Structure and evolution of ultra-massive white dwarfs in general relativity

Abstract

Context. Ultra-massive white dwarfs (M 1.05M ) are of utmost importance in view of the role they play in type Ia supernovae explosions, merger events, the existence of high magnetic field white dwarfs, and the physical processes in the Super Asymptotic Giant Branch phase. Aims. We present the first set of constant rest-mass ultra-massive oxygen/neon white dwarf cooling tracks with masses M > 1.29M which fully take into account the effects of general relativity on their structural and evolutionary properties. Methods. We have computed the full evolution sequences of 1.29, 1.31, 1.33, 1.35, and 1.369 M white dwarfs with the La Plata stellar evolution code, LPCODE. For this work, the standard equations of stellar structure and evolution have been modified to include the effects of general relativity. Specifically, the fully general relativistic partial differential equations governing the evolution of a spherically symmetric star are solved in a way they resemble the standard Newtonian equations of stellar structure. For comparison purposes, the same sequences have been computed but for the Newtonian case. Results. According to our calculations, the evolutionary properties of the most massive white dwarfs are strongly modified by general relativity effects. In particular, the resulting stellar radius is markedly smaller in the general relativistic case, being up to 25% smaller than predicted by the Newtonian treatment for the more massive ones. We find that oxygen/neon white dwarfs more massive than 1.369 M become gravitationally unstable with respect to general relativity effects. When core chemical distribution due to phase separation on crystallization is considered, such instability occurs at somewhat lower stellar masses, 1.36M . In addition, cooling times for the most massive white dwarf sequences result in about a factor of two smaller than in the Newtonian case at advanced stages of evolution. Finally, a sample of white dwarfs has been identified as ideal candidates to test these general relativistic effects. Conclusions. We conclude that the general relativity effects should be taken into account for an accurate assessment of the structural and evolutionary properties of the most massive white dwarfs. These new ultra-massive white dwarf models constitute a considerable improvement over those computed in the framework of the standard Newtonian theory of stellar interiors.

Newtonian pulsations of relativistic ONe-core ultra-massive DA white dwarfs

“…On the other hand, the oscillation kinetic energy of the modes is higher in the relativistic case, since the WDs are more compact and dense than in the Newtonian case. Finally, the rate of change of periods is larger for the relativistic case, since the cooling timescale is shorter due to relativistic effects, in comparison with the Newtonian case (Althaus et al, 2022).…”

“…We have generated ultra-massive WD model sequences with ONe cores taking into account the full effects General Relativity employing the LPCODE stellar evolution code (Althaus et al, 2022). We considered realistic initial chemical profiles as predicted by the progenitor evolutionary history (Siess, 2007(Siess, , 2010Camisassa et al, 2019), and computed model sequences of 1.29, 1.31, 1.33, 1.35, and 1.369M ⊙ WDs.…”

An Extremely Massive White Dwarf Escaped from the Hyades Star Cluster

“…Though we typically expect ultramassive WDs to have oxygen-neon (ONe) cores (Siess 2007), we focused on initial estimates for CO models, for consistency across the WD sequence. This leads to an overestimation of the predicted mass compared with ONe models (see Figure 6 of Camisassa et al 2022), in particular for WDs above 1.29 M e , where general relativistic effects become significant (see Figure 7 of Althaus et al 2022). That said, the difference is negligible at 1.05 M e , so the candidate selection requirements for followup are not overly impacted.…”

“…While potential channels exist for WDs with CO cores to enter the ultramassive regime without igniting carbon (Camisassa et al 2022;Wu et al 2022), these channels are unlikely to maintain CO cores into the mass range of WD1. Using the determined g log and T eff , we estimate the mass and cooling age using the ONe core WD cooling models of Althaus et al (2022), chosen due to their incorporation of full general relativistic effects on WD structure in their model evolution, which are thought to be particularly impactful for WDs above 1.29 M e (Althaus et al 2022) We summarize the astrometric, spectroscopic, and derived properties of WD1 in Table 1.…”

Relativistic Corrections in White Dwarf Asteroseismology

“…Recently, the effects of GR on ultra-massive WDs have been considered (Mathew & Nandy 2017;Carvalho et al 2018;Nunes et al 2021;Althaus et al 2022). GR effects on the internal structure become more significant as M approaches M C , the stellar radius shrinks, and the compactness parameter grows.…”