Ionization within the Local Cavity by Hot White Dwarfs

ABSTRACT.We present estimates of the contribution from 33 nearby (d < 100 pcd<100  pc) hot DA white dwarfs to the total ionization of the Local Cavity. The EUV spectra of these white dwarfs have been derived using non-LTE stellar atmosphere models (with the inclusion of metals where necessary) to calculate the incident flux at the Earth using up-to-date distances and interstellar column densities. Our model spectra give a very good approximation to the spectra obtained with observations from the NASA Extreme Ultraviolet Explorer satellite. The number of Lyman continuum photons per second emitted by each of the white dwarfs for wavelengths over the 30–912 A range was then derived. Using these numbers, the respective sizes of the ionized Stromgren spheres of HII were then calculated for each star under the assumption of ambient interstellar electron density values of ne = 0.04 cm-3ne=0.04  cm-3 and 0.10 cm-30.10  cm-3. The location and sizes of these ionized regions have been plotted on a 3D map of the neut...