Carbon dredge-up required to explain the Gaia white dwarf colour-magnitude bifurcation

The Gaia colour--magnitude diagram reveals a striking separation between hydrogen-atmosphere white dwarfs and their helium-atmosphere counterparts throughout a significant portion of the white dwarf cooling track. However, pure-helium atmospheres have Gaia magnitudes that are too close to the pure-hydrogen case to explain this bifurcation. To reproduce the observed split in the cooling sequence, it has been shown that trace amounts of hydrogen and/or metals must be present in the helium-dominated atmospheres of hydrogen-deficient white dwarfs. Yet, a complete explanation of the Gaia bifurcation that takes into account known constraints on the spectral evolution of white dwarfs has thus far not been proposed. In this work, we attempt to provide such a holistic explanation by performing population synthesis simulations coupled with state-of-the-art model atmospheres and evolutionary calculations that account for element transport in the envelopes of white dwarfs. By relying on empirically grounded assumptions, these simulations successfully reproduce the bifurcation. We show that the convective dredge-up of optically undetectable traces of carbon from the deep interior is crucial to account for the observations. Neither the convective dilution/mixing of residual hydrogen nor the accretion of hydrogen or metals can be the dominant drivers of the bifurcation. Finally, we emphasize the importance of improving theoretical models for the average ionization level of carbon in warm dense helium, which governs the shape of the diffusive tail of carbon and in turn the predicted amount of dredged-up carbon.Comment: Accepted for publication in MNRAS, minor changes following reports from reviewer

The ubiquity of carbon dredge-up in hydrogen-deficient white dwarfs as revealed by GALEX

The convective dredge-up of carbon from the interiors of hydrogen-deficient white dwarfs has long been invoked to explain the presence of carbon absorption features in the spectra of cool DQ stars ($T_{\rm eff} < 10{,}000\,{\rm K}$). It has been hypothesized that this transport process is not limited to DQ white dwarfs and also operates, albeit less efficiently, in non-DQ hydrogen-deficient white dwarfs within the same temperature range. This non-DQ population is predominantly composed of DC white dwarfs, which exhibit featureless optical spectra. However, no direct observational evidence of ubiquitous carbon pollution in DC stars has thus far been uncovered. In this Letter, we analyze data from the Galaxy Evolution Explorer (GALEX) to reveal the photometric signature of ultraviolet carbon lines in most DC white dwarfs in the $8500\,{\rm K} \leq T_{\rm eff} \leq 10{,}500\,{\rm K}$ temperature range. Our results show that the vast majority of hydrogen-deficient white dwarfs experience carbon dredge-up at some point in their evolution.Comment: Accepted for publication in MNRAS Letter

Toward Precision Cosmochronology: A New C/O Phase Diagram for White Dwarfs

The continuous cooling of a white dwarf is punctuated by events that affect its cooling rate. Probably the most significant of those is the crystallization of its core, a phase transition that occurs once the C/O interior has cooled down below a critical temperature. This transition releases latent heat as well as gravitational energy due to the redistribution of the C and O ions during solidification, thereby slowing down the evolution of the white dwarf. The unambiguous observational signature of core crystallization - a pile-up of objects in the cooling sequence - was recently reported. However, existing evolution models struggle to quantitatively reproduce this signature, casting doubt on their accuracy when used to measure the ages of stellar populations. The timing and amount of the energy released during crystallization depend on the exact form of the C/O phase diagram. Using the advanced Gibbs-Duhem integration method and state-of-the-art Monte Carlo simulations of the solid and liquid phases, we have obtained a very accurate version of this phase diagram, allowing a precise modeling of the phase transition. Despite this improvement, the magnitude of the crystallization pile-up remains underestimated by current evolution models. We conclude that latent heat release and O sedimentation alone are not sufficient to explain the observations and that other unaccounted physical mechanisms, possibly $^{22}$Ne phase separation, play an important role.Comment: 4 pages, 2 figures. Accepted for publication in A&A Letter

