The Red Rectangle: Its Shaping Mechanism and Its Source of Ultraviolet Photons

The proto planetary Red Rectangle (RR) nebula is powered by HD 44179, a spectroscopic binary (P = 318 d), in which a luminous post-asymptotic giant branch (AGB) component is the primary source of both luminosity and current mass loss. Here, we present the results of a seven-year, eight-orbit spectroscopic monitoring program of HD 44179, designed to uncover new information about the source of the Lyman/far-ultraviolet (far-UV) continuum in the system as well as the driving mechanism for the bipolar outflow producing the current nebula. Our observations of the Hα line profile around the orbital phase of superior conjunction reveal the secondary component to be the origin of the fast (max. v ~ 560 km s^-1) bipolar outflow in the RR. The outflow was previously inferred on the basis of a single, broad Hα emission line profile. The variation of total Hα flux from the central H II region with orbital phase also identifies the secondary or its surroundings as the source of the far-UV ionizing radiation in the system. The estimated mass of the secondary (~0.94 M _sun) and the speed of the outflow suggest that this component is a main-sequence star and not a white dwarf, as previously suggested. We identify the source of the Lyman/far-UV continuum in the system as the hot, inner region (T _max >= 17,000 K) of an accretion disk surrounding the secondary, fed by Roche lobe overflow from the post-AGB primary at a rate of about 2-5 × 10^-5 M _sun yr^-1. The speed of the accretion-driven, bipolar outflow was found to be strongly modulated by the intercomponent separation in the highly eccentric (e = 0.37) orbit, with maximum speeds occurring near periastron and minimum speeds found around apastron. The total luminosity of the accretion disk around the secondary is estimated to be at least 300 L _sun, about 5% of the luminosity of the entire system.