scholarly journals Radio Emission from Flares in Single Late-type Stars

1995 ◽  
Vol 151 ◽  
pp. 22-31
Author(s):  
Arnold O. Benz
Keyword(s):  
Radio Emission ◽  
High Energy ◽  
Primary Energy ◽  
Relativistic Electrons ◽  
Late Type ◽  
Loss Cone ◽  
Magnetically Trapped ◽  
Stellar Flares ◽  
Gyrosynchrotron Emission ◽  
M Stars

AbstractRadio observations provide the most direct information on non-thermal electrons in stellar flares and in the coronae of late-type stars. Radio emissions of single main-sequence F, G, and of many K stars have recently been discovered, in addition to the well-known dwarf M stars. Their long-duration radio flares with low circular polarization, slow variations and broad bandwidth can be attributed to gyrosynchrotron emission of mildly relativistic electrons. The same holds for the low-level (‘quiescent’) radio emission. On the other hand, highly polarized radio flares of M stars have been interpreted by coherent emissions from loss-cone instabilities of magnetically trapped electrons. These conjectures are consistent with recent VLBI observations. The identification of the radio emission process allows to estimate the high-energy component of the flare and compare it to the total flare energy. The weakly polarized radio emission may serve as a proxy for hard X-ray signatures of relativistic electrons. The fraction of primary energy released into energetic electrons then appears to be large and similar to solar flares.

scholarly journals Quiescent Microwave Emission from Late-Type Stars

1994 ◽  
Vol 142 ◽  
pp. 743-751
Author(s):  
Manuel Güdel
Keyword(s):  
Microwave Emission ◽  
Radio Continuum ◽  
Acceleration Process ◽  
Relativistic Electrons ◽  
Late Type ◽  
Model Calculations ◽  
X Ray ◽  
Gyrosynchrotron Emission ◽  
Crucial Information ◽  
High Level

AbstractA diversity of stellar classes has been detected to be prolific sources of low-level, “quiescent” microwave radiation. This emission is, in most cases, attributed to the persistent presence of mildly relativistic electrons in the coronae. Frequent or continuous particle acceleration is required to maintain a high level of gyrosynchrotron emission. In this paper, observations relevant to our understanding of quiescent microwave emission from stars are reviewed, with emphasis on nondegenerate, late-type stars. Though the nature of the acceleration process remains unknown, a large amount of crucial information is presently available, in particular observations of timescales of slow variations, estimates of particle energies, model calculations based on spectral observations, and correlative studies with soft X-ray emission.Subject headings: acceleration of particles — radio continuum: stars — stars: activity — stars: coronae — stars: late-type — stars: magnetic fields

scholarly journals Analytic Approximation to the Bounce-average Drift Angle for Gyrosynchrotron-emitting Electrons in the Magnetosphere of V471 Tauri

1999 ◽  
Vol 16 (2) ◽  
pp. 139-146 ◽  
Author(s):  
Jennifer Nicholls ◽  
Michelle C. Storey
Keyword(s):  
Binary System ◽  
Radio Emission ◽  
Numerical Modelling ◽  
Interaction Region ◽  
Opening Angle ◽  
Relativistic Electrons ◽  
Analytic Approximation ◽  
Number Density ◽  
Drift Angle ◽  
Gyrosynchrotron Emission

AbstractNumerical modelling (Nicholls & Storey 1999) suggests that the eclipse of a wedge of enhanced number density of mildly relativistic electrons is responsible for the variations in quiescent radio emission of the binary system V471 Tauri. In the model, the wedge of enhanced density is created by electrons accelerated in the interaction region of the magnetospheres of the two stars, which subsequently drift in azimuth while emitting gyrosynchrotron emission. We present here an analytic approximation to the opening angle of the wedge of enhanced density and show that it is consistent with the opening angle derived from numerical modelling for reasonable values of the input parameters.

scholarly journals Evidence for Magnetic Fields in the Outflow from T Tau S

1997 ◽  
Vol 182 ◽  
pp. 475-480
Author(s):  
T.P. Ray ◽  
T.W.B. Muxlow ◽  
D.J. Axon ◽  
A. Brown ◽  
D. Corcoran ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Radio Emission ◽  
Direct Evidence ◽  
Relativistic Electrons ◽  
Radio Flux ◽  
Optical Star ◽  
Gyrosynchrotron Emission ◽  
Left And Right ◽  
T Tau ◽  
Opposite Helicity

