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1. Multi-gigayear White Dwarf Cooling Delays from Clustering-enhanced Gravitational Sedimentation

   https://doi.org/10.3847/1538-4357/abb5a5  

   Bauer, Evan B.; Schwab, Josiah; Bildsten, Lars; Sihao Cheng, 思浩 (October 2020, The Astrophysical Journal)

   Abstract Cooling white dwarfs (WDs) can yield accurate ages when theoretical cooling models fully account for the physics of the dense plasma of WD interiors. We use MESA to investigate cooling models for a set of massive and ultramassive WDs (0.9–1.3 ) for which previous models have failed to match kinematic age indicators based on Gaia DR2. We find that the WDs in this population can be explained as C/O cores experiencing unexpectedly rapid 22 Ne sedimentation in the strongly liquid interior just prior to crystallization. We propose that this rapid sedimentation is due to the formation of solid clusters of 22 Ne in the liquid C/O background plasma. We show that these heavier solid clusters sink faster than individual 22 Ne ions and enhance the sedimentation heating rate enough to dramatically slow WD cooling. MESA models including our prescription for cluster formation and sedimentation experience cooling delays of ≈4 Gyr on the WD Q branch, alleviating tension between cooling ages and kinematic ages. This same model then predicts cooling delays coinciding with crystallization of 6 Gyr or more in lower-mass WDs (0.6–0.8 ). Such delays are compatible with, and perhaps required by, observations of WD populations in the local 100 pc WD sample and the open cluster NGC 6791. These results motivate new investigations of the physics of strongly coupled C/O/Ne plasma mixtures in the strongly liquid state near crystallization and tests through comparisons with observed WD cooling. 

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2. Fundamental Tests of White Dwarf Cooling Physics with Wide Binaries

   https://doi.org/10.3847/1538-4357/ad6303  

   Barrientos, Manuel; Kilic, Mukremin; Bergeron, Pierre; Blouin, Simon; Brown, Warren R; Andrews, Jeff J (September 2024, The Astrophysical Journal)

   Abstract We present follow-up spectroscopy and a detailed model atmosphere analysis of 29 wide double white dwarfs, including eight systems with a crystallized C/O core member. We use the state-of-the-art evolutionary models to constrain the physical parameters of each star, including the total age. Assuming that the members of wide binaries are coeval, any age difference between the binary members can be used to test the cooling physics for white dwarf stars, including potential delays due to crystallization and²²Ne distillation. We use our control sample of 14 wide binaries with noncrystallized members to show that this method works well; the control sample shows an age difference of only ΔAge = −0.03 ± 0.15 Gyr between its members. For the eight crystallized C/O core systems we find a cooling anomaly of ΔAge = ${1.13}_{-1.07}^{+1.20}$ Gyr. Even though our results are consistent with a small additional cooling delay (∼1 Gyr) from²²Ne distillation and other neutron-rich impurities, the large uncertainties make this result not statistically significant. Nevertheless, we rule out cooling delays longer than 3.6 Gyr at the 99.7% (3σ) confidence level for 0.6–0.9M_⊙white dwarfs. Further progress requires larger samples of wide binaries with crystallized massive white dwarf members. We provide a list of subgiant + white dwarf binaries that could be used for this purpose in the future. 

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3. Carbon Burning Cannot Explain Puffy Hypervelocity White Dwarfs

   https://doi.org/10.1088/1538-3873/ae05cc  

   Yamaguchi, Natsuko; El-Badry, Kareem; Wong, Tin_Long Sunny; Shen, Ken J (September 2025, Publications of the Astronomical Society of the Pacific)

