Planet-Mediated Precision-Reconstruction of the Evolution of the Cataclysmic Variable HU Aquarius

Abstract

Cataclysmic variables (CVs) are binaries in which a compact white dwarf accretes material from a low-mass companion star. The discovery of two planets in orbit around the CV HU Aquarii opens unusual opportunities for understanding the formation and evolution of this system. In particular the orbital parameters of the planets constrains the past and enables us to reconstruct the evolution of the system through the common-envelope phase. During this dramatic event the entire hydrogen envelope of the primary star is ejected, passing the two planets on the way. The observed eccentricities and orbital separations of the planets in HU Aqr enable us to limit the common-envelope parameter $\alpha \lambda = 0.45\pm 0.17$ or $\gamma = 1.77\pm0.02$ and measure the rate at which the common envelope is ejected, which turns out to be copious. The mass in the common envelope is ejected from the binary system at a rate of ${\dot m} = 1.9\pm 0.3\,\MSun/yr$. The reconstruction of the initial conditions for HU Aqr indicates that the primary star had a mass of $M_{\rm ZAMS} = 1.6\pm0.2$\,\MSun\, and a $m_{\rm ZAMS} = 0.47\pm 0.04$\,\MSun\, companion in a $a=25$--160\,\RSun\, (best value $a=97$\,\RSun) binary. The two planets were born with an orbital separation of $a_a=541\pm44$\,\RSun\, and $a_b=750\pm72$\,\Rsun\, respectively. After the common envelope, the primary star turns into a $0.52\pm0.01$\,\MSun\, helium white dwarf, which subsequently accreted $\sim 0.30$\,\MSun\, from its Roche-lobe filling companion star, grinding it down to its current observed mass of $0.18\,\MSun$.