12 research outputs found
Ultra-Wideband Detection of 22 Coherent Radio Bursts on M Dwarfs
Coherent radio bursts detected from M dwarfs have some analogy with solar
radio bursts, but reach orders of magnitude higher luminosities. These events
trace particle acceleration, powered by magnetic reconnection, shock fronts
(such as formed by coronal mass ejections, CMEs), and magnetospheric currents,
in some cases offering the only window into these processes in stellar
atmospheres. We conducted a 58-hour, ultra-wideband survey for coherent radio
bursts on 5 active M dwarfs. We used the Karl G. Jansky Very Large Array (VLA)
to observe simultaneously in three frequency bands covering a subset of 224-482
MHz and 1-6 GHz, achieving the widest fractional bandwidth to date for any
observations of stellar radio bursts. We detected 22 bursts across 13 epochs,
providing the first large sample of wideband dynamic spectra of stellar
coherent radio bursts. The observed bursts have diverse morphology, with
durations ranging from seconds to hours, but all share strong (40-100%)
circular polarization. No events resemble solar Type II bursts (often
associated with CMEs), but we cannot rule out the occurrence of radio-quiet
stellar CMEs. The hours-long bursts are all polarized in the sense of the
x-mode of the star's large-scale magnetic field, suggesting they are cyclotron
maser emission from electrons accelerated in the large-scale field, analogous
to auroral processes on ultracool dwarfs. The duty cycle of luminous coherent
bursts peaks at 25% at 1-1.4 GHz, declining at lower and higher frequencies,
indicating source regions in the low corona. At these frequencies, active M
dwarfs should be the most common galactic transient source.Comment: 48 pages, 23 figures. Submitted to Ap
First Detection of Thermal Radio Emission from Solar-Type Stars with the Karl G. Jansky Very Large Array
We present the first detections of thermal radio emission from the
atmospheres of solar-type stars {\tau} Cet, {\eta} Cas A, and 40 Eri A. These
stars all resemble the Sun in age and level of magnetic activity, as indicated
by X-ray luminosity and chromospheric emission in calcium-II H and K lines. We
observed these stars with the Karl G. Jansky Very Large Array with
sensitivities of a few {\mu}Jy at combinations of 10.0, 15.0, and 34.5 GHz.
{\tau} Cet, {\eta} Cas A, and 40 Eri A are all detected at 34.5 GHz with
signal-to-noise ratios of 6.5, 5.2, and 4.5, respectively. 15.0-GHz upper
limits imply a rising spectral index greater than 1.0 for {\tau} Cet and 1.6
for {\eta} Cas A, at the 95% confidence level. The measured 34.5-GHz flux
densities correspond to stellar disk-averaged brightness temperatures of
roughly 10,000 K, similar to the solar brightness temperature at the same
frequency. We explain this emission as optically- thick thermal free-free
emission from the chromosphere, with possible contributions from coronal
gyroresonance emission above active regions and coronal free-free emission.
These and similar quality data on other nearby solar-type stars, when combined
with ALMA observations, will enable the construction of temperature profiles of
their chromospheres and lower transition regions.Comment: 9 pages, 2 figures. Accepted for publication in Ap
Ultra-wideband Detection of 22 Coherent Radio Bursts on M Dwarfs
Coherent radio bursts detected from M dwarfs have some analogy with solar radio bursts but reach orders of magnitude higher luminosities. These events trace particle acceleration, powered by magnetic reconnection, shock fronts (such as those formed by coronal mass ejections (CMEs)), and magnetospheric currents, in some cases offering the only window into these processes in stellar atmospheres. We conducted a 58 hr ultra-wideband survey for coherent radio bursts on five active M dwarfs. We used the Karl G. Jansky Very Large Array to observe simultaneously in three frequency bands covering a subset of 224–482 MHz and 1–6 GHz, achieving the widest fractional bandwidth to date for any observations of stellar radio bursts. We detected 22 bursts across 13 epochs, providing the first large sample of wideband dynamic spectra of stellar coherent radio bursts. The observed bursts have diverse morphology, with durations ranging from seconds to hours, but all share strong (40%–100%) circular polarization. No events resemble solar Type II bursts (often associated with CMEs), but we cannot rule out the occurrence of radio-quiet stellar CMEs. The hours-long bursts are all polarized in the sense of the x-mode of the star's large-scale magnetic field, suggesting that they are cyclotron maser emission from electrons accelerated in the large-scale field, analogous to auroral processes on ultracool dwarfs. The duty cycle of luminous coherent bursts peaks at 25% at 1–1.4 GHz, declining at lower and higher frequencies, indicating source regions in the low corona. At these frequencies, active M dwarfs should be the most common galactic transient source
