765 research outputs found
The First Empirical Mass Loss Law for Population II Giants
Using the Spitzer IRAC camera we have obtained mid-IR photometry of the red
giant branch stars in the Galactic globular cluster 47 Tuc. About 100 stars
show an excess of mid-infrared light above that expected from their
photospheric emission. This is plausibly due to dust formation in mass flowing
from these stars. This mass loss extends down to the level of the horizontal
branch and increases with luminosity. The mass loss is episodic, occurring in
only a fraction of stars at a given luminosity. Using a simple model and our
observations we derive mass loss rates for these stars. Finally, we obtain the
first empirical mass loss formula calibrated with observations of Population II
stars. The dependence on luminosity of our mass loss rate is considerably
shallower than the widely used Reimers Law. The results presented here are the
first from our Spitzer survey of a carefully chosen sample of 17 Galactic
Globular Clusters, spanning the entire metallicity range from about one
hundredth up to almost solar
HST - WFPC2 photometry of the globular cluster ngc 288: binary systems, blue stragglers and very blue stars
We report on new WFPC2 observations of the globular cluster NGC 288, focusing
our attention on peculiar stars. A very pronounced binary sequence, paralleling
the ordinary Main Sequence (MS) is clearly observed in the Color Magnitude
Diagram (CMD) and a huge relative fraction of Blue Straggler Stars is measured.
The dataset offers the opportunity of studying the evolution of a large
population of binaries (and binary evolution by-products) in a low density
environment, where the evolution of such systems is not dominated by collisions
and encounters. Three (very) Extreme Horizontal Branch Stars have been found,
all lying outside of the cluster core.Comment: 6 pages, 3 figures, in press in the chemical evolution of the Milky
Way: stars versus clusters, F. Matteucci and F. Giovannelli eds, Kluwe
BVI Photometry and the Luminosity Function of the Globular Cluster M92
We present new BVI ground-based photometry and VI space-based photometry for
the globular cluster M92 (NGC 6341) and examine luminosity functions in B, V,
and I containing over 50,000 stars ranging from the tip of the red giant branch
to several magnitudes below the main sequence turn off. Once corrected for
completeness, the observed luminosity functions agree very well with
theoretical models and do not show stellar excesses in any region of the
luminosity function. Using reduced chi squared fitting, the new M92 luminosity
function is shown to be an excellent match to the previously published
luminosity function for M30. These points combine to establish that the
"subgiant excess" found in previously published luminosity functions of
Galactic globular clusters are due to deficiencies in the stellar models used
at that time. Using up to date stellar models results in good agreement between
observations and theory.
Several statistical methods are presented to best determine the age of M92.
These methods prove to be insensitive to the exact choice of metallicity within
the published range. Using [Fe/H]=-2.17 to match recent studies we find an age
of 14.2 plus or minus 1.2 Gyr for the cluster.Comment: 22 pages, 13 figures, 3 tables, accepted for publication in A
Learning in neural networks with material synapses
We discuss the long term maintenance of acquired memory in synaptic connections of a perpetually learning electronic device. This is affected by ascribing each synapse a finite number of stable states in which it can maintain for indefinitely long periods. Learning uncorrelated stimuli is expressed as a stochastic process produced by the neural activities on the synapses. In several interesting cases the stochastic process can be analyzed in detail, leading to a clarification of the performance of the network, as an associative memory, during the process of uninterrupted learning. The stochastic nature of the process and the existence of an asymptotic distribution for the synaptic values in the network imply generically that the memory is a palimpsest but capacity is as low as log N for a network of N neurons. The only way we find for avoiding this tight constraint is to allow the parameters governing the learning process (the coding level of the stimuli; the transition probabilities for potentiation and depression and the number of stable synaptic levels) to depend on the number of neurons. It is shown that a network with synapses that have two stable states can dynamically learn with optimal storage efficiency, be a palimpsest, and maintain its (associative) memory for an indefinitely long time provided the coding level is low and depression is equilibrated against potentiation. We suggest that an option so easily implementable in material devices would not have been overlooked by biology. Finally we discuss the stochastic learning on synapses with variable number of stable synaptic states
Event-driven simulation of spiking neurons with stochastic dynamics
We present a new technique, based on a proposed event-based strategy (Mattia & Del Giudice, 2000), for efficiently simulating large networks of simple model neurons. The strategy was based on the fact that interactions among neurons occur by means of events that are well localized in time (the action potentials) and relatively rare. In the interval between two of these events, the state variables associated with a model neuron or a synapse evolved deterministically and in a predictable way. Here, we extend the event-driven simulation strategy to the case in which the dynamics of the state variables in the inter-event intervals are stochastic. This extension captures both the situation in which the simulated neurons are inherently noisy and the case in which they are embedded in a very large network and receive a huge number of random synaptic inputs. We show how to effectively include the impact of large background populations into neuronal dynamics by means of the numerical evaluation of the statistical properties of single-model neurons under random current injection. The new simulation strategy allows the study of networks of interacting neurons with an arbitrary number of external afferents and inherent stochastic dynamics
Apgar score or birthweight in Chihuahua dogs born by elective Caesarean section : which is the best predictor of the survival at 24 h after birth?
