Quantitative spectroscopy of OB-stars in the optical and the infrared

Massive OB stars are the most luminous stellar objects (10e5 to a few 10e6 L⊙). Although being rare by number they play a dominant role in the chemical and dynamical evolution of galaxies through their input of energy, momentum, and nuclear processed material into the interstellar medium by means of stellar winds, eruptions, and (final) explosions. The luminosity of hot massive stars is the key ingredient to the driving of a dense (10e−6 - 10e−5 M⊙/yr) and fast (up to 3,000 km/s) outflow lasting a lifetime. This mass loss imprints unambiguous signatureson the spectral energy distribution and spectral lines received from these objects. The goal of this thesis was to investigate and to apply recent, improved methods for spectral diagnostics in the optical and the infrared by means of unified model atmospheres, comprising the entire sub- and supersonic structure from the pseudo-hydrostatic photosphere to the stellar wind. In particular, we used the nlte-model atmospheres code fastwind, which is highly computational efficient and updated to comprise an adequate though approximate treatment of metal line opacity effects, i.e., metal line-blocking/-blanketing. First we have tested this code by a comparison with alternative codes (e.g., cmfgen by Hillier & Miller 1998 and WM-Basic by Pauldrach et al. 2001), particularly in terms of temperature stratification, fluxes, and number of ionizing photons. In almost all cases we obtained very similar results. Also for the H/He lines which could be only compared to cmfgen, the coincidence between the codes is remarkable, except for a subtle discrepancy concerning the He i singlets, where, in a restricted temperature range, cmfgen predicts weaker singlet lines. Having tested the improved model atmospheres code we began our study with a re-analysis of the Galactic O-star sample presented by Puls et al. 1996 (at that time using pure H/He models) to investigate the influence of line-blocking/-blanketing. This re-analysis (by means of profile fitting of photospheric and wind lines from H and He) resulted in a significant re-definition of the effective temperature scale due to this line-blanketing effect. We obtain lower effective temperatures (up to 8,000 K, depending on spectral type and luminosity class) in combination with a reduction in either gravity or helium abundance, thus, making it possible to assign a new Teff - log g and Teff - spectral type calibration as a function of luminosity class. Further, by calculating new spectroscopic masses and comparing them with previous results we find a significant reduction in the so-called mass discrepancy (Herrero et al. 1992), where the latter describes the unfortunate situation that spectroscopically derived masses are lower than those resulting from stellar evolution calculations. For stars below 50 M⊙ a systematic trend is retained such that the spectroscopically derived masses are smaller by approx. 10 M⊙ compared to the evolutionary ones. Moreover, the wind momentum luminosity relation (WLR) changes because of lower luminosities and almost unmodified wind-momentum rates. Still present, however, is a separation of the WLR as a function of luminosity class, in contrast to theoretical simulations which do not predict such a dependence. From simple arguments and using stellar samples of different sizes, we find strong indications that for most supergiants the mass-loss rate is over-estimated by a factor of 2 to 3, whereas the mass-loss-estimates for luminosity class III and V objects are consistent with our own theoretical expectations and those by others. The over-estimate is interpreted as an effect of wind-clumping, and our argumentation is based on the assumption that the material in the lower wind region is un-clumped, in accordance with theoretical predictions. As a final step we have analyzed a large sample of OB stars by means of H and K band spectroscopy in the infrared (IR) regime, with the primary goal to investigate to what extent a lone near IR-spectroscopy is able to recover stellar and wind parameters derived in the optical. Due to the substantial progress in ground-based IR instrumentation in the past decade and the extension of model atmosphere codes to the infrared wavelength regime, IR spectroscopy has become a powerful diagnostics for the investigation of young and massive stars lying deeply embedded in the dust-enshrouded environment of molecular clouds or the Galactic centre, allowing us to take a first step in the direction of a pure IR analysis. For the stars analyzed we obtain well-agreeing results between the optical and the infrared, except for the line cores of Br gamma in early O stars with significant mass loss, which, again, might indicate the presence of clumping effects. Having derived the stellar and wind parameters from the IR, we are now able to constrain the observational requirements to perform a pure IR-analysis. Most important is a very high S/N ratio, as the lines to be investigated are extremely shallow, and a very good resolution, in addition to an adequately large set of strategic lines. Given this prerequisite, spectral analyses based on pure IR data could, indeed, be successfully used as an alternative or support to traditional methods, and will allow us to proceed towards our ultimate goal of analyzing very young and highly obscured objects just emanating from their birth places

X-ray spectral diagnostics of activity in massive stars

X-rays give direct evidence of instabilities, time-variable structure, and shock heating in the winds of O stars. The observed broad X-ray emission lines provide information about the kinematics of shock-heated wind plasma, enabling us to test wind-shock models. And their shapes provide information about wind absorption, and thus about the wind mass-loss rates. Mass-loss rates determined from X-ray line profiles are not sensitive to density-squared clumping effects, and indicate mass-loss rate reductions of factors of 3 to 6 over traditional diagnostics that suffer from density-squared effects. Broad-band X-ray spectral energy distributions also provide mass-loss rate information via soft X-ray absorption signatures. In some cases, the degree of wind absorption is so high that the hardening of the X-ray SED can be quite significant. We discuss these results as applied to the early O stars zeta Pup (O4 If), 9 Sgr (O4 V((f))), and HD 93129A (O2 If*).Comment: To appear in the proceedings of IAU 272: Active OB Star

