240 research outputs found
On emission-line spectra obtained from evolutionary synthesis models I. Dispersion in the ionising flux and Lowest Luminosity Limits
(abriged) Stellar clusters with the same general physical properties (e.g.,
total mass, age, and star-formation mode) may have very different stellar mass
spectra due to the incomplete sampling of the underlying mass function; such
differences are especially relevant in the high-mass tail due to the smaller
absolute number of massive stars. The dispersion in the number of massive stars
also produces a dispersion in the properties of the corresponding ionising
spectra. In this paper, we lay the bases for the future analysis of this effect
by evaluating the dispersion in the ionising fluxes of synthetic spectra. As an
important consequence, we found that the intensities of synthetic fluxes at
different ionisation edges are strongly correlated, a fact suggesting that no
additional dispersion will result from the inclusion of sampling effects in the
analysis of diagnostic diagrams; this is true for HII regions on all scales.
Additionally, we find convincing suggestions that the He II lines are strongly
affected by sampling, and so cannot be used to constrain the evolutionary
status of stellar clusters. We also establish the range of applicability of
synthesis models set by the Lowest Luminosity Limit for the ionising flux, that
is the lowest limit in cluster mass for which synthesis models can be applied
to predict ionising spectra. This limit marks the boundary between the
situations in which the ionising flux is better modeled with a single star as
opposed to a star cluster; this boundary depends on the metallicity and age,
ranging from 10^3 to more than 10^6 Mo. As a consequence, synthesis models
should not be used to try to account for the properties of clusters with
smaller masses.Comment: Replaced with accepted versio
On surface brightness fluctuations: probabilistic and statistical bases I: Stellar population and theoretical SBF
This work aims to provide a theoretical formulation of Surface Brightness
Fluctuations (SBF) in the framework of probabilistic synthesis models, and to
distinguish between the different distributions involved in the SBF definition.
RESULTS: We propose three definitions of SBF: (i) stellar population SBF, which
can be computed from synthesis models and provide an intrinsic metric of fit
for stellar population studies; (ii) theoretical SBF, which include the stellar
population SBF plus an additional term that takes into account the distribution
of the number of stars per resolution element psi(N); theoretical SBF coincide
with Tonry & Schneider (1998) definition in the very particular case that
psi(N) is assumed to be a Poisson distribution. However, the Poisson
contribution to theoretical SBF is around 0.1% of the contribution due to the
stellar population SBF, so there is no justification to include any reference
to Poisson statistics in the SBF definition; (iii) observational SBF, which are
those obtained in observations that are distributed around the theoretical SBF.
Finally, we show alternative ways to compute SBF and extend the application of
stellar population SBF to defining a metric of fitting for standard stellar
population studies. CONCLUSIONS: We demostrate that SBF are observational
evidence of a probabilistic paradigm in population synthesis, where integrated
luminosities have an intrinsic distributed nature, and they rule out the
commonly assumed deterministic paradigm of stellar population modeling.Comment: A&A accepte
Confidence limits of evolutionary synthesis models III. On time-integrated quantities
Evolutionary synthesis models are a fundamental tool to interpret the
properties of observed stellar systems. In order to achieve a meaningful
comparison between models and real data, it is necessary to calibrate the
models themselves, i.e. to evaluate the dispersion due to the discreteness of
star formation as well as the possible model errors. In this paper we show that
linear interpolations in the log M - log t_k plane, that are customary in the
evaluation of isochrones in evolutionary synthesis codes, produce unphysical
results. We also show that some of the methods used in the calculation of
time-integrated quantities (kinetic energy, and total ejected masses of
different elements) may produce unrealistic results. We propose alternative
solutions to solve both problems. Moreover, we have quantified the expected
dispersion of these quantities due to stochastic effects in stellar
populations. As a particular result, we show that the dispersion in the 14N/12C
ratio increases with time.Comment: 11 pages, 8 figures, accepted by A&
Confidence levels of evolutionary synthesis models III: On sampling and Poissonian fluctuations
In terms of statistical fluctuations, stellar population synthesis models are
only asymptotically correct in the limit of a large number of stars, where
sampling errors become asymptotically small. When dealing with stellar
clusters, starbursts, dwarf galaxies or stellar populations within pixels,
sampling errors introduce a large dispersion in the predicted integrated
properties of these populations. We present here an approximate but generic
statistical formalism which allows a very good estimation of the uncertainties
and confidence levels in any integrated property, bypassing extensive Monte
Carlo simulations, and including the effects of partial correlations between
different observables. Tests of the formalism are presented and compared with
proper estimates. We derive the minimum mass of stellar populations which is
required to reach a given confidence limit for a given integrated property. As
an example of this general formalism, which can be included in any synthesis
code, we apply it to the case of young (t < 20 Myr) starburst populations. We
show that, in general, the UV continuum is more reliable than other continuum
bands for the comparison of models with observed data. We also show that
clusters where more than 10^5 Mo have been transformed into stars have a
relative dispersion of about 10% in Q(He+) for ages smaller than 3 Myr. During
the WR phase the dispersion increases to about 25% for such massive clusters.