The photospheres of the hottest fastest stars in the Galaxy

We perform nonlocal thermodynamic equilibrium (NLTE) model atmosphere analyses of the three hottest hypervelocity stars (space velocities between $\approx$ 1500-2800 km s$^{-1}$) known to date, which were recently discovered spectroscopically and identified as runaways from Type Ia supernovae. The hottest of the three (J0546$+$0836, effective temperature $T_\mathrm{eff}$ = 95,000 $\pm$ 15,000 K, surface gravity log g = $5.5 \pm 0.5$) has an oxygen-dominated atmosphere with a significant amount of carbon (C = $0.10 \pm 0.05$, O = $0.90 \pm 0.05$, mass fractions). Its mixed absorption+emission line spectrum exhibits photospheric absorption lines from O V and O VI as well as O III and O IV emission lines that are formed in a radiation-driven wind with a mass-loss rate of the order of $10^{-8}$ $M_\odot$ yr$^{-1}$. Spectroscopically, J0546$+$0836 is a [WC]-PG1159 transition-type pre-white dwarf. The second object (J0927$-$6335) is a PG1159-type white dwarf with a pure absorption-line spectrum dominated by C III/C IV and O III/O IV. We find $T_\mathrm{eff}$ = 60,000 $\pm$ 5000 K, log g = $7.0 \pm 0.5$, and a carbon- and oxygen-dominated atmosphere with C = $0.47 \pm 0.25$, O = $0.48 \pm 0.25$, and possibly a minute amount of helium (He = $0.05 \pm 0.05$). Comparison with post-AGB evolutionary tracks suggests a mass of $M\approx0.5$ $M_\odot$ for both objects, if such tracks can safely be applied to these stars. We find the third object (J1332$-$3541) to be a relatively massive ($M=0.89 M_\odot$) hydrogen-rich (DAO) white dwarf with $T_\mathrm{eff}$ = 65,657 $\pm$ 2390 K, log g = $8.38 \pm 0.08$, and abundances H = $0.65 \pm 0.04$ and He = $0.35 \pm 0.04$. We discuss our results in the context of the "dynamically driven double-degenerate double-detonation" (D$^6$) scenario proposed for the origin of these stars.Comment: Accepted for publication in A&

Van Roosbroeck's equations with topological terms: the case of Weyl semimetals

Van Roosbroeck's equations constitute a versatile tool to determine the dynamics of electrons under time- and space-dependent perturbations. Extensively utilized in ordinary semiconductors, their potential to model devices made from topological materials remains untapped. Here, we adapt van Roosbroeck's equations to theoretically study the bulk response of a Weyl semimetal to an ultrafast and spatially localized light pulse in the presence of a quantizing magnetic field. We predict a transient oscillatory photovoltage that originates from the chiral anomaly. The oscillations take place at the plasma frequency (THz range) and are damped by intervalley scattering and dielectric relaxation. Our results illustrate the ability of van Roosbroeck's equations to unveil the interplay between electronic band topology and fast carrier dynamics in microelectronic devices.Comment: 5 pages + appendix. Version accepted for publicatio

The photospheres of the hottest fastest stars in the Galaxy

We perform nonlocal thermodynamic equilibrium (NLTE) model atmosphere analyses of the three hottest hypervelocity stars (space velocities between ≈1500–2800 km s−1) known to date, which were recently discovered spectroscopically and identified as runaways from Type Ia supernovae. The hottest of the three (J0546+0836, effective temperature Teff = 95 000 ± 15 000 K, surface gravity lo

An emerging and enigmatic spectral class of isolated DAe white dwarfs

Two recently discovered white dwarfs, WDJ041246.84$+$754942.26 and WDJ165335.21$-$100116.33, exhibit H$\alpha$ and H$\beta$ Balmer line emission similar to stars in the emerging DAHe class, yet intriguingly have not been found to have detectable magnetic fields. These white dwarfs are assigned the spectral type DAe. We present detailed follow-up of the two known DAe stars using new time-domain spectroscopic observations and analysis of the latest photometric time-series data from TESS and ZTF. We measure the upper magnetic field strength limit of both stars as $B < 0.05$ MG. The DAe white dwarfs exhibit photometric and spectroscopic variability, where in the case of WDJ041246.84$+$754942.26 the strength of the H$\alpha$ and H$\beta$ emission cores varies in anti-phase with its photometric variability over the spin period, which is the same phase relationship seen in DAHe stars. The DAe white dwarfs closely cluster in one region of the Gaia Hertzsprung-Russell diagram together with the DAHe stars. We discuss current theories on non-magnetic and magnetic mechanisms which could explain the characteristics observed in DAe white dwarfs, but additional data are required to unambiguously determine the origin of these stars.Comment: 20 pages, 16 figures. Accepted for publication in MNRA