Hot horizontal branch stars in NGC 288 – effects of diffusion and stratification on their atmospheric parameters^{⋆,⋆⋆,⋆⋆⋆}

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
e-mail: smoehler@eso.org
² Institut für Theoretische Physik und Astrophysik, Olshausenstraße 40, 24118 Kiel, Germany
³ Georg-August-Universität, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: dreizler@astro.physik.uni-goettingen.de
⁴ Département de Physique et d’Astronomie, Université de Moncton, Moncton, New Brunswick, E1A 3E9, Canada
e-mail: francis.leblanc@umoncton.ca; viktor.khalak@umoncton.ca
⁵ Département de physique, Université de Montréal, Montréal, Québec, H3C 3J7, Canada
e-mail: michaudg@umontreal.ca; jacques.richer@umontreal.ca
⁶ NASA Goddard Space Flight Center, Exploration of the Universe Division, Code 667, Greenbelt MD 20771, USA
e-mail: allen.sweigart@gmail.com
⁷ Stellar Astrophysics Centre, Department of Physics & Astronomy, University of Århus, Ny Munkegade 120, 8000 Århus C, Denmark
e-mail: fgj@phys.au.dk

Received: 31 October 2013
Accepted: 11 March 2014

Abstract

Context. NGC 288 is a globular cluster with a well-developed blue horizontal branch (HB) covering the u-jump that indicates the onset of diffusion. It is therefore well suited to study the effects of diffusion in blue HB stars.

Aims. 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.

Methods. We defined a new photometric index sz from uvby measurements that is gravity-sensitive between 8000 K and 12 000 K. Using medium-resolution spectra and Strömgren 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.

Results. The atmospheric parameters and masses of the hot HB stars in NGC 288 show a behaviour seen also in other clusters for temperatures between 9000 K and 14 000 K. Outside this temperature range, however, they instead follow the results found for such stars in ω 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 M_⊙. 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.

Conclusions. 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.