Keywords

We here report a probable detection of a stellar coronal mass ejection (CME) in active M dwarf KIC 8093473 by performing an analysis on its time resolved X-ray spectra observed by the XMM-Newton satellite. Compared to the value at the quiescent state and the interstellar one, our spectral modeling returns a marginal (and probably evolving) excess of hydrogen column density in the flare state at a significance level of 1σ, which can be understood by an additional absorption due to a flare-associate CME. The CME mass is then estimated to be ∼7 × 10¹⁸–2 × 10²⁰ g according to the ice cream cone model.

We present observations of the active M-dwarf binary AT Mic (dM4.5e+dM4.5e) obtained with the orbital observatory AstroSat. During 20 ks of observations, in the far-ultraviolet (130–180 nm) and soft X-ray (0.3–7 keV) spectral ranges, we detected both quiescent emission and at least five flares on different components of the binary. The X-ray flares were typically longer than and delayed (by 5–6 minutes) with respect to their ultraviolet counterparts, in agreement with the Neupert effect. Using X-ray spectral fits, we estimated the parameters of the emitting plasma. The results indicate the presence of a hot multi-thermal corona with average temperatures in the range of ∼7–15 MK and emission measure of ∼(2.9–4.5) × 10⁵² cm⁻³; both the temperature and the emission measure increased during the flares. The estimated abundance of heavy elements in the corona of AT Mic is considerably lower than at the Sun (∼0.18–0.34 of the solar photospheric value); the coronal abundance increased during the flares due to chromospheric evaporation. The detected flares had the energies of ∼10³¹–10³² erg; the energy-duration relations indicate the presence of magnetic fields stronger than in typical solar flares.

The intensity of the green line (Fe xiv 5303 Å) is the strongest in the visible spectrum of the solar corona, and this line has been used as long-term powerful diagnostic tools for studying the coronal configurations and hot plasma dynamics. However, it remains unclear and an open question whether there exists close relationship between the green line intensities and the coronal extreme ultraviolet (EUV) line emissions for various coronal structures. In this paper, we use the green-line data by the Lijiang YOGIS Lyot coronagraph and the EUV data from the Solar Dynamics Observatory/Atmospheric Imaging Assembly instruments in order to perform direct comparisons and analyses, based on two algorithms developed to extract particular features in the low corona. It is found that, among the correlation coefficients obtained between the intensities of 5303 Å and the EUV lines, the coefficients between the green line and the 211 Å wavelength for different coronal structures and different limb locations always keep the highest values (ranging from 0.89 to 0.99), which has not been reported before. This result can be helpful and promising to link together the various physical processes involved at different heights in the corona by precisely tracking the bright loops or other features observed in 5303 Å above the limb down to the correct surface locations revealed by the 211 Å data. Furthermore, the ground-based observations of the coronal green line and the space-based EUV observations at 211 Å can advantageously complement each other when there is a need.

The compact, nonthermal emission in DoAr21 has been studied with the Very Long Baseline Array (VLBA) to investigate the possibility that the residuals of the astrometry fitting are due to the reflex motion induced by a possible companion. We find that the fitting of VLBA astrometric observations of DoAr21 improves significantly by adding the orbital motions of three companions. We obtain an improved distance to the source of 134.6 ± 1.0 pc, and estimate that the central star, DoAr21, has a mass of about 2.04 ± 0.70 M_⊙. We suggest that DoAr21 represents a unique case where two substellar companions, DoAr21b and DoAr21c (m_b ∼ 35.6 ± 27.2 M_jup and m_c ∼ 44.0 ± 13.6 M_jup, respectively), have been found to be associated with a relatively low-mass, pre-main sequence star. In addition, we find that this WTTau star is an astrometric double system, having a low-mass star companion, DoAr21B (m_B ∼ 0.35 ± 0.12 M_⊙), in a relatively eccentric orbit. The orbit of this low-mass stellar companion is compact, while the brown dwarfs are located in external orbits. DoAr21c has the strongest astrometric signature in the periodogram, while DoAr21B has a weak but significant signature. On the other hand, the astrometric signature of DoAr21b does not appear in the periodogram, however, this brown dwarf was directly detected in some of the VLBA observations. The estimated orbital periods of DoAr21B, DoAr21b, and DoAr21c are P_B ∼ 92.92 ± 0.02, P_b ∼ 450.9 ± 3.8, and P_c ∼ 1013.5 ± 25.3 days, respectively. Since the estimated age of this young star is about 0.4–0.8 Myr, the detected brown dwarf companion is among the youngest companions observed to date.