We further find that the most reliable observable for the determination of the
WR population is the ratio of the luminosity of the WR bump over the Hbeta
luminosity. A fraction of the observed scatter in the integrated properties of
clusters and starbursts can be accounted for by sampling fluctuations.Comment: accepted by A&A. 13 figures, a&a style packages see also
http://www.laeff.esa.es/users/mcs for tables (by end October) and for high
resolution figure
The connection between missing AGB stars and extended horizontal branches
Recent surveys confirm early results about a deficiency or even absence of
CN-strong stars on the asymptotic giant branch (AGB) of globular clusters
(GCs), although with quite large cluster-to-cluster variations. In general,
this is at odds with the distribution of CN band strengths among first ascent
red giant branch (RGB) stars. Norris et al. proposed that the lack of CN-strong
stars in some clusters is a consequence of a smaller mass of these stars that
cannot evolve through the full AGB phase. In this short paper we found that the
relative frequency of AGB stars can change by a factor of two between different
clusters. We also find a very good correlation between the minimum mass of
stars along the horizontal branch (Gratton et al. 2010) and the relative
frequency of AGB stars, with a further dependence on metallicity. We conclude
that indeed the stars with the smallest mass on the HB cannot evolve through
the full AGB phase, being AGB-manque'. These stars likely had large He and N
content, and large O-depletion. We then argue that there should not be AGB
stars with extreme O depletion, and few of them with a moderate one.Comment: 5 Pages, 2 figures, A&A Accepte
M75, a Globular Cluster with a Trimodal Horizontal Branch. I. Color-Magnitude Diagram
Deep UBVI photometry for a large field covering the distant globular cluster
M75 (NGC 6864) is presented. We confirm a previous suggestion (Catelan et al.
1998a) that M75 possesses a bimodal horizontal branch (HB) bearing striking
resemblance to the well-known case of NGC 1851. In addition, we detect a third,
smaller grouping of stars on the M75 blue tail, separated from the bulk of the
blue HB stars by a gap spanning about 0.5 mag in V. Such a group of stars may
correspond to the upper part of a very extended, though thinly populated, blue
tail. Thus M75 appears to have a trimodal HB. The presence of the "Grundahl
jump" is verified using the broadband U filter. We explore the color-magnitude
diagram of M75 with the purpose of deriving the cluster's fundamental
parameters, and find a metallicity of [Fe/H] = -1.03 +/- 0.17 dex and -1.24 +/-
0.21 dex in the Carretta & Gratton (1997) and Zinn & West (1984) scales,
respectively. We discuss earlier suggestions that the cluster has an
anomalously low ratio of bright red giants to HB stars. A differential age
analysis with respect to NGC 1851 suggests that the two clusters are
essentially coeval.Comment: 19 pages, 15 figures, emulateapj5/apjfonts style. Astronomical
Journal, in press. This version contains some very low-resolution figures,
due to the size constraints of astro-ph. We strongly encourage the interested
reader to download instead the preprint with full-resolution figures, which
can be found at http://www.astro.puc.cl/~mcatelan
RR Lyrae variables in Galactic globular clusters: IV. Synthetic HB and RR Lyrae predictions
We present theoretical predictions concerning horizontal branch stars in
globular clusters, including RR Lyrae variables, as derived from synthetic
procedures collating evolutionary and pulsational constraints. On this basis,
we explore the predicted behavior of the pulsators as a function of the
horizontal branch morphology and over the metallicity range Z=0.0001 to 0.006,
revealing an encouraging concordance with the observed distribution of
fundamentalised periods with metallicity. Theoretical relations connecting
periods to K magnitudes and BV or VI Wesenheit functions are presented, both
appearing quite independent of the horizontal branch morphology only with Z
greater or equal than 0.001. Predictions concerning the parameter R are also
discussed and compared under various assumptions about the horizontal branch
reference luminosity level.Comment: 11 pages, 10 figures. Accepted for publication in "Astronomy and
Astrophysics
Confidence limits of evolutionary synthesis models. IV Moving forward to a probabilistic formulation
Synthesis models predict the integrated properties of stellar populations.
Several problems exist in this field, mostly related to the fact that
integrated properties are distributed. To date, this aspect has been either
ignored (as in standard synthesis models, which are inherently deterministic)
or interpreted phenomenologically (as in Monte Carlo simulations, which
describe distributed properties rather than explain them). We approach
population synthesis as a problem in probability theory, in which stellar
luminosities are random variables extracted from the stellar luminosity
distribution function (sLDF). We derive the population LDF (pLDF) for clusters
of any size from the sLDF, obtaining the scale relations that link the sLDF to
the pLDF. We recover the predictions of standard synthesis models, which are
shown to compute the mean of the sLDF. We provide diagnostic diagrams and a
simplified recipe for testing the statistical richness of observed clusters,
thereby assessing whether standard synthesis models can be safely used or a
statistical treatment is mandatory. We also recover the predictions of Monte
Carlo simulations, with the additional bonus of being able to interpret them in
mathematical and physical terms. We give examples of problems that can be
addressed through our probabilistic formalism. Though still under development,
ours is a powerful approach to population synthesis. In an era of resolved
observations and pipelined analyses of large surveys, this paper is offered as
a signpost in the field of stellar populations.Comment: Accepted by A&A. Substantially modified with respect to the 1st
draft. 26 pages, 14 fig
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