   Abstract Several hypervelocity white dwarfs (HVWDs) with space velocities of ≳1000 km s⁻¹have recently been discovered. One possible origin of these stars is the dynamically driven double-degenerate double-detonation (D6) scenario, in which an accreting sub-Chandrasekhar mass carbon-oxygen (CO) WD detonates as a SN Ia. In this scenario, the less massive WD may survive its companion’s detonation and be ejected as a HVWD. Most of the observed HVWDs are hotter and puffier than normal WDs, perhaps due to their recent proximity to a SN. In this work, we test whether these properties can be explained by long-lived stable carbon (C) burning in the interiors of CO WD donors triggered by a SN shock. We model the long-term evolution of CO WDs following rapid energy injection using 1D models. We find that stable C burning can be ignited in CO WDs with masses of 0.95–1.10M_⊙if SN energy penetrates sufficiently deeply. The resulting born-again stars settle on the C-burning main sequence while they convert their interiors from C and O to Ne and Mg, where they have temperatures and radii similar to some of the observed HVWDs. However, the timescale over which C-burning WDs remain inflated is ≲10⁵yr, which is at least an order of magnitude shorter than the kinematic ages of observed hot HVWDs. We conclude that observed HVWDs are unlikely to be inflated by C burning. The stellar evolution of observed HVWDs remains an open problem. 

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4. Massive White Dwarfs in the 100 pc Sample: Magnetism, Rotation, Pulsations, and the Merger Fraction

   https://doi.org/10.3847/1538-4357/ad6905  

   Jewett, Gracyn; Kilic, Mukremin; Bergeron, Pierre; Moss, Adam; Blouin, Simon; Brown, Warren R; Kosakowski, Alekzander; Toonen, Silvia; Agüeros, Marcel A (October 2024, The Astrophysical Journal)

   Abstract We present a detailed model atmosphere analysis of massive white dwarfs withM> 0.9M_⊙andT_eff≥ 11,000 K in the Montreal White Dwarf Database 100 pc sample and the Pan-STARRS footprint. We obtained follow-up optical spectroscopy of 109 objects with no previous spectral classification in the literature. Our spectroscopic follow-up is now complete for all 204 objects in the sample. We find 118 normal DA white dwarfs, including 45 massive DAs near the ZZ Ceti instability strip. There are no normal massive DBs: the six DBs in the sample are strongly magnetic and/or rapidly rotating. There are 20 massive DQ white dwarfs in our sample, and all are found in the crystallization sequence. In addition, 66 targets are magnetic (32% of the sample). We use magnetic white dwarf atmosphere models to constrain the field strength and geometry using offset dipole models. We also use magnetism, kinematics, and rotation measurements to constrain the fraction of merger remnant candidates among this population. The merger fraction of this sample increases from 25% for 0.9–1M_⊙white dwarfs to 49% for 1.2–1.3M_⊙. However, this fraction is as high as ${78}_{-7}^{+4}$ % for 1.1–1.2M_⊙white dwarfs. Previous works have demonstrated that 5%–9% of high-mass white dwarfs stop cooling for ∼8 Gyr due to the²²Ne distillation process, which leads to an overdensity of Q-branch stars in the solar neighborhood. We demonstrate that the overabundance of the merger remnant candidates in our sample is likely due to the same process. 

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5. Distillation of ⁵⁶ Fe in Ultramassive O–Ne White Dwarfs

   https://doi.org/10.3847/1538-4357/acbfaa  

   Caplan, Matthew E.; Blouin, Simon; Freeman, Ian F. (April 2023, The Astrophysical Journal)

   Abstract When white dwarfs freeze, the plasma mixtures inside them undergo separation processes that can produce radical changes in the composition profile of the star. The abundance of neutron-rich elements, such as 22 Ne or 56 Fe, determines whether or not the first crystals are more or less dense than the surrounding fluid and thus whether they sink or float. These processes have now been studied for C–O–Ne and C–O–Fe mixtures, finding that distillation and precipitation processes are possible in white dwarfs. In this work, we calculate the phase diagram of more complicated O–Ne–Fe mixtures and make predictions for the internal structure of the separated white dwarf. There are two possible outcomes determined by a complicated interplay between the Ne abundance, the 22 Ne fraction, and the 56 Fe abundance. Either Fe distills to form an inner core because the first O–Ne solids are buoyant, or an O–Ne inner core forms and Fe accumulates in the liquid until Fe distillation begins and forms an Fe shell. In the case of an Fe shell, a Rayleigh–Taylor instability may arise and overturn the core. In either case, Fe distillation may only produce a cooling delay of order 0.1 Gyr, as these processes occur early at high white dwarf luminosities. Fe inner cores and shells may be detectable through asteroseismology and could enhance the yield of neutron-rich elements such as 55 Mn and 58 Ni in supernovae. 

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