arXiv subjects

Daniel Schaerer

Publications and source records attributed to Daniel Schaerer.

209 records · Page 12Linked to original sources

arXiv1998

Massive Star Evolution in Different Environments

We review the properties of massive star evolution in different environments, where the major environmental factor is metallicity. Comparisons between evolutionary models and observations of massive OB, WR stars and related objects are presented. We also review several observations asking for future improvements of stellar models and theoretical developments in this respect. We summarize evolutionary scenarios for the most massive stars and try to clarify recent questions regarding their evolutionary status as core-H or core-He burning objects. Another environmental effect, which might affect stellar evolution is a cluster environment with a high stellar density. As test cases of massive star evolution in dense clusters we summarize recent work on the densest known resolved young clusters: R136, NGC 3603, and the three Galactic Center star clusters (the central cluster, Quintuplet and "Arches" cluster). For the central cluster we present new comparisons between stellar parameters of emission line stars derived by Najarro et al. (1994, 1997), and appropriate evolutionary models. From their parameters we argue that most of these stars can be regarded as WNL stars, and do hence not necessarily represent a peculiar class. We suggest that some apparent differences with well known WR stars can be understood in terms of their core burning stage and/or other changes due to a high metallicity. Based on our present knowledge we conclude that in young clusters with central stellar densities up to rho_c ~ 10.**(5-6) Msun/pc**3 no compelling evidence for a secondary effect influencing the evolution of massive stars has yet been found.

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arXiv1998

Massive star populations in I Zw 18: A probe of stellar evolution in the early universe

We present a study of the gaseous and stellar emission in I Zw18, the most metal-poor star-forming galaxy known. Archival HST WFPC2 and FOS data have been used to analyze the spatial distribution of [OIII], Halpha, and HeII 4686. The latter is used to identify Wolf-Rayet stars found by ground-based spectroscopy and to locate nebular HeII emission. Most of the HeII emission is associated with the NW stellar cluster, displaced from the surrounding shell-like [OIII] and Halpha emission. We found evidence for HeII sources compatible with 5-9 WNL stars and/or compact nebular HeII emission as well as residual diffuse emission. New evolutionary tracks and synthesis models at the appropriate metallicity predict a mass limit M_WR ~90 Msun for WR stars to become WN and WC/WO. The observed equivalent widths of the WR lines are in good agreement with an instantaneous burst model with a Salpeter IMF extending up to M_up ~ 120-150 Msun. Our model is also able to fully reproduce the observed equivalent widths of nebular HeII emission due to the presence of WC/WO stars. This finding together with the spatial distribution of nebular HeII further supports the hypothesis that WR stars are responsible for nebular HeII emission in extra-galactic HII regions. Finally we discuss the implications on stellar mass loss, chemical yields, final stellar masses, and the ionizing flux of starburst galaxies at very low metallicities.

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arXiv1998

Searching for WR stars in I Zw 18 -- The origin of HeII emission

I Zw 18 is the most metal poor star-forming galaxy known and is an ideal laboratory to probe stellar evolution theory at low metallicities. Using archival HST WFPC2 imaging and FOS spectroscopy we were able to improve previous studies. We constructed a continuum free HeII map, which was used to identify Wolf-Rayet (WR) stars recently found by ground-based spectroscopy and to locate diffuse nebular emission. Most of the HeII emission is associated with the NW stellar cluster, clearly displaced from the surrounding shell-like [OIII] and Halpha emission. We found evidence for HeII sources, compatible with 5--9 WNL stars and/or compact nebular HeII emission, as well as residual diffuse emission. Only one of them is outside the NW cluster. We have calculated evolutionary tracks for massive stars and synthesis models at the appropriate metallicity (Z ~ 0.02 Zsun). These single star models predict a mass limit M_WR ~ 90 Msun for WR stars to become WN and WC/WO. For an instantaneous burst model with a Salpeter IMF extending up to M_up ~ 120-150 Msun our model predictions are in reasonable agreement with the observed equivalent widths. Our model is also able to fully reproduce the observed equivalent widths of nebular HeII emission due to the presence of WC/WO stars. This quantitative agreement and the spatial correlation of nebular HeII with the stellar cluster and the position of WR stars shown from the ground-based spectra further supports the hypothesis that WR stars are responsible for nebular HeII emission in extra-galactic HII regions. (Abridged abstract)

