ABSTRACT. We discuss the problem of assigning an effective temperature to models of stellar evolution in the Wolf-Rayet stage. We conclude that it is best to use models with different velocity laws depending on the stellar temperature. We follow the simplest approach and propose to use a standard law with ß= 1 for the cooler and ß=2 for the hotter Wolf-Rayet stars. We present energy distributions from non-LTE model calculations with a ß=2 velocity law. The new calculations, together with published results based on ß= 1, allow one to synthesize realistic spectra of starburst regions. The flux tables are available in digital form from the authors.

All together indicate a stratification in the ionized gas, usually interpreted in terms of nuclear photoionization as the driving ionization mechanism. However, CL profiles show various peculiarities: they are broader by a factor of 2 than lower ionization lines, the broadening being in terms of asymmetric blue wings, and their centroid position at the nucleus is blueshifted by a few hundred km s À1 . Moreover, in NGC 1386 and NGC 1068, a doublepeaked [Fe vii] line is detected in the nuclear and extended coronal region, this being the first report of this type of profile in CLs in active galactic nuclei. If interpreted as outflow signatures, the total broadening of the lines at zerointensity levels implies gas velocities up to 2000 km s À1. Although the stratification of ions across the coronal region means that photoionization is the main power mechanism, the high velocities deduced from the profiles, the relatively large spatial extension of the emission, and the results from photoionization models indicate that an additional mechanism is at work. We suggest that shocks generated by the outflow could provide the additional required power for line formation.

Helium and hydrogen recombination lines observed in low-metallicity, extragalactic H II regions provide the data used to infer the primordial helium mass fraction, Y P . The ionization corrections for unseen neutral helium (or hydrogen) are usually assumed to be absent; i.e., the ionization correction factor is taken to be unity (icf ≡ 1). In this paper we revisit the question of the icf for H II regions ionized by clusters of young, hot, metal-poor stars. Our key result is that for the H II regions used in the determination of Y P , there is a "reverse" ionization correction: icf < 1. We explore the effect on the icf of more realistic inhomogeneous H II region models and find that for those regions ionized by young stars, with "hard" radiation spectra, the icf is reduced further below unity. In Monte Carlos using H II region data from the literature (Izotov and Thuan 1998) we estimate a reduction in the published value of Y P of order 0.003, which is roughly twice as large as the quoted statistical error in the Y P determination. 1997) we have explored the magnitude of the contribution to systematic error in Y P from possible temperature fluctuations in the extragalactic H II regions, concluding that they may have a significant effect on the determination of the primordial abundance of helium, comparable to or even greater than the statistical uncertainties. Here we turn our attention to another potential source of systematic error: the ionization correction for unseen neutral hydrogen and/or helium in H II regions.Following Peimbert & Costero (1969), an empirical method is usually employed to derive the abundances from the observed emission-line intensities. The electron density and temperature of the gas are obtained from various emission-line intensity ratios and they

This paper presents an analysis of the concentration of the hydrogen molecule inside the ionized region of planetary nebulae. The equations corresponding to the ionization and chemical equilibria of H, H+, H-, H2, H2+, and H3+ are coupled with the equations of ionization and thermal balance for a photoionized atomic gas. Forty different reactions related to the formation or the destruction of these species are included. The presence of dust is taken into account, since grains act as catalysts for the production of H2, as well as shield the molecules against the stellar ionizing radiation. We analyze the effect of the stellar ionizing continuum, as well as of the gas and grain properties on the calculated H2 mass. It is shown that a significant concentration of H2 can survive inside the ionized region of planetary nebulae, mostly in the inner region of the recombination zone. The total H2 to total hydrogen mass ratio inside the ionized region increases with the central star temperature, and, depending on the PN physical conditions, it can be of the order of 10^-6 or even higher. The increase of the recombination zone with the stellar temperature can account for such correlation. This can explain why the H2 emission is more frequently observed in bipolar planetary nebulae (Gatley's rule), since this kind of object has typically hotter stars. Applying our results for the planetary nebula NGC 6720, we obtain an H2 to hydrogen mass ratio similar to the value obtained from the observed H2 line emission.Comment: 13 pages, 4 figures. Accepted for publication in Ap