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.Comment: 32 pages, LaTeX. All the Figures and the summary Table are located at http://www.stsci.edu/science/starburst99/, ApJ accepte

Abstract. We present new calibrations of stellar parameters of O stars at solar metallicity taking non-LTE, wind, and lineblanketing effects into account. Gravities and absolute visual magnitudes are derived from results of recent spectroscopic analyses. Two types of effective temperature scales are derived: one from a compilation based on recent spectroscopic studies of a sample of massive stars -the "observational scale" -and the other from direct interpolations on a grid of non-LTE spherically extended line-blanketed models computed with the code CMFGEN (Hillier & Miller 1998) -the "theoretical scale". These T eff scales are then further used together with the grid of models to calibrate other parameters (bolometric correction, luminosity, radius, spectroscopic mass and ionising fluxes) as a function of spectral type and luminosity class. Compared to the earlier calibrations of Vacca et al. (1996) the main results are: -The effective temperature scales of dwarfs, giants and supergiants are cooler by 2000 to 8000 K, the theoretical scale being slightly cooler than the observational one. The reduction is the largest for the earliest spectral types and for supergiants. -Bolometric corrections as a function of T eff are reduced by 0.1 mag due to line blanketing which redistributes part of the UV flux in the optical range. For a given spectral type the reduction of BC is larger for early types and for supergiants. Typically BCs derived using the theoretical T eff scale are 0.40 to 0.60 mag lower than that of Vacca et al. (1996), whereas the differences using the observational T eff scale are somewhat smaller. -Luminosities are reduced by 0.20 to 0.35 dex for dwarfs, by ∼0.25 for all giants and by 0.25 to 0.35 dex for supergiants.The reduction is essentially the same for both T eff scales. It is independent of spectral type for giants and supergiants and is slightly larger for late type than for early type dwarfs. -Lyman continuum fluxes are reduced. Our theoretical values for the hydrogen ionising photon fluxes for dwarfs are 0.20 to 0.80 dex lower than those of Vacca et al. (1996), the difference being larger at late spectral types. For giants the reduction is of 0.25 to 0.55 dex, while for supergiants it is of 0.30 to 0.55 dex. Using the observational T eff scale leads to smaller reductions at late spectral types. The present results should represent a significant improvement over previous calibrations, given the detailed treatment of non-LTE line-blanketing in the expanding atmospheres of massive stars.

Abstract. We present realistic models for massive Population III stars and stellar populations based on non-LTE model atmospheres, recent stellar evolution tracks and up-to-date evolutionary synthesis models, with the aim to study their spectral properties, including their dependence on age, star formation history, and IMF. A comparison of plane parallel non-LTE model atmospheres and comoving frame calculations shows that even in the presence of some putative weak mass loss, the ionising spectra of metal-free populations differ little or negligibly from those obtained using plane parallel non-LTE models. As already discussed by Tumlinson & Shull (2000), the main salient property of Pop III stars is their increased ionising flux, especially in the He + continuum (>54 eV). The main result obtained for individual Pop III stars is the following: due to their redward evolution off the zero age main sequence (ZAMS) the spectral hardness measured by the He + /H ionising flux is decreased by a factor ∼2 when averaged over their lifetime. If such stars would suffer strong mass loss, their spectral appearance could, however, remain similar to that of their ZAMS position. The main results regarding integrated stellar populations are: -for young bursts and the case of a constant SFR, nebular continuous emission -neglected in previous studiesdominates the spectrum redward of Lyman-α if the escape fraction of ionising photons out of the considered region is small or negligible. In consequence predicted emission line equivalent widths are considerably smaller than found in earlier studies, whereas the detection of the continuum is eased. Nebular line and continuous emission strongly affect the broad band photometric properties of Pop III objects; -due to the redward stellar evolution and short lifetimes of the most massive stars, the hardness of the ionising spectrum decreases rapidly, leading to the disappearance of the characteristic He ii recombination lines after ∼3 Myr in instantaneous bursts; -He ii λ1640, Hα (and other) line luminosities usable as indicators of the star formation rate are given for the case of a constant SFR. For obvious reasons such indicators depend strongly on the IMF; -due to an increased photon production and reduced metal yields, the relative efficiency of ionising photon energy to heavy element rest mass production, η, of metal-poor and metal-free populations is increased by factors of ∼4 to 18 with respect to solar metallicity and for "standard" IMFs; -the lowest values of η ∼ 1.6-2.2% are obtained for IMFs exclusively populated with high mass stars (M low > ∼ 50 M ). If correct, the yields dominated by pair creation SNae then predict large overabundances of O/C and Si/C compared to solar abundance ratios. Detailed results are given in tabular form and as fit formulae for easy implementation in other calculations. The predicted spectra will be used to study the detectability of Pop III galaxies and to derive optimal search strategies for such objects.

