Non-resonant grain acceleration in MHD turbulence

We discuss a new type of dust acceleration mechanism that acts in a turbulent magnetized medium. The magnetohydrodynamic turbulence can accelerate grains through resonant as well as non-resonant interactions. We show that the magnetic compression provides higher velocities for super-Alfvénic turbulence and can accelerate an extended range of grains in warm media compared to gyroresonance. While fast modes dominate the acceleration for the large grains, slow modes can be important for submicron grains. We provide comprehensive discussion of all the possible grain acceleration mechanisms in interstellar medium. We show that supersonic velocities are attainable for Galactic dust grains. We discuss the consequence of the acceleration. The implications for extinction curve, grain alignment, chemical abundance etc. are provided.     

Stellar Inversions

Inversions of oscillation frequencies have proved to be extremelypowerful in studying the structure of the Sun. We examine theconditions under which we can invert oscillation frequenciesof other stars to study their structure. We show that despite thevery limited number of modes that are expected to be observed in otherstars, we can perform inversions that will give localized informationof the stellar core. These results can limit the space of admissiblemodels of a given star.     

Predicting spectral features in galaxy spectra from broad-band photometry

We explore the prospects of predicting emission-line features present in galaxy spectra given broad-band photometry alone. There is a general consent that colours, and spectral features, most notably the 4000 Å break, can predict many properties of galaxies, including star formation rates and hence they could infer some of the line properties. We argue that these techniques have great prospects in helping us understand line emission in extragalactic objects and might speed up future galaxy redshift surveys if they are to target emission-line objects only. We use two independent methods, Artificial Neural Networks (based on the ANNz code) and Locally Weighted Regression (LWR), to retrieve correlations present in the colour N -dimensional space and to predict the equivalent widths present in the corresponding spectra. We also investigate how well it is possible to separate galaxies with and without lines from broad-band photometry only. We find, unsurprisingly, that recombination lines can be well predicted by galaxy colours. However, among collisional lines some can and some cannot be predicted well from galaxy colours alone, without any further redshift information. We also use our techniques to estimate how much information contained in spectral diagnostic diagrams can be recovered from broad-band photometry alone. We find that it is possible to classify active galactic nuclei and star formation objects relatively well using colours only. We suggest that this technique could be used to considerably improve redshift surveys such as the upcoming Fibre Multi Object Spectrograph (FMOS) survey and the planned Wide Field Multi Object Spectrograph (WFMOS) survey.     

Methods of searching for planets around pulsars

We analyse different methods of searching for planets around neutron stars by timing observations of pulsars. To this end, we study a few interesting models describing TOA residual variations that are observed, or could be observed, and which can mimic planets. We carry out a detailed theoretical analysis of the behaviour of these methods in the situations mentioned. We show that it is very helpful to look at these phenomena as some kind of quasi-periodic variations of residuals of time of arrival of pulsar pulses. We demonstrate that such a model-independent approach leads to promising conclusions that can be useful when analysing timing observations of pulsars to find planets or to prove that observed phenomena are of planetary origin.     

Wavelength dependence of angular diameters of M giants: an observational perspective

We discuss the wavelength dependence of angular diameters of M giants from an observational perspective. Observers cannot directly measure an optical-depth radius for a star, despite this being a common theoretical definition. Instead, they can use an interferometer to measure the square of the fringe visibility. We present new plots of the wavelength-dependent centre-to-limb variation (CLV) of intensity of the stellar disc as well as visibility for Mira and non-Mira M giant models. We use the terms 'CLV spectra' and 'visibility spectra' for these plots. We discuss a model-predicted extreme limb-darkening effect (also called the narrow-bright-core effect) in very strong TiO bands which can lead to a misinterpretation of the size of a star in these bands. We find no evidence as yet that this effect occurs in real stars. Our CLV spectra can explain the similarity in visibilities of R Dor (M8IIIe) that have been observed recently, despite the use of two different passbands. We compare several observations with models, and find that the models generally underestimate the observed variation in visibility with wavelength. We present CLV and visibility spectra for a model that is applicable to the M supergiant α Ori.     

Ion and relativistic electron acceleration by Alfven and whistler turbulence in solar flares

Solar Physics - We show that protons can be accelerated to several GeV in ≲10 s by Alfven turbulence whose energy density is greater than a few erg/cm3. We also show that electrons can be...     

Maser Emission of Relativistic Electrons Spiralling and Drifting in Curved Magnetic Fields

Synchro-curvature radiation describes the emission from a relativistic charged par- ticle which is moving and spiralling in a curved magnetic field. We investigate the maser emission for synchro-curvature radiation including drift of the guiding center of the radiating electron. It is shown that under some conditions the absorption coefficient can be negative, so maser can happen. These conditions are different from those needed for maser emission of curvature radiation including drift of the charged particles. We point out that our results, in- cluding the emissivity, can reduce to these of curvature radiation. Previously it was found that synchro-curvature radiation can not generate maser in vacuum, but we argue that synchro- curvature radiation including drift can generate maser even in vacuum. We discuss the possi- bilities of the potential applications of the synchro-curvature maser in modeling gamma ray bursts and pulsars.     

