Spectral evolution of hot hybrid white dwarfs: II. Photometry

TL;DR

This study complements the spectral analysis of hot DA/DAO white dwarfs by performing a comprehensive photometric investigation: archival time-series data reveal variability in four of 32 objects, while SED fitting with Gaia parallaxes yields radii, luminosities, and gravity masses and uncovers infrared excesses in several systems. Notably, WD 0232+035 displays a 4.23-day irradiation-driven period, and WD 1342+443 shows a 1.87-day photometric period with weak emission lines indicating irradiation; mid-IR excesses are modeled with cool dust or companions, suggesting possible irradiated substellar components. The analysis also tests whether metal line blanketing can reconcile Kiel and gravity masses, finding that it does not, and in some cases slightly worsens the agreement, while reinforcing a luminosity-dependent He abundance trend and a persistent separation between DAO and DA WDs in the HR diagram. Overall, the work highlights the complex interplay between photometric variability, circumstellar dust, binary interactions, and atmospheric modeling in the hottest WDs, and points to future infrared and time-resolved spectroscopic follow-ups to fully characterize these systems.

Abstract

We present a photometric analysis of 19 DA and 13 DAO white dwarfs (WDs) with effective temperatures exceeding 60 kK, building on the spectral analysis reported in the first paper of this two-part study. By examining archival light curves for periodic signals, we identify that four of the 32 objects ($13^{+8}_{-4}$%) exhibit photometric variability. Spectral energy distribution (SED) fitting allowed us to derive radii, luminosities, and gravity masses, as well as to characterise the infrared excesses observed in six sources. A notable discovery is the identification of a 1.87 d period in the ZTF light curves of WD1342+443 and weak emission lines in the optical spectra of this star, which strongly indicate an irradiation effect system. Our SED fit indicates the presence of cool dust, which must be located farther from the star, and that any companion with a spectral type earlier than L2.0 would appear in the SED. This leads us to speculate that WD1342+443 might have an irradiated, sub-stellar companion. We also highlight that we uncovered, for the first time, a 4.23 d photometric period in the well-known, close DA+dM binary WD0232+035, based on TESS data. We find that the phase and amplitude of the light curve variations are consistent with expectations from an irradiation effect. Intriguingly, we detected an additional, mysterious period at 1.39 d, which is approximately one-third of the orbital period. Moreover, we revisited the longstanding discrepancy between Kiel and gravity masses for the hottest WDs. To address this, we explored fully metal line blanketed model atmospheres as a potential solution, contrasting them with the results from pure H and H+He models. Our results show that including metal opacities does not resolve the discrepancy - in fact, it slightly deteriorates the agreement. Finally, we reaffirm the previously observed correlation between He abundance and luminosity.

Figures (8)

• Figure 1: Phase-averaged TESS (orange) and phase-averaged ZTF (green and red) light curves of the variable objects. Peak-to-peak amplitudes were measured by fitting a harmonic series, shown by the black lines.
• Figure 2: Spectral energy distribution fits to the sample objects (top of each panel) and residuals to the fits (bottom of each panel). The top panels show the SED fits to a DAO (left column) and a DA (right column) WD that do not show an IR excess. The remaining panels (second row: DAO; third and fourth row: DA) depict objects with a near- and/or mid-IR excess modelled with our best fit TMAP models from Paper I and either a best-fit late-type star model from the PHOENIX grid or one or two blackbody component(s). Dots represent observations from different bands, whereas residuals are shown by crosses. Each colour represents a single survey or mission. The dark purple line shows the best-fit (combined) model. Blue and purple areas show the fluxes from the WD and the cool component, respectively. For clarity, the y axis of all panels are shown in the form $f_{\lambda}$$\lambda^{3}$, except for WD 1342+443, WD 2218+706, and WD 2350$-$706 where $f_{\lambda}$ is shown on a logarithmic scale.
• Figure 3: Time-series TESS photometry (grey) of WD 0232+035. Each panel represents a TESS sector. A fit of a harmonic series, as a combination of two significant periods (4.23 d and 1.39 d), to the entire TESS dataset is overplotted in black.
• Figure 4: Comparison between gravity and Kiel masses of DAO (blue) and DA (red) WDs. The dashed line represents a 1:1 comparison. Gravity masses were calculated using the radii and surface gravities derived by employing metal line blanketed models in the SED fits and spectral analysis, respectively.
• Figure 5: Comparison of gravity masses obtained using metal line blanketed models and pure H and/or H+He models. The dashed line represents a 1:1 comparison.