The Timescale Correlation Method: Distances to Planetary Nebulae with Halos

We present deep narrow-band CCD images of six PNe obtained with the Palomar 5 m telescope in the light of [N II] λ6584 + Hα and [O III] λ5007. Several previously undetected structures are found, including faint multiple envelope structures surrounding three of the targets. The inclusion of these sources with data for other multiple envelope PNe published in the literature permit a statistical evaluation of the relationship between PN "shells" and the thermal pulses of the PN nucleus. In this paper, we investigate the dynamical timescale between successive envelopes, τ_dyn, and the nuclear interpulse timescale, τ_ip. Our results show that the deviations from the relation τ_dyn = τ_ip can be explained by the uncertainty in the Shklovsky distance to individual PNe, which is σ_dsh/d_sh ~2. By imposing the constraint that τ_dyn = τ_ip, we find that it is possible to derive a PN distance indicator, which we name the timescale correlation distance, d_TC. The derivation for d_TC is independent of ad hoc and often unsupported assumptions inherent to the Shklovsky method.