The α-element abundances in the most oxygen-poor planetary nebula PNG 135.9+55.9

¹ Observatorio Astronómico Nacional, Instituto de Astronomía, UNAM, PO Box 439027, San Diego, CA 92143-9027, USA e-mail: {richer, gag}@astrosen.unam.mx
² LUTH, Observatoire de Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France e-mail: grazyna.stasinska@obspm.fr
³ University of Leicester, University Road, Leicester LE1 7RH, UK e-mail: {rfj, pdd}@star.le.ac.uk
⁴ Canada-France-Hawaii Telescope Corp., PO Box 1597, Kamuela, HI 96743, USA e-mail: veillet@cfht.hawaii.edu
⁵ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38200, La Laguna, Tenerife, Spain e-mail: cgc@ll.iac.es
⁶ Centro Astronómico Hispano Alemán, C/Jesús Durbán Remón 2-2, 04004, Almería, Spain e-mail: prada@caha.es

Corresponding author: M. Richer, richer@astrosen.unam.mx

Received: 26 June 2002
Accepted: 9 September 2002

Abstract

We present extensive new spectroscopy and imaging of PNG 135.9+55.9. We use these data as constraints to photoionization models to derive limits on the oxygen abundance. We find that PNG 135.9+55.9 has an oxygen abundance less than 1/50 of the solar value. Our models favour a value of between 5.8 and 6.5 dex, confirming that PNG 135.9+55.9 is the most oxygen-poor planetary nebula known (Tovmassian et al. [CITE]). We also derive , , and . Although the value of Ne/O is nominally high, it need not imply that the progenitor of PNG 135.9+55.9 converted any of its initial oxygen abundance to neon. The helium abundance appears to be very low, , but a precise determination will require a much more detailed study. We find that is lower than expected and perhaps variable, a finding for which we have no clear explanation.