Shear-induced instability and arch filament eruption: A magnetohydrodynamic (MHD) numerical simulation

We investigate, via a two-dimensional (nonplanar) MHD simulation, a situation wherein a bipolar magnetic field embedded in a stratified solar atmosphere (i.e., arch-filament-like structure) undergoes symmetrical shear motion at the footpoints. It was found that the vertical plasma flow velocities grow exponentially leading to a new type of global MHD-instability that could be characterized as a Dynamic Shearing Instability, with a growth rate of about 8{ovV} _A a, where {ovV} _A is the average Alfvén speed and a ^–1 is the characteristic length scale. The growth rate grows almost linearly until it reaches the same order of magnitude as the Alfvén speed. Then a nonlinear MHD instability occurs beyond this point. This simulation indicates the following physical consequences: the central loops are pinched by opposing Lorentz forces, and the outer closed loops stretch upward with the vertically-rising mass flow. This instability may apply to arch filament eruptions (AFE) and coronal mass ejections (CMEs).To illustrate the nonlinear dynamical shearing instability, a numerical example is given for three different values of the plasma beta that span several orders of magnitude. The numerical results were analyzed using a linearized asymptotic approach in which an analytical approximate solution for velocity growth is presented. Finally, this theoretical model is applied to describe the arch filament eruption as well as CMEs.     

Nucleosynthesis in SNIa

The final products of a SNIa explosion critically depend on the degree of neutronization of the incinerated material and on the total amount of burned material. Here we study their dependence on the velocity of the burning front and on the density at which the thermonuclear runaway starts. The abundances of⁵⁴Cr,^{54, 58}Fe,^{58, 62}Ni provide some constraints to the possible values of model parameters.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.     

Explosive hydrogen-burning model of solar flares

A model of solar flare is proposed, taking into account the high temperature (10⁹K) produced by the shock wave generated by the hydromagnetic wave at the junction of sunspot and the area just outside it and subsequent explosive hydrogen burning, producing the desired 10^28–29 ergs of a solar flare.     

Mass motions associated with Hα active region arch structures

We have studied mass motions associated with active region arch structures from observations of a developing active region near the center of the solar disk. We present a method for the computation of the line-of-sight velocity from photographs at H ± 0.5 under the assumption of Beckers' cloud model and reasonable assumptions about the Doppler width and optical depth of the arches. Some arches show motions typical to arch filaments (the material moves towards the observer near the apex of the arch and away from the observer near the footpoints), while in others the velocity field is more complex. Assuming a symmetric loop, we reconstructed the velocity vector along an arch filament. The results are consistent with the picture where material is draining out of the filament while the whole structure is ascending with a velocity near that of the apex, which does not exceed 10 km s^–1. The motion is systematically slower than expected from a free-fall model.     

Angular momentum transport in pre-main-sequence stars of intermediate mass

Pre-Main-Sequence stars with masses between 2 and 5 M (Herbig Ae/Be stars) have radiative subphotospheric envelopes. However, they possess strong stellar winds and show definite signs of activity which could be linked to surface magnetic field. Therefore, they must lose angular momentum at a significant rate.We investigate the effect of such angular momentum losses on the internal structure of these stars, and on the distribution of angular velocity inside them. This paper presents a preliminary analysis guided by an analogy with laboratory and geophysical fluids. We propose that the friction exerted at the stellar surface by the angular momentum losses produces a mixed layer below the surface, separated from the unperturbed interior by an interface. Using scaling laws established by experimental studies of sheared stratified fluids, we discuss a simplified model for the evolution of the mixed layer.Although this model is still too preliminary to allow quantitative predictions, we show that for a reasonable choice of parameters, the mixed layer penetrates into the stellar interior on a time-scale of 10⁶ years, comparable to the Kelvin time-scale for the Herbig Ae/Be stars.     

