Indications of a Large Fraction of Spectroscopic Binaries among Nuclei of Planetary Nebulae

Previous work indicates that about 10% of planetary-nebula nuclei (PNNi) arephotometrically variable short-period binaries with periods of hours to a fewdays. These systems have most likely descended from common-envelope (CE)interactions in initially much wider binaries. Population-synthesis studiessuggest that these very close pairs could be the short-period tail of a muchlarger post-CE binary population with periods of up to a few months. We haveinitiated a radial-velocity (RV) survey of PNNi with the WIYN 3.5-m telescopeand Hydra spectrograph, which is aimed at discovering these intermediate-periodbinaries. We present initial results showing that 10 out of 11 well-observedPNNi have variable RVs, suggesting that a significant binary population may bepresent. However, further observations are required because we have as yet beenunable to fit our sparse measurements with definite orbital periods, andbecause some of the RV variability might be due to variations in the stellarwinds of some of our PNNi.Comment: 11 pages, 1 table, no figures. Accepted by the Astrophysical Journal Letter