Formation and Evolution of Binary Planetary Nebula Nuclei and Related Objects

Abstract

A study is made of the evolution of close binaries in which the primary first fills its Roche lobe after the exhaustion of helium at its center and before the onset of thermal pulses. Initial masses are in the range 3-6 M<SUB>0</SUB>. Also examined is the evolution of a 1 M<SUB>sun</SUB> model which fills its Roche lobe on the first giant branch when the mass of its helium degenerate core reaches 0.4 M<SUB>0</SUB>. In all cases, a common envelope scenario is assumed, and mass is removed from the model primary on a timescale shorter than the initial thermal timescale of the envelope of the primary until the model contracts within a final Roche lobe of predetermined size. After the removal of the common envelope, systems are very close binaries in which the evolved remnant has either a carbon-oxygen (CO) core and a helium-burning shell (progenitor of mass 2.3-8 M<SUB>sun</SUB>) or a helium core and a hydrogen-burning shell (progenitor of mass 1-2.3 M<SUB>sun</SUB>). The more massive remnants fill their Roche lobes for an extended period, transferring first hydrogen-rich material at a rate ∼10<SUP>-8±1</SUP> M<SUB>sun</SUB> yr<SUP>-1</SUP> for up to 10<SUP>6</SUP> yr, and then helium at a rate ∼10<SUP>-6±1</SUP> M<SUB>sun</SUB> yr<SUP>-1</SUP> for (1-4) × 10<SUP>5</SUP> yr. As much as ∼0.4 M<SUB>sun</SUB> of nearly pure helium can be transferred to an accretor. <P />A possible real analog of the hydrogen-transferring models is U Sco, although, in many instances, a hydrogen-transferring episode may be bypassed or considerably shortened if the hot remnant blows a strong enough wind. Possible observational analogs of helium transferring models are bright ultrasoft X-ray sources like CAL 83 and CAL 87 in the Large Magellanic Cloud and some classes of supernova precursors. Our models help to explain the number and properties of hot helium OB subdwarfs. <P />A method for estimating initial orbital periods of binary planetary nebula nuclei is introduced and used to infer the initial characteristics of binary systems which have produced close binary central stars, precataclysmic variables, and related systems. Using existing estimates of their current masses, we find that the initial primaries of V477 Lyr, UU Sge, KV Vel, and MT Ser (all central stars of planetary nebulae) filled their Roche lobes during the early asymptotic giant branch (EAGB) stage. As EAGB stars, these primaries had a small, degenerate CO nucleus, a massive helium shell, and an extended hydrogen-rich envelope. Most of the hydrogen-rich envelope was ejected in a common envelope event, and the planetary nebula now around the system is the ejected common envelope. The ionizing remnants of the primaries have a small, but growing, CO nucleus and a massive (≲0.2 M<SUB>sun</SUB>) helium shell. The progenitor of the primary component of V651 Mon, another central star of a common envelope planetary nebula, probably filled its Roche lobe during the first red giant phase, after the mass of its degenerate helium core reached ∼0.4 M<SUB>sun</SUB>. Wind mass loss may be responsible for the fact that the hot central star lies well within its Roche lobe. <P />An examination and interpretation of the properties of several precataclysmic systems and binaries with components having peculiar surface compositions provide further insight into the consequences of mass loss and mass transfer in close binaries. Included in the study are systems containing helium supergiants, helium subdwarf O stars, S stars, Ba stars, and main-sequence carbon stars.