Asteroseismology of a star cooled by neutrino emission : the pulsating pre-white dwarf PG 0122+200

Observation of g-mode pulsations in the variable pre-white dwarf (GW Virginis) stars provides a unique means to probe their interiors and to study the late stages of stellar evolution. Multisite campaigns have in several cases proved highly successful in decoding preÈwhite dwarf light curves. Three previous attempts to untangle the pulsation spectrum of the coolest GW Virginis star, PG 0122+200, confirmed the existence of multiple g-modes but left the fundamental period spacing and therefore the star's mass and luminosity in doubt. We present an analysis based on new observations of PG 0122+200 obtained during a Whole Earth Telescope (WET) campaign conducted in the fall of 1996. Although our coverage was, because of bad weather, far poorer than in previous WET campaigns, we confirm the previous result that PG 0122+200 rotates once in 1.6±0.1 days. The most likely period spacing supported by the data implies a mass of 0.69±0.03 Mʘ. Based on the best seismology we can currently do, the cooling of PG 0122+200 is dominated by neutrino losses. This is not true for all pre-white dwarf stars and makes PG 0122+200 the prime candidate for learning useful physics. Constraints placed on the cooling rate of PG 0122+200 by future measurement of dII/dt could provide a unique test of the standard theory of lepton interactions in the (experimentally unexplored) region of phase-space appropriate to pre-white dwarf interiors

Asteroseismology of a star cooled by neutrino emission : the pulsating pre-white dwarf PG 0122+200

Observation of g-mode pulsations in the variable pre-white dwarf (GW Virginis) stars provides a unique means to probe their interiors and to study the late stages of stellar evolution. Multisite campaigns have in several cases proved highly successful in decoding preÈwhite dwarf light curves. Three previous attempts to untangle the pulsation spectrum of the coolest GW Virginis star, PG 0122+200, confirmed the existence of multiple g-modes but left the fundamental period spacing and therefore the star's mass and luminosity in doubt. We present an analysis based on new observations of PG 0122+200 obtained during a Whole Earth Telescope (WET) campaign conducted in the fall of 1996. Although our coverage was, because of bad weather, far poorer than in previous WET campaigns, we confirm the previous result that PG 0122+200 rotates once in 1.6±0.1 days. The most likely period spacing supported by the data implies a mass of 0.69±0.03 Mʘ. Based on the best seismology we can currently do, the cooling of PG 0122+200 is dominated by neutrino losses. This is not true for all pre-white dwarf stars and makes PG 0122+200 the prime candidate for learning useful physics. Constraints placed on the cooling rate of PG 0122+200 by future measurement of dII/dt could provide a unique test of the standard theory of lepton interactions in the (experimentally unexplored) region of phase-space appropriate to pre-white dwarf interiors

Normal modes and discovery of high-order cross-frequencies in the DBV white dwarf GD 358

We present a detailed mode identification performed on the 1994 Whole Earth Telescope (WET) run on GD 358. The results are compared with that obtained for the same star from the 1990 WET data. The two temporal spectra show very few qualitative differences, although amplitude changes are seen in most modes, including the disappearance of the mode identified as k = 14 in the 1990 data. The excellent coverage and signal-to-noise ratio obtained during the 1994 run lead to the secure identification of combination frequencies up to fourth order, i.e. peaks that are sums or differences of up to four parent frequencies, including a virtually complete set of second-order frequencies, as expected from harmonic distortion. We show how the third-order frequencies are expected to affect the triplet structure of the normal modes by back-interacting with them. Finally, a search for l = 2 modes was unsuccessful, not verifying the suspicion that such modes had been uncovered in the 1990 data set

Normal modes and discovery of high-order cross-frequencies in the DBV white dwarf GD 358

We present a detailed mode identification performed on the 1994 Whole Earth Telescope (WET) run on GD 358. The results are compared with that obtained for the same star from the 1990 WET data. The two temporal spectra show very few qualitative differences, although amplitude changes are seen in most modes, including the disappearance of the mode identified as k = 14 in the 1990 data. The excellent coverage and signal-to-noise ratio obtained during the 1994 run lead to the secure identification of combination frequencies up to fourth order, i.e. peaks that are sums or differences of up to four parent frequencies, including a virtually complete set of second-order frequencies, as expected from harmonic distortion. We show how the third-order frequencies are expected to affect the triplet structure of the normal modes by back-interacting with them. Finally, a search for l = 2 modes was unsuccessful, not verifying the suspicion that such modes had been uncovered in the 1990 data set

