Quantitative spectroscopy

The main long-term goal of the project started with this thesis, is to develop (and subsequently, of course, to apply) reliable methods for quantitative spectroscopy of hot, massive stars with winds. Only if our diagnostic tools are sufficiently well developed may we with some confidence draw conclusions on various observed properties of these objects, such as effective temperatures, chemical abundances, and mass-loss rates.

From the results of Chapters 4 and 5 we have learned that the description of clumping included in present-day, state-of-the-art, unified model atmospheres may not be adequate under certain cir- cumstances. Using the basic methods developed in those chapters, we intend to updateFASTWIND (Sect. 1.5.5) shortly, with a more appropriate description of wind clumping, accounting both for the presence of optically thick clumps and for a non-monotonic velocity field. A treatment of X-rays will be implemented as well. Thereafter we shall be ready to derive empirical mass-loss rates, as well as structure properties, of unprecedented reliability for large samples of hot, massive stars, by means of multi-diagnostic, multi-wavelength studies.

With this, we hope to set a new standard for empirical mass-loss rates from hot, massive stars, and shed light on current conflicts between observations and theory (e.g., the clumping in the inner wind and the velocity spans of the clumps, see previous sections). Most importantly, we aim to answer the key question: Can we trust mass-loss rates currently in use in models of massive star evolution? Also, chemical surface abundances of stars provide constraints on both stellar and galactic evolution models as well as probe the conditions both in the present-day and in the early Universe. The mass- loss rate is important when deriving chemical surface abundances of hot stars with significant wind

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strengths, for the modeled ionization equilibria and synthesized profiles are both affected by mass loss. As an example, nitrogen is a key element for constraining massive star evolution as well as galactic chemical evolution. However, first results from the VLT-FLAMES project (see Chapter 2) revealed apparent conflicts between observed nitrogen surface abundances in B-stars and the predictions from stellar interior models including rotational mixing (Hunter et al., 2007, 2009). On the other hand, primary nitrogen, presumably produced by strong rotational mixing in massive stars, is needed in galactic chemical evolution models, to reproduce the observed abundance patterns of the very old low-mass, metal-poor, halo stars that have survived until today (Chiappini et al., 2006). Nitrogen model atoms appropriate for O stars are currently being developed within our group (PhD thesis of J. Rivero Gonz´alez), and reliable atmospheric structures including the stellar wind (and accounting for the effects of wind clumping, see above) will be essential in order to use these model atoms for deriving abundances of desired accuracy, and in the extension to put further constraints on the evolution models.

The importance of adequate model atoms in quantitative spectroscopy was also demonstrated in a side project of this thesis, in which we used the NLTE approach for trace elements to model and analyze infra-red Mg I emission lines in late-type stars (Chapter 6). Although observations of these lines so far have been scarce indeed, the advent of NASA’s airborne Stratospheric Observatory for Infrared Astronomy (SOFIA)1opens up for more regular observations in the future. Then it might finally be possible to explore these lines’ long recognized potential to detect upper photospheric magnetic fields, due to Zeeman splitting, as well as to use them to, e.g., empirically test the efficiency of collisions with neutral hydrogen, which generally are a great factor of uncertainty in NLTE spectral analyses of cool stellar atmospheres, and to which the lines are very sensitive.

A future major (and somewhat exciting) application for quantitative spectroscopy of stars might be analyses of AB-supergiants in distant galaxies beyond the local group. These stars are the intrinsically brightest (‘normal’) stars in the optical, and are thus ideal objects to study when pushing the distance limits for observations of individual stars. From studying single stars in environments very different from our own we may obtain invaluable information not only about stellar and wind properties, but also about chemical compositions of and even distances to2 their host galaxies (for a summary of first results, see Kudritzki et al., 2008). However, at such distances only the very brightest stars are accessible to us, stars which indeed have strong and powerful winds. Therefore it is critical that winds be considered in these analyses. Currently FASTWIND is used only for the B supergiants, whereas the hybrid NLTE approach (see Sect. 1.5.3) still is utilized for the A supergiants. Actually, also present-day unified model atmospheres cannot reproduce the observed Hα line profiles from local A-supergiants, which are observed as un-saturated lines but modeled as saturated ones. Since Hα behaves like a quasi-resonance line in this domain (e.g., Kudritzki & Puls, 2000), this behavior might be explained by the presence of optically thick clumps, in analogy with our findings for UV resonance lines in O stars (Chapters 4 and 5). We will explore this possibility anon, by making appropriate updates ofFASTWIND.

1 _{which recently, finally, has made its ‘first light flight’, see http://www.sofia.usra.edu/}

2 _{Via a purely spectroscopic distance indicator, the so-called flux-weighted gravity - luminosity relation, FGLR, see Ku-}

122 CHAPTER 7. SUMMARY AND OUTLOOK