Populations of OB-type stars in galaxies

One of the challenges for stellar astrophysics is to reach the point at which we can undertake reliable spectral synthesis of unresolved populations in young, star-forming galaxies at high redshift. Here I summarise recent studies of massive stars in the Galaxy and Magellanic Clouds, which span a range of metallicities commensurate with those in high-redshift systems, thus providing an excellent laboratory in which to study the role of environment on stellar evolution. I also give an overview of observations of luminous supergiants in external galaxies out to a remarkable 6.7 Mpc, in which we can exploit our understanding of stellar evolution to study the chemistry and dynamics of the host systems.Comment: 9 pages, 3 figures, invited review at IAU Symposium 272 (eds Neiner, Wade, Meynet & Peters

Modeling the near-infrared lines of O-type stars

We use a grid of 30 line-blanketed unified stellar photosphere and wind models for O-type stars; computed with the code CMFGEN in order to evaluate its potential in the near-infrared spectral domain. The grid includes dwarfs, giants and supergiants. We analyse the equivalent width behaviour of the 20 strongest lines of hydrogen and helium in spectral windows that can be observed using ground-based instrumentation and compare the results with observations. Our main findings are that: i) HeI/HeII line ratios in the J, H and K bands correlate well with the optical ratio employed in spectral classification, and can therefore be used to determine the spectral type; ii) in supergiant stars the transition from the stellar photosphere to the wind follows a shallower density gradient than the standard approach followed in our models, which can be mimicked by adopting a lower gravity in our prescription of the density stratification. iii) the Brackett gamma line poses a number of peculiar problems which partly might be related to wind clumping, and iv) the Brackett alpha line is an excellent mass-loss indicator. For the first and last item we provide quantitative calibrations.Comment: 14 pages, 7 figures, accepted by A&

The star formation process in the Magellanic Clouds

The Magellanic Clouds offer unique opportunities to study star formation both on the global scales of an interacting system of gas-rich galaxies, as well as on the scales of individual star-forming clouds. The interstellar media of the Small and Large Magellanic Clouds and their connecting bridge, span a range in (low) metallicities and gas density. This allows us to study star formation near the critical density and gain an understanding of how tidal dwarfs might form; the low metallicity of the SMC in particular is typical of galaxies during the early phases of their assembly, and studies of star formation in the SMC provide a stepping stone to understand star formation at high redshift where these processes can not be directly observed. In this review, I introduce the different environments encountered in the Magellanic System and compare these with the Schmidt-Kennicutt law and the predicted efficiencies of various chemo-physical processes. I then concentrate on three aspects that are of particular importance: the chemistry of the embedded stages of star formation, the Initial Mass Function, and feedback effects from massive stars and its ability to trigger further star formation.Comment: 12pages, 5figures, invited review at the IAUS 256, The Magellanic System: Stars, Gas, and Galaxies, eds. Jacco van Loon, Joana Oliveir

The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds

Aims. We compare the predictions of evolutionary models for early-type stars with atmospheric parameters, projected rotational velocities and nitrogen abundances estimated for a sample of Be-type stars. Our targets are located in 4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11 region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346. Methods. Atmospheric parameters and photospheric abundances have been determined using the non-LTE atmosphere code tlusty. Effective temperature estimates were deduced using three different methodologies depending on the spectral features observed; in general they were found to yield consistent estimates. Gravities were deduced from Balmer line profiles and microturbulences from the Si iii spectrum. Additionally the contributions of continuum emission from circumstellar discs were estimated. Given its importance in constraining stellar evolutionary models, nitrogen abundances (or upper limits) were deduced for all the stars analysed. Results. Our nitrogen abundances are inconsistent with those predicted for targets spending most of their main sequence life rotating near to the critical velocity. This is consistent with the results we obtain from modelling the inferred rotational velocity distribution of our sample and of other investigators. We consider a number of possibilities to explain the nitrogen abundances and rotational velocities of our Be-type sample.Comment: 14 pages, 9 figures, submitted to A&

First VLT/X-shooter spectroscopy of early-type stars outside the Local Group

As part of the VLT/X-shooter science verification, we obtained the first optical medium-resolution spectrum of a previously identified bright O-type object in NGC55, an LMC-like galaxy at a distance of \sim2.0 Mpc. Based on the stellar and nebular spectrum, we investigate the nature and evolutionary status of the central object(s) and its influence on the surrounding interstellar medium. We conclude that the source, NGC55_C1_31, is a composite object, likely a stellar cluster, which contains one or several hot (T_eff \simeq 50000 K) WN stars with a high mass-loss rate (\sim3 \times 10^{-5} M_\odot yr^{-1}) and a helium-rich composition (N_He/N_H = 0.8). The visual flux is dominated by OB-type (super)giant stars with T_eff \sim< 35000 K, solar helium abundance (N_He/N_H = 0.1), and mass-loss rate \sim2 \times 10^{-6} M_\odot yr^{-1}. The surrounding H II region has an electron density n_e < 10^2 cm^{-3} and an electron temperature T(OIII) \simeq 11500 \pm 600 K. The oxygen abundance of this region is [O/H] = 8.18 \pm 0.03 which corresponds to Z = 0.31 \pm 0.04 Z_\odot. We observed no significant gradients in T(OIII), n_e or [O/H] on a scale of 73 pc extending in four directions from the ionising source. The properties of the HII region can be reproduced by a CLOUDY model which uses the central cluster as ionising source, thus providing a self-consistent interpretation of the data. We also report on the serendipitous discovery of HeII nebular emission associated with the nearby source NGC55_C2_35, a feature usually associated with strong X-ray sources.Comment: 12 pages, 10 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society; the definitive version will be available at wwww.blackwell-synergy.co

