Abstract
NGC288 is a globular cluster with a well-developed blue horizontal branch covering the u-jump that indicates the onset of diffusion. It is therefore well suited to study the effects of diffusion in blue horizontal branch (HB) stars. We compare observed abundances with predictions from stellar evolution models calculated with diffusion and from stratified atmospheric models. We verify the effect of using stratified model spectra to derive atmospheric parameters. In addition, we investigate the nature of the overluminous blue HB stars around the u-jump. We defined a new photometric index sz from uvby measurements that is gravity-sensitive between 8000K and 12000K. Using medium-resolution spectra and Stroemgren photometry, we determined atmospheric parameters (T_eff, log g) and abundances for the blue HB stars. We used both homogeneous and stratified model spectra for our spectroscopic analyses. The atmospheric parameters and masses of the hot HB stars in NGC288 show a behaviour seen also in other clusters for temperatures between 9000K and 14000K. Outside this temperature range, however, they instead follow the results found for such stars in omega Cen. The abundances derived from our observations are for most elements (except He and P) within the abundance range expected from evolutionary models that include the effects of atomic diffusion and assume a surface mixed mass of 10^-7 M0. The abundances predicted by stratified model atmospheres are generally significantly more extreme than observed, except for Mg. When effective temperatures, surface gravities, and masses are determined with stratified model spectra, the hotter stars agree better with canonical evolutionary predictions. Our results show definite promise towards solving the long-standing problem of surface gravity and mass discrepancies for hot HB stars, but much work is still needed to arrive at a self-consistent solution.
Highlights
Low-mass stars that burn helium in a core of about 0.5 M and hydrogen in a shell populate a roughly horizontal region in the optical colour−magnitude diagrams of globular clusters, which has earned them the name horizontal branch (HB) stars
NGC 288 is a globular cluster with a well-developed blue horizontal branch (HB) covering the u-jump that indicates the onset of diffusion
Our results show definite promise towards solving the long-standing problem of surface gravity and mass discrepancies for hot HB stars, but much work is still needed to arrive at a self-consistent solution
Summary
Low-mass stars that burn helium in a core of about 0.5 M and hydrogen in a shell populate a roughly horizontal region in the optical colour−magnitude diagrams of globular clusters, which has earned them the name horizontal branch (HB) stars (ten Bruggencate 1927). A large photometric survey of many globular clusters by Grundahl et al (1999) demonstrated that the Strömgren u-brightness of blue HB stars suddenly increases near 11 500 K This u-jump is attributed to a sudden increase in the atmospheric metallicity of the blue HB stars to super-solar values that is caused by the radiative levitation of heavy elements. Recent evolutionary models of HB stars that include atomic diffusion calculated by Michaud et al (2008, 2011) can reasonably well reproduce the abundance anomalies of several elements observed in blue HB stars. These models do not treat the atmospheric region in detail. A variation in helium this small is too small to be detected by our analysis
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