24 research outputs found
A Panchromatic View of Brown Dwarf Aurorae
Stellar coronal activity has been shown to persist into the low-mass star
regime, down to late M-dwarf spectral types. However, there is now an
accumulation of evidence suggesting that at the end of the main sequence there
is a transition in the nature of the magnetic activity from chromospheric and
coronal to planet-like and auroral, from local impulsive heating via flares and
MHD wave dissipation to energy dissipation from strong large-scale
magnetospheric current systems. We examine this transition and the prevalence
of auroral activity in brown dwarfs through a compilation of multi-wavelength
surveys of magnetic activity, including radio, X-ray, and optical. We compile
the results of those surveys and place their conclusions in the context of
auroral emission as the consequence of large-scale magnetospheric current
systems that accelerate energetic electron beams and drive the particles to
impact the cool atmospheric gas. We explore the different manifestation of
auroral phenomena in brown dwarf atmospheres, like H, and define their
distinguishing characteristics. We conclude that large amplitude photometric
variability in the near infrared is most likely a consequence of clouds in
brown dwarf atmospheres, but that auroral activity may be responsible for
long-lived stable surface features. We report a connection between auroral
H emission and quiescent radio emission in ECMI pulsing brown dwarfs,
suggesting a potential underlying physical connection between the quiescent and
auroral emissions. We also discuss the electrodynamic engines powering brown
dwarf aurorae and the possible role of satellites around these systems to both
power the aurorae and seed the magnetosphere with plasma.Comment: 26 pages, 17 figures, and 2 tables; accepted to Ap
The Occurrence Rate of Quiescent Radio Emission for Ultracool Dwarfs using a Generalized Semi-Analytical Bayesian Framework
We present a generalized analytical Bayesian framework for calculating the
occurrence rate of steady emission (or absorption) in astrophysical objects. As
a proof-of-concept, we apply this framework to non-flaring quiescent radio
emission in ultracool ( M7) dwarfs. Using simulations, we show that our
framework recovers the simulated radio occurrence rate to within 1-5% for
sample sizes of 10-100 objects when averaged over an ensemble of trials and
simulated occurrence rates for our assumed luminosity distribution models. In
contrast, existing detection rate studies may under-predict the simulated rate
by 51-66% because of sensitivity limits. Using all available literature results
for samples of 82 ultracool M dwarfs, 74 L dwarfs, and 23 T/Y dwarfs, we find
that the maximum-likelihood quiescent radio occurrence rate is between
- %, depending on the luminosity prior that we
assume. Comparing each spectral type, we find occurrence rates of
- % for M dwarfs, -
% for L dwarfs, and - % for T/Y
dwarfs. We rule out potential selection effects and speculate that age and/or
rotation may account for tentative evidence that the quiescent radio occurrence
rate of L dwarfs may be suppressed compared to M and T/Y dwarfs and phenomenon.
Finally, we discuss how we can harness our occurrence rate framework to
carefully assess the possible physics that may be contributing to observed
occurrence rate trends
Binarity Enhances the Occurrence Rate of Radiation Belt Emissions in Ultracool Dwarfs
Despite a burgeoning set of ultracool dwarf (M7) radio detections,
their radio emissions remain enigmatic. Open questions include the plasma
source and acceleration mechanisms for the non-auroral "quiescent" component of
these objects' radio emissions, which can trace Jovian synchrotron radiation
belt analogs. Ultracool dwarf binary systems can provide test beds for
examining the underlying physics for these plasma processes. We extend a
recently developed occurrence rate calculation framework to compare the
quiescent radio occurrence rate of binary systems to single objects. This
generalized and semi-analytical framework can be applied to any set of
astrophysical objects conceptualized as unresolved binary systems with
approximately steady-state emission or absorption. We combine data available in
the literature to create samples of 179 single ultracool dwarfs (82 M dwarfs,
74 L dwarfs, and 23 T/Y dwarfs) and 27 binary ultracool dwarf systems. Using
these samples, we show that quiescent radio emissions occur in
- per cent of binaries where both components are ultracool
dwarfs, depending on priors. We also show that binarity enhances the ultracool
dwarf quiescent radio occurrence rate relative to their single counterparts.
