Asteroseismology of the PG 1159 star PG 0122+200

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 = 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 ≤ log(q y ) ≤ −5.3. The comparison of the mode trapping amplitudes among GW Vir stars suggests that the mass-loss efficiency must decrease significantly below T eff ≤ 140 kK. We measure an average period spacing of 22.9 s from which we derive a mass of 0.59 ± 0.02 M . From the triplets we measure a mean rotational splitting of 3.74 µHz and a rotational period of 1.55 days. We derive an upper limit to the magnetic field of B ≤ 4 × 10 3 G. The luminosity (log L/L = 1.3 ± 0.5) and the distance (D = 0.7 +1.0 −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 ± 0.2 confirming that the PG 0122+200 evolutionary time scale should be dominated by neutrino cooling. A measurement ofṖ for the largest amplitude untrapped modes should verify this prediction.

White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Line Widths: Kepler Observations of 27 Pulsating DA White Dwarfs through K2 Campaign 8

“…References. -(0) This work; (1) Giammichele et al (2016); (2) Dolez et al (2006); (3) Fu et al (2013); (4) Su et al (2014); (5) Pfeiffer et al (1996); (6) Bognár et al (2016); (7) Bradley (2001); (8) Castanheira et al (2013); (9) Kepler et al (1995); (10) Pech & Vauclair (2006); (11) Fu et al (2007); (12) Bond et al (1996); (13) Charpinet et al (2009); (14) Vauclair et al (2002); (15) Kawaler et al (1995); (16) Østensen et al (2011); (17) Hermes et al (2017b); (18) Bell et al (2015); (19) Greiss et al (2014); (20) icantly simplifies mode identification and affords us the opportunity to probe internal rotation for 20 of the 27 DAVs we present here. For example, Figure 1 shows the five dipole (ℓ = 1) modes in EPIC 201802933 (SDSSJ1151+0525) identified from their splittings.…”

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

“…The resulting averaged rotational splitting of 3.405 μHz translates into an average rotation period of 1.70 days. This is a slightly longer rotation period than the 1.55 days found by Fu et al (2007). In addition, the nonlinear effect invoked by Goupil et al (1998) also implies a time modulation of the departure to equal splitting within the triplets in the intermediate regime.…”

“…Those frequencies are seen in the power spectrum of all the timeseries. We interpret the 1558.671 μHz (641.6 s period) as a component of one additional = 1 mode to the series listed in Fu et al (2007) in which the lowest frequency mode was at 1636.25 μHz (611.15 s). This new mode corresponds to the k = 25 order, = 1 mode of the best-fit model of Córsico et al (2007), which is found at 633.1 s; however, we detect only one frequency, so we do not know its m value.…”

“…Discovered as a GW Vir variable star by Bond & Grauer (1987), PG 0122+200 was observed more intensively in 1986 (O'Brien et al 1996) and then, through a number of multisite campaigns in 1990 (Vauclair et al 1995), in 1996 when it was the priority target of a WET campaign (O'Brien et al 1998), as well as in 2001and 2002(Fu et al 2007. From the cumulative pulsation periods observed during these campaigns, it was possible to identify 23 periods as = 1 modes, composed of seven triplets and two single modes, and to derive a precise value of the average period spacing, ΔP = 22.9 s (Fu et al 2007).…”

“…From the cumulative pulsation periods observed during these campaigns, it was possible to identify 23 periods as = 1 modes, composed of seven triplets and two single modes, and to derive a precise value of the average period spacing, ΔP = 22.9 s (Fu et al 2007). These results were used by Córsico et al (2007) to derive precise constraints on the PG 0122+200 parameters, where the most important ones for estimating the neutrino luminosity are the total mass M = 0.556 +0.009 −0.014 M and the effective temperature (T eff = 81540 (+800, -1400) K) or, alternatively, the luminosity (log (L * /L ) = 1.14 (+0.02, -0.04)).…”