Search

Neuromorphic computing is the implementation of neural networks in hardware, in a manner mimicking the human brain. We present a superconducting synaptic circuit for use in such a system. The proposed system makes use of quantum flux parametron architecture to attain both excitatory and inhibitory weightings, and rapid single flux quantum architecture to adjust the magnitude of the synapse weighting. We show that the proposed synaptic circuit is capable of being tuned to eight different weightings, and these adjustments can all be done on the fly. We also demonstrate the synapse's capabilities in full neuron architecture using simple perceptron logic. The result is successfully training the system to emulate simple Boolean logic gates, from scratch.

Context.The stellar mass loss rate is important for the rotational evolution of a star and for its interaction with the circumstellar environment. The analysis of astrospheric absorption features enables an empirical determination of mass loss rates of cool stars other than the Sun.

Observational and theoretical investigations provide evidence for non-uniform spot and magnetic flux distributions on rapidly rotating stars, which have a significant impact on their angular momentum loss rate through magnetised winds. Supplementing the formalism of MacGregor & Brenner (1991) with a latitude-dependent magnetised wind model, we analyse the effect of analytically prescribed surface distributions of open magnetic flux with different shapes and degrees of non-uniformity on the rotational evolution of a solar-like star. The angular momentum redistribution inside the star is treated in a qualitative way, assuming an angular momentum transfer between the rigidly-rotating radiative and convective zones on a constant coupling timescale of $15{~{\rm Myr}}$; for the sake of simplicity we disregard interactions with circumstellar disks. We find that non-uniform flux distributions entail rotational histories which differ significantly from those of classical approaches, with differences cumulating up to $200\%$during the main sequence phase. Their impact is able to mimic deviations of the dynamo efficiency from linearity of up to $40\%$and nominal dynamo saturation limits at about 35 times the solar rotation rate. Concentrations of open magnetic flux at high latitudes thus assist in the formation of very rapidly rotating stars in young open clusters, and ease the necessity for a dynamo saturation at small rotation rates. However, since our results show that even minor amounts of open flux at intermediate latitudes, as observed with Zeeman-Doppler imaging techniques, are sufficient to moderate this reduction of the AM loss rate, we suggest that non-uniform flux distributions are a complementary rather than an alternative explanation for very rapid stellar rotation.

An explanation of the observed saturation of X-ray emission with increasing rotation rate of late-type stars is suggested. The suggestion is based on a simple model for the closed field line region of the magnetized stellar corona (stellar magnetosphere). The model assumes a magnetic dipole field aligned with the rotation axis confining a co-rotating, isothermal plasma. The effects of a stellar wind and of toroidal fields are neglected in the model. The closed field line region in the model is defined by those field lines on which the ratio of plasma pressure to magnetic pressure (plasma β) does not exceed one. Using scaling relations of surface magnetic field strength, surface density and coronal temperature with angular velocity, the model is used to calculate the stellar X-ray emission measure as a function of rotation rate. The results are in good agreement with the saturation in X-ray emission with rotation that is observed in G-K stars. The dependence of the results on the parameters of the model will be discussed.

A recent unambiguous detection of X-ray rotational modulation of the supersaturated star VXR45 ($P=0.223$days) has shown that its corona has discrete dark and bright X-ray regions. We suggest that due to the rapid rotation, the X-ray emitting corona has been centrifugally stripped away, creating open field regions that are dark in X-rays. This leads naturally both to a significant rotational modulation in X-rays but also to the lower X-ray luminosity of supersaturated stars compared to those rotating more slowly. To demonstrate the effect, we take as an example a more slowly rotating star for which surface magnetograms are available. We extrapolate the potential coronal magnetic field based on these magnetograms and determine for a hydrostatic, isothermal atmosphere the structure of the density and of the optically-thin X-ray emission. We show that if the rotation rate of this star were increased, the magnitude of the X-ray luminosity would decrease while its rotational modulation would increase in a way that is consistent with the recent observations of VXR45.

We argue that contemporary magnetic fields in the radiative cores of solar-type stars are relics of hydromagnetic dynamos operating over the pre-main sequence epoch when the core formed. Simulations of differential rotation and dynamos of a 1 $M_\odot$star are performed for a sequence of evolutionary stages of the core-growth epoch. A big difference in the dynamo-generated field between the initial and final stages of the core formation is predicted. An axisymmetric and cyclic field similar to the present day Sun is excited in a star with a mature radiative zone. An infant Sun with its core just starting to grow generates a nonaxisymmetric global field which is steady in a rotating frame. A qualitative explanation for the nonaxisymmetry is suggested. A model for the field capturing by the growing core is designed. The internal field is most efficiently captured over the initial phase when the core grows most rapidly. A nonaxisymmetric global relic field with an amplitude somewhat below 1 G is predicted for a main-sequence star of solar mass. An interpretation of the active longitude phenomenon as observed on the Sun and stars in terms of the nonaxisymmetric relic field is suggested.

