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41.
Deborah P. Ruffle Eric Herbst † 《Monthly notices of the Royal Astronomical Society》2001,324(4):1054-1062
Solid CO2 is observed to be an abundant interstellar ice component towards both quiescent clouds and active star-forming regions. Our recent models of gas–grain chemistry, appropriate for quiescent regions, severely underproduce solid CO2 at the single assumed gas density and temperature. In this paper, we investigate the sensitivity of our model results to changes in these parameters. In addition, we examine how the nature of the grain surface affects the results and also consider the role of the key surface reaction between O and CO. We conclude that the observed high abundance of solid CO2 can be reproduced at reasonable temperatures and densities by models with diffusive surface chemistry, provided that the diffusion of heavy species such as O occurs efficiently. 相似文献
42.
Results are presented from a model of molecular line formation in collapsing star-forming cores. The study includes, for the first time, a self-consistent chemical and dynamical model which is then directly coupled to an appropriate radiative transfer model. The assumptions of chemical uniformity or simple monotonic variations within such cores are shown to be unacceptable. The results show that the abundance variations and the line profiles are highly sensitive to the assumed values of the free parameters in the chemical model. Extreme caution is therefore advised in the quantitative analysis of emission-line profiles from infall sources. The implied degeneracy can be overcome by multiple line-of-sight observations of many species and transitions. 相似文献
43.
A. Megier S. Aiello B. Barsella S. Casu J. Kreowski 《Monthly notices of the Royal Astronomical Society》2001,326(3):1095-1100
We relate the equivalent widths of the major diffuse interstellar bands (DIBs) near 5797 and 5780 Å with different colour excesses, normalized by E ( B − V ) , which characterize the growth of interstellar extinction in different wavelength ranges. It is demonstrated that the two DIBs correlate best with different parts of the extinction curve, and the ratio of these diffuse bands is best correlated with the far-ultraviolet (UV) rise. A number of peculiar lines of sight are also found, indicating that the carriers of some DIBs and the far-UV extinction can be separated in certain environments, e.g. towards the Per OB2 association. 相似文献
44.
Alexandre Faure Jonathan Tennyson 《Monthly notices of the Royal Astronomical Society》2001,325(1):443-448
Molecular R -matrix calculations are performed to give rotational excitation rates for electron collisions with linear molecular ions. Results are presented for CO+ , HCO+ , NO+ and H2 + up to electron temperatures of 10 000 K. De-excitation rates and critical electron densities are also given. It is shown that the widely used Coulomb–Born approximation is valid for Δ j =1 transitions when the molecular ion has a dipole greater than about 2D, but otherwise is not reliable for studying electron-impact rotational excitation. In particular, transitions with Δ j >1 are found to have appreciable rates and are found to be entirely dominated by short-range effects. 相似文献
45.
C. R. Purcell R. Balasubramanyam M. G. Burton A. J. Walsh V. Minier M. R. Hunt-Cunningham L. L. Kedziora-Chudczer S. N. Longmore T. Hill I. Bains P. J. Barnes A. L. Busfield P. Calisse N. H. M. Crighton S. J. Curran T. M. Davis J. T. Dempsey G. Derragopian B. Fulton M. G. Hidas M. G. Hoare J.-K. Lee E. F. Ladd S. L. Lumsden T. J. T. Moore M. T. Murphy R. D. Oudmaijer M. B. Pracy J. Rathborne S. Robertson A. S. B. Schultz J. Shobbrook P. A. Sparks J. Storey T. Travouillion 《Monthly notices of the Royal Astronomical Society》2006,367(2):553-576
46.
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 12 CO( 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
47.
Phase-referenced observations of 13 star-forming regions in the 2 Π1/2 , J = 1/2 transition of rotationally excited OH at 4765 MHz have been carried out using MERLIN. Two of the regions were also observed at 4750 MHz and one at 4660 MHz. There were 10 maser detections at 4765 MHz and three non-detections. There were no detections at 4750 and 4660 MHz. The 4765-MHz masers have brightness temperatures of ∼107 K at MERLIN resolution (∼50 mas). Several cases of 4765-MHz masers overlapping in position and velocity with 1720- and 1665-MHz masers are reported. There are also isolated 4765-MHz masers with peak flux densities ≥30 times that of any ground-state counterpart. Most of the 4.7-GHz maser spots are unresolved at 50-mas angular resolution, but in four of the nearest sources the maser spots are resolved, indicating a characteristic size for 4765-MHz maser regions of ∼100 au. In W3(OH) we discovered that 20 per cent of the 4765-MHz emission comes from a narrow low-brightness filament that stretches north–south for ∼1.0 arcec (∼2200 au) between two previously known 4765-MHz maser spots. The filament appears in projection against the H ii region and has a brightness temperature of ∼4 × 105 K . There are matching absorption features in mainline transitions of highly excited OH. The filament may trace a shock front in a rotating disc. 相似文献
48.
49.
50.
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