Helium recombination spectra as temperature diagnostics for planetary nebulae |
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Authors: | Y Zhang † X-W Liu Y Liu R H Rubin ‡ |
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Institution: | Department of Astronomy, Peking University, Beijing 100871, P. R. China;Department of Physics, Guangzhou University, Guangzhou 510405, P. R. China;NASA Ames Research Center, Moffett Field, CA 94035-1000, USA |
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Abstract: | Electron temperatures derived from the He i recombination line ratios, designated T e(He i ), are presented for 48 planetary nebulae (PNe). We study the effect that temperature fluctuations inside nebulae have on the T e(He i ) value. We show that a comparison between T e(He i ) and the electron temperature derived from the Balmer jump of the H i recombination spectrum, designated T e(H i ), provides an opportunity to discriminate between the paradigms of a chemically homogeneous plasma with temperature and density variations, and a two-abundance nebular model with hydrogen-deficient material embedded in diffuse gas of a 'normal' chemical composition (i.e. ~solar), as the possible causes of the dichotomy between the abundances that are deduced from collisionally excited lines and those deduced from recombination lines. We find that T e(He i ) values are significantly lower than T e(H i ) values, with an average difference of 〈 T e(H i ) ? T e(He i )〉= 4000 K . The result is consistent with the expectation of the two-abundance nebular model but is opposite to the prediction of the scenarios of temperature fluctuations and/or density inhomogeneities. From the observed difference between T e(He i ) and T e(H i ), we estimate that the filling factor of hydrogen-deficient components has a typical value of 10?4. In spite of its small mass, the existence of hydrogen-deficient inclusions may potentially have a profound effect in enhancing the intensities of He i recombination lines and thereby lead to apparently overestimated helium abundances for PNe. |
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Keywords: | ISM: general planetary nebulae: general |
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