Discovery of Two Pulsating Extremely Low-mass Pre-white Dwarf Candidates in the TESS Eclipsing Binaries

Abstract

We report the discovery of two new pulsating extremely low-mass pre-white dwarf (pre-ELMV) candidates in the Transiting Exoplanet Survey Satellite (TESS) eclipsing binaries, TIC 149160359 and TIC 416264037. Their light curves show a typical feature of EL CVn-type binaries. The light-curve modeling indicates that they are both detached systems with very low-mass ratios (q ≃ 0.1). Based on the photometric solutions, the masses and radii of the two main-sequence primary components are estimated, and those of the secondaries are deduced. The results show that the less-massive components of the two binaries are both probably thermally bloated, pre-ELMVs. Apart from the eclipsing light changes, short-period light variations are clearly shown in their residual light curves. We have made the Fourier analysis of their light-curve residuals with the Period04 program. TIC 149160359 was found to pulsate in 21 independent frequencies, 17 of which are between 21 and 35 day−1 and the others are between 63 and 77 day−1. The Fourier amplitude spectrum of TIC 416264037 also shows two frequency concentration ranges. Out of nine independent frequencies, seven reside within the low-frequency range of 12.5–19.9 day−1. Two pulsating signals, f 4 = 122.2698 day−1 and f 10 = 112.3603 day−1, were detected in the high-frequency region. These low-frequency signals that are detected on TIC 149160359 and TIC 416264037 are probably due to the intrinsic pulsations of their δ Sct-type primary components. However, the high-frequency signals are likely to come from the pulsations of the pre-ELM WD components. This brings the number of pre-ELMV candidates to 12.

A new instability domain of CNO-flashing low-mass He-core stars on their early white-dwarf cooling branches

“…Although we are aware that detecting pulsations (and rates of change of periods) in this type of objects is not an easy task, we consider that searches for low-amplitude variability are worth doing. As already shown by the results from the Transiting Exoplanet Survey Satellite (TESS, Ricker et al 2015) in the case of the two new pre-ELMVs reported by Wang et al (2020), and as the future space missions like Plato (Piotto 2018) and Cheops (Moya et al 2018) will probably show, the continuous improvement in the quality of the observations is likely to help in this regard.…”

“…We have included a sample of ELM WD stars (shown with red asterisks; Brown et al 2010Brown et al , 2013Brown et al , 2016Brown et al , 2020Vennes et al 2011;Gianninas et al 2015;Kawka et al 2015;Pelisoli & Vos 2019). Also, we have included the location of the known ELMVs (Hermes et al 2012(Hermes et al , 2013bKilic et al 2015Kilic et al , 2018Bell et al 2017Bell et al , 2018Pelisoli et al 2018) marked with lightblue squares with dots, and pre-ELMVs (Maxted et al 2013(Maxted et al , 2014Gianninas et al 2016;Wang et al 2020), indicated with black circles with dots (both regions have been emphasized with light-blue and light-red shaded areas, respectively). In addition, we have included the location of some sdBV stars (Green et al 2011), BLAPs (Pietrukowicz et al 2017), as well as High-Gravity (HG)-BLAPs (Kupfer et al 2019) indicated with green circles, and orange and pink triangles, respectively (their regions emphasized with green, orange and pink shaded areas, respectively).…”

“…The discovery of their probable precursors, the so-called low-mass pre-WDs, has triggered the interest in this type of objects because of the possibility of studying the evolution of the progenitors that lead to the WD phase. Even more interesting, the detection of multi-periodic brightness variations in low-mass WDs (Hermes et al 2012(Hermes et al , 2013bKilic et al 2015Kilic et al , 2018Bell et al 2017Bell et al , 2018Pelisoli et al 2018), and low-mass pre-WDs (Maxted et al 2013(Maxted et al , 2014Gianninas et al 2016;Wang et al 2020) has brought about new classes of pulsating stars known as ELMVs and pre-ELMVs, respectively. It has allowed the study of their stellar interiors with the tools of asteroseismology, similar to the case of other pulsating WDs such as ZZ Ceti stars or DAVs -pulsating WDs with H-rich atmospheresand V777 Her or DBVs -pulsating WDs with He-rich atmospheres (Winget & Kepler 2008;Fontaine & Brassard 2008;Althaus et al 2010;Córsico et al 2019).…”

“…Red asterisks represent ELM (and pre-ELM) stars(Brown et al 2010(Brown et al , 2013(Brown et al , 2016(Brown et al , 2020Vennes et al 2011;Gianninas et al 2015;Kawka et al 2015;Pelisoli & Vos 2019). Light-blue squares with dots represent pulsating low-mass WDs (ELMVs)(Hermes et al 2012(Hermes et al , 2013bKilic et al 2015Kilic et al , 2018Bell et al 2017Bell et al , 2018Pelisoli et al 2018), while black circles with dots correspond to pulsating low-mass pre-WDs (pre-ELMVs)(Maxted et al 2013(Maxted et al , 2014Gianninas et al 2016;Wang et al 2020). Green circles correspond to sdBVs(Green et al 2011), orange triangles represent BLAPs(Pietrukowicz et al 2017), and pink triangles correspond to HG (High Gravity)-BLAPs(Kupfer et al 2019).…”

Space-Based Photometry of Binary Stars: From Voyager to TESS

“…Wang et al [320] used the TESS database to identify two new EL CVn systems, which are short-period EBs containing a normal A star and a hot low-mass pre-white-dwarf [321,322]. In both cases δ Scuti pulsations were detected from the A star and higher-frequency pulsations were also found which might arise from the secondary component.…”