arXiv subjects

Daniel Schaerer

Publications and source records attributed to Daniel Schaerer.

At least 199 records · Page 11Linked to original sources

arXiv2000

Database of Geneva stellar evolution tracks and isochrones for UBVRIJHKLL'M, HST-WFPC2, Geneva and Washington photometric systems

We have used an updated version of the empirically and semi-empirically calibrated BaSeL library of synthetic stellar spectra of Lejeune et al. (1997, 1998) and Westera et al. (1999) to calculate synthetic photometry in the UBVRIJHKLL'M, HST-WFPC2, Geneva, and Washington systems for the entire set of non-rotating Geneva stellar evolution models covering masses from 0.4-0.8 to 120-150 Msun and metallicities Z=0.0004 (1/50 Zsun) to 0.1 (5 Zsun). The results are provided in a database which includes all individual stellar tracks and the corresponding isochrones covering ages from 10^3 yr to 16--20 Gyr in time steps of Delta(log t)= 0.05 dex. The database also includes a new grid of stellar tracks of very metal-poor stars (Z=0.0004) from 0.8 - 150 Msun calculated with the Geneva stellar evolution code. The full database will be available in electronic form at the CDS (http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/(vol)/(page)) and at http://webast.ast.obs-mip.fr/stellar/.

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arXiv2000

On ionisation effects and abundance ratios in damped Lyman-alpha systems

The similarity between observed velocity structures of Al III and singly ionised species in damped Lyman-alpha systems (DLAs) suggests the presence of ionised gas in the regions where most metal absorption lines are formed. To explore the possible implications of ionisation effects we construct a simplified two-region model for DLAs consisting of an ionisation bounded region with an internal radiation field and a neutral region with a lower metal content. Within this framework we find that ionisation effects are important. If taken into account, the element abundance ratios in DLAs are quite consistent with those observed in Milky Way stars and in metal-poor H II regions in blue compact dwarf galaxies. In particular we cannot exclude the same primary N origin in both DLAs and metal-poor galaxies. From our models no dust depletion of heavy elements needs to be invoked; little depletion is however not excluded.

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arXiv2000

Massive star populations and the IMF in metal-rich starbursts

We present new spectroscopic observations of Mkn 309, a starburst galaxy with one of the largest WR populations known. A highly super solar metallicity is derived. Using additional objects we analyse a sample of five metal-rich WR galaxies with the main goal of constraining the basic properties of the massive star populations (IMF slope, M_up) and the star formation history (age, burst duration) of these objects by quantitative comparisons with evolutionary synthesis models. The following main results are obtained: 1) The observations are well explained by extended bursts of star formation or a superposition of several bursts. Ages and burst durations are estimated. This naturally explains both the observed WR populations (including WN and WC stars) and the presence of red supergiants. 2) The fitted SEDs indicate that the stellar light suffers from a smaller extinction than that of the gas, confirming independent earlier findings. 3) All the considered observational constraints are compatible with a Salpeter IMF extending to masses >~ 40 Msun. Adopting a conservative approach we derive a LOWER LIMIT of Mup >~ 30 Msun for the Salpeter IMF. From more realistic assumptions on the metallicity and SF history we favour a lower limit Mup >~ 30-40 Msun, which is also in agreement with Hbeta equivalent width measurements of metal-rich HII regions in spiral galaxies indicating an upper mass cut-off of at least ~ 35 - 50 Msun. Steep IMF slopes (alpha >~ 3.3) are very unlikely. (abridged/modified abstract)

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arXiv2000

Massive Star Forming Regions: from diagnostic tools to derived properties

The current state-of-the-art of multi-wavelength diagnostic tools (evolutionary synthesis, photoionisation models) for massive star forming regions (HII regions, starbursts, etc.) and some of their input physics (especially model atmospheres) is reviewed. Analysis of stellar populations based on integrated spectra from both stellar features and nebular emission lines from the UV to IR are summarised. We stress the importance of "template" studies at various scales (from individual stars to well studied galaxies) and various wavelengths, to understand the processes operating in massive star forming regions and to reliably derive their properties.