Meter- to Millimeter Emission from Cool Stellar Systems : Latest Results, Synergies Across the Spectrum, and Outlook for the Next Decade
Splinter session summary, to appear in the proceedings of the 20th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (ed. S. J. Wolk)Radio observations of cool stellar systems provide unique information on their magnetic fields, high-energy processes, and chemistry. Buoyed by powerful new instruments (e.g. ALMA, JVLA, LOFAR), advances in related fields (e.g., the Gaia astrometric revolution), and above all a renewed interest in the relevant stellar astrophysics, stellar radio astronomy is experiencing a renaissance. In this splinter session, participants took stock of the present state of stellar radio astronomy to chart a course for the field's future
Constraining the Physical Properties of Stellar Coronal Mass Ejections with Coronal Dimming: Application to Far Ultraviolet Data of Eridani
Coronal mass ejections (CMEs) are a prominent contributor to solar system
space weather and might have impacted the Sun's early angular momentum
evolution. A signal diagnostic of CMEs on the Sun is coronal dimming: a drop in
coronal emission, tied to the mass of the CME, that is the direct result of
removing emitting plasma from the corona. We present the results of a coronal
dimming analysis of Fe XII 1349 A and Fe XXI 1354 A emission from
Eridani ( Eri), a young K2 dwarf, with archival far-ultraviolet
observations by the Hubble Space Telescope's Cosmic Origins Spectrograph.
Following a flare in February 2015, Eri's Fe XXI emission declined
by %. Although enticing, a scant 3.8 min of preflare observations
allows for the possibility that the Fe XXI decline was the decay of an earlier,
unseen flare. Dimming nondetections following each of three prominent flares
constrain the possible mass of ejected Fe XII-emitting (1 MK) plasma to less
than a few g. This implies that CMEs ejecting this much or more
1 MK plasma occur less than a few times per day on Eri. On the Sun,
g CMEs occur once every few days. For Eri, the mass loss
rate due to CME-ejected 1 MK plasma could be , well below
the star's estimated 30 mass loss rate (wind + CMEs). The
order-of-magnitude formalism we developed for these mass estimates can be
broadly applied to coronal dimming observations of any star.Comment: 27 pages, 22 figures, accepted to Ap
Stellar Coronal and Wind Models: Impact on Exoplanets
Surface magnetism is believed to be the main driver of coronal heating and
stellar wind acceleration. Coronae are believed to be formed by plasma confined
in closed magnetic coronal loops of the stars, with winds mainly originating in
open magnetic field line regions. In this Chapter, we review some basic
properties of stellar coronae and winds and present some existing models. In
the last part of this Chapter, we discuss the effects of coronal winds on
exoplanets.Comment: Chapter published in the "Handbook of Exoplanets", Editors in Chief:
Juan Antonio Belmonte and Hans Deeg, Section Editor: Nuccio Lanza. Springer
Reference Work
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Resolved imaging confirms a radiation belt around an ultracool dwarf.
Radiation belts are present in all large-scale Solar System planetary magnetospheres: Earth, Jupiter, Saturn, Uranus and Neptune1. These persistent equatorial zones of relativistic particles up to tens of megaelectron volts in energy can extend further than ten times the planets radius, emit gradually varying radio emissions2-4 and affect the surface chemistry of close-in moons5. Recent observations demonstrate that very low-mass stars and brown dwarfs, collectively known as ultracool dwarfs, can produce planet-like radio emissions such as periodically bursting aurorae6-8 from large-scale magnetospheric currents9-11. They also exhibit slowly varying quiescent radio emissions7,12,13 hypothesized to trace low-level coronal flaring14,15 despite departing from empirical multiwavelength flare relationships8,15. Here we present high-resolution imaging of the ultracool dwarf LSR J1835 + 3259 at 8.4 GHz, demonstrating that its quiescent radio emission is spatially resolved and traces a double-lobed and axisymmetrical structure that is similar in morphology to the Jovian radiation belts. Up to 18 ultracool dwarf radii separate the two lobes, which are stably present in three observations spanning more than one year. For plasma confined by the magnetic dipole of LSR J1835 + 3259, we estimate 15 MeV electron energies, consistent with Jupiters radiation belts4. Our results confirm recent predictions of radiation belts at both ends of the stellar mass sequence8,16-19 and support broader re-examination of rotating magnetic dipoles in producing non-thermal quiescent radio emissions from brown dwarfs7, fully convective M dwarfs20 and massive stars18,21