[Fe III] lines in the planetary nebula NGC 2392 Yong Zhang , Xuan Fang , Xiao-Wei Liu , Sun Kwok 1 2 2,3 1 Department of Physics, The University of Hong Kong, Hong Kong, China 1 2 Department of Astronomy, School of Physics, Peking University, Beijing, China 3 Kavli Institute for Astronomy and Astrophysics at Peking University, Beijing, ChinaPROJECT INFORMATION ABSTRACT Eskimo Nebula, NGC 2392, is a young double-shell planetary nebula (PN). Its intrinsic structure and shaping mechanism are still not fully understood. We Fig. 1 HST image of NGC 2392. present new optical spectroscopy of NGC 2392. The slit was placed at two different The [O III] is shown as blue, the locations to obtain the spectra of the inner and outer shells. Several [Fe III] lines are Ha is shown as green, and the [N clearly detected. We find that these [Fe III] lines mostly originate from the inner shell. II] is shown as red. The red lines Therefore, we suggest that NGC 2392 might have an intrinsic structure similar to Ant represent the position of the long nebula Mz 3, which exhibits a number of [Fe III] lines from the central dense regions. In slit, and the close green boxes this scenario, the inner and outer shells correspond to the central emission core and the represent the position of the outer lobes of Mz 3, respectively. echelle slit.OBSERVATIONS AND DATA REDUCTION The observations were carried out on Dec. 1-4, 2010 with the Yunnan 2.4m telescope,located at Gao Meigu Observatory, China. The telescope was equipped with the YunnanFaint Object Spectrograph and Camera (YFOSC). A 2000X4000 CCD chip were used, leadingto a pixel size of 0.28 arcsec. Long-slit and echelle spectra of two different nebular positions(inner and outer regions) were obtained. A slit width of 1 arcsce was used throughout thelong-slit observations. A 0.58”X0.5” short-slit was used for the echelle observations. Thepositions of the slits are shown in Figure 1. For each spectrum, three 1800s and one 60sexposures were taken. The spectra were reduced using the IRAF package, following theusual steps. HeAr and FeAr lamps were used for wavelength calibration. Absolute fluxcalibration was obtained by observing the standard stars G191-B2B. The 1D spectra wereobtained by integrating along the slit. Figure 2 gives the long-slit spectra, which cover a Fig. 3 Echelle spectra of the innerwavelength range from 3300Å to 10000Å at a resolution of 7Å. The echelle spectra have a (black) and outer (red) regions NGCresolution of 3Å. Figure 3 shows the order covering the wavelength position of [Fe III] lines. 2392. Note that [Fe III] lines are only Fig. 2 Long slit spectra ofTable 1 lists the detected lines and their fluxes. visible in the inner region. NGC 2392.RESULTS Fig. 4 Plasma diagnostic diagram. The diagnostics are labelled by the ions: [S II] 6731/6716. [Cl A total of about 70 atomic lines are detected. The spectra of inner regions exhibit less III] 5537/5517, [O II]lines than those of outer regions (see Figure 2 and Table 1) due to strong continuum resulted (7320+7330)/(3726+3729), [N II]from scattered stellar light. We carry out plasma diagnostic using forbidden line ratios and (6548+6584)/5754, [O III]the Balmer discontinuity (Figure 4). The resultant electron temperatures and densities are (4959+5007)/4363, and [Fe III]given in Table 2. The inner regions show slightly higher temperatures and densities. The 4658/4701.electron temperature of the outer regions obtained from the Balmer discontinuity is onlyslightly lower than that derived from the [O III] forbidden line ratio. The most intriguing result of our observations is the detection of several [Fe III] lines. Asshown in Figure 3, these [Fe III] lines are particularly strong in inner regions. In Table 3, wecompare the observed flux ratios of [Fe III] lines from the same upper levels with thetheoretical values, and find that they are in good agreement. In our previous paper (Zhang &Liu, 2002, MNRAS, 337, 499), we found that the bipolar PN Mz 3 displays strong iron lines inthe central dense regions. [Fe III] lines have high critical densities and thus can be used as aprobe of high-density nebular regions. In the inner regions of NGC 2392, the electron density Fig. 5 Schematic drawing of adeduced from [Fe III] forbidden line ratio is higher than those from other line ratios, bipolar PN, showing oursuggesting that these iron lines might arise from denser regions where other forbidden lines proposition that NGC 2392 andare collisionally deexcited. Consequently, we hypothesize that NGC 2392 and Mz 3 have a Mz 3 have the same intrinsicsimilar intrinsic structure. Figure 5 illustrates that NGC 2392 and Mz 3 are a pole- and edge- structure, but are viewed inon view of a bipolar PN, respectively. The structure of NGC 2392 has been much discussed in different perspectives.the literature. We argue that if a model reflect the intrinsic nebular structure we shouldidentify corresponding PNs in different perspectives. Our model can also provide a natural explanation for the ionization and thermal structuresof NGC 2392, whose ionization degree increases and temperature fluctuations decrease Table 1. Detected lines. Table 2. Plasma diagnostics. Table 4. Ionic abundances.radically outward. According to our model, the inner dense regions are like a torus which isoptically thick to Ly photons. The high-ionization outer disk is actually the projection ofoutflows, in whose direction the Ly optical depth is low. The spectra of the inner regionssample nebular regions with a longer length along the line of sight (Figure 5), and thus showlarger temperature fluctuations. The materials in the inner and outer regions may originate from different mass loss Table 3. Comparison ofprocesses. From the abundance calculations (Table 4), we find that the outer regions might observed and predicted intensityhave a lower iron abundance, suggesting that these materials were probably ejected at a ratios of pairs of [Fe III] comingearlier stage, and iron has been condensed into dust grains. from the same upper level. In future studies, we will further verify our hypothesis by constructing 3D spatio-kinematicmodels using the SHAPE software developed by Steffen & López (2006, RevMexAA, 42, 99).CONTACT: We wish to thank the staff of Gao Meigu Observatory for their help with the observations. We also thank Chin-Hao Hsia for plotting Figure 1.Yong Zhang Support for this work was partially provided by the Research Grants Council of the Hong Kong under grants HKU7028/07P and the Seedzhangy96@hku.hk Funding Programme for Basic Research in HKU (200909159007).