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Centro de Astrofísica da Universidade do Porto

Advanced course on chromospheric activity

Giancarlo Pace

The chromosphere is the layer of a late-type star between the temperature minimum, where the temperature starts increasing outwards, and the transition region, where a very steep temperature gradient occurs. In the Sun it is seen as a reddish ring during a total eclipse. Its occurrence is due to the presence of a convective envelope and a magnetic field. The energy emitted in the chromosphere is referred to as chromospheric activity. Chromospheric activity varies at different time scales and some stars show activity cycles with periods of the order of the decade. One such cycles is the well known 11-year solar cycle. Among the various phenomena associated with the chromosphere, spots stand out as the most easily detectable in the Sun and in nearby active stars, allowing us to measure rotation period in the latter. Systematic observations of Sun spots date back to the time of Galileo, and they continue to provide precious insight to the nature of chromospheric activity. Their 400-year old record of observations allowed us to detect the Maunder minimum, a time of exceptionally low solar activity. Chromospheric activity in stars received a lot of attention due to its alleged potential as age indicator and, more recently, because of its effect on high-precision radial velocity measurements. In this course I will both explore the present theoretical understanding of chromospheric activity, going deep into the physics behind magnetic fields in stars and related phenomena, and review observations and their impact in other fields of astrophysics.

Sessão 1: The theoretical aspect
23 abril 2013

A minimum background on stellar atmospheres necessary to proceed on, a definition of chromospheric activity and of all the phenomena associated with it (spot, prominence, spicule, flares, plage...), and the physical processes responsible for their manifestation, which include the generation of a magnetic field in stars.

Sessão 2: The phenomenological and observational aspect
30 abril 2013

Period measurement through light-curve modulation from the spot, evolution of rotation with time (gyrochronology), evolution of chromospheric activity with time, interference with high-precision radial velocity measurements, and the Wilson-Bappu effect.