Mode identification of oscillations of Delta Scuti stars using multicolor photometry and high resolution spectroscopy
Creators: name: Adassuriya, Janaka
Publisher: name: Zenodo
Year: 2022

description: Delta Scuti stars are pulsating stars. They are transitional objects where<br> asteroseismology could help to study the physics of the stellar interior. The pulsation<br> drives by an opacity mechanism with periodicities, typically of a few hours. The<br> central or shell hydrogen burning stage Delta Scuti stars are placed where the classical<br> instability strip crosses the main sequence of the Hertzsprung-Russell diagram. Both<br> radial and non-radial pulsations occur in Delta Scuti stars. With the increase of<br> ground-based, and most importantly, the advent of space-based photometry, this<br> particular class of stars became better studied with their complex frequency spectra.<br> The high-quality continuous space-based light curves mainly reveal the pulsation<br> frequencies of both pressure modes (p-mode) and gravity modes (g-mode). These<br> modes should be identified through mode identification.<br> The analysis of high temporal resolution ground and space-based photometric<br> observations of SZ Lyncis, a binary star one of whose components is high amplitude<br> Delta Scuti, is presented. UBVR photometric observations were obtained from<br> Mt. Abu Infrared Observatory (24.65° N, 72.78° E) in India and Fairborn Observatory<br> (31.68° N, 110.88° W) in Arizona. In addition, the archival observations from the<br> Wide Angle Search for Planets (WASP) project were used. Furthermore, the<br> continuous, high-quality light curve from the TESS project was extensively used for<br> the analysis. The well-resolved light curve from Transiting Exoplanet Survey Satellite<br> (TESS) reveals the presence of 23 frequencies with four independent modes, 13<br> harmonics of the primary pulsation frequency of 8.296943 ± 0.000002 d<sup>−1</sup>, and their<br> combinations. The frequency 8.296 d<sup>−1</sup> is identified as the fundamental radial mode<br> primarily by the amplitude ratio method. The estimated pulsation constant using the<br> frequency 8.296 d<sup>-1</sup> also claims the existence of this fundamental mode. The<br> frequencies 14.535 d<sup>-1</sup>, 32.620 d<sup>-1</sup>, and 4.584 d<sup>−1</sup> are newly discovered for SZ Lyn.<br> Out of these three, 14.535 d<sup>-1</sup>and 32.620 d<sup>−1</sup> are identified as non-radial low order<br> p-modes, and 4.584 d<sup>−1</sup> could indicate presence of a g-mode in SZ Lyn. The ultimate<br> purpose of frequency determination and mode identification is to derive accurate<br> physical parameters of SZ Lyn. The optimization of the generated stellar pulsation<br> models with the observed frequencies produced a set of physical parameters to<br> represent SZ Lyn. The theoretical models correspond to 7500 K ≤ T<sub>eff</sub> ≤ 7800 K and<br> log (g) = 3.81 ± 0.06. The mass of SZ Lyn estimated using evolutionary sequences is<br> close to 1.7–2.0 M<sub>⊙</sub>. The period-density relation calculates a mean density (ρ) of<br> 0.1054 ± 0.0016 g cm<sup>−3</sup>.<br> Five more Delta Scuti stars observed by the Kepler mission were also analyzed for<br> frequencies and thereby identified the oscillation modes. KIC 4077032 and KIC<br> 8623953 have contrasting frequency spectra and thus emphasize the diversity of the<br> Delta Scuti group. Though the fundamental radial mode is the highest amplitude<br> dominant mode, KIC 4077032 shows a secondary amplitude mode as its fundamental<br> mode. On the other hand, KIC 8623953 has the highest amplitude for the fundamental<br> mode, and this mode does not agree with the well-established range of the pulsation<br> constant (Q) of the fundamental mode of Delta Scuti stars. The splitting of<br> frequencies due to rotation was identified in both KIC 9700322 and KIC 11754974.<br> This rotational splitting confirmed the availability of l = 2 quintuplet mode in KIC<br> 9700322 and KIC 11754974. Furthermore, the rotational periods were determined to<br> be 6.19 days and 3.83 days for KIC 9700322 and KIC 11754974, respectively. On<br> the contrary, KIC 9845907 is a purely radial pulsator where all frequencies fit into<br> l = 0 modes. In common, large numbers of harmonics and linear combinations of<br> these independent frequencies are also present in all five Kepler targets.<br> The spectroscopic observation of SZ Lyn was carried out at the Mount Abu<br> Observatory from October to December 2016. Although the low-resolution<br> spectroscopy of SZ Lyn is insufficient to resolve the low amplitude frequencies, the<br> fundamental radial mode is prominently recovered from the spectroscopic data.