A Not-So-Compact Companion: Massive, Oversize White Dwarf in a Post-Common Envelope Eclipsing Binary

TL;DR

2M07515777+1807352 is a post-common-envelope WD+MS eclipsing binary with a 10.3-day orbit and a massive WD of $M_{WD}=1.08\,M_{\odot}$, whose WD radius of $R_{WD}\approx1.54\,R_{\oplus}$ is anomalously large relative to WD-mass relations. The authors combine RVs from APOGEE and NEID, broadband SED fitting, and TESS photometry, modeling the system with PHOEBE to derive precise orbital and stellar parameters, including a near-edge-on inclination and a WD radius constrained by eclipse geometry. They detect super-synchronous rotation of the MS companion ($P_{rot}\approx6$ d) and find no evidence for a third star or an accretion disk, while speckle imaging rules out a nearby companion. COSMIC population synthesis indicates the system most plausibly formed via CEE while the WD progenitor was on the AGB, with TP-AGB or early-AGB channels producing similar end states; the long period implies that additional energy sources beyond orbital energy may have contributed to envelope ejection in wide PCEBs. Collectively, the results provide empirical constraints on CEE energetics, eccentricity retention, and formation pathways for massive-WD post-CE binaries, with implications for the late stages of binary evolution and potential refinements to WD-structure models.

Abstract

We provide a detailed characterization of 2M07515777+1807352, a post-common envelope eclipsing binary system with a 10.3 d, nearly, but not quite, circular orbit (e = 0.02). This system consists of a massive white dwarf (WD) ($1.08$ M$_{\odot}$) and a 4400 K main-sequence companion (0.66 M$_{\odot}$). This WD is among the most massive known within post-common envelope binary systems. We also find, through both spectral energy distribution and $\it{TESS}$ light curve analyses, that the WD has a radius of $1.54\pm 0.07 R_{\oplus}$, roughly $12σ$ larger than the expected value from WD mass-radius relationships. Both the Lomb-Scargle analysis and the $v \sin{i}$ of the system indicate the main-sequence companion to be super-synchronously rotating at a period of $\sim$6 days, which may suggest accretion occurred during the evolution of the system. This binary also shares similar physical characteristics with six other post-common envelope systems hosting massive WDs, which may point to a shared formation pathway. We model the history of this system with COSMIC and find that it likely formed through an episode of common envelope evolution following the onset of mass transfer when the progenitor primary was on either the early or the thermally pulsing stages of the asymptotic giant branch. As a result of its properties, the study of 2M07515777+1807352 can provide new insights regarding many key outstanding questions in our understanding of common envelope evolution.

Figures (9)

• Figure 1: Orbit solution for 2M07515777 (P = 10.3 d, Table \ref{['tab:all_param']}) as a function of phase (top panel). RV measurements are shown from both APOGEE (open circles) and NEID (filled circles), along with the orbital velocity curve. The center-of-mass velocity, $\gamma$, is shown as a dashed line. In the middle panel, we show the residuals corresponding to the best-fit orbit along with (bottom panel) the residuals of a completely circular (e=0) orbit of the same period to demonstrate the significance of the non-zero eccentricity. The residuals of the best-fit orbit are consistent with the RV measurement errors reported by NEID.
• Figure 2: Spectral energy distribution of 2M07515777. Red symbols represent the observed photometric measurements, where the horizontal bars represent the effective width of the passband. Blue symbols are the model fluxes from the best-fit 2-component atmosphere model (red curve for the K dwarf, cyan curve for the WD, grey curve for the two combined). The inset shows the absolute flux-calibrated Gaia spectrophotometry as a grey swath overlaid on the 2-component best-fit model.
• Figure 3: Detrended TESS light curves for 2M07515777, including sectors 44-46 (left) and 71-72 (right), both showing periodic variation.
• Figure 4: The detrended phase-folded TESS light curves of the eclipse and associated errors for 2M07515777 are shown in gray, including sectors 44-46 (upper panel) and 71-72 (lower panel). We also show the binned data with black markers. Overplotted in pink is the resulting fit from PHOEBE using the Nelder-Mead optimizer.
• Figure 5: Lomb-Scargle periodogram of all combined TESS light curve sectors, indicating a peak period of 6.0449 days.