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Theoretical Physics Letters
2023 ° 13(05) ° 0697-1296
https://www.wikipt.org/tphysicsletters
DOI: 10.1490/369888.0687tpl
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In 2020, a publication presented the first-light results for 18 known ZZ Ceti stars observed by the TESS space telescope during the first survey observations of the southern ecliptic hemisphere. However, in the meantime, new measurements have become available from this field, in many cases with the new, 20s ultrashort cadence mode. Aims. We investigated the similarities and differences in the pulsational behaviour of the observed stars between the two observational seasons and searched for new pulsation modes for asteroseismology. Methods. We performed a Fourier analysis of the light curves using the standard pre-whitening process and compared the results with frequencies obtained from the earlier data. Utilising the 2018 version of the White Dwarf Evolution Code, we also performed an asteroseismic analysis of the different stars. We searched for models with seismic distances in the vicinity of the Gaia geometric distances. Results. We detected several new possible pulsation modes of the studied pulsators. In the case of HE 0532-5605, we found a similar brightening phase to the one presented in the 2020 first-light paper, which means this phenomenon is recurring. Therefore, HE 0532- 5605 appears to be a new outbursting DAV star. We also detected a lower-amplitude brightening phase in the star WD J0925+0509. However, this case has proven to be the result of the passage of a Solar System object in the foreground. We accept asteroseismic model solutions for six stars.
ZZ Ceti or DAV stars are short-period (P ∼ 100 − 1500 s), low-amplitude (A ∼ 0.1%) variables, which means precise and short-exposure-time measurements are required both from the ground and space in order to study their pulsations. Their atmospheres are dominated by hydrogen, and they form the most populous group of pulsating white dwarf stars, lying in the 10 500– 13 000 K effective temperature range. Their pulsation modes are low-spherical-degree (ℓ = 1 and 2), and low-to-mid radialorder g-modes. The κ − γ mechanism (Dolez & Vauclair 1981; Winget et al. 1982) in combination with the convective driving mechanism (Brickhill 1991; Goldreich & Wu 1999) is responsible for the excitation of the observed pulsations. Despite the relatively narrow instability strip of the DAV stars, they can show a large variety of pulsational properties, evolving from the hot (blue) to the cool (red) edge of the instability domain. Hotter DAVs show fewer and lower-amplitude pulsation modes than the cooler ones. Furthermore, in this latter case, above about 800 s in period, we usually do not detect one single peak at a given frequency in the Fourier transform of the light curve, but a large number of peaks under a broad envelope, which is reminiscent of stochastically driven oscillations. We refer the reader to Hermes et al. (2017) for a detailed description of both the DAV instability strip and this interesting phenomenon based on observations with the Kepler space telescope. For reviews of the theoretical and observational aspects of studies of white dwarf pulsators, we also recommend the papers of Winget & Kepler (2008), Fontaine & Brassard (2008), Althaus et al. (2010), Córsico et al. (2019), and Córsico (2020). We also mention the so-called outburst phenomena in cool DAV stars, which appears as an increase in the stellar flux of ZZ Ceti stars close to the red edge of the instability strip. These outburst events were discovered using the measurements of the Kepler space telescope; see the papers of Keaton Bell and his collaborators: Bell et al. (2015, 2016, 2017), and also Hermes et al. (2015). Such phenomena suggest that the average brightness of the star increases relatively quickly (in about 1 hour) and by at least several per cent, and remains in this state for several hours or even longer, sometimes for even more than 1 day. After that, the stellar brightness decreases to the initial value; the outburst event repeats after several days or weeks. The duration and occurrence of these events is irregular and unpredictable. Bell et al. (2017) discuss a possible explanation for the outbursts: non-linear mode coupling, which can transfer energy from a driven parent mode into two daughter modes. In this case, these otherwise damped daughter modes will deposit the additional energy at the base of the convection zone, and we observe the resulting surface heating of the star. This pulsational energy-transfer mechanism could explain the observed location of the cool edge of the ZZ Ceti instability strip, which should be much cooler according to theoretical calculations. However, Montgomery et al. (2020) raised another possibility, namely that phase shifts of the travelling waves reflected from the outer turning point being close to the convection zone could also be relevant in explaining the outburst phenomenon. This paper focuses on the study of ZZ Ceti stars observed by the Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2015). TESS was launched on 18 April 2018, and during its two-year primary mission, it provided 30 minute (long-)cadence full-frame images from almost the entire sky, and 120 second (short-)cadence observations on selected targets. The main goal of the mission is to find exoplanets at bright nearby stars with the transit method, but the time sampling of the observations also allows us to examine the pulsations of stars in the observed fields. Due to their short periods, only the short cadence mode is suitable for studying the light variations of compact pulsators. The first-light papers of the TESS Asteroseismic Science Consortium (TASC) Compact Pulsators Working Group (WG#8), presented for example by Bell et al. (2019), Charpinet et al. (2019), and Bognár et al. (2020), clearly demonstrate the suitability of TESS measurements for compact pulsators. Moreover, TESS observations have significantly raised (by about 20 per cent) the number of known DAV stars (Romero et al. 2022). Fortunately, the Extended Mission was approved for 2020– 2022, with some modifications, for example in the cadence of\ the observations. The full-frame image cadence was reduced to 10 minutes, and a new, 20-second ultrashort cadence mode was implemented. The latter in particular was a welcome addition given the short periods seen in compact pulsators. This paper is the continuation of our work published in 2020 (Bognár et al. 2020, hereafter P01). We detail the main goals of the present work and some points about the light curve reduction process in Sect. 2. Section 3 presents the results of the lightcurve analyses, while Sect. 4 summarises our asteroseismic investigations. Finally, the summary and conclusions are presented in Sect. 5.
