Ionization within the Local Cavity by Hot White Dwarfs
Abstract
We present estimates of the contribution from 33 nearby (d < 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 Å range was then derived. Using these numbers, the respective sizes of the ionized Strömgren spheres of HII were then calculated for each star under the assumption of ambient interstellar electron density values of ne = 0.04 cm-3 and 0.10 cm-3. The location and sizes of these ionized regions have been plotted on a 3D map of the neutral absorption contours of the local ISM, as derived for both in-plane and out of plane directions. A similar procedure was also carried out for the determination of the sizes of the ionized HeII regions surrounding the same 33 white dwarfs. We conclude that only a few of the very hottest DA white dwarfs located within the Local Cavity can possess appreciably sized ionized Strömgren spheres of HII and HeII. The size of these ionized regions can potentially range between 20 pc and 40 pc in diameter, dependent on the ambient value of the electron density of the interstellar medium. However, the volumetric filling factor of these ionized spheres within the Local Cavity is small (∼6%), even at the lowest value of electron density considered in this study (i.e., ne = 0.04 cm-3). Thus, the contribution from DA white dwarfs to the ionization of both the local interstellar cloud complex and other clouds located within the Local Cavity is of secondary importance compared to the ionization contribution provided by the two early B-type stars of ɛ and β CMa.
- Publication:
-
Publications of the Astronomical Society of the Pacific
- Pub Date:
- June 2013
- DOI:
- 10.1086/671190
- Bibcode:
- 2013PASP..125..644W