Hot flashers and He dwarfs in galactic globulars

Free access article

Issue		A&A Volume 457, Number 2, October II 2006


Page(s)		569 - 573
Section		Stellar structure and evolution
DOI		http://dx.doi.org/10.1051/0004-6361:20064875

A&A 457, 569-573 (2006)
DOI: 10.1051/0004-6361:20064875

Hot flashers and He dwarfs in galactic globulars

M. Castellani¹, V. Castellani¹ and P. G. Prada Moroni^{2, 3}

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 $M_{\odot}$ with Z= 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 M= 0.4975 to M= 0.4845 ( $\pm$ 0.0005) $M_{\odot}$ , all models igniting the He-flash with a mass of the H-rich envelope as given by $M_{\rm e}$ = 0.00050 $\pm$ 0.00002 $M_{\odot}$ . 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 $\log L/L_{\odot}=-1$ 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.

Key words: stars: evolution -- stars: white dwarfs -- stars: mass-loss

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