ROSAT X-Ray Observations of Late-Type Evolved Stars: On the Relationship between Coronal Temperatures and Luminosities

We present ROSAT Position Sensitive Proportional Counters (PSPC) X-ray observations of three near-solar-mass stars, in different evolutionary phases beyond the main sequence: eta Sco (F3 III-IV), iota Vir (F6 III), and HD 74772 (G5 III). All three of these nearby, presumably single stars have been detected, and we have collected enough counts to perform a detailed analysis of their soft X-ray spectra. While the X-ray spectra of eta Sco and HD 74772 can be fitted with Raymond-Smith thermal models with temperatures around 2 x 10⁶ K, the high signal-to-noise spectrum of iota Vir provides unambiguous evidence of a multitemperature plasma, with a two-temperature best-fit model with components at approximately 2 x 10⁶ K and 8 x 10⁶ K. Evidence of some hot plasma (T approximately 10⁷ K) has been also found for HD 74772. The present data, compared with spectral fitting results for other late-type stars observed with the Einstein Observatory, indicate that the low X-ray luminosity giants (L_x is less than 5 x 10²⁸ ergs/s) do not share with the higher X-ray luminosity stars of the same class the property of having substantial amount of 10⁷ K plasma. Moreover, our results confirm the trend of increasing X-ray luminosities with increasing coronal temperatures.