The exotic eclipsing nucleus of the ring planetary nebula SuWt 2

Abstract

SuWt 2 is a planetary nebula (PN) consisting of an edge-on thin bright ionized ring, and perpendicular fainter bipolar lobes. The central star (12^th magnitude) is an eclipsing binary, which is however too cool to ionize the PN; IUE spectra show no evidence for a hot, evolved PN central star, that could have first created and then ionised the nebula. We followed the central binary of SuWt 2 (NSV 19992) for over a decade with photometry and spectroscopy, and from these data obtained light and radial velocity curves and built an SED, from all of which we measured the stellar and system parameters. The two components are nearly identical A1 V stars, each of mass ∼2.7 M_⊙. Both are in the short-lived “blue-hook” evolutionary phase that occurs between the main sequence and the Hertzsprung gap, with an age of about 520 Myr. One puzzle is that the stars' rotational velocities are different from each other, and considerably slower than synchronous with the orbital period. We find that it is possible that the center-of-mass velocity of the eclipsing pair is varying with time, suggesting that there is an unseen third orbiting body in the system.