Hot flashers and He dwarfs in galactic globulars

¹ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy e-mail: m.castellani@oa-roma.inaf.it
² Dipartimento di Fisica, Università di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
³ INFN, Sezione di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy

Received: 18 January 2006
Accepted: 1 June 2006

Abstract

Context.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.

Aims.Our aim has been to test the theoretical results vis-a-vis different assumptions about the efficiency of mass loss.

Methods.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 0.86  with 0.0015.

Results.We find reasonable agreement with previous evolutionary investigations, showing that for the given metallicity late hot flashers are predicted to cover the mass range 0.4975 to 0.4845 (±0.0005) , all models igniting the He-flash with a mass of the H-rich envelope as given by M_e = 0.00050 ±0.00002 . 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-manqué He white dwarf. We predict that these structures will cool down, reaching the luminosity in a time five times longer at the very least than the corresponding cooling time of a normal CO white dwarf.

Conclusions.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.