884 research outputs found
Photospheric and chromospheric activity in four young solar-type stars
We present a photometric and spectroscopic study of four G-K dwarfs, namely
HD 166, epsilon Eri, chi1 Ori and kappa1 Cet. In three cases, we find a clear
spatial association between photospheric and chromospheric active regions. For
chi1 Ori we do not find appreciable variations of photospheric temperature, and
chromospheric Halpha emission. We applied a spot/plage model to the observed
rotational modulation of temperature and flux to derive spot/plage parameters
and to reconstruct a rough three-dimensional map of the outer atmosphere of
kappa1 Cet, HD 166 and epsilon Eri.Comment: 12 pages, 3 tables, 9 figures. Submitted to Ap
Time-independent approximations for periodically driven systems with friction
The classical dynamics of a particle that is driven by a rapidly oscillating
potential (with frequency ) is studied. The motion is separated into a
slow part and a fast part that oscillates around the slow part. The motion of
the slow part is found to be described by a time-independent equation that is
derived as an expansion in orders of (in this paper terms to the
order are calculated explicitly). This time-independent equation
is used to calculate the attracting fixed points and their basins of
attraction. The results are found to be in excellent agreement with numerical
solutions of the original time-dependent problem.Comment: 5 pages, 4 figures. Revised version. Minor change
C IV fluxes from the Sun as a star, and the correlation with magnetic flux
A total of 144 C IV wavelength 1548 Solar Maximum Mission (SMM)-UVSP spectroheliograms of solar plages were analyzed, some of which are series of exposures of the same region on the same day. Also analyzed were the C IV wavelength 1551 rasters of plages and C IV wavelength 1548 rasters of the quiet sun. The sample contained data on 17 different plages, observed on 50 different days. The center-to-limb variations of the active regions show that the optical thickness effects in the C IV wavelength 1548 line can be neglected in the conversion from intensity to flux density. As expected for the nearly optically thin situation, the C IV wavelength 1548 line is twice as bright as the C IV 1551 line. The average C IV wavelength 1548 flux density for a quiet region is 2700 ergs/cm/s and, with surprisingly little scatter, 18,000 erg/cm/s for plages. The intensity histograms of rasters obtained at disk center can be separated into characteristic plage and quiet sun contributions with variable relative filling factors. The relationship between the C IV and magnetic flux densities for spatially resolved data is inferred to be almost the same, with only an additional factor of order unity in the constant of proportionality
Discovery of a massive supercluster system at
Superclusters are the largest relatively isolated systems in the cosmic web.
Using the SDSS BOSS survey we search for the largest superclusters in the
redshift range .
We generate a luminosity-density field smoothed over
to detect the large-scale over-density regions. Each individual over-density
region is defined as single supercluster in the survey. We define the
superclusters in the way that they are comparable with the superclusters found
in the SDSS main survey.
We found a system we call the BOSS Great Wall (BGW), which consists of two
walls with diameters 186 and 173 Mpc, and two other major superclusters
with diameters of 64 and 91 Mpc. As a whole, this system consists of
830 galaxies with the mean redshift 0.47. We estimate the total mass to be
approximately . The morphology of the
superclusters in the BGW system is similar to the morphology of the
superclusters in the Sloan Great Wall region.
