We report on two millimeter flares detected by the Atacama Large Millimeter/submillimeter Array at 220 GHz from AU Mic, a nearby M dwarf. The larger flare had a duration of only ∼35 s, with peak LR = ...2 × 1015 erg s−1 Hz−1, and lower limit on linear polarization of . We examine the characteristics common to these new AU Mic events and those from Proxima Cen previously reported in MacGregor et al.-namely short durations, negative spectral indices, and significant linear polarization-to provide new diagnostics of conditions in outer stellar atmospheres and details of stellar flare particle acceleration. The event rates (∼20 and 4 events days−1 for AU Mic and Proxima Cen, respectively) suggest that millimeter flares occur commonly but have been undetected until now. Analysis of the flare observing frequency and consideration of possible incoherent emission mechanisms confirms the presence of MeV electrons in the stellar atmosphere occurring as part of the flare process. The spectral indices point to a hard distribution of electrons. The short durations and lack of pronounced exponential decay in the light curve are consistent with formation in a simple magnetic loop, with radio emission predominating from directly precipitating electrons. We consider the possibility of both synchrotron and gyrosynchrotron emission mechanisms, although synchrotron is favored given the linear polarization signal. This would imply that the emission must be occurring in a low density environment of only modest magnetic field strength. A deeper understanding of this newly discovered and apparently common stellar flare mechanism awaits more observations with better-studied flare components at other wavelengths.
ABSTRACT Observations of debris disks offer a window into the physical and dynamical properties of planetesimals in extrasolar systems through the size distribution of dust grains. In particular, the ...millimeter spectral index of thermal dust emission encodes information on the grain size distribution. We have made new VLA observations of a sample of seven nearby debris disks at 9 mm, with resolution and ∼5 Jy beam−1rms. We combine these with archival ATCA observations of eight additional debris disks observed at 7 mm, together with up-to-date observations of all disks at (sub)millimeter wavelengths from the literature, to place tight constraints on the millimeter spectral indices and thus grain size distributions. The analysis gives a weighted mean for the slope of the power-law grain size distribution, , of , with a possible trend of decreasing q for later spectral type stars. We compare our results to a range of theoretical models of collisional cascades, from the standard self-similar, steady-state size distribution (q = 3.5) to solutions that incorporate more realistic physics such as alternative velocity distributions and material strengths, the possibility of a cutoff at small dust sizes from radiation pressure, and results from detailed dynamical calculations of specific disks. Such effects can lead to size distributions consistent with the data, and plausibly the observed scatter in spectral indices. For the AU Mic system, the VLA observations show clear evidence of a highly variable stellar emission component; this stellar activity obviates the need to invoke the presence of an asteroid belt to explain the previously reported compact millimeter source in this system.
We present Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm (230 GHz) observations of the HD 32297 and HD 61005 debris disks, two of the most iconic debris disks because of their dramatic ...swept-back wings seen in scattered light images. These observations achieve sensitivities of 14 and 13 Jy beam−1 for HD 32297 and HD 61005, respectively, and provide the highest resolution images of these two systems at millimeter wavelengths to date. By adopting a Markov Chain Monte Carlo modeling approach, we determine that both disks are best described by a two-component model consisting of a broad (ΔR/R > 0.4) planetesimal belt with a rising surface density gradient and a steeply falling outer halo aligned with the scattered light disk. The inner and outer edges of the planetesimal belt are located at 78.5 8.1 au and 122 3 au for HD 32297, and 41.9 0.9 au and 67.0 0.5 au for HD 61005. The halos extend to 440 32 au and 188 8 au, respectively. We also detect 12CO J = 2-1 gas emission from HD 32297 co-located with the dust continuum. These new ALMA images provide observational evidence that larger, millimeter-sized grains may also populate the extended halos of these two disks previously thought to only be composed of small, micron-sized grains. We discuss the implications of these results for potential shaping and sculpting mechanisms of asymmetric debris disks.
