Wind variability in intermediate luminosity B supergiants

We discuss wind variability in intermediate luminosity B supergiants. Because these are not the hottest, most luminous or just about other kind of -est stars, they have not attracted much attention. However, since their wind lines are well-developed, but unsaturated, they make ideal diagnostics for time dependent wind studies. We show that their wind profiles undergo large scale variability and that this phenomenon is quite common. We also discuss a time series for the B0.5 Ib HD 64760 which suggests the long sought after connection between wind and photospheric activity. Newly developed analysis techniques needed to extract physical information from the wind lines are discussed, and results are applied to the time series.     

Non-radial stellar oscillations: a perspective from potential scattering (I)

This article is the first in a series designed to gain insight into the stellar oscillation problem from a somewhat novel point of view: that of potential scattering, well-known in the quantum mechanical literature. In this paper the known theoretical foundations are developed and applied in the context of the astrophysical problem, wherein the star itself (rather than any portion of it) is the potential which scatters waves and traps them. The basis for the identification of a precisely defined scattering problem is the existence of a linear Schrödinger equation associated both globally (Section 2) and locally (Section 8) with the nonlinear eigenvalue equation for nonradial stellar pulsations. The paper is also designed to be a fairly complete account of the relevant mathematical topics that are germane to a study of this kind. This paper is dedicated to the memory of Professor Zdenèk Kopal, who was a great source of professional encouragement to me during the last fifteen years of his life.     

Rosat wide field camera data and the temperature of the interstellar medium

The shadowing effects of the molecular clouds in the nearby interstellar medium on the soft x-ray background has been investigated, using ROSAT WFC data in conjunction with previous rocket B and C band surveys. Shadowing over a 5° extent occurs only for a few percent of the sky, but the mixed model of the ISM is supported.Detailed modelling of the Draco shadowing region shows little evidence for a multi-temperature, hot ISM component.     

A Yohkoh search for “black-light flares”

Calculations which predict that a phenomenon analogous to stellar negative pre-flares could also exist on the Sun were published by Hénouxet al. (1990), and Aboudarhamet al., (1990), who showed that at the beginning of a solar white-light flare (WLF) event an electron beam can cause a transient darkening before the WLF emission starts, under certain conditions. They named this event a black light flare (BLF). Such a BLF event should appear as diffuse dark patches lasting for about 20 seconds preceding the WLF emission, which would coincide with intense and impulsive hard X-ray bursts. The BLF location would be at (or in the vicinity of) the forthcoming bright patches. Their predicted contrast depends on the position of the flare on the solar disc and on the wavelength band of the observation.TheYohkoh satellite provided white-light data from the aspect camera of the SXT instrument (Tsunetaet al., 1991), at 431 nm and with a typical image interval of 10–12 s. We have studied nine white-light flares observed with this instrument, with X-ray class larger than M6. We have found a few interesting episodes, but no unambiguous example of the predicted BLF event. This study, although the best survey to date, was not ideal from the observational point of view. We therefore encourage further searches. Successful observations of this phenomenon on the Sun would greatly strengthen our knowledge of the lower solar atmosphere and its effects on solar luminosity variations.     

Geomorphic evolution of a part of Middle Ganga Plain,Bihar: A case study using remote sensing data

An insight into the geomorphic evolution of any area can be obtained by detailed landform mapping. In the present study, an area in the Middle Ganga Plain has been selected for the study using mainly remote sensing data. Various fluvial landforms have been mapped and the changes in planform of rivers over approximately 50 years have been evaluated. Both fluvial processes and tectonic activities are considered to have collectively influenced the migration of the rivers in this region. Digital enhancements of Landsat MSS and TM data are found to be quite useful in identification and mapping of subtle fluvial palaeofeatures. The present study demonstrates the utility of remote sensing in examining the geomorphic evolution of the area.     

