570 research outputs found
A search for radio emission from exoplanets around evolved stars
The majority of searches for radio emission from exoplanets have to date
focused on short period planets, i.e., the so-called hot Jupiter type planets.
However, these planets are likely to be tidally locked to their host stars and
may not generate sufficiently strong magnetic fields to emit electron cyclotron
maser emission at the low frequencies used in observations (typically >150
MHz). In comparison, the large mass-loss rates of evolved stars could enable
exoplanets at larger orbital distances to emit detectable radio emission. Here,
we first show that the large ionized mass-loss rates of certain evolved stars
relative to the solar value could make them detectable with the Low Frequency
Array (LOFAR) at 150 MHz ( = 2 m), provided they have surface magnetic
field strengths >50 G. We then report radio observations of three long period
(>1 au) planets that orbit the evolved stars Gem, Dra, and
UMi using LOFAR at 150 MHz. We do not detect radio emission from any
system but place tight 3 upper limits of 0.98, 0.87, and 0.57 mJy on
the flux density at 150 MHz for Gem, Dra, and UMi,
respectively. Despite our non-detections these stringent upper limits highlight
the potential of LOFAR as a tool to search for exoplanetary radio emission at
meter wavelengths.Comment: 9 pages, 3 figure
Counter-rotating Accretion Disks
We consider accretion disks consisting of counter-rotating gaseous components
with an intervening shear layer. Configurations of this type may arise from the
accretion of newly supplied counter-rotating gas onto an existing co-rotating
gas disk. For simplicity we consider the case where the gas well above the disk
midplane is rotating with angular rate and that well below has the
same properties but is rotating with rate . Using the Shakura-Sunyaev
alpha turbulence model, we find self-similar solutions where a thin (relative
to the full disk thickness) equatorial layer accretes very rapidly, essentially
at free-fall speed. As a result the accretion speed is much larger than it
would be for an alpha disk rotating in one direction. Counter-rotating
accretion disks may be a transient stage in the formation of counter-rotating
galaxies and in the accretion of matter onto compact objects.Comment: 7 pages, 3 figures, aas2pp4.sty, submitted to Ap
Sub-arcsecond high sensitivity measurements of the DG~Tau jet with e-MERLIN
We present very high spatial resolution deep radio continuum observations at
5 GHz (6 cm) made with e-MERLIN of the young stars DG Tau A and B. Assuming it
is launched very close (~=1 au) from the star, our results suggest that the DG
Tau A outflow initially starts as a poorly focused wind and undergoes
significant collimation further along the jet (~=50 au). We derive jet
parameters for DG Tau A and find an initial jet opening angle of 86 degrees
within 2 au of the source, a mass-loss rate of 1.5x10^-8 solar masses/yr for
the ionised component of the jet, and the total ejection/accretion ratio to
range from 0.06-0.3. These results are in line with predictions from MHD
jet-launching theories.Comment: Accepted MNRAS Letter
Tentative Evidence for Relativistic Electrons Generated by the Jet of the Young Sun-like Star DG Tau
Synchrotron emission has recently been detected in the jet of a massive
protostar, providing further evidence that certain jet formation
characteristics for young stars are similar to those found for highly
relativistic jets from AGN. We present data at 325 and 610 MHz taken with the
GMRT of the young, low-mass star DG Tau, an analog of the Sun soon after its
birth. This is the first investigation of a low-mass YSO at at such low
frequencies. We detect emission with a synchrotron spectral index in the
proximity of the DG Tau jet and interpret this emission as a prominent bow
shock associated with this outflow. This result provides tentative evidence for
the acceleration of particles to relativistic energies due to the shock impact
of this otherwise very low-power jet against the ambient medium. We calculate
the equipartition magnetic field strength (0.11 mG) and particle energy
(4x10^40 erg), which are the minimum requirements to account for the
synchrotron emission of the DG Tau bow shock. These results suggest the
possibility of low energy cosmic rays being generated by young Sun-like stars.Comment: 19 pages, 2 figures, accepted for publication in ApJ Letter
Size-dependent reversal of grains in perpendicular magnetic recording media measured by small-angle polarized neutron scattering
Polarized small-angle neutron scattering has been used to measure the magnetic structure of a CoCrPt–SiOx thin-film data storage layer, contained within a writable perpendicular recording media, at granular (<10 nm) length scales. The magnetic contribution to the scattering is measured as the magnetization is reversed by an external field, providing unique spatial information on the switching process. A simple model of noninteracting nanomagnetic grains provides a good description of the data and an analysis of the grain-size dependent reversal provides strong evidence for an increase in magnetic anisotropy with grain diameter
