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2000ApJ...541..218S - Astrophys. J., 541, 218-233 (2000/September-3)
Photoionization of galactic halo gas by old supernova remnants.
SLAVIN J.D., McKEE C.F. and HOLLENBACH D.J.
Abstract (from CDS):
We consider the ionization of the clouds observed toward the halo star HD 93521, for which there are no O stars close to the line of sight. Along this line of sight, two groups of clouds (densities ∼0.3-1 cm–3 and temperatures ∼7000 K) are observed at ∼0 km.s–1 (``slow'') and ~-50 km.s–1 (``fast''). We show that the observed ionization can be explained successfully by our model EUV/soft X-ray flux from cooling hot gas. In particular, we can match the Hα intensity, the S++/S+ ratio, and the C+* column. Our models show that it is possible to account for the observed ionization without invoking exotic ionization mechanisms such as decaying neutrinos (Sciama). Our value for X-ray opacity along this line of sight is somewhat larger than the average for the halo found by Arabadjis & Bregman, but we do not regard the difference as significant. From observations of the ratios of columns of C+* and either S+ or H0, we are able to estimate the thermal pressure in the clouds. The slow clouds require high (∼104 cm–3 K) thermal pressures to match the NC+*/NS+^_ ratio. Additional heating sources are required for the slow clouds to maintain their ∼7000 K temperatures at these pressures, as found by Reynolds, Haffner, & Tufte. We also estimate the fraction of the line of sight that is occupied by warm ionized and neutral gas and find it to be small, only 6%-9%.
Abstract Copyright: ∼
Journal keyword(s): Galaxy: Halo - ISM: Clouds - ISM: General - ISM: Supernova Remnants - X-Rays: ISM
Simbad objects: 7
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