The evolutionary status of the peculiar variable star V348 Sagittarii

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Abstract
We present new long-slit and narrow-band imaging observations of the nebula surrounding the peculiar variable star V348 Sgr. These show that the overall ionization state of the nebula is low, but the extent, structure and physical conditions are all typical of old planetary nebulae. An analysis of the abundances shows a pronounced helium enrichment, indicating that the system is highly evolved. Further clues to the past evolution of the system are provided by models which demonstrate that the ionization structure of the nebula is incompatible with ionization by a central star of temperature 20000 K and luminosity 3500 L⊙ as suspected for V348 Sgr. As there are no indications of a hotter star in the system, we are forced to accept that the star must have been hotter in the very recent past (nebular recombination time-scale is 120 < τrec < 2300 yr). Given all the observational facts, the evolutionary status of V348 Sgr is interpreted as a white dwarf that has suffered a last thermal pulse, reigniting a helium-burning shell. The lower limit for τrec is probably just consistent with the post-pulse crossing time required to reach the AGB, while the upper limit suggests the star may have already reached the AGB and is currently evolving to higher temperatures again. The present data do not allow us to differentiate between these two scenarios.

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