On the sensitivity of HeI singlet lines to the FeIV model atom in O stars

Recent calculations and analyses of O star spectra have revealed discrepancies between theory and observations, and between different theoretical calculations, for the strength of optical HeI singlet transitions.We investigate the source of these discrepancies. Using a non-LTE radiative transfer code we have undertaken detailed test calculations for a range of O star properties. Our principal test model has parameters similar to those of the O9V star, 10 Lac. We show that the discrepancies arise from uncertainties in the radiation field in the HeI resonance transition near 584Angs. The radiation field at 584Angs. is influenced by model assumptions, such as the treatment of line-blanketing and the adopted turbulent velocity, and by the FeIV atomic data. It isshown that two FeIV transitions near 584Angs can have a substantial influence on the strength of the HeI singlet transitions. Because of the difficulty of modeling the HeI singlet lines, particularly in stars with solar metalicity, the HeI triplet lines should be preferred in spectral analyses. These lines are much less sensitive to model assumptions.Comment: 7 pages, 9 figures, accepted for publication in A&

Improved velocity law parameterization for hot star winds

The velocity law of hot star winds is usually parameterized via the so-called beta velocity law. Although this parameterization stems from theoretical considerations, it is not the most accurate description of the wind velocity law that follows from hydrodynamical calculations. We show that the velocity profile of our hydrodynamical wind models is described much better by polynomial approximation. This approximation provides a better fit than the beta velocity law already for the same number of free parameters.Comment: 3 pages, 2 figures, accepted for publication in Astronomy & Astrophysic

Hydrogen and helium line formation in OB dwarfs and giants. A hybrid non-LTE approach

Aims: Hydrogen and helium line spectra are crucial diagnostic features for the quantitative analysis of OB stars. We compute synthetic spectra based on a hybrid non-LTE approach in order to test the ability of these models to reproduce high-resolution and high-S/N spectra of dwarf and giant stars and also to compare them with published grids of non-LTE (OSTAR2002) and LTE (Padova) models. Methods: Our approach solves the restricted non-LTE problem based on classical line-blanketed LTE model atmospheres. State-of-the-art model atoms and line-broadening theories are employed to model the H and He I/II spectra over the entire optical range and in the near-IR. Results: The synthetic spectra match almost all measurable hydrogen and helium lines observed in six test stars over a wide spectral range from the Balmer limit to the NIR, except for only a few well-understood cases. Our approach reproduces other published non-LTE calculations, however avoids inconsistencies with the modelling of the He I singlets recently discussed in the literature. It improves on the published LTE models in many aspects: non-LTE strengthening and the use of improved line-broadening data result in overall significant differences in the line profiles and equivalent widths of the Balmer and helium lines. Where possible, systematic effects on the stellar parameter determination are quantified, e.g. gravities derived from the Hgamma wings may be overestimated by up to ~0.2 dex at our upper temperature boundary in LTE. (abridged)Comment: 25 pages, 19 figures. Modified according to suggestions of the referee. Accepted for publication in A&A. Several figures in low resolution. A high-resolution pdf version of the preprint can be downloaded from http://www.sternwarte.uni-erlangen.de/~ai97/preprints/HHe_nieva.pd

Modelling the clumping-induced polarimetric variability of hot star winds

Clumping in the winds of massive stars may significantly reduce empirical mass-loss rates, and which in turn may have a large impact on our understanding of massive star evolution. Here, we investigate wind-clumping through the linear polarization induced by light scattering off the clumps. Through the use of an analytic wind clumping model, we predict the time evolution of the linear polarimetry over a large parameter space. We concentrate on the Luminous Blue Variables, which display the greatest amount of polarimetric variability and for which we recently conducted a spectropolarimetric survey. Our model results indicate that the observed level of polarimetric variability can be reproduced for two regimes of parameter space: one of a small number of massive, optically-thick clumps; and one of a very large number of low-mass clumps. Although a systematic time-resolved monitoring campaign is required to distinguish between the two scenarios, we currently favour the latter, given the short timescale of the observed polarization variability. As the polarization is predicted to scale linearly with mass-loss rate, we anticipate that all hot stars with very large mass-loss rates should display polarimetric variability. This is consistent with recent findings that intrinsic polarization is more common in stars with strong H$\alpha$ emission.Comment: 12 pages, 11 figures, accepted to A&

