2,219 research outputs found
On the impact of Helium abundance on the Cepheid Period-Luminosity and Wesenheit relations and the Distance Ladder
This work analyses the effect of the Helium content on synthetic
Period-Luminosity Relations (PLRs) and Period-Wesenheit Relations (PWRs) of
Cepheids and the systematic uncertainties on the derived distances that a
hidden population of He-enhanced Cepheids may generate. We use new stellar and
pulsation models to build a homogeneous and consistent framework to derive the
Cepheid features. The Cepheid populations expected in synthetic color-magnitude
diagrams of young stellar systems (from 20 Myr to 250 Myr) are computed in
several photometric bands for Y = 0.25 and Y = 0.35, at a fixed metallicity (Z
= 0.008). The PLRs appear to be very similar in the two cases, with negligible
effects (few %) on distances, while PWRs differ somewhat, with systematic
uncertainties in deriving distances as high as about 7% at log P < 1.5.
Statistical effects due to the number of variables used to determine the
relations contribute to a distance systematic error of the order of few
percent, with values decreasing from optical to near-infrared bands. The
empirical PWRs derived from multi-wavelength datasets for the Large Magellanic
Cloud (LMC) is in a very good agreement with our theoretical PWRs obtained with
a standard He content, supporting the evidence that LMC Cepheids do not show
any He effect
Multipopulation aftereffects on the color-magnitude diagram and Cepheid variables of young stellar systems
Context: The evidence of a multipopulation scenario in Galactic globular
clusters raises several questions about the formation and evolution of the two
(or more) generations of stars. These populations show differences in their age
and chemical composition. These differences are found in old- and intermediate-
age stellar clusters in the Local Group. The observations of young stellar
systems are expected to present footprints of multiple stellar populations.
Aims: This theoretical work intends to be a specific step in exploring the
space of the observational indicators of multipopulations, without covering all
the combinations of parameters that may contribute to the formation of multiple
generations of stars in a cluster or in galaxy. The goal is to shed light on
the possible observational features expected by core He-burning stars that
belong to two stellar populations with different original He content and ages.
Methods: The tool adopted was the stellar population synthesis. We used new
stellar and pulsation models to construct a homogeneous and consistent
framework. Synthetic color-magnitude diagrams (CMDs) of young- and
intermediate-age stellar systems (from 20 Myr up to 1 Gyr) were computed in
several photometric bands to derive possible indicators of double populations
both in the observed CMDs and in the pulsation properties of the Cepheids.
Results: We predict that the morphology of the red/blue clump in VIK bands
can be used to photometrically indicate the two stellar populations in a rich
assembly of stars if there is a significant difference in their original He
content. Moreover, the period distribution of the Cepheids appears to be widely
affected by the coeval multiple generations of stars within stellar systems. We
show that the Wesenheit relations may be affected by the helium content of the
Cepheids.Comment: in press on A&
Fast model predictive control for hydrogen outflow regulation in ethanol steam reformers
© 20xx IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.In the recent years, the presence of alternative power sources, such as solar panels, wind farms, hydropumps
and hydrogen-based devices, has significantly increased. The reasons of this trend are clear: contributing to
a reduction of gas emissions and dependency on fossil fuels. Hydrogen-based devices are of particular interest due
to their significant efficiency and reliability. Reforming technologies are among the most economic and efficient ways
of producing hydrogen. In this paper we consider the regulation of hydrogen outflow in an ethanol steam reformer
(ESR). In particular, a fast model predictive control approach based on a finite step response model of the process
is proposed. Simulations performed using a more realistic non-linear model show the effectiveness of the proposed
approach in driving the ESR to different operating conditions while fulfilling input and output constraints.Peer ReviewedPostprint (author's final draft
Early metal enrichment in high-redshift quasars
Quasars are powerful systems whose spectrum is rich of metal features that
allow us to investigate the chemical evolution of galaxies at very high
redshift, even close to the reionization epoch. I review the main observational
constraints on the metallicity of quasars host galaxies at high redshift and
discuss the implications and issues for models of galaxy evolution in the early
universe.Comment: 8 pages, invited review at the workshop "Probing Stellar Populations
out to the Distant Universe
Surface Brightness Fluctuations from archival ACS images: a stellar population and distance study
We derive Surface Brightness Fluctuations (SBF) and integrated magnitudes in
the V- and I-bands using Advanced Camera for Surveys (ACS) archival data. The
sample includes 14 galaxies covering a wide range of physical properties:
morphology, total absolute magnitude, integrated color. We take advantage of
the latter characteristic of the sample to check existing empirical
calibrations of absolute SBF magnitudes both in the I- and V-passbands.
