569 research outputs found
Space station WP-04 power system. Volume 2: Study results
Results of the phase B study contract for the definition of the space station Electric Power System (EPS) are presented in detail along with backup information and supporting data. Systems analysis and trades, preliminary design, advanced development, customer accommodations, operations planning, product assurance, and design and development phase planning are addressed. The station design is a hybrid approach which provides user power of 25 kWe from the photovoltaic subsystem and 50 kWe from the solar dynamic subsystem. The electric power is distributed to users as a utility service; single phase at a frequency of 20 kHz and voltage of 440VAC. The solar array NiH2 batteries of the photovoltaic subsystem are based on commonality to those used on the co-orbiting and solar platforms
Space station WP-04 power system. Volume 1: Executive summary
Major study activities and results of the phase B study contract for the preliminary design of the space station Electrical Power System (EPS) are summarized. The areas addressed include the general system design, man-tended option, automation and robotics, evolutionary growth, software development environment, advanced development, customer accommodations, operations planning, product assurance, and design and development phase planning. The EPS consists of a combination photovoltaic and solar dynamic power generation subsystem and a power management and distribution (PMAD) subsystem. System trade studies and costing activities are also summarized
Electron beam induced radio emission from ultracool dwarfs
We present the numerical simulations for an electron-beam-driven and
loss-cone-driven electron-cyclotron maser (ECM) with different plasma
parameters and different magnetic field strengths for a relatively small region
and short time-scale in an attempt to interpret the recent discovered intense
radio emission from ultracool dwarfs. We find that a large amount of
electromagnetic field energy can be effectively released from the beam-driven
ECM, which rapidly heats the surrounding plasma. A rapidly developed
high-energy tail of electrons in velocity space (resulting from the heating
process of the ECM) may produce the radio continuum depending on the initial
strength of the external magnetic field and the electron beam current. Both
significant linear polarization and circular polarization of electromagnetic
waves can be obtained from the simulations. The spectral energy distributions
of the simulated radio waves show that harmonics may appear from 10 to
70 ( is the electron plasma frequency) in the
non-relativistic case and from 10 to 600 in the relativistic
case, which makes it difficult to find the fundamental cyclotron frequency in
the observed radio frequencies. A wide frequency band should therefore be
covered by future radio observations.Comment: 10 pages, 19 figures, accepted for publication in the Astrophysical
Journa
Mapping radio emitting-region on low-mass stars and brown dwarfs
Strong magnetic activity in ultracool dwarfs (UCDs, spectral classes later than M7) have emerged from a number of radio observations, including the periodic beams. The highly (up to 100%) circularly polarized nature of the emission point to an effective amplification mechanism of the high-frequency electromagnetic waves – the electron cyclotron maser (ECM) instability. Several anisotropic velocity distibution models of electrons, including the horseshoe distribution, ring shell distribution and the loss-cone distribution, are able to generate the ECM instability. A magnetic-field-aligned electric potential would play an significant role in the ECM process. We are developing a theoretical model in order to simulate ECM and apply this model to map the radio-emitting region on low-mass stars and brown dwarfs
Sporadic Long-term Variability in Radio Activity from a Brown Dwarf
Radio activity has been observed in a large variety of stellar objects,
including in the last few years, ultra-cool dwarfs. To explore the extent of
long-term radio activity in ultra-cool dwarfs, we use data taken over an
extended period of 9 hr from the Very Large Array of the source 2MASS
J05233822-1403022 in September 2006, plus data taken in 2004. The observation
taken in September 2006 failed to detect any radio activity at 8.46 GHz. A
closer inspection of earlier data reveals that the source varied from a null
detection on 3 May 2004, to 95 Jy on 17 May 2004, to 230 Jy
on 18 June 2004. The lack of detection in September 2006 suggests at least a
factor of ten flux variability at 8.46 GHz. Three short photometric runs did
not reveal any optical variability. In addition to the observed pulsing nature
of the radio flux from another ultra-cool source, the present observations
suggests that ultra-cool dwarfs may not just be pulsing but can also display
long-term sporadic variability in their levels of quiescent radio emission. The
lack of optical photometric variability suggests an absence of large-scale
spots at the time of the latest VLA observations, although small very high
latitude spots combined with a low inclination could cause very low amplitude
rotational modulation which may not be measurable. We discuss this large
variability in the radio emission within the context of both gyrosynchrotron
emission and the electron-cyclotron maser, favoring the latter mechanism.Comment: 7 pages, 2 figures, 1 table, accepted for publication in A&A Letter
The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science case, survey design and initial results
