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

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