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arXiv2000

Super star clusters as probes of massive star evolution and the IMF at extreme metallicities

Young super star clusters and young compact massive star forming regions can provide useful information on their burst properties (age, burst duration, SFR), the upper end of the IMF and yield new constraints on the evolution of massive stars. Through the study of their stellar populations we can in particular extend our knowledge on massive stars to extreme metallicities unavailable for such stars in the Local Group. Here we summarise the main results from recent studies on two metallicity extremes: Wolf-Rayet and O star populations in very metal-poor BCD, and metal-rich compact nuclear SF regions.

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arXiv1999

Gamma-ray line emission from OB associations and young open clusters

OB associations and young open clusters constitute the most prolific nucleosynthesis sites in our Galaxy. The combined activity of stellar winds and core-collapse supernovae ejects significant amounts of freshly synthesised nuclei into the interstellar medium. Radioactive isotopes, such as 26Al or 60Fe, that have been co-produced in such events may eventually be observed by gamma-ray instruments through their characteristic decay-line signatures. In order to study such signatures and to derive constraints on the involved nucleosynthesis processes, we developed a multi-wavelength evolutionary synthesis model for massive star associations. We apply this model to associations and clusters in the well-studied Cygnus region for which we re-determined the stellar census based on photometric and spectroscopic data. In particular we study the relation between 1.809 MeV gamma-ray line emission and ionising flux, since the latter has turned out to provide an excellent tracer of the global galactic 1.809 MeV emission. We compare our model to COMPTEL 1.8 MeV gamma-ray line observations from which we derive limits on the relative contributions from massive stars and core-collapse supernovae to the actual 26Al content in this region. Based on our model we make predictions about the expected 26Al and 60Fe line signatures in the Cygnus region. These predictions make the Cygnus region a prime target for the future INTEGRAL mission.

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arXiv1999

What heats the bright HII regions in I Zw 18?

We have used the radiation field from a starburst population synthesis model appropriate for the brightest HII region of IZw 18 to perform a photoionization model analysis of this object. We have investigated whether, with the observed nebular density distribution as revealed by the HST images and a total stellar radiation compatible with the observed UV flux, the constraints represented by the observed ionization structure, the HeII/Hbeta ratio, the Halpha flux and the electron temperature indicated by the [OIII] 4363/5007 ratio can be reproduced. We have found that, even taking into account strong deviations from the adopted spectral energy distribution of the ionizing radiation and the effect of additional X-rays, the photoionization models yield too low a 4363/5007 ratio by about 30 %. This discrepancy is significant and poses an interesting problem, which cannot be solved by expected inaccuracies in the atomic data. The missing energy may be of the same order of magnitude as the one provided by the stellar photons or lower, depending on the way it acts on the 4363 line. Elemental abundance determinations in I Zw 18 are affected by this problem. Before the nature of the missing heating source and its interactions with the nebular gas are better understood it is, however, not possible to estimate the typical uncertainties by which standard empirical methods are affected. Other topics discussed include: nebular HeII emission, photon leakage, [OI] 6300 emission, reddening, ionization correction factors. (abridged abstract)

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arXiv1999

Wolf-Rayet galaxies as probes of young stellar systems

Wolf-Rayet (WR) galaxies provide detailed information on massive star populations in starbursts and thereby represent ideal objects to determine accurate ages for young systems, to measure the burst duration, and to probe the upper end of the IMF. WR galaxies play also a particular role in a variety of studies on star formation in Seyfert2/LINERS, dust production and local chemical enrichment, temperature fluctuations in HII regions, gaz outflows and X-ray emission etc. Different age indicators for young starbursts and WR galaxies are discussed. We summarise recent work on the burst properties of WR galaxies, massive star populations at different metallicities, and the use of WR galaxies as benchmarks for multi-wavelength models of starbursts and photoionisation models including IR fine structure lines.

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arXiv1999

Determining Star Formations Rates: Methods and Uncertainties

Methods for the determination of star formation rates (SFR) from integrated populations are reviewed. We discuss the assumptions and underlying uncertainties (e.g. IMF slope, M_up, metallicity, SF history etc.) used in the calibrations of UV, FIR, Halpha, and [OII] indicators. The ``universality'' of empirically calibrated indicators such as [OII] is examined. We also present preliminary results from a theoretical study with the aim to understand the systematics of forbidden line indicators and to provide new SFR indicators including optical to far-IR lines.

