Large granulation cells on the surface of the giant star π1 Gruis.

Convection plays a major part in many astrophysical processes, including energy transport, pulsation, dynamos and winds on evolved stars, in dust clouds and on brown dwarfs. Most of our knowledge about stellar convection has come from studying the Sun: about two million convective cells with typical sizes of around 2,000 kilometres across are present on the surface of the Sun-a phenomenon known as granulation. But on the surfaces of giant and supergiant stars there should be only a few large (several tens of thousands of times larger than those on the Sun) convective cells, owing to low surface gravity. Deriving the characteristic properties of convection (such as granule size and contrast) for the most evolved giant and supergiant stars is challenging because their photospheres are obscured by dust, which partially masks the convective patterns. These properties can be inferred from geometric model fitting, but this indirect method does not provide information about the physical origin of the convective cells. Here we report interferometric images of the surface of the evolved giant star π1 Gruis, of spectral type S5,7. Our images show a nearly circular, dust-free atmosphere, which is very compact and only weakly affected by molecular opacity. We find that the stellar surface has a complex convective pattern with an average intensity contrast of 12 per cent, which increases towards shorter wavelengths. We derive a characteristic horizontal granule size of about 1.2 × 1011 metres, which corresponds to 27 per cent of the diameter of the star. Our measurements fall along the scaling relations between granule size, effective temperature and surface gravity that are predicted by simulations of stellar surface convection.

[Causes of corrosive mechanical defects in isoelastic endoprostheses]. / Príciny korozne mechanických poruch izoelastických endoprotéz.

Within the framework of evaluation of endoprostheses of the hip joint reoperated at the orthopaedic clinic of the Institute for Postgraduate Medical Training, Faculty Hospital Na Bulovce in Prague in the course of six years the authors made a more detailed analysis of the group of isoelastic POLDI endoprostheses which displayed obvious signs of corrosion. The specimens from this group were subjected to evaluation focused on corrosion, metallographic and fractographic evaluation. In impaired wires of isoelastic endoprostheses the fracture surfaces have a fatigue character where a corrosion fatigue mechanism may be assumed as well fracture with a dominant mechanical component of stress. From the analysis of corrosion ensues that the connections of the wire shank with the head of the endoprosthesis are affected by crevice corrosion, the revealed fractures and cracks are most probably associated with corrosion fatigue. The construction of isoelastic prostheses of this type is the cause of corrosion processes which lead to mechanical failure. This should be the reason to refuse them for patients of any age group.