skip to main content


Title: CONSTRAINTS ON AURORAL RADIO EMISSION FROM Y DWARFS
As an initial pilot study of magnetism in Y dwarfs, we have observed the three known IR variable Y dwarfs WISE J085510.83-071442.5, WISE J140518.40+553421.4, and WISEP J173835.53+273258.9 with the Karl G. Jansky Very Large Array (VLA) from 4-8 GHz to investigate the presence of quiescent radio emission as a proxy for highly circularly polarized radio emission associated with large-scale auroral currents. Measurements of magnetic fields on Y dwarfs, currently only possible by observing auroral radio emission, are essential for constraining fully convective magnetic dynamo models. We do not detect any pulsed or quiescent radio emission, down to rms noise levels of 7.2 uJy for WISE J085510.83-071442.5, 2.2 uJy for WISE J140518.40+553421.4, and 3.2 uJy for WISEP J173835.53+273258.9. The fractional detection rate of radio emission from T dwarfs is <10% and suggests that a much larger sample of deep observations of Y dwarfs is needed to rule out radio emission in the Y dwarf population. The significance of a single detection provides strong motivation for such a search.  more » « less
Award ID(s):
1654815
NSF-PAR ID:
10051225
Author(s) / Creator(s):
Date Published:
Journal Name:
Astrophysical journal
ISSN:
1538-4357
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    We present the detection of rotationally modulated, circularly polarized radio emission from the T8 brown dwarf WISE J062309.94−045624.6 between 0.9 and 2.0 GHz. We detected this high-proper-motion ultracool dwarf with the Australian SKA Pathfinder in 1.36 GHz imaging data from the Rapid ASKAP Continuum Survey. We observed WISE J062309.94−045624.6 to have a time and frequency averaged StokesIflux density of 4.17 ± 0.41 mJy beam−1, with an absolute circular polarization fraction of 66.3% ± 9.0%, and calculated a specific radio luminosity ofLν∼ 1014.8erg s−1Hz−1. In follow-up observations with the Australian Telescope Compact Array and MeerKAT we identified a multipeaked pulse structure, used dynamic spectra to place a lower limit ofB> 0.71 kG on the dwarf’s magnetic field, and measured aP= 1.912 ± 0.005 hr periodicity, which we concluded to be due to rotational modulation. The luminosity and period we measured are comparable to those of other ultracool dwarfs observed at radio wavelengths. This implies that future megahertz to gigahertz surveys, with increased cadence and improved sensitivity, are likely to detect similar or later-type dwarfs. Our detection of WISE J062309.94−045624.6 makes this dwarf the coolest and latest-type star observed to produce radio emission.

     
    more » « less
  2. We have used the Jansky VLA to observe a sample of 5 known aurorally emitting late L and T dwarfs ranging in age from 0.2-3.4 Gyr. We observed each target for seven hours, extending to higher frequencies than previously attempted for objects in this sample. We establish proportionally higher limits on maximum surface magnetic field strengths while simultaneously placing constraints on rotation periods through detections of repeating pulses. Observations at 8{12 GHz yield measurements of 3.7{4.1 kG localized field strengths (corresponding to minimum mean surface fields between 2.7{2.9 kG) on four of our targets, including the archetypal cloud variable T2.5 dwarf SIMP J01365663+0933473 recently proposed to be a possible planetary-mass object in the Carina-Near moving group. We detect a circularly polarized radio pulse at 15{16.5 GHz for the T6.5 dwarf 2MASS 10475385+2124234, corresponding to a localized 5.6 kG field strength and minimum mean surface field of 4.0 kG. For the same object, we also tentatively detect a circularly polarized radio pulse at 16.5{18 GHz corresponding to a localized 6.2 kG field strength and minimum mean surface field of 4.4 kG. We measure rotation periods between 1.44-2.88 hr for all targets, supporting i) the emerging consensus in convective dynamo models that rapid rotation may be important for producing strong dipole fields and/or ii) rapid rotation is a key ingredient for driving the current systems powering auroral radio emission. We do not detect a clear cutoff in the pulsed emission for any targets, which would correspond to a maximum local surface magnetic field strength. However, we do observe evidence of variable structure in the frequency-dependent timeseries of our targets on timescales shorter than a rotation period, suggesting a higher degree of variability in the current systems near the surfaces of brown dwarfs, where emission at the highest frequencies are expected to probe. Finally, we find that old brown dwarfs may generate fields as strong as young brown dwarfs. 
    more » « less
  3. ABSTRACT Active M dwarfs are known to produce bursty radio emission, and multiwavelength studies have shown that solar-like magnetic activity occurs in these stars. However, coherent bursts from active M dwarfs have often been difficult to interpret in the solar activity paradigm. We present Australian Square Array Pathfinder (ASKAP) observations of UV Ceti at a central frequency of 888 MHz. We detect several periodic, coherent pulses occurring over a time-scale consistent with the rotational period of UV Ceti. The properties of the pulsed emission show that they originate from the electron cyclotron maser instability, in a cavity at least 7 orders of magnitude less dense than the mean coronal density at the estimated source altitude. These results confirm that auroral activity can occur in active M dwarfs, suggesting that these stars mark the beginning of the transition from solar-like to auroral magnetospheric behaviour. These results demonstrate the capabilities of ASKAP for detecting polarized, coherent bursts from active stars and other systems. 
    more » « less
  4. Abstract

