W. F. Wall
Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Apartados Postales 51 and 216, 72000 Puebla, Pue., México (wwall@inaoep.mx).
Received 2005 December 1
Accepted 2006 February 7
]]> RESUMEN
Se presenta un tratamiento que explica mejor el factor de conversión N(H2)/I(CO) y que incluye la transferencia radiactiva. A primera vista, incluir la transferencia radiativa parece superfluo para una línea ópticamente gruesa como CO J = 1 → 0. No obstante, dado que el medio interestelar es inhomogéneo, los fragmentos de gas (es decir, grumos) todavía pueden ser ópticamente delgadas hacia sus bordes y en las alas de los perfiles de la línea. El tratamiento estadístico de Martin et al. (1984) de la transferencia radiactiva a través una nube molecular con grumos se usa para derivar una expresión para el factor de conversión que supera los defectos de las explicaciones más tradicionales basadas en Dickman et al. (1986). Por un lado, el tratamiento presentado aquí posiblemente representa un avance importante al entender el factor de conversión N(H2)/I(CO) pero, por otro lado, tiene sus propios defectos, que son discutidos aquí brevemente.
ABSTRACT
A treatment that better explains the N(Fi2)/I(CO) conversion factor is given that includes radiative transfer. At first glance, involving radiative transfer seems superfluous for an optically thick line such as CO J = 1 → 0 line. However, given that the interstellar medium is inhomogeneous, the individual gas fragments (i.e., clumps) can still be optically thin toward their edges and in the wings of their line profiles. The statistical treatment by Martin et al. (1984) of the radiative transfer through a clumpy molecular cloud is used to derive an expression for the conversion factor that overcomes the shortcomings in the more traditional explanations based on Dickman et al. (1986). While possibly representing important step forward in understanding the N(H2)/I(CO) conversion factor, the treatment here also has shortcomings of its own that are briefly discussed.
Key Words: ISM: MOLECULES — ISM: DUST — ISM: ORION.
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ACKNOWLEDGEMENTS
]]> This work was supported by CONACyT grants #211290-5-0008PE and #202-PY.44676 to W. F. W. at INAOE. I am very grateful to W. T. Reach for his comments and support. I thank T. A. .D. Paglione, G. MacLeod, E. Vazquez Semadeni, and others for stimulating and useful discussions. The author is grateful to R. Maddalena and T. Dame, who supplied the map of the peak 12CO J = 1 → 0 line strengths and provided important calibration information.
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