An Exploration of the Paradigm for the 2–3 Hour Period Gap in Cataclysmic Variables
Bibliographic record
Abstract
We critically examine the basic paradigm for the origin of the 2-3 hr period gap in cataclysmic variables (CVs), i.e., binary systems in which a white dwarf accretes from a relatively unevolved, low-mass donor star. The observed orbital period distribution for ~300 CVs shows that these systems typically have orbital periods, P orb , in the range of ~80 minutes to ~8 hr but a distinct dearth of systems with 2 ≲ P orb (hr) ≲ 3. This latter feature of the period distribution is often referred to as the "period gap." The conventional explanation for the period gap involves a thermal bloating of the donor star for P orb ≳ 3 hr due to mass transfer rates that are enhanced over those that could be driven by gravitational radiation (GR) losses alone (e.g., magnetic braking). If for some reason the supplemental angular momentum losses become substantially reduced when P orb decreases below ~3 hr, the donor star will relax thermally and shrink inside of its Roche lobe. This leads to a cessation of mass transfer until GR losses can bring the system into Roche lobe contact again at P orb ~ 2 hr. We carry out an extensive population synthesis study of CVs, starting from ~3 × 10 6 primordial binaries and evolving some ~2 × 10 4 surviving systems through their CV phase. In particular we study current-epoch distributions of CVs in the P orb , R 2 - P orb , M 2 - P orb , q - P orb , T eff - P orb , and L 2 - P orb planes, where is the mass transfer rate, q is the mass ratio M 2 / M 1 , and M 2 , R 2 , T eff , and L 2 are the donor star mass, radius, effective temperature, and luminosity, respectively. This work presents a new perspective on theoretical studies of the long-term evolution of CVs. In particular, we show that if the current paradigm is correct, the secondary masses in CVs just above the period gap should be as much as ~50% lower than would be inferred if one assumes a main-sequence radius-mass relation for the donor star. We quantify the M 2 - P orb relations expected from models wherein the donor stars are thermally bloated. Finally, we propose specific observations, involving the determination of secondary masses in CVs, that would allow for a definitive test of the currently accepted model (i.e., interrupted thermal bloating) for the period gap in CVs.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.003 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.002 |
| Scholarly communication | 0.002 | 0.004 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.002 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".