Astronomy and Astrophysics, volume 457, 569-573 (2006/10-2)
Hot flashers and He dwarfs in galactic globulars.
CASTELLANI M., CASTELLANI V. and PRADA MORONI P.G.
Abstract (from CDS):
We revisit the evolutionary scenario for hot flasher low-mass structures, where mass loss delays the He flash till the initial phases of their white dwarf cooling sequence. Our aim has been to test the theoretical results vis-a-vis different assumptions about the efficiency of mass loss. To this purpose, we present evolutionary models covering a fine grid of masses, as obtained assuming a single episode of mass loss in a red giant model of 0.86M☉ with Z=0.0015. We find reasonable agreement with previous evolutionary investigations, showing that for the given metallicity late hot flashers are predicted to cover the mass range M=0.4975 to M=0.4845(±0.0005)M☉, all models igniting the He-flash with a mass of the H-rich envelope as given by Me=0.00050±0.00002M☉. The ignition mechanism is discussed in some detail, showing the occurrence of a bifurcation in the evolutionary history of stellar structures at the lower-mass limit for He ignition. Below such a critical mass, the structures miss the He ignition, cooling down as a hot flasher-manque He white dwarf. We predict that these structures will cool down, reaching the luminosity logL/L☉=-1 in a time five times longer at the very least than the corresponding cooling time of a normal CO white dwarf. On very general grounds, one expects that old stellar clusters with a sizeable population of hot flashers should produce at least a similar amount of slow-cooling He white dwarfs. According to this result, in a cluster where 20% of red giants escape the He burning phase, one expects roughly twice as white dwarfs than in a normal cluster where all red giants undergo their He flash.