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arXiv1999

On the origin of [OIV] emission in Wolf-Rayet galaxies

We propose that the emission of the high excitation [OIV] line observed with ISO in NGC 5253 and II Zw 40 is due to the presence of hot Wolf-Rayet (WR) stars in these objects. We construct a consistent evolutionary synthesis and photoionization model which successfully reproduces the constraints on their massive star content and the relevant optical and IR emission lines including [OIV] 25.9 micron. Our explanation for the origin of [OIV] is supported empirically by: 1) the simultaneous presence of nebular HeII and [OIV] in these objects, and 2) the close relation between nebular HeII and WR stars in extragalactic HII regions. Photoionization by hot WR stars is mainly expected to be of importance in young low metallicity galaxies. Alternate mechanisms are likely at the origin of [OIV] emission in other objects.

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arXiv1999

Starburst99: Synthesis Models for Galaxies with Active Star Formation

Starburst99 is a comprehensive set of model predictions for spectrophotometric and related properties of galaxies with active star formation. The models are an improved and extended version of the data set previously published by Leitherer & Heckman (1995). We have upgraded our code by implementing the latest set of stellar evolution models of the Geneva group and the model atmosphere grid compiled by Lejeune et al. (1997). Several predictions which were not included in the previous publication are shown here for the first time. The models are presented in a homogeneous way for five metallicities between Z = 0.040 and 0.001 and three choices of the initial mass function. The age coverage is 10^6 to 10^9 yr. We also show the spectral energy distributions which are used to compute colors and other quantities. The full data set is available for retrieval at http://www.stsci.edu/science/starburst99/. This website allows users to run specific models with non-standard parameters as well. We also make the source code available to the community.

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arXiv1999

New catalogue of Wolf-Rayet galaxies and high-excitation extra-galactic HII regions

We present a new compilation of Wolf-Rayet (WR) galaxies and extra-galactic HII regions showing BROAD HeII emission drawn from the literature. Relevant information on the presence of other broad emission lines (NIII 4640, CIV 5808 and others) from WR stars of WN and WC subtypes, and other existing broad nebular lines is provided. In total we include 139 known WR galaxies. Among these, 57 objects show both broad HeII and CIV features. In addition to the broad (stellar) HeII emission, a NEBULAR HeII component is well established (suspected) in 44 (54) objects. We find 19 extra-galatic HII regions without WR detections showing nebular HeII emission. The present sample can be used for a variety of studies on massive stars, interactions of massive stars with the ISM, stellar populations, starburst galaxies etc. The data is accessible electronically and will be updated periodicaly.

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arXiv1998

Populations of WC and WN stars in WR galaxies: new constraints on starburst parameters

Detection and analysis of Wolf-Rayet (WR) star populations in young starbursts is a powerful tool to reveal the properties of the star-forming events. We present new high S/N optical spectra of a sample of well-known WR galaxies, in which we detected for the first time the spectral signature of WC stars. The detailed comparison of WN and WC star populations derived in these galaxies, with the most recent model predictions, allows to put tight constraints on starburst parameters, such as the age and duration of the burts, and the IMF.

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arXiv1998

Models for WR and O Star Populations in Starbursts: New Developments and Applications to Large Samples of WR Galaxies

We summarise recent developments on synthesis models for massive star populations with a particular emphasis on Wolf-Rayet (WR) stars. Quantitative analysis of the stellar content of WR galaxies are reviewed. Comparing observations of WR galaxies from various samples with synthesis models we derive constraints on their burst properties. The observations indicate very short burst periods and are generally compatible with a Salpeter IMF and a large upper mass cut-off. The use of the Hbeta equivalent width as an age indicator works well for WR galaxies. We briefly summarise comparisons of stellar populations in super star clusters which also provide useful contraints on evolutionary models for massive stars e.g. at very low metallicities inaccessible in the Local Group. In particular the observed WN and WC populations favour the high mass loss models of Meynet et al. (1994). Finally we review recent work on the origin of nebular HeII emission. From the new catalogue of WR galaxies and high excitation HII regions of Schaerer et al. (1998) we find a close relation between the presence of nebular HeII and WR stars. The analysis of individual WR galaxies including I Zw 18 supports the suggestion of Schaerer (1996) that hot WR stars are responsible for the hard ionizing flux.

