains why the same value may apply to dark anddiffuse gas. Section 6 discusses which molecular emission diagnosticsmight actually be used to distinguish between the COcontributions from diffuse and dark gas. Sections 7 and 8 presenta brief summary and discuss how our concept of the ISM mightchange when a substantial portion of the observed CO emissionis ascribed to diffuse rather than dense molecular gas.2.2. Hi absorptionThis is mostly taken from the VLA results of Dickey et al.(1983) but a line profile for B2251+158 (3C 454.3) was madeavailable on the website of John Dickey and we took new Hiabsorption profiles toward J0008+686, J0102+584,B0528+134,B0736+017, J2007+404, J2023+318 and B2145+067 at theVLA in 2005 May and July.2.3. HCO + absorptionWe used results from the PdBI’s HCO + survey of Lucas & Liszt(1996) along with the slightly more recent results of Liszt &Lucas (2000) and a few additional profiles that were taken at thePdBI in 2001–2004.The rotational excitation of HCO + above the cosmicmicrowave background is very weak in diffusegas (Liszt & Lucas 1996) so that N HCO + =1.12 × 10 12 cm −2 (∫ τ(HCO + )dv/1kms −1) for an assumedHCO + permanent dipole moment of 3.889 Debye. This dipolemoment is slightly smaller than the value used in most of ourprevious work (4.07 D), increasing the inferred HCO + columndensities by 10%.2. Observational materialThedatausedinthisworkaregiveninTablesE.1andE.2ofAppendix E (available online).2.1. E B−VValues of the total reddening E B−V along the line of sight arefrom the work of Schlegel et al. (1998) at a spatial resolution of6 ′ . The claimed rms error of these measurements is a percentage(16%) of the value. To convert to column density we usethe equivalence N H = N(H I)+ 2N H2 = 5.8 × 10 21 H cm −2 E B−Vestablished by Bohlin et al. (1978) andRachford et al. (2009).Typically A V = E B−V /3.1 (Spitzer 1978).2.4. J = 1–0 CO emissionAll the results quoted here are from the ARO12m antenna at1 ′ resolution, placed on a main-beam scale by dividing the nativeTr∗ values by 0.85. Most of these profiles were used onthe Tr ∗ scale in our earlier work (Liszt & Lucas 1996, 1998,2000) but profiles toward sources with Hi absorption and lackingHCO + absorption data (noted in Fig. 1) and toward sourceswith J-names in Tables C.1 and C.2 are new. The velocity resolutionwas typically 0.1 km s −1 and all spectra were taken infrequency-switching mode and deconvolved (folded) using theEKHL algorithm (Liszt 1997a). Where upper limits on CO emissionare shown, they are plotted symbolically at very conservativevalues taken over much wider ranges than are occupied byPage 2 of 10