The Pre-He White Dwarfs in Eclipsing Binaries. V. TIC 399725538

TL;DR

This study combines time-series echelle spectroscopy with high-cadence TESS photometry to characterize TIC 399725538, a short-period EL CVn-type eclipsing binary. The primary A is a ≈$1.93 m~M_sol$, ≈$1.92 m~R_sol$ star with $T_{ m eff} oughly7194$ K, while the secondary B is a ≈$0.211 m~M_sol$ pre-ELM WD with $T_{ m eff} oughly10935$ K, implying a detached system ($P_{ m orb} oughly1.2933$ d) formed through non-conservative mass transfer. B is underluminous for its mass when compared to He-core WD evolutionary tracks, highlighting potential gaps in WD formation models, whereas A shows potential $oldsymbol{ m 9 Dor}$/$oldsymbol{ m 7}$ Dor pulsations, with $f_1 oughly0.0848$ day$^{-1}$ identified in residuals. The system lies in the thick-disk population, reinforcing the role of mass-transfer pathways in diverse Galactic environments and underscoring the need for higher-cadence, high-resolution follow-up to fully map the pulsational and binary evolution properties of EL CVn binaries.

Abstract

We present echelle spectra of TIC 399725538 obtained in Korea and Thailand to investigate the physical properties and evolutionary scenarios of EL CVn-type binaries. The time-series spectra yielded the radial velocities (RVs) of the primary component and its atmospheric parameters, $T_{\rm eff,A}=7194\pm70$ K and $v_{\rm A}\sin i=68\pm9$ km s$^{-1}$. Joint modeling of our RVs and the archival TESS data provided component masses of $M_{\rm A}=1.930\pm0.054$ $M_\odot$ and $M_{\rm B}=0.211\pm0.005$ $M_\odot$, radii of $R_{\rm A}=1.922\pm0.020$ $R_\odot$ and $R_{\rm B}=0.207\pm0.005$ $R_\odot$, and luminosities of $L_{\rm A}=8.87\pm0.39$ $L_\odot$ and $L_{\rm B}=0.546\pm0.034$ $L_\odot$. The surface gravity and distance derived from this modeling are consistent with the model-independent $\log g_{\rm B}$ obtained from the direct observables and with the Gaia distance. The third light of $l_3=0.136\pm0.003$ comes mainly from two neighboring stars, TIC 399725539 and TIC 399725544. Comparison with stellar models indicates that TIC 399725538 A lies within the instability region of $δ$ Sct$-$$γ$ Dor hybrids, whereas its extremely low-mass companion is markedly underluminous compared to theoretical white dwarf (WD) counterparts. Multifrequency analysis of the binary-subtracted residual lights revealed three significant signals, two of which correspond to aliases at two and four times the dominant frequency $f_1=0.0848$ day$^{-1}$. The $f_1$ frequency is likely a $γ$ Dor-type pulsation arising from the brighter A component, though further confirmation is required. Our results demonstrate that TIC 399725538 is a short-period EL CVn system belonging to the thick-disk population, consisting of a main-sequence $γ$ Dor pulsator and a helium-core WD precursor formed through stable mass transfer.

Figures (7)

• Figure 1: TESS observations of TIC 399725538 distributed in BJD (top panel) and orbital phase (second panel). The blue and green circles are individual measurements for Sectors 32 and 71, respectively, and the red solid curve represents the synthetic model obtained through our W-D fit. The third and bottom panels show the residual lights corresponding to the binary model curve.
• Figure 2: Sample of the BF profiles. The black lines are the observed BFs for two orbital phases ($\phi$), with a single peak representing the primary component. A rotational broadening function applied to this peak is plotted as the solid blue line. The vertical dotted lines represent the radial velocity of the binary center of mass, and the green arrows point to the RV positions of the secondary companion predicted by our binary modeling.
• Figure 3: Radial velocities of TIC 399725538 A. The solid curves represent the results from a consistent light and RV curve analysis, and the dotted line denotes the systemic velocity of $+$111.12 km s$^{-1}$. The lower panel displays the residuals between observations and models.
• Figure 4: Reconstructed spectrum of TIC 399725538 A. The black and red solid lines represent the disentangled and best-fitting synthetic spectra, respectively. The lower panel displays the residuals between the two.
• Figure 5: The top panel shows the light curve residuals of TESS S71 distributed in BJD. The middle and bottom panels are amplitude spectra before and after prewhitening all three frequencies extracted from the full residuals using the PERIOD04 code. The synthetic model curve for these frequencies is indicated by the red solid line in the top panel. The vertical dotted lines in the middle panel indicate the positions of the extracted frequencies and the red line in the bottom panel represents five times the noise spectrum.