Pulsating low-mass white dwarfs in the frame of new evolutionary sequences. IV. The secular rate of period change

CALCAFERRO, L. M.; A. H. CÓRSICO; ALTHAUS, L. G.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2017 vol. 600

0004-6361

Context. An increasing number of low-mass(M⋆/M⊙ ≲ 0.45) and extremelylow-mass (ELM, M⋆/M⊙ ≲ 0.18-0.20)white-dwarf stars are being discovered in the field of the Milky Way.Some of these stars exhibit long-period g-mode pulsations, and arecalled ELMV variable stars. Also, some low-mass pre-white dwarf starsshow short-period p-mode (and likely radial-mode) photometricvariations, and are designated as pre-ELMV variable stars. The existenceof these new classes of pulsating white dwarfs and pre-white dwarfsopens the prospect of exploring the binary formation channels of theselow-mass white dwarfs through asteroseismology. Aims: We aim topresent a theoretical assessment of the expected temporal rates ofchange of periods (\dot{Π}) for such stars, based on fullyevolutionary low-mass He-core white dwarf and pre-white dwarf models. Methods: Our analysis is based on a large set of adiabaticperiods of radial and nonradial pulsation modes computed on a suite oflow-mass He-core white dwarf and pre-white dwarf models with massesranging from 0.1554 to 0.4352 M⊙, which were derived bycomputing the non-conservative evolution of a binary system consistingof an initially 1 M⊙ ZAMS star and a 1.4M⊙ neutron star companion. Results: We computedthe secular rates of period change of radial (ℓ = 0) and nonradial(ℓ = 1,2) g and p modes for stellar models representative of ELMVand pre-ELMV stars, as well as for stellar objects that are evolvingjust before the occurrence of CNO flashes at the early cooling branches.We find that the theoretically expected magnitude of \dot{Π} of gmodes for pre-ELMVs is by far larger than for ELMVs. In turn, \dot{Π}of g modes for models evolving before the occurrence of CNO flashes arelarger than the maximum values of the rates of period change predictedfor pre-ELMV stars. Regarding p and radial modes, we find that thelarger absolute values of \dot{Π} correspond to pre-ELMV models. Conclusions: We conclude that any eventual measurement of a rate ofperiod change for a given pulsating low-mass pre-white dwarf or whitedwarf star could shed light about its evolutionary status. Also, in viewof the systematic difficulties in the spectroscopic classification ofstars of the ELM Survey, an eventual measurement of \dot{Π} couldhelp to confirm that a given pulsating star is an authentic low-masswhite dwarf and not a star from another stellar population.