4,863 research outputs found
Theory of coupled neuronal-synaptic dynamics
In neural circuits, synaptic strengths influence neuronal activity by shaping
network dynamics, and neuronal activity influences synaptic strengths through
activity-dependent plasticity. Motivated by this fact, we study a
recurrent-network model in which neuronal units and synaptic couplings are
interacting dynamic variables, with couplings subject to Hebbian modification
with decay around quenched random strengths. Rather than assigning a specific
role to the plasticity, we use dynamical mean-field theory and other techniques
to systematically characterize the neuronal-synaptic dynamics, revealing a rich
phase diagram. Adding Hebbian plasticity slows activity in chaotic networks and
can induce chaos in otherwise quiescent networks. Anti-Hebbian plasticity
quickens activity and produces an oscillatory component. Analysis of the
Jacobian shows that Hebbian and anti-Hebbian plasticity push locally unstable
modes toward the real and imaginary axes, explaining these behaviors. Both
random-matrix and Lyapunov analysis show that strong Hebbian plasticity
segregates network timescales into two bands with a slow, synapse-dominated
band driving the dynamics, suggesting a flipped view of the network as synapses
connected by neurons. For increasing strength, Hebbian plasticity initially
raises the complexity of the dynamics, measured by the maximum Lyapunov
exponent and attractor dimension, but then decreases these metrics, likely due
to the proliferation of stable fixed points. We compute the marginally stable
spectra of such fixed points as well as their number, showing exponential
growth with network size. In chaotic states with strong Hebbian plasticity, a
stable fixed point of neuronal dynamics is destabilized by synaptic dynamics,
allowing any neuronal state to be stored as a stable fixed point by halting the
plasticity. This phase of freezable chaos offers a new mechanism for working
memory.Comment: 20 pages, 9 figure
Prospects for joint radio telescope and gravitational wave searches for astrophysical transients
The radio skies remain mostly unobserved when it comes to transient
phenomena. The direct detection of gravitational waves will mark a major
milestone of modern astronomy, as an entirely new window will open on the
universe. Two apparently independent phenomena can be brought together in a
coincident effort that has the potential to boost both searches. In this paper
we will outline the scientific case that stands behind these future joint
observations and will describe the methods that might be used to conduct the
searches and analyze the data. The targeted sources are binary systems of
compact objects, known to be strong candidate sources for gravitational waves.
Detection of transients coincident in these two channels would be a significant
smoking gun for first direct detection of gravitational waves, and would open
up a new field for characterization of astrophysical transients involving
massive compact objects.Comment: 12 pages, Amaldi 8 Conference (New York, 2009) proceedings pape
On the nature of candidate luminous blue variables in M33
Luminous blue variables (LBVs) are expected to play an important role in
massive stellar evolution as well as being the progenitors of some of the most
luminous supernovae known. In this paper we provide a multiwavelength study of
the population of (candidate) LBVs identified within M33. New spectra provide
an observational baseline of >4yr with respect to published data, which is well
suited to identifying LBV outbursts. Multi-epoch optical and mid-IR surveys of
M33 further constrain the variability of the sample and permit a search for
dusty circumstellar ejecta. Spectroscopic and photometric variability appears
common amongst the sample, although in many cases further observations will be
required to determine its physical origin. Nevertheless, we report a new
outburst of M33 Var C, while the transition of the WNLh star B517 to a cooler B
supergiant phase between 1993-2010 confirms an LBV classification.
Proof-of-concept quantitative analysis is provided for Romano's star; the
results being consistent with the finding that its bolometric luminosity varies
during its LBV excursions. The combination of the temperature and luminosity of
two stars, the B hypergiant [HS80] 110A and the cool hypergiant B324, appears
to be in violation of the empirical Humphreys-Davidson limit. Mid-IR
observations demonstrate that a number of candidates appear associated with hot
circumstellar dust, although no objects as extreme as Eta Carinae are
identified. The combined multiwavelength dataset suggests that the population
of LBVs studied is contaminated by stars demonstrating the B[e] phenomenon. Of
these, a subset of optically faint, low luminosity stars associated with hot
dust are of particular interest since they appear similar to the likely
progenitors of SN 2008S and the 2008 NGC300 transient, albeit suffering less
intrinsic extinction. [ABRIDGED]Comment: 23 pages, 14 figures, accepted for publication in Astronomy and
Astrophysic
Continuous-wave gravitational radiation from pulsar glitch recovery
Nonaxisymmetric, meridional circulation inside a neutron star, excited by a
glitch and persisting throughout the post-glitch relaxation phase, emits
gravitational radiation. Here, it is shown that the current quadrupole
contributes more strongly to the gravitational wave signal than the mass
quadrupole evaluated in previous work. We calculate the signal-to-noise ratio
for a coherent search and conclude that a large glitch may be detectable by
second-generation interferometers like the Laser Interferometer
Gravitational-Wave Observatory. It is shown that the viscosity and
compressibility of bulk nuclear matter, as well as the stratification
length-scale and inclination angle of the star, can be inferred from a
gravitational wave detection in principle.Comment: 19 pages, 4 figures, accepted for publication in MNRA
A multi-wavelength census of star formation activity in the young embedded cluster around Serpens/G3-G6
Aims. The aim of this paper is to characterise the star formation activity in
the poorly studied embedded cluster Serpens/G3-G6, located ~ 45' (3 pc) to the
south of the Serpens Cloud Core, and to determine the luminosity and mass
functions of its population of Young Stellar Objects (YSOs).
