Alignment of the Angular Momentum Vectors of Planetary Nebulae in the Galactic Bulge

Abstract

We use high-resolution H α images of 130 planetary nebulae (PNe) to investigate whether there is a preferred orientation for PNe within the Galactic Bulge. The orientations of the full sample have an uniform distribution. However, at a significance level of 0.01, there is evidence for a non-uniform distribution for those planetary nebulae with evident bipolar morphology. If we assume that the bipolar PNe have an unimodal distribution of the polar axis in Galactic coordinates, the mean Galactic position angle is consistent with 90°, i.e. along the Galactic plane, and the significance level is better than 0.001 (the equivalent of a 3.7σ significance level for a Gaussian distribution). The shapes of PNe are related to angular momentum of the original star or stellar system, where the long axis of the nebula measures the angular momentum vector. In old, low-mass stars, the angular momentum is largely in binary orbital motion. Consequently, the alignment of bipolar nebulae that we have found indicates that the orbital planes of the binary systems are oriented perpendicular to the Galactic plane. We propose that strong magnetic fields aligned along the Galactic plane acted during the original star formation process to slow the contraction of the star forming cloud in the direction perpendicular to the plane. This would have produced a propensity for wider binaries with higher angular momentum with orbital axes parallel to the Galactic plane. Our findings provide the first indication of a strong, organized magnetic field along the Galactic plane that impacted on the angular momentum vectors of the resulting stellar population.