1,205 research outputs found
Optical spectroscopy of candidate Alpha Persei white dwarfs
As part of an investigation into the high mass end of the initial mass-final
mass relation we performed a search for new white dwarf members of the nearby
(172.4 pc), young (80-90 Myr) Persei open star cluster. The
photometric and astrometric search using the UKIRT Infrared Deep Sky Survey and
SuperCOSMOS sky surveys discovered 14 new white dwarf candidates. We have
obtained medium resolution optical spectra of the brightest 11 candidates using
the William Herschel Telescope and confirmed that while 7 are DA white dwarfs,
3 are DB white dwarfs and one is an sdOB star, only three have cooling ages
within the cluster age, and from their position on the initial mass-final mass
relation, it is likely none are cluster members. This result is disappointing,
as recent work on the cluster mass function suggests that there should be at
least one white dwarf member, even at this young age. It may be that any white
dwarf members of Per are hidden within binary systems, as is the case
in the Hyades cluster, however the lack of high mass stars within the cluster
also makes this seem unlikely. One alternative is that a significant level of
detection incompleteness in the legacy optical image survey data at this
Galactic latitude has caused some white dwarf members to be overlooked. If this
is the case, Gaia will find them.Comment: 8 pages, 7 Figures, 3 Tables. Accepted for publication in MNRA
The population of hot subdwarf stars studied with Gaia II. The Gaia DR2 catalogue of hot subluminous stars
Based on data from the ESA Gaia Data Release 2 (DR2) and several
ground-based, multi-band photometry surveys we compiled an all-sky catalogue of
hot subluminous star candidates selected in Gaia DR2 by means of
colour, absolute magnitude and reduced proper motion cuts. We expect the
majority of the candidates to be hot subdwarf stars of spectral type B and O,
followed by blue horizontal branch stars of late B-type (HBB), hot post-AGB
stars, and central stars of planetary nebulae. The contamination by cooler
stars should be about . The catalogue is magnitude limited to Gaia
and covers the whole sky. Except within the Galactic plane
and LMC/SMC regions, we expect the catalogue to be almost complete up to about
. The main purpose of this catalogue is to serve as input
target list for the large-scale photometric and spectroscopic surveys which are
ongoing or scheduled to start in the coming years. In the long run, securing a
statistically significant sample of spectroscopically confirmed hot subluminous
stars is key to advance towards a more detailed understanding of the latest
stages of stellar evolution for single and binary stars.Comment: 13 pages, A&A, accepte
Resonance induced by repulsive interactions in a model of globally-coupled bistable systems
We show the existence of a competition-induced resonance effect for a generic
globally coupled bistable system. In particular, we demonstrate that the
response of the macroscopic variable to an external signal is optimal for a
particular proportion of repulsive links. Furthermore, we show that a resonance
also occurs for other system parameters, like the coupling strength and the
number of elements. We relate this resonance to the appearance of a multistable
region, and we predict the location of the resonance peaks, by a simple
spectral analysis of the Laplacian matrix
Modeling rhythmic patterns in the hippocampus
We investigate different dynamical regimes of neuronal network in the CA3
area of the hippocampus. The proposed neuronal circuit includes two fast- and
two slowly-spiking cells which are interconnected by means of dynamical
synapses. On the individual level, each neuron is modeled by FitzHugh-Nagumo
equations. Three basic rhythmic patterns are observed: gamma-rhythm in which
the fast neurons are uniformly spiking, theta-rhythm in which the individual
spikes are separated by quiet epochs, and theta/gamma rhythm with repeated
patches of spikes. We analyze the influence of asymmetry of synaptic strengths
on the synchronization in the network and demonstrate that strong asymmetry
reduces the variety of available dynamical states. The model network exhibits
multistability; this results in occurrence of hysteresis in dependence on the
conductances of individual connections. We show that switching between
different rhythmic patterns in the network depends on the degree of
synchronization between the slow cells.Comment: 10 pages, 9 figure
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