Asteroseismology of RXJ 2117+3412, the hottest pulsating PG 1159 star 01/2002; DOI: 10.1051/0004-6361:20011483

ABSTRACT The pulsating PG 1159 planetary nebula central star RXJ 2117+3412 has been observed over three successive seasons of a multisite photometric campaign. The asteroseismological analysis of the data, based on the 37 identified $\ell=1$ modes among the 48 independent pulsation frequencies detected in the power spectrum, leads to the derivation of the rotational splitting, the period spacing and the mode trapping cycle and amplitude, from which a number of fundamental parameters can be deduced. The average rotation period is $1.16\pm 0.05$ days. The trend for the rotational splitting to decrease with increasing periods is incompatible with a solid body rotation. The total mass is 0.56$^{+0.02}_{-0.04}$ $M_{\odot}$ and the He-rich envelope mass fraction is in the range 0.013–0.078 $M_{*}$. The luminosity derived from asteroseismology is log($L/L_{\odot})= 4.05$ $^{+0.23}_{-0.32}$ and the distance 760 $^{+230}_{-235}$ pc. At such a distance, the linear size of the planetary nebulae is $2.9\pm 0.9$ pc. The role of mass loss on the excitation mechanism and its consequence on the amplitude variations is discussed.

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    ABSTRACT: We present a summary of what is currently known about the three distinct families of isolated pulsating white dwarfs. These are the GW Vir stars (He/C/O-atmosphere stars with Teff ≃ 120,000 K), the V777 Her stars (He-atmosphere, Teff ≃ 25,000 K), and the ZZ Ceti stars (H-atmosphere, Teff ≃ 12,000 K), all showing multiperiodic luminosity variations caused by low-order and low-degree g-mode instabilities. We also provide, in an Appendix, a very brief overview of the newly found evidence in favor of the existence of a fourth category of oscillating white dwarfs bearing strong similarities with these families of pulsators. We begin our survey with a short historical introduction, followed by a general discussion of pulsating white dwarfs as compact pulsators. We then discuss the class properties of these objects, including an updated census. We next focus on the instability domains for each family of pulsators in the log g - Teff diagram, and present their time-averaged properties in more detail. This is followed by a section on excitation physics, i.e., the causes of the pulsational instabilities, with emphasis on the common properties of the different types of pulsator. We then discuss the time-dependent properties of the pulsating white dwarfs featuring, among other things, a brief "picture tour" across the ZZ Ceti instability strip. We next review the methods used to infer or constrain the angular geometry of a pulsation mode in a white dwarf. These include multicolor photometry and time-resolved spectroscopy, the exploitation of the nonlinear features in the observed light curves, and rotational splitting. We also consider basic adiabatic asteroseismology starting with a discussion of the reaction of the period spectrum to variations of model parameters. We next review the various asteroseismological inferences that have so far been claimed for white dwarfs. We also discuss the potential of exploiting the rates of period change. We finally provide some concluding remarks, including a list with several suggestions for future progress in the field.
    Publications of the Astronomical Society of the Pacific 09/2008; 120:1043-1096. · 3.69 Impact Factor
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    ABSTRACT: Context.The variable pre-white dwarf PG 1159 stars (GW Vir) are $g$-mode non-radial pulsators. Asteroseismology puts strong constraints on their global parameters and internal structure. PG 0122+200 defines the red edge of the instability strip and its evolutionary timescale is predicted to be dominated by neutrino emission. Its study offers the opportunity to better understand the instability mechanism and to validate the physics of the neutrino production in dense plasma.Aims.To achieve such a goal requires determining precisely its fundamental parameters. This is the goal of this paper.Methods.We present new multi-site photometric observations obtained in 2001 and 2002. Together with previous data, they allow us to detect 23 frequencies, composed of 7 triplets and 2 single frequencies, which are used to constrain its internal structure and derive its fundamental parameters.Results.All the observed frequencies correspond to $\ell$=1 $g$-modes. The period distribution shows a signature of mode trapping from which we constrain the He-rich envelope mass fraction to be -6.0$\leq \log(q_{y}) \leq$ -5.3. The comparison of the mode trapping amplitudes among GW Vir stars suggests that the mass-loss efficiency must decrease significantly below $T_{\rm eff}\leq$ 140 kK. We measure an average period spacing of 22.9 s from which we derive a mass of 0.59$\pm$0.02 $M_{\odot}$. From the triplets we measure a mean rotational splitting of 3.74 $\mu$Hz and a rotational period of 1.55 days. We derive an upper limit to the magnetic field of $B\leq4\times10^{3}$ G. The luminosity ($\log L/L_{\odot}$ = 1.3$\pm$0.5) and the distance ($D = 0.7^{\rm +1.0}_{\rm -0.4}$ kpc) are only weakly constrained due to the large uncertainty on the spectroscopically derived surface gravity and the absence of a measured parallax.Conclusions.From the asteroseismic mass, the ratio of the neutrino luminosity on the photon luminosity is 1.6$\pm$0.2 confirming that the PG 0122+200 evolutionary time scale should be dominated by neutrino cooling. A measurement of $\dot{P}$ for the largest amplitude untrapped modes should verify this prediction.
    Astronomy and Astrophysics 01/2007; · 5.08 Impact Factor
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    ABSTRACT: Aims.We searched for photometric variability in a sample of hot, hydrogen-deficient planetary nebula nuclei (PNNi) with “PG 1159” or “O VI” spectral type, most of them embedded in a bipolar or elliptical planetary nebula envelope (PNe). These characteristics may indicate the presence of a hidden close companion and an evolution affected by episodes of interaction between them.Methods.We obtained time-series photometry from a sample of 11 candidates using the Nordic Optical Telescope (NOT) with the Andalucía Faint Object Spectrograph and Camera (ALFOSC), modified with our own control software to be able to observe in a high-speed multi-windowing mode. The data were reduced on-line with the real time photometry (RTP) code, which allowed us to detect periodic variable stars with small amplitudes from CCD data in real time. We studied the properties of the observed modulation frequencies to investigate their nature.Results.We report the first detection of low-amplitude pulsations in the PNNi VV 47, NGC 6852, and Jn 1. In addition, we investigated the photometric variability of NGC 246. Time-series analysis shows that the power spectra of VV 47, NGC 6852, and NGC 246 are variable on time scales of hours. Power spectra from consecutive nights of VV 47 and NGC 6852 show significant peaks in different frequency regions. The same type of variability is present in NGC 246 in 2 observing runs separated by 3 days. Changes are also found in the power spectra of VV 47 and NGC 246 during the same night. The VV 47 power spectra are peculiar since they present modulation frequencies in a wide range from 175 to 7600 $\mu$Hz. This is different from the previously known pulsating PNNi where no frequencies are found above ~3000 $\mu$Hz. The high-frequency modulation observed in VV 47 may be due to $g$-modes triggered by the $\epsilon$-mechanism, observed for the first time. 01/2006;

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