GW Vir Instability Strip Research Articles

The GW Vir Instability Strip in Light of New Observations of PG 1159 Stars: Discovery of Pulsations in the Central Star of A72 and Variability of RX J0122.9–7521

We present the results of new time-series photometric observations of 29 pre–white dwarf stars of PG 1159 spectral type, carried out in the years 2014–2022. For the majority of stars, a median noise level in Fourier amplitude spectra of 0.5–1.0 mmag was achieved. This allowed the detection of pulsations in the central star of planetary nebula A72 (Abell 72), consistent with g modes excited in GW Vir stars, and variability in RX J0122.9–7521 that could be due to pulsations, binarity, or rotation. For the remaining stars from the sample that were not observed to vary, we placed upper limits for variability. After combination with literature data, our results place the fraction of pulsating PG 1159 stars within the GW Vir instability strip at 36%. An updated list of all known PG 1159 stars is provided, containing astrometric measurements from the recent Gaia DR3 data, as well as information on physical parameters, variability, and nitrogen content. Those data are used to calculate luminosities for all PG 1159 stars to place the whole sample on the theoretical Hertzsprung–Russell diagram for the first time in that way. The pulsating stars are discussed as a group, and arguments are given that the traditional separation of GW Vir pulsators in “DOV” and “PNNV” stars is misleading and should not be used.

The Missing Link? Discovery of Pulsations in the Nitrogen-rich PG 1159 Star PG 1144+005

Abstract Up to 98% of all single stars will eventually become white dwarfs—stars that link the history and future evolution of the Galaxy, and whose previous evolution is engraved in their interiors. Those interiors can be studied using asteroseismology, utilizing stellar pulsations as seismic waves. The pulsational instability strips of DA and DB white dwarf stars are pure, allowing for the important generalization that their interior structure represents that of all DA and DB white dwarfs. This is not the case for the hottest pulsating white dwarfs, the GW Vir stars: only about 50% of white dwarfs in this domain pulsate. Several explanations for the impurity of the GW Vir instability strip have been proposed, based on different elemental abundances, metallicity, and helium content. Surprisingly, there is a dichotomy that only stars rich in nitrogen, which by itself cannot cause pulsation driving, pulsate—the only previous exception being the nitrogen-rich nonpulsator PG 1144+005. Here, we report the discovery of pulsations in PG 1144+005 based on new observations. We identified four frequency regions: 40, 55, 97, and 112 day−1 with low and variable amplitudes of about 3–6 mmag and therefore confirm the nitrogen dichotomy. As nitrogen is a trace element revealing the previous occurrence of a very late thermal pulse (VLTP) in hot white dwarf stars, we speculate that it is this VLTP that provides the interior structure required to make a GW Vir pulsator.

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

Context. The PG 1159 pre-white dwarf stars experiment a rapidly cooling phase with a time scale of a few 106 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 l = 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 μ Hz (400 s), its variations are best fitted by a combination of two terms: one long term with a time scale of 5.4 × 104 years, which is significantly shorter than the predicted evolutionary time scale of 8 × 106 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.

ON THE POSSIBLE EXISTENCE OF SHORT-PERIODg-MODE INSTABILITIES POWERED BY NUCLEAR-BURNING SHELLS IN POST-ASYMPTOTIC GIANT BRANCH H-DEFICIENT (PG1159-TYPE) STARS

