Sudden and steady orbital period changes across the classical nova eruptions of DQ Her and BT Mon

Abstract

ABSTRACT I report two new measures of the sudden change in the orbital period (P) across the nova eruption (ΔP) and the steady period change in quiescence ($\dot{P}$) for classical novae (CNe) DQ Her and BT Mon. The fractional changes (ΔP/P) in parts per million (ppm) are −4.46 ± 0.03 for DQ Her and +39.6 ± 0.5 for BT Mon. For BT Mon, the ΔP/P value is not large enough (i.e. >1580 ppm) to allow for hibernation in this system. The negative ΔP/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 ± 0.02 for DQ Her and −2.3 ± 0.1 for BT Mon, all with units of 10−11 d cycle−1. 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 ΔP for BT Mon imposes an additional positive period change of +0.60 × 10−11 d cycle−1 when averaged over the eruption cycle, so this system actually has a long-term rise in P.