378 research outputs found
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
In-flight calibration of STEREO-B/WAVES antenna system
The STEREO/WAVES (SWAVES) experiment on board the two STEREO spacecraft
(Solar Terrestrial Relations Observatory) launched on 25 October 2006 is
dedicated to the measurement of the radio spectrum at frequencies between a few
kilohertz and 16 MHz. The SWAVES antenna system consists of 6 m long orthogonal
monopoles designed to measure the electric component of the radio waves. With
this configuration direction finding of radio sources and polarimetry (analysis
of the polarization state) of incident radio waves is possible. For the
evaluation of the SWAVES data the receiving properties of the antennas,
distorted by the radiation coupling with the spacecraft body and other onboard
devices, have to be known accurately. In the present context, these properties
are described by the antenna effective length vectors. We present the results
of an in-flight calibration of the SWAVES antennas using the observations of
the nonthermal terrestrial auroral kilometric radiation (AKR) during STEREO
roll maneuvers in an early stage of the mission. A least squares method
combined with a genetic algorithm was applied to find the effective length
vectors of the STEREO Behind (STEREO-B)/WAVES antennas in a quasi-static
frequency range () which fit best to the model
and observed AKR intensity profiles. The obtained results confirm the former
SWAVES antenna analysis by rheometry and numerical simulations. A final set of
antenna parameters is recommended as a basis for evaluations of the SWAVES
data
Polarised radio pulsations from a new T dwarf binary
Brown dwarfs display Jupiter-like auroral phenomena such as magnetospheric
H emission and coherent radio emission. Coherent radio emission is a
probe of magnetospheric acceleration mechanisms and provides a direct
measurement of the magnetic field strength at the emitter's location, both of
which are difficult to access by other means. Observations of the coldest brown
dwarfs (spectral types T and Y) are particularly interesting as their
magnetospheric phenomena may be very similar to those in gas-giant exoplanets.
Here we present 144 MHz radio and infrared adaptive optics observations of the
brown dwarf WISEP J101905.63+652954.2 made using the LOFAR and Keck telescopes
respectively. The radio data shows pulsed highly circularly polarised emission
which yields a rotation rate of hr. The infrared imaging
reveals the source to be a binary with a projected separation of
mas between components of spectral type T5. and T7.. With a
simple "toy model" we show that the radio emission can in principle be powered
by the interaction between the two dwarfs with a mass-loss rates of at least
times the Jovian value. WISEP J101905.63+652954.2 is interesting because
it is the first pulsed methane dwarf detected in a low radio-frequency search.
Unlike previous gigahertz-frequency searches that were only sensitive to
objects with kiloGauss fields, our low-frequency search is sensitive to surface
magnetic fields of Gauss and above which might reveal the coldest
radio-loud objects down to planetary mass-scales.Comment: Accepted for publication in A&
Magnetospheric Emission from Extrasolar Planets
The magnetospheric emissions from extrasolar planets represent a science
frontier for the next decade. All of the solar system giant planets and the
Earth produce radio emissions as a result of interactions between their
magnetic fields and the solar wind. In the case of the Earth, its magnetic
field may contribute to its habitability by protecting its atmosphere from
solar wind erosion and by preventing energetic particles from reaching its
surface. Indirect evidence for at least some extrasolar giant planets also
having magnetic fields includes the modulation of emission lines of their host
stars phased with the planetary orbits, likely due to interactions between the
stellar and planetary magnetic fields. If magnetic fields are a generic
property of giant planets, then extrasolar giant planets should emit at radio
wavelengths allowing for their direct detection. Existing observations place
limits comparable to the flux densities expected from the strongest emissions.
