2,327 research outputs found
Quiescent Radio Emission from Southern Late-type M Dwarfs and a Spectacular Radio Flare from the M8 Dwarf DENIS 1048-3956
We report the results of a radio monitoring program conducted at the
Australia Telescope Compact Array to search for quiescent and flaring emission
from seven nearby Southern late-type M and L dwarfs. Two late-type M dwarfs,
the M7 V LHS 3003 and the M8 V DENIS 1048-3956, were detected in quiescent
emission at 4.80 GHz. The observed emission is consistent with optically thin
gyrosynchrotron emission from mildly relativistic (~1-10 keV) electrons with
source densities n_e ~ 10 G magnetic fields. DENIS
1048-3956 was also detected in two spectacular, short-lived flares, one at 4.80
GHz (peak f_nu = 6.0+/-0.8 mJy) and one at 8.64 GHz (peak f_nu = 29.6+/-1.0
mJy) approximately 10 minutes later. The high brightness temperature (T_B >~
10^13 K), short emission period (~4-5 minutes), high circular polarization
(~100%), and apparently narrow spectral bandwidth of these events imply a
coherent emission process in a region of high electron density (n_e ~
10^11-10^12 cm^-3) and magnetic field strength (B ~ 1 kG). If the two flare
events are related, the apparent frequency drift in the emission suggests that
the emitting source either moved into regions of higher electron or magnetic
flux density; or was compressed, e.g., by twisting field lines or gas motions.
The quiescent fluxes from the radio-emitting M dwarfs violate the Gudel-Benz
empirical radio/X-ray relations, confirming a trend previously noted by Berger
et al. (abridged)Comment: 28 pages, 8 figures, accepted for publication in Ap
Awareness Of Racial And Ethnic Health Disparities Has Improved Only Modestly Over A Decade
Abstract available at publisher's web site.http://dx.doi.org/10.1377/hlthaff.2010.070
A broadband FFT spectrometer for radio and millimeter astronomy
The core architecture, tests in the lab and first results of a Fast Fourier
Transform (FFT) spectrometer are described. It is based on a commercially
available fast digital sampler (AC240) with an on-board Field Programmable Gate
Array (FPGA). The spectrometer works continuously and has a remarkable total
bandwidth of 1 GHz, resolved into 16384 channels. The data is sampled with 8
bits, yielding a dynamic range of 48 dB. An Allan time of more than 2000 s and
an SFDR of 37 dB were measured. First light observations with the KOSMA
telescope show a perfect spectrum without internal or external spurious
signals.Comment: Astronomy and Astrophysics, in pres
Electrically Switchable Photonic Molecule Laser
We have studied the coherent intercavity coupling of the evanescent fields of
the whispering gallery modes of two terahertz quantum-cascade lasers
implemented as microdisk cavities. The electrically pumped single-mode
operating microcavities allow to electrically control the coherent mode
coupling for proximity distances of the cavities up to 30-40 \mu\m. The optical
emission of the strongest coupled photonic molecule can be perfectly switched
by the electrical modulation of only one of the coupled microdisks. The
threshold characteristics of the strongest coupled photonic molecule
demonstrates the linear dependence of the gain of a quantum-cascade laser on
the applied electric field.Comment: 4 pages, 4 figure
Graphene microwave transistors on sapphire substrates
We have developed metal-oxide graphene field-effect transistors (MOGFETs) on
sapphire substrates working at microwave frequencies. For monolayers, we obtain
a transit frequency up to ~ 80 GHz for a gate length of 200 nm, and a power
gain maximum frequency of about ~ 3 GHz for this specific sample. Given the
strongly reduced charge noise for nanostructures on sapphire, the high
stability and high performance of this material at low temperature, our MOGFETs
on sapphire are well suited for a cryogenic broadband low-noise amplifier
Castor A and Castor B resolved in a simultaneous Chandra and XMM-Newton observation
We present a simultaneous Chandra and XMM-Newton observation of the Castor
sextett, focusing on Castor A and Castor B, two spectroscopic binaries with
early-type primaries. Of the present day X-ray instruments only Chandra can
isolate the X-ray lightcurves and spectra of A and B. We compare the Chandra
observation with XMM-Newton data obtained simultaneously. Albeit not able to
resolve Castor A and Castor B from each other, the higher sensitivity of
XMM-Newton allows for a quantitative analysis of their combined high-resolution
spectrum. He-like line triplets are used to examine the temperature and the
density in the corona of Castor AB. The temporal variability of Castor AB is
studied using data collected with the European Photon Imaging Camera onboard
XMM-Newton. Strong flare activity is observed, and combining the data acquired
simultaneously with Chandra and XMM-Newton each flare can be assigned to its
host. Our comparison with the conditions of the coronal plasma of other stars
shows that Castor AB behave like typical late-type coronal X-ray emitters
supporting the common notion that the late-type secondaries within each
spectroscopic binary are the sites of the X-ray production.Comment: accepted for publication in A&
Dynamics of Circumstellar Disks II: Heating and Cooling
We present a series of 2-d () hydrodynamic simulations of marginally
self gravitating disks around protostars using an SPH code. We implement simple
dynamical heating and we cool each location as a black body, using a
photosphere temperature obtained from the local vertical structure. We
synthesize SEDs from our simulations and compare them to fiducial SEDs derived
from observed systems. These simulations produce less distinct spiral structure
than isothermally evolved systems, especially in the inner third of the disk.
