Magnetic Activity of Dwarf G, K, & M Stars

As part of the ''Sun in Time'' Program, Guinan, DeWarf, Güdel (PSI), and Ribas have been carrying out multi-wavelength (X-ray to near-IR) observations of solar-type (G0 V-G5 V) stars with different ages. These stars serve as proxies for the Sun (and other solar type stars) and cover ages that include most of the Sun's main sequence life time. This program addresses a variety of topics that include: the study of short and long term magnetic evolution; the physics and energy transfer mechanisms of the chromosphere, transition region, and corona; and the evolution of the XUV spectral irradiance of the Sun and of the high energy radiation on paleo-planetary environments and atmospheres. As part of this program, excellent correlations were found among age, rotation period, and magnetically generated coronal X-ray and EUV emissions, Transition Region and Chromospheric FUV-NUV emissions. For example, for this narrow spectral range of solar type stars, the coronal X-ray emission of young main sequence early G stars are $\sim 100-1000$ times stronger than stars near the Sun's age of 4.6 Gyr.

This program recently has been expanded to include samples of nearby dK and dM stars. As part of the initial program, undergraduate students Joseph Drescher, Kelly Kolb, Joleen Miller, Laurie Barge and Ryan Hamilton have compiled the properties of new candidates. The stars selected for study have well determined parallaxes, colors, spectral types and also have measures of age ( = rotation) sensitive measures such as Lx, Ca II HK, Mg II hk emission fluxes. As was done in the ''Sun in Time'' program, most younger stars selected are members of clusters or moving groups. The ages of the some of the older, less active stars are estimated using isochronal fits. The ages of some of the dM stars were estimated from kinematical considerations or associations with nearby hotter stars. The initial results of this program and relations between magnetic activity indicators (such Lx or Ca II) and age (rotation) have been presented at the May 2003 AAS Meeting in Nashville. Tight relations for the late G and K stars were found as long as the physical properties of the stars (colors, $T_{\rm eff}$, or spectral types) are kept very narrow. The implications of this program for identifying late type stars that might be suitable for life were also presented.