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White dwarf (WD) stars evolve simply and predictably, making them reliable age indicators. However, self-consistent validation of the methods for determining WD total ages has yet to be widely performed. This work uses 1565 wide (>100 au) WD+WD binaries and 24 new triples containing at least two WDs to test the accuracy and validity of WD total age determinations. For these 1589 wide double WD binaries and triples, we derive the total age of each WD using photometric data from all-sky surveys, in conjunction with Gaia parallaxes and current hydrogen atmosphere WD models. Ignoring the initial-to-final mass relation and considering only WD cooling ages, we find that roughly 21%–36% of the more massive WDs in a system have a shorter cooling age. Since more massive WDs should be born as more massive main-sequence stars, we interpret this unphysical disagreement as evidence of prior mergers or the presence of an unresolved companion, suggesting that roughly 21%–36% of wide WD+WD binaries were once triples. Among the 423 wide WD+WD pairs that pass high-fidelity cuts, we find that 25% total age uncertainties are generally appropriate for WDs with masses >0.63M_⊙and temperatures <12,000 K and provide suggested inflation factors for age uncertainties for higher-mass WDs. Overall, WDs return reliable stellar ages, but we detail cases where the total ages are least reliable, especially for WDs <0.63M_⊙.

 

We explore changes in the adiabatic low-order g-mode pulsation periods of 0.526, 0.560, and 0.729M_⊙carbon–oxygen white dwarf models with helium-dominated envelopes due to the presence, absence, and enhancement of²²Ne in the interior. The observed g-mode pulsation periods of such white dwarfs are typically given to 6−7 significant figures of precision. Usually white dwarf models without²²Ne are fit to the observed periods and other properties. The rms residuals to the ≃150−400 s low-order g-mode periods are typically in the range ofσ_rms≲ 0.3 s, for a fit precision ofσ_rms/P≲ 0.3%. We find average relative period shifts of ΔP/P≃ ±0.5% for the low-order dipole and quadrupole g-mode pulsations within the observed effective temperature window, with the range of ΔP/Pdepending on the specific g-mode, abundance of²²Ne, effective temperature, and the mass of the white dwarf model. This finding suggests a systematic offset may be present in the fitting process of specific white dwarfs when²²Ne is absent. As part of the fitting processes involves adjusting the composition profiles of a white dwarf model, our study on the impact of²²Ne can provide new inferences on the derived interior mass fraction profiles. We encourage routinely including²²Ne mass fraction profiles, informed by stellar evolution models, to future generations of white dwarf model-fitting processes.

 

All ten LIGO/Virgo binary black hole (BH-BH) coalescences reported following the O1/O2 runs have near-zero effective spins. There are only three potential explanations for this. If the BH spin magnitudes are large, then: (i) either both BH spin vectors must be nearly in the orbital plane or (ii) the spin angular momenta of the BHs must be oppositely directed and similar in magnitude. Then there is also the possibility that (iii) the BH spin magnitudes are small. We consider the third hypothesis within the framework of the classical isolated binary evolution scenario of the BH-BH merger formation. We test three models of angular momentum transport in massive stars: a mildly efficient transport by meridional currents (as employed in the Geneva code), an efficient transport by the Tayler-Spruit magnetic dynamo (as implemented in the MESA code), and a very-efficient transport (as proposed by Fuller et al.) to calculate natal BH spins. We allow for binary evolution to increase the BH spins through accretion and account for the potential spin-up of stars through tidal interactions. Additionally, we update the calculations of the stellar-origin BH masses, including revisions to the history of star formation and to the chemical evolution across cosmic time. We find that we can simultaneously match the observed BH-BH merger rate density and BH masses and BH-BH effective spins. Models with efficient angular momentum transport are favored. The updated stellar-mass weighted gas-phase metallicity evolution now used in our models appears to be key for obtaining an improved reproduction of the LIGO/Virgo merger rate estimate. Mass losses during the pair-instability pulsation supernova phase are likely to be overestimated if the merger GW170729 hosts a BH more massive than 50  M ⊙ . We also estimate rates of black hole-neutron star (BH-NS) mergers from recent LIGO/Virgo observations. If, in fact. angular momentum transport in massive stars is efficient, then any (electromagnetic or gravitational wave) observation of a rapidly spinning BH would indicate either a very effective tidal spin up of the progenitor star (homogeneous evolution, high-mass X-ray binary formation through case A mass transfer, or a spin- up of a Wolf-Rayet star in a close binary by a close companion), significant mass accretion by the hole, or a BH formation through the merger of two or more BHs (in a dense stellar cluster).