Additional sensitivity at low radio frequencies coupled with algorithmic
improvements likely will enable a new means of detection and characterization
of extrasolar planets within the next decade.Comment: Science white paper for Astro2010; submitted to PSF pane
MOVES – I. The evolving magnetic field of the planet-hosting star HD189733
HD189733 is an active K dwarf that is, with its transiting hot Jupiter, among the most studied exoplanetary systems. In this first paper of the Multiwavelength Observations of an eVaporating Exoplanet and its Star (MOVES) programme, we present a 2-yr monitoring of the large-scale magnetic field of HD189733. The magnetic maps are reconstructed for five epochs of observations, namely 2013 June–July, 2013 August, 2013 September, 2014 September and 2015 July, using Zeeman–Doppler imaging. We show that the field evolves along the five epochs, with mean values of the total magnetic field of 36, 41, 42, 32 and 37 G, respectively. All epochs show a toroidally dominated field. Using previously published data of Moutou et al. and Fares et al., we are able to study the evolution of the magnetic field over 9 yr, one of the longest monitoring campaigns for a given star. While the field evolved during the observed epochs, no polarity switch of the poles was observed. We calculate the stellar magnetic field value at the position of the planet using the potential field source surface extrapolation technique. We show that the planetary magnetic environment is not homogeneous over the orbit, and that it varies between observing epochs, due to the evolution of the stellar magnetic field. This result underlines the importance of contemporaneous multiwavelength observations to characterize exoplanetary systems. Our reconstructed maps are a crucial input for the interpretation and modelling of our MOVES multiwavelength observations.Publisher PDFPeer reviewe
Overview of Saturn lightning observations
The lightning activity in Saturn's atmosphere has been monitored by Cassini
for more than six years. The continuous observations of the radio signatures
called SEDs (Saturn Electrostatic Discharges) combine favorably with imaging
observations of related cloud features as well as direct observations of
flash-illuminated cloud tops. The Cassini RPWS (Radio and Plasma Wave Science)
instrument and ISS (Imaging Science Subsystem) in orbit around Saturn also
received ground-based support: The intense SED radio waves were also detected
by the giant UTR-2 radio telescope, and committed amateurs observed SED-related
white spots with their backyard optical telescopes. Furthermore, the Cassini
VIMS (Visual and Infrared Mapping Spectrometer) and CIRS (Composite Infrared
Spectrometer) instruments have provided some information on chemical
constituents possibly created by the lightning discharges and transported
upward to Saturn's upper atmosphere by vertical convection. In this paper we
summarize the main results on Saturn lightning provided by this
multi-instrumental approach and compare Saturn lightning to lightning on
Jupiter and Earth.Comment: 10 pages, 6 figures, 2 tables; Proc. PRE VII conference Graz Sept.
201
Direct radio discovery of a cold brown dwarf
Magnetospheric processes seen in gas-giants such as aurorae and
circularly-polarized cyclotron maser radio emission have been detected from
some brown dwarfs. However, previous radio observations targeted known brown
dwarfs discovered via their infrared emission. Here we report the discovery of
BDR J1750+3809, a circularly polarized radio source detected around 144 MHz
with the LOFAR telescope. Follow-up near-infrared photometry and spectroscopy
show that BDR J1750+3809 is a cold methane dwarf of spectral type T
at a distance of . The quasi-quiescent radio spectral
luminosity of BDR J1750+3809 is which is over two orders of magnitude larger than that
of the known population of comparable spectral type. This could be due to a
preferential geometric alignment or an electrodynamic interaction with a close
companion. In addition, as the emission is expected to occur close to the
electron gyro-frequency, the magnetic field strength at the emitter site in BDR
J1750+3809 is , which is comparable to planetary-scale
magnetic fields. Our discovery suggests that low-frequency radio surveys can be
employed to discover sub-stellar objects that are too cold to be detected in
infrared surveys.Comment: Accepted for publication in ApJ
Surveying the Dynamic Radio Sky with the Long Wavelength Demonstrator Array
This paper presents a search for radio transients at a frequency of 73.8 MHz
(4 m wavelength) using the all-sky imaging capabilities of the Long Wavelength
Demonstrator Array (LWDA). The LWDA was a 16-dipole phased array telescope,
located on the site of the Very Large Array in New Mexico. The field of view of
the individual dipoles was essentially the entire sky, and the number of
dipoles was sufficiently small that a simple software correlator could be used
to make all-sky images. From 2006 October to 2007 February, we conducted an
all-sky transient search program, acquiring a total of 106 hr of data; the time
sampling varied, being 5 minutes at the start of the program and improving to 2
minutes by the end of the program. We were able to detect solar flares, and in
a special-purpose mode, radio reflections from ionized meteor trails during the
2006 Leonid meteor shower. We detected no transients originating outside of the
solar system above a flux density limit of 500 Jy, equivalent to a limit of no
more than about 10^{-2} events/yr/deg^2, having a pulse energy density >~ 1.5 x
10^{-20} J/m^2/Hz at 73.8 MHz for pulse widths of about 300 s. This event rate
is comparable to that determined from previous all-sky transient searches, but
at a lower frequency than most previous all-sky searches. We believe that the
LWDA illustrates how an all-sky imaging mode could be a useful operational
model for low-frequency instruments such as the Low Frequency Array, the Long
Wavelength Array station, the low-frequency component of the Square Kilometre
Array, and potentially the Lunar Radio Array.Comment: 20 pages; accepted for publication in A
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