Electron temperature determination from nebular continuum emission in planetary nebulae and the importance of temperature fluctuations

Abstract

Electron temperatures derived from the observed Balmer discontinuity are presented for 14 planetary nebulae. Electron temperatures deduced from the Paschen discontinuity are also obtained for four of them. The new measurements, together with those published by others, are compared to temperatures deduced from the nebular-to-auroral forbidden line ratio of [O III], Te([O III]na). It is shown that, on average, Te(Bal) tends to be lower than the Te([O III]na) for the same object, indicating the presence of large temperature fluctuations. Except for a few planetary nebulae with t2 > 0.10, t2 = 0.03 is a representative value. Possible mechanisms that could be responsible for such large temperature fluctuations are discussed. The observations also reveal, for the first time, two extremely weak discontinuities, one of He I at λ3422 in NGC 7009 and another of He II at λ5694 in Me 2–1.