ACTIVITY VERSUS ROTATION IN THE EXTREME-ULTRAVIOLET

This paper presents extreme ultraviolet (EUV) fluxes for 74 main-seque nce stars observed with the Extreme Ultraviolet Explorer (EUVE). The s ample of stars was chosen on the basis of rotational period and includ es 23 new EUV detections. These detections are in addition to the ROSA T WFC and EUVE catalogs and have increased the number of main-sequence stars detected in the extreme ultraviolet by similar to 12%. We prese nt an activity-rotation relation in the EUV and show that in a sample of stars with different effective temperatures the Rossby number is a better parameter for describing the levels of EUV emission as compared to the rotational period. This is also known to be the case for the c hromospheric emission. A comparison with the emission in the Mg II h a nd k lines shows that saturation in the EUV occurs in lower Rossby num bers than does chromospheric saturation, where saturation in X-rays oc curs in the same Rossby numbers as in the EUV. The radiative losses in the EUV are comparable to the radiative losses in X-rays. The EUVE ob servations indicate that the low activity dM star Gl 685 has a cool co rona with a characteristic temperature of < 2.5 x 10(6) K. A large amo unt of the coronal radiative losses in low activity dwarfs is expected to emerge in EUV wavelengths.