Studies of accretion flows around rotating black holes – II. Standing shocks in the pseudo-Kerr geometry

Standing, propagating or oscillating shock waves are common in accretion and winds around compact objects. We study the topology of all possible solutions using the pseudo-Kerr geometry. We present the parameter space spanned by the specific energy and angular momentum and compare it with that obtained from the full general relativity to show that the potential can work satisfactorily in fluid dynamics also, provided the polytropic index is suitably modified. We then divide the parameter space depending on the nature of the solution topology. We specifically study the nature of the standing Rankine–Hugoniot shocks. We also show that as the Kerr parameter is increased, the shock location generally moves closer to the black hole. In future, these solutions can be used as guidelines to test numerical simulations around compact objects.     

Cosmic rays from the galactic center

We examine the possibility that recent data on cosmic ray anisotropies presented by the AGASA group may lead to the conclusion that our Galactic Center is a major source of the highest energy cosmic rays in our galaxy. We discuss how such a source would contribute to the magnitude and directional properties of the observed flux when measured against a background of extragalactic cosmic rays. We do this using the results of previous propagation calculations and our own more recent calculations which are specifically for a Galactic Center source.We find that the AGASA data can indeed be plausibly interpreted in this way and also that an argument can be made that the Galactic Center has the appropriate physical properties for acceleration to energies of the order of 10¹⁸ eV. We show that data from the SUGAR array are compatible with the AGASA result.     

Theoretical Models for Producing Circularly Polarized Radiation in Extragalactic Radio Sources

We discuss the production of circular polarization in compact radio sources both by the intrinsic mechanism and by Faraday conversion. We pay particular attention to the magnetic field structure, considering partially ordered fields and Laing sheets, and distinguishing between uniform and unidirectional fields. (The latter can be constrained by flux conservation arguments.) In most cases, Faraday conversion is the more important mechanism. Conversion operates on Stokes U, which can be generated by internal Faraday rotation, or by magnetic field fluctuations, which can therefore produce circular polarization even in a pure pair plasma. We also show that the spectrum of circular polarization in an inhomogeneous jet can be quite different from that in a uniform source, being flat or even inverted.     

Structure through colour: a pixel approach towards understanding galaxies

We present a study of pixel colour–magnitude diagrams (pCMDs) for a sample of 69 nearby galaxies chosen to span a wide range of Hubble types. Our goal is to determine how useful a pixel approach is for studying galaxies according to their stellar light distributions and content. The galaxy images were analysed on a pixel-by-pixel basis to reveal the structure of the individual pCMDs. We find that the average surface brightness (or projected mass density) in each pixel varies according to galaxy type. Early-type galaxies exhibit a clear 'prime sequence' and some pCMDs of face-on spirals reveal 'inverse-L' structures. We find that the colour dispersion at a given magnitude is found to be approximately constant in early-type galaxies but this quantity varies in the mid and late types. We investigate individual galaxies and find that the pCMDs can be used to pick out morphological features. We discuss the discovery of 'Red Hooks' in the pCMDs of six early-type galaxies and two spirals and postulate their origins. We develop quantitative methods to characterize the pCMDs, including measures of the blue-to-red light ratio and colour distributions of each galaxy and we organize these by morphological type. We compare the colours of the pixels in each galaxy with the stellar population models of Bruzual & Charlot to calculate star formation histories for each galaxy type and compare these to the stellar mass within each pixel. Maps of pixel stellar mass and mass-to-light ratio are compared to galaxy images. We apply the pCMD technique to three galaxies in the Hubble Ultra Deep Field to test the usefulness of the analysis at high redshift. We propose that these results can be used as part of a new system of automated classification of galaxies that can be applied at high redshift.     

On accretion flow penetration of magnetospheres

We address the problem of plasma penetration of astrophysical magnetospheres, an important issue in a wide variety of contexts, ranging from accretion in cataclysmic variables to flows in protostellar systems. We point out that under well-defined conditions, penetration can occur without any turbulent mixing (driven, for example, by Rayleigh–Taylor or Kelvin–Helmholtz instabilities) caused by charge polarization effects, if the inflowing plasma is bounded in the direction transverse to both the flow velocity and the magnetic field. Depolarization effects limit the penetration depth, which nevertheless can, under specific circumstances, be comparable to the size of the magnetosphere. We discuss the effect of ambient medium on plasma propagation across the stellar magnetic field and determine the criteria for deep magnetosphere penetration. We show that, under conditions appropriate to magnetized white dwarfs in AM Her type cataclysmic variables, charge polarization effects can lead to deep penetration of the magnetosphere.     