We present a pulsational stability analysis of hot post-asymptotic giant branch (AGB) H-deficient pre-white dwarf stars with active He-burning shells. The stellar models employed are state-of-the-art equilibrium structures representative of PG1159 stars derived from the complete evolution of the progenitor stars, through the thermally pulsing AGB phase and born-again episode. On the basis of fully nonadiabatic pulsation computations, we confirmed theoretical evidence for the existence of a separate PG1159 instability strip in the log Teff–log g diagram characterized by short-period g-modes excited by the � -mechanism. This instability strip partially overlaps the already known GW Vir instability strip of intermediate/long-period g-modes destabilized by the classical κ-mechanism acting on the partial ionization of C and/or O in the envelope of PG1159 stars. We found that PG1159 stars characterized by thick He-rich envelopes and located inside this overlapping region could exhibit both short and intermediate/ long periods simultaneously. As a natural application of our results, we study the particular case of VV 47, a pulsating planetary nebula nucleus (PG1159 type) that is particularly interesting because it has been reported to exhibit a rich and complex pulsation spectrum including a series of unusually short pulsation periods. We found that the long periods exhibited by VV 47 can be readily explained by the classical κ-mechanism, while the observed shortperiod branch below ≈300 s could correspond to modes triggered by the He-burning shell through the � -mechanism, although more observational work is needed to confirm the reality of these short-period modes. Were the existence of short-period g-modes in this star convincingly confirmed by future observations, VV 47 could be the first known pulsating star in which both the κ-mechanism and the � -mechanism of mode driving are simultaneously operating.

High-resolution ultraviolet spectroscopy of PG 1159-035 with HST and FUSE

Context. PG 1159-035 is the prototype of the PG 1159 spectral class which consists of extremely hot hydrogen-deficient (pre-) white dwarfs. It is also the prototype of the GW Vir variables, which are non-radial g-mode pulsators. The study of PG 1159 stars reveals insight into stellar evolution and nucleosynthesis during AGB and post-AGB phases. Aims. We perform a quantitative spectral analysis of PG 1159-035 focusing on the abundance determination of trace elements. Methods. We have taken high-resolution ultraviolet spectra of PG 1159-035 with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. They are analysed with non-LTE line blanketed model atmospheres. Results. We confirm the high effective temperature with high precision (T eff = 140000 ± 5000 K) and the surface gravity of log g = 7. For the first time we assess the abundances of silicon, phosphorus, sulfur, and iron. Silicon is about solar. For phosphorus we find an upper limit of solar abundance. A surprisingly strong depletion of sulfur (2% solar) is discovered. Iron is not detected, suggesting an upper limit of 30% solar. This coincides with the Fe deficiency found in other PG 1159 stars. We redetermine the nitrogen abundance and find it to be lower by one dex compared to previous analyses. Conclusions. The sulfur depletion is in contradiction with current models of AGB star intershell nucleosynthesis. The iron deficiency confirms similar results for other PG 1159 stars and is explained by the conversion of iron into heavier elements by n-capture in the s-processing environment of the precursor AGB star. However, the extent of the iron depletion is stronger than predicted by evolutionary models. The relatively low nitrogen abundance compared to other pulsating PG 1159 stars weakens the role of nitrogen as a distinctive feature of pulsators and non-pulsators in the GW Vir instability strip.

Detection of non-radial g-mode pulsations in the newly discovered PG 1159 star HE 1429-1209

We performed time-series photometry of the PG 1159-type star HE 1429-1209, which was recently discovered in the ESO SPY survey. We show that the star is a low-amplitude (≈0.05 mag) non-radial g-mode pulsator with a period of 919 s. HE 1429-1209 is among the hottest known post-AGB stars ( K) and, together with the known pulsator RX J2117.1+3412, it defines empirically the blue edge of the GW Vir instability strip in the HRD at high luminosities.

HST spectroscopy of PG 1159 stars: the boundary of the GW Vir instability strip

HST spectroscopy of PG 1159 stars: the boundary of the GW Vir instability strip

Confining the Edges of the GW Vir Instability Strip

We report on our NLTE model atmosphere analyses of PG 1159 stars. The results enable us to confine the location of the GWVir instability region in the Hertzsprung-Russell diagram. The analysis of a spectrum of the non-pulsator PG 1520+525 taken with the EUVE satellite in comparison with HST data of the pulsating protoype PG 1159-035 (=GW Vir) locates the blue edge of the instability strip near Teff=140 000 K for stars in the respective luminosity range.

Confining the Edges of the GW Vir Instability Strip

AbstractGround based optical spectroscopy as well as EUVE and HST observations are used for model atmosphere analyses of PG1159 stars in order to confine the GW Vir instability strip.