A search for photometric variability in planetary nebulae and related objects with the ROTSE-IIId archive

Abstract

Binarity shapes a substantial fraction of planetary nebulae (PNe), and time resolved photometry remains the most productive route to identifying binary central stars (CSPNe). We have searched the ROTSE–IIId archive for variability among 93 light curves (90 objects, owing to duplicate catalogue entries) of PNe and related objects listed in the HASH planetary nebula database observed between 2004 and 2012, of which 51 satisfied our quality criteria and were searched with a generalised Lomb–Scargle periodogram. Only half the objects are classified as PNe, and variability is stronger among the mimics than among the nebulae themselves. Injection and recovery simulations place the detection boundary at a few hundredths of a magnitude only for the best sampled light curves. Five of the ten graded variables are PNe, four of them catalogued central stars. A period of 0.9647 d recurs in four unrelated targets, from the beat between the nightly sampling and the sidereal lunar month, and rejecting such frequencies leaves blind zones in the search. Of seven targets with published periods, one is recovered outright, one lies below the blind search threshold, two were excluded by the quality cuts and three fail for reasons of baseline or amplitude. The recovered object, the 21.3 d variable in K 1-6 (P = 21.3081 ± 0.0039 d, Tmax  = HJD 2454125.560), was listed as a true PN but has been shown to be photoionised interstellar material around a hierarchical triple, and so illustrates contamination rather than binarity. Archives of metre class robotic telescopes thus serve this field less as discovery instruments than as suppliers of epochs.