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Assuming that the interchange convection of magnetic flux elements is the physical cause for the existence of filamentary penumbrae in sunspots, we investigate the behavior of an individual fibril embedded in the deep penumbra. The fibril is approximated by a thin magnetic flux tube which evolves dynamically in the environment given by the global magnetostatic model of a sunspot. Our simulation shows that the flux tube, initially positioned at the penumbra--quiet Sun boundary in the model, will rise through its deep penumbra developing a flow along the tube that points upward beneath the photosphere, and radially outward above the photosphere. Our results suggest that a bright filament may be formed by an extended tail of a penumbral grain. Such filaments are optically thick, hotter than the surroundings, and elevated above a darker background. An upflow in penumbral grains bends horizontally outwards above the photosphere and gradually cools down due to radiative losses leading to a tail that gradually darkens. The plasma flow inside the flux tube then becomes transparent and the tube constitutes a thin elevated flow channel, that can reproduce the observed features of the Evershed effect. We present also a new acceleration mechanism for the Evershed flow. It is demonstrated that a local surplus of gas pressure develops inside the tube as it rises through the specific (superadiabatic and magnetized) penumbral background. The resulting gradient of the gas pressure can drive the flow along the tube., Comment: 15 pages, 9 POSTSCRIPT figures. A & A, Accepted on 26 June 1998

Relying on the assumption that the interchange convection of magnetic flux tubes is the physical cause for the existence of sunspot penumbrae, we propose a model in which the dynamical evolution of a thin magnetic flux tube reproduces the Evershed effect and the penumbral fine structure such as bright and dark filaments and penumbral grains. According to our model, penumbral grains are the manifestation of the footpoints of magnetic flux tubes, along which hot subphotospheric plasma flows upwards with a few km/s. Above the photosphere the hot plasma inside the tube is cooled by radiative losses as it flows horizontally outwards. As long as the flowing plasma is hotter than the surroundings, it constitutes a bright radial filament. The flow confined to a thin elevated channel reaches the temperature equilibrium with the surrounding atmosphere and becomes optically thin near the outer edge of the penumbra. Here, the tube has a height of approximately 100 km above the continuum and the flow velocity reaches up to 14 km/s. Such a flow channel can reproduce the observed signatures of the Evershed effect., Comment: 5 pages, 2 figures, accepted for publication in ApJ letters

The motion of an individual magnetic flux tube inside the penumbra of a sunspot is studied numerically. Here, we present preliminary results. The thin flux tube approximation together with a simplified radiative heat exchange with the surroundings is used to study the evolution of a flux tube embedded into a background given by a global magneto-static sunspot model. The investigation is undertaken in order to verify the conjecture that convection in sunspot penumbrae occurs by an interchange of magnetic flux tubes. The code being developed can be used to study dynamic aspects of filamentary structure in the penumbra: the temporal and spatial fluctuations of the temperature and the magnetic field, the motion of bright penumbral grains, or the Evershed effect. Here we present the evolution of a wave formed by the tube whose fragment emerges in the penumbral photosphere and migrates towards the umbra. The properties of this wave show qualitative features of the observed bright penumbral grains with corresponding upward velocity and its correlation with brightness and the inclination of the magnetic field, and also of the Evershed effect., Comment: 6 pages, tex, two figures (not included), submitted to Astrophysical Letters and Comments, compressed postscript file including figures is available at ftp://sirius.astrouw.edu.pl/pub/submit/crj/tube.ps.gz

Fifteen variable stars have been discovered during four nights of monitoring of NGC 7789. It was possible to determine periods for ten of these variables. Seven objects are W UMa-type binaries, in this number five are members of the cluster. Among the remaining variables, there are three further close binaries, one delta Scuti which is a Blue Straggler in the cluster, and one multi-modal variable (with indeterminable periods). One of the binaries showed an eclipse-like event; this binary is a good candidate for determination of masses of its components at the turn-off point of the cluster., Comment: A&A Suppl. in press, 7 pages, uuencoded compressed PostScript, OAUW-94-9

The conjecture that energy transport in sunspot penumbrae occurs through convection by an interchange of magnetic flux tubes is used in order to construct a magnetostatic model for a deep penumbra which is part of a global sunspot model. The model describes an overall mechanical equilibrium between the three stratifications: of the umbra, the penumbra, and of the quiet sun, all of them being treated as stellar like envelopes. Local differences between the gas pressure in the neighbouring stratifications are balanced by the magnetic forces confined to two current sheets: the magnetopause and the peripatopause, whose shapes are determined by the solution of a free boundary problem. A self consistent coupling between global magnetostatic and thermal equilibrium is achieved by the assumption of a perfect thermal isolation of the umbra and a parametrized inflow of the heat into the penumbra. Characteristic parameters observed in sunspots have been imposed as surface constraints for the model. Then only models with a total magnetic flux larger than about 3x10^21 Mx turn out to be feasible. Smaller tubes would require more heat flux than is available in the solar convection zone if they contained an extended penumbra. The solutions obtained indicate that the penumbra contains 50-60% of the total magnetic flux of the spot and has a depth comparable with half the radius of the spot. About 60% of the solar heat flux impinging on the magnetopause seems to be transmitted into the penumbra and conveyed to the photosphere by the interchange convection which can be maintained if the average inclination of the magnetic field exceeds about 30deg. Such a penumbra forming an extended, Comment: A&A 290,295; 23 pages, uuencoded compressed PostScript, figures on request (crj@hydra.astrouw.edu.pl), MPA-801