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arXiv1997

New Models for Wolf-Rayet and O Star Populations in Young Starbursts

Using the latest stellar evolution models, theoretical stellar spectra, and a compilation of observed emission line strengths from Wolf-Rayet (WR) stars, we construct evolutionary synthesis models for young starbursts. We explicitly distinguish between the various WR subtypes (WN, WC, WO), and we treat O and Of stars separately. We provide detailed predictions of UV and optical emission line strengths for both the WR stellar lines and the major nebular hydrogen and helium emission lines, as a function of several input parameters related to the starburst episode. We also derive the theoretical frequency of WR-rich starbursts. We then discuss: nebular HeII 4686 emission, the contribution of WR stars to broad Balmer line emission, techniques used to derive the WR and O star content from integrated spectra, and explore the implications of the formation of WR stars through mass transfer in close binary systems in instantaneous bursts. The observational features predicted by our models allow a detailed quantitative determination of the massive star population in a starburst region (particularly in so-called "WR galaxies") from its integrated spectrum and provide a means of deriving the burst properties (e.g., duration, age) and the parameters of the initial mass function of young starbursts. (Abridged abstract)

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arXiv1997

The ionizing fluxes of early type stars and their impact on HII regions and the `Diffuse Ionized Gas'

We discuss recent results on the ionizing fluxes of O stars obtained from our "combined stellar structure and atmosphere models" (CoStar) accounting for stellar winds, non-LTE effects and line blanketing. The implications on the ionization structure of HII regions are summarized, and observational constraints on the ionizing spectra and the total ionizing photon fluxes are presented. Using our CoStar models we derive new consistent predictions for the H and HeI ionizing fluxes of steady-state massive star populations. Implications for the interpretation of observations of `Diffuse Ionized Gas' in galaxies are discussed. Finally we present preliminary model calculations aiming to improve our current predictions further and compare our results to recent results from the Munich group.

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arXiv1997

Detection of Wolf-Rayet stars of WN and WC subtype in Super Star Clusters of NGC 5253

We present spectroscopic observations of the central star clusters in NGC 5253 the aim of which is to search for WC stars. Our observations show the presence of Wolf-Rayet (WR) stars not only of WN but also of WC subtype in two star forming regions corresponding to the maximum optical and UV emission. The massive star population we derive is consistent with young bursts of ~3 and 4 Myr. The region of maximum optical emission is found to provide the dominant contribution of the ionizing flux as opposed to the less extinguished region of maximum UV brightness. The presence of WR stars near the N-enriched regions found by Walsh & Roy (1987, 1989) and Kobulnicky et al. (1997) suggests they are a possible source of N. It is presently unclear whether or not our detection of WC stars is compatible with the normal observed He/O and C/O abundance ratios.

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arXiv1997

Fundamental stellar parameters of zeta Pup and gamma^2 Vel from HIPPARCOS data

We report parallax measurements by the HIPPARCOS satellite of zeta Puppis and gamma^2 Velorum. The distance of zeta Pup is d=429 (+120/ -77) pc, in agreement with the commonly adopted value to Vela OB2. However, a significantly smaller distance is found for the gamma^2 Vel system: d=258 (+41/-31) pc. The total mass of gamma^2 Vel derived from its parallax, the angular size of the semi-major axis as measured with intensity interferometry, and the period is M(WR+O)=29.5 (+/-15.9) Msun. This result favors the orbital solution of Pike et al. (1983) over that of Moffat et al. (1986). The stellar parameters for the O star companion derived from line blanketed non-LTE atmosphere models are: Teff=34000 (+/-1500) K, log L/Lsun=5.3 (+/-0.15) from which an evolutionary mass of M=29 (+/-4) Msun and an age of 4.0 (+0.8/-0.5) Myr is obtained from single star evolutionary models. With non-LTE model calculations including He and C we derive a luminosity log L/Lsun~4.7 (+/-0.2) for the WR star. The mass-luminosity relation of hydrogen-free WR stars implies a mass of M(WR)~5 (+/-1.5) Msun. From our data we favor an age of ~10 Myr for the bulk of the Vela OB2 stars. Evolutionary scenarios for zeta Pup and gamma^2 Vel are discussed in the light of our results.

