First results from a two-dimensional numerical study of the buoyant rise of twisted magnetic flux tubes in the solar convection zone are presented. We show in detail the process by which the transverse component of the field can suppress the splitting of the rising tube into two vortex filaments. For the suppression to be effective, the pitch angle

We present a study of the old and intermediate-age star formation history (SFH) of the Local Group dwarf irregular galaxy (DIG) NGC 6822. It has been quantitatively obtained from the comparison of the observed [(V-I)_0_, M_I_] CM diagram with a set of model CM diagrams, computed assuming different evolutionary scenarios. The comparison has been

We present deep UBVRI CCD photometry of the resolved stellar content of the Local Group Dwarf Irregular Galaxy NGC 6822. The main advantage of this new data is that the color-magnitude (CM) diagrams obtained contain stars of virtually all ages and metallicities and hence are a record of the full star formation history of the galaxy. The wide field

The existence of systematic spectral-fit residuals in ROSAT PSPC spectra, and their dependence on time, is by now a well established fact. This paper describes how those residuals may be related to second order variations of the gain of the PSPC as a function of energy and time. As a result, the energy scale used for the interpretation of PSPC

This paper presents an atlas containing 59 low-resolution supernova spectra which correspond to 27 type Ia supernovae. These 59 spectra spectra which correspond to 27 type Ia supernovae. These 59 spectra refer to different ages along the postmaximum type Ia evolution, covering from the photospheric to the old (~>300 days) nebular phases. These

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