We focused on the results of the ultrashort cadence (20 s) mode TESS observations of nine white dwarf stars known as ZZ Ceti variables. These stars were presented in P01, but in that case the shortest cadence mode available was only 120 s. The new TESS observations allow us to determine the pulsation modes without the Nyquist ambiguities of the 120 s measurements, and to compare the frequency contents between the different observational cycles. We note that we investigated the measurements on 18 previously known ZZ Ceti stars in P01, but more than half of them did not show periodic light variations in the TESS observations. We explain this as mainly resulting from the faintness of these targets, in combination with the crowding of the large TESS pixels. Considering the new observations presented in this paper, we find significant pulsation frequencies in seven stars. Additionally, we detect one-one brightening episodes in the light curves of two targets. In the case of the first, HE 0532-5605, we detect a similar brightening phase to that detected earlier and described in P01, that is, the phenomenon is recurring, implying that HE 0532-5605 is most probably a new outbursting ZZ Ceti star. However, the observed brightening of the other star, WD J0925+0509, was extrinsic, and caused by a passing minor planet crossing the photometric aperture of the star in the TESS images. Using the effective temperature and surface gravity values for HE 0532-5605 provided in Bognár & Sódor (2016) (11 510 K and 8.42 dex, respectively); and placing it on the Teff– log g diagram presented for example by Hermes et al. (2017) (their Fig. 3), we see that the star can be found near the red edge of the empirical ZZ Ceti instability strip. In this respect, HE 0532-5605 is similar to other ZZ Cetis, showing outbursts. Below, we briefly summarise our results for the different stars showing pulsations based on the new ultrashort cadence TESS observations: Ross 548: We can clearly detect the four highest-amplitude frequencies using the ultrashort cadence data, without the Nyquist alias ambiguities of the 120 s cadence observations. EC 23487-2424: The dominant frequency is different from the one detected in the previous TESS data. Some of the peaks seem to be unstable in amplitude, frequency, or phase, producing additional peaks around them. BPM 31594: We find almost all the frequencies detected previously, as well as several additional frequencies. BPM 30551: Only new frequencies are detected, although one of them very near to another detected in the first TESS data set in P01. MCT 0145-2211: Only new frequencies are detected, none of the old ones appeared in the new TESS data, almost as if we observed a different star. L 19-2: One already known frequency, and four additional frequencies in the 20 s cadence data. HS 1013+0321: The first TESS data set did not show pulsational light variations, while in the new TESS data, we detect all three frequencies reported earlier in the literature. We also detect a new peak, which appears to be the result of rotational frequency splitting. The newly detected frequencies impose stronger constraints on asteroseismological modelling. We performed a preliminary asteroseismic analysis of the stars that show pulsational light variations, as the ultimate goal of our efforts to detect as many pulsation modes as we possibly can is to learn more about the internal structure of the target stars and their non-pulsating counterparts, and about the dynamical processes operating in them. We succeeded in finding models with parameters in the vicinity of the Gaia geometric distances. Here, we demonstrate the high value of the new, ultrashort cadence mode observations in studying white dwarf variables, and the continuation of these measurements could be extremely valuable to the white dwarf community.
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