We have observed at 5GHz the T Tau system with high resolution (≲0″.1) using the Multi-Element Radio Linked Interferometer (MERLIN) based at Jodrell Bank. Both the optical star (T Tau N) and its well-known infrared companion (T Tau S) were detected. The radio emission from T Tau S was found to be roughly extended in the direction of what is thought to be its outflow axis. More importantly we discovered that this radio emission split up into two spatially separated lobes of opposite helicity in the left and right-circular polarization channels. The circularly polarized lobes appear to straddle the star so that the “flow” and the “counterflow” were of opposite helicity. Such observations are the first direct evidence for the presence of magnetic fields in extended outflows that we are aware of. The radio flux appears to be due to gyrosynchrotron emission from mildly relativistic electrons (γ≈2–3). These electrons may have been accelerated in shocks close to the source. Using reasonable assumptions, the inferred magnetic fields strengths are surprisingly large (≳ several gauss) at distances of approximately 10-20 AU from their source. This is consistent with the magnetic fields being part of a collimated flow.

Quiescent Non-thermal Radio Emission from Stellar Systems

1995 ◽  
Vol 12 (2) ◽  
pp. 174-179 ◽  
Author(s):  
Michelle C. Storey ◽  
R. G. Hewitt
Keyword(s):  
Magnetic Field ◽  
Radio Emission ◽  
Three Dimensional ◽  
Emission Region ◽  
Relativistic Electrons ◽  
Thermal Radio ◽  
T Tauri ◽  
Thermal Radio Emission ◽  
Magnetic Field Model ◽  
Gyrosynchrotron Emission

AbstractNon-thermal radio emission has been detected from dMe stars, RS CVn binaries and W T Tauri stars. Polarisation and intensity measurements of the quiescent (i.e. non-flaring) emission indicate that the emission is gyrosynchrotron emission from mildly relativistic electrons spiralling in a magnetic field. A three-dimensional dipole magnetic field model for the stellar field is presented and the quiescent gyrosynchrotron emission from such a model is calculated and compared with observations. The model can account for many phenomenological features of quiescent emission. Quantitative comparisons of model results with observations indicate that the electron distribution in the emission region may be a magnetic mirroring distribution.

scholarly journals Radio Evidence for Nonthermal Particle Acceleration on Stars of Late Spectral Type

1994 ◽  
Vol 142 ◽  
pp. 753-764
Author(s):  
Kenneth R. Lang
Keyword(s):  
Particle Acceleration ◽  
Radio Emission ◽  
Binary Stars ◽  
Main Sequence ◽  
Late Type ◽  
T Tauri ◽  
Rs Cvn Stars ◽  
Vlbi Data ◽  
Flare Stars ◽  
M Stars

AbstractRadio evidence for nonthermal particle acceleration on stars of late spectral type (G, K, and M) is reviewed, including the pre-main-sequence T Tauri stars, the dwarf M flare stars, and the RS CVn binary stars. Both the long-lasting radio emission and the transient radio flares can be attributed to nonthermal electrons accelerated in magnetically confined coronae, radiating by either incoherent gyrosynchrotron radiation or by coherent processes. Related observations of the Sun provide the framework for interpreting the radio emission of late-type stars. We review recent VLBI observations of single and binary stars, observations of rapid acceleration and dynamic spectra of dwarf M flare stars, and models for long-lasting radio emission from dwarf M flare stars and RS CVn stars. The VLBI data provide evidence for nonthermal particle acceleration in large-scale magneto-spheres several times larger than the stellar radius for T Tauri and RS CVn stars. In contrast, rapid radio flares on dwarf M stars require sources much smaller than the visible star in size, and this is consistent with the VLBI data.Dynamic spectra suggest particle acceleration by relativistic electron beams on dwarf M stars. The long-lived radio radiation of dwarf M stars and RS CVn stars requires more-or-less continuous particle acceleration over periods of hours, perhaps by persistent low-amplitude flaring.Subject headings: acceleration of particles — binaries: close — radio continuum: stars — stars: flare — stars: late-type — stars: pre-main-sequence

scholarly journals Flares in Late-type Stars: X-Ray

1995 ◽  
Vol 151 ◽  
pp. 148-157
Author(s):  
Roberto Pallavicini
Keyword(s):  
Magnetic Energy ◽  
Primary Energy ◽  
Energy Scale ◽  
Electromagnetic Spectrum ◽  
X Rays ◽  
Late Type ◽  
X Ray ◽  
Stellar Flares ◽  
Optical Continuum ◽  
Diagnostic Capabilities

