Constraining the evolution of cataclysmic variables via the masses and accretion rates of their underlying white dwarfs

Abstract

We report on the masses (MWD), effective temperatures (${T_\mathrm{eff}}$) and secular mean accretion rates ($\langle \dot{M} \rangle$) of 43 cataclysmic variable (CV) white dwarfs, 42 of which were obtained from the combined analysis of their Hubble Space Telescope ultraviolet data with the parallaxes provided by the Early Third Data Release of the Gaia space mission, and one from the white dwarf gravitational redshift. Our results double the number of CV white dwarfs with an accurate mass measurement, bringing the total census to 89 systems. From the study of the mass distribution, we derive $\langle M_\mathrm{WD} \rangle = 0.81^{+0.16}_{-0.20}\, \mathrm{M_\odot }$, in perfect agreement with previous results, and find no evidence of any evolution of the mass with orbital period. Moreover, we identify five systems with MWD < 0.5M⊙, which are most likely representative of helium-core white dwarfs, showing that these CVs are present in the overall population. We reveal the presence of an anti-correlation between the average accretion rates and the white dwarf masses for the systems below the 2 − 3 h period gap. Since $\langle \dot{M} \rangle$ reflects the rate of system angular momentum loss, this correlation suggests the presence of an additional mechanism of angular momentum loss that is more efficient at low white dwarf masses. This is the fundamental concept of the recently proposed empirical prescription of consequential angular momentum loss (eCAML) and our results provide observational support for it, although we also highlight how its current recipe needs to be refined to better reproduce the observed scatter in ${T_\mathrm{eff}}$ and $\langle \dot{M} \rangle$, and the presence of helium-core white dwarfs.

The Accreting White Dwarfs in Cataclysmic Variables

“…Moreover, the authors of [20] obtained accurate mass accretion rates for many intermediate polars, with the important conclusion that the mass accretion rate in these objects does not depend on the orbital period. This was further corroborated in [16]. The WD effective surface temperature as a function of the orbital period from [15] (red and blue dots) and from [14] (green, non-magnetic system).…”

“…Unfortunately, there is a paucity of data points above the period gap (Figure 1), and, in addition, many WD masses are not known with accuracy. However, thanks to a large project using the Hubble Space Telescope, accurate masses are at last being secured [15,16] together with the accurate CV white dwarf masses derived from totally eclipsing binaries [17].…”

“…Even more to the point, the number of CVs with robust white dwarf mass determinations above the CV period gap is severely limited for meaningful comparisons with robust CV white dwarf mass determinations below the gap, as emphasized in both [29] and [17]. Pala et al [16] have found little evidence to suggest that CV white dwarfs gain mass despite the occurrence of hundreds to thousands of nova explosions.…”

“…The mass of the white dwarf follows immediately from the white dwarf mass-radius law of (e.g., [2,[30][31][32]) to obtain the white dwarf mass with a precision ≈5% (see recent results in Figures 2-5). Accurate Gaia distances lead to accurate values of M wd and T eff [16,25]. Due to compressional heating of the WD, the accretion rate, Ṁ, averaged over the thermal timescale of the WD envelope is then obtained from T eff [33].…”

Triggering micronovae through magnetically confined accretion flows in accreting white dwarfs

“…This then translates to column masses in the range 5 × 10 −12 𝑀 ⊙ < 𝑀 col < 5 × 10 −11 𝑀 ⊙ . For AWDs with 𝑀 WD ≈ 0.8𝑀 ⊙ (typical for AWDs: Zorotovic et al 2011;Pala et al 2022) the corresponding fractional accretion area required by the model would then be 𝑓 ≈ 10 −6 . Although this value is low when compared to those inferred from X-ray observations of other magnetic AWDs (Hellier 1997; Lopes de Oliveira & Mukai 2019), it is still allowed by models of magnetically channeled accretion flows where material precipitates on to the polar cap in discrete filaments (King 1995;Frank et al 2002).…”