28 research outputs found
Wolf-Rayet Stars in Starburst Galaxies
Wolf-Rayet stars have been detected in a large number of galaxies
experiencing intense bursts of star formation. All stars initially more massive
than a certain, metallicity-dependent, value are believed to experience the
Wolf-Rayet phase at the end of their evolution, just before collapsing in
supernova explosion. The detection of Wolf-Rayet stars puts therefore important
constraints on the evolutionary status of starbursts, the properties of their
Initial Mass Functions and their star formation regime. In this contribution we
review the properties of galaxies hosting Wolf-Rayet stars, with special
emphasis on the factors that determine their presence and evolution, as well as
their impact on the surrounding medium.Comment: Contribution to the Proceedings of the JENAM 99 conference "The
interplay between massive stars and the ISM", held in Toulouse in September
7-11, 1999. 10 pages, 5 figures. Requires elsart.cls latex macr
Gamma-ray line emission from OB associations and young open clusters
OB associations and young open clusters constitute the most prolific
nucleosynthesis sites in our Galaxy. The combined activity of stellar winds and
core-collapse supernovae ejects significant amounts of freshly synthesised
nuclei into the interstellar medium. Radioactive isotopes, such as 26Al or
60Fe, that have been co-produced in such events may eventually be observed by
gamma-ray instruments through their characteristic decay-line signatures. In
order to study such signatures and to derive constraints on the involved
nucleosynthesis processes, we developed a multi-wavelength evolutionary
synthesis model for massive star associations. We apply this model to
associations and clusters in the well-studied Cygnus region for which we
re-determined the stellar census based on photometric and spectroscopic data.
In particular we study the relation between 1.809 MeV gamma-ray line emission
and ionising flux, since the latter has turned out to provide an excellent
tracer of the global galactic 1.809 MeV emission. We compare our model to
COMPTEL 1.8 MeV gamma-ray line observations from which we derive limits on the
relative contributions from massive stars and core-collapse supernovae to the
actual 26Al content in this region. Based on our model we make predictions
about the expected 26Al and 60Fe line signatures in the Cygnus region. These
predictions make the Cygnus region a prime target for the future INTEGRAL
mission.Comment: 4 pages, 1 figure, Proc. 5th Compton Symposiu
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&
Young LMC clusters: the role of red supergiants and multiple stellar populations in their integrated light and CMDs
The optical integrated spectra of three LMC young stellar clusters (NGC 1984,
NGC 1994 and NGC 2011) exhibit concave continua and prominent molecular bands
which deviate significantly from the predictions of single stellar population
(SSP) models. In order to understand the appearance of these spectra, we create
a set of young stellar population (MILES) models, which we make available to
the community. We use archival International Ultraviolet Explorer integrated UV
spectra to independently constrain the cluster masses and extinction, and rule
out strong stochastic effects in the optical spectra. In addition, we also
analyze deep colour-magnitude diagrams of the clusters to provide independent
age determinations based on isochrone fitting. We explore hypotheses including
age-spreads in the clusters, a top-heavy initial mass function, different SSP
models and the role of red supergiant stars (RSG). We find that the strong
molecular features in the optical spectra can only be reproduced by modeling an
increased fraction of about 20 per cent by luminosity of RSG above what is
predicted by canonical stellar evolution models. Given the uncertainties in
stellar evolution at Myr ages, we cannot presently rule-out the presence of Myr
age-spreads in these clusters. Our work combines different wavelengths as well
as different approaches (resolved data as well as integrated spectra for the
same sample) in order to reveal the complete picture. We show that each
approach provides important information but in combination can we better
understand the cluster stellar populations.Comment: Accepted for publication in MNRA
Multi-task Bias-Variance Trade-off Through Functional Constraints
Multi-task learning aims to acquire a set of functions, either regressors or
classifiers, that perform well for diverse tasks. At its core, the idea behind
multi-task learning is to exploit the intrinsic similarity across data sources
to aid in the learning process for each individual domain. In this paper we
draw intuition from the two extreme learning scenarios -- a single function for
all tasks, and a task-specific function that ignores the other tasks
dependencies -- to propose a bias-variance trade-off. To control the
relationship between the variance (given by the number of i.i.d. samples), and
the bias (coming from data from other task), we introduce a constrained
learning formulation that enforces domain specific solutions to be close to a
central function. This problem is solved in the dual domain, for which we
propose a stochastic primal-dual algorithm. Experimental results for a
multi-domain classification problem with real data show that the proposed
procedure outperforms both the task specific, as well as the single
classifiers
Crucial aspects of the initial mass function (I): The statistical correlation between the total mass of an ensemble of stars and its most massive star
Our understanding of stellar systems depends on the adopted interpretation of
the IMF, phi(m). Unfortunately, there is not a common interpretation of the
IMF, which leads to different methodologies and diverging analysis of
observational data.We study the correlation between the most massive star that
a cluster would host, mmax, and its total mass into stars, M, as an example
where different views of the IMF lead to different results. We assume that the
IMF is a probability distribution function and analyze the mmax-M correlation
within this context. We also examine the meaning of the equation used to derive
a theoretical M-char_mmax relationship, N x int[Char_mmax-mup] phi(m) dm = 1
with N the total number of stars in the system, according to different
interpretations of the IMF. We find that only a probabilistic interpretation of
the IMF, where stellar masses are identically independent distributed random
variables, provides a self-consistent result. Neither M nor N, can be used as
IMF scaling factors. In addition, Char_mmax is a characteristic maximum stellar
mass in the cluster, but not the actual maximum stellar mass. A -Char_mmax
correlation is a natural result of a probabilistic interpretation of the IMF;
however, the distribution of observational data in the N (or M)-cmmax plane
includes a dependence on the distribution of the total number of stars, N (and
M), in the system, Phi(N), which is not usually taken into consideration. We
conclude that a random sampling IMF is not in contradiction to a possible
mmax-M physical law. However, such a law cannot be obtained from IMF algebraic
manipulation or included analytically in the IMF functional form. The possible
physical information that would be obtained from the N (or M)-mmax correlation
is closely linked with the Phi(M) and Phi(N) distributions; hence it depends on
the star formation process and the assumed.Comment: Accepted by A&A; 16 pages, 10 figure