Other
Scientific paper
Dec 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aas...197.4211d&link_type=abstract
American Astronomical Society, 197th AAS Meeting, #42.11; Bulletin of the American Astronomical Society, Vol. 32, p.1467
Other
Scientific paper
We explore potential solutions to a major puzzle about the diffuse ionized medium (DIM) -- finding an ionization mechanism for H and He that is consistent with both its sources and the observed spectral anomaly in the weak He I λ5876 /Hα intensity ratio. The most promising mechanism appears to be photoionization by LyC radiation emitted from OB associations; only ~ 8% of the ionizing photons need escape into the Reynolds layer to sustain the photoionization rate. However, He I λ5876 /Hα is observed to be much less than that observed in classical H II regions, requiring the radiation field to be much softer than that expected from OB associations. The inferred ratio of helium ionizing photons to hydrogen ionizing photons, Q1/Q_0 < ~ 0.03 (Reynolds & Tufte 1995, ApJ, 439, L17), is a factor of 7--10 below values obtained from OB association models. Either the intrinsic spectrum of OB associations has Q1/Q_0 < ~ 0.03, or the radiation is softened between the association and the DIM. The intrinsic spectrum can be this soft if the OB association contains only B stars with T eff < ~ 35,000 K (Sembach et al. 2000, ApJ, 528, 310). The ionizing photon luminosity from such associations would then be much too low to sustain the DIM unless ≳ 90% of the emitted photons penetrated into the DIM. Although softening the LyC radiation through reprocessing by cold dense gas has been suggested, we find that this process is inefficient, owing to the relatively high He I photoionization cross section (λ <= 504 Å). Another promising possibility is that the LyC radiation is filtered by small dust grains in the DIM, whose extinction curve increases with decreasing wavelength between the ionization edges of H I (912 Å) and He I (504 Å). However, it may prove difficult to obtain an extinction curve that rises sufficiently rapidly at short wavelengths, to provide adequate softening without overly attenuating the total Q0. This work was supported by the NASA Astrophysical Theory Program at the University of Colorado, grant NAG5-4063.
Dove James B.
Maloney Philip R.
Shull Michael J.
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