SIMBAD references

We have used the He I discontinuities at 3421 Å to determine the electron temperatures, designated T_e(He I), for a sample of five Galactic planetary nebulae (PNe). We compared T_e(He I) with the electron temperatures derived from the hydrogen Balmer jump at 3646 Å, designated T_e(H I), and found that T_e(He I) are generally lower than T_e(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 T_e(He I)/T_e(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.