46 research outputs found
GMRT detections of low-mass young stars at 323 and 608 MHz
We present the results of a pathfinder project conducted with the Giant
Metrewave Radio Telescope (GMRT) to investigate protostellar systems at low
radio frequencies. The goal of these investigations is to locate the break in
the free-free spectrum where the optical depth equals unity in order to
constrain physical parameters of these systems, such as the mass of the ionised
gas surrounding these young stars. We detect all three target sources, L1551
IRS 5 (Class I), T Tau and DG Tau (Class II), at frequencies 323 and 608 MHz
(wavelengths 90 and 50 cm, respectively). These are the first detections of low
mass young stellar objects (YSOs) at such low frequencies. We combine these new
GMRT data with archival information to construct the spectral energy
distributions for each system and find a continuation of the optically thin
free-free spectra extrapolated from higher radio frequencies to 323 MHz for
each target. We use these results to place limits on the masses of the ionised
gas and average electron densities associated with these young systems on
scales of ~1000 au. Future observations with higher angular resolution at lower
frequencies are required to constrain these physical parameters further.We thank the staff of the GMRT who have made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. REA, TPR and CPC acknowledge support from Science Foundation Ireland under grant 13/ERC/I2907. AMS gratefully acknowledges support from the European Research Council under grant ERC-2012-StG-307215 LODESTONE. DAG thanks the Science and Technology Facilities Council for support. We thank the anonymous referee for their helpful and constructive comments to clarify this manuscript.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw70
Anomalous microwave emission from spinning nanodiamonds around stars
Several interstellar environments produce 'anomalous microwave emission'
(AME), with brightness peaks at tens-of-gigahertz frequencies. The
emission's origins are uncertain -- rapidly spinning nanoparticles could
emit electric-dipole radiation, but the polycyclic aromatic hydrocarbons
that have been proposed as the carrier are now found not to correlate
with Galactic AME signals. The difficulty is in identifying co-spatial
sources over long lines of sight. Here we identify AME in three
proto-planetary discs. These are the only known systems that host
hydrogenated nanodiamonds, in contrast to the very common detection of
polycyclic aromatic hydrocarbons. Using spectroscopy, the nanodiamonds
are located close to the host stars, at physically well-constrained
temperatures. Developing disc models, we reproduce the emission with
diamonds 0.75--1.1 nm in radius, holding <= 1-2% of the carbon budget.
Ratios of microwave emission to stellar luminosity are approximately
constant, allowing nanodiamonds to be ubiquitous but emitting below
detection thresholds in many star systems. This result is compatible
with the findings with similar-sized diamonds found within Solar System
meteorites. As nanodiamond spectral absorption is seen in interstellar
sightlines, these particles are also a candidate for generating
galaxy-scale AME
A GMRT survey of regions towards the Taurus molecular cloud at 323 and 608 MHz
We present observations of three active sites of star formation in the Taurus
Molecular Cloud complex taken at 323 and 608 MHz (90 and 50 cm, respectively)
with the Giant Metrewave Radio Telescope (GMRT). Three pointings were observed
as part of a pathfinder project, targeted at the young stellar objects (YSOs)
L1551 IRS 5, T Tau and DG Tau (the results for these target sources were
presented in a previous paper). In this paper, we search for other YSOs and
present a survey comprising of all three fields; a by-product of the large
instantaneous field of view of the GMRT. The resolution of the survey is of
order 10 arcsec and the best rms noise at the centre of each pointing is of
order Jy beam at 323 MHz and Jy beam at 608
MHz. We present a catalogue of 1815 and 687 field sources detected above
at 323 and 608 MHz, respectively. A total of 440 sources
were detected at both frequencies, corresponding to a total unique source count
of 2062 sources. We compare the results with previous surveys and showcase a
sample of extended extragalactic objects. Although no further YSOs were
detected in addition to the target YSOs based on our source finding criteria,
these data can be useful for targeted manual searches, studies of radio
galaxies or to assist in the calibration of future observations with the Low
Frequency Array (LOFAR) towards these regions.Science Foundation Ireland [Grant ID: 13/ERC/I2907], Science and Technology Facilities Council, European Research Council [Grant ID: ERC-2012-StG-307215 LODESTONE]This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw184
Constraining the nature of DG Tau A’s thermal and non-thermal radio emission
DG Tau A, a class-II young stellar object (YSO) displays both thermal,
and non-thermal, radio emission associated with its bipolar jet. To
investigate the nature of this emission, we present sensitive (sigma ~ 2
microJy/beam), Karl G.\ Jansky Very Large Array (VLA) 6 and 10 GHz
observations. Over 3.81 yr, no proper motion is observed towards the
non-thermal radio knot C, previously thought to be a bowshock. Its
quasi-static nature, spatially-resolved variability and offset from the
central jet axis supports a scenario whereby it is instead a stationary
shock driven into the surrounding medium by the jet. Towards the
internal working surface, knot A, we derive an inclination-corrected,
absolute velocity of 258 +/- 23 km/s. DG Tau A's receding counterjet
displays a spatially-resolved increase in flux density, indicating a
variable mass loss event, the first time such an event has been observed
in the counterjet. For this ejection, we measure an ionised mass loss
rate of (3.7 +/- 1.0) * 10**8 Msun/yr during the event. A
contemporaneous ejection in the approaching jet isn't seen, showing it
to be an asymmetric process. Finally, using radiative transfer
modelling, we find that the extent of the radio emission can only be
explained with the presence of shocks, and therefore reionisation, in
the flow. Our modelling highlights the need to consider the relative
angular size of optically thick, and thin, radio emission from a jet, to
the synthesised beam, when deriving its physical conditions from its
spectral index
A LOFAR DETECTION of the LOW-MASS YOUNG STAR T TAU at 149 MHz
© 2017 Published by Elsevier B.V. Radio observations of young stellar objects (YSOs) enable the study of ionized plasma outflows from young protostars via their free-free radiation. Previous studies of the low-mass young system T Tau have used radio observations to model the spectrum and estimate important physical properties of the associated ionized plasma (local electron density, ionized gas content, and emission measure). However, without an indication of the low-frequency turnover in the free-free spectrum, these properties remain difficult to constrain. This paper presents the detection of T Tau at 149 MHz with the Low Frequency Array (LOFAR)-the first time a YSO has been observed at such low frequencies. The recovered total flux indicates that the free-free spectrum may be turning over near 149 MHz. The spectral energy distribution is fitted and yields improved constraints on local electron density ((7.2 ± 2.1) × 103 cm-3), ionized gas mass ( ± × -1.0 1.8 10-6Ṁ), and emission measure ((1.67 ± 0.14) × 105 pc cm-6)
AMI-CL J0300+2613: A Galactic anomalous-microwave-emission ring masquerading as a galaxy cluster
The Arcminute Microkelvin Imager (AMI) carried out a blind survey for galaxy
clusters via their Sunyaev-Zel'dovich effect decrements between 2008 and 2011.
