Aims. We aimed at finding a physical explanation for the occurrence of macroturbulence in the atmospheres of hot massive stars, a phenomenon found in observations for more than a decade but that remains unexplained. Methods: We computed time series of line profiles for evolved massive stars broadened by rotation and by hundreds of low-amplitude

Collisional excitation of neutral hydrogen atoms can significantly increase the intensity of Balmer lines with respect to pure recombination. If this effect is not taken into account, the abundance analysis of these objects returns biased values, and the bias may be significant if accuracies better than a few percent are required. The most affected

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Comet Hale-Bopp defines a milestone event for cometary science: it is the first "really big" comet observed with modern equipment on the ground and from space and due to that; it is considered the new reference object in cometary sciences. At the beginning of a new era in spacecraft exploration of comets and five years after Hale-Bopp's perihelion