In the dog, the correct management of parturition and the prompt neonatal evaluation and assistance
can reduce the perinatal mortality rates that are particularly high in toy breeds. Newborn evaluation and factors
addressing prognosis are pivotal to guarantee the correct neonatal assistance. Assessment of the Apgar score with
viability classification and birthweight are recognized as predictors for neonatal survival in dogs, but breed-specific
data are needed for a more feasible application in the dog species, in which wide differences among breeds are
known. The present study aimed therefore to: (a) assess the role of Apgar score and birthweight as predictors for the survival of Chihuahua newborn puppies in the first 24 h of life; (b) to assess a cut-off of the Apgar score and birthweight values that can predict the survival of Chihuahua newborn puppies in the first 24 h after birth; (c) to assess the possible effect played by maternal parity, newborn gender and litter-size on Apgar score in Chihuahua newborn puppies, in order to provide breed-specific data for a better neonatal assistance..Data obtained from 176 normal developed Chihuahua puppies born by elective Caesarean section, showed that 62%, 28% and 10% of puppies were classified in the Apgar score classes 7\u201310, 4\u20136 and 0\u20133, respectively, with survival at 24 h after birth of 97%, 96%, 39%, in the three Apgar classes of viability, respectively. Apgar score was a better predictor for survival at 24 h after birth than birthweight (AUC 0.93, P < 0.0001; AUC 0.69, P < 0.01, respectively). Litter-size of 7 puppies/litter plays a negative effect on Apgar score. Apgar score is a better predictor of survival at 24 h than birthweight, and the best cut-off of Apgar score for survival at 24 h after birth is 4, with 96% sensitivity and 77%
specificity. The different proportion of \u201cnormal viable\u201d and \u201cless viable\u201d neonates in comparison to other studies
highlights that Chihuahua puppies born by elective Caesarean section should be carefully evaluated at birth to provide correct assistance
Different Characteristics of the Bright Branches of the Globular Clusters M3 and M13
We carried out wide-field BVI CCD photometric observations of the GCs M3 and
M13 using the BOAO 1.8 m telescope equipped with a 2K CCD. We present CMDs of
M3 and M13. We have found AGB bumps at V = 14.85 for M3 at V = 14.25 for M13.
It is found that AGB stars in M3 are more concentrated near the bump, while
those in M13 are scattered along the AGB sequence. We identified the RGB bump
of M3 at V = 15.50 and that of M13 at V = 14.80. We have estimated the ratios R
and R2 for M3 and M13 and found that of R for M3 is larger than that for M13
while R2's for M3 and M13 are similar when only normal HB stars are used in R
and R2 for M13. However, we found that R's for M3 and M13 are similar while R2
for M3 is larger than that for M13 when all the HB stars are included in R and
R2 for M13. We have compared the observed RGB LFs of M3 and M13 with the
theoretical RGB LF of Bergbusch & VandenBerg at the same radial distances from
the cluster centers as used in R and R2 for M3 and M13. We found "extra stars"
belonging to M13 in the comparison of the observed RGB LF of M13 and the
theoretical RGB LF of Bergbusch & VandenBerg. In the original definition of R
of Buzzoni et al., N(HB) corresponds to the lifetime of HB stars in the RR
Lyrae instability strip at log T_eff = 3.85. So, the smaller R value resulting
for M13 compared with that for M3 in the case where only normal HB stars are
included in R and R2 for M13 may be partially caused by "extra stars", and the
similar R's for M3 and M13 in the case where the all HB stars are included in R
and R2 for M13 may be caused by "extra stars" in the upper RGB of M13. If
"extra stars" in the upper RGB of M13 are caused by an effective "deep mixing"
these facts support the contention that an effective "deep mixing" could lead
to different HB morphologies between M3 and M13 and subsequent sequences.Comment: 24 pages, 7 figures, to be published in the A
The Star Formation History of the Carina Dwarf Galaxy
We have analyzed deep B and V photometry of the Carina dwarf spheroidal
reaching below the old main-sequence turnoff to about V = 25. Using simulated
color-magnitude diagrams to model a range of star formation scenarios, we have
extracted a detailed, global star formation history. Carina experienced three
significant episodes of star formation at about 15 Gyr, 7 Gyr, and 3 Gyr.
Contrary to the generic picture of galaxy evolution, however, the bulk of star
formation, at least 50%, occured during the episode 7 Gyr ago, which may have
lasted as long as 2 Gyr. For unknown reasons, Carina formed only 10-20% of its
stars at an ancient epoch and then remained quiescent for more than 4 Gyr. The
remainder (~30%) formed relatively recently, only 3 Gyr ago. Interest in the
local population of dwarf galaxies has increased lately due to their potential
importance in the understanding of faint galaxy counts. We surmise that objects
like Carina, which exhibits the most extreme episodic behavior of any of the
dwarf spheroidal companions to the Galaxy, are capable of contributing to the
observed excess of blue galaxies at B = 24 only if the star formation occurred
instantaneously.Comment: 23 pages of text, 20 figures, 8 tables. AJ, in pres
Influence of synaptic depression on memory storage capacity
Synaptic efficacy between neurons is known to change within a short time
scale dynamically. Neurophysiological experiments show that high-frequency
presynaptic inputs decrease synaptic efficacy between neurons. This phenomenon
is called synaptic depression, a short term synaptic plasticity. Many
researchers have investigated how the synaptic depression affects the memory
storage capacity. However, the noise has not been taken into consideration in
their analysis. By introducing "temperature", which controls the level of the
noise, into an update rule of neurons, we investigate the effects of synaptic
depression on the memory storage capacity in the presence of the noise. We
analytically compute the storage capacity by using a statistical mechanics
technique called Self Consistent Signal to Noise Analysis (SCSNA). We find that
the synaptic depression decreases the storage capacity in the case of finite
temperature in contrast to the case of the low temperature limit, where the
storage capacity does not change
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