Stellar Evolution and Variability in the Pre-ZAHB 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 RGB and is followed by a series of secondary flashes before the star settles on the zero-age HB (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). We investigate the possibility that some pre-ZAHB stars may cross the instability strip (IS), thus becoming variable. In particular, it has been suggested that some RR Lyrae stars (RRL) with high period change rates (P-dot) may in fact be pre-ZAHB stars. We present the first theoretical effort devoted to interpreting some of the high-P-dot stars as pre-ZAHB pulsators. We constructed a grid of evolutionary tracks using the Garching Stellar Evolution Code for a chemical composition appropriate to the case of the globular cluster M3, where a number of stars with high P-dot values are found. We follow each star's pre-ZAHB evolution in detail, compute periods and P-dot values for the stars lying inside the IS, and produce pre-ZAHB Monte Carlo simulations for the case of M3. Our results indicate that one should expect of order 1 pre-ZAHB star for every 60 or so bona-fide HB stars in M3. Among the pre-ZAHB stars, ~22% are expected to fall within the IS boundaries, presenting RRL-like pulsations. On average, the pre-ZAHB pulsators are expected to have longer periods than the bona-fide HB pulsators, and 76% of them are predicted to show negative P-dot values. The most likely P-dot value for the pre-ZAHB variables is ~-0.3 d/Myr, but more extreme P-dot values are also possible: 38% of the variables are predicted to have P-dot < -0.8 d/Myr. It appears likely, therefore, that some RRL stars in M3 with high (absolute) P-dot values are pre-ZAHB pulsators in disguise. (abridged)

8 pages, 8 figures. A&A, in press

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Cited by in corpus (19)

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• Acoustic Signatures of the Helium Core Flash
• Long-term photometric monitoring of RR Lyr stars in M3
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• Long-term photometric monitoring of Messier 5 variables: I. Period changes of RR Lyrae stars
• RR Lyrae stars and the horizontal branch of NGC 5904 (M5)
• Pulsational instabilities driven by the $\epsilon$ mechanism in hot pre-horizontal branch stars I. The Hot-Flasher Scenario
• Period Change Similarities among the RR Lyrae Variables in Oosterhoff I and Oosterhoff II Globular Systems
• On the serendipitous discovery of a Li-rich giant in the globular cluster NGC 362
• Period Change Rates in Large Magellanic Cloud Cepheids Revisited
• A New Overview of Secular Period Changes of RR Lyrae stars inM5
• Selected topics in the evolution of low-mass stars
• Humps and bumps: The effects of shocks on the optical light curves of fundamental-mode RR Lyrae stars
• Fundamental parameters of RR Lyrae stars from multicolour photometry and Kurucz atmospheric models -- III. SW And, DH Peg, CU Com, DY Peg
• Pulsation Period Changes as a Tool to Identify Pre-Zero Age Horizontal Branch Stars
• Period changes in the RR Lyrae stars of NGC 6171 (M107)
• Evolution and period change in RR Lyr variables of the globular cluster M 3
• Fundamental parameters of RR Lyrae stars from multicolour photometry and Kurucz atmospheric models -- II. Adaptation to double-mode stars
• RR Lyrae stars with variable mean magnitudes