    Detection of low-frequency (≤1.4 GHz) radio emission from stellar and planetary systems can lead to new insights into stellar activity, extrasolar space weather, and planetary magnetic fields. In this work, we investigate three large field-of-view surveys at 74 MHz, 150 MHz, and 1.4 GHz, as well as a myriad of multiwavelength ancillary data, to search for radio emission from about 2600 stellar objects, including about 800 exoplanetary systems, 600 nearby low-mass stars, and 1200 young stellar objects located in the Taurus and Upper Scorpius star-forming regions. The selected sample encompasses stellar spectral types from B to L and distances between 5 and 300 pc. We report the redetection of five stars at 1.4 GHz, one of which also shows emission at 150 MHz. Four of these are low- and intermediate-mass young stars, and one is the evolved starαSco. We also observe radio emission at the position of a young brown dwarf at 1.4 GHz and 150 MHz. However, due to the large astrometric uncertainty of radio observations, a follow-up study at higher angular resolution would be required to confirm whether the observed emission originates from the brown dwarf itself or a background object. Notably, all of the selected radio sources are located in nearby star-forming regions. Furthermore, we use image stacking and statistical methods to derive upper limits on the average quiescent radio luminosity of the families of objects under investigation. These analyses provide observational constraints for large-scale searches for current and ongoing low-frequency radio emissions from stars and planets.

     
    more » « less
  5. Abstract

    Coronal magnetic fields are well known to be one of the crucial parameters defining coronal physics and space weather. However, measuring the global coronal magnetic fields remains challenging. The polarization properties of coronal radio emissions are sensitive to coronal magnetic fields. While they can prove to be useful probes of coronal and heliospheric magnetic fields, their usage has been limited by technical and algorithmic challenges. We present a robust algorithm for precise polarization calibration and imaging of low-radio frequency solar observations and demonstrate it on data from the Murchison Widefield Array, a Square Kilometre Array (SKA) precursor. This algorithm is based on theMeasurement Equationframework, which forms the basis of all modern radio interferometric calibration and imaging. It delivers high-dynamic-range and high-fidelity full-Stokes solar radio images with instrumental polarization leakages <1%, on par with general astronomical radio imaging, and represents the state of the art. Opening up this rewarding, yet unexplored, phase space will enable multiple novel science investigations and offer considerable discovery potential. Examples include detection of low-level circular polarization from thermal coronal emission to estimate large-scale quiescent coronal fields; polarization of faint gyrosynchrotron emissions from coronal mass ejections for robust estimation of plasma parameters; and detection of the first-ever linear polarization at these frequencies. This method has been developed with the SKA in mind and will enable a new era of high-fidelity spectropolarimetric snapshot solar imaging at low radio frequencies.

     
    more » « less