2.2 Nebular spectrum and reddening

Table 2 represents a full list of observed lines and their measured fluxes from different apertures ($10$arcsec$\times$$20$arcsec) taken from field 2: (A) the ring and (B) the inside of the shell. Fig. 1 (bottom panel) shows the location and area of each aperture in the nebula. The top and bottom panels of Fig. 2 show the extracted blue and red spectra after integration over the aperture located on the ring with the strongest lines truncated so the weaker features can be seen. The emission line identification, laboratory wavelength, multiplet number, the transition with the lower- and upper-spectral terms, are given in columns 1-4 of Table 2, respectively. The observed fluxes of the interior and ring, and the fluxes after correction for interstellar extinction are given in columns 5-8. Columns 9 and 10 present the integrated and dereddened fluxes after integration over two apertures (A and B). All fluxes are given relative to H$\beta$, on a scale where ${\rm H}\beta = 100$.

Table 2: Observed and dereddened relative line fluxes, on a scale where ${\rm H}\beta = 100$. The integrated observed H($\beta$) flux was dereddened using $c({\rm H}\beta )$ to give an integrated dereddened flux. Uncertain (errors of 20%) and very uncertain (errors of 30%) values are followed by “:” and “::”, respectively. The symbol `*' denotes doublet emission lines.