Non-LTE Line Formation in the Near-IR: Hot Stars

Line-formation calculations in the Rayleigh-Jeans tail of the spectral energy distribution are complicated by an amplification of non-LTE effects. For hot stars this can make quantitative modelling of spectral lines in the near-IR challenging. An introduction to the modelling problems is given and several examples in the context of near-IR line formation for hydrogen and helium are discussed.Comment: 16 pages, 13 figure

Nitrogen line spectroscopy of O-stars -- I. Nitrogen III emission line formation revisited

This is the first paper in a series dealing with optical Nitrogen spectroscopy of O-type stars, aiming at the analysis of Nitrogen abundances. We implemented a new Nitrogen model atom into the NLTE atmosphere/spectrum synthesis code FASTWIND, and compare the resulting optical NIII lines at 4634/40/42 A with other predictions, mostly from Mihalas & Hummer (1973, ApJ 179, 827,`MH'), and from the alternative code CMFGEN. Using similar model atmospheres as MH (not blanketed and wind-free), we are able to reproduce their results, in particular the triplet emission lines. According to MH, these should be strongly related to dielectronic recombination (DR) and the drain by certain two-electron transitions. However, using realistic, fully line-blanketed atmospheres at solar abundances, the key role of DR controlling these emission features is superseded -- for O-star conditions -- by the strength of the stellar wind and metallicity. In the case of wind-free models, the resulting lower ionizing EUV-fluxes severely suppress the emission. As the mass-loss rate is increased, pumping through the NIII resonance line(s) in the presence of a near-photospheric velocity field results in a net optical triplet line emission. A comparison with results from CMFGEN is mostly satisfactory, except for the range 30 kK < Teff < 35 kK, where CMFGEN triggers the triplet emission at lower Teff than FASTWIND. This effect could be traced down to line overlap effects between the NIII and OIII resonance lines that so far cannot be simulated by FASTWIND. Since the efficiency of DR and `two electron drain' strongly depends on the degree of line-blanketing/-blocking, we predict the emission to become stronger in a metal-poor environment, though lower wind-strengths and Nitrogen abundances might counteract this effect. Weak winded stars should display less triplet emission than stars with `normal' winds.Comment: Accepted by Astronomy & Astrophysics. Main paper: 18 pages, 16 figures; Online-appendix: 6 pages, 14 figure

Chemical composition of Galactic OB stars II. The fast rotator Z Oph

Z Oph, HD149757, is an O9.5 Vnn star with a very high projected rotational velocity (vsini >= 340 km\s). It is also a classical runaway star due to its high proper motion. We perform a quantitative analysis of its optical spectrum in order to measure important observables of the star such as its mass, effective temperature, luminosity and He, C, N, and O abundances. Comparing these observed values to those predicted by the rotating evolutionary models of the Geneva group we find that none of the two sets of models is capable of reproducing the characteristics of the star. Nevertheless, due to its runaway nature, the reason for this discrepancy may be that the star is not the result of the evolution of a single object, but the product of the evolution of a close binary system.Comment: Accepted for publication in A&

Atmospheric NLTE-models for the spectroscopic analysis of blue stars with winds : II. Line-blanketed models

We present new or improved methods for calculating NLTE, line-blanketed model atmospheres for hot stars with winds (spectral types A to O), with particular emphasis on fast performance. These methods have been implemented into a previous, more simple version of the model atmosphere code FASTWIND (Santolaya-Rey et al. 1997) and allow us to spectroscopically analyze large samples of massive stars in a reasonable time-scale, using state-of-the-art physics. Although this updated version of the code has already been used in a number of recent investigations, the corresponding methods have not been explained in detail so far, and no rigorous comparison with results from alternative codes has been performed. This paper intends to address both topics. In particular, we describe our (partly approximate) approach to solve the equations of statistical equilibrium for those elements that are primarily responsible for line-blocking and blanketing, as well as an approximate treatment of the line-blocking itself, which is based on a simple statistical approach using suitable means of line opacities and emissivities. Both methods are validated by specific tests. Furthermore, we comment on our implementation of a consistent temperature structure. In the second part, we concentrate on a detailed comparison with results from two codes used in alternative spectroscopical investigations, namely CMFGEN (Hillier & Miller 1998) and WM-Basic (Pauldrach et al. 2001). All three codes predict almost identical temperature structures and fluxes for λ > 400 Å, whereas at lower wavelengths a number of discrepancies are found. Particularly in the HeII continua, where fluxes and corresponding numbers of ionizing photons react extremely sensitively to subtle differences in the models, we consider any uncritical use of these quantities (e.g., in the context of nebula diagnostics) as unreliable. Optical H/He lines as synthesized by FASTWIND are compared with results from CMFGEN, obtaining a remarkable coincidence, except for the HeI singlets in the temperature range between 36 000 to 41 000 K for dwarfs and between 31 000 to 35 000 K for supergiants, where CMFGEN predicts much weaker lines. Consequences of these discrepancies are discussed. Finally, suggestions are presented as to adequately parameterize model-grids for hot stars with winds, with only one additional parameter compared to standard grids from plane-parallel, hydrostatic models.Facultad de Ciencias Astronómicas y Geofísica