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

TL;DR

This study measures the magnitude difference between the main sequence turn off and the red giant branch bump in Galactic globular clusters, revealing a significant discrepancy between observed data and theoretical predictions.

Contribution

It provides new empirical measurements of these features using HST data and compares them with stellar evolution models, highlighting a notable inconsistency.

Findings

Theoretical red giant branch bump magnitudes are too bright by about 0.2 mag.

The magnitude difference diagnostic is less affected by horizontal branch uncertainties.

The discrepancy suggests potential issues in current stellar evolution models.

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 [$\alpha$/Fe], employing photometric data collected with the Advanced Camera for Survey on board 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, the theoretical red giant branch bump magnitudes being too bright by on average $\sim0.2$~mag. This corroborates results from the more widely studied magnitude difference between horizontal branch and red giant bump, avoiding the well known problems associated to the determination of the horizontal branch level from colour magnitude diagrams, and to uncertainties in the luminosity of horizontal branch stellar models. We briefly discuss several potential solutions of this discrepancy.