Radio Continuum Observations of G-M Giants and Supergiants and Inferred Ionized Mass Loss Rates

Abstract

We have conducted a 6 cm survey of 33 nearby G-M giants and supergiants with the NRAO VLA.* We have detected α¹ Her (M5 II) at 0.9 mJy and α Boo (K2 IIIp) at 0.27 mJy, and obtained upper limits for the remaining 31 stars significantly lower than most previous values. If we interpret both detections as being due to radio emission from partially ionized stellar winds, we derive ionized mass loss rates of 2 × 10^-9 M_Θ yr^-1 (α¹ Her) and 7 × 10^-11 M_Θ yr^-1 (α Boo). We infer an ionization fraction for the wind of α¹ Her of 1.4%, significantly higher than the upper limits of 0.045 and 0.012% for the two other M5 stars (RX Boo and W Hya) that were observed. The weak radio source identified with a Boo appears to share the high proper motion of the star (~2.5 per year), confirming its association with the K giant. This is probably another example of a stellar wind-type source, but we cannot rule out at this time the possibility that the radio emission is from a static, extended chromosphere (T_e ~ 10⁴ K) of angular diameter about 1.7 times the photospheric angular diameter.