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

A&A 518, A45 (2010)tel-00726959, version 1 - 31 Aug 2012W CO [K-km s -1 ]1010.11 10⌡⌠ τ(HCO + )dv [km s -1 ]Fig. C.1. Integrated CO J = 1–0 brightness plotted against theintegrated HCO + J = 1–0 optical depth. N HCO + = 1.12 ×10 12 cm −2 (∫ τ(HCO + )dv/1 kms −1) . See Appendix D.do this in the main discussion. In Fig. C.1 we show the variationof W CO with ∫ τ(HCO + )dv. CO appears reliably at detectablelevels W CO> ∼ 0.3Kkms −1 , N CO> ∼ 3 × 10 14 cm −2when N HCO + > ∼ 3 × 10 11 cm −2 or N H2> ∼ N HCO +/3 × 10 −9 =10 20 cm −2 .IfX HCO + = 3 × 10 −9 the ensemble mean values〈W CO 〉 = 3.45 K km s −1 , 〈∫ τ(HCO + )dv 〉 = 2.38 km s −1 implyW CO = 1Kkms −1 per 2.6 × 10 20 H 2 cm −2 , just 30% above thatderived in Sect. 3.5.The near linearity of the N CO –N HCO + relationship in Fig. C.1results from bulk averaging over whole lines of sight: giventhe same general mix of conditions, an ensemble of richer andpoorer or shorter and longer sightlines will show proportionalitiesbetween almost any two quantities in this way. As shownin Fig. A.1 there is no such proportionality on a per-componentbasis. In detail, and with much scatter, the overall chemical variationis approximately N CO ∝ (N H2 ) 2 (Liszt 2007b; Sheffer et al.2008).Appendix D: Calculating the CO brightnessfrom galactic survey resultsThe statistics of observing the clumpy galactic molecular clouddistribution are Poisson (Gordon & Burton 1976; Burton &Gordon 1978) so the integrated CO brightness W CO (r) accumulatedwhen traversing a path of length r in the galactic plane isW CO (r) = W CO0 (1 − exp (−r/λ))(D.1)where W CO0 is the characteristic brightness of a clump (GMC)and λ is the geometric mean free path between clumps.Although it is possible to derive W CO0 and λ separately, galacticsurvey results are given in terms of a hybrid quantity A CO whoseunits are K km s −1 per kpc corresponding to evaluating W CO (r)when r ≪ λ, i.e.W CO (r) = (W CO0 /λ) r = A CO r.ThecoefficientA CO is closely related to the mean density: just convertW CO to N H2 . For H I the integrated brightness per unit distanceis directly converted into a mean density n(H I), if it is assumedthat the gas is optically thin.The brightness of the CO cloud ensemble viewed verticallythrough the galactic disk is then just A CO Δz, whereΔz is theequivalent thickness of the disk. For a Gaussian vertical distributionwith dispersion σ z , Δz = (2π) 1/2 σ z .ReferencesBigiel, F., Leroy, A., Walter, F., et al. 2008, AJ., 136, 2846Bohlin, R. C., Savage, B. D., & Drake, J. F. 1978, ApJ, 224, 132Bothwell, M. S., Kennicutt, R. C., & Lee, J. C. 2009, MNRAS, 400, 154Burgh, E. B., France, K., & McCandliss, S. R. 2007, ApJ, 658, 446Burton, W. B., & Gordon, M. A. 1978, A&A, 63, 7Cox, D. P. 2005, ARA&A, 43, 337Dickey, J. M., Kulkarni, S. R., Heiles, C. E., & Van Gorkom, J. H. 1983, ApJSS,53, 591Draine, B. T., Dale, D. A., Bendo, G., et al. 2007, ApJ, 663, 866Falgarone, E., Pineau Des Forêts, G., Hily-Blant, P., & Schilke, P. 2006, A&A,452, 511Garwood, R. W., & Dickey, J. M. 1989, ApJ, 338, 841Goldreich, P., & Kwan, J. 1974, ApJ, 189, 441Goldsmith, P. F., Heyer, M., Narayanan, G., et al. 2008, ApJ, 680, 428Gordon, M. A., & Burton, W. B. 1976, ApJ, 208, 346Hartmann, D., & Burton, W. B. 1997, Atlas of galactic neutral hydrogen(Cambridge; New York: Cambridge University Press)Heiles, C., & Troland, T. H. 2003, ApJ, 586, 1067Helfer, T. T., & Blitz, L. 1997, ApJ, 478, 233Leroy, A. K., Walter, F., Brinks, E., et al. 2008, AJ, 136, 2782Liszt, H. 1997a, A&ASS, 124, 183Liszt, H. S. 1982, ApJ, 262, 198Liszt, H. S. 1995, ApJ, 442, 163Liszt, H. S. 1997, A&A, 322, 962Liszt, H. S. 2007a, A&A, 476, 291Liszt, H. S. 2007b, A&A, 461, 205Liszt, H., & Lucas, R. 2002, A&A, 391, 693Liszt, H. S., & Lucas, R. 1994, ApJ, 431, L131Liszt, H. S., & Lucas, R. 1996, A&A, 314, 917Liszt, H. S., & Lucas, R. 1998, A&A, 339, 561Liszt, H. S., & Lucas, R. 2000, A&A, 355, 333Liszt, H. S., Burton, W. B., & Bania, T. M. 1981, ApJ, 246, 74Liszt, H. S., Pety, J., & Tachihara, K. 2009, A&A, 499, 503Lucas, R., & Liszt, H. S. 1994, A&A, 282, L5Lucas, R., & Liszt, H. S. 1996, A&A, 307, 237McCall, B. J., Hinkle, K. H., Geballe, T. R., et al. 2002, ApJ, 567, 391Morton, D. C. 1975, ApJ, 197, 85Münch, I. G. 1952, ApJ, 116, 575Pety, J., Lucas, R., & Liszt, H. S. 2008, A&A, 489, 217Rachford, B. L., Snow, T. P., Destree, J. D., et al. 2009, ApJSS, 180, 125Savage, B. D., Drake, J. F., Budich, W., & Bohlin, R. C. 1977, ApJ, 216, 291Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525Sheffer, Y., Rogers, M., Federman, S. R., et al. 2008, ApJ, 687, 1075Sheffer, Y., Rogers, M., Federman, S. R., Lambert, D. L., & Gredel, R. 2007,ApJ, 667, 1002Sofia, U. J., Lauroesch, J. T., Meyer, D. M., & Cartledge, S. I. B. 2004, ApJ, 605,272Sonnentrucker, P., Welty, D. E., Thorburn, J. A., & York, D. G. 2007, ApJSS,168, 58Spitzer, L. 1978, Physical processes in the interstellar medium (New York:Wiley-Interscience)Tachihara, K., Abe, R., Onishi, T., Mizuno, A., & Fukui, Y. 2000, Publ. Astron.Soc. Jpn., 52, 1147Van Dishoeck, E. F., & Black, J. H. 1986, ApJSS, 62, 109Watson, W. D., Anicich, V. G., & Huntress, W. T., J. 1976, ApJ, 205, L165Weselak, T., Galazutdinov, G. A., Beletsky, Y., & Krełowski, J. 2010, Mon. Not.R. Astron. Soc., 402, 1991Wu, J., Evans, II, N. J., Gao, Y., et al. 2005, ApJ, 635, L173Young, J. S., & Scoville, N. 1982, ApJ, 258, 467Pages 9 to 10 are available in the electronic edition of the journal at http://www.aanda.orgPage 8 of 10