137 research outputs found
Out-of-plane fluctuation conductivity of layered superconductors in strong electric fields
The non-Ohmic effect of a high electric field on the out-of-plane
magneto-conductivity of a layered superconductor near the superconducting
transition is studied in the frame of the Langevin approach to the
time-dependent Ginzburg-Landau equation. The transverse fluctuation
conductivity is computed in the self-consistent Hartree approximation for an
arbitrarily strong electric field and a magnetic field perpendicular to the
layers. Our results indicate that high electric fields can be effectively used
to suppress the out-of-plane fluctuation conductivity in high-temperature
superconductors and a significant broadening of the transition induced by a
strong electric field is predicted. Extensions of the results are provided for
the case when the electric field is applied at an arbitrary angle with respect
to the layers, as well as for the three-dimensional anisotropic regime of a
strong interlayer coupling.Comment: to be published in Phys. Rev.
Debris Disks around Solar-Type Stars: Observations of the Pleiades with Spitzer Space Telescope
We present Spitzer MIPS observations at 24 um of 37 solar-type stars in the
Pleiades and combine them with previous observations to obtain a sample of 71
stars. We report that 23 stars, or 32 +/- 6.8%, have excesses at 24 um at least
10% above their photospheric emission. We compare our results with studies of
debris disks in other open clusters and with a study of A stars to show that
debris disks around solar-type stars at 115 Myr occur at nearly the same rate
as around A-type stars. We analyze the effects of binarity and X-ray activity
on the excess flux. Stars with warm excesses tend not to be in equal-mass
binary systems, possibly due to clearing of planetesimals by binary companions
in similar orbits. We find that the apparent anti-correlations in the incidence
of excess and both the rate of stellar rotation and also the level of activity
as judged by X-ray emission are statistically weak.Comment: 34 pages; accepted for publication in ApJ; new version included
corrections of typos, etc to match published versio
Gravity Indicators in the Near-Infrared Spectra of Brown Dwarfs
We investigate the sensitivity to temperature and gravity of the strong
absorption features in the J- and K-band spectra of substellar objects. We
compare the spectra of giants and young M dwarfs (of low gravity) to field M
and L dwarfs (of high gravity) and to model spectra from the Lyon group. We
find that low-resolution spectra of M4 - M9 stars and young brown dwarfs at R ~
350 and S/N > 70 can determine the spectral type to a precision of pm 1
subtype, using the H2O and CO bands, and can measure the surface gravity to pm
0.5 dex, using the atomic lines of KI and NaI. This result points toward the
development of photometric spectral indices to separate low-mass members from
foreground and background objects in young clusters and associations. We also
emphasize the complexity of the interpretation of the empirical quantities
(e.g., spectral types) in terms of the physical variables (e.g., temperature,
opacities) in the cool atmospheres of young brown dwarfs.Comment: Accepted to ApJ, 44 pages & 10 figures, preliminary results reported
in astro-ph/020822
Spitzer 24 micron Survey of Debris Disks in the Pleiades
We performed a 24 micron 2 Deg X 1 Deg survey of the Pleiades cluster, using
the MIPS instrument on Spitzer. Fifty four members ranging in spectral type
from B8 to K6 show 24 micron fluxes consistent with bare photospheres. All Be
stars show excesses attributed to free-free emission in their gaseous
envelopes. Five early-type stars and four solar-type stars show excesses
indicative of debris disks. We find a debris disk fraction of 25 % for B-A
members and 10 % for F-K3 ones. These fractions appear intermediate between
those for younger clusters and for the older field stars. They indicate a decay
with age of the frequency of the dust-production events inside the planetary
zone, with similar time scales for solar-mass stars as have been found
previously for A-stars.Comment: accepted to Ap
Near and Mid-IR Photometry of the Pleiades, and a New List of Substellar Candidate Members
We make use of new near and mid-IR photometry of the Pleiades cluster in
order to help identify proposed cluster members. We also use the new photometry
with previously published photometry to define the single-star main sequence
locus at the age of the Pleiades in a variety of color-magnitude planes.
The new near and mid-IR photometry extend effectively two magnitudes deeper
than the 2MASS All-Sky Point Source catalog, and hence allow us to select a new
set of candidate very low mass and sub-stellar mass members of the Pleiades in
the central square degree of the cluster. We identify 42 new candidate members
fainter than Ks =14 (corresponding to 0.1 Mo). These candidate members should
eventually allow a better estimate of the cluster mass function to be made down
to of order 0.04 solar masses.