Finally, we discuss potential implications for the underlying drivers of
ultracool dwarf quiescent radio emissions, including possible plasma sources
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Radio Emission from Binary Ultracool Dwarf Systems
Well-characterized binary systems will provide valuable opportunities to study the conditions that are necessary for the onset of both auroral and nonauroral magnetospheric radio emission in the ultracool dwarf regime. We present new detections of nonauroral "quiescent" radio emission at 4–8 GHz of the three ultracool dwarf binary systems GJ 564 BC, LP 415-20, and 2MASS J21402931+1625183. We also tentatively detect a highly circularly polarized pulse at 4–6 GHz that may indicate aurorae from GJ 564 BC. Finally, we show that the brightest binary ultracool dwarf systems may be more luminous than predictions from single-object systems.
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On the Correlation between L Dwarf Optical and Infrared Variability and Radio Aurorae
Photometric variability attributed to cloud phenomena is common in L/T
transition brown dwarfs. Recent studies show that such variability may also
trace aurorae, suggesting that localized magnetic heating may contribute to
observed brown dwarf photometric variability. We assess this potential
correlation with a survey of 17 photometrically variable brown dwarfs using the
Karl G. Jansky Very Large Array (VLA) at 4 -- 8 GHz. We detect quiescent and
highly circularly polarized flaring emission from one source, 2MASS
J17502484-0016151, which we attribute to auroral electron cyclotron maser
emission. The detected auroral emission extends throughout the frequency band
at 5 -- 25, and we do not detect evidence of a cutoff. Our
detection confirms that 2MASS J17502484-0016151 hosts a magnetic field strength
of 2.9 kG, similar to those of other radio-bright ultracool dwarfs. We
show that H emission continues to be an accurate tracer of auroral
activity in brown dwarfs. Supplementing our study with data from the
literature, we calculate the occurrence rates of quiescent emission in L dwarfs
with low- and high-amplitude variability and conclude that high amplitude O/IR
variability does not trace radio magnetic activity in L dwarfs.Comment: 26 pages, 5 figures, 6 table
On the Correlation between L Dwarf Optical and Infrared Variability and Radio Aurorae
Photometric variability attributed to cloud phenomena is common in L/T transition brown dwarfs. Recent studies show that such variability may also trace aurorae, suggesting that localized magnetic heating may contribute to observed brown dwarf photometric variability. We assess this potential correlation with a survey of 17 photometrically variable brown dwarfs using the Karl G. Jansky Very Large Array at 4–8 GHz. We detect quiescent and highly circularly polarized flaring emission from one source, 2MASS J17502484-0016151, which we attribute to auroral electron cyclotron maser emission. The detected auroral emission extends throughout the frequency band at ~5–25σ, and we do not detect evidence of a cutoff. Our detection confirms that 2MASS J17502484-0016151 hosts a magnetic field strength of ≥2.9 kG, similar to those of other radio-bright ultracool dwarfs. We show that Hα emission continues to be an accurate tracer of auroral activity in brown dwarfs. Supplementing our study with data from the literature, we calculate the occurrence rates of quiescent emission in L dwarfs with low- and high-amplitude variability and conclude that high-amplitude optical and infrared variability does not trace radio magnetic activity in L dwarfs
A Survey for Hα Emission from Late L Dwarfs and T Dwarfs
Recently, studies of brown dwarfs have demonstrated that they possess strong magnetic fields and have the potential to produce radio and optical auroral emissions powered by magnetospheric currents. This emission provides the only window on magnetic fields in the coolest brown dwarfs and identifying additional benchmark objects is key to constraining dynamo theory in this regime. To this end, we conducted a new red optical (6300–9700 Å) survey with the Keck telescopes looking for Hα emission from a sample of late L dwarfs and T dwarfs. Our survey gathered optical spectra for 29 targets, 18 of which did not have previous optical spectra in the literature, greatly expanding the number of moderate-resolution (R ~ 2000) spectra available at these spectral types. Combining our sample with previous surveys, we confirm an Hα detection rate of 9.2±^(3.5)_(2.1)% for L and T dwarfs in the optical spectral range of L4–T8. This detection rate is consistent with the recently measured detection rate for auroral radio emission from Kao et al., suggesting that geometrical selection effects due to the beaming of the radio emission are small or absent. We also provide the first detection of Hα emission from 2MASS 0036+1821, previously notable as the only electron cyclotron maser radio source without a confirmed detection of Hα emission. Finally, we also establish optical standards for spectral types T3 and T4, filling in the previous gap between T2 and T5