The magnitude difference between the main sequence turn off and the red giant branch bump in Galactic globular clusters

¹ INAF – Osservatorio Astronomico di Teramo, via M. Maggini, 64100 Teramo, Italy
e-mail: cassisi@oa-teramo.inaf.it; pietrinferni@oa-teramo.inaf.it
² Centro de Estudios de Fisica del Cosmos de Aragon (CEFCA), 44001 Teruel, Spain
e-mail: amarin@cefca.es
³ Departamento de Astrofisica, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁴ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead CH41 1LD, UK
e-mail: ms@astro.livjm.ac.uk
⁵ Departamento de Astrofísica, Universidad de La Laguna, Tenerife, Spain
⁶ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
e-mail: aparicio@iac.es; monelli@iac.es

Received: 4 November 2010
Accepted: 25 November 2010

Abstract

We present new measurements of the magnitude of the main sequence turn off and the red giant branch bump in the luminosity function of a sample of Galactic globular clusters with updated estimates of [Fe/H] and [α/Fe], employing photometric data collected with the Advanced Camera for Survey onboard the HST. We compare measured and predicted values of the magnitude difference between these two features, a rarely employed diagnostic of the internal structure of low-mass stars at the beginning of their red giant evolution. Our analysis discloses a clear discrepancy between theory and observations, because the theoretical red giant branch bump magnitudes are too bright by on average  ~0.2 mag. This corroborates results from the more widely studied magnitude difference between horizontal branch and red giant bump, avoiding the well known problems associated with determining the horizontal branch level from colour-magnitude diagrams and with uncertainties in the luminosity of horizontal branch stellar models. We briefly discuss several potential solutions of this discrepancy.