We present ALMA mosaic observations at 1.3 mm (223 GHz) of the Fomalhaut system with a sensitivity of 14 Jy/beam. These observations provide the first millimeter map of the continuum dust emission ...from the complete outer debris disk with uniform sensitivity, enabling the first conclusive detection of apocenter glow. We adopt an MCMC modeling approach that accounts for the eccentric orbital parameters of a collection of particles within the disk. The outer belt is radially confined with an inner edge of 136.3 0.9 au and width of 13.5 1.8 au. We determine a best-fit eccentricity of 0.12 0.01. Assuming a size distribution power-law index of q = 3.46 0.09, we constrain the dust absorptivity power-law index β to be 0.9 < β < 1.5. The geometry of the disk is robustly constrained with inclination 65 6 0 3, position angle 337 9 0 3, and argument of periastron 22 5 4 3. Our observations do not confirm any of the azimuthal features found in previous imaging studies of the disk with Hubble Space Telescope, SCUBA, and ALMA. However, we cannot rule out structures ≤10 au in size or that only affect smaller grains. The central star is clearly detected with a flux density of 0.75 0.02 mJy, significantly lower than predicted by current photospheric models. We discuss the implications of these observations for the directly imaged Fomalhaut b and the inner dust belt detected at infrared wavelengths.
AA Tau is the archetype for a class of stars with a peculiar periodic photometric variability thought to be related to a warped inner disk structure with a nearly edge-on viewing geometry. We present ...high resolution (∼0 2) ALMA observations of the 0.87 and 1.3 mm dust continuum emission from the disk around AA Tau. These data reveal an evenly spaced three-ringed emission structure, with distinct peaks at 0 34, 0 66, and 0 99, all viewed at a modest inclination of 59 1 0 3 (decidedly not edge-on). In addition to this ringed substructure, we find non-axisymmetric features, including a "bridge" of emission that connects opposite sides of the innermost ring. We speculate on the nature of this "bridge" in light of accompanying observations of HCO+ and 13CO (J = 3-2) line emission. The HCO+ emission is bright interior to the innermost dust ring, with a projected velocity field that appears rotated with respect to the resolved disk geometry, indicating the presence of a warp or inward radial flow. We suggest that the continuum bridge and HCO+ line kinematics could originate from gap-crossing accretion streams, which may be responsible for the long-duration dimming of optical light from AA Tau.
We present observations of the HD 15115 debris disk from the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 mm that capture this intriguing system with the highest resolution (0 6 or 29 ...au) at millimeter wavelengths to date. This new ALMA image shows evidence for two rings in the disk separated by a cleared gap. By fitting models directly to the observed visibilities within a Markov Chain Monte Carlo framework, we are able to characterize the millimeter continuum emission and place robust constraints on the disk structure and geometry. In the best-fit model of a power-law disk with a Gaussian gap, the disk inner and outer edges are at 43.9 5.8 au (0 89 0 12) and 92.2 2.4 au (1 88 0 49), respectively, with a gap located at 58.9 4.5 au (1 2 0 10) with a fractional depth of 0.88 0.10 and a width of 13.8 5.6 au (0 28 0 11). Because we do not see any evidence at millimeter wavelengths for the dramatic east-west asymmetry seen in scattered light, we conclude that this feature most likely results from a mechanism that only affects small grains. Using dynamical modeling and our constraints on the gap properties, we are able to estimate a mass for the possible planet sculpting the gap to be 0.16 0.06 MJup.
We present 1.3 mm observations of the debris disk surrounding the HR 8799 multi-planet system from the Submillimeter Array to complement archival ALMA observations that spatially filtered away the ...bulk of the emission. The image morphology at 3 8 (150 au) resolution indicates an optically thin circumstellar belt, which we associate with a population of dust-producing planetesimals within the debris disk. The interferometric visibilities are fit well by an axisymmetric radial power-law model characterized by a broad width, ΔR/R 1. The belt inclination and orientation parameters are consistent with the planet orbital parameters within the mutual uncertainties. The models constrain the radial location of the inner edge of the belt to au. In a simple scenario where the chaotic zone of the outermost planet b truncates the planetesimal distribution, this inner edge location translates into a constraint on the planet b mass of MJup. This mass estimate is consistent with infrared observations of the planet luminosity and standard hot-start evolutionary models, with the uncertainties allowing for a range of initial conditions. We also present new 9 mm observations of the debris disk from the Very Large Array and determine a millimeter spectral index of 2.41 0.17. This value is typical of debris disks and indicates a power-law index of the grain size distribution q = 3.27 0.10, close to predictions for a classical collisional cascade.
