A double white dwarf with a paradoxical origin?

TL;DR

This study analyzes a double white dwarf system using Hubble and SDSS data, revealing an unexpected formation history where the low-mass white dwarf appears older than the massive one, challenging existing binary evolution models.

Contribution

First detailed UV spectral analysis of this double white dwarf system, providing new insights into their masses, temperatures, and evolutionary ages, highlighting a paradox in binary star evolution.

Findings

Massive white dwarf has a cooling age of about 1 Gyr.

Low-mass white dwarf's age likely exceeds 5 Gyrs.

Results challenge current binary evolution theories.

Abstract

We present Hubble Space Telescope UV spectra of the 4.6 h period double white dwarf SDSS J125733.63+542850.5. Combined with Sloan Digital Sky Survey optical data, these reveal that the massive white dwarf (secondary) has an effective temperature T2 = 13030 +/- 70 +/- 150 K and a surface gravity log g2 = 8.73 +/- 0.05 +/- 0.05 (statistical and systematic uncertainties respectively), leading to a mass of M2 = 1.06 Msun. The temperature of the extremely low-mass white dwarf (primary) is substantially lower at T1 = 6400 +/- 37 +/- 50 K, while its surface gravity is poorly constrained by the data. The relative flux contribution of the two white dwarfs across the spectrum provides a radius ratio of R1/R2 = 4.2, which, together with evolutionary models, allows us to calculate the cooling ages. The secondary massive white dwarf has a cooling age of about 1 Gyr, while that of the primary low-mass white dwarf is likely to be much longer, possibly larger than 5 Gyrs, depending on its mass and the strength of chemical diffusion. These results unexpectedly suggest that the low-mass white dwarf formed long before the massive white dwarf, a puzzling discovery which poses a paradox for binary evolution.