Electron Temperatures of Planetary Nebulae Determined from the He I Discontinuities

📝 Abstract

We have used the He I discontinuities at 3421A 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 3646A, designated Te(H I), and found that Te(He I) are generally lower than Te(H I). There are two possible interpretations, a) the presence of substantial He+2 zone, or b) 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 modelling 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 modelling work.

💡 Analysis

We have used the He I discontinuities at 3421A 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 3646A, designated Te(H I), and found that Te(He I) are generally lower than Te(H I). There are two possible interpretations, a) the presence of substantial He+2 zone, or b) 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 modelling 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 modelling work.

📄 Content

arXiv:0901.1411v1 [astro-ph.SR] 11 Jan 2009 Electron Temperatures of Planetary Nebulae Determined from the He i Discontinuities Y. Zhang1, H.-B. Yuan2, C.-T. Hua3, X.-W. Liu2,4, J. Nakashima1, and S. Kwok1 ABSTRACT We have used the He i discontinuities at 3421 ˚A to determine the electron temperatures, designated Te(He i), for a sample of five Galactic planetary nebu- lae (PNe). We compared Te(He i) with the electron temperatures derived from the hydrogen Balmer jump at 3646 ˚A, designated Te(H i), and found that Te(He i) are generally lower than Te(H i). There are two possible interpretations, a) the presence of substantial He2+ zone, or b) the presence of hydrogen-deficient cold clumps within diffuse nebulae. A series of photoionization models were con- structed 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 modelling shows that the presence of a small amount of hydrogen-deficient in- clusions seems to be able to reproduce the observed intensities of He i discon- tinuities. 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 modelling work. Subject headings: ISM: general — planetary nebulae: general 1. Introduction The accurate determination of chemical abundances of planetary nebulae (PNe) is of fun- damental importance to the understanding of the nucleosynthesis of low- and intermediate- mass stars and the chemical evolution of galaxies. However, one of the main problems in nebular astrophysics is that the heavy element abundances derived from collisionally excited 1Department of Physics, University of Hong Kong, Hong Kong; zhangy96@hku.hk 2Department of Astronomy, Peking University, Beijing 100871, China 3Observatoire Astronomique de Marseille-Provence Laboratoire d’Astrophysique de Marseille Pˆole de l’Etoile Site de Chˆateau-Gombert 38, rue Fr´ed´eric Joliot-Curie 13388 Marseille cedex 13, France 4Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China – 2 – lines (CEL) are often lower than those derived from optical recombination lines (ORLs). The typical ORL/CEL abundance discrepancy factor (ADF) is ∼2 for general PNe. The most extreme case to date is Hf 2-2, which has a ADF of about 70 (Liu et al. 2006). Recent reviews on this problem have been presented by Liu (2006) and Peimbert & Peimbert (2006) (see also the references therein). There are two possible solutions: (1) the presence of tem- perature and density variations in chemically homogeneous nebulae; (2) the two-component nebular model with hydrogen-deficient inclusions embedded in the diffuse nebula. In order to test the two scenarios, we need to further investigate nebular physical conditions, in par- ticular for the regions that ORLs originate in. For this purpose, new plasma diagnostic tools rather than the classical CEL diagnostics are badly required. Zhang et al. (2004) applied the hydrogen Balmer jump to determine the electron tem- peratures – hereafter Te(H i) – for a large sample of PNe. They found that Te(H i) is systematically lower than that derived from the ratio of nebular to auroral lines of [O iii] – hereafter Te(O iii), consistent with previous results by Liu & Danziger (1993). The dis- crepancies between Te(H i) and Te(O iii) were first studied by Peimbert (1967, 1971), who found that temperature variations within nebulae may lead to higher Te(O iii) compared to Te(H i). To quantitatively study the problem, they defined the mean square tempera- ture variation, t2. Photoionization models of chemically and spatially homogeneous nebulae yielded typical values of t2 between 0.003–0.015 (see Peimbert & Peimbert 2006). However, the observations by Liu & Danziger (1993) and Zhang et al. (2004) indicated to considerably large temperature variations, which are far beyond the predictions of typical photoionization models. This can be ascribed to either additional energy inputs for chemically homogeneous nebulae or chemical inhomogeneities. Zhang et al. (2005a,b) developed a method to use the He i recombination line ratios to diagnose electron temperatures – hereafter Te(He i). The comparison between Te(He i) and Te(H i) is essential to discriminate between the two-abundance model and chemically homo- geneous nebulae with temperature and density variations as responsible for the CEL/ORL abundance problem since the two scenarios predict different relations between Te(He i) and Te(H i). From a study of a sample of 48 PNe, Zhang et al. (2005a) (hereafter Paper i) found that Te(He i) is significantly lower than Te(H i), in favor of the two-abundance model. How- ever, there are two arguable problems. Peimbert & Peimbert (2006) found that for some PNe the Te(He i) values derived from the He i λλ3889, 4471, and 7069 are different with those derived from the He i λλ6678,72

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