The thermal force (TF) is an exchange force mediated by Coulomb collisions between electrons and ions in a heat-conducting astrophysical plasma, is one of three, non-inertial, balancing terms in the parallel component of the generalized Ohm's law, and is magnetic field aligned with a size that scales with and is parallel to the dimensionless heat flux. The TF (i) increases the size of E_∥ above that implied by the electron pressure divergence; (ii) deepens the electrostatic trap for electrons about the Sun; (iii) strengthens the electron kurtosis and skewness, further levitating ions out of their gravitational well, (iv) constrains the heat flow in a plasma where parallel currents are preempted; and (v) is shown to be directly measurable using the full electron velocity distribution function above and below thermal energies. (vi) The usually ignored TF modifies all species internal energy equations; it enhances the rate of conduction cooling by the electrons, increases the ion entropy, and forestalls adiabatic behavior. Using estimates at 1 au this effect is especially strong in the higher speed wind U > 400 km s⁻¹ regime. (vii) On rather general grounds any physical heat transport is accompanied by an underlying TF; in almost all known cases of modeling astrophysical plasmas this dependence is ignored or demonstrably incorrect. It follows that attempts to predict species specific pressures without inclusion of the TF is futile.

Solving the problem of fast eruptive events in magnetically dominated astrophysical plasmas requires the use of particularly well adapted numerical tools. Indeed, the central mechanism based on magnetic reconnection is determined by a complex behavior with quasi-singular forming current layers enriched by their associated small-scale magnetic islands called plasmoids. A new code is thus presented for the solution of two-dimensional dissipative magnetohydrodynamics (MHD) equations in cartesian geometry specifically developed to this end. A current–vorticity formulation representative of an incompressible model is chosen in order to follow the formation of the current sheets and the ensuing magnetic reconnection process. A finite-element discretization using triangles with quadratic basis functions on an unstructured grid is employed, and implemented via a highly adaptive characteristic-Galerkin scheme. The adaptivity of the code is illustrated on simplified test equations and finally for magnetic reconnection associated with the nonlinear development of the tilt instability between two repelling current channels. Varying the Lundquist number S has allowed us to study the transition between the steady-state Sweet–Parker reconnection regime (for S ≲ 10⁴) and the plasmoid-dominated reconnection regime (for S ≳ 10⁵). The implications for the understanding of the mechanism explaining the fast conversion of free magnetic energy in astrophysical environments such as the solar corona are briefly discussed.

Type II radio bursts are observed in the Sun in association with many coronal mass ejections (CMEs). In view of this association, there has been an expectation that, by scaling from solar flares to the flares that are observed on M dwarfs, radio emission analogous to solar type II bursts should be detectable in association with M dwarf flares. However, several surveys have revealed that this expectation does not seem to be fulfilled. Here we hypothesize that the presence of larger global field strengths in low-mass stars, suggested by recent magnetoconvective modeling, gives rise to such large Alfvén speeds in the corona that it becomes difficult to satisfy the conditions for the generation of type II radio bursts. As a result, CMEs propagating in the corona/wind of flare stars are expected to be "radio-quiet" as regards type II bursts. In view of this, we suggest that, in the context of type II bursts, scaling from solar to stellar flares is of limited effectiveness.

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.

For more than a decade, Alpha Centauri AB (G2 V+K1 V) has been observed by Chandra, in a long-term program to follow coronal (T ∼ 10⁶ K) activity cycles of the two sunlike stars. Over 2008.4–2017.8, 19 HRC-I exposures were taken, each about 10 ks in duration, and spaced about six months apart. Beyond monitoring the AB X-ray luminosities, the HRC-I sequence represents a unique decadal record of the dozen, or so, serendipitous X-ray sources in the α Cen field, which is at low Galactic latitude and thus dominated by nearby stars. For the present study, the 10 brightest candidates were considered. Only a handful of these were persistent; most were variable, some highly so, flaring in a few epochs, weak or absent in the others. All 10 X-ray sources have Gaia objects within about 2''; mostly late-type dwarfs, but a few giants. However, two of the proposed optical counterparts have statistically significant offsets, and possible conflicts between the X-ray and optical properties. Another of the candidates brightened by a factor of 100 in X-rays during a single exposure, briefly attaining super-flare status. The Gaia counterpart is anomalously blue for its absolute G-magnitude and likely is a WD+dM pair. To the extent that the low Galactic latitude field is representative, the Chandra time-domain view emphasizes that the high-energy stellar sky is biased toward transient sources, so any snapshot survey surely will miss many of the most interesting objects.

Heat flux suppression in collisionless plasmas for a large range of plasma β is explored using two-dimensional particle-in-cell simulations with a strong, sustained thermal gradient. We find that a transition takes place between whistler-dominated (high-β) and double-layer-dominated (low-β) heat flux suppression. Whistlers saturate at small amplitude in the low beta limit and are unable to effectively suppress the heat flux. Electrostatic double layers (DLs) suppress the heat flux to a mostly constant factor of the free-streaming value once this transition happens. The DL physics is an example of ion–electron coupling and occurs on a scale of roughly the electron Debye length. The scaling of ion heating associated with the various heat flux driven instabilities is explored over the full range of β explored. The range of plasma-βs studied in this work makes it relevant to the dynamics of a large variety of astrophysical plasmas, including the intracluster medium of galaxy clusters, hot accretion flows, stellar and accretion disk coronae, and the solar wind.