Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS

VI. Asteroseismology of the GW Vir-type central star of the Planetary Nebula NGC 246

¹ Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, (1900) La Plata, Argentina
e-mail: lcalcaferro@fcaglp.unlp.edu.ar
² Instituto de Astrofísica La Plata, CONICET-UNLP, Paseo del Bosque s/n, (1900) La Plata, Argentina
³ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland
⁴ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Instituto de Física, Universidade Federal do Rio Grande do Sul, 91501-970 Porto-Alegre, RS, Brazil
⁶ Department of Physics, Queens College, City University of New York, Flushing, NY 11367, USA
⁷ Department of Physics and Astronomy, Iowa State University Ames, Ames, IA 50011, USA
⁸ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany

Received: 25 December 2023
Accepted: 23 February 2024

Abstract

Context. Significant advances have been achieved through the latest improvements in the photometric observations accomplished by the recent space missions, which substantially boost the study of pulsating stars via asteroseismology. The TESS mission has already proven to be of particular relevance for pulsating white dwarf and pre-white dwarf stars.

Aims. We report a detailed asteroseismic analysis of the pulsating PG 1159 star NGC 246 (TIC 3905338), which is the central star of the planetary nebula NGC 246, based on high-precision photometric data gathered by the TESS space mission.

Methods. We reduced TESS observations of NGC 246 and performed a detailed asteroseismic analysis using fully evolutionary PG 1159 models computed accounting for the complete prior evolution of their progenitors. We constrained the mass of this star by comparing the measured mean period spacing with the average of the computed period spacings of the models, and we also employed the observed individual periods to search for a seismic stellar model.

Results. We extracted a total of 17 periodicities from the TESS light curves from the two sectors where NGC 246 was observed. All the oscillation frequencies are associated with g-mode pulsations, with periods spanning from ∼1460 to ∼1823 s. We found a constant period spacing of ΔΠ = 12.9 s, which allowed us to deduce that the stellar mass is higher than ∼0.87 M_⊙ if the period spacing is assumed to be associated with ℓ = 1 modes, and that the stellar mass is ∼0.568 M_⊙ if it is associated with ℓ = 2 modes. The less massive models are more consistent with the distance constraint from Gaia parallax. Although we were not able to find a unique asteroseismic model for this star, the period-to-period fit analyses suggest a high stellar mass (≳0.74 M_⊙) when the observed periods are associated with modes with ℓ = 1 only, and both a high and an intermediate stellar mass (≳0.74 M_⊙ and ∼0.57 M_⊙, respectively) when the observed periods are associated with modes with a mixture of ℓ = 1, 2.