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

Abstract

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.

Chemical Abundances and Rotation Velocities of Blue Horizontal‐Branch Stars in Six Globular Clusters

“…21), with hints of a slight rise, no more than 0.5 dex on average, among the hottest stars of M13 and M15. This pattern of overabundant iron and normal magnesium that we observe in our stars matches the prior results of Glaspey et al (1989) and the recent work by Moehler et al (1999) on the southern metal-poor globular NGC 6752. Moehler et al observe 42 BHB stars at medium spectral resolution and derive [Fe/H] and [Mg/H] from the strongest lines of each species.…”

“…Underabundances of helium have been observed previously in many hot, evolved stars, including subdwarf B (sdB) stars in the field and the analogous EHB stars in globular clusters (Baschek 1975, p. 101;Heber 1987;Moehler et al 2000). Michaud, Vauclair, & Vauclair (1983), building on the original suggestion by Greenstein, Truran, & Cameron (1967), explained these underabundances as a result of gravitational settling of helium.…”

“…For stars cooler than $11,500 K, Grundahl et al found good agreement between the observed and theoretical BHB loci, but the hotter stars' u magnitudes were brighter than predictions by $0.4 mag. These u-jumps have now been found in every one of 14 globular clusters studied by Grundahl et al (1999), suggesting that some mechanism, not included in the stellar models, affects all HB stars hotter than 11,500 K. Similarly, surface gravities derived from Balmer and helium line profile fitting (Moehler, Heber, & DeBoer 1995;Moehler, Heber, & Durrell 1997a;Moehler, Heber, & Rupprecht 1997b;Moehler et al 2003) exhibit a systematic offset toward lower log g for 12;000 K < T eff < 20; 000 K, while stars outside this temperature range agree well with theoretical predictions. The magnitude of this '' low-gravity jump '' is small, but it appears consistently in a number of different clusters and has proven difficult to account for via luminosity evolution or higher helium content, again suggesting that the influence of some additional stellar characteristic is not fully accounted for in the models.…”

“…The cooler one, at T eff ' 10;000 K, showed [Fe/H ¼ À1:5, as expected for this cluster, but the hotter star, at 16,000 K, was found to be strongly enhanced in iron and depleted in helium. More comprehensive surveys of abundances of BHB stars in this temperature range were done by Behr et al (1999) in M13, and Moehler et al (1999) in NGC 6752. In both cases, a sharp transition from normal abundances to strongly metal-enhanced, helium-depleted photospheres was found at T eff ' 11;500 K. Shortly thereafter, Behr et al (2000a) found a nearly identical pattern in M15. In this paper, we undertake a full reanalysis of our complete data set, parts of which were previously published by Cohen & McCarthy (1997) and Behr et al (1999Behr et al ( , 2000aBehr et al ( , 2000b.…”

Observations of the hot horizontal-branch stars in the metal-rich bulge globular cluster NGC 6388

“…Previous experience with the analysis of hot stars in globular clusters has shown that low surface gravities are often associated with impossibly small HB masses, thus indicating that the surface gravities are not reliable. To carry out this consistency check, we have estimated masses for all of our target stars using the approach outlined by Moehler et al (2000). Figure 6 compares these masses with the theoretical ZAHB masses for both canonical and helium-rich compositions.…”