AAS 195th Meeting, January 2000
Session 44. Late Stages of Stellar Evolution
Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall

## [44.01] Evidence for Non-standard Post-turnoff Stellar Evolution from HIPPARCOS F Stars

A. A. Suchkov (STScI)

We find that HIPPARCOS F stars above the locus of the turnoffs in the \log Te - MV diagram have kinematics that is too old for the age derived from isochrone fitting, suggesting that the observed absolute magnitudes of these stars are, on average, brighter than predicted by the standard'' stellar evolution modeling. The anomalous brightness can partly be attributed to unidentified binaries (Suchkov &\ McMaster, 1999, ApJ, 542, L99). However, the combined flux from the normally evolving components of a binary cannot explain the magnitude of the effect; much of it appears to be due to a truly enhanced luminosity of individual stars, implying non-standard, slower stellar evolution.

Analysis of age and kinematics of stars for which we can derive two independent estimates of absolute magnitude, one from the HIPPARCOS parallax and the Johnson V magnitude, MV, and another from the dereddened uvby color index c0, Mc_0, seems to indicate that the anomalous luminosity of the post-turnoff stars is associated mostly with binaries. This is in line with another recent finding that has revealed age discrepancy for components of eclipsing binaries similar to that reported here: the evolved, brighter component in these binaries looks in many cases younger than the fainter component (Popper 1997, AJ, 114, 1195). This obviously suggests that normal stellar evolution is modified in (close) binaries. However, anomalous luminosity, hence non-standard stellar evolution of evolved single stars as well cannot be ruled out at this point.

Finally, there is an indication that the brightness anomaly is more pronounced in old and metal poor stars. However, since older stars are typically more metal poor, additional analysis is required to establish which of the two factors plays a role in that effect.

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