Sky reconstruction for the Tianlai cylinder array

We apply our sky map reconstruction method for transit type interferometers to the Tianlai cylinder array.The method is based on spherical harmonic decomposition,and can be applied to a cylindrical array as well as dish arrays and we can compute the instrument response,synthesized beam,transfer function and noise power spectrum.We consider cylinder arrays with feed spacing larger than half a wavelength and,as expected,we find that the arrays with regular spacing have grating lobes which produce spurious images in the reconstructed maps.We show that this problem can be overcome using arrays with a different feed spacing on each cylinder.We present the reconstructed maps,and study the performance in terms of noise power spectrum,transfer function and beams for both regular and irregular feed spacing configurations.     

On the spin paradigm and the radio dichotomy of quasars

We investigate whether models based on the assumption that jets in quasars are powered by rotating black holes can explain the observed radio dichotomy of quasars. We show that in terms of the 'spin paradigm' models, radio-loud quasars could be objects in which the rotation rate of the black hole corresponds to an equilibrium between spin-up by accretion and spin-down by the Blandford–Znajek mechanism. Radio-quiet quasars could be hosting black holes with an average spin much smaller than the equilibrium one. We discuss possible accretion scenarios which can lead to such a bimodal distribution of black hole spins.     

Can late-type active stars be explained by a dipole magnetic trap?

In this paper we review four different types of X-ray and/or radio observations of active late-type stars. We then consider if a single magnetic source configuration – a toroidal dipole magnetic trap – can possibly explain these various different observations. We conclude that, indeed, dipole magnetic confinement (similar to the magnetic configurations of the Earth's radiation belts and the case of Jupiter and the Io torus) can explain all the diverse observational data. We take this to be very strong observational support for this type of magnetic confinement scheme. We also consider that this magnetic configuration is only likely to be established and maintained in the most active stars.     

Determination of force-free factor α and forecast of proton flares

We evaluated the force-free factor α for 18 well-observed proton flare regions during 1967–1972 according to the degree of twist in their neutral line. We found that, on the day of the flare, α ≥ 0.34 for flares of Class 1 or greater and ≥ 0.50 for Class 2 or greater and that α always increased over the one or two days before the flare. We therefore suggest that α can be used in forecasting proton events. We also outline a squeezing force-free field model of large flares which can better explain certain observed facts.     

Magnetic shear in flaring regions

We have evaluated the shear angle of the neutral line of the non-potential magnetic field for one or two days prior to and after the flare event for 10 cases. We have used the H filament positions to evaluate the shear in the neutral line. We find from the samples we have studied that it is the change in the shear that occurs a day prior to the flare that can lead to the event. This change can be in either direction, i.e., it can be a large increase from a small value or a decrease from a large initial value. Thus it is the change in the shear angle that seems to be a deciding criterion for a flare to occur and not a large value for the shear angle itself. We have one instance where there was no significant change in the shear angle over a period of a few days and this region, although similar to other active regions studied, did not produce any flare activity.     

Pseudo-viscous modelling of self-gravitating discs and the formation of low mass ratio binaries

We present analytic models for the local structure of self-regulated self-gravitating accretion discs that are subject to realistic cooling. Such an approach can be used to predict the secular evolution of self-gravitating discs (which can usefully be compared with future radiation hydrodynamical simulations) and to define various physical regimes as a function of radius and equivalent steady state accretion rate. We show that fragmentation is inevitable, given realistic rates of infall into the disc, once the disc extends to radii >70 au (in the case of a solar mass central object). Owing to the outward redistribution of disc material by gravitational torques, we also predict fragmentation at >70 au even in the case of low angular momentum cores which initially collapse to a much smaller radius. We point out that 70 au is close to the median binary separation and propose that such delayed fragmentation, at the point that the disc expands to >70 au, ensures the creation of low mass ratio companions that can avoid substantial further growth and consequent evolution towards unit mass ratio. We thus propose this as a promising mechanism for producing low mass ratio binaries, which, while abundant observationally, are severely underproduced in hydrodynamical models.     

Coupled thermal-orbital evolution of the early Moon

Coupled thermal-orbital histories of early lunar evolution are considered in a simple model. We consider a plagioclase lid, overlying a magma ocean, overlying a solid mantle. Tidal dissipation occurs in the plagioclase lid and heat transport is by conduction and melt migration. We find that large orbital eccentricities can be obtained in this model. We discuss possible consequences of this phase of large eccentricities for the shape of the Moon and geochronology of lunar samples. We find that the orbit can pass through the shape solution of Garrick-Bethell et al. (Garrick-Bethell, I., Wisdom, J., Zuber, M. [2006]. Science 313, 652), but we argue that the shape cannot be maintained against elastic deformation as the orbit continues to evolve.