Daniel Schaerer

Bio: Daniel Schaerer is an academic researcher from University of Geneva. The author has contributed to research in topics: Stars & Metallicity. The author has an hindex of 16, co-authored 32 publications receiving 6093 citations. Previous affiliations of Daniel Schaerer include Space Telescope Science Institute & Centre national de la recherche scientifique.

Starburst99: Synthesis Models for Galaxies with Active Star Formation

Abstract: 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. 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. 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 106—109 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 a Web site, which allows users to run specific models with nonstandard parameters as well. We also make the source code available to the community.

STELIB: A library of stellar spectra at R 2000 ?;??

Abstract: We present STELIB ? , a new spectroscopic stellar library, available at http://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneous library of 249 stellar spectra in the visible range (3200 to 9500 A), with an intermediate spectral resolution (<3 A) and sampling (1 A). This library includes stars of various spectral types and luminosity classes, spanning a relatively wide range in metallicity. The spectral resolution, wavelength and spectral type coverage of this library represents a substantial improvement over previous libraries used in population synthesis models. The overall absolute photo- metric uncertainty is 3%.

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

Abstract: Using the latest stellar evolution models, theoretical stellar spectra, and a compilation of observed emission line strengths from Wolf-Rayet (W-R) stars, we construct evolutionary synthesis models for young starbursts. We explicitly distinguish between the various W-R subtypes (WN, WC, WO), whose relative frequency is a strong function of metallicity, and we treat O and Of stars separately. We calculate the numbers of O and W-R stars produced during a starburst and provide detailed predictions of UV and optical emission line strengths for both the W-R 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 W-R-rich starbursts. Our models predict that nebular He II λ4686 emission from a low-metallicity starburst should be associated with the presence of WC/WO stars and/or hot WN stars evolving to become WC/WO stars. In addition, W-R stars contribute to broad components beneath the nebular Balmer lines; the broad W-R component may constitute several percent of the total flux in the line. We review the various techniques used to derive the W-R and O star content from integrated spectra, assess their accuracy, and propose two new formulae to estimate the W-R/O number ratio from UV or optical spectra. We also explore the implications of the formation of W-R stars through mass transfer in close binary systems in instantaneous bursts. While the formation of W-R stars through Roche lobe overflow prolongs the W-R-dominated phase, there are clear observational signatures that allow us to distinguish the phases in which W-R stars are formed predominantly through single or binary star channels. In particular at low metallicities, when massive close binaries contribute significantly to the formation of W-R stars, the binary-dominated phase is expected to occur at ages corresponding to relatively low Hβ equivalent widths. 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 "W-R galaxies") from its integrated spectrum and provide a means of deriving the burst properties (e.g., duration and age) and the parameters of the initial mass function of young starbursts. The model predictions should provide the most reliable determinations to date. They can also be used to test current theories of massive star evolution and atmospheres and to investigate the variation in stellar properties with metallicity.

Starburst99: Synthesis Models for Galaxies with Active Star Formation

Abstract: 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 this http URL. This website allows users to run specific models with non-standard parameters as well. We also make the source code available to the community.

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

Abstract: We present a new compilation of Wolf-Rayet (WR) galaxies and extra-galactic Hii regions showing broad He ii 4686 emission drawn from the literature. Relevant information on the presence of other broad emis- sion lines (N iii 4640, C iv 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 He ii 4686 and C iv 5808 features. In addition to the broad (stellar) He ii 4686 emission, a nebular He ii component is well es- tablished (suspected) in 44 (54) objects. We nd 19 extra-galatic Hii regions without WR detections showing nebular He ii 4686 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 periodically.