Bow shocks can be formed around planets due to their interaction with the coronal medium of the host stars. The net velocity of the particles impacting on the planet determines the orientation of the shock. At the Earth's orbit, the (mainly radial) solar wind is primarily responsible for the formation of a shock facing towards the Sun. However, for close‐in planets that possess high Keplerian velocities and are frequently located at regions where the host star's wind is still accelerating, a shock may develop ahead of the planet. If the compressed material is able to absorb stellar radiation, then the signature of bow shocks may be observed during transits. Bow‐shock models have been investigated in a series of papers (Llama et al. 2011; Vidotto et al. 2010, 2011a,b) for known transiting systems. Once the signature of a bow‐shock is observed, one can infer the magnetic field intensity of the transiting planet. Here, we investigate the potential to use this model to detect magnetic fields of (hypothetical) planets orbiting inside the habitable zone of M‐dwarf stars. For these cases, we show, by means of radiative transfer simulations, that the detection of bow‐shocks of planets surrounding M‐dwarf stars may be more difficult than for the case of close‐in giant planets orbiting solar‐type stars (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Context. The search for radio emission from extra-solar planets has so far been unsuccessful. Much of the effort in modelling the predicted emission has been based on the analogy with the well-known emission from Jupiter. Unlike Jupiter, however, many of the targets of these radio searches are so close to their parent stars that they may well lie inside the stellar magnetosphere.Aims. For these close-in planets we determine which physical processes dominate the radio emission and compare our results to those for large-orbit planets that are that are immersed in the stellar wind.Methods. We have modelled the reconnection of the stellar and planetary magnetic fields. We calculate the extent of the planetary magnetosphere if it is in pressure balance with its surroundings and determine the conditions under which reconnection of the stellar and planetary magnetic fields could provide the accelerated electrons necessary for the predicted radio emission. Results. We show that received radio fluxes of tens of mJy are possible for exoplanets in the solar neighbourhood that are close to their parent stars if their stars have surface field strengths above 1?10?G. We show that for these close-in planets, the power of the radio emission depends principally on the ratio (Nc/B*1/3)2where Ncis the density at the base of the stellar corona, and?B*is the stellar surface magnetic field strength.Conclusions. Radio emission is most likely to be detected from planets around stars with high-density coronae, which are therefore likely to be bright X-ray sources. The dependence of stellar coronal density on stellar rotation rate and effective temperature is crucial in predicting radio fluxes from exoplanets.

We have investigated magnetostatic equilibria for coronal loops embedded in a potential magnetic field on a rotating star. We find that for any given star, there is a maximum value of the plasma pressure inside a single loop, above which no equilibrium exists. This maximum internal pressure depends on the ratio of the temperatures inside and outside the loop, and on the ratio of the plasma pressure to the magnetic pressure at the base of the external field. Thus, any loop of a large-scale field which is heated or cooled to a different temperature from its immediate surroundings, or which experiences a change in its internal pressure may eventually lose equilbrium. For some values of the base pressure and temperature ratio the relation between summit height and footpoint separation is double-valued. As the summit height of a loop is increased, its footpoint separation increases to a critical value, then decreases to zero at the maximum possible summit height. At the critical footpoint separation the slope of the loop height-footpoint separation relation becomes infinite, and no equilibrium solution exists for greater footpoint separations.

Several members of the Actinomycetales, including the medically important mycobacteria, produce 1-D-myo-inosityl-2-(N-acetyl-l-cysteinyl)amino-2-deoxy-α-d-glucopyranoside (trivial name mycothiol) as their principal low-molecular-mass thiol. The pseudo-disaccharide component of mycothiol, 1-D-myo-inosityl-2-amino-2-deoxy-α-D-glucopyranoside (α-D-GI), was synthesized by ligation of 1-D,l-2,3,4,5,6-penta-O-acetyl-myo-inositol to 3,4,6-tri-O-acetyl-2-deoxy-2-(2,4-dinitrophenylamino)-α-D-glu-copyranosyl bromide to give, in the first instance, an isomeric mixture of α- and β-linked pseudo-disaccharides. The α-coupled d,D and D,l isomers, α-d-GI and α-l-GI respectively, were purified from the mixture by TLC, followed by removal of the protecting groups. A cell-free extract of Mycobacterium smegmatis catalysed the ligation of cysteine, acetate and α-D-GI in the presence of ATP and Mg2+ to form mycothiol, as judged by HPLC. When no acetate was added to the incubation mixture, an additional thiol accumulated. In the presence of [14C]acetate no radiolabel was recovered in this species, but only in mycothiol. The additional thiol was isolated as the bimane derivative, and 1H and 1H–1H COSY NMR spectra confirmed its identity as desacetylmycothiol. A more complete conversion of desacetylmycothiol into mycothiol was achieved in the presence of acetyl-S-CoA. These results indicate that the biosynthesis of mycothiol proceeds by the sequential addition of cysteine and acetate to α-D-GI. The inositol moiety appears to be an important determinant of specificity, since α-L-GI was poorly utilized.

