The period and amplitude changes in the coolest GW Virginis variable star (PG 1159-type) PG 0122+200

Abstract

Context. The PG 1159 pre-white dwarf stars experiment a rapidly cooling phase with a time scale of a few 10 6 years. Theoretical models predict that the neutrinos produced in their core should play a dominant role in the cooling, mainly at the cool end of the PG 1159 sequence. Measuring the evolutionary time scale of the coolest PG 1159 stars could offer a unique opportunity to empirically constrain the neutrino emission rate. Aims. A subgroup of the PG 1159 stars are nonradial pulsators, the GW Vir type of variable stars. They exhibit g-mode pulsations with periods of a few hundred seconds. As the stars cool, the pulsation frequencies evolve according to the change in their internal structure. It was anticipated that the measurement of their rate of change would directly determine the evolution time scale and so constrain the neutrino emission rates. As PG 0122+200 (BB Psc) defines the red edge of the GW Vir instability strip, it is a good candidate for such a measurement. Methods. The pulsations of PG 0122+200 have been observed during 22 years from 1986 to 2008, through the fast photometry technique. We used those data to measure the rate of change of its frequencies and amplitudes. Results. Among the 24 identified = 1 modes, the frequency and amplitude variations have been obtained for the seven largest amplitude ones. We find changes of their frequency of much larger amplitudes and shorter time scales than the one predicted by theoretical models that assume that the cooling dominates the frequency variations. In the case of the largest amplitude mode at 2497 μHz (400 s), its variations are best fitted by a combination of two terms: one long term with a time scale of 5.4 × 10 4 years, which is significantly shorter than the predicted evolutionary time scale of 8 × 10 6 years; and one additional periodic term with a period of either 261 or 211 days. Some other mechanism(s) than the cooling must be responsible for such variations. We suggest that the resonant coupling induced within triplets by the star rotation could be such a mechanism. As a consequence, no useful constraints on the neutrino emission rate can presently be derived as long as the dominant mechanism is not properly understood. Conclusions. The temporal variations in the pulsation frequencies observed in PG 0122+200 cannot be simply attributed to the cooling of the star, regardless of the contribution of the neutrino losses. Our results suggest that the resonant coupling induced by the rotation plays a dominant role which must be further investigated.

Amplitude and frequency variations of oscillation modes in the pulsating DB white dwarf star KIC 08626021

“…The theoretical exploration of these mechanisms was extensively developed in Buchler et al (1995Buchler et al ( , 1997, but was almost interrupted more than a decade ago because of the lack of clear observational evidence of such phenomena, as a result of the difficulty in capturing amplitude or frequency variations that occur on timescales of months to years from ground-based observatories. Nevertheless, the presence of resonant couplings within rotationally split mode triplets was proposed for the first time as the explanation for the frequency and amplitude of longterm variations observed in the GW Vir pulsator PG 0122+200 (Vauclair et al 2011) from successive campaigns on this object. This suggests that pulsating white dwarfs could be among the best candidates to detect and test the nonlinear resonant coupling theory.…”

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

“…Althaus et al (2008) have found that this discrepancy could be alleviated if PG1159−035 is characterized by a thin He-rich envelope, leading to remarkably large magnitudes of the rates of period change. A measurement ofΠ in another GW Vir star, PG0112+200, has been carried out by Vauclair et al (2011). The derived rates of period change are much larger than those predicted by theoretical models (Córsico et al 2007), calling for the presence of other mechanism(s) apart from neutrino cooling to explain the disagreement.…”

“…The derived rates of period change are much larger than those predicted by theoretical models (Córsico et al 2007), calling for the presence of other mechanism(s) apart from neutrino cooling to explain the disagreement. In particular, a mechanism that could be playing a dominant role is resonant mode coupling induced by the rotation (Vauclair et al 2011). A cautionary note regarding the interpretation of the measured rates of period change in pulsating WDs is needed here.…”

“…A cautionary note regarding the interpretation of the measured rates of period change in pulsating WDs is needed here. The studies by Hermes et al (2013b) for the DAV star WD 0111+0018, and Vauclair et al (2011) for the GW Vir star PG0112+200 (among others), indicate that our understanding of the rates of period change in pulsating WDs is far from complete, and this should be kept in mind when usingΠ in the applications mentioned before.…”

Limits on the neutrino magnetic dipole moment from the luminosity function of hot white dwarfs

“…Unfortunately, up to now it has been impossible to determine the rate of period change generated by secular cooling in DOVs or DBVs (Vauclair et al 2011;Chote et al 2013;Dalessio et al 2013), although some hints have recently been found by Redaelli et al (2011). Blinnikov & Dunina-Barkovskaya (1994) showed that the early WDLF of Fleming et al (1986) implied that the magnetic dipole moment is μ ν < ∼ 10 −11 μ B .…”