Photospheres of OB stars are representative of the interstellar material from which they were born due to their relative youth. The evolutionary characteristics of blue massive stars imply that these objects and the associated ionized nebulae - H11 regions - must share the same chemical composition.1
Traditionally, chemical abundance studies in spiral and irregular galaxies have been based on the emission line spectra of H 11 regions. This is logical, since H ii regions are luminous and have high surface brightness (in the emission lines) relative to individual stars in galaxies. Therefore, it is relatively easy to obtain high-quality spectroscopic observational data, even with small and medium-sized
1 There are certain cases in which this is not completely fulfilled. (1) Strong stellar mass-loss may expose already-contaminated underlying layers on the surface of the star; (2) Authors of several studies of OBA-type stars have found observational evidence of stellar-surface contamination by products from the CNO bicycle; rotating models by Maeder & Meynet (2000) predict that mixing of nuclear processed material at the surface will increase with stellar mass, age, initial rotational velocity and decreasing metallicity. (3) Certain elements in the nebular material can be depleted onto dust grains; in this case the gas-phase abundances derived through a spectroscopic study of the H ii region could be somewhat lower than the stellar ones.
telescopes. This has made possible both the detailed study of individual nebulae (Esteban et al. 2004) and the determination of radial gradients in the Milky Way (Shaver et al. 1983; Afflerbach et al. 1997; Esteban et al. 2005) and other spiral galaxies (see Chapter 17 in these proceedings), imposing observational constraints on the chemical-evolution models of these galaxies (see Chapter 43 in these proceedings).
Although this is a commonly used methodology, it is not without some difficulties and problems. For example, it is known that the optical recombination lines (ORLs) of ionized nebulae indicate higher abundances than do collisionally excited lines (CELs); temperature fluctuations, density condensations, and abundance inho-mogeneities have been proposed as explanations to solve the ORL/CEL problem; however, none of these explanations is completely satisfactory (Esteban 2002). I refer the reader to Chapter 17 for a review of the use and limitations of strong-line methods in the determination of nebular abundances. Stasinska (2005) has recently shown that, for metal-rich nebulae, the derived abundances based on direct measurements of the electron temperature (Te) may deviate systematically from the real ones. Finally, one must keep in mind other sources of uncertainties in the nebular abundance determination such as the atomic data, reddening corrections, and the possible depletion of elements by their accretion onto dust.
Among the stellar objects, blue massive stars can easily be identified at large distances due to their high luminosities. Therefore, massive stars offer a unique opportunity to study present-day chemical abundances in spiral and irregular galaxies as an alternative method to classical H ii-region studies. However, the amount of energy released by these stars is so large that it produces dramatic effects on the star itself: these stellar objects undergo mass outflows throughout their lifetimes (so-called stellar winds) and their atmospheres depart from LTE conditions, two facts that make their modeling quite complex. It has not been until very recently that the development of massive-star model atmospheres and the growth of computational efficiency have allowed a reliable abundance analysis of these objects. Nowadays, it is feasible; however, one must take into account that there are some effects that can affect the final results and must be treated carefully: (1) the hypothesis governing the stellar-atmosphere modeling (LTE versus NLTE, plane-parallel versus spherical models, inclusion of line-blanketing effects), (2) atomic models and atomic data; and (3) establishment of the stellar parameters and microturbulence.
Nebular and stellar methodologies are now working in tandem to advance our knowledge of the metallicity content of irregular and spiral galaxies (from the Milky Way to far beyond the Local Group). Since they sample similar spatial and temporal distributions, these objects offer us a unique framework in which the reliability of the abundances derived using both methodologies can be tested.
0 « Rolleston et al. (2000) a Daflon & Cunha (2004)
0 « Rolleston et al. (2000) a Daflon & Cunha (2004)
Figure 11.1. A comparison of three recent determinations of the O abundance gradient in the Milky Way. Although the comparison of values found in the literature for the gradient obtained by means of massive-star and H ii-region studies seems to be in agreement within the intrinsic uncertainties (see the text), this might not be the case when comparing results from individual studies. Note also the offset in absolute abundances resulting from the three studies.
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