In a recent Letter, Litvinenko, Forbes, and Priest (1996) claimed that the rate of the very fast flux-pile up reconnection is severely limited by the effects of plasma pressure. They considered however only two-dimensional, zero-vorticity flows. Here we show that this limitation is a feature of these restrictive assumptions and can be removed by relaxing either of them.

To explain the observed intermingling of polarities in the magnetic field distributions of rapidly rotating stars, surface magnetic flux transport models demand the presence of fast meridional flows.We combine simulations of the pre-eruptive and post-eruptive magnetic flux transport in cool stars to investigate the influence of a fast meridional circulation on the latitudinal eruption pattern of magnetic flux tubes and on the polar magnetic field properties. Magnetic flux tubes rising through the convection zone experience an enhanced latitude-dependent poleward deflection through meridional flows, which renders the wings of stellar butterfly diagrams convex. The larger amount of magnetic flux emerging at higher latitudes supports the intermingling of opposite polarities of polar magnetic fields and yields magnetic flux densities in the polar regions about 20% higher than in the case disregarding the pre-eruptive deflection. Taking the pre-eruptive evolution of magnetic flux into account therefore eases the need for the fast meridional flows predicted by previous investigations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We show that magnetic field annihilation depends strongly on the behaviour of the vorticity and is quite different in 2D and 3D. In 3D the vorticity can be increased locally by the stretching of vortex lines (an effect that is absent in 2D). This leads to the onset ofcellular flow at quite low vorticities and the fragmentation of the simple current sheet.

Context.The soft X-ray emission from high density plasma observed in several CTTS is usually associated with the accretion process. However, it is still unclear whether this high density “cool” plasma is heated in the accretion shock, or if it is coronal plasma fed or modified by the accretion process.Aims.We conducted a coordinated quasi-simultaneous optical and X-ray observing campaign of the CTTS V2129 Oph. In this paper, we analyze Chandra grating spectrometer data and attempt to correlate the observed X-ray emitting plasma components with the characteristics of the accretion process and the stellar magnetic field constrained by simultaneous optical observations.Methods.We analyze a 200 ks Chandra/HETGS observation, subdivided into two 100 ks segments, of the CTTS V2129 Oph. For the two observing segments corresponding to two different phases within one stellar rotation, we measure the density of the cool plasma component and the emission measure distribution.Results.The X-ray emitting plasma covers a wide range of temperatures: from 2 up to 34 MK. The cool plasma component of V2129 Oph(T≈ 3−4 MK) varies between the two segments of the Chandra observation: during the first observing segment high density plasma (\hbox{$\log N_{\rm e} = 12.1^{+0.6}_{-1.1}$}) with high EMat ~3−4 MK is present, whereas, during the second segment, this plasma component has lower EMand lower density (log Ne< 11.5), although the statistical significance of these differences is marginal. Hotter plasma components, T≥ 10 MK, vary on short timescales (~10 ks), which are typical of coronal plasma. A clear flare, detected during the first segment, might be located in a large coronal loop (half length >3 R⋆). Conclusions.Our observation provides additional confirmation that the dense cool plasma at a few MK in CTTS is material heated in the accretion shock. The variability of this cool plasma component on V2129 Ophmay be explained in terms of X-rays emitted in the accretion shock and seen with different viewing angles at the two rotational phases probed by our observation. In particular, during the first time interval a direct view of the shock region is possible, while, during the second, the accretion funnel itself intersects the line of sight to the shock region, preventing us from observing the accretion-driven X-rays.

Rapidly rotating solar type stars display varying amounts of chromospheric emission in their Balmer lines. The profiles of these lines often display rapid variability. Since the mid-1980s, high-cadence echelle spectroscopy of numerous such stars has revealed that some of the variability is caused by concentrations of neutral hydrogen, trapped in the stellar coronal magnetic field at distances up to a few stellar radii from the rotation axis. In this paper we review the observations and the tomographic techniques that can be applied to mapping these "slingshot prominence" systems. We discuss the possible mechanisms for formation and mechanical support of these condensations, in the light of recent efforts to map 3-dimensional stellar coronal magnetic fields and X-ray emission measure distributions, with the help of Zeeman-Doppler imaging.