Can Field OB Stars Ionize the WIM?

Given that the WIM appears to be ionized by OB stars, do the Lyman continuum photons leak from HII regions, or are they due to field OB stars? The first possibility agrees with several characteristics of the WIM: an increase in forbidden line strengths compared to the Balmer lines towards lower H intensities, and the concentration of H emission from the WIM near HII regions. Ferguson et al. (1996a) argue that leakage has to occur because field OB stars may not contribute enough ionizing photons. However, a careful census of the field star population is required. An O star in a low-density environment will have a large Strömgren sphere: about 150 pc for an O8 star in medium with density 0.2 cm, twice that for an O5 star. The concentration of OB field stars in the general areas near HII regions could well give rise to similar spectral characteristics for the WIM as the leaking HII region model. Elmegreen (these proceedings) proposes a fractal HII region model, which implies that every OB star inside an HII region acts like a field OB star, a picture that would also account for the observed properties.

We are addressing this problem by analyzing far-UV images of the stellar light, obtained with the Ultraviolet Imaging Telescope on the ASTRO-1 and ASTRO-2 missions, in conjunction with the H images of the WIM. We test if the far-UV to H intensity ratios across galactic disks are consistent with those expected from luminous stars. An example is shown in Figure 3.

  
Figure: Ratio of the Lyman continuum to far-UV (1520Å) luminosity in HII regions and DIG (=WIM) regions in M33, compared to models from Hill et al. (1995). The Lyman continuum luminosity is inferred from the H luminosity. The average ratio for the WIM appears consistent with that predicted for a steady state star formation rate (from Hoopes & Walterbos 1997).

The data appear consistent with ionization of the WIM by field stars. However, there are several complications. Extinction effects are troublesome in analyzing far-UV data. Some of the far-UV light in regions of WIM could be due to light scattered from OB stars inside HII regions. Also, while models such as shown in Figure 3 predict sufficient Lyman continuum output to ionize the WIM, we need to determine if the ionizing stars are actually present. We are doing this by analyzing HST far-UV images of selected regions in nearby galaxies. Finally, the ionization stage of helium is also critical in testing the viability of field stars as the source of ionization.

© Copyright Astronomical Society of Australia 1997