On the Nature of Jets from a Main Sequence Companion at the Onset of Common Envelope Evolution

Abstract

Abstract I consider a flow structure by which main sequence companions that enter a common envelope evolution (CEE) with giant stars might launch jets even when the accreted gas has a sub-Keplerian specific angular momentum. I first show that after a main sequence star enters the envelope of a giant star the specific angular momentum of the accreted gas is sub-Keplerian but still sufficiently large for the accreted gas to avoid two conical-like openings along the two opposite polar directions. I suggest that the high-pressure zone that the accreted gas builds around the main sequence equatorial plane accelerates outflows along these polar openings. Most of the inflowing gas is deflected to the polar outflows, i.e., two oppositely directed jets. The actual mass that the main sequence star accretes is only a small fraction, ≈0.1, of the inflowing gas. However, the gravitational energy that this gas releases powers the inflow-outflow streaming of gas and adds energy to the common envelope ejection. This flow structure might take place during a grazing envelope evolution if it occurs, during the early CEE and possibly in some post-CEE cases. This study increases the parameter space for main sequence stars to launch jets. Such jets might shape some morphological features in planetary nebulae, add energy to mass removal in CEE and power some intermediate luminosity optical transients.