The first detection, known as AMI-CL J0300+2613, has been reobserved with AMI
equipped with a new digital correlator with high dynamic range. The combination
of the new AMI data and more recent high-resolution sub-mm and infra-red maps
now shows the feature in fact to be a ring of positive dust-correlated Galactic
emission, which is likely to be anomalous microwave emission (AME). If so, this
is the first completely blind detection of AME at arcminute scales
AMI-LA observations of the SuperCLASS supercluster
We present a deep survey of the SuperCLASS super-cluster - a region of sky
known to contain five Abell clusters at redshift - performed using
the Arcminute Microkelvin Imager (AMI) Large Array (LA) at 15.5GHz. Our
survey covers an area of approximately 0.9 square degrees. We achieve a nominal
sensitivity of Jy beam toward the field centre, finding 80
sources above a threshold. We derive the radio colour-colour
distribution for sources common to three surveys that cover the field and
identify three sources with strongly curved spectra - a high-frequency-peaked
source and two GHz-peaked-spectrum sources. The differential source count (i)
agrees well with previous deep radio source count, (ii) exhibits no evidence of
an emerging population of star-forming galaxies, down to a limit of 0.24mJy,
and (iii) disagrees with some models of the 15GHz source population.
However, our source count is in agreement with recent work that provides an
analytical correction to the source count from the SKADS Simulated Sky,
supporting the suggestion that this discrepancy is caused by an abundance of
flat-spectrum galaxy cores as-yet not included in source population models
Tracking of an electron beam through the solar corona with LOFAR
© ESO 2018. The Sun's activity leads to bursts of radio emission, among other phenomena. An example is type-III radio bursts. They occur frequently and appear as short-lived structures rapidly drifting from high to low frequencies in dynamic radio spectra. They are usually interpreted as signatures of beams of energetic electrons propagating along coronal magnetic field lines. Here we present novel interferometric LOFAR (LOw Frequency ARray) observations of three solar type-III radio bursts and their reverse bursts with high spectral, spatial, and temporal resolution. They are consistent with a propagation of the radio sources along the coronal magnetic field lines with nonuniform speed. Hence, the type-III radio bursts cannot be generated by a monoenergetic electron beam, but by an ensemble of energetic electrons with a spread distribution in velocity and energy. Additionally, the density profile along the propagation path is derived in the corona. It agrees well with three-fold coronal density model by (1961, ApJ, 133, 983)
The JCMT BISTRO Survey: The Magnetic Field of the Barnard 1 Star-Forming Region
This is the final version. Available from American Astronomical Society / IOP Publishing via the DOI in this record.We present the POL-2 850 um linear polarization map of the Barnard 1 clump in the Perseus molecular cloud complex
from the B-fields In STar-forming Region Observations (BISTRO) survey at the James Clerk Maxwell Telescope. We find a trend of decreasing polarization fraction as a function of total intensity, which we link to depolarization effects towards higher density regions of the cloud. We then use the polarization data at 850 um to infer the plane-of-sky orientation of the large-scale magnetic field in Barnard 1. This magnetic field runs North-South across most of the cloud, with the exception of B1-c where it turns more East-West. From the dispersion of polarization angles, we calculate a turbulence correlation length of 5.0 +/- 2.5 arcsec (1500 au), and a turbulent-to-total magnetic energy ratio
of 0.5 +/- 0.3 inside the cloud. We combine this turbulent-to-total magnetic energy ratio with observations of NH3
molecular lines from the Green Bank Ammonia Survey (GAS) to estimate the strength of the plane-of-sky component
of the magnetic field through the Davis-Chandrasekhar-Fermi method. With a plane-of-sky amplitude of 120 +/- 60 uG
and a criticality criterion lambda_c = 3.0 +/- 1.5, we find that Barnard 1 is a supercritical molecular cloud with a magnetic field nearly dominated by its turbulent component.National Research Foundation of Korea (NRF)National Key R&D Program of ChinaNational Natural Science Foundation of China (NSFC