Temperature fluctuations and chemical homogeneity in the planetary nebula NGC 4361

Abstract

We have obtained deep long-slit spectra of NGC 4361 between λλ4000 and 4975 at a spectral resolution of 1.5 A. The spectra were used to map the C II, C III and C IV recombination lines and the resulting C2+/H+, C3+/H+ and C4+/H+ ionic abundances across the nebular surface. Our results confirm the high recombination-line C abundance derived by Torres-Peimbert, Peimbert & Pena – the C2+/H+ and C3+/H+ ionic abundances derived from recombination lines are respectively a factor of ∼15 and ∼5 higher than the corresponding values derived from the UV collisionally excited C III]λ1908 and C IVλ1549 lines. The recombination-line C abundance is found to be constant across the nebula within the measurement uncertainties (≲0.2 dex), as opposed to the model proposed by Torres-Peimbert, Peimbert & Pena, which predicts a C-rich inner zone with a C/H abundance ratio two orders of magnitude higher than in the outer regions. We have also detected the N IIIλ4379 and O IVλ4632 recombination lines from NGC 4361. The N3+/H+ abundance ratio derived from the λ4379 line is at least a factor of 10 higher than the upper limit from the UV collisionally excited N IV]λ1487 line. There is clear evidence of temperature variations across the nebular surface, with an amplitude of ±1000 K around a mean value of 18000 K, probably produced by shock heating. The observed temperature fluctuations are, however, too small to have a significant effect on the ionic abundances derived from collisionally excited lines. Observations with higher spatial and spectral resolution might quantify better the amplitude of temperature fluctuations and their effects on abundance determination. We have obtained the first reliable measurement of the abundance of fluorine for a planetary nebula. From the [F IV]λ4060 line, a F/H abundance ratio of 4.5×10−8 is found. The [F/O] and C/O abundance ratios derived for NGC 4361 are roughly in agreement with the correlation found between these two quantities among red giant and AGB stars.