2.3. Abundances: Uniformities and Uncertainties
Generally one would like to use the abundances measured in an HII region as a probe of the global ISM abundances. Often, one has the opportunity to measure several HII regions in a galaxy, and, thus, test this assumption. The starting assumption is that the HII region is the result of the photoionization of the surrounding ISM by one or more newly formed massive stars. A second implicit assumption is that the ionized gas of the HII region has not been enriched by stellar winds or ejecta from the exciting star.
There is substantial theoretical and observational evidence that the abundances derived from the HII region surrounding a single star will not be significantly altered by the presence of a stellar wind. Theoretically, it is difficult to get the 106 K stellar wind created ``bubble" to mix with the surrounding 104 K gas (Weaver et al. 1977; Dyson & Smith 1985; Tenorio-Tagle 1996). Also, the observation that many Galactic WR nebulae (Esteban et al. 1992) and WR nebulae in M33 (Esteban et al. 1994) show normal ISM abundances, implies that even in the presence of strong winds, the abundances of the surrounding H II region are not altered. Observations of individual giant HII regions have generally found the abundances to be uniform throughout (e.g., Skillman 1985; Díaz et al. 1987; Rosa & Mathis 1987; Gonzalez-Delgado et al. 1994).
Exceptions have been observed in HII regions excited by single stars in which ejected stellar material is ionized by the exciting star. The signature of this is enhanced N and He (typically factors of 5 - 10 and 20 percent respectively) with an associated O under-abundance (typically a factor of 2 - 5). These have been observed in our Galaxy (see Esteban et al. 1992; and references therein) and in the LMC (Heydari-Malayeri, Melnick, & van Drom 1990; Garnett & Chu 1994). Thus, while there is some observational evidence for local ``pollution'' associated with a individual WR star or a Luminous Blue Variable star (e.g, Carina, Davidson et al. 1986), it does not appear to be a large effect when observing nebulae excited by a large cluster of stars.
We will return to this theme in Section 4.1, where I review the Ph.D. thesis work of Chip Kobulnicky (1997) - an observational exploration into this problem of pollution and mixing timescales within the realm of irregular galaxies.