422 research outputs found
On recent SFR calibrations and the constant SFR approximation
Star Formation Rate (SFR) inferences are based in the so-called constant SFR
approximation, where synthesis models are require to provide a calibration; we
aims to study the key points of such approximation to produce accurate SFR
inferences. We use the intrinsic algebra used in synthesis models, and we
explore how SFR can be inferred from the integrated light without any
assumption about the underling Star Formation history (SFH). We show that the
constant SFR approximation is actually a simplified expression of more deeper
characteristics of synthesis models: It is a characterization of the evolution
of single stellar populations (SSPs), acting the SSPs as sensitivity curve over
different measures of the SFH can be obtained. As results, we find that (1) the
best age to calibrate SFR indices is the age of the observed system (i.e. about
13Gyr for z=0 systems); (2) constant SFR and steady-state luminosities are not
requirements to calibrate the SFR; (3) it is not possible to define a SFR
single time scale over which the recent SFH is averaged, and we suggest to use
typical SFR indices (ionizing flux, UV fluxes) together with no typical ones
(optical/IR fluxes) to correct the SFR from the contribution of the old
component of the SFH, we show how to use galaxy colors to quote age ranges
where the recent component of the SFH is stronger/softer than the older
component.
Particular values of SFR calibrations are (almost) not affect by this work,
but the meaning of what is obtained by SFR inferences does. In our framework,
results as the correlation of SFR time scales with galaxy colors, or the
sensitivity of different SFR indices to sort and long scale variations in the
SFH, fit naturally. In addition, the present framework provides a theoretical
guide-line to optimize the available information from data/numerical
experiments to improve the accuracy of SFR inferences.Comment: A&A accepted, 13 pages, 4 Figure
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
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 documentsâ markup structure
Incidence of lesions on Fungiidae corals in the eastern Red Sea is related to water temperature and coastal pollution
Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Marine Environmental Research 98 (2014): 29-38, doi:10.1016/j.marenvres.2014.04.002.As sea surface temperatures rise and the global human population increases, large-scale
field observations of marine organism health and water quality are increasingly
necessary. We investigated the health of corals from the family Fungiidae using visual
observations in relation to water quality and microbial biogeochemistry parameters along
1300 km of the Red Sea coast of Saudi Arabia. At large scales, incidence of lesions
caused by unidentified etiology showed consistent signs, increasing significantly from the
northern to southern coast and positively correlated to annual mean seawater
temperatures. Lesion abundance also increased to a maximum of 96% near the populous
city of Jeddah. The presence of lesioned corals in the region surrounding Jeddah was strongly correlated with elevated concentrations of ammonium and changes in microbial
communities that are linked to decreased water quality. This study suggests that both high
seawater temperatures and nutrient pollution may play an indirect role in the formation of
lesions on corals.This research was supported by Award No. USA 00002 to K. Hughen by King Abdullah
University of Science and Technology (KAUST) and a WHOI Ocean Life Institute
postdoctoral scholar fellowship to A. Apprill
Nuclear activity and massive star formation in the low luminosity AGN NGC4303: Chandra X-ray observations
We present evidence of the co-existence of either an AGN or an ultraluminous
X-ray source (ULX), together with a young super stellar cluster in the 3
central parsecs of NGC4303. The galaxy contains a low luminosity AGN and hosts
a number of starburst regions in a circumnuclear spiral, as well as in the
nucleus itself. A high spatial resolution Chandra image of this source reveals
that the soft X-ray emission traces the ultraviolet nuclear spiral down to a
core, which is unresolved both in soft and hard X-rays. The astrometry of the
X-ray core coincides with the UV core within the Chandra positioning accuracy.
The total X-ray luminosity of the core, 1.5*10^{39} erg/s, is similar to that
from some LINERs or from the weakest Seyferts detected so far. The soft X-rays
in both the core and the extended structure surrounding it can be well
reproduced by evolutionary synthesis models (which include the emission
expected from single stars, the hot diffuse gas, supernova remnants and binary
systems), consistent with the properties of the young stellar clusters
identified in the UV. The hard X-ray tail detected in the core spectrum,
however, most likely requires the presence of an additional source. This
additional source could either be a weak active nucleus black hole or an
ultraluminous X-ray object. The implications of these results are discussed.Comment: 37 pages, 7 figures, ApJ accepte
On the Optimization of Broad-Band Photometry for Galaxy Evolution Studies
We have derived the uncertainties to be expected in the derivation of galaxy
physical properties (star formation history, age, metallicity, reddening) when
comparing broad-band photometry to the predictions of evolutionary synthesis
models. We have obtained synthetic colors for a large sample (9000) of
artificial galaxies assuming different star formation histories, ages,
metallicities, reddening values, and redshifts. The colors derived have been
perturbed by adopting different observing errors, and compared back to the
evolutionary synthesis models grouped in different sets. The comparison has
been performed using a combination of Monte Carlo simulations, a Maximum
Likelihood Estimator and Principal Component Analysis. After comparing the
input and derived output values we have been able to compute the uncertainties
and covariant degeneracies between the galaxy physical properties as function
of (1) the set of observables available, (2) the observing errors, and (3) the
galaxy properties themselves. In this work we have considered different sets of
observables, some of them including the standard Johnson/Cousins (UBVRI) and
Sloan Digital Sky Survey (SDSS) bands in the optical, the 2 Micron All Sky
Survey (2MASS) bands in the near-infrared, and the Galaxy Evolution Explorer
(GALEX) bands in the UV, at three different redshifts, z=0.0, 0.7, and 1.4.
This study is intended to represent a basic tool for the design of future
projects on galaxy evolution, allowing an estimate of the optimal band-pass
combinations and signal-to-noise ratios required for a given scientific
objective.Comment: 20 pages, 9 postscript figures, 3 tables, accepted for publication in
A
Lipid biomarkers in Symbiodinium dinoflagellates : new indicators of thermal stress
Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Coral Reefs 32 (2013): 923-934, doi:10.1007/s00338-013-1076-3.Lipid content and fatty acid profiles of corals and their dinoflagellate endosymbionts are known to vary in response to high temperature stress. To better understand the heat stress response in these symbionts, we investigated cultures of Symbiodinium goreauii type C1 and Symbiodinium clade subtype D1 grown under a range of temperatures and durations. The predominant lipids produced by Symbiodinium are palmitic (C16) and stearic (C18) saturated fatty acids and their unsaturated analogs, docosahexaenoic (C22:6, n-3) polyunsaturated fatty acid (PUFA), and a variety of sterols. The relative amount of unsaturated acids within the C18 fatty acids in Symbiodinium tissue decreases in response to thermal stress. Prolonged exposure to high temperature also causes a decrease in abundance of fatty acids relative to sterols. These shifts in fatty acids and sterols are common to both types C1 and D1, but the apparent thermal threshold of lipid changes is lower for type C1. This work indicates that ratios among free fatty acids and sterols in Symbiodinium can be used as sensitive indicators of thermal stress. If the Symbdionium lipid stress response is unchanged in hospite, the algal heat stress biomarkers we have identified could be measured to detect thermal stress within the coral holobiont.. These results provide new insights into the potential role of lipids in the overall Symbiodinium thermal stress response.This research was supported by Award No. USA 00002 to K. Hughen made by King Abdullah University of Science and Technology (KAUST).2014-12-0
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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