We also use new IRAC data, in particular the images obtained at 8 um, in
order to comment briefly on interstellar dust in and near the Pleiades. We
confirm, as expected, that -- with one exception -- a sample of low mass stars
recently identified as having 24 um excesses due to debris disks do not have
significant excesses at IRAC wavelengths. However, evidence is also presented
that several of the Pleiades high mass stars are found to be impacting with
local condensations of the molecular cloud that is passing through the Pleiades
at the current epoch.Comment: Accepted to ApJS; data tables and embedded-figure version available
at http://spider.ipac.caltech.edu/staff/stauffer/pleiades07
Peculiarities and variations in the optical spectrum of the post-AGB star V448Lac=IRAS22223+4327
Repeated observations with high spectral resolution acquired in 1998-2008 are
used to study the temporal behavior of the spectral line profiles and velocity
field in the atmosphere and circumstellar envelope of the post-AGB star
V448Lac. Asymmetry of the profiles of the strongest absorption lines with
low-level excitation potentials less 1eV and time variations of these profiles
have been detected, most prominently the profiles of the resonance lines of
BaII, YII, LaII, SiII. The peculiarity of these profiles can be explained using
a superposition of stellar absorption line and shell emission lines. Emission
in the (0;1) 5635A Swan band of the C2 molecule has been detected in the
spectrum of V448Lac for the first time. The core of the Halpha line displays
radial velocity variations with an amplitude ~8 km/s. Radial velocity
variations displayed by weakest metallic lines with lower amplitudes, 1-2 km/s,
may be due to atmospheric pulsations. Differential line shifts, 0 -- 8 km/s,
have been detected on various dates. The position of the molecular spectrum is
stationary in time, indicating a constant expansion velocity of the
circumstellar shell, Vexp=15.2 km/s, as derived from the C2 and NaI lines.Comment: 19 pages, 8 figures, 1 tabl
Monte Carlo calculation of the current-voltage characteristics of a two dimensional lattice Coulomb gas
We have studied the nonlinear current-voltage characteristic of a two
dimensional lattice Coulomb gas by Monte Carlo simulation. We present three
different determinations of the power-law exponent of the nonlinear
current-voltage characteristic, . The determinations rely on
both equilibrium and non-equilibrium simulations. We find good agreement
between the different determinations, and our results also agree closely with
experimental results for Hg-Xe thin film superconductors and for certain single
crystal thin-film high temperature superconductors.Comment: late
Altered Behaviour, Dopamine and Norepinephrine Regulation in Stressed Mice Heterozygous in TPH2 Gene
Gene-environment interaction (GxE) determines the vulnerability of an individual to a spectrum of stress-related neuropsychiatric disorders. Increased impulsivity, excessive aggression, and other behavioural characteristics are associated with variants within the tryptophan hydroxylase-2 (Tph2) gene, a key enzyme in brain serotonin synthesis. This phenotype is recapitulated in naïve mice with complete, but not with partial Tph2 inactivation. Tph2 haploinsufficiency in animals reflects allelic variation of Tph2 facilitating the elucidation of respective GxE mechanisms. Recently, we showed excessive aggression and altered serotonin brain metabolism in heterozygous Tph2-deficient male mice (Tph2+/−) after predator stress exposure. Here, we sought to extend these studies by investigating aggressive and anxiety-like behaviours, sociability, and the brain metabolism of dopamine and noradrenaline. Separately, Tph2+/− mice were examined for exploration activity in a novel environment and for the potentiation of helplessness in the modified swim test (ModFST). Predation stress procedure increased measures of aggression, dominancy, and suppressed sociability in Tph2+/− mice, which was the opposite of that observed in control mice. Anxiety-like behaviour was unaltered in the mutants and elevated in controls. Tph2+/− mice exposed to environmental novelty or to the ModFST exhibited increased novelty exploration and no increase in floating behaviour compared to controls, which is suggestive of resilience to stress and despair. High-performance liquid chromatography (HPLC) revealed significant genotype-dependent differences in the metabolism of dopamine, and norepinephrine within the brain tissue. In conclusion, environmentally challenged Tph2+/− mice exhibit behaviours that resemble the behaviour of non-stressed null mutants, which reveals how GxE interaction studies can unmask latent genetically determined predispositions. © 2020 The Authors.The authors' work reported here was supported by Deutsche Forschungsgemeinschaft (DFG:CRC TRR58A1/A5), DAAD (to ES), the European Union's Seventh Framework Programme (FP7/2007–2013) under Grant No.602805 (Aggressotype) and the Horizon 2020 Research and Innovation Programme under Grant No.728018 (Eat2beNICE) (to KPL and TS) and the President's program of PhD Exchange of RF-2017 (to TS and DA). We appreciate the valuable technical help of Natalia Bazhenova, Drs. Alexander Trofimov and Natalia Markova with this project
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