Aims: We study the equilibrium and stability of twisted magnetic flux tubes with mass flows along the field lines. Then, we focus on the stability and oscillatory modes of magnetic tubes with uniform twist B0 = B0(r/p eϕ + ez) in a zero-β plasma, surrounded by a uniform, purely longitudinal field. Methods: First we investigate the possible

I report on the discovery of two faint (~26.8 R mag arcsec-2) tidal tails around the shell elliptical galaxy NGC 3656 (Arp 155). This galaxy had previously been interpreted as a case of accretion, or minor merger. The two tidal tails are inconsistent with a minor merger and point instead to a disk-disk major merger origin. NGC 3656 extends Toomre's

Aims: We present here two H II region catalogues with azimuthal resolution for the two grand design galaxies NGC 628 and NGC 6946. With the help of these catalogues, we study several properties of the star-forming processes occurring in spiral galaxies. Methods: We obtained direct imaging in the narrow-band filters centred at Hα, Hβ, [O II]λ3727

We investigate the effects of interactions between ions and neutrals on the chromosphere and overlying corona using 2.5D radiative MHD simulations with the Bifrost code. We have extended the code capabilities implementing ion–neutral interaction effects using the generalized Ohm’s law, i.e., we include the Hall term and the ambipolar diffusion

Context. Spectropolarimetry at high spatial and spectral resolution is a basic tool to characterize the magnetic properties of the solar atmosphere. Aims: We introduce the KIS/IAA Visible Imaging Polarimeter (VIP), a new post-focus instrument that upgrades the TESOS spectrometer at the German Vacuum Tower Telescope (VTT) into a full vector

A brief description of the HEXAFLEX system is presented, which has been contrived to carry out two-dimensional spectroscopy (spectral imaging) in combination with the FLEX spectrograph at the 4.2 m William Herschel Telescope. Some astronomical illustrative examples are shown.