Zeeman-doppler imaging: A new option for magnetic field study of Ap and solar-type stars

In the task of studying stellar magnetic fields, polarimetric methods have been intensively used in Ap stars. But the observational material classically used to reconstruct stellar magnetic structures (average longitudinal magnetic field as a function of rotational phase) is not rich enough in spatial information to derive geometries more complex than centered or decentered dipoles.In solar-type stars, all evidences of activity recently detected on their surfaces (starspots, flares, ...) indicate they are most likely magnetic stars. But polarimetric methods have always failed in these stars, probably due to the complex magnetic topologies encountered which even prevented until now a simple detection (Borra, Edwards, and Mayor, 1984). With the Zeeman broadening measurement technique proposed by Robinson (1980), no reliable results can be derived for rapid rotators, which are otherwise presumed to be the best candidates for magnetic detections. Once more, if temperature inhomogeneity charts are already available for solar-type stars (Vogt, 1987), spatial information on their magnetic distributions has conversely not yet been obtained.The new option, recently proposed by Semel (1989) and qualified by Donati, Semel, and Praderie (1989), is based on the rotational modulation study of a rapid rotator Stokes parameter V(), obtained with both high spectral resolution R, and high signal-to-noise ratio S/N. Since the magnetic information used refers to localized strips on the stellar disc (as a consequence of the star rotation), multipolar structures can thus be resolved.A new instrumentation and observing procedure have been defined for ZDI, in order to obtain very high S/N data. The method has been successfully tested on two bright magnetic Ap stars: a magnetic detection was obtained on UMa and a 15-point phase coverage of ² CVn is available for the reconstruction of complete 2D abundance and magnetic mappings of its photosphere.Concerning solar-type stars, a numerical simulation was carried out in order to determine the observational constraints required for the detection of typical magnetic field similar to those reported in slow rotators with the Robinson method (Saar, 1988). The specifications needed are S/N 400 per 40 mÅ pixel and R - 6 × 10⁴.     

Re-analysis of close binary systems

Line-forming regions around close binaries with strong winds ( /4r _* v 10^–4 g cm^–2) are large in extent compared with the stars, large enough to screen them. Their orbitally-modulated Doppler shifts can overestimate the mass function, because of a larger rotational lever arm. In particular, most of the black-hole candidates need not involve companions more massive than a neutron star.The solar-wind problem is reconsidered. An extrapolation to Wolf-Rayet stars suggests that their winds are centrifugally driven. Their mass-loss rates tend to have been overestimated.Seemingly single (massive) stars can hide a (compact) companion.     

Effects of rotation on colours and line indices of stars

We have analysed the broad-bandUBV colours and the intermediate banduvby colours of Persei, Pleiades, and the Scorpio-Centaurus association for rotation effects. An attempt was made to see if we can discriminate normal single stars from that of binary and peculiar stars after taking the observed rotation effects into account. It is found that the spread in the observed colours does not allow in general such a discrimination except that the objects with large reddening are double-lined binaries, peculiar stars or emission-lined objects. The few normal stars in these three clusters with such large reddening are listed as they are likely to belong to one of the above classes.On leave of absence from Assumption College, Changanacherry, Kerala.     

Solar-type stars in Cepheus OB 2: Hα emission stars and IRAS point sources

A survey for emission line stars was carried out in 1980 with the Schmidt telescope of Konkoly Observatory in the region of IC 1396 (Kun, 1986a, hereafter referred to as Paper I). This work was aimed at a preliminary mapping of the probable medium mass members of the association Cepheus OB 2. The stars of the region have been followed since then with the same telescope in order to obtain a more complete view on the stellar content of this giant star-forming region. The common variability of the H emission strength makes the sense of repeating such surveys. The new survey resulted in the discovery of 65 further emission stars. Equatorial coordinates and finding charts are given for them.PhotographicBVRI photometry was carried out for all known emission stars of the field. The colourmagnitude and two-colour diagrams derived from theB, V, R, andI magnitudes show that these stars are probably F-G-type (1.5M <M<3M ), partly pre-Main-Sequence members of Cep OB 2, whereas a few of them may be distant red giants.The youngest objects of the region can be found among the IRAS point sources. Their properties are discussed on the basis of IRAS data alone.     

Isolation of integrals for centroid orbits in stellar system models

In this paper we present two solutions for isolating integrals of the centroid motion in a non-stationary stellar system under the ellipsoidal hypothesis with point-axial symmetry.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.