Stellar population synthesis at the resolution of 2003

Abstract: We present a new model for computing the spectral evolution of stellar populations at ages between 100,000 yr and 20 Gyr at a resolution of 3 A across the whole wavelength range from 3200 to 9500 A for a wide range of metallicities. These predictions are based on a newly available library of observed stellar spectra. We also compute the spectral evolution across a larger wavelength range, from 91 A to 160 micron, at lower resolution. The model incorporates recent progress in stellar evolution theory and an observationally motivated prescription for thermally-pulsing stars on the asymptotic giant branch. The latter is supported by observations of surface brightness fluctuations in nearby stellar populations. We show that this model reproduces well the observed optical and near-infrared colour-magnitude diagrams of Galactic star clusters of various ages and metallicities. Stochastic fluctuations in the numbers of stars in different evolutionary phases can account for the full range of observed integrated colours of star clusters in the Magellanic Clouds. The model reproduces in detail typical galaxy spectra from the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS). We exemplify how this type of spectral fit can constrain physical parameters such as the star formation history, metallicity and dust content of galaxies. Our model is the first to enable accurate studies of absorption-line strengths in galaxies containing stars over the full range of ages. Using the highest-quality spectra of the SDSS EDR, we show that this model can reproduce simultaneously the observed strengths of those Lick indices that do not depend strongly on element abundance ratios [abridged].

Starburst99: Synthesis Models for Galaxies with Active Star Formation

Abstract: 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. 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. 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 106—109 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 a Web site, which allows users to run specific models with nonstandard parameters as well. We also make the source code available to the community.

The Origin of the Mass-Metallicity Relation: Insights from 53,000 Star-forming Galaxies in the Sloan Digital Sky Survey

Abstract: We utilize Sloan Digital Sky Survey imaging and spectroscopy of ~53,000 star-forming galaxies at z ~ 0.1 to study the relation between stellar mass and gas-phase metallicity. We derive gas-phase oxygen abundances and stellar masses using new techniques that make use of the latest stellar evolutionary synthesis and photoionization models. We find a tight (?0.1 dex) correlation between stellar mass and metallicity spanning over 3 orders of magnitude in stellar mass and a factor of 10 in metallicity. The relation is relatively steep from 108.5 to 1010.5 M? h, in good accord with known trends between luminosity and metallicity, but flattens above 1010.5 M?. We use indirect estimates of the gas mass based on the H? luminosity to compare our data to predictions from simple closed box chemical evolution models. We show that metal loss is strongly anticorrelated with baryonic mass, with low-mass dwarf galaxies being 5 times more metal depleted than L* galaxies at z ~ 0.1. Evidence for metal depletion is not confined to dwarf galaxies but is found in galaxies with masses as high as 1010 M?. We interpret this as strong evidence of both the ubiquity of galactic winds and their effectiveness in removing metals from galaxy potential wells.

Cosmic Star-Formation History

Abstract: Over the past two decades, an avalanche of data from multiwavelength imaging and spectroscopic surveys has revolutionized our view of galaxy formation and evolution. Here we review the range of complementary techniques and theoretical tools that allow astronomers to map the cosmic history of star formation, heavy element production, and reionization of the Universe from the cosmic "dark ages" to the present epoch. A consistent picture is emerging, whereby the star-formation rate density peaked approximately 3.5 Gyr after the Big Bang, at z~1.9, and declined exponentially at later times, with an e-folding timescale of 3.9 Gyr. Half of the stellar mass observed today was formed before a redshift z = 1.3. About 25% formed before the peak of the cosmic star-formation rate density, and another 25% formed after z = 0.7. Less than ~1% of today's stars formed during the epoch of reionization. Under the assumption of a universal initial mass function, the global stellar mass density inferred at any epoch matches reasonably well the time integral of all the preceding star-formation activity. The comoving rates of star formation and central black hole accretion follow a similar rise and fall, offering evidence for co-evolution of black holes and their host galaxies. The rise of the mean metallicity of the Universe to about 0.001 solar by z = 6, one Gyr after the Big Bang, appears to have been accompanied by the production of fewer than ten hydrogen Lyman-continuum photons per baryon, a rather tight budget for cosmological reionization.

Star Formation in the Milky Way and Nearby Galaxies

Abstract: We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star-formation rates are discussed, and updated prescriptions for calculating star-formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds.