Recently, Dr. CHEN Xinghao from the Yunnan Observatories of the Chinese Academy of Sciences and his cooperators have successfully identified three distinct frequency sequences corresponding to l = 0, 1, and 2 for the δ Scuti star TIC 120857354. Based on these identifications, they accurately determined evolutionary status of the star by using the p-g mixed modes with l=2. This study was published in Monthly Notices of the Royal Astronomical Society.
δ Scuti stars, also known as dwarf Cepheid variables, are a class of pulsating stars that have garnered significant interest within the astronomical community. As variable stars, they serve as distance indicators, helping astronomers measure the scale of the universe by determining the distances to nearby galaxies. Moreover, their rich pulsations provide a testing ground for theories of stellar structure and evolution.
In asteroseimology, the spherical harmonic indices are crucial parameters that describe the angular dependence of the modes in the stars. Accurately identifying these indices is essential for modeling the star’s interior structure and understanding its evolutionary state. For δ Scuti stars, their periods are low radial orders that deviate from the equal frequency spacing relationship of solar-like variables and the equal period spacing relationship of pulsating white dwarfs, thus mode identifications is a longstanding difficulty for asteroseismology of δ Scuti stars.
Over the past four decades, four primary methods of mode identification have been applied to δ Scuti stars, including the multicolour photometric method, the spectroscopic method, the echelle diagram method, and rotational splitting method. The application of the first two methods is significantly constrained by the requirement for high temporal resolutions and short exposure times; even so, they can only identify a small number of modes. The echelle diagram method predominantly identifies modes with low spherical harmonic degrees, specifically l=0 and l=1 sequences, while higher degree sequences are often absent, such as l= 2. In contrast to the aforementioned methods, the rotational splitting method stands out for its ability to effectively distinguish frequencies with different spherical harmonic indices. This method capitalizes on the splitting of modes due to the star’s rotation, facilitating the identification of modes.
Based on TESS data, the researchers conducted an in-depth analysis of the high-amplitude δ Scuti star TIC120857354. Its frequency spectrum is mainly characterized by radial fundamental mode pulsations and radial first overtone frequencies, accompanied by 18 independent low-amplitude pulsation frequencies. Using the KS test method, they identified two significant frequency spacings: the rotational splitting spacing of 2.4 μHz and the large frequency spacing of 74.6 μHz.
Based on the two spacings, the researchers successfully identified three frequency sequences for l = 0, 1, and 2. The l=2 sequence was found to be similar to the l=0 sequence, in that as the node number decreases, the frequency spacing also decreases. Further analysis suggested that the evanescent zone between the g-mode propagation region and the p-mode propagation region for the l=2 modes is thinner than that for the l=1 modes, making the l=2 modes more likely to exhibit a mixed characteristic of p-g modes. This may be the reason why l=2 modes were difficult to detect in previous studies. Moreover, the study highlights that previous research may have underestimated the important role of l=2 modes in probing the internal structure and evolution of stars. Using l=2 modes, they accurately determined the size of the convective core of TIC120857354.
Contact:
CHEN Xinghao
Yunnan Observatories, CAS
Email: chenxinghao@ynao.ac.cn
