Pulsation Period Changes as a Tool to Identify Pre-Zero Age Horizontal Branch Stars

TL;DR

This paper explores the potential of using pulsation period changes to identify pre-zero age horizontal branch stars, which are difficult to detect due to their short duration and overlapping characteristics with other stellar phases.

Contribution

It introduces a method to detect pre-ZAHB stars via their pulsation period changes, supported by stellar models and simulations relevant to the globular cluster M3.

Findings

Some RR Lyrae stars with high period change rates are likely pre-ZAHB pulsators.

Pre-ZAHB stars can be distinguished by their rapid period change rates.

The study provides a new approach to identify a short-lived stellar phase.

Abstract

One of the most dramatic events in the life of a low-mass star is the He flash, which takes place at the tip of the red giant branch (RGB) and is followed by a series of secondary flashes before the star settles into the zero-age horizontal branch (ZAHB). Yet, no stars have been positively identified in this key evolutionary phase, mainly for two reasons: first, this pre-ZAHB phase is very short compared to other major evolutionary phases in the life of a star; and second, these pre-ZAHB stars are expected to overlap the loci occupied by asymptotic giant branch (AGB), HB and RGB stars observed in the color-magnitude diagram (CMD). We investigate the possibility of detecting these stars through stellar pulsations, since some of them are expected to rapidly cross the Cepheid/RR Lyrae instability strip in their route from the RGB tip to the ZAHB, thus becoming pulsating stars along the way. As a consequence of their very high evolutionary speed, some of these stars may present anomalously large period change rates. We constructed an extensive grid of stellar models and produced pre-ZAHB Monte Carlo simulations appropriate for the case of the Galactic globular cluster M3 (NGC 5272), where a number of RR Lyrae stars with high period change rates are found. Our results suggest that some -- but certainly not all -- of the RR Lyrae stars in M3 with large period change rates are in fact pre-ZAHB pulsators.