1995A&A...302..775G

A sample of F and G main sequence stars and slightly evolved F and G stars, selected as the apparently strongest X-ray sources in their class as detected in the ROSAT All-Sky Survey (RASS), has been observed in microwaves to search for coronae with strong heating and populations of nonthermal particles. The microwave flux densities were observed with the VLA at 8.4GHz. Radio emission has been detected from nine targets, in both luminosity classes V and IV. Since known or unknown cool companions in binary systems may cause spurious results, we have checked the available spectroscopic and astrometric data, including unpublished CORAVEL observations. There is at least one detected object in each of the four spectral and luminosity classes of stars, FIV, FV, GIV, and GV for which no known companion can be made responsible for the observed emission. A very luminous X-ray and radio source is identified with the F0 V star HD 12230, a member of the Pleiades Moving Group with an age of the order of 50-70Myr. HD 129333 (EK Dra), a G0 V target presumably of the same age, is detected also, and the X-ray and radio modulations agree with the optically measured rotation. On the other hand, three very old stars that are leaving the main sequence and are moving towards the subgiant luminosity class are found to be strong X-ray and radio emitters; in the case of HD 20010, an F8 IV star, the hypothetical existence of an unknown spectroscopic companion would contradict astrometric data. These stars appear to define a new class of radio-luminous coronal stars. The observed microwave flux densities agree with the ratio of radio to X-ray fluxes of other active coronal stars. We report sensitive upper limits for all non-detections, up to an order of magnitude lower than in previous surveys. These observations yield first systematic evidence that stars close to the solar spectral type can maintain considerable nonthermal electron populations in their coronae, possibly due to a mechanism that involves coronal heating. They provide the crucial link between the study of the solar corona and of active coronal stars (the "solar-stellar connection"), and bridge the remaining gaps on the radio main sequence between the cooler stars and chemically peculiar Ap stars. Further, they support the view that young, near-Zero-Age Main-Sequence (ZAMS) stars are able to continually produce luminous radio emission after their arrival on the ZAMS. The strong activity resurgence in the sample of old stars moving off the main sequence may be related to an increase in convective turnover time as the internal structuring of the stars changes; this is of potential interest for the study of the stellar interior of evolved stars.