298 research outputs found
A probabilistic formulation of evolutionary synthesis models: implications for SED fittings
Evolutionary synthesis models (ESM) have been extensively used to obtain the
star formation history in galaxies by means of SED fitting. Implicit in this
use of ESM is that (a) for given evolutionary parameters, the shape of the SED
is fixed whatever the size of the observed cluster (b) all regions of the
observed SED have the same weight in the fit. However, Nature does not follow
these two assumptions, as is implied by the existence of Surface Brightness
Fluctuations in galaxies and as can be shown by simple logical arguments.Comment: 2 pages, 2 figure, going to be published in the proceedings of the
IAU Symp. 241, "Stellar Populations as Building Blocks of Galaxies
PyNeb: a new tool for analyzing emission lines. I. Code description and validation of results
Analysis of emission lines in gaseous nebulae yields direct measures of
physical conditions and chemical abundances and is the cornerstone of nebular
astrophysics. Although the physical problem is conceptually simple, its
practical complexity can be overwhelming since the amount of data to be
analyzed steadily increases; furthermore, results depend crucially on the input
atomic data, whose determination also improves each year. To address these
challenges we created PyNeb, an innovative code for analyzing emission lines.
PyNeb computes physical conditions and ionic and elemental abundances, and
produces both theoretical and observational diagnostic plots. It is designed to
be portable, modular, and largely customizable in aspects such as the atomic
data used, the format of the observational data to be analyzed, and the
graphical output. It gives full access to the intermediate quantities of the
calculation, making it possible to write scripts tailored to the specific type
of analysis one wants to carry out. In the case of collisionally excited lines,
PyNeb works by solving the equilibrium equations for an n-level atom; in the
case of recombination lines, it works by interpolation in emissivity tables.
The code offers a choice of extinction laws and ionization correction factors,
which can be complemented by user-provided recipes. It is entirely written in
the python programming language and uses standard python libraries. It is fully
vectorized, making it apt for analyzing huge amounts of data. The code is
stable and has been benchmarked against IRAF/NEBULAR. It is public, fully
documented, and has already been satisfactorily used in a number of published
papers.Comment: 17 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysics. Typos and reference list corrected in this versio
Modeling the ionizing spectra of H ii regions: individual stars versus stellar ensembles
Aims. We study how IMF sampling affects the ionizing flux and emission line
spectra of low mass stellar clusters. Methods. We performed 2 x 10^6 Monte
Carlo simulations of zero-age solar-metallicity stellar clusters covering the
20 - 10^6 Mo mass range. We study the distribution of cluster stellar masses,
Mclus, ionizing fluxes, Q(H0), and effective temperatures, Tclus. We compute
photoionization models that broadly describe the results of the simulations and
compare them with photoionization grids. Results. Our main results are: (a) A
large number of low mass clusters (80% for Mclus = 100 Mo) are unable to form
an H ii region. (b) There are a few overluminous stellar clusters that form H
ii regions. These overluminous clusters preserve statistically the mean value
of obtained by synthesis models, but the mean value cannot be used as a
description of particular clusters. (c) The ionizing continuum of clusters with
Mclus < 10^4 Mo is more accurately described by an individual star with
self-consistent effective temperature(T*) and Q(H0) than by the ensemble of
stars (or a cluster Tclus) produced by synthesis models. (d)Photoionization
grids of stellar clusters can not be used to derive the global properties of
low mass clusters. Conclusions. Although variations in the upper mass limit,
mup, of the IMF would reproduce the effects of IMF sampling, we find that an ad
hoc law that relates mup to Mclus in the modelling of stellar clusters is
useless, since: (a) it does not cover the whole range of possible cases, and
(b) the modelling of stellar clusters with an IMF is motivated by the need to
derive the global properties of the cluster: however, in clusters affected by
sampling effects we have no access to global information of the cluster but
only particular information about a few individual stars.Comment: A&A in pres
The distance to the C component of I Zw 18 and its star formation history: A probabilistic approach
We analyzed the resolved stellar population of the C component of the
extremely metal-poor dwarf galaxy Izw18 in order to evaluate its distance and
star formation history as accurately as possible. In particular, we aimed at
answering the question of whether this stellar population is young. We
developed a probabilistic approach to analyzing high-quality photometric data
obtained with the Advanced Camera for Surveys of the Hubble Space Telescope.
This approach gives a detailed account of the various stochastic aspects of
star formation. We carried out two successive models of the stellar population
of interest, paying attention to how our assumptions could affect the results.
We found a distance to the C component of I Zw 18 as high as 27 Mpc, a
significantly higher value than those cited in previous works. The star
formation history we inferred from the observational data shows various
interesting features: a strong starburst that lasted for about 15 Myr, a more
moderate one that occurred approx 100 Myr ago, a continuous process of star
formation between both starbursts, and a possible episode of low level star
formation at ages over 100 Myr. The stellar population studied is likely approx
125 Myr old, although ages of a few Gyr cannot be ruled out. Furthermore,
nearly all the stars were formed in the last few hundreds of Myr.Comment: 13 pags., 17 (low-resolution) Figs. Accepted by A&
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