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arXiv1996

Combined stellar structure and atmosphere models for massive stars. IV. The impact on the ionization structure of single star HII regions

We study the impact of modern stellar atmospheres that take into account the effects of stellar winds, departures from LTE and line blanketing ("CoStar" models) on the ionization structure of HII regions. Results from a large grid of photoionization models are presented. Due to a flatter energy distribution in the HeI continuum, compared to the widely used Kurucz models, generally higher ionic ratios are obtained. We find that N+/O+ and Ne++/O++ can be safely used as direct indicators of N/O and Ne/O abundance ratios in HII regions, over a wide range of astrophysical situations. The roughly constant observed value of Ne++/O++ ionic ratios in Galactic HII regions is naturally reproduced by photoionization models using CoStar fluxes, while Kurucz models at solar metallicity fail to reproduce this behaviour. This gives support to ionizing fluxes from non-LTE atmospheres including stellar winds and line blanketing. However, we also point out that tests of stellar atmosphere models from observations of HII regions are hampered by a lack of strong constraints on the ionization parameter.

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arXiv1996

Combined stellar structure and atmosphere models for massive stars. III. Spectral evolution and revised ionising fluxes of O3-B0 stars

We provide an extensive set of theoretical spectral energy distributions of massive stars derived from our "combined stellar structure and atmosphere models". The calculations covering the entire main sequence evolution for initial masses M_i=20 - 120 Msun (O3-B0 stars of all luminosity classes) are used for a systematic study of the ionizing fluxes of O and early B stars. We demonstrate the importance of accounting simultaneously for non-LTE effects, line blanketing and stellar winds. The main results from our spectra are the following: (1) The flux in the HeII continuum is increased by 2 to 3 (3 to 6) orders of magnitudes compared to predictions from plane parallel non-LTE (LTE) model atmospheres. (2) The flux in the HeI continuum is increased due to non-LTE effects. However, we find that it is also influenced by wind effects. The combined effect of a mass outflow and line blanketing leads to a flatter energy distribution in the HeI continuum, which confirms the results of Sellmaier et al. (1996) for a wider range of stellar parameters. (3) The flux in the Lyman continuum is also modified due to line blanketing and the presence of a stellar winds, although to a lesser degree than the spectrum at higher energies. We derive revised ionizing fluxes for O3 to B0 stars based on the recent Teff and gravity calibrations of Vacca et al. (1996). The total number of Lyman continuum photons is found to be slightly lower than previous derivations. Due to the increased flux in the HeI continuum the hardness ratio of the HeI to H continuum is increased by 1.6 to 2.5 depending on spectral type and luminosity class. A critical discussion of current model assumptions shows that for stars of types later than B0, having weak stellar winds, reliable predictions of ionizing fluxes are not yet possible.

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arXiv1996

WR Populations in Starbursts: WN and WC Subtypes and the Role of Binaries

We present the first results of a new set of population synthesis models, which utilize the latest stellar evolutionary tracks, recent non-LTE atmosphere models which include stellar winds, and observed line strengths in WR spectra to predict the relative strengths of various WN and WC/WO emission features in the spectra of starburst galaxies. Our results will be used to derive accurate numbers of WN and WC stars in starburst galaxies. We also analyze the frequency and the WN and WC content of WR-rich galaxies in low metallicity samples; the theoretical predictions are found to be in good agreement with the observed frequencies. We also discuss the possible role of massive close binaries in starburst regions. If the starburst regions are formed in relatively instantaneous bursts we argue that, given their young age as derived from emission lines equivalent widths, (1) in the majority of the observed WR galaxies massive close binaries have not contributed significantly to the WR population, and (2) nebular HeII 4686 emission is very unlikely due to massive X-ray binaries.

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arXiv1996

About the initial mass function and HeII emission in young starbursts

We demonstrate that it is crucial to account for the evolution of the starburst population in order to derive reliable numbers of O stars from integrated spectra for burst ages t > 2 - 3 Myr. In these cases the method of Vacca & Conti (1992) and Vacca (1994) systematically underestimates the number of O stars. Therefore the current WR/O number ratios in Wolf-Rayet (WR) galaxies are overestimated. This questions recent claims about flat IMF slopes (alpha ~ 1-2) in these objects. If the evolution of the burst is properly treated we find that the observations are indeed compatible with a Salpeter IMF, in agreement with earlier studies. Including recent predictions from non-LTE, line blanketed model atmospheres which account for stellar winds, we synthesize the nebular and WR HeII 4686 emission in young starbursts. For metallicities 1/5 <= Z/Z_sun <= 1 we predict a strong nebular HeII emission due to a significant fraction of WC stars in early WR phases of the burst. For other metallicities broad WR emission will always dominate the HeII emission. Our predictions of the nebular HeII intensity agree well with the observations in WR galaxies and an important fraction of the giant HII regions where nebular HeII is detected. We propose further observational tests of our result.

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