Region				Interior		Ring		Total
Line	$\lambda_{\rm lab}$(Å)	Mult	Transition	$F(\lambda)$	$I(\lambda)$	$F(\lambda)$	$I(\lambda)$	$F(\lambda)$	$I(\lambda)$
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
3726 $[$O II$]$	3726.03	F1	$2{\rm p}^{3}\,{}^{4}{\rm S}_{3/2}-2{\rm p}^{3}\,{}^{2}{\rm D}_{3/2}$	$183 \pm 54$	$307 \pm 91$	$576 \pm 172$	$815 \pm 244$	$479 \pm 143$	$702\pm 209$
3729 $[$O II$]$	3728.82	F1	$2{\rm p}^{3}\,{}^{4}{\rm S}_{3/2}-2{\rm p}^{3}\,{}^{2}{\rm D}_{5/2}$	*	*	*	*	*	*
3869 $[$Ne III$]$	3868.75	F1	$2{\rm p}^{4}\,{}^{3}{\rm P}_{2}-2{\rm p}^{4}\,{}^{1}{\rm D}_{2}$	128.93::	199.42::	144.31::	195.22::	145.82::	204.57::
3967 $[$Ne III$]$	3967.46	F1	$2{\rm p}^{4}\,{}^{3}{\rm P}_{1}-2{\rm p}^{4}\,{}^{1}{\rm D}_{2}$	-	-	15.37::	20.26::	-	-
4102 H$\delta$	4101.74	H6	$2{\rm p}\,{}^{2}{\rm P}-6{\rm d}\,{}^{2}{\rm D}$	-	-	16.19:	20.55:	16.97:	22.15:
4340 H$\gamma$	4340.47	H5	$2{\rm p}\,{}^{2}{\rm P}-5{\rm d}\,{}^{2}{\rm D}$	24.47::	31.10::	30.52:	36.04:	31.69:	38.18:
4363 $[$O III$]$	4363.21	F2	$2{\rm p}^{2}\,{}^{1}{\rm D}_{2}-2{\rm p}^{2}\,{}^{1}{\rm S}_{0}$	37.02::	46.58::	5.60	6.57	5.15	6.15
4686 He II	4685.68	3-4	$3{\rm d}\,{}^{2}{\rm D}-4{\rm f}\,{}^{2}{\rm F}$	80.97	87.87	29.98	31.72	41.07	43.76
4861 H$\beta$	4861.33	H4	$2{\rm p}\,{}^{2}{\rm P}-4{\rm d}\,{}^{2}{\rm D}$	100.00	100.00	100.00	100.00	100.00	100.00
4959 $[$O III$]$	4958.91	F1	$2{\rm p}^{2}\,{}^{3}{\rm P}_{1}-2{\rm p}^{2}\,{}^{1}{\rm D}_{2}$	390.90	373.57	173.63	168.27	224.48	216.72
5007 $[$O III$]$	5006.84	F1	$2{\rm p}^{2}\,{}^{3}{\rm P}_{2}-2{\rm p}^{2}\,{}^{1}{\rm D}_{2}$	1347.80	1259.76	587.22	560.37	763.00	724.02
5412 He II	5411.52	4-7	$4{\rm f}\,{}^{2}{\rm F}-7{\rm g}\,{}^{2}{\rm G}$	19.33	15.01	5.12	4.30	6.90	5.68
5755 $[$N II$]$	5754.60	F3	$2{\rm p}^{2}\,{}^{1}{\rm D}_{2}-2{\rm p}^{2}\,{}^{1}{\rm S}_{0}$	7.08:	4.90:	13.69	10.61	10.17	7.64
5876 He I	5875.66	V11	$2{\rm p}\,{}^{3}{\rm P}-3{\rm d}\,{}^{3}{\rm D}$	-	-	11.51	8.69	8.96	6.54
6548 $[$N II$]$	6548.10	F1	$2{\rm p}^{2}\,{}^{3}{\rm P}_{1}-2{\rm p}^{2}\,{}^{1}{\rm D}_{2}$	115.24	63.13	629.36	414.79	513.64	321.94
6563 H$\alpha$	6562.77	H3	$2{\rm p}\,{}^{2}{\rm P}-3{\rm d}\,{}^{2}{\rm D}$	524.16	286.00	435.14	286.00	457.70	286.00
6584 $[$N II$]$	6583.50	F1	$2{\rm p}^{2}\,{}^{3}{\rm P}_{2}-2{\rm p}^{2}\,{}^{1}{\rm D}_{2}$	458.99	249.05	1980.47	1296.67	1642.12	1021.68
6678 He I	6678.16	V46	$2{\rm p}\,{}^{1}{\rm P}_{1}-3{\rm d}\,{}^{1}{\rm D}_{2}$	-	-	3.30	2.12	2.68	1.63
6716 $[$S II$]$	6716.44	F2	$3{\rm p}^{3}\,{}^{4}{\rm S}_{3/2}-3{\rm p}^{3}\,{}^{2}{\rm D}_{5/2}$	60.63	31.77	131.84	84.25	116.21	70.36
6731 $[$S II$]$	6730.82	F2	$3{\rm p}^{3}\,{}^{4}{\rm S}_{3/2}-3{\rm p}^{3}\,{}^{2}{\rm D}_{3/2}$	30.08	15.70	90.39	57.61	76.98	46.47
7005 [Ar V]	7005.40	F1	$3{\rm p}^{2}\,{}^{3}{\rm P}-3{\rm p}^{2}\,{}^{1}{\rm D}$	5.46:	2.66:	-	-	-	-
7136 $[$Ar III$]$	7135.80	F1	$3{\rm p}^{4}\,{}^{3}{\rm P}_{2}-3{\rm p}^{4}\,{}^{1}{\rm D}_{2}$	31.81	15.03	26.22	15.59	27.75	15.51
7320 $[$O II$]$	7319.40	F2	$2{\rm p}^{3}\,{}^{2}{\rm D}_{5/2}-2{\rm p}^{3}\,{}^{2}{\rm P}$	18.84	8.54	9.00	5.20	10.96	5.93
7330 $[$O II$]$	7329.90	F2	$2{\rm p}^{3}\,{}^{2}{\rm D}_{3/2}-2{\rm p}^{3}\,{}^{2}{\rm P}$	12.24	5.53	4.50	2.60	6.25	3.37
7751 $[$Ar III$]$	7751.43	F1	$3{\rm p}^{4}\,{}^{3}{\rm P}_{1}-3{\rm p}^{4}\,{}^{1}{\rm D}_{2}$	46.88	19.38	10.97	5.95	19.05	9.60
9069 $[$S III$]$	9068.60	F1	$3{\rm p}^{2}\,{}^{3}{\rm P}_{1}-3{\rm p}^{2}\,{}^{1}{\rm D}_{2}$	12.32	4.07	13.27	6.16	13.34	5.65
$c({\rm H}\beta )$				-	0.822	-	0.569	-	0.638

