4 Summary and discussions

In this paper, we present the spatially-resolved observations of M2-42 obtained with the WiFeS on the ANU 2.3 m telescope. Using the velocity-resolved channel maps derived from the [NII] $ \lambda $6584 emission line, a morpho-kinematic model has been developed which includes different morphological components of the nebula: a dense torus and a pair of asymmetric bipolar outflows in opposite directions. From the HWHM method, the torus is found to expand slowly at $ 20$kms$ ^{-1}$, almost in agreement with $ 15$kms$ ^{-1}$ derived by Akras & López (2012). From the reconstruction model, the trail of bipolar outflows was found to go along the direction of (GPA, $ i$) $ =$ ( $ 112^{\circ}$, $ -82^{\circ}$), which is very similar to the inclination of $ i=77^{\circ}$ derived by Akras & López (2012) based on the SPM long-slit data. We find a “jet” expansion velocity of $ 120 \pm 40$kms$ ^{-1}$ with respect to the nebular center, which is higher than the value of $ 70$kms$ ^{-1}$ estimated by Akras & López (2012). Moreover, we found that the SW jet, which moves toward us, has possibly a bow shock structure relating to the interaction with ISM (see e.g. Wareing et al., 2007).

An empirical analysis of the nebular spectra separately integrated over the three different regions shows that the mean density of the jets is a factor of five lower than that in the main shell. Although the abundances of singly-ionized helium and doubly-ionized oxygen are almost the same in both the shell and the jets, the singly-ionized nitrogen and sulfur abundances derived from the jets are about three times higher than those obtained from the main shell. The similar ionization characteristics have been found in collimated jets emerged from other PNe (see e.g. Balick et al., 1994; Balick et al., 1993).

Nearly 10% of Galactic PNe have been found to have the small-scale low-ionization structures in opposite directions on both sides of their central stars. Around half of them are fast, highly collimated outflows with velocities of 30-200 kms$ ^{-1}$ relative to the main bodies, so called FLIERs (Balick et al., 1993; Balick et al., 1994; Balick et al., 1998). Previously, Balick et al. (1994) claimed the presence of nitrogen enrichment by factors of 2-5 in the FLIERs of some PNe. However, Gonçalves et al. (2003) suggested that empirically derived nitrogen overabundance seen in FLIERs are a result of inaccurate ionization correction factors applied in the empirical analysis. Gonçalves et al. (2006) constructed a chemically homogeneous photoionization model of NGC7009, which can reproduce the ionization characteristics of its shell and FLIERs. Similarly, the enhancement of N$ ^{+}$/H$ ^{+}$ and S$ ^{+}$/H$ ^{+}$ in the FLIERs of M2-42 could be attributed to the geometry and density distribution rather than chemical inhomogeneities.

The previous observations of M2-42 showed that its central star is of wels type (DePew et al., 2011). Moreover, we found that its stellar spectrum might be similar to the WN8 subclass of van der Hucht (2001) based on $ I($NIII $ )\gtrsim I($HeII). The terminal wind velocity was also estimated to be about 640 kms$ ^{-1}$. However, our observations did not cover the NII and NIV lines, which are necessary for the WN classification. This typical stellar characteristics and its point-symmetric morphology could be a result of a common-envelope evolutionary phase (see e.g. Nordhaus & Blackman, 2006). Currently, there is no evidence for binarity in M2-42. We believe that further observations of its central star will help develop a better stellar classification and also shed light on the mechanism producing its FLIERs.

Ashkbiz Danehkar