207 research outputs found
A secondary clump of red giant stars: why and where
Based on the results of detailed population synthesis models, it was recently
claimed that the clump of red giants in the colour-magnitude diagram (CMD) of
composite stellar populations should present an extension to lower
luminosities, which goes down to about 0.4 mag below the main clump. This
feature is made of stars just massive enough for having ignited helium in
non-degenerate conditions. In this paper, we go into more details about the
origin and properties of this feature. We first compare the clump theoretical
models with data for clusters of different ages and metallicities, basically
confirming the predicted behaviours. We then refine the previous models in
order to show that: (i) The faint extension is expected to be clearly separated
from the main clump in the CMD of metal-rich populations, defining a secondary
clump by itself. (ii) It should be present in all galactic fields containing ~1
Gyr old stars and with mean metallicities higher than about Z=0.004. (iii) It
should be particularly strong, if compared to the main red clump, in galaxies
which have increased their star formation rate in the last Gyr or so of their
evolution. In fact, secondary clumps similar to the model predictions are
observed in the CMD of nearby stars from Hipparcos data, and in those of some
LMC fields observed to date. There are also several reasons why this secondary
clump may be missing or hidden in other observed CMDs of galaxy fields (e.g.
photometric errors or differential absorption larger than 0.2 mag).
Nonetheless, this structure may provide important constraints to the star
formation history of Local Group galaxies. We comment also on the intrinsic
luminosity variation and dispersion of clump stars, which may limit their use
as either absolute or relative distance indicators.Comment: 20 pages with 11 figures, submitted to MNRA
Broad-band photometric evolution of star clusters
I briefly introduce a database of models that describe the evolution of star
clusters in several broad-band photometric systems. Models are based on the
latest Padova stellar evolutionary tracks - now including the alpha-enhanced
case and improved AGB models - and a revised library of synthetic spectra from
model atmospheres. As of today, we have revised isochrones in
Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Thuan-Gunn, and Washington
systems. Several other filter sets are included in a preliminary way, like
those used by the EIS and SDSS projects. The database contains also integrated
magnitudes of single-burst stellar populations and Monte-Carlo simulations that
show the stochastic dispersion of the colours as a function of cluster mass,
age, and metallicity. The models are useful for several kinds of studies,
including estimates of masses and ages of extragalactic star clusters observed
by means of broad-band photometry.Comment: 5 pages, to appear in Extragalactic Star Clusters, IAU Symp 207, eds.
E.K. Grebel, D. Geisler, D. Minniti. The isochrone data is in
http://pleiadi.pd.astro.it/~lgirardi/isoc_photsys.htm
Fine structure of the red clump in Local Group galaxies
Some fine structures can nowadays be identified in the high-quality
colour-magnitude diagrams (CMD) of Local Group galaxies. The clump of red
giants, for instance, may present a significant colour spread, and extensions
to both brighter and fainter luminosities. Such features are predicted by
population synthesis models which consider stars in the complete relevant
ranges of ages and metallicities, and are potentially useful for constraining
the star formation histories of the parent galaxies over scales of gigayears.
We briefly comment the cases of fields in the Magellanic Clouds, M31, and the
local CMD from Hipparcos.Comment: To appear in Proc. of the VLT Opening Symposium (Parallel Workshop 2:
Star-Way to the Universe
Towards simulating the photometry, chemistry, mass loss and pulsational properties of AGB star populations in resolved galaxies
Extended and updated grids of TP-AGB tracks have been implemented in the
TRILEGAL population synthesis code, which generates mock stellar catalogues for
a galaxy given its mass, distance, star formation history and age-metallicity
relation, including also the Milky Way foreground population. Among the stellar
parameters that are simulated, we now include the surface chemistry, mass-loss
rates, pulsation modes and periods of LPVs. This allows us to perform a series
of consistency checks between AGB model predictions and observations, that we
are just starting to explore. We present a few examples of model--data
comparisons, mostly regarding the near-infrared and variability data for AGB
stars in the Magellanic Clouds.Comment: 5 pages, 3 figures, contributed talk to the workshop "Why galaxies
care about AGB stars", Vienna, August 7-11, 200
The third dredge-up and the carbon star luminosity functions in the Magellanic Clouds
We investigate the formation of carbon stars as a function of the stellar
mass and parent metallicity. Theoretical modelling is based on an improved
scheme for treating the third dredge-up in synthetic calculations of thermally
pulsing asymptotic giant branch (TP-AGB) stars. In this approach, the usual
criterion (based on a constant minimum core mass for the occurrence of
dredge-up, M_c^min) is replaced by one on the minimum temperature at the base
of the convective envelope, T_b^dred, at the stage of the post-flash luminosity
maximum. Envelope integrations then allow determination of M_c^min as a
function of stellar mass, metallicity, and pulse strength (see Wood 1981), thus
inferring if and when dredge-up first occurs. Moreover, the final possible shut
down of the process is predicted. Extensive grids of TP-AGB models were
computed using this scheme. We present and discuss the calibration of the two
dredge-up parameters (lambda and T_b^dred) aimed at reproducing the carbon star
luminosity function (CSLF) in the LMC. It turns out that the faint tail is
almost insensitive to the history of star formation rate (SFR) in the parent
galaxy (it is essentially determined by T_b^dred), in contrast to the bright
wing which may be more affected by the details of the recent SFR. Once the
faint end is reproduced, the peak location is a stringent calibrator of lambda.
The best fit to the observed CSLF in the LMC is obtained with Z=0.008,
lambda=0.50, log(T_b^dred)=6.4, and a constant SFR up to 5x10^8 yr ago. A good
fit to the CSLF in the SMC is then easily derived from the Z=0.004 models, with
a single choice of parameters, and a constant SFR over the entire significant
age interval. The results are consistent with the theoretical expectation that
the third dredge-up is more efficient at lower Zs.Comment: 22 pages with 15 figures, to appear in A&
Is the galactic disk older than the halo?
Aim of this study is to infer the age of the Galactic Disk by means of the
ages of old open clusters, and comment on some recent claims that the Galactic
Disk can be older than the Halo. To this purpose, we analyze the
Color-Magnitude Diagrams (CMDs) of six very old clusters, namely NGC 188, NGC
6791, Collinder 261, Melotte 66, Berkeley 39 and Berkeley 17, and determine
their ages. For each cluster we use the most recent photometric and
spectroscopic data and metallicity estimates. The ages are derived from the
isochrone fitting method using the stellar models of the Padua library
(Bertelli et al . 1994, Girardi et al. 1999). We find that the ages of these
clusters fall in the range 4 to 9-10 Gyr: Melotte 66 is the youngest whereas
NGC 6791 and Berkeley 17 have ages of about 9-10 Gyr. Previous estimates for
Berkeley 17 indicated an age as old as 12 Gyr, almost falling within the range
of classical globular clusters. In our analysis, this cluster is always very
old but perhaps somewhat younger than in previous studies. However we call
attention on the fact that the above ages are to be taken as provisional
estimates, because of the many uncertaintes still affecting stellar models in
the mass range 1.0 to 1.5 Mo. Despite this drawback of extant theory of stellar
structure, if NGC 6791 and Berkeley 17 set the limit to the age of the Galactic
Disk, this component of the Milky Way can be as old as 9-10 Gyr, but surely
younger than the Galactic Halo, at least as inferred from recent determinations
of the age of globular clusters. Finally, it is worth recalling that open
clusters can only provide a lower limit to the age of the Galactic Disk, while
other indicators - like White Dwarfs - are perhaps more suited to this task.Comment: 16 pages, 11 figure, accepted for publication in MNRA
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