Resonance line radiation originating from a region with well-developed plasma turbulence

This study considers the influence of the effects of scattering due to Langmuir turbulent pulsations in the transfer of radiation in the spectral lines. The transfer equation of radiation in spectral lines, by taking into account scattering due to Langmuir turbulent pulsations, is written in a form convenient for application by numerical methods.The profile's intensity for a plane-parallel finite isothermal slab of a turbulent plasma in the case of complete redistribution of scattering by an atom are obtained. Numerical studies show that in this case with the broadening of spectral lines and the decreasing of self-reversal, the Langmuir frequency _pe is of the same order as the electronic Doppler width _De. Creation of the line satellites when _pe is larger than the line width is shown with the aid of numerical methods.     

Radiation-hydrodynamic waves and global solar oscillations

We investigate nonadiabatic hydrodynamic waves in a nongrey, radiating, thermally conducting, homogeneous atmosphere in LTE with a finite mean free path of photons. Avoiding the Eddington approximation the remaining simplifications in the basic equations are discussed, the generalized dispersion relation is analysed, and some wave properties in a grey model are studied. The properties of waves in a stratified atmosphere are analysed as well. In connection with the predicted properties of the nonadiabatic waves we discuss observations ofp-modes by measuring brightness fluctuations.     

Stellar irradiance variations due to surface temperature inhomogeneities

Observations of rotational modulation of continuum brightness and photospheric and chromospheric spectral-line profiles of late-type stars indicate the presence of very inhomogeneous surface temperature distributions. We present three stellar examples (VY Ari, HR 7275, HU Vir) where time-series photometry is used to trace the evolution of spotted regions. Simultaneous spectroscopy and Doppler imaging for one of the three stars (HU Virgo, Fig. 1) makes it possible to compute the temperature distribution of the photosphere and the relative intensity distribution of parts of the chromosphere (from CaII K and H line profiles). The combination of time-series spot modeling and Doppler imaging enabled us to determine thesign and amount of differential surface rotation on HU Vir. We found a big, cool polar spot (see figure below) and a differential (surface) rotation law where higher-latitude regions rotate faster than lower-latitude regions (opposite to what we see on the Sun). Currently, this ensemble of techniques - time-series photometry and photospheric and chromospheric Doppler imaging - is only applicable to stars overactive by approximately a factor of 100 as compared to the active Sun, e.g. the evolved components in RS CVn-type binaries and some rapidly-rotating, single, pre-main sequence stars or giant stars. Stellar rotation is a fundamental parameter for (magnetic) activity. Starspots, or any other surface inhomogeneities, allow one to derive very precise stellar rotation rates and, if coupled with seismological observations of solar-type stars, could provide information on the internal angular momentum distribution in overactive late-type stars.To be published in Astronomy & Astrophysics.     

Effect of perturbed potentials on the non-linear stability of libration pointL 4 in the restricted problem

The non-linear stability of the libration pointL ₄ in the restricted problem has been studied when there are perturbations in the potentials between the bodies. It is seen that the pointL ₄ is stable for all mass ratios in the range of linear stability except for three mass ratios depending upon the perturbing functions. The theory is applied to the following four cases:

Particle acceleration and the decay of soft X-ray non-thermal line broadening in solar flares

The relationship between the production of -ray emitting particles and non-thermal soft X-ray line broadening is investigated. A model of particle acceleration via the stochastic interaction with MHD turbulence is assumed and the time development of the wave energy density derived under the condition of energy conservation between waves and particles. The inferred numbers and energy distribution of accelerated protons for four -ray flares are used to define the wave energy density and its temporal development. The presence of Alfvén wave turbulence is considered as the source of the non-thermal motions in the ambient plasma. These motions are observed as excess widths in the soft X-ray line emission from these events. The decay of the waves via the particle acceleration process is compared with the observed decays of this non-thermal line broadening. Our results show that both the -ray emission and excess soft X-ray line widths in these flares can be explained by the single physical phenomenon of Alfvén wave turbulence.