Uv Line Profiles of Biv from a 10-Ps Krf-Laser-Produced Plasma

We use a KrF laser that generates pulses up to 8 × 10¹⁴ watt/cm² when focused onto a boron nitride target. We measured the line profile of the near UV, n = 0, n = 2 transitions of helium-like boron and we studied the triplet 1s2p³P-1s2s³S at 2825.85 (J = 1), 2821.68 (J = 2) and 2824.57 (J = 0) Å, at various positions from the target surface, in order to correlate details of this profile with the presence of self-generated magnetic fields and/or dynamic turbulence among other processes. We fitted the measured profiles to a 0-D model, that includes, Stark, Doppler, Zeeman and instrumental broadening. The effect of the magnetic field was included using an intermediate field calculations. Preliminary results of these measurements are presented and discussed.     

Observations of Stellar Winds

The different methods for the study of stellar winds are discussed: (1) P Cygni profiles (2) atomic emission lines (3) infrared and radio free-free emission (4) molecular emission lines (5) infrared radiation by dust. For each of these methods we describe some characteristic observations and we discuss the way in which these data can be used to derive the mass loss rate and the velocity distribution of the wind. We discuss these from the basic physical point of view in order to obtain an understanding of the basic physical processes involved. This revised version was published online in July 2006 with corrections to the Cover Date.     

Arm and Interarm Star Formation in Spiral Galaxies

We present an outline of our study of the effects of star formation on the different components of the interstellar medium in the discs of spiral galaxies, both globally and as a function of arm and interarm environment. We are in the process of obtaining images of 57 spiral galaxies at low inclinations, and analysing them to study the distribution of recent massive star formation, old stars, young stars, gas and dust. We will dissect the images into arm and interarm regions and compare and contrast the morphology and scale lengths within these regions inH_α, HI, the near infrared, optical and (where available) CO. Modelling will show how the scale lengths are affected by star formation, how this differs between arms and interarms, and whether the Schmidt Law varies from the global values in the arm and interarm regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Viscous damping of nonlinear magneto-acoustic waves

The motivation for the present work is discussed, and in Section 2 the nonlinear Burger's equation is derived for wave propagation in a compressible unstratified viscous fluid permeated by a magnetic field (initially constant), using the reductive perturbation method of Taniuti and Wei (1968). An analytic solution of the equation is given (after Sakai, 1972) and the angular behaviour is shown for certain parameters describing the nonlinearity and damping of the system, for both fast- and slow-mode disturbances.     

Vibrational stability of stars in thermal imbalance: A solution in terms of asymptotic expansions

The solution of the partial differential equation describing the ‘non-isentropic’ oscillations of a star in thermal imbalance has been obtained in terms of asymptotic expansions up to the first order in the parameterII/t _s, whereII is the adiabatic pulsation period for the fundamental mode andt _s , a secular time scale of the order of the Kelvin-Helmholtz time. Use has been made of the zeroth order ‘isentopic’ solution derived in I. The solution obtained allows one to derive unambiguously a general integral expression for the coefficient of vibrational stability for arbitrary stellar models in thermal imbalance. The physical interpretation of this stability coefficient is discussed and its generality and its simplicity are stressed. Application to some simple analytic stellar models in homologous and nonhomologous contraction enables one to recover, in a more straightforward manner, results obtained by Coxet al. (1973). Aizenman and Cox (1974) and Davey (1974). Finally, we emphasize that the inclusion of the effects of thermal imbalance in the stability calculations of realistic evolutionary sequences of stellar models, not considered up to now by the other authors, is quite easy and straightforward with the simple formula derived here.     

The cascading of cosmic-ray nuclei in various media

More than a thousand interactions of primary heavy nuclei of the cosmic radiation with charge ≥10 and energy >1 GeV nucleon^?1 in nuclear emulsion have been studied with emphasis on how the primary nucleus fragments. It has been determined that the cases of multiple successive fragmentations that have been observed do not occur more frequently than expected. The fragmentation ofZ>20 nuclei does depend on the target nucleus to some extent so it is important to try to separate the interactions in emulsion by theirN _h (number of evaporation prongs). The fragmentation of ₈ ¹⁶ O at 2.1 GeV nucleon^?1 measured at the Bevalac shows a similar dependence on target nucleus. By using data from these new interactions combined with published data we have simulated on a computer nuclear cascades in both emulsion and air. Results on these cascades are given for both primary silicon and primary iron nuclei. These results are used to discuss the fluctuations expected in extensive air showers produced by heavy primary nuclei.     

Inductive electric field of a time-dependent ring current

The inductive electric field generated by a time-dependent symmetric ring current has been investigated. The symmetric ring current was modelled by a population of protons drifting in a magnetic dipole field. The interaction of these protons with ion-cyclotron waves was assumed to be the dominant energy loss process for the ring current protons, at least under certain conditions. The calculation, with spectral densities for the ion-cyclotron waves that are based on experimental data, showed that an azimuthal inductive electric field of as much as 0.25 mV/m can be produced by this mechanism. Furthermore there is evidence that if the spectral density of the waves is substantially larger than the one adopted here, the electric field might increase to the order of 1.0 mV/m or more.     

Photometry of AR Lac

Visible photometry of the RS CVn system AR Lac is presented. We find some interesting features in the light curves. Also a possible oscillation with period 1.05 hr arises from the data. A new determination of the geometrical parameters of the system is given.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.