New whole earth telescope observations of cd-247599 : steps towards delta scuti star seismology

92 h of new Whole Earth Telescope observations have been acquired for the δ Scuti star CD -24 7599. All the seven pulsation modes reported by Handler et al. are confirmed. However, significant amplitude variations which are not caused by beating of closely spaced frequencies occurred within two years. Analysing the combined data of both WET runs, we detect six further pulsation modes, bringing the total number up to 13. We also examine our data for high-frequency pulsations similar to those exhibited by rapidly oscillating Ap stars, but we do not find convincing evidence for variability in this frequency domain. From new colour photometry and spectroscopy we infer that CD-24 7599 is a hot mainsequence δ Scuti star with approximately solar metallicity and ν sin i = 52 ± 2 km s -1. We cannot yet propose a definite pulsation mode identification, but we report the detection of a characteristic frequency spacing between the different modes. We ascribe it to the simultaneous presence of l = 1 and l = 2 modes of consecutive radial order. A comparison of this frequency spacing with frequencies of solar-metallicity models, as well as stability analysis, allows us to constrain tightly the evolutionary state of CD - 24 7599. It is in the first half of its main-sequence evolution, and has a mass of 1.85 ± 0.05 M̛ and a mean density of p = 0.246 ± 0.020 P̛. This yields a seismological distance of 650 ± 70 pc, which i~ as accurate as distance determinations for 0 Scuti stars observed in clusters. Most of the pulsation modes are pure p modes of radial order k = 4-6, but the gl mode of l = 2 is likely to be excited and observed as well. Since a significant contribution to this mode's kinetic energy comes from the outer part of the convective core, CD-247599 becomes particularly interesti1)g for testing convective overshooting theories

Asteroseismology of RXJ 2117+3412, the hottest pulsating PG 1159 star

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 l = 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 ± 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 Mѳ and the He-rich envelope mass fraction is in the range 0.013–0.078 M*. The luminosity derived from asteroseismology is log(L/Lѳ) = 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 ± 0.9 pc. The role of mass loss on the excitation mechanism and its consequence on the amplitude variations is discussed

New whole earth telescope observations of cd-247599 : steps towards delta scuti star seismology

92 h of new Whole Earth Telescope observations have been acquired for the δ Scuti star CD -24 7599. All the seven pulsation modes reported by Handler et al. are confirmed. However, significant amplitude variations which are not caused by beating of closely spaced frequencies occurred within two years. Analysing the combined data of both WET runs, we detect six further pulsation modes, bringing the total number up to 13. We also examine our data for high-frequency pulsations similar to those exhibited by rapidly oscillating Ap stars, but we do not find convincing evidence for variability in this frequency domain. From new colour photometry and spectroscopy we infer that CD-24 7599 is a hot mainsequence δ Scuti star with approximately solar metallicity and ν sin i = 52 ± 2 km s -1. We cannot yet propose a definite pulsation mode identification, but we report the detection of a characteristic frequency spacing between the different modes. We ascribe it to the simultaneous presence of l = 1 and l = 2 modes of consecutive radial order. A comparison of this frequency spacing with frequencies of solar-metallicity models, as well as stability analysis, allows us to constrain tightly the evolutionary state of CD - 24 7599. It is in the first half of its main-sequence evolution, and has a mass of 1.85 ± 0.05 M̛ and a mean density of p = 0.246 ± 0.020 P̛. This yields a seismological distance of 650 ± 70 pc, which i~ as accurate as distance determinations for 0 Scuti stars observed in clusters. Most of the pulsation modes are pure p modes of radial order k = 4-6, but the gl mode of l = 2 is likely to be excited and observed as well. Since a significant contribution to this mode's kinetic energy comes from the outer part of the convective core, CD-247599 becomes particularly interesti1)g for testing convective overshooting theories