289 research outputs found
26Al yields from rotating Wolf--Rayet star models
We present new Al stellar yields from rotating Wolf--Rayet stellar
models which, at solar metallicity, well reproduce the observed properties of
the Wolf-Rayet populations. These new yields are enhanced with respect to
non--rotating models, even with respect to non--rotating models computed with
enhanced mass loss rates. We briefly discuss some implications of the use of
these new yields for estimating the global contribution of Wolf-Rayet stars to
the quantity of Al now present in the Milky Way.Comment: 6 pages, 2 figures, to appear in New Astronomy Review
On biases in the predictions of stellar population synthesis models
Sampling fluctuations in stellar populations give rise to dispersions in
observables when a small number of sources contribute effectively to the
observables. This is the case for a variety of linear functions of the spectral
energy distribution (SED) in small stellar systems, such as galactic and
extragalactic HII regions, dwarf galaxies or stellar clusters. In this paper we
show that sampling fluctuations also introduce systematic biases and
multi-modality in non-linear functions of the SED, such as luminosity ratios,
magnitudes and colours. In some cases, the distribution functions of rational
and logarithmic quantities are bimodal, hence complicating considerably the
interpretation of these quantities in terms of age or evolutionary stages.
These biases can be only assessed by Monte Carlo simulations. We find that
biases are usually negligible when the effective number of stars, Neff, which
contribute to a given observable is larger than 10. Bimodal distributions may
appear when Neff is between 10 and 0.1. Predictions from any model of stellar
population synthesis become extremely unreliable for small Neff values,
providing an operational limit to the applicability of such models for the
interpretation of integrated properties of stellar systems. In terms of stellar
masses, assuming a Salpeter Initial Mass Function in the range 0.08 -- 120 Mo,
Neff=10 corresponds to about 10**5 Mo (although the exact value depends on the
age and the observable). This bias may account, at least in part, for claimed
variations in the properties of the stellar initial mass function in small
systems, and arises from the discrete nature of small stellar populations.Comment: 16 pages, 14 figures. Accpeted for publication in MNRA
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
A Methodology for Simulated Experiments in Interactive Search
Interactive information retrieval has received much attention in recent years, e.g. [7]. Furthermore, increased activity in developing interactive features in search systems used across existing popular Web search engines suggests that interactive systems are being recognised as a promising next step in assisting information search. One of the most challenging problems with interactive systems however remains evaluation.
We describe the general specifications of a methodology for conducting controlled and reproducible experiments in the context of interactive search. It was developed in the AutoAdapt project1 focusing on search in intranets, but the methodology is more generic than that and can be applied to interactive Web search as well. The goal of this methodology is to evaluate the ability of different algorithms to produce domain models that provide accurate suggestions for query modifications. The AutoAdapt project investigates the application of automatically constructed adaptive domain models for providing suggestions for query modifications to the users of an intranet search engine. This goes beyond static models such as the one employed to guide users who search the Web site of the University of Essex which is based on a domain model that has been built in advance using the documents’ markup structure
Evaluation of the graphical interface to a World Wide Web-based college course offered through the UNC-CH School of Public Health Department of Maternal and Child Health
This study describes a questionnaire survey of and interviews with students of the UNC-CH School of Public Health Department of Maternal and Child Health. The survey and interviews were conducted to evaluate the design of an interactive study guide offered as part of a World Wide Web-based college course. The results of the study identified a discrepancy in the perceived success of the design of the study guide between students who used the guide for the college course and those who used the guide as a supplemental educational resource only. Based on the results, the investigator suggests a number of recommendations to improve the experience of all students using the study guide
The Luminous Starburst Ring in NGC 7771: Sequential Star Formation?
Only two of the twenty highly luminous starburst galaxies analyzed by Smith
et al. exhibit circumnuclear rings of star formation. These galaxies provide a
link between 10^11 L_sun systems and classical, less-luminous ringed systems.
We report the discovery of a near-infrared counterpart to the nuclear ring of
radio emission in NGC 7771. A displacement between the ~10 radio bright clumps
and the ~10 near-infrared bright clumps indicates the presence of multiple
generations of star formation. The estimated thermal emission from each radio
source is equivalent to that of ~35000 O6 stars. Each near-infrared bright knot
contains ~5000 red supergiants, on average. The stellar mass of each knot is
estimated to be ~10^7 M_sun. The implied time-averaged star formation rate is
\~40 M_sun per yr. Several similarities are found between the properties of
this system and other ringed and non-ringed starbursts. Morphological
differences between NGC 7771 and the starburst + Seyfert 1 galaxy NGC 7469
suggest that NGC 7771 may not be old enough to fuel an AGN, or may not be
capable of fueling an AGN. Alternatively, the differences may be unrelated to
the presence or absence of an AGN and may simply reflect the possibility that
star formation in rings is episodic.Comment: accepted for publication in The Astrophysical Journal (10 January
1999); 48 pages including 13 figures; AAS LaTe
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