Low-frequency GMRT observations of ultra-cool dwarfs

Abstract

Observations of radio emission in about 10 per cent of ultra-cool dwarfs (UCDs) indicate the presence of strong, persistent magnetic fields in these stars. These results are in contrast to early theoretical expectations on fully-convective dynamos, and to other tracers of magnetic activity, such as H α and X-ray luminosity. Radio-frequency observations have been key to physically characterising UCD magnetospheres, although explaining the diverse behaviour within them remains challenging. Most radio-frequency studies of UCDs have been conducted in the 4-8 GHz band, where traditional radio interferometers are typically most sensitive. Hence, the nature of UCD radio emission at low frequencies ($\lesssim 1.4\mathrm{GHz}$) remains relatively unexplored, but can probe optically thick emission, and regions of lower magnetic field strengths -- regimes not accessible to higher-frequency observations. In this work, we present the results from Giant Metrewave Radio Telescope observations of nine UCDs taken at $\sim 610$ and $1300\mathrm{MHz}$. These are the first observations of UCDs in this frequency range to be published in the literature. Using these observations, we are able to constrain the coronal magnetic field strength and electron number density of one of the targets to $1\lesssim B\lesssim 90\mathrm{G}$ and $4\lesssim \log (N_e)\lesssim 10$, respectively. We do not detect the flaring emission observed at higher frequencies, to a limit of a few millijanskys. These results show that some UCDs can produce low-frequency radio emission, and highlights the need for simultaneous multi-wavelength radio observations to tightly constrain the coronal and magnetospheric properties of these stars.

arXiv commentAccepted for publication in Monthly Notices of the Royal Astronomical Society. 10 pages, 5 figures

References in corpus (58)

• Astropy: A Community Python Package for Astronomy
• emcee: The MCMC Hammer
• The NumPy array: a structure for efficient numerical computation
• Gaia Data Release 2. Summary of the contents and survey properties
• The SIMBAD astronomical database
• The VizieR database of Astronomical Catalogues
• The GMRT 150 MHz All-sky Radio Survey: First Alternative Data Release TGSS ADR1
• The Theory of Brown Dwarfs and Extrasolar Giant Planets
• Magnetic fields of non-degenerate stars
• Theory of Low-Mass Stars and Substellar Objects
• The Hawaii Infrared Parallax Program. I. Ultracool Binaries and the L/T Transition
• New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the Main Sequence
• Rotation and Activity in Mid-M to L Dwarfs
• Ionospheric Calibration of Low Frequency Radio Interferometric Observations using the Peeling Scheme: I. Method Description and First Results
• Spectroscopic Properties of Cool Stars in the SDSS: An Analysis of Magnetic Activity and a Search for Subdwarfs
• Simulations of dynamo action in fully convective stars
• The Brown Dwarf Kinematics Project (BDKP) I. Proper Motions and Tangential Velocities for a Large Sample of Late-type M, L and T Dwarfs
• The Brown Dwarf Kinematics Project (BDKP). III. Parallaxes for 70 Ultracool Dwarfs
• Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs
• Compact continuum source-finding for next generation radio surveys
• Activity in Very Cool Stars: Magnetic Dissipation in late-M and L Dwarf Atmospheres
• Radio Observations of a Large Sample of Late-M, L, and T Dwarfs: The Distribution of Magentic Field Strengths
• Source finding in the era of the SKA (precursors): Aegean 2.0
• Flaring Up All Over -- Radio Activity in Rapidly-Rotating Late-Type M and L Dwarfs
• Rotational Modulation of the Radio Emission from the M9 Dwarf TVLM 513-46546: Broadband Coherent Emission at the Substellar Boundary?
• Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. III. X-ray, Radio, and H-alpha Activity Trends in M and L Dwarfs
• Explaining the coexistence of large-scale and small-scale magnetic fields in fully convective stars
• Auroral radio emission from late L and T dwarfs: A new constraint on dynamo theory in the substellar regime
• The Radio Activity-Rotation Relation of Ultracool Dwarfs
• Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf
• The Magnetic Properties of an L Dwarf Derived from Simultaneous Radio, X-ray, and H-alpha Observations
• Trends in Ultracool Dwarf Magnetism. I. X-Ray Suppression and Radio Enhancement
• Quiescent Radio Emission from Southern Late-type M Dwarfs and a Spectacular Radio Flare from the M8 Dwarf DENIS 1048-3956
• Polarized Synchrotron Emissivities and Absorptivities for Relativistic Thermal, Power-Law, and Kappa Distribution Functions
• Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM513-46546
• An X-ray Flare Detected on the M8 Dwarf VB 10
• Discovery of Five Very Low Mass Close Binaries, Resolved in the Visible with Lucky Imaging
• Magnetic cycles in a dynamo simulation of fully convective M-star Proxima Centauri
• Periodic Radio Emission from the M7 Dwarf 2MASS J13142039+1320011: Implications for the Magnetic Field Topology
• The Radio Spectrum of TVLM513-46546: Constraints on the Coronal Properties of a Late M Dwarf
• Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. IV. The Active, Young Binary NLTT 33370 AB (=2MASS J13142039+1320011)
• Wideband Dynamic Radio Spectra of Two Ultra-cool dwarfs
• Sporadic Long-term Variability in Radio Activity from a Brown Dwarf
• Volume-limited radio survey of ultracool dwarfs
• A Mini-survey of Ultracool Dwarfs at 4.9 GHz
• First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
• Radio detections of southern ultracool dwarfs
• The First Millimeter Detection of a Non-Accreting Ultracool Dwarf
• An Astrometric Search for a Sub-stellar Companion of the M8.5 Dwarf TVLM 513-46546 Using Very Long Baseline Interferometry
• The magnetosphere of the ultracool dwarf DENIS 1048-3956
• High-Precision Radio and Infrared Astrometry of LSPM J1314+1320AB - I: Parallax, Proper Motions, and Limits on Planets
• Simulating radio emission from Low Mass Stars
• Characterization of the Benchmark Binary NLTT 33370
• 325 MHz VLA Observations of Ultracool Dwarfs TVLM 513-46546 and 2MASS J0036+1821104
• Electron beam induced radio emission from ultracool dwarfs
• A LOFAR mini-survey for low-frequency radio emission from the nearest brown dwarfs
• Modelling the environment around five ultracool dwarfs via the radio domain
• Constraining Direction-Dependent Instrumental Polarisation: A New Technique for Polarisation Angle Calibration

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