Electron temperatures of planetary nebulae determined from the He I discontinuities.
ZHANG Y., YUAN H.-B., HUA C.-T., LIU X.-W., NAKASHIMA J. and KWOK S.
Abstract (from CDS):
We have used the He I discontinuities at 3421 Å to determine the electron temperatures, designated Te(He I), for a sample of five Galactic planetary nebulae (PNe). We compared Te(He I) with the electron temperatures derived from the hydrogen Balmer jump at 3646 Å, designated Te(H I), and found that Te(He I) are generally lower than Te(H I). There are two possible interpretations, (1) the presence of substantial He2+ zone, or (2) the presence of hydrogen-deficient cold clumps within diffuse nebulae. A series of photoionization models were constructed to test the two scenarios. We found that the observed Te(He I)/Te(H I) discrepancies are beyond the predictions of chemically homogeneous models. Our modeling shows that the presence of a small amount of hydrogen-deficient inclusions seems to be able to reproduce the observed intensities of He I discontinuities. We stress the value of He I discontinuities in investigating nebular physical conditions. Albeit with some observational and technical limitations, He I discontinuities should be considered in future modeling work.
ISM: general - planetary nebulae: general
NGC 2240 is a misprint for NGC 2440.