Sudden and Steady Orbital Period Changes Across the Classical Nova Eruptions of DQ Her and BT Mon

TL;DR

This study measures orbital period changes across nova eruptions in DQ Her and BT Mon, revealing unexpected negative and positive shifts that challenge existing models and suggest additional mechanisms at play.

Contribution

The paper provides the first measurements of long-term period change rates in classical novae, highlighting discrepancies with magnetic braking predictions and proposing alternative explanations.

Findings

DQ Her shows a negative fractional period change, counter to hibernation models.

BT Mon exhibits a positive fractional period change, not large enough for hibernation.

Measured long-term period change rates differ significantly from magnetic braking predictions.

Abstract

I report two new measures of the sudden change in the orbital period ($P$) across the nova eruption ($\Delta$P) and the steady period change in quiescence ($\dot{P}$) for classical novae (CNe) DQ Her and BT Mon. The fractional changes ($\Delta$P/P) in parts-per-million (ppm) are $-$4.46$\pm$0.03 for DQ Her and +39.6$\pm$0.5 for BT Mon. For BT Mon, the $\Delta$P/P value is not large enough (i.e., $>$1580 ppm) to allow for Hibernation in this system. The {\it negative} $\Delta$P/P for DQ Her is a confident counterexample of the Hibernation model for the evolution of cataclysmic variables. Further, published models of period changes by nova eruptions do not allow for such a negative value, so some additional mechanism is required, with this perhaps being due to asymmetric ejection of material. My program has also measured the first long-term $\dot{P}$ for CNe, with 0.00$\pm$0.02 for DQ Her and $-$2.3$\pm$0.1 for BT Mon, all with units of $10^{-11}$ days/cycle. These can be directly compared to the predictions of the Magnetic Braking model, where the long-term average $\dot{P}$ is a single universal function of $P$. The predicted values are -0.027 for DQ Her and -0.33 for BT Mon. For both novae, the measured $\dot{P}$ is significantly far from the predictions for Magnetic Braking. Further, the observed $\Delta$P for BT Mon imposes an additional {\it positive} period change of +0.60$\times$10$^{-11}$ days/cycle when averaged over the eruption cycle, so this system actually has a long-term {\it rise} in $P$.