Radio-continuum observations of a variety of cool stars

We present radio-continuum observations made at 2 and/or 6 cm of 26 cool stars (F0 and later), including 10 F-K main-sequence stars and 16 F-M giant and supergiant stars. The data were obtained as part of several Very Large Array² observing programs. 22 of the observed stars were not detected as sources and have typical 3σ upper limits of ∼0.35 mJy at 2 cm and ∼0.15-0.60 mJy at 6 cm. The failure to detect any of the F-K dwarf stars reconfirms that such stars have intrinsically weak (log L_ν < 13-14 at 6 cm) radio emission, even if they have rotational periods and/or x-ray luminosities that are similar to those of the (radio detected) RS CVn binary systems. The evolved stars that were detected as radio sources include two M giants (g Her and R Lyr) and the infrared carbon star IRC + 10216. The results of this program when combined with those of our previous studies suggest the existence of a discontinuity in the average ionization fractions of the winds of red giants that occurs at about a spectral type of M6 III. This is also the point at which significant dust formation is observed in red giants. Giants earlier than M6 have f_ion ∼10^-2, while cooler giants have f_ion <≤10^-4. The radio source associated with IRC + 10216 may be partially resolved at 2 cm; the brightness temperature at 2 cm, assuming that this is the case, is 1920 K. If the source is in fact unresolved, then the "true" brightness temperature must be greater than this. We discuss alternate models for the 2 and 6 cm radio emission of IRC + 10216 and conclude that the 6 cm emission is consistent with free-free emission from an expanding, partially ionized (f_ion≤10^-4) wind with an ionized mass loss rate of ∼3×10^-9 Script M sign_⊙ yr^-1.