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Register a new userThe Missing Link? Discovery of Pulsations in the Nitrogen-rich PG 1159 Star PG 1144+005
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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 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 non-pulsator 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.
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PG 1159-035, a pre-white dwarf with T_eff=140,000 K, is the prototype of both two classes: the PG1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. In this work, we used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods and identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M = 0.59 +/- 0.02 solar masses for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at r_c = 0.83 +/- 0.05 stellar radius. From the multiplet splitting, we calculated the rotational period P_rot = 1.3920 +/- 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for l=1 modes and less than 50% for l=2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars.
Previous studies aiming at the iron-abundance determination in three PG 1159 stars (K 1-16, PG 1159-035, NGC 7094) and a [WC]-PG 1159 transition star (Abell 78) have revealed that no object shows any iron line in the UV spectrum. The stars are iron-deficient by at least 1 dex, typically. A possible explanation is that iron nuclei were transformed by neutron captures into heavier elements (s-process), however, the extent of the iron-destruction would be much stronger than predicted by AGB star models. But if n-captures are the right explanation, then we should observe an enrichment of trans-iron elements. In this paper we report on our search for a possible nickel overabundance in one of the four Fe deficient PG 1159 stars, namely the central star NGC 7094. We are unable to identify any nickel line in HST and FUSE spectra and conclude that Ni is not overabundant. It is conceivable that iron was transformed into even heavier elements, but their identification suffers from the lack of atomic data.
We discuss theoretical AGB predictions for hydrogen-deficient PG 1159 stars and Sakurais object, which show peculiar enhancements in He, C and O, and how these enhancements may be understood in the framework of a very late thermal pulse nucleosynthetic event. We then discuss the nucleosynthesis origin of rare subclasses of presolar grains extracted from carbonaceous meteorites, the SiC AB grains showing low 12C/13C in the range 2 to 10 and the very few high-density graphite grains with 12C/13C around 10.
Stellar post asymptotic giant branch (post-AGB) evolution can be completely altered by a final thermal pulse (FTP) which may occur when the star is still leaving the AGB (AFTP), at the departure from the AGB at still constant luminosity (late TP, LTP) or after the entry to the white-dwarf cooling sequence (very late TP, VLTP). Then convection mixes the He-rich material with the H-rich envelope. According to stellar evolution models the result is a star with a surface composition of $mathrm{H}approx,20,$% by mass (AFTP), $approx 1,$% (LTP), or (almost) no H (VLTP). Since FTP stars exhibit intershell material at their surface, spectral analyses establish constraints for AGB nucleosynthesis and stellar evolution. We performed a spectral analysis of the so-called hybrid PG 1159-type central stars (CS) of the planetary nebulae Abell 43 and NGC7094 by means of non-local thermodynamical equilibrium models. We confirm the previously determined effective temperatures of $T_mathrm{eff} = 115,000pm 5,000,$K and determine surface gravities of $log (g,/,mathrm{cm/s^2}) = 5.6pm 0.1$ for both. From a comparison with AFTP evolutionary tracks, we derive stellar masses of $0.57^{+0.07}_{-0.04},M_odot$ and determine the abundances of H, He, and metals up to Xe. Both CS are likely AFTP stars with a surface H mass fraction of $0.25 pm 0.03$ and $0.15 pm 0.03$, respectively, and a Fe deficiency indicating subsolar initial metallicities. The light metals show typical PG 1159-type abundances and the elemental composition is in good agreement with predictions from AFTP evolutionary models. However, the expansion ages do not agree with evolution timescales expected from the AFTP scenario and alternatives should be explored.
We present the detection of non-radial oscillations in a hot, helium-atmosphere white dwarf using 78.7 d of nearly uninterrupted photometry from the Kepler space telescope. With an effective temperature >30,000 K, PG 0112+104 becomes the hottest helium-atmosphere white dwarf known to pulsate. The rich oscillation spectrum of low-order g-modes includes clear patterns of rotational splittings from consecutive sequences of dipole and quadrupole modes, which can be used to probe the rotation rate with depth in this highly evolved stellar remnant. We also measure a surface rotation rate of 10.17404 hr from an apparent spot modulation in the K2 data. With two independent measures of rotation, PG 0112+104 provides a remarkable test of asteroseismic inference.
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