6 Conclusions

Our new, extensive observations of PNG 135.9+55.9 confirm its unusual nature. Spectroscopy covering the wavelength interval 3400-9700 Å reveals lines of only H I, He II, [O III], and [Ne III]. Deep H$\alpha$ imaging was used to derive the radial density distribution.

Using these data as constraints, we constructed a new set of nebular models from which we derived the abundance of oxygen and the Ne/O, S/O, and Ar/O abundance ratios. We confirm the extremely low value of the oxygen abundance, which we find to be less than 1/50 of the solar value: our models favour a value of $12 + \log {\rm O}/{\rm H}$ between 5.8 and 6.5 dex. The distance implied by these models places PNG 135.9+55.9 in the Milky Way halo, in accordance with its radial velocity (Tovmassian et al. 2001). The models also imply nebular masses in the range expected.

For the $\alpha$-element ratios, we find ${\rm Ne}/{\rm O} = 0.5 \pm 0.3$, ${\rm S}/{\rm O} < 0.094$, and ${\rm Ar}/{\rm O} < 0.23$. The Ne/O ratio may be somewhat higher than is commonly found in planetary nebulae in the Milky Way disk (e.g., Henry 1989; Kingsburgh & Barlow 1994). One possibility is that the progenitor of PNG 135.9+55.9 converted some of its O to Ne. It is also possible that the anomalous Ne/O ratio is the result of discrete chemical enrichment in the very early evolution of the galaxy (e.g., Burris et al. 2000). Regardless, of the cause, any conversion of O to Ne has been modest and does not affect our conclusion that PNG 135.9+55.9 is the progeny of an intrinsically very oxygen-poor star.

An unusual characteristic of PNG 135.9+55.9 is its low H $\alpha/{\rm H}\beta$ ratio, for which we find no clear explanation. Despite its low metallicity and the concomitant high electron temperature that should result in collisionally excited Balmer lines of H I, PNG 135.9+55.9 has an ${\rm H}\alpha/{\rm H}\beta$ ratio typically below 3. Furthermore, ${\rm H}\alpha/{\rm H}\beta$appears to be variable between observing runs and even within a single night. One possible explanation for both the low ${\rm H}\alpha/{\rm H}\beta$ ratio and its variability is if PNG 135.9+55.9 contains an accretion disk, though the evidence is not convincing. At any rate, this issue does not appear to affect our conclusions regarding the chemical abundances.

We also measure a low ${\rm He}/{\rm H}$ ratio of $\sim$0.08. This makes PNG 135.9+55.9 interesting as a probe of the pregalactic He abundance. However, the derivation of a very precise He abundance will require the resolution of a number of outstanding issues, including the ${\rm H}\alpha/{\rm H}\beta$ problem, the foreground reddening, and the internal temperature structure.

Acknowledgements

MGR thanks Anabel Arrieta, Leonid Georgiev, Felipe Montalvo, and Salvador Monrroy for their able assistance with the observations at SPM. GT, GS, MGR, and CV are grateful for the receipt of Director's discretionary time at the CFHT. We are grateful to the WHT staff for the spectrum obtained through service time. GS acknoledges useful discussions with Y. Izotov, J. M. Huré and M. Mouchet. PDD is a PPARC-supported PDRA. MGR acknowledges financial support from DGAPA project IN100799 and CONACyT project 37214-E. GS acknowledges financial support from DGAPA project IN114601. GT and GS acknowledge financial support from CONACyT project 34521-E.