Molecular line intensities as measures of cloud masses – I. Sensitivity of CO emissions to physical parameter variations     

T. A. Bell  E. Roueff  S. Viti  D. A. Williams 《Monthly notices of the Royal Astronomical Society》2006,371(4):1865-1872

A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H₂, the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the  ¹²CO( J = 1 → 0)  emission line, using a CO-to-H₂ conversion factor, X . Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different from those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor.  相似文献   

Helicoidal magneto-electron waves in interstellar molecular clouds     

S. I.Bastrukov  J.Yang  D. V.Podgainy 《Monthly notices of the Royal Astronomical Society》2002,330(4):901-906

We investigate the evolution of the magnetic flux density in a magnetically supported molecular cloud driven by Hall and Ohmic components of the electric field generated by the flows of thermal electrons. Particular attention is given to the wave transport of the magnetic field in a cloud whose gas dynamics is dominated by electron flows; the mobility of neutrals and ions is regarded as heavily suppressed. It is shown that electromagnetic waves penetrating such a cloud can be converted into helicons – weakly damped, circularly polarized waves in which the densities of the magnetic flux and the electron current undergo coherent oscillations. These waves are interesting in their own right, because for electron magnetohydrodynamics the low-frequency helicoidal waves have the same physical significance as the transverse Alfvén waves do for a single-component magnetohydrodynamics. The latter, as is known, are considered to be responsible for the widths of molecular lines detected in dark, magnetically supported clouds. From our numerical estimates for the group velocity and the rate of dissipation of helicons it follows that a possible contribution of these waves to the broadening of molecular lines is consistent with the conditions typical of dark molecular clouds.  相似文献   

Low velocity shock-cloud encounters I.     

András Horváth Jr.  L. Viktor Tóth 《Astrophysics and Space Science》1995,233(1-2):169-173

Shocks propagating in the interstellar medium (ISM) play an important role in the life of molecular clouds. Through a theoretical study of interaction between clouds and shocks we can understand, for example, the density distribution of observed molecular clouds and the first steps of star formation. The only way to study of interaction in detail is via a numerical hydrodynamical simulation. In this paper we present the first results of a hydrocode which is able to follow the processes after the collision between the cloud and shock front.Our main theoretical result is that the chemical processes (e.g. H₂ dissociation) can affect the dynamical processes significantly. Global parameters of the cloud are calculated for the comparision of the simulation and the observations.  相似文献   

The formation of molecular clouds in spiral galaxies     

C. L. Dobbs  I. A. Bonnell  J. E. Pringle 《Monthly notices of the Royal Astronomical Society》2006,371(4):1663-1674

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Molecular line mapping of the giant molecular cloud associated with RCW 106 – I. ¹³CO     

I. Bains  T. Wong  M. Cunningham  P. Sparks  D. Brisbin  P. Calisse  J. T. Dempsey  G. Deragopian  S. Ellingsen  B. Fulton  F. Herpin  P. Jones  Y. Kouba  C. Kramer  E. F. Ladd  S. N. Longmore  J. McEvoy  M. Maller  V. Minier  B. Mookerjea  C. Phillips  C. R. Purcell  A. Walsh  M. A. Voronkov  M. G. Burton 《Monthly notices of the Royal Astronomical Society》2006,367(4):1609-1628

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Spiral shocks, triggering of star formation and the velocity dispersion in giant molecular clouds     

I. A. Bonnell  C. L. Dobbs  T. P. Robitaille  J. E. Pringle 《Monthly notices of the Royal Astronomical Society》2006,365(1):37-45

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Shocked molecular hydrogen towards the Tornado nebula     

J. S. Lazendic  M. Wardle  M. G. Burton  F. Yusef-Zadeh  A. J. Green  J. B. Whiteoak 《Monthly notices of the Royal Astronomical Society》2004,354(2):393-400

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Supersonic turbulence, filamentary accretion and the rapid assembly of massive stars and discs     

Robi Banerjee  Ralph E. Pudritz  Dave W. Anderson 《Monthly notices of the Royal Astronomical Society》2006,373(3):1091-1106

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Properties of dust and detection of Hα emission in LDN 1780     

C. del Burgo  L. Cambrésy 《Monthly notices of the Royal Astronomical Society》2006,368(3):1463-1478

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The effect of dissipation on the generation of density structure by magnetohydrodynamic waves     

A. J. Lim  S. A. E. G. Falle  T. W. Hartquist 《Monthly notices of the Royal Astronomical Society》2005,357(2):461-470

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