For each spatially resolved emission line profile, we extracted flux intensity, central wavelength (or centroid velocity), and FWHM (or velocity dispersion). Each emission line profile for each spaxel is fitted to a single Gaussian curve using the MPFIT routine (Markwardt, 2009), an IDL version of the MINPACK-1 FORTRAN code (Moré, 1977), which applies the Levenberg-Marquardt technique to the non-linear least-squares problem. Flux intensity maps of key emission lines of field 2 are shown in Fig.3 for $[$O III$]$ $\lambda$5007, H$\alpha$ $\lambda$6563, $[$N II$]$ $\lambda$6584 and $[$S II$]$ $\lambda$6716; the same ring morphology is visible in the $[$N II$]$ map as seen in Fig.1. White contour lines in the figures depict the distribution of the narrow-band emission of H$\alpha$ and $[$N II$]$ taken with the ESO 3.6 m telescope, which can be used to distinguish the borders between the ring structure and the inside region. We excluded the stellar continuum offset from the final flux maps using MPFIT, so spaxels show only the flux intensities of the nebulae.

Figure: The observed optical spectrum from an aperture $10$arcsec$\times$$20$arcsec taken from field 2 located on the east ring of the PN SuWt 2 and normalized such that $F({\rm H}\beta )=100$.

The H$\alpha$ and H$\beta$ Balmer emission-line fluxes were used to derive the logarithmic extinction at H$\beta$, $c({\rm H}\beta)=\log[I({\rm H}\beta)/F({\rm H}\beta)]$, for the theoretical line ratio of the case B recombination ( $T_{\rm e}=10\,000$K and $N_{\rm e}=100$ cm$^{-3}$; Hummer & Storey, 1987). Each flux at the central wavelength was corrected for reddening using the logarithmic extinction $c({\rm H}\beta )$ according to

$$I(\lambda)=F(\lambda)\,10^{c({\rm H}\beta)[1+f(\lambda)]},$$ (1)

where $F(\lambda)$ and $I(\lambda)$ are the observed and intrinsic line flux, respectively, and $f(\lambda)$ is the standard Galactic extinction law for a total-to-selective extinction ratio of $R_V \equiv A(V)/E(B-V)=3.1$ (Seaton, 1979b; Seaton, 1979a; Howarth, 1983).

Figure: Undereddened flux maps for field 2 (see Fig. 1) of the PN SuWt 2: $[$O III$]$ $\lambda$5007, H$\alpha$ $\lambda$6563, $[$N II$]$ $\lambda$6584 and $[$S II$]$ $\lambda$6716. The flux is derived from single Gaussian profile fits to the emission line at each spaxel. The white contour lines show the distribution of the narrow-band emission of H$\alpha$ and [NII] in arbitrary unit taken with the ESO 3.6-m telescope. North is up and east is towards the left-hand side. Flux unit is in $10^{-15}$ ergs${}^{-1}$cm${}^{-2}$spaxel${}^{-1}$.

Accordingly, we obtained an extinction of $c({\rm H}\beta)=0.64$ [ $E(B-V) = 0.44$] for the total fluxes (column 9 in Table 2). Our derived nebular extinction is in good agreement with the value found by Exter et al. (2010), $E(B-V) = 0.40$ for the central star, though they obtained $E(B-V) = 0.56$ for the nebula. It may point to the fact that all reddening is not due to the interstellar medium (ISM), and there is some dust contribution in the nebula. Adopting a total observed flux value of log$F$(H$\alpha$)=$-11.69$ ergcm${}^{-2}$s${}^{-1}$ for the ring and interior structure (Frew et al., 2013a; Frew et al., 2013b; Frew, 2008) and using $c({\rm H}\beta)=0.64$, lead to the dereddened H$\alpha$ flux of log$I$(H$\alpha$)=$-11.25$ ergcm${}^{-2}$s${}^{-1}$.

According to the strength of He II $\lambda$4686 relative to H$\beta$, the PN SuWt 2 is classified as the intermediate excitation class with ${\rm EC}=6.6$ (Dopita & Meatheringham, 1990) or ${\rm EC}=7.8$ (Reid & Parker, 2010). The EC is an indicator of the central star effective temperature (Reid & Parker, 2010; Dopita & Meatheringham, 1991). Using the $T_{\rm eff}$-EC relation of Magellanic Cloud PNe found by Dopita & Meatheringham (1991), we estimate $T_{\rm eff}=143$kK for ${\rm EC}=6.6$. However, we get $T_{\rm eff}=177$kK for ${\rm EC}=7.8$ according to the transformation given by Reid & Parker (2010) for Large Magellanic Cloud PNe.