Hot HB stars in globular clusters – Physical parameters and consequences for theory^*

VI. The second parameter pair M 3 and M 13

¹ Institut für Theoretische Physik und Astrophysik, Abteilung Astrophysik, Leibnizstraße 15, 24098 Kiel, Germany
² Dr. Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
³ SSAI, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA e-mail: landsman@mpb.gsfc.nasa.gov
⁴ NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA e-mail: sweigart@bach.gsfc.nasa.gov
⁵ Institute of Physics and Astronomy, Aarhus University, Ny Munkegade, 8000 Aarhus C, Denmark e-mail: fgj@ifa.au.dk

Corresponding author: S. Moehler, moehler@astrophysik.uni-kiel.de

Received: 4 October 2002
Accepted: 23 April 2003

Abstract

We present the results of spectroscopic analyses of hot horizontal branch (HB) stars in M 13 and M 3, which form a famous “second parameter” pair. From the spectra and Strömgren photometry we derived – for the first time in M 13 – atmospheric parameters (effective temperature and surface gravity). For stars with Strömgren temperatures between 10 000 and 12 000 K we found excellent agreement between the atmospheric parameters derived from Strömgren photometry and those derived from Balmer line profile fits. However, for cooler stars there is a disagreement in the parameters derived by the two methods, for which we have no satisfactory explanation. Stars hotter than 12 000 K show evidence for helium depletion and iron enrichment, both in M 3 and M 13. Accounting for the iron enrichment substantially improves the agreement with canonical evolutionary models, although the derived gravities and masses are still somewhat too low. This remaining discrepancy may be an indication that scaled-solar metal-rich model atmospheres do not adequately represent the highly non-solar abundance ratios found in blue HB stars affected by diffusion. We discuss the effects of an enhancement in the envelope helium abundance on the atmospheric parameters of the blue HB stars, as might be caused by deep mixing on the red giant branch or primordial pollution from an earlier generation of intermediate mass asymptotic giant branch stars.