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arXiv1998

New light on the origin of nebular HeII emission in young starbursts

We review the observations of nebular HeII 4686 emission in Local Group objects and extra-galactic HII regions. Different ionization mechanisms proposed to explain the unusually high excitation are summarized. The models of Schaerer (1996) and Schaerer & Vacca (1998) predict nebular HeII emission due to hot WN and WC/WO stars, in good agreement with observations of Wolf-Rayet (WR) galaxies showing WR signatures and/or nebular HeII. Comparisons of synthesis models with observations suggest that nebular HeII emission may be related to WR stars in all cases. The case of I Zw 18 is discussed. It is shown that the observed WR lines and nebular HeII can be reproduced with an instantaneous burst model and a Salpeter IMF. This supports the suggestion that WR stars are responsible for nebular HeII emission.

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arXiv1998

The impact of new ionizing fluxes on ISO observations of HII regions and starbursts

Extensive grids of photoionization models have been calculated for single star HII regions and evolving starbursts. We illustrate the predictions for IR fine structure lines which are used to analyse the stellar content, and derive properties such as the age and IMF. The impact of recent ionizing fluxes on the IR lines are shown. First comparisons of our starburst models with IR-diagnostics and the ISO observations of Genzel et al. (1998) are also presented.

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arXiv1998

Populations of WC and WN stars in Wolf-Rayet galaxies

We report the detection of WC stars in 5 Wolf-Rayet (WR) galaxies: He 2-10, NGC 3049, NGC 3125, NGC 5253 and Tol 89. The faint broad CIV5808 line requires sufficiently high S/N to be detected explaining the non-detection of this WC feature in previous observations. From the WR emission lines we conclude that all regions (except NGC 3049) contain a mixed population of WNL, and early WC stars. A spatial offset between the nebular emission and stellar light is observed in 2 objects. Due to age differences the nebular structures around the WR regions are possibly smaller than those mainly energized by SNe. The spatial distribution of WR stars follows the stellar continuum. We derive the absolute number of WR stars of different subtypes. The WC/WN ratios have typical values between 0.2-0.4, and show no clear trend with metallicity (Z). For low-Z objects, these values are larger than the observed WC/WN ratios in Local Group objects, but compatible with expectations for short bursts of if stellar evolution models with high mass loss are used. The ages of the starburst regions are 3-6 Myr; the burst duration must not exceed 2-4 Myr to account for the high population of WR stars, even if emission line stars similar to those in R136 and NGC 3603 are common in starbursts. Within the uncertainties most of the observed quantities are reproduced by models with a Salpeter IMF. Although some WR lines are stronger than predicted, no clear case requiring a significantly flatter IMF is found. IMF slopes much steeper than Salpeter may, however, not be compatible with our data. (Abridged abstract)

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arXiv1998

Evolutionary synthesis modeling of red supergiant features in the near-infrared

We present evolutionary synthesis models applied to near--infrared spectral features observed in the spectra of young Magellanic Cloud clusters and starburst galaxies. The temporal evolution of the first and second overtones of CO at 2.29 micron (2-0 bandhead) and 1.62 micron (6-3 bandhead) and of the (U-B), (B-V) and (J-K) colors are investigated. We find that the current evolutionary tracks of massive stars with sub-solar chemical composition in the red supergiant phase are not reliable for any synthesis of the temporal evolution of infrared stellar features. The high sensitivity of the selected infrared features to the atmospheric parameters of cool stars allows us to place constraints on the temperature and the fraction of time spent in the red part of the Hertzsprung-Russell diagram by massive stars during their core-helium burning phase. We derive a set of empirically calibrated spectrophotometric models by adjusting the red supergiant parameters such that the properties of the observed templates are reproduced.

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