A unified model for the evolution of cataclysmic variables

Sarkar, Arnab; Tout, Christopher A.

doi:10.1093/mnras/stac1187

“…We note importantly that no unique M 2,gap exists for all CVs, as is known from observations and theoretical computations (e.g. Knigge et al 2011;Sarkar & Tout 2022). This is because for unevolved CV donors, M 2,gap and the lower end of the period gap, at which mass transfer resumes (P gap,− ), depend on M 1 and the MB strength above the period gap.…”

Section: Binary Evolution Calculationmentioning

confidence: 69%

Magnetic braking below the cataclysmic variable period gap and the observed dearth of period bouncers

Sarkar,

Rodriguez,

Ginzburg

et al. 2024

A&A

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Context. Period bouncers are cataclysmic variables (CVs) that have evolved past their orbital period minimum. The strong disagreement between theory and observations of the relative fraction of period bouncers is a severe shortcoming in the understanding of CV evolution. Aims. We test the implications of the hypothesis that magnetic braking (MB), which is suggested to be an additional angular momentum loss (AML) mechanism for CVs below the period gap (Porb ≲ 120 min), weakens around their period minimum. Methods. We computed the evolution of CV donors below the period gap using the MESA code, assuming that the evolution of the system is driven by AML due to gravitational wave radiation (GWR) and MB. We parametrised the MB strength as AMLMB = κAMLGWR. We computed two qualitatively different sets of models, one in which κ is a constant and another in which κ depends on stellar parameters in such a way that the value of κ decreases as the CV approaches the period minimum (Porb ≈ 80 min), beyond which κ ≈ 0. Results. We find that two crucial effects drive the latter set of models. (1) A decrease in κ as CVs approach the period minimum stalls their evolution so that they spend a long time in the observed period minimum spike (80 ≲ Porb/min ≲ 86). Here, they become difficult to distinguish from pre-bounce systems in the spike. (2) A strong decrease in the mass-transfer rate makes them virtually undetectable as they evolve further. So, the CV stalls around the period minimum and then “disappears”. This reduces the number of detectable bouncers. Physical processes, such as dynamo action, white dwarf magnetism, and dead zones, may cause such a weakening of MB at short orbital periods. Conclusions. The weakening MB formalism provides a possible solution to the problem of the dearth of detectable period bouncers in CV observational surveys.

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“…We have used a physically motivated formalism for this magnetic braking mechanism, discussed in detail by Sarkar & Tout (2022), known as the double dynamo (DD) model, and compared it to the empirical magnetic braking formulae of Rappaport et al (1983) and its modification by Knigge et al (2011). We show that the time-scales of angular momentum loss and subsequent mass loss are shorter with the DD model than with the other two models.…”

Section: Discussionmentioning

confidence: 99%

“…The first is that of Sarkar & Tout (2022). We use their formula for AML by magnetic braking, AML MB , their equation ( 25), with the three free parameters (𝛼, 𝛽, 𝛾) = (4.6, 0.08, 3.2) 1 .…”

Section: The Dependence Of the Secular Evolution Of CV Systems On The...mentioning

confidence: 99%

Evolved cataclysmic variables as progenitors of AM CVn stars

Sarkar¹,

Ge²,

Tout³

2023

Preprint

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We model cataclysmic variables (CVs) with solar metallicity donors (𝑋 = 0.7, 𝑍 = 0.02) that evolve to form AM CVn stars through the Evolved CV formation channel using various angular momentum loss mechanisms by magnetic braking (AML MB ). We find that the time-scale for AML MB in our double-dynamo (DD) model is shorter than that of previously used empirical formulae. Owing to the shorter time-scales, a larger parameter space of initial conditions evolves to form AM CVn stars with the DD model than with other models. We perform an analysis of the expected number of AM CVn stars formed through the Evolved CV channel and find about 3 times as many AM CVn stars as reported before. We evolve these systems in detail with the Cambridge stellar evolution code (STARS) and show that evolved CVs populate a region with orbital period 𝑃 orb 5.5 hr. We evolve our donors beyond their orbital period minimum and find that a significant number become extremely H-exhausted systems. This makes them indistinguishable from systems evolved from the He-star and the White Dwarf (WD) channels in terms of the absence of H in their spectra. We also compare the masses, mass-transfer rates of the donor, and the orbital period with observations. We find that the state of the donor and the absence of H in systems such as YZ LMi and V396 Hya match with our modelled trajectories, while systems such as CR Boo and HP Lib match with our modelled tracks if their actual donor mass lies on the lower-end of the observed mass range.

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“…(5) Population studies based on the standard evolutionary model predict spatial densities 1-2 orders of magnitude larger than observed values (Zorotovic & Schreiber 2020). Although additional angular momentum loss mechanisms and models (Patterson 1998;Knigge et al 2011;Schreiber et al 2016;Pala et al 2017;Zorotovic & Schreiber 2017Belloni et al 2018;Liu & Li 2019;Metzger et al 2021;Sarkar & Tout 2022) were suggested to solve the disagreements, the only published, self-consistent simulations of CV evolution were performed by Hillman et al (2020), whose multigigayear models of novae take into account every nova eruption's thermonuclear runaway, mass and angular momentum losses, feedback due to irradiation, variable mass transfer rate (M  ), orbital size, and period changes. Hillman et al (2020) reproduced the observed range of mass transfer rates at a given orbital period, with large and cyclic kiloyear to megayear timescale changes.…”

Section: Introductionmentioning

confidence: 99%

Galactic Model Parameters and Spatial Density of Cataclysmic Variables in the Gaia Era: New Constraints on Population Models

Canbay¹,

Bilir

²

,

Özdönmez

³

et al. 2023

AJ

3

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The spatial distribution, Galactic model parameters, and luminosity function of cataclysmic variables (CVs) are established using reestimated trigonometric parallaxes from Gaia DR3. The data sample of 1587 CVs in this study is claimed to be suitable for Galactic model parameter estimation as the distances are based on trigonometric parallaxes, and the Gaia DR3 photometric completeness limits were taken into account when the sample was created. According to the analysis, the scale height of all CVs increases from 248 ± 2 to 430 ± 4 pc toward shorter periods near the lower limit of the period gap and suddenly drops to 300 ± 2 pc for the shortest orbital period CVs. The exponential scale heights of all CVs and the magnetic systems are found to be 375 ± 2 and 281 ± 3 pc, respectively, considerably larger than those suggested in previous observational studies. The local spatial density of all CVs and the magnetic systems in the sample are 6.8 − 1.1 + 1.3 × 10−6 and 2.1 − 0.4 + 0.5 × 10 − 6 pc−3, respectively. Our measurements strengthen the 1–2 order of magnitude discrepancy between the CV spatial densities predicted by population synthesis models and observations. It is likely that this discrepancy is due to objects undetected by CV surveys, such as systems with very low M ̇ and ones in the period gap. A comparison of the luminosity function of white dwarfs with the luminosity function of all CVs in this study show that 500 times the luminosity function of CVs fits very well to the luminosity function of white dwarfs. We conclude that the estimations and data sample in this study can be confidently used for further analyses of CVs.

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A unified model for the evolution of cataclysmic variables

Cited by 10 publications

References 23 publications

Magnetic braking below the cataclysmic variable period gap and the observed dearth of period bouncers

Magnetic braking below the cataclysmic variable period gap and the observed dearth of period bouncers

Evolved cataclysmic variables as progenitors of AM CVn stars

Galactic Model Parameters and Spatial Density of Cataclysmic Variables in the Gaia Era: New Constraints on Population Models

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