Infrared-Faint Radio Sources: A New Population of High-redshift Radio Galaxies
We present a sample of 1317 Infrared-Faint Radio Sources (IFRSs) that, for
the first time, are reliably detected in the infrared, generated by
cross-correlating the Wide-Field Infrared Survey Explorer (WISE) all-sky survey
with major radio surveys. Our IFRSs are brighter in both radio and infrared
than the first generation IFRSs that were undetected in the infrared by the
Spitzer Space Telescope. We present the first spectroscopic redshifts of IFRSs,
and find that all but one of the IFRSs with spectroscopy has z > 2. We also
report the first X-ray counterparts of IFRSs, and present an analysis of radio
spectra and polarization, and show that they include Gigahertz-Peaked Spectrum,
Compact Steep Spectrum, and Ultra-Steep Spectrum sources. These results,
together with their WISE infrared colours and radio morphologies, imply that
our sample of IFRSs represents a population of radio-loud Active Galactic
Nuclei at z > 2. We conclude that our sample consists of lower-redshift
counterparts of the extreme first generation IFRSs, suggesting that the fainter
IFRSs are at even higher redshift.Comment: 23 pages, 17 figures. Submitted to MNRA
Radio Galaxy Zoo: The Distortion of Radio Galaxies by Galaxy Clusters
We study the impact of cluster environment on the morphology of a sample of
4304 extended radio galaxies from Radio Galaxy Zoo. A total of 87% of the
sample lies within a projected 15 Mpc of an optically identified cluster.
Brightest cluster galaxies (BCGs) are more likely than other cluster members to
be radio sources, and are also moderately bent. The surface density as a
function of separation from cluster center of non-BCG radio galaxies follows a
power law with index out to (Mpc), which
is steeper than the corresponding distribution for optically selected galaxies.
Non-BCG radio galaxies are statistically more bent the closer they are to the
cluster center. Within the inner (Mpc) of a cluster,
non-BCG radio galaxies are statistically more bent in high-mass clusters than
in low-mass clusters. Together, we find that non-BCG sources are statistically
more bent in environments that exert greater ram pressure. We use the
orientation of bent radio galaxies as an indicator of galaxy orbits and find
that they are preferentially in radial orbits. Away from clusters, there is a
large population of bent radio galaxies, limiting their use as cluster
locators; however, they are still located within statistically overdense
regions. We investigate the asymmetry in the tail length of sources that have
their tails aligned along the radius vector from the cluster center, and find
that the length of the inward-pointing tail is weakly suppressed for sources
close to the center of the cluster.Comment: 23 pages, 17 figures, 2 tables. Supplemental data files available in
The Astronomical Journal or contact autho
Investigating episodic accretion in a very low-mass young stellar object
Very low-mass Class I protostars have been investigated very little thus far.
Variability of these young stellar objects (YSOs) and whether or not they are
capable of strong episodic accretion is also left relatively unstudied. We
investigate accretion variability in IRS54, a Class I very low-mass protostar
with a mass of M ~ 0.1 - 0.2 M. We obtained spectroscopic
and photometric data with VLT/ISAAC and VLT/SINFONI in the near-infrared (,
, and bands) across four epochs (2005, 2010, 2013, and 2014). We used
accretion-tracing lines (Pa and Br) and outflow-tracing lines
(H and [FeII] to examine physical properties and kinematics of the object.
A large increase in luminosity was found between the 2005 and 2013 epochs of
more than 1 magnitude in the band, followed in 2014 by a steep decrease.
Consistently, the mass accretion rate () rose by an order of
magnitude from ~ 10 M yr to ~ M
yr between the two early epochs. The visual extinction () has also
increased from ~ 15 mag in 2005 to ~ 24 mag in 2013. This rise in in
tandem with the increase in is explained by the lifting up of a
large amount of dust from the disc of IRS54, following the augmented accretion
and ejection activity in the YSO, which intersects our line of sight due to the
almost edge-on geometry of the disc. Because of the strength and timescales
involved in this dramatic increase, this event is believed to have been an
accretion burst possibly similar to bursts of EXor-type objects. IRS54 is the
lowest mass Class I source observed to have an accretion burst of this type,
and therefore potentially one of the lowest mass EXor-type objects known so
far
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