Aims. The development of a general code for 3D Lyα radiation transfer in galaxies to understand the diversity of Lyα line profiles observed in star-forming galaxies and related objects. Methods. Using a Monte Carlo technique, we developed a 3D Lyα radiation transfer code that allows for prescribed arbitrary hydrogen density, ionisation, temperature structures, dust distributions, arbitrary velocity fields, and UV photon sources. As a first test and application we examined the Lyα line profiles predicted for several simple geometrical configurations and their dependence on the main input parameters. Results. Overall, we find line profiles reaching from doubly peaked symmetric emission to symmetric Voigt (absorption) in static configurations with increasing dust content, and asymmetric red-(blue-) shifted emission lines with a blue (red) counterpart ranging from absorption to emission (with increasing line/continuum strength) in expanding (infalling) media. In particular we find the following results to be interesting for the interpretation of Lyα profiles from galaxies. 1) Standard Lyα absorption line fitting of global spectra of galaxies may lead to an underestimation of the true hydrogen column density in certain geometrical conditions; 2) Normal (inverted) P-Cygni-like Lyα profiles can be obtained in expanding (infalling) media from objects without any intrinsic Lyα emission, as a natural consequence of radiation transfer effects; 3) The formation and the detailed shape of Lyα profiles resulting from expanding shells has been thoroughly revised. In particular we find that, for sufficiently large column densities (N H > ∼ 10 20 cm −2 ), the position of the main Lyα emission peak is quite generally redshifted by approximately twice the expansion velocity. This is in excellent agreement with the observations of z ∼ 3 LBGs, which show that Lyα is redshifted by ∼2V exp , where V exp is the expansion velocity measured from the interstellar absorption lines blueshifted with respect to the stellar redshift. This finding also indicates that large-scale, fairly symmetric shell structures must be a good description of the outflows in LBGs.

Abstract. Using new sets of stellar evolution models at very low metallicities (Z = 10 −7 , 10 −5 ) and previously published grids we examine spectral properties of the ionising continua, the Lyman-break, and the Lyα and He  λ1640 recombination lines in starbursts. The metallicity dependence of these properties, especially the transition from primordial galaxies (Population III) to currently observed metallicities, is examined for various IMFs and star formation histories. For the average properties of starbursts, approximated by a model with constant star formation, the main findings are:-The Lyman continuum flux Q(H) increases with decreasing metallicity. For a universal Salpeter IMF from 1-100 M the enhancement reaches typically a factor of ∼3 between solar metallicity and Pop III objects. -While for metallicities Z > ∼ 1/50 Z the amplitude of the Lyman-break depends little on Z, a reduction by a factor ∼2 is found at lower metallicities, due to the strong increase of the average stellar temperature. -Using theoretical models and empirical constraints we discuss the expected evolution of the hardness of He + to H ionising photons, Q(He + )/Q(H), with metallicity and possible uncertainties. Over the metallicity range Z = 0 to ∼10 −4 the hardness decreases from log(Q(He + )/Q(H)) ∼ −1.4 . . . −2.3 by ∼1.5-2 or more orders of magnitude, depending strongly on the upper mass cut-off of the IMF. From empirical constraints we derive a hardness log(Q(He + )/Q(H)) ∼ -3.2 to -2.6 for metal-poor starbursts (1/25 < ∼ Z/Z < ∼ 1/4) and softer spectra for higher metallicities. We also provide a simple estimate of the possible impact of hot WR like stars on Q(He + )/Q(H) at very low metallicities (Z < ∼ 10 −4 ). -Calibrations for star formation rate determinations from various recombination lines at all metallicities and for various IMFs are derived.For young bursts the maximum Lyα equivalent width is shown to increase strongly with decreasing metallicity from W(Lyα) ∼ 250-350 Å at Z > ∼ 1/50 Z to 400-850 Å or higher at Z between 10 −5 and 0 (Pop III) for the same Salpeter IMF. However, for well known reasons, the Lyα emission predicted likely represents an upper limit. Non-negligible He  λ1640 emission due to stellar photoionisation appears to be limited to very small metallicities (log(Z/Z ) < ∼ −5.3) and Population III objects. The predictions, available on the Web through the CDS and at http://webast.ast.obs-mip.fr/sfr/, should be useful for a variety of studies regarding high redshift galaxies, cosmological reionisation, and others.