Abstract

Context. 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 and is followed by a series of secondary flashes before the star settles on the zero-age horizontal branch (ZAHB). Yet, no stars have ever been positively identified in this key phase in the life of a low-mass star (hereafter the "pre-ZAHB" phase). Aims. In this paper, we investigate the possibility that at least some pre-ZAHB stars may cross the instability strip, thus becoming variable stars whose properties might eventually lead to positive identifications. In particular, it has been suggested that some of the RR Lyrae stars with high period change rates () may in fact be pre-ZAHB stars. Here we present the first theoretical effort devoted to interpreting at least some of the high- stars as pre-ZAHB pulsators. Methods. We constructed an extensive grid of evolutionary tracks using the Garching Stellar Evolution Code (GARSTEC) for a chemical composition appropriate to the case of the globular cluster M 3 (NGC 5272), where a number of stars with high values are found. We follow each star's pre-ZAHB evolution in detail, and compute the periods and period change rates for the stars lying inside the instability strip, also producing pre-ZAHB Monte Carlo simulations that are appropriate for the case of M 3. Results. Our results indicate that one should expect of order 1 pre-ZAHB star for every 60 or so bona fide HB stars in M 3. Among the pre-ZAHB stars, approximately 22% are expected to fall within the boundaries of the instability strip, presenting RR Lyrae-like pulsations. On average, these pre-ZAHB pulsators are expected to have longer periods than the bona fide HB pulsators, and 76% of them are predicted to show negative Ṗ values. While the most likely value for the pre-ZAHB variables is ≃-0.3 d/Myr, more extreme values are also possible: 38% of the variables are predicted to have Ṗ < -0.8 d/Myr. Conclusions. It appears likely, therefore, that some - but certainly not all - of the RR Lyrae stars in M 3 with high (absolute) Ṗ values are in fact pre-ZAHB pulsators in disguise. © 2008 ESO.