The Structure of Very Cool, Helium-Rich White Dwarf Atmospheres.

Details The Structure of Very Cool, Helium-Rich White Dwarf Atmospheres. The Structure of Very Cool, Helium-Rich White Dwarf Atmospheres.

Mar 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982phdt........28k&link_type=abstract

Thesis (PH.D.)--UNIVERSITY OF WASHINGTON, 1982.Source: Dissertation Abstracts International, Volume: 42-12, Section: B, page: 48

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Fifteen model atmospheres of very cool white dwarfs have been calculated with effective temperatures from 6000 K to 2500 K in steps of 250 K, log g = 8, and pure He chemical composition. The equation of state and the opacities used are based on a hot Thomas-Fermi model of the helium gas. For the energy transfer we allowed for electron conduction along with radiation and convection. We found that pressure ionization and electron degeneracy become important in the photospheres of cool white dwarfs for T(,eff) < 4000 K. Electron conduction also becomes important for these temperatures resulting in (a) a large reduction in the temperature gradient in the layers above the convection zone so that these upper layers are practically isothermal and (b) a reduction of the height of the top of the convection zone so that as we go to lower T(,eff) the upper boundary of the convection zone sinks deeper into the atmosphere because of the increasing efficiency of conduction. The densities of the atmospheres of very cool white dwarfs are relatively large even at very low optical depths. This implies that close to the surface of these objects there is a quite massive but very transparent layer which adds very little to the optical depth of the atmosphere. The emerging monochromatic fluxes of our models with T(,eff) < 4500 K show that as compared to blackbodies very cool white dwarfs emit more radiation in the short wavelength range and less in the long wavelength range and thus they appear bluer than blackbodies with the same temperature. Thus, the temperatures of some very cool white dwarfs may have been overestimated since they were determined by comparing their colors to blackbody colors. For example for the white dwarf VB 11, whose T(,eff) had been estimated as higher than 4000 K, from our models we estimate a T(,eff) of 3750 K. If this result is correct then VB 11 is probably the coolest known white dwarf.

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