White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up

ALTHAUS, L. G.; CAMISASSA, M. E.; MILLER BERTOLAMI, M. M.; CÓRSICO, A. H.; GARCÍA-BERRO, E.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2015 vol. 576 p. 9 - 20

0004-6361

Context. White dwarfs are nowadays routinely used as reliablecosmochronometers, allowing several stellar populations to be dated. Aims: We present new white dwarf evolutionary sequences forlow-metallicity progenitors. This is motivated by the recent findingthat residual H burning in low-mass white dwarfs resulting from Z =0.0001 progenitors is the main energy source over a significant part oftheir evolution. Methods: White dwarf sequences have been derivedfrom full evolutionary calculations that take the entire history ofprogenitor stars into account, including the thermally pulsing and thepost-asymptotic giant branch (AGB) phases. Results: We show thatfor progenitor metallicities in the range 0.00003 ≲ Z ≲ 0.001,and in the absence of carbon enrichment from the occurrence of a thirddredge-up episode, the resulting H envelope of the low-mass white dwarfsis thick enough to make stable H burning the most important energysource even at low luminosities. This has a significant impact on whitedwarf cooling times. This result is independent of the adopted mass-lossrate during the thermally-pulsing and post-AGB phases and in theplanetary nebulae stage. Conclusions: We conclude that in theabsence of third dredge-up episodes, a significant part of the evolutionof low-mass white dwarfs resulting from low-metallicity progenitors isdominated by stable H burning. Our study opens the possibility of usingthe observed white dwarf luminosity function of low-metallicity globularclusters to constrain the efficiency of third dredge up episodes duringthe thermally-pulsing AGB phase of low-metallicity progenitors.