Additionally, by comparing our SBF and color data with the Teramo-SPoT simple
stellar population models, and other recent sets of population synthesis
models, we discuss the feasibility of stellar population studies based on
fluctuation magnitudes analysis. The main result of this study is that
multiband optical SBF data and integrated colors can be used to significantly
constrain the chemical composition of the dominant stellar system in the
galaxy, but not the age in the case of systems older than 3 Gyr.
SBF color gradients are also detected and analyzed. These SBF gradient data,
together with other available data, point to the existence of mass dependent
metallicity gradients in galaxies, with the more massive objects showing a
non--negligible SBF versus color gradient. The comparison with models suggests
that such gradients imply more metal rich stellar populations in the galaxies'
inner regions with respect to the outer ones.Comment: ApJ Accepte
The Cepheids of NGC1866: A Precise Benchmark for the Extragalactic Distance Scale and Stellar Evolution from Modern UBVI Photometry
We present the analysis of multiband time-series data for a sample of 24
Cepheids in the field of the Large Magellanic Cloud cluster NGC1866. Very
accurate BVI VLT photometry is combined with archival UBVI data, covering a
large temporal window, to obtain precise mean magnitudes and periods with
typical errors of 1-2% and of 1 ppm, respectively. These results represent the
first accurate and homogeneous dataset for a substantial sample of Cepheid
variables belonging to a cluster and hence sharing common distance, age and
original chemical composition. Comparisons of the resulting multiband
Period-Luminosity and Wesenheit relations to both empirical and theoretical
results for the Large Magellanic Cloud are presented and discussed to derive
the distance of the cluster and to constrain the mass-luminosity relation of
the Cepheids. The adopted theoretical scenario is also tested by comparison
with independent calibrations of the Cepheid Wesenheit zero point based on
trigonometric parallaxes and Baade-Wesselink techniques. Our analysis suggests
that a mild overshooting and/or a moderate mass loss can affect
intermediate-mass stellar evolution in this cluster and gives a distance
modulus of 18.50 +- 0.01 mag. The obtained V,I color-magnitude diagram is also
analysed and compared with both synthetic models and theoretical isochrones for
a range of ages and metallicities and for different efficiencies of core
overshooting. As a result, we find that the age of NGC1866 is about 140 Myr,
assuming Z = 0.008 and the mild efficiency of overshooting suggested by the
comparison with the pulsation models.Comment: 13 pages, 10 figures, accepted in MNRAS (2016 January 14
Spatial and Temporal Dynamics in the Ionic Driving Force for GABAA Receptors
It is becoming increasingly apparent that the strength of GABAergic synaptic transmission is dynamic. One parameter that can establish differences in the actions of GABAergic synapses is the ionic driving force for the chloride-permeable GABAA receptor (GABAAR). Here we review some of the sophisticated ways in which this ionic driving force can vary within neuronal circuits. This driving force for GABAARs is subject to tight spatial control, with the distribution of Cl− transporter proteins and channels generating regional variation in the strength of GABAAR signalling across a single neuron. GABAAR dynamics can result from short-term changes in their driving force, which involve the temporary accumulation or depletion of intracellular Cl−. In addition, activity-dependent changes in the expression and function of Cl− regulating proteins can result in long-term shifts in the driving force for GABAARs. The multifaceted regulation of the ionic driving force for GABAARs has wide ranging implications for mature brain function, neural circuit development, and disease
- …