Strong Microwave Radiation from Solar-Twin'' GV Stars
Session 46 -- Late Type Stars
Display presentation, Wednesday, 9:20-6:30, Heller Lounge Room

## [46.07] Strong Microwave Radiation from Solar-Twin'' GV Stars

M. G\"udel (JILA/NIST\&CU), J.H.M.M. Schmitt (MPE Garching), A.O. Benz (ETH Z\"urich)

We report the detection of four solar-type main-sequence G stars as strong, steady 8.5 GHz VLA microwave sources. The targets were X-ray selected based on a previously reported relation between quiescent X-ray and microwave luminosities ($L_X$ and $L_R$) of active stars. $L_X$ was obtained from the ROSAT All-Sky Survey. The fluxes of the radio detections ($6\leq \sigma \leq 13$) match our predictions within $\sim$0.05 -- 0.2 dex (for age estimates, see references below): \begin{center} \begin{tabular}{lllllll} star & spect. & d(pc) & flux (mJy) & log$L_R$ & log$L_X$ & age (yrs) \\ & & & & & & \\ Gl 97 & G1V & 13 & 0.28$\pm$0.035 & 13.8 & 28.9 & $\sim 2\cdot 10^9$ \\ Gl 755 & G5V & 19 & 0.19$\pm$0.031 & 13.9 & 29.4 & ... \\ Gl 559.1 & dG0e & 21 & 0.34$\pm$0.025 & 14.3 & 29.6 & $\sim 0.07\cdot 10^9$ \\ HR 9107 & G2V & 29 & 0.19$\pm$0.030 & 14.3 & 29.5 & $\sim 10\cdot 10^9$ \\ \end{tabular} \end{center} Gl 97 (see, e.g., Soderblom ApJS 53,1) and Gl 755 are single MS stars. Gl 559.1 is a very rapidly rotating, chromospherically extremely active young star probably just settling on the main sequence (Soderblom \& Clements AJ 93, 920; Elias \& Dorren AJ 100, 818). A widely separated companion has been suspected (Duquennoy \& Mayor A\&A 248, 485), but we reason that the radio emission comes from the G star. The surprise detection is HR 9107, a metal-deficient, high space velocity, old-disk population star just leaving the MS (see Deliyannis et al. ApJS 73, 21).\\ Brightness temperature estimates based on an optically thin plasma likely suggest nonthermal emission, probably gyrosynchrotron as on other active stars. These detections extend the dichotomy between active and inactive stars into the range of solar-type stars. We are currently proposing detailed investigations of these stars. This research is supported by the Swiss National Science Foundation, NASA, CU, and NIST; the NRAO VLA is supported by Associated Universities, Inc. and the US NSF.