How Gaps in Time-Series Data Affect Asteroseismic Interpretation

Most pulsating white dwarf stars pulsate with many periods, each of which is a probe of their interior, which has made asteroseismolgy of these stars an active field. However, disentangling the multiple periodicities requires long, uninterrupted strings of data. We briefly describe the history of multi-site observing campaigns that culminated in the development of the Whole Earth Telescope in the late 1980s that still functions today. Through examples from the May 1990 campaign on GD 358, we show how critical it is to eliminate periodic gaps in data to greatly reduce aliasing in Fourier Transforms normally used to analyze the frequency content of pulsating white dwarfs. We close with a brief description of space satellite-based data, along with the advantages and disadvantages of these data compared to ground-based data.

EPIC 228782059: Asteroseismology of What Could Be the Coolest Pulsating Helium-atmosphere White Dwarf (DBV) Known

We present analysis of a new pulsating helium-atmosphere (DB) white dwarf, EPIC 228782059, discovered from 55.1 days of K2 photometry. The long-duration, high-quality light curves reveal 11 independent dipole and quadruple modes, from which we derive a rotational period of 34.1 ± 0.4 hr for the star. An optimal model is obtained from a series of grids constructed using the White Dwarf Evolution Code, which returns M * = 0.685 ± 0.003M ⊙, T eff = 21,910 ± 23 K, and log g = 8.14 ± 0.01 dex. These values are comparable to those derived from spectroscopy by Koester & Kepler (20,860 ± 160 K, and 7.94 ± 0.03 dex). If these values are confirmed or better constrained by other independent works, it would make EPIC 228782059 one of the coolest pulsating DB white dwarf stars known, and would be helpful for testing different physical treatments of convection, and to further investigate the theoretical instability strip of DB white dwarf stars.

Constraining the secular variation of the gravitational constant using white-dwarf stars spectra

Context Astrophysical observations play a critical role in the possibility of variations in fundamental physical constants. One of the ways of probing these variations would be based on the evolution of the white-dwarf stars. Aims We use the spectrum of white-dwarf star G191-B2B to find an upper limit on the possible deviation of the gravitational constant with strong gravitational fields. Methods We analyze archive observation of the Hubble Space Telescope Imaging Spectrograph (HSTIS) to determine the possible cosmological deviation of the gravitational constant from the observed gravitational redshift. Results Our analysis provided a strong estimate on an upper bound on the possible space-time variation of the gravitational constant ̇⁄ = (0.238 ± 2.959) × 10 yr comparing with previous results. Conclusions The obtained result in this study offers the possibility of testing parameters of modern unification theories.

White dwarf stars in modified gravity

A few questions related to white dwarfs’ physics is posed. It seems that the modified gravity framework can be a good starting point to provide alternative explanations to cooling processes, their age determination, and Chandrasekhar mass limits. Moreover, we have also obtained the Chandrasekhar limit coming from Palatini [Formula: see text] gravity provided by a simple Lane–Emden model.