The BGW is one of the most extended and massive system of superclusters yet
found in the Universe.Comment: 4 pages, accepted as a letter in A&
Flux- and volume-limited groups/clusters for the SDSS galaxies: catalogues and mass estimation
We provide flux-limited and volume-limited galaxy group and cluster
catalogues, based on the spectroscopic sample of the SDSS data release 10
galaxies. We used a modified friends-of-friends (FoF) method with a variable
linking length in the transverse and radial directions to identify as many
realistic groups as possible. The flux-limited catalogue incorporates galaxies
down to m_r = 17.77 mag. It includes 588193 galaxies and 82458 groups. The
volume-limited catalogues are complete for absolute magnitudes down to M_r =
-18.0, -18.5, -19.0, -19.5, -20.0, -20.5, and -21.0; the completeness is
achieved within different spatial volumes, respectively. Our analysis shows
that flux-limited and volume-limited group samples are well compatible to each
other, especially for the larger groups/clusters. Dynamical mass estimates,
based on radial velocity dispersions and group extent in the sky, are added to
the extracted groups. The catalogues can be accessed via http://cosmodb.to.ee
and the Strasbourg Astronomical Data Center (CDS).Comment: 16 pages, 18 figures, 2 tables, accepted for publication in A&
Wavelet analysis of the formation of the cosmic web
According to the modern cosmological paradigm galaxies and galaxy systems
form from tiny density perturbations generated during the very early phase of
the evolution of the Universe. Using numerical simulations we study the
evolution of phases of density perturbations of different scales to understand
the formation and evolution of the cosmic web. We apply the wavelet analysis to
follow the evolution of high-density regions (clusters and superclusters) of
the cosmic web. We show that the positions of maxima and minima of density
waves (their spatial phases) almost do not change during the evolution of the
structure. Positions of extrema of density perturbations are the more stable,
the larger is the wavelength of perturbations. Combining observational and
simulation data we conclude that the skeleton of the cosmic web was present
already in an early stage of structure evolution.Comment: 12 pages, 8 figures, revised versio
Semiclassical theory of a quantum pump
In a quantum charge pump, the periodic variation of two parameters that
affect the phase of the electronic wavefunction causes the flow of a direct
current. The operating mechanism of a quantum pump is based on quantum
interference, the phases of interfering amplitudes being modulated by the
external parameters. In a ballistic quantum dot, there is a minimum time before
which quantum interference can not occur: the Ehrenfest time. Here we calculate
the current pumped through a ballistic quantum dot when the Ehrenfest time is
comparable to the mean dwell time. Remarkably, we find that the pumped current
has a component that is not suppressed if the Ehrenfest time is much larger
than the mean dwell time.Comment: 14 pages, 8 figures. Revised version, minor change
An integrative approach based on probabilistic modelling and statistical inference for morpho-statistical characterization of astronomical data
This paper describes several applications in astronomy and cosmology that are
addressed using probabilistic modelling and statistical inference
A Blind Search for Magnetospheric Emissions from Planetary Companions to Nearby Solar-type Stars
This paper reports a blind search for magnetospheric emissions from planets
around nearby stars. Young stars are likely to have much stronger stellar winds
than the Sun, and because planetary magnetospheric emissions are powered by
stellar winds, stronger stellar winds may enhance the radio luminosity of any
orbiting planets. Using various stellar catalogs, we selected nearby stars (<~
30 pc) with relatively young age estimates (< 3 Gyr). We constructed different
samples from the stellar catalogs, finding between 100 and several hundred
stars. We stacked images from the 74-MHz (4-m wavelength) VLA Low-frequency Sky
Survey (VLSS), obtaining 3\sigma limits on planetary emission in the stacked
images of between 10 and 33 mJy. These flux density limits correspond to
average planetary luminosities less than 5--10 x 10^{23} erg/s. Using recent
models for the scaling of stellar wind velocity, density, and magnetic field
with stellar age, we estimate scaling factors for the strength of stellar
winds, relative to the Sun, in our samples. The typical kinetic energy carried
by the stellar winds in our samples is 15--50 times larger than that of the
Sun, and the typical magnetic energy is 5--10 times larger. If we assume that
every star is orbited by a Jupiter-like planet with a luminosity larger than
that of the Jovian decametric radiation by the above factors, our limits on
planetary luminosities from the stacking analysis are likely to be a factor of
10--100 above what would be required to detect the planets in a statistical
sense. Similar statistical analyses with observations by future instruments,
such as the Low Frequency Array (LOFAR) and the Long Wavelength Array (LWA),
offer the promise of improvements by factors of 10--100.Comment: 11 pages; AASTeX; accepted for publication in A
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