Flare-like brightenings, similar to those observed on the Sun but on a larger energy scale, are observed in a variety of late-type stars, including classical M dwarf flare stars, RS CVn-type binaries and pre-main sequence (PMS) objects. These events, which are observable over the whole electromagnetic spectrum, are interpreted as due to rapid release of magnetic energy at the star surface. Comprehensive reviews of stellar flares have been presented by Haisch &: Rodonò (1989), Pettersen (1991), Haisch et al. (1991), Pallavicini (1992a), as well as in contributions at this conference. Here I will discuss X-ray emission, with emphasis on flares on M dwarfs and RS CVn binaries.X-ray observations are important because they provide information on the high-temperature coronal portion of the flare and, at least in principle, could give clues as to the primary energy release and particle acceleration. In the stellar case there are severe limitations of the diagnostic capabilities of X-ray observations, in comparison to the solar case. In fact, stellar flares have up to now been observed only at soft X-ray energies (≤ 10 keV) where thermal processes greatly dominate over non-thermal ones (optical continuum observations have to be used in this case as a proxy for hard X-rays). Moreover, stellar X-ray observations are spatially unresolved and so far of poor spectral resolution. Thus, the observational data that are readily accessible are only light curves, average temperatures T and volume integrated emission measures EM = ∫ n2dV, but there is no direct information on parameters such as density, volume and detailed geometry.

scholarly journals High-sensitivity radio study of the non-thermal stellar bow shock EB27

2021 ◽  
Author(s):  
Paula Benaglia ◽  
Santiago del Palacio ◽  
Christopher Hales ◽  
Marcelo E Colazo
Keyword(s):  
Radio Emission ◽  
Massive Star ◽  
High Sensitivity ◽  
Bow Shock ◽  
Relativistic Electrons ◽  
Polarimetric Observation ◽  
Very Large Array ◽  
Thermal Radio Emission ◽  
The Magnetic Field ◽  
Linear Polarisation

Abstract We present a deep radio-polarimetric observation of the stellar bow shock EB27 associated to the massive star BD+43○3654. This is the only stellar bow shock confirmed to have non-thermal radio emission. We used the Jansky Very Large Array in S band (2–4 GHz) to test whether this synchrotron emission is polarised. The unprecedented sensitivity achieved allowed us to map even the fainter regions of the bow shock, revealing that the more diffuse emission is steeper and the bow shock brighter than previously reported. No linear polarisation is detected in the bow shock above 0.5%, although we detected polarised emission from two southern sources, probably extragalactic in nature. We modeled the intensity and morphology of the radio emission to better constrain the magnetic field and injected power in relativistic electrons. Finally, we derived a set of more precise parameters for the system EB27–BD+43○3654 using Gaia Early Data Release 3, including the spatial velocity. The new trajectory, back in time, intersects the core of the Cyg OB2 association.

scholarly journals Giant planets around AF and M stars

2013 ◽  
Vol 8 (S299) ◽  
pp. 64-65
Author(s):  
Julien Rameau ◽  
Gaël Chauvin ◽  
Anne-Marie Lagrange ◽  
Philippe Delorme ◽  
Justine Lannier
Keyword(s):  
Giant Planets ◽  
Early Type ◽  
Late Type ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Planetary Mass ◽  
Nearby Stars ◽  
Low Mass ◽  
M Stars ◽  
The One

AbstractWe present the results of two three-year surveys of young and nearby stars to search for wide orbit giant planets. On the one hand, we focus on early-type and massive, namely β Pictoris analogs. On the other hand, we observe late type and very low mass stars, i.e., M dwarfs. We report individual detections of new planetary mass objects. According to our deep detection performances, we derive the observed frequency of giant planets between these two classes of parent stars. We find frequency between 6 to 12% but we are not able to assess a/no correlation with the host-mass.

Sign in / Sign up

Export Citation Format

Share Document