Methods. Multi-wavelength broadband photometry was obtained to sample the
near and mid-IR spectral energy distributions to separate YSOs from field stars
and classify the YSO evolutionary stage. ISOCAM mapping in the two filters LW2
(5-8.5 um) and LW3 (12-18 um) of a 19' x 16' field was combined with JHKs data
from 2MASS, Ks data from Arnica/NOT, and L' data from SIRCA/NOT. Continuum
emission at 1.3 mm (IRAM) and 3.6 cm (VLA) was mapped to study the cloud
structure and the coldest/youngest sources. Deep narrow band imaging at the
2.12 um S(1) line of H2 from NOTCam/NOT was obtained to search for signs of
bipolar outflows.
Results. We have strong evidence for a stellar population of 31 Class II
sources, 5 flat-spectrum sources, 5 Class I sources, and two Class 0 sources.
Our method does not sample the Class III sources. The cloud is composed of two
main dense clumps aligned along a ridge over ~ 0.5 pc plus a starless core
coinciding with absorption features seen in the ISOCAM maps. We find two
S-shaped bipolar collimated flows embedded in the NE clump, and propose the two
driving sources to be a Class 0 candidate (MMS3) and a double Class I (MMS2).
For the Class II population we find a best age of ~ 2 Myr and compatibility
with recent Initial Mass Functions (IMFs) by comparing the observed Class II
luminosity function (LF), which is complete to 0.08 L_sun, to various model LFs
with different star formation scenarios and input IMFs.Comment: 18 pages, 16 figures, 3 online tables, accepted by A&
Response to novel objects and foraging tasks by common marmoset (Callithrix Jacchus) female Pairs
Many studies have shown that environmental enrichment can significantly improve the psychological well-being of captive primates, increasing the occurrence of explorative behavior and thus reducing boredom. The response of primates to enrichment devices may be affected by many factors such as species, sex, age, personality and social context. Environmental enrichment is particularly important for social primates living in unnatural social groupings (i.e. same-sex pairs or singly housed animals), who have very few, or no, benefits from the presence of social companions in addition to all the problems related to captivity (e.g. increased inactivity). This study analyses the effects of enrichment devices (i.e. novel objects and foraging tasks) on the behavior of common marmoset (Callithrix jacchus) female pairs, a species that usually lives in family groups. It aims to determine which aspects of an enrichment device are more likely to elicit explorative behaviors, and how aggressive and stress-related behaviors are affected by its presence. Overall, the marmosets explored foraging tasks significantly longer than novel objects. The type of object, which varied in size, shape and aural responsiveness (i.e. they made a noise when the monkey touched them), did not affect the response of the monkeys, but they explored objects that were placed higher in the enclosure more than those placed lower down.Younger monkeys were more attracted to the enrichment devices than the older ones. Finally, stress-related behavior (i.e. scratching) significantly decreased when the monkeys were presented with the objects; aggressive behavior as unaffected. This study supports the importance of environmental enrichment for captive primates and shows that in marmosets its effectiveness strongly depends upon the height of the device in the enclosure and the presence of hidden food. The findings can be explained ifone considers the foraging behavior of wild common marmosets. Broader applications for the research findings are suggested in relation to enrichment
On the nature of the galactic early-B hypergiants
Despite their importance to a number of astrophysical fields, the lifecycles
of very massive stars are still poorly defined. In order to address this
shortcoming, we present a detailed quantitative study of the physical
properties of four early-B hypergiants (BHGs); Cyg OB2 #12, zeta Sco, HD190603
and BP Cru. These are combined with an analysis of their long-term
spectroscopic and photometric behaviour in order to determine their
evolutionary status. The long-term datasets revealed that they are remarkably
stable over long periods (>40yr), with the possible exception of zeta Sco prior
to the 20th century, in contrast to the typical excursions that characterise
luminous blue variables (LBVs). Zeta Sco, HD190603 and BP Cru possess physical
properties intermediate between B supergiants and LBVs; we therefore suggest
that BHGs are the immediate descendants and progenitors (respectively) of such
stars (for initial masses in the range ~30-60Msun). In contrast, while the wind
properties of Cyg OB2 #12 are consistent with this hypothesis, the combination
of extreme luminosity and spectroscopic mass (~110Msun) and comparatively low
temperature means it cannot be accommodated in such a scheme. Likewise, despite
its co-location with several LBVs above the Humphreys-Davidson (HD) limit, the
lack of long term variability and its unevolved chemistry apparently excludes
such an identification. Since such massive stars are not expected to evolve to
such cool temperatures, the properties of Cyg OB2 #12 are difficult to
understand under current evolutionary paradigms. [ABRIDGED]Comment: 36 pages, 19 figures (of which 17 pages are online supplemental
material). Accepted for publication in Astronomy and Astrophysic
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