2.1 Physical and chemical conditions

Electron temperature $T_{\rm e}$ and electron density $N_{\rm e}$ for the different regions of M2-42 are presented in Table 2. The electron temperatures and densities were obtained using the EQUIB code (Howarth & Adams, 1981) from the [NII] nebular to auroral line ratio and the [SII] doublet line ratio, respectively. The electron temperature ${\it T}_{\rm e}$([NII]) $_{\rm corr}$ was corrected for recombination contribution to the auroral line using the formula given by Liu et al. (2000) and the ionic abundance ${\rm N^{++}}/{\rm H^{+}}$ derived from the NII lines. The values of $N_{\rm e}($[SII]$)=3150$cm$^{-3}$ and $T_{\rm e}($[NII] $)_{\rm corr}=9600$K are in agreement with $N_{\rm e}($[SII]$)= 3240$cm$^{-3}$ and $T_{\rm e}($[NII]$)=9350$K derived by Wang & Liu (2007). Additionally, we determined the physical conditions of the NE and SW jets. The jets show a mean electron temperature of $8840 \pm 180$K, which is 760K lower than that of the main shell, whereas their mean electron density of $595 \pm 125$cm$^{-3}$ is by a factor of five lower than that of the main shell.

Table 2 also lists the ionic abundances X${}^{i+}$/H${}^{+}$ derived from collisionally excited lines (CELs) and optical recombination lines (ORLs). We used the EQUIB code to calculate the ionic abundances. We adopted the physical conditions, $T_{\rm e}$ ( ${\it T}_{\rm e}$ $_{\rm corr}$ for the main shell) and $N_{\rm e}$, derived from CELs. The atomic data sets used for plasma diagnostics and abundances analysis are the same as those used by Danehkar (2014, Chapter 3).

Our value of He$^{+}$/H $^{+}=0.105$ for the main shell is in good agreement with He$^{+}$/H$^{+}$ = 0.107 derived by Wang & Liu (2007). However, they derived O$^{++}$/H$^{+}$ =  $5.27 \times 10^{-4}$, which is twice our value. This could be due to the different atomic data used by them. Our values of N$^{+}$/H$^{+}$, S$^{+}$/H$^{+}$ and Ar$^{3+}$/H$^{+}$ are in reasonable agreement with N$^{+}$/H$^{+}$ =  $1.03 \times 10^{-4}$, S$^{+}$/H$^{+}$ =  $4.96\times 10^{-7}$ and Ar$^{3+}$/H$^{+}$ =  $1.59 \times 10^{-7}$ obtained by Wang & Liu (2007). Note that a slit with a width of $2\hbox{$^{\prime\prime}$}$ used by Wang & Liu (2007) is not completely related to the main shell. We see that the abundance discrepancy factor for O$^{++}$, ${\rm ADF}({\rm O}^{++}) \equiv ({\rm O}^{++}/{\rm H}^{+})_{\rm ORL} / ({\rm O}^{++}/{\rm H}^{+})_{\rm CEL}= 3.14$, is in agreement with ${\rm ADF}({\rm O}^{++})=2.09$ (Wang & Liu, 2007). Moreover, our abundance ratio of (N$^{++}$/O $^{++})_{\rm ORL}=0.388$ derived from ORLs is in excellent agreement with (N$^{++}$/O $^{++})_{\rm ORL}=0.399$ obtained by Wang & Liu (2007). Although He$^{+}$/H$^{+}$ and O$^{++}$/H$^{+}$ derived from the jets are similar to those of the main shell, N$^{+}$/H$^{+}$ and S$^{+}$/H$^{+}$ derived from the jets are about three times higher than those of the main shell. These ionization features of the bipolar collimated jets are typical of fast, low-ionization emission regions (FLIERs; Balick et al., 1993; Balick et al., 1994; Balick et al., 1998).

Table 2: Electron temperature ${\it T}_{\rm e}$, electron density ${\it N}_{\rm e}$ and ionic abundances derived from the dereddened fluxes listed in Table 1.

Parameter	Main Shell	NE Jet	SW Jet
${\it T}_{\rm e}$([NII])(K)	10270	9020	8660
${\it T}_{\rm e}$([NII]) $_{\rm corr}$(K)	9600	-	-
${\it N}_{\rm e}$([SII])(cm$^{-3}$)	3150	470	720
(He$^{+}$/H$^{+}$) $_{\rm ORL}$	0.105	0.107	0.110
(N$^{+}$/H$^{+}$) $_{\rm CEL} \times 10^{5}$	0.764	2.912	2.236
(O$^{++}$/H$^{+}$) $_{\rm CEL} \times 10^{4}$	2.606	2.469	3.208
(S$^{+}$/H$^{+}$) $_{\rm CEL} \times 10^{6}$	0.347	1.150	1.040
(S$^{++}$/H$^{+}$) $_{\rm CEL} \times 10^{6}$	3.116	-	-
(Ar$^{3+}$/H$^{+}$) $_{\rm CEL} \times 10^{7}$	1.871	-	-
(N$^{++}$/H$^{+}$) $_{\rm ORL} \times 10^{4}$	3.175	-	-
(O$^{++}$/H$^{+}$) $_{\rm ORL} \times 10^{4}$	8.185	-	-