ABSTRACT:

The outer atmospheres of late-type stars are characterized by strong emission in excess of the stellar photosphere. Through its relation with rotation the origin of this activity in a stellar dynamo was recognized long ago. Yet, the strength of magnetic activity has not been calibrated across the whole electromagnetic spectrum and its dependence on stellar mass and age has remained widely elusive. Most previous efforts have concentrated on solar-type stars because of their relative brightness and the direct connection with the Sun. Here, I present recent observational studies of magnetic activity on M stars. Low-mass stars are interesting planet hosts because they exist in large numbers, they are long-lived, and they provide a lower contrast between stellar/planet properties enhancing the chance for planet detection. The high-energy emission related with magnetic activity may be crucial for the evolution of planetary atmospheres, in particular for M stars which are notoriously strongly active. I present a wide range of diagnostics of chromospheric and coronal emission in the X-ray, UV and optical band. The data discussed comprise both photometric observations (obtained with GALEX, ROSAT and XMM-Newton) and spectroscopic measurements (obtained with X-Shooter@VLT). The aim of these studies is to establish connections between the emissions in different energy bands and determine how they change throughout the stellar evolution.