Some publications related to stellar population synthesis and starburst galaxies

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• Carbon star populations in systems with different metallicities: statistics in Local Group galaxies.
  M. Mouhcine, A. Lançon
  MNRAS 338, 572 (2003)
  We present evolutionary population synthesis models for the study of the cool and luminous intermediate age stellar populations in resolved galaxies with particular emphasis on carbon star populations. We study the effects of the star formation history, the age and the metallicity on the populations of intermediate mass stars.
  Chemically consistent models are used to study the effect of the star formation history on the relations between carbon star population properties and global parameters of the parent galaxy (age, metallicity). Our models are able to account, for the first time, for those correlations, as observed in the galaxies of the Local Group. For stellar populations older than about 1 Gyr, the properties of carbon star populations are linked to the current metallicity in a way that is quite independent of the star formation scenario. The number ratio of carbon stars to late-type M stars forms a metallicity sequence along which stellar populations with very different star formation histories are found.
  For the same populations, we find that both the mean bolometric luminosity of carbon stars and their normalized number to the luminosity of the parent galaxy are quite independent of metallicity over a large range in metallicity. This is in good agreement with the observational constraints.
• The modelling of intermediate-age stellar populations. II. Average spectra for upper AGB stars and their use.
  A. Lançon, M. Mouhcine
  A&A 393, 167 (2002)
  The upper Asymptotic Giant Branch (AGB) is populated with oxygen rich and carbon rich Long Period Variables (LPVs). n this paper, the library of individual spectra of Lançon & Wood (2000) is used to construct averages that can be incorporated conveniently in population synthesis work. The connection between such spectra and stellar evolution tracks is discussed.
  We further address: (i) the bolometric corrections and temperature scales needed to associate a spectrum with a given point on a theoretical stellar evolution track (or isochrone), (ii) the simplifying assumptions that will be implicitely made when using the average spectra, (iii) potential biases in the sample of Lançon & Wood and their effects, (iv) the small contribution of LPVs to the interstellar hydrogen emission lines in galaxies. It is emphasized that an a posteriori calibration of the effective temperature scale remains necessary, until consistent models for the evolution, the pulsation and the spectral appearance of LPVs become available.
• The modelling of intermediate-age stellar populations. I. Near-IR properties
  M. Mouhcine, A. Lançon
  A&A 393, 149 (2002)
  In this paper, we discuss how the integrated properties of intermediate-age single-burst populations, especially in the near-infrared, behave as a function of age and metallicity. Our models take into account all stellar evolutionary phases that affect the evolution of the integrated optical and near-infrared spectrum of such a population. Particular care was dedicated to the Asymptotic Giant Branch (AGB) stars, which can be dominant at near-infrared wavelengths. We take into account the relevant physical processes that control the evolution through the thermally pulsing AGB, namely (i) the mass-loss, (ii) the third dredge-up, and (iii) the envelope burning. We use the model to evaluate the AGB-termination luminosity, carbon star properties as function of initial metallicity and initial mass, and the contribution of these stars to the integrated light. In the isochrones presented in this paper the lifetime and the nature of the AGB stars (oxygen-rich or carbon-rich) are established as consequences of the interplay between the physical processes that control the AGB star evolution.
  We confirm that, in contrast with the classical point of view, no sharp optical/near-infrared colour jump occurs when AGB stars start to dominate the stellar population. The envelope burning process that affects massive AGB stars, making them overluminous with respect to early standard core mass-luminosity relations, causes a smoothing of the colour evolution for stellar systems dominated by those stars. We reanalyze the observational strategy proposed by Lançon 1999 (below) to identify intermediate-age stellar populations in post-starburst spectra using our new model sets.
• First spectroscopic evidence for carbon stars outside the local group: Properties of a massive star cluster in NGC 7252.
  M. Mouhcine, A. Lançon, C. Leitherer, D. Silva, M.A.T. Groenewegen
  A&A 393, 101 (2002)
  We present near-infrared [1-2.3 micron] spectroscopy of the massive intermediate age star cluster W3 in the merger remnant galaxy NGC 7252, obtained with SOFI on the NTT. This cluster has an age when the integrated near-infrared properties of a stellar population are dominated by the cool and luminous Asymptotic Giant Branch (AGB). We compare the data with predictions from new evolutionary synthesis models (above). The comparison of the data to the models clearly shows that carbon stars are present: for the first time, carbon rich star spectral features are thus detected directly outside the Local Group galaxies. [...]
• An inverse method to recover the SFR and reddening properties from spectra of galaxies.
  J.-L. Vergely, A. Lançon, M. Mouhcine
  A&A 394, 807 (2002)
  We develop a non-parametric inverse method to investigate the star formation rate, the metallicity evolution and the reddening properties of galaxies based on their spectral energy distributions. When low resolution SEDs are available in the UV, optical and near-IR together, we conclude that it is possible to constrain the star formation rate and the effective dust optical depth simultaneously with a S/N ratio fo 25. With excellent S/N ratios, it would also be possible to constrain the age-metallicity relation.
  We apply the method to the nuclear starburst of the interacting galaxy NGC 7714. We confirm the finding of Lançon et al. 2001 (below): the SED suggests that enhanced star formation started several 100 Myr ago, and increased the total stellar mass in the central regions by 10-20 percent. Dynamical models suggest that closest approach of NGC 7715 occured 100-200 Myr ago (Struck & Smith 2003). It requires unpleasant fine-tuning to make the two timescales agree, so the latest interaction with NGC 7715 might not have been the most important event in the life of NGC 7714.

