Rings in the haloes of planetary nebulae^*

¹ Isaac Newton Group of Telescopes, Ap. de Correos 321, 38700 Sta. Cruz de la Palma, Spain
² Departamento de Astrofísica, Universidad Complutense, 28040, Madrid, Spain e-mail: pat@astrax.fis.ucm.es
³ Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands e-mail: mellema@strw.LeidenUniv.nl
⁴ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain e-mail: corrado@ll.iac.es
⁵ Cerro Tololo Inter-American Observatory, NOAO-AURA, Casilla 603, La Serena, Chile e-mail: hschwarz@ctio.noao.edu

Corresponding author: R. Corradi, rcorradi@ing.iac.es

Received: 30 September 2003
Accepted: 17 December 2003

Abstract

We present a search for rings or arcs in the haloes of planetary nebulae (PNe). We discovered such structures in eight PNe, tripling the sample of PNe with known rings. This shows that, contrary to what was believed to date, the occurrence of mass loss fluctuations with timescales of 10²–10³ yrs at the end of the asymptotic giant branch phase (AGB) is common. We estimate a lower limit of the occurrence rate of rings in PN haloes to be ~35%. Using these new detections and the cases previously known, we discuss the statistical properties of ring systems in PNe haloes. We estimate that the mass modulation producing the rings takes place during the last 10 000 or 20 000 yrs of AGB evolution. In PNe, the spacing between rings ranges from <0.01 pc to 0.06 pc, significantly larger than those seen in proto-PNe. This, together with the finding of a possible positive correlation of spacing with the post-AGB age of the nebulae, suggests that the spacing of the rings increases with time. These properties, as well as the modest surface brightness amplitudes of rings, are consistent with the predictions of the dust-driven wind instability model explored by Meijerink et al. ([CITE]), but do not immediately exclude other proposed models.