High-resolution Optical Observations of Extreme-ultraviolet–selected Active Late-type Stars

We present high-resolution optical spectra of EUV-selected late-type stars. We have detected seven new spectroscopic binaries, including a pair of F stars and an RS Canum Venaticorum candidate. Many of the stars in our sample show remarkable H emission with equivalent widths up to 8 Å. Several sources show strong He i 5876 Å emission. We use cross-correlation techniques to estimate line-of-sight rotational velocities and find that they range from less than 5 to over 80 km s 1. Several of the stars show a strong Li 6707 Å line, which we use to determine their Li abundances. The EUV-to-bolometric luminosity, a measure of the coronal heating efficiency, is as high as 10 3. The present sample extends the EUV-selected late-type stars to lower Rossby numbers and makes the coronal saturation boundary better pronounced. Our results indicate that there is a drop in the EUV emission from the corona in late M dwarfs possibly because of a change in the nature of the magnetic dynamo in fully convective stars.