• Multiwavelength Study of the Starburst Galaxy NGC 7714. II: The Balance between Young, Intermediate Age and Old Stars.
  A. Lançon, J.D. Goldader, C. Leitherer, R.M. González Delgado
  ApJ 552, 150 (2001)
  We combine existing multiwavelength data (incl. an HST/GHRS UV spectrum, an optical spectrum, far-IR, Xray and radio fluxes) with new HST/WFPC2 images, near-IR photometry and K band spectroscopy. We use these data to constrain the young, the intermediate age and the old stellar populations in the central 330 pc of the starburst galaxy NGC 7714. [...] We find that the young burst responsible for the UV light is only a small part of an extended episode of enhanced star formation (SF) [...]. The mass of young and intermediate age stars thus formed equals at least 10% of the mass locked in pre-existing stars of the underlying galaxy nucleus [...]. The spectrophotometric SF timescale is long compared to the ~110 Myr elapsed since closest contact with NGC 7715. The trigger of the starburst remains elusive. NGC 7714 owes its brightness in the UV to a few low extinction lines of sight towards young stars. [...] The different extinction estimates obtained from different indicators result naturally from the coexistence of populations with various ages and obscurations. The near-IR continuum image looks smoothest, as a consequence of lower sensitivity to extinction and of a larger contribution of old stars. We compare the nuclear properties of NGC 7714 with results from studies in larger apertures. We emphasize that the global properties of starburst galaxies are the result of the averaging over many lines of sight with diverse properties in terms of obscuration and stellar ages.

• Can we Age-date Starbursts?
  A. Lançon
  in Starburst Galaxies Near and Far, Ringberg Meeting, eds. L. Tacconi and D. Lutz (in press).
  Age dating starbursts is an exercise with many caveats. We attempt to summarise a discussion session that was lead along a rather optimistic guideline: the aim was to highlight that current age estimates, despite undeniable uncertainties, do provide constraints on the physics of starbursts. In many cases, better starburst theories will be needed before the improvement of empirical timelines becomes crucial.

• Stochastic Fluctuations in the Spectrophotometric Properties of Star Clusters.
  A. Lançon, M. Mouhcine
  in Massive Stellar Clusters, ASP Conf. Ser. 211
  Integrated spectrophotometric properties of stellar systems are intrinsically dispersed due to the stochastic nature of the small numbers of bright stars they contain. Among clusters, only the most massive ones can be used individually for comparison with the mean properties predicted by population synthesis calculations. The appropriate minimal masses depend, among others, on the waveband or colour index studied and on age. Selected indices (near-IR CO and H2O, EW(Halpha)) are used to illustrate the asymmetric nature of the probability distribution of observable properties and their dependence on cluster mass.

• A Critical Look at the Role of AGB Stars in Stellar Population Synthesis
  A. Lançon
  Review in Asymptotic Giant Branch Stars, IAU Symposium 191 (1999)
  Asymptotic giant branch stars are essential contributors to the near and mid-IR emission of intermediate age (0.1-1 Gyr old) stellar populations. Detecting this light will set constraints on the star formation history in galaxies and, conversely, the search for AGB signatures in well studied populations will help us reduce some of the still large uncertainties in AGB models. This paper reviews how AGB stars are currently included in population synthesis models and which spectral features can be used to identify their emission in galaxy light; targets for observational tests are suggested, and some observational and theoretical difficulties are discussed.

• How to search for AGB stars in near-IR post-starburst spectra
  A. Lançon, M. Mouhcine, M. Fioc, D. Silva
  A&A Letter, 1999
  This paper suggests near-IR narrow band colour indices to be used to identify AGB contributions (O-rich and C-rich) in the integrated emission of stellar populations.

• Towards Time-Averaged 0.5-2.5 micron Spectra of Variable AGB Stars for Population Synthesis
  A. Lançon
  in Extragalactic Astronomy in the Infrared, XXXIInd Rencontres de Moriond, March 1997 (Editions Frontières)
  Based on extensive spectroscopic observations of low effective temperatures stars (Lançon and Wood, 1997 ), some current difficulties faced in the interpretation of near-IR absorption features in the integrated spectra of galaxies are discussed. Variable AGB stars, on which the observational programme focuses, are expected to contribute a significant fraction of the K band emission of 0.1 to 1 Gyr old stellar populations. The absence of simple correlations between near-IR molecular absorption indices and broad band colours is demonstrated. Time-averaged spectra for these stars are in preparation.

• The Near-IR Spectral Evolution of Dusty Starburst Galaxies
  A. Lançon, B. Rocca-Volmerange - New Astronomy, 1, 215 (1996)
  The new features of the evolutionary population synthesis models (near-IR stellar energy distribution, ISM contributions with emphasis on the near-IR lines of He and H) - The analysis of the spectrum of NGC 1614, a luminous IRAS galaxy hosting a powerful starburst (history, relative contributions of the burst and the underlying population, dust distribution, constraints on the IMF)

• A Database for Galaxy Evolution Modeling
  C. Leitherer et. al. - PASP, 108, 996 (1996)
  The CDrom accompanying this article contains selected model predictions regarding the evolution of near-IR spectra of galaxies (spectra and spectral indices for various star-formation histories and IMFs).

lancon@astro.u-strasbg.fr