The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution (≈2.”5), sensitivity (a 1σ goal of 70 μJy/beam in the coadded data), full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2–4 GHz). The first observations began in 2017 September, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hr of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (decl. > −40°), a total of 33 885 deg². The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an "on the fly" interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations
Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf
[Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical
observation of the L dwarf binary 2MASSW J0746+20. We detect strong radio
emission, dominated by short-duration periodic pulses at 4.86 GHz with
P=124.32+/-0.11 min. The stability of the pulse profiles and arrival times
demonstrates that they are due to the rotational modulation of a B~1.7 kG
magnetic field. A quiescent non-variable component is also detected, likely due
to emission from a uniform large-scale field. The H-alpha emission exhibits
identical periodicity, but unlike the radio pulses it varies sinusoidally and
is offset by exactly 1/4 of a phase. The sinusoidal variations require
chromospheric emission from a large-scale field structure, with the radio
pulses likely emanating from the magnetic poles. While both light curves can be
explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out
a symmetric dipole topology since it would result in a phase lag of 1/2
(poloidal field) or zero (toroidal field). We therefore conclude that either
(i) the field is dominated by a quadrupole configuration, which can naturally
explain the 1/4 phase lag; or (ii) the H-alpha and/or radio emission regions
are not trivially aligned with the field. Regardless of the field topology, we
use the measured period along with the known rotation velocity (vsini=27 km/s),
and the binary orbital inclination (i=142 deg), to derive a radius for the
primary star of 0.078+/-0.010 R_sun. This is the first measurement of the
radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides
a constraint on the mass-radius relation below 0.1 M_sun. We find that the
radius is about 30% smaller than expected from theoretical models, even for an
age of a few Gyr.Comment: Submitted to Ap
A Mini-survey of Ultracool Dwarfs at 4.9 GHz
A selection of ultracool dwarfs are known to be radio active, with both
gyrosynchrotron emission and the electron cyclotron maser instability being
given as likely emission mechanisms. To explore whether ultracool dwarfs
previously undetected at 8.5 GHz may be detectable at a lower frequency. We
select a sample of fast rotating ultracool dwarfs with no detectable radio
activity at 8.5 GHz, observing each of them at 4.9 GHz. From the 8 dwarfs in
our sample, we detect emission from 2MASS J07464256+2000321, with a mean flux
level of 286 24 . The light-curve of 2MASS J07464256+2000321, is
dominated towards the end of the observation by a very bright, 100 %
left circularly polarized burst during which the flux reached 2.4 mJy. The
burst was preceded by a raise in the level of activity, with the average flux
being 160 in the first hour of observation rising to
400 in the 40 minutes before the burst. During both periods,
there is significant variability. The detection of 100% circular polarization
in the emission at 4.9 GHz points towards the electron cyclotron maser as the
emission mechanism. However, the observations at 4.9 GHz and 8.5 GHz were not
simultaneous, thus the actual fraction of dwarfs capable of producing radio
emission, as well as the fraction of those that show periodic pulsations is
still unclear, as indeed are the relative roles played by the electron
cyclotron maser instability versus gyrosynchrotron emission, therefore we
cannot assert if the previous non-detection at 8.5 GHz was due to a cut-off in
emission between 4.9 and 8.4 GHz, or due to long term variability
First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
We report the first direct detection of a strong, 5 kG magnetic field on the
surface of an active brown dwarf. LSR J1835+3259 is an M8.5 dwarf exhibiting
transient radio and optical emission bursts modulated by fast rotation. We have
detected the surface magnetic field as circularly polarized signatures in the
819 nm sodium lines when an active emission region faced the Earth. Modeling
Stokes profiles of these lines reveals the effective temperature of 2800 K and
log gravity acceleration of 4.5. These parameters place LSR J1835+3259 on
evolutionary tracks as a young brown dwarf with the mass of 554 M and age of 224 Myr. Its magnetic field is at least 5.1 kG and covers
at least 11% of the visible hemisphere. The active region topology recovered
using line profile inversions comprises hot plasma loops with a vertical
stratification of optical and radio emission sources. These loops rotate with
the dwarf in and out of view causing periodic emission bursts. The magnetic
field is detected at the base of the loops. This is the first time that we can
quantitatively associate brown dwarf non-thermal bursts with a strong, 5 kG
surface magnetic field and solve the puzzle of their driving mechanism. This is
also the coolest known dwarf with such a strong surface magnetic field. The
young age of LSR J1835+3259 implies that it may still maintain a disk, which
may facilitate bursts via magnetospheric accretion, like in higher-mass T
Tau-type stars. Our results pave a path toward magnetic studies of brown dwarfs
and hot Jupiters.Comment: ApJ, in pres
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