THE BANANA PROJECT. III. SPIN-ORBIT ALIGNMENT IN THE LONG-PERIOD ECLIPSING BINARY NY CEPHEI

Abstract

Binaries are not always neatly aligned. Previous observations of the DI Her\nsystem showed that the spin axes of both stars are highly inclined with respect\nto one another and the orbital axis. Here we report on a measurement of the\nspin-axis orientation of the primary star of the NY Cep system, which is\nsimilar to DI Her in many respects: it features two young early-type stars (~6\nMyr, B0.5V+B2V), in an eccentric and relatively long-period orbit (e=0.48,\nP=15.d3). The sky projections of the rotation vector and the spin vector are\nwell-aligned (beta_p = 2 +- 4 degrees), in strong contrast to DI Her. Although\nno convincing explanation has yet been given for the misalignment of DI Her,\nour results show that the phenomenon is not universal, and that a successful\ntheory will need to account for the different outcome in the case of NY Cep.\n