Some publications related to stellar population synthesis and starburst galaxies
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
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
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
First spectroscopic evidence for carbon stars outside
the local group: Properties of a massive star cluster in
M. Mouhcine, A. Lançon, C. Leitherer, D. Silva, M.A.T.
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?
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
Stochastic Fluctuations in the Spectrophotometric Properties of
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
Review in Asymptotic Giant Branch Stars, IAU Symposium 191
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
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,
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).