The brightness of the red giant branch tip

Theoretical framework, a set of reference models, and predicted observables

¹ Institute of Space Sciences (IEEC-CSIC) Campus UAB, Carrer de Can Magrans, s/n, 08193, Barcelona Spain
e-mail: aldos@ice.csic.es
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 86748 Garching, Germany
³ INAF–Osservatorio Astronomico di Teramo, via M. Maggini, 64100 Teramo, Italy
⁴ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK

Received: 18 April 2017
Accepted: 27 June 2017

Abstract

Context. The brightness of the tip of the red giant branch is a useful reference quantity for several fields of astrophysics. An accurate theoretical prediction is needed for such purposes.

Aims. We provide a solid theoretical prediction for the brightness of the tip of the red giant branch, valid for a reference set of standard physical assumptions, and mostly independent of numerical details.

Methods. We examine the dependence on physical assumptions and numerical details for a wide range of metallicities and masses and based on two different stellar evolution codes. We adjust differences between the codes to treat the physics as identically as possible. After we have succeeded in reproducing the tip brightness between the codes, we present a reference set of models based on the most up to date physical inputs, but neglecting microscopic diffusion, and convert theoretical luminosities to observed infrared colours suitable for observations of resolved populations of stars and include analytic fits to facilitate their use.

Results. We find that consistent use of updated nuclear reactions, including an appropriate treatment of the electron screening effects, and careful time-stepping on the upper red giant branch are the most important aspects to bring initially discrepant theoretical values into agreement. Small but visible differences remain unexplained for very low metallicities and mass values at and above 1.2 M_⊙, corresponding to ages younger than 4 Gyr. The colour transformations introduce larger uncertainties than the differences between the two stellar evolution codes.

Conclusions. We demonstrate that careful stellar modelling allows an accurate prediction for the luminosity of the red giant branch tip. Differences to empirically determined brightnesses may result either from insufficient colour transformations or from deficits in the constitutional physics. We present the best-tested theoretical reference values to date.