Pattern are similar further from the star but do not collapse into condensed
objects. The photosphere temperature is well fit to a power law in radius with
index , which is very steep. Far from the star, internal heating
( work and shocks) are not responsible for generating a large fraction of
the thermal energy contained in the disk matter. Gravitational torques
responsible for such shocks cannot transport mass and angular momentum
efficiently in the outer disk. Within 5--10 AU of the star, rapid break
up and reformation of spiral structure causes shocks, which provide sufficient
dissipation to power a larger fraction of the near IR energy output. The
spatial and size distribution of grains can have marked consequences on the
observed near IR SED and can lead to increased emission and variability on
year time scales. When grains are vaporized they do not reform
into a size distribution similar to that from which most opacity calculations
are based. With rapid grain reformation into the original size distribution,
the disk does not emit near infrared photons. With a plausible modification to
the opacity, it contributes much more.Comment: Accepted by ApJ, 60pg incl 24 figure
Collisional damping rates for plasma waves
The distinction between the plasma dynamics dominated by collisional
transport versus collective processes has never been rigorously addressed until
recently. A recent paper [Yoon et al., Phys. Rev. E 93, 033203 (2016)]
formulates for the first time, a unified kinetic theory in which collective
processes and collisional dynamics are systematically incorporated from first
principles. One of the outcomes of such a formalism is the rigorous derivation
of collisional damping rates for Langmuir and ion-acoustic waves, which can be
contrasted to the heuristic customary approach. However, the results are given
only in formal mathematical expressions. The present Brief Communication
numerically evaluates the rigorous collisional damping rates by considering the
case of plasma particles with Maxwellian velocity distribution function so as
to assess the consequence of the rigorous formalism in a quantitative manner.
Comparison with the heuristic ("Spitzer") formula shows that the accurate
damping rates are much lower in magnitude than the conventional expression,
which implies that the traditional approach over-estimates the importance of
attenuation of plasma waves by collisional relaxation process. Such a finding
may have a wide applicability ranging from laboratory to space and
astrophysical plasmas.Comment: 5 pages, 2 figures; Published in Physics of Plasmas, volume/Issue
23/6. Publisher: AIP Publishing LLC. Date: Jun 1, 2016. URL:
http://aip.scitation.org/doi/10.1063/1.4953802 Rights managed by AIP
Publishing LL
Constraining planet structure from stellar chemistry: the cases of CoRoT-7, Kepler-10, and Kepler-93
We explore the possibility that the stellar relative abundances of different
species can be used to constrain the bulk abundances of known transiting rocky
planets. We use high resolution spectra to derive stellar parameters and
chemical abundances for Fe, Si, Mg, O, and C in three stars hosting low mass,
rocky planets: CoRoT-7, Kepler-10, and Kepler-93. These planets follow the same
line along the mass-radius diagram, pointing toward a similar composition. The
derived abundance ratios are compared with the solar values. With a simple
stoichiometric model, we estimate the iron mass fraction in each planet,
assuming stellar composition. We show that in all cases, the iron mass fraction
inferred from the mass-radius relationship seems to be in good agreement with
the iron abundance derived from the host star's photospheric composition. The
results suggest that stellar abundances can be used to add constraints on the
composition of orbiting rocky planets.Comment: A&A Letters, in pres
Density functional study of the actinide nitrides
The full potential all electron linearized augmented plane wave plus local
orbitals (FP-LAPW + lo) method, as implemented in the suite of software WIEN2K,
has been used to systematically investigate the structural, electronic, and
magnetic properties of the actinide compounds AnN (An = Ac, Th, Pa, U, Np, Pu,
Am). The theoretical formalism used is the generalized gradient approximation
to density functional theory (GGA-DFT) with the Perdew-Burke-Ernzerhof (PBE)
exchange-correlation functional. Each compound has been studied at six levels
of theory: non-magnetic (NM), non-magnetic with spin-orbit coupling (NM+SOC),
ferromagnetic (FM), ferromagnetic with spin-orbit coupling (FM+SOC),
anti-ferromagnetic (AFM), and anti-ferromagnetic with spin-orbit coupling
(AFM+SOC). The structural parameters, bulk moduli, densities of states, and
charge distributions have been computed and compared to available experimental
data and other theoretical calculations published in the literature. The total
energy calculations indicate that the lowest energy structures of AcN, ThN, and
PaN are degenerate at the NM+SOC, FM+SOC, and AFM+SOC levels of theory with
vanishing total magnetic moments in the FM+SOC and AFM+SOC cases, making the
ground states essentially non-magnetic with spin-orbit interaction. The ground
states of UN, NpN, PuN, and AmN are found to be FM+SOC at the level of theory
used in the present computations. The nature of the interactions between the
actinide metals and nitrogen atom, and the implications on 5f electron
delocalization and localization are discussed in detail.Comment: 5 tables, 12 figure
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