Abstract
We place lower limits on the obliquities between debris disks and their host stars for 31 systems by comparing their disk and stellar inclinations. While previous studies did not find ...evidence for misalignment, we identify six systems with minimum obliquities falling between ∼30°and 60°, indicating that debris disks can be significantly misaligned with their stars. These high-obliquity systems span a wide range of stellar parameters with spectral types K through A. Previous works have argued that stars with masses below 1.2
M
⊙
(spectral types of ∼F6) have magnetic fields strong enough to realign their rotation axes with the surrounding disk via magnetic warping; given that we observe high obliquities for relatively low-mass stars, magnetic warping alone is likely not responsible for the observed misalignments. Yet, chaotic accretion is expected to result in misalignments of ∼20° at most and cannot explain the larger obliquities found in this work. While it remains unclear how primordial misalignment might occur and what role it plays in determining the spin–orbit alignment of planets, future work expanding this sample is critical toward understanding the mechanisms that shape these high-obliquity systems.
Abstract
A 20 s cadence Transiting Exoplanet Survey Satellite monitoring campaign of 226 low-mass flare stars during Cycle 3 recorded 3792 stellar flares of ≥10
32
erg. We explore the time-resolved ...emission and substructure in 440 of the largest flares observed at high signal-to-noise, 97% of which released energies of ≥10
33
erg. We discover degeneracy present at 2 minute cadence between sharply peaked and weakly peaked flares is common, although 20 s cadence breaks these degeneracies. We better resolve the rise phases and find 46% of large flares exhibit substructure during the rise phase. We observe 49 candidate quasi-periodic pulsations (QPP) and confirm 17 at ≥3
σ
. Most of our QPPs have periods less than 10 minutes, suggesting short-period optical QPPs are common. We find QPPs in both the rise and decay phases of flares, including a rise-phase QPP in a large flare from Proxima Cen. We confirm that the Davenport et al. template provides a good fit to most classical flares observed at high cadence, although 9% favor Gaussian peaks instead. We characterize the properties of complex flares, finding 17% of complex flares exhibit “peak-bump” morphologies composed of a large, highly impulsive peak followed by a second, more gradual Gaussian peak. We also estimate the UVC surface fluences of temperate planets at flare peak and find one-third of 10
34
erg flares reach the D90 dose of
Deinococcus radiodurans
in just 20 s in the absence of an atmosphere.
Abstract
Debris disks are scaled-up analogs of the Kuiper Belt in which dust is generated by collisions between planetesimals. In the
collisional cascade
model of debris disks, the dust lost to ...radiation pressure and winds is constantly replenished by grinding collisions between planetesimals. The model assumes that collisions are destructive and involve large velocities; this assumption has not been tested beyond our solar system. We present 0.″25 (≈2.4 au) resolution observations of the
λ
= 450
μ
m dust continuum emission from the debris disk around the nearby M dwarf AU Microscopii with the Atacama Large Millimeter/submillimeter Array. We use parametric models to describe the disk structure, and a Monte Carlo Markov Chain (MCMC) algorithm to explore the posterior distributions of the model parameters; we fit the structure of the disk to both our data and archival
λ
= 1.3 mm data (Daley et al. 2019), from which we obtain two aspect ratio measurements at 1.3 mm (
h
1300
=
0.025
−
0.002
+
0.008
) and at 450
μ
m (
h
450
=
0.019
−
0.001
+
0.006
), as well as the grain-size distribution index
q
= 3.03 ± 0.02. Contextualizing our aspect ratio measurements within the modeling framework laid out in Pan & Schlichting (2012), we derive a power-law index of velocity dispersion as a function of grain size
p
= 0.28 ± 0.06 for the AU Mic debris disk. This result implies that smaller bodies are more easily disrupted than larger bodies by collisions, which is inconsistent with the strength regime usually assumed for such small bodies. Possible explanations for this discrepancy are discussed.
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