384 research outputs found
The Search for Signatures Of Transient Mass Loss in Active Stars
The habitability of an exoplanet depends on many factors. One such factor is
the impact of stellar eruptive events on nearby exoplanets. Currently this is
poorly constrained due to heavy reliance on solar scaling relationships and a
lack of experimental evidence. Potential impacts of Coronal Mass Ejections
(CMEs), which are a large eruption of magnetic field and plasma from a star,
are space weather and atmospheric stripping. A method for observing CMEs as
they travel though the stellar atmosphere is the type II radio burst, and the
new LOw Frequency ARray (LOFAR) provides a means for detection. We report on 15
hours of observation of YZ Canis Minoris (YZ CMi), a nearby M dwarf flare star,
taken in LOFAR's beam-formed observation mode for the purposes of measuring
transient frequency-dependent low frequency radio emission. The observations
utilized Low-Band Antenna (10-90 MHz) or High-Band Antenna (110-190 MHz) for
five three-hour observation periods. In this data set, there were no confirmed
type II events in this frequency range. We explore the range of parameter space
for type II bursts constrained by our observations Assuming the rate of shocks
is a lower limit to the rate at which CMEs occur, no detections in a total of
15 hours of observation places a limit of shocks/hr for YZ CMi due to the stochastic nature of the events and
limits of observational sensitivity. We propose a methodology to interpret
jointly observed flares and CMEs which will provide greater constraints to CMEs
and test the applicability of solar scaling relations
Numerical simulations of unbounded cyclotron-maser emissions
Numerical simulations have been conducted to study the spatial growth rate and emission topology of the cyclotron-maser instability responsible for stellar/planetary auroral magnetospheric radio emission and intense non-thermal radio emission in other astrophysical contexts. These simulations were carried out in an unconstrained geometry, so that the conditions existing within the source region of some natural electron cyclotron masers could be more closely modelled. The results have significant bearing on the radiation propagation and coupling characteristics within the source region of such non-thermal radio emissions
GMRT radio observations of the transiting extrasolar planet HD189733b at 244 and 614 MHz
We report a sensitive search for meter-wavelength emission at 244 and 614 MHz
from HD189733b, the nearest known extrasolar transiting planet of `hot-Jupiter'
type. To discriminate any planetary emission from possible stellar or
background contributions, we observed the system for 7.7 hours encompassing the
planet's eclipse behind the host star. These GMRT observations provide very low
(3 sigma) upper limits of 2 mJy at 244 MHz and 160 micro-Jy at 614 MHz. These
limits are, respectively, about 40 and 500 times deeper than those reported
recently at a nearby frequency of 340 MHz. Possible explanations of our
non-detection include: (1) the Earth being outside the planet's emission beam;
(2) its highly variable emission with more rapid flaring than the temporal
sampling in our observations; (3) the planetary emission being intrinsically
too weak; or more likely, (4) the emission being predominantly at lower
frequencies because of a weak planetary magnetic field. We briefly discuss
these possibilities and the constraints on this exo-planetary system
environment.Comment: Accepted for publication in A&A letter
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
Fine structures of radio bursts from flare star AD Leo with FAST observations
Radio bursts from nearby active M-dwarfs have been frequently reported and
extensively studied in solar or planetary paradigms. Whereas, their
sub-structures or fine structures remain rarely explored despite their
potential significance in diagnosing the plasma and magnetic field properties
of the star. Such studies in the past have been limited by the sensitivity of
radio telescopes. Here we report the inspiring results from the high
time-resolution observations of a known flare star AD Leo with the
Five-hundred-meter Aperture Spherical radio Telescope (FAST). We detected many
radio bursts in the two days of observations with fine structures in the form
of numerous millisecond-scale sub-bursts. Sub-bursts on the first day display
stripe-like shapes with nearly uniform frequency drift rates, which are
possibly stellar analogs to Jovian S-bursts. Sub-bursts on the second day,
however, reveal a different blob-like shape with random occurrence patterns and
are akin to solar radio spikes. The new observational results suggest that the
intense emission from AD Leo is driven by electron cyclotron maser instability
which may be related to stellar flares or interactions with a planetary
companion.Comment: 25 pages, 12 figures, accepted for publication in Ap
Properties of Saturn Kilometric Radiation measured within its source region
On 17 October 2008, the Cassini spacecraft crossed the southern sources of
Saturn kilometric radiation (SKR), while flying along high-latitude nightside
magnetic field lines. In situ measurements allowed us to characterize for the
first time the source region of an extra-terrestrial auroral radio emission.
Using radio, magnetic field and particle observations, we show that SKR sources
are surrounded by a hot tenuous plasma, in a region of upward field-aligned
currents. Magnetic field lines supporting radio sources map a continuous,
high-latitude and spiral-shaped auroral oval observed on the dawnside,
consistent with enhanced auroral activity. Investigating the Cyclotron Maser
Instability (CMI) as a mechanism responsible for SKR generation, we find that
observed cutoff frequencies are consistent with radio waves amplified
perpendicular to the magnetic field by hot (6 to 9 keV) resonant electrons,
measured locally
GMRT search for 150 MHz radio emission from the transiting extrasolar planets HD189733b and HD209458b
We report a sensitive search for meter-wavelength emission at 150 MHz from
two prominent transiting extrasolar planets, HD189733b and HD209458b. To
distinguish any planetary emission from possible stellar or background
contributions, we monitored these systems just prior to, during, and after the
planet's eclipse behind the host star. No emission was detected from HD209458b
with a 3-sigma upper limit of 3.6 mJy. For HD189733b we obtain a 3-sigma upper
limit of 2.1 mJy and a marginal 2.7-sigma detection of about 1.9+/-0.7 mJy from
a direction just 13" from the star's coordinates (i.e., within the beam), but
its association with the planet remains unconfirmed. Thus, the present GMRT
observations provide unprecedentedly tight upper limits for meter wavelengths
emissions from these nearest two transiting type exoplanets. We point out
possible explanations of the non-detections and briefly discuss the resulting
constraints on these systems.Comment: To be published in Astronomy & Astrophysic
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
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