Computer Science – Sound
Scientific paper
Apr 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........33d&link_type=abstract
PhD Thesis, Instituto de Astronomia- UNAM, Mexico, D. F., Mexico, 1998.
Computer Science
Sound
Scientific paper
Molecular Clouds are the sites were stars are formed. The birth of a star results in a strong UV flux that propagates through the cold molecular material, dissociating and ionizing the gas. A shell of ionized gas (an HII region) forms around the star, both of which are encapsulated by a shell of photo-dissociated gas (the PDR). The extent of these regions depends mainly on the effective temperature of the star, the cloud density and the opacity of the dust grains -- to a lesser extent on the metallicity of the star. In this thesis we calculate the rate of dissociating photons produced by main sequence stars of different spectral types and metallicities. The stellar fluxes are obtained using the LTE atmosphere models of Kurucz (1993, CD-ROOM 13; for stars with 7,500 K <= Teff <= 50,000 K) and the N-LTE atmosphere models of Aufdenberg et al. (1998, ApJ, 498, 837; for stars with 30,000 K <= Teff <= 51,230 K). In both cases we find that OB stars have a comparable rate of ionizing and dissociating photons. For cooler stars the dissociation rates are well above the ionization rates; the former becoming negligible when the Teff <= 13,000 K. Metallicity effects are only important for stars with Teff <= 15,000 K. In this case the dissociating rates increase approximately .5 dex as the metallicity goes from solar to 0.01 solar. Using a radiative transfer code and the Kurucz models we calculate the size of the HII region and PDR for uniform density clouds (n(H) = 10, 103 and 105 cm-3). The size of these regions are calculated for a medium where dust is optically thin to the UV radiation (regions with large grains; Landgraf & Grun 1997, Astro. Ph/11190) and a medium where dust is optically thick to the UV radiation. The results show that in an optically thin medium the PDRs are at least one order of magnitude larger than the HII regions; this difference is reduced to ~0.5 dex in a medium where dust is optically thick to the UV radiation. In both cases the ratio of the size of the PDR to the size of the HII region increases as the Teff of the star decreases. The dust opacity, on the other hand, becomes negligible for low cloud densities and for low mass stars.We also derived an analytical approximation to the size of the PDR for both media. The approximations differ with the numerical results by less than 30% (in the optically thin case and for Teff >= 13000 K) and by less than 10% in the optically thick case. We also explored the effects of the PDR on the destruction of molecular clouds. Following the work by Franco, Shore & Tenorio-Tagle (1994, ApJ,436,745), and considering only the static case, we find that their estimates of the number of O stars that could be formed within a molecular cloud of given mass are reduced by approximately 30%. Note however, that the formation time of the H2 molecule is ~104 times the HI recombination time and the expansion of the HII region will modify the effects of the PDR on the destruction of the molecular cloud. In any case, we conclude that intermediate mass stars contribute significantly to the overall HI produced in a burst of star formation, and the formation of PDRs reduces even more the number of O stars that can be produced by a molecular cloud. In order to explore the dynamics of the composite region (HII+PDR) we constructed a spherical hydrodynamical code with radiative transfer. This study was limited to the case of high-density cloud cores (n(H) >= 105 cm-3) where LTE is reached in shorth times and we can assume a constant temperature for the HII region and the PDR. In general the HII + PDR composite region includes two fronts, an ionization front (IF) and a dissociation front (DF). The expansion of the HII region follows the well known evolutionary phases described in earlier works. The evolution of the PDR can be summarized as follows: a shock front (DSF) is produced ahead of the DF due to the pressure differences between the atomic and molecular gas. The DF initially propagates supersonically, slowing down as it reaches its equilibrium radius. At approximately 0.6 times the hydrogen molecule formation time the DSF becomes evident (at this time the ionization shock has advanced into the PDR). Since the DF extends over several grid zones, the DSF in our model initially is formed at the internal border of the DF, increasing the density there and shielding the molecules ahead it. The DF then is left behind the shock front and the DSF continues to advance until it reaches pressure equilibrium with the molecular gas. We also found that, since dust absorption reduces considerably the amount of ionizing photons, the ultracompact HII region (UCHII) reaches pressure equilibrium with the ambient gas in short times compared to the case where dust is optically thin to UV radiation. This results in an UCHII with a size ~ 10-2 times the size in the optically thin case. After this time the ISF advances into the PDR loosing strength until it becomes a sound wave. At these densities, the ISF is weakened to almost a sound wave before it reaches the DF and does not have a considerable effect on the expansion of the PDR.
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