UV Spectroscopy of Metal-Poor Massive Stars in the Small Magellanic Cloud

The Hubble Space Telescope has provided the first clear evidence for weaker winds of metal-poor massive stars in the Small Magellanic Cloud, confirming theoretical predictions of the metallicity dependence of mass-loss rates and wind terminal velocities. For lower luminosity O-type stars however, derived mass-loss rates are orders of magnitude lower than predicted, and are at present unexplained.Comment: 4 pages, 3 figures. To appear in 'The Impact of HST on European Astronomy', Eds., G. De Marchi & F.D. Macchetto, Astrophysics & Space Science, Springe

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

Evolution of progenitor stars of Type Ibc supernovae and long gamma-ray bursts

We discuss how rotation and binary interactions may be related to the diversity of type Ibc supernovae and long gamma-ray bursts. After presenting recent evolutionary models of massive single and binary stars including rotation, the Tayler-Spruit dynamo and binary interactions, we argue that the nature of SNe Ibc progenitors from binary systems may not significantly differ from that of single star progenitors in terms of rotation, and that most long GRB progenitors may be produced via the quasi-chemically homogeneous evolution at sub-solar metallicity. We also briefly discuss the possible role of magnetic fields generated in the convective core of a massive star for the transport of angular momentum, which is potentially important for future stellar evolution models of supernova and GRB progenitors.Comment: 6 pages, 4 figures, to appear in IAU Symp. 250, Massive Stars as Cosmic Engines, Kauai (HI), 12/2007, ed. F. Bresolin, P. Crowther, & J. Pul

The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55

The quantitative study of the physical properties and chemical abundances of large samples of massive blue stars at different metallicities is a powerful tool to understand the nature and evolution of these objects. Their analysis beyond the Milky Way is challenging, nonetheless it is doable and the best way to investigate their behavior in different environments. Fulfilling this task in an objective way requires the implementation of automatic analysis techniques that can perform the analyses systematically, minimizing at the same time any possible bias. As part of the ARAUCARIA project we carry out the first quantitative spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55 at 1.94 Mpc away. By applying the methodology developed in this work, we derive their stellar parameters, chemical abundances and provide a characterization of the present-day metallicity of their host galaxy. Based on the characteristics of the stellar atmosphere/line formation code FASTWIND, we designed and created a grid of models for the analysis of massive blue supergiant stars. Along with this new grid, we implemented a spectral analysis algorithm. Both tools were specially developed to perform fully consistent quantitative spectroscopic analyses of low spectral resolution of B-type supergiants in a fast and objective way. We present the main characteristics of our FASTWIND model grid and perform a number of tests to investigate the reliability of our methodology. The automatic tool is applied afterward to a sample of 12 B-type supergiant stars in NGC55, deriving the stellar parameters and abundances. The results indicate that our stars are part of a young population evolving towards a red supergiant phase. The derived chemical composition hints to an average metallicity similar to the one of the Large Magellanic Cloud, with no indication of a spatial trend across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&

The VLT-FLAMES survey of massive stars: Wind properties and evolution of hot massive stars in the LMC

[Abridged] We have studied the optical spectra of 28 O- and early B-type stars in the Large Magellanic Cloud, 22 of which are associated with the young star-forming region N11. Stellar parameters are determined using an automated fitting method, combining the stellar atmosphere code FASTWIND with the genetic-algorithm optimisation routine PIKAIA. Results for stars in the LH9 and LH10 associations of N11 are consistent with a sequential star formation scenario, in which activity in LH9 triggered the formation of LH10. Our sample contains four stars of spectral type O2, of which the hottest is found to be ~49-54 kK (cf. ~45-46 kK for O3 stars). The masses of helium-enriched dwarfs and giants are systematically lower than those implied by non-rotating evolutionary tracks. We interpret this as evidence for efficient rotationally-enhanced mixing, leading to the surfacing of primary helium and to an increase of the stellar luminosity. This result is consistent with findings for SMC stars by Mokiem et al. For bright giants and supergiants no such mass-discrepancy is found, implying that these stars follow tracks of modestly (or non-)rotating objects. Stellar mass-loss properties were found to be intermediate to those found in massive stars in the Galaxy and the SMC, and comparisons with theoretical predictions at LMC metallicity yielded good agreement over the luminosity range of our targets, i.e. 5.0 < log L/L(sun) < 6.1