The composition of massive white dwarfs and their dependence on C-burning modeling

Abstract

Context. Recent computations of the interior composition of ultra-massive white dwarfs (WDs) have suggested that some WDs could be composed of neon (Ne)-dominated cores. This result is at variance with our previous understanding of the chemical structure of massive WDs, where oxygen is the predominant element. In addition, it is not clear whether some hybrid carbon (C) oxygen (O)-Ne WDs might form when convective boundary mixing is accounted for during the propagation of the C-flame in the C-burning stage. Both the Ne-dominated and hybrid CO-Ne core would have measurable consequences for asteroseismological studies based on evolutionary models. Aims. In this work, we explore in detail to which extent differences in the adopted micro- and macro-physics can explain the different final WD compositions that have been found by different authors. Additionally, we explore the impact of such differences on the cooling times, crystallization, and pulsational properties of pulsating WDs. Methods. We performed numerical simulations of the evolution of intermediate massive stars from the zero age main sequence to the WD stage varying the adopted physics in the modeling. In particular, we explored the impact of the intensity of convective boundary mixing during the C-flash, extreme mass-loss rates, and the size of the adopted nuclear networks on the final composition, age, as well crystallization and pulsational properties of WDs. Results. In agreement with previous authors, we find that the inclusion of convective boundary mixing quenches the carbon flame leading to the formation of hybrid CO-Ne cores. Based on the insight coming from 3D hydro-dynamical simulations, we expect that the very slow propagation of the carbon flame will be altered by turbulent entrainment affecting the inward propagation of the flame. Also, we find that Ne-dominated chemical profiles of massive WDs recently reported appear in their modeling due to a key nuclear reaction being overlooked. We find that the inaccuracies in the chemical composition of the ultra-massive WDs recently reported lead to differences of 10% in the cooling times and degree of crystallization and about 8% in the period spacing of the models once they reach the ZZ Ceti instability strip.

Stellar Neutrino Emission across the Mass–Metallicity Plane

“…Neutrino emission from nuclear reactions dominate whenever H and He burn; otherwise, neutrinos from thermal processes generally dominate (Farag et al 2020). For example, light curves for M ZAMS 8 M e in Figure 2 have the minimum mass for C ignition and those for M ZAMS 10 M e have the minimum mass for Ne ignition (Becker & Iben 1979, 1980García-Berro et al 1997;Farmer et al 2015;De Gerónimo et al 2022). For these advanced burning stages L ν in Figure 2 becomes nearly vertical and greatly exceeds L γ .…”

Dimming the Lights: 2D Simulations of Deflagrations of Hybrid C/O/Ne White Dwarfs Using FLASH

“…A ZAMS star with a metallicity of Z = 0.02 was evolved using the set of controls from Farmer et al (2015) and the 29-species network sagb_NeNa_MgAl.net. This network notably includes the 12 C( 12 C,p) 23 Na reaction of the carbon-burning process, which was shown by De Gerónimo et al (2022) to have a strong impact on the 20 Ne mass fraction of the end-state WD. The ZAMS star evolves along the main sequence until it uses up the hydrogen in its core, transitions to the red giant branch, burns through its core helium, becomes a superasymptotic giant branch star, and ignites carbon burning offcenter from the C/O core.…”