Prominence formation and ejection in cool stars

The observational signatures of prominences have been detected in single and binary G and K type stars for many years now, but recently this has been extended to the M dwarf regime. Prominences carry away both mass and angular momentum when they are ejected and the impact of this mass on any orbiting planets may be important for the evolution of exoplanetary atmospheres. By means of the classification used in the massive star community, that involves knowledge of two parameters (the co-rotation and Alfvén radii, r_K and r_A), we have determined which cool stars could support prominences. From a model of mechanical support, we have determined that the prominence mass m_p/M_*=(E_M/E_G)(r_*/r_K)²F where E_MB²_⋆r³_⋆ and E_G=GM²_⋆/r_⋆ are magnetic and gravitational energies and F is a geometric factor. Our calculated masses and ejection frequencies (typically 101⁶−10¹⁷g and 0.4 d, respectively) are consistent with observations and are sufficient to ensure that an exoplanet orbiting in the habitable zone of an M dwarf could suffer frequent impacts.