Computer Science
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........29s&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF TORONTO (CANADA), 1995.Source: Dissertation Abstracts International, Volume: 56-12, Section: B, pa
Computer Science
Scientific paper
The infrared solar spectrum exhibits emission lines near 12 μm from the Mg scI high-l Rydberg transitions 6g - 7h and 6h - 7i. Chang et al. (1991) demonstrated that the emission arises from small deviations in the populations of these Rydberg levels from their thermodynamic equilibrium values. In this thesis, the possible operation of this emission mechanism is investigated in the B stars by performing non-LTE radiative transfer calculations for the high-l Rydberg transitions of Mg scII and O scI. Highly realistic atomic models are employed, complete in energy levels and radiative transitions far into the Rydberg regime. For Mg scII, the collisional excitation rates are improved by computing collision strengths in a 10 state close-coupling approximation using the R-matrix method. The collisional excitation rates derived from these collisions strengths include the full effects of autoionizing resonances and have an expected accuracy of +/-10% for transitions between levels lying low in energy in the close-coupling expansion. For Mg scII, wide-ranging infrared emission is found, spanning the entire range of B spectral types. The emission is caused by the same mechanism operative in the Rydberg levels of Mg scI in the sun. Small divergences between the Rydberg departure coefficients produce rising monochromatic source functions and emission. Flux profiles of the Mg scII high-l ( Delta n = +1) transitions from n = 4 and 5 show an emission peak superposed on wider absorption trough, similar in form to the solar Mg scI lines, while for higher n, the profiles are in full emission. The strongest emission is predicted for transitions from n = 5, 6, and 7 and strongly increases for lower surface gravities where the rates of thermalizing collisions are lower. The emission strengths reach maxima of Flambda /Fc ~ 1.15 and Wlambda ~ -0.1 A. Transitions from higher n exhibit progressively lower continuum contrasts due to the steep rise with wavelength of the continuous opacity in the infrared and increased Stark broadening. The largest source of potential uncertainty affecting the emission strengths is the uncertain scale of the collisional excitation rates between the Rydberg levels. However, reasonable variations of these rates does not eliminate the emission. Although small divergences occur between the Rydberg departure coefficients of O scI, wide-ranging infrared emission is not predicted. Only small self-reversals in the cores of the high-l transitions from n = 4 and 5 are seen and then only at the lowest surface gravities. The failure of O scI to produce significant emission, or more precisely, significant Rydberg population divergences, can be attributed to the lack of strong ultraviolet photoionization rates from its lower energy levels, the increased collisional coupling between its more closely spaced Rydberg levels, and the longer wavelengths of its Rydberg transitions. Considerable uncertainty exists in the prediction of the absolute infrared line strengths of O scI due to uncertainty in the exact treatment or radiative transfer in the resonance line and in the magnitude of the collisional excitation rates among the Rydberg levels. However, these uncertainties do not alter the basic conclusion of no significant emission from the high-l Rydberg transitions of O scI in B stars.
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