2 Observations and data reduction

Integral-field spectra of SuWt 2 were obtained during two observing runs in 2009 May and 2012 August with the Wide Field Spectrograph (WiFeS; Dopita et al., 2007). WiFeS is an image-slicing Integral Field Unit (IFU) developed and built for the ANU 2.3-m telescope at the Siding Spring Observatory, feeding a double-beam spectrograph. WiFeS samples 0.5 arcsec along each of twenty five $38$ arcsec $\times$ $1$ arcsec slits, which provides a field-of-view of $25$ arcsec$\times$ $38$ arcsec and a spatial resolution element of $1.0$ arcsec$\times$ $0.5$ arcsec (or $1\arcsec \times 1\arcsec$ for y-binning=2). The spectrograph uses volume phase holographic gratings to provide a spectral resolution of $R=3000$ (100 kms${}^{-1}$ full width at half-maximum, FWHM), and $R=7000$ (45 kms${}^{-1}$ FWHM) for the red and blue arms, respectively. Each grating has a different wavelength coverage. It can operate two data accumulation modes: classical and nod-and-shuffle (N&S). The N&S accumulates both object and nearby sky-background data in either equal exposures or unequal exposures. The complete performance of the WiFeS has been fully described by Dopita et al. (2007); Dopita et al. (2010).

Figure: Top panel: narrow-band filter image of the PN SuWt 2 on a logarithmic scale in H$\alpha$ and [NII]6584Å taken with the European Southern Observatory (ESO) 3.6-m telescope (programme ID 055.D-0550). The rectangles correspond to the WiFeS fields of view used for our study: 1 (red) and 2 (blue); see Table 1 for more details. Bottom panels: Spatial distribution maps of flux intensity and continuum of $[$ O III$]$ $\lambda$ 5007 for field 2 and locations of apertures ($10$ arcsec$\times$ $20$ arcsec) used to integrate fluxes, namely `A' the ring and `B' the interior structure. The white contour lines show the distribution of the above narrow-band H$\alpha$ emission in arbitrary unit. North is up and east is towards the left-hand side.

Our observations were carried out with the B3000/R3000 grating combination and the RT560 dichroic using N&S mode in 2012 August; and the B7000/R7000 grating combination and the RT560 dichroic using the classical mode in 2009 May. This covers $\lambda \lambda$ 3300-5900 Å in the blue channel and $\lambda \lambda$ 5500-9300 Å in the red channel. As summarized in Table 1, we took two different WiFeS exposures from different positions of SuWt 2; see Fig. 1 (top). The sky field was collected about 1 arcmin away from the object. To reduce and calibrate the data, it is necessary to take the usual bias frames, dome flat-field frames, twilight sky flats, `wire' frames and arc calibration lamp frames. Although wire, arc, bias and dome flat-field frames were collected during the afternoon prior to observing, arc and bias frames were also taken through the night. Twilight sky flats were taken in the evening. For flux calibration, we also observed some spectrophotometric standard stars.

Table 1: Journal of SuWt 2 Observations at the ANU 2.3-m Telescope.

Field	1	2
Instrument	WiFeS	WiFeS
Wavelength Resolution	$\sim 7000$	$\sim 3000$
Wavelength Range (Å)	4415-5589,	3292-5906,
	5222-7070	5468-9329
Mode	Classical	N&S
Y-Binning	1	2
Object Exposure (s)	$900$	$1200$
Sky Exposure (s)	-	$600$
Standard Star	LTT3218	LTT9491,
		HD26169
$v_{\rm LSR}$ correction	$-5.51$	$-25.77$
Airmass	$1.16$	$1.45$
Position (see Fig.1)	13:55:46.2	13:55:45.5
	$-59$ :22:57.9	$-59$ :22:50.3
Date (UTC)	16/05/09	20/08/12

2.1 WiFeS data reduction

The WiFeS data were reduced using the WIFES pipeline (updated on 2011 November 21), which is based on the Gemini IRAF³ package (version 1.10; IRAF version 2.14.1) developed by the Gemini Observatory for the integral-field spectroscopy.

Each CCD pixel in the WiFeS camera has a slightly different sensitivity, giving pixel-to-pixel variations in the spectral direction. This effect is corrected using the dome flat-field frames taken with a quartz iodine (QI) lamp. Each slitlet is corrected for slit transmission variations using the twilight sky frame taken at the beginning of the night. The wavelength calibration was performed using Ne-Ar arc exposures taken at the beginning of the night and throughout the night. For each slitlet the corresponding arc spectrum is extracted, and then wavelength solutions for each slitlet are obtained from the extracted arc lamp spectra using low-order polynomials. The spatial calibration was accomplished by using so called `wire' frames obtained by diffuse illumination of the coronagraphic aperture with a QI lamp. This procedure locates only the centre of each slitlet, since small spatial distortions by the spectrograph are corrected by the WiFeS cameras. Each wavelength slice was also corrected for the differential atmospheric refraction by relocating each slice in $x$ and $y$ to its correct spatial position.

In the N&S mode, the sky spectra are accumulated in the unused 80 pixel spaces between the adjacent object slices. The sky subtraction is conducted by subtracting the image shifted by 80 pixels from the original image. The cosmic rays and bad pixels were removed from the raw data set prior to sky subtraction using the IRAF task LACOS_IM of the cosmic ray identification procedure of van Dokkum (2001), which is based on a Laplacian edge detection algorithm. However, a few bad pixels and cosmic rays still remained in raw data, and these were manually removed by the IRAF/STSDAS task IMEDIT.

We calibrated the science data to absolute flux units using observations of spectrophotometric standard stars observed in classical mode (no N&S), so sky regions within the object data cube were used for sky subtraction. An integrated flux standard spectrum is created by summing all spectra in a given aperture. After manually removing absorption features, an absolute calibration curve is fitted to the integrated spectrum using third-order polynomials. The flux calibration curve was then applied to the object data to convert to an absolute flux scale. The $[$ O I $] \lambda$ 5577Å night sky line was compared in the sky spectra of the red and blue arms to determine a difference in the flux levels, which was used to scale the blue spectrum of the science data. Our analysis using different spectrophotometric standard stars (LTT9491 and HD26169) revealed that the spectra at the extreme blue have an uncertainty of about 30% and are particularly unreliable for faint objects due to the CCD's poor sensitivity in this area.

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 $R_V \equiv A(V)/E(B-V)=3.1$ (Howarth, 1983; Seaton, 1979a; Seaton, 1979b).

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., 2013b; Frew et al., 2013a; 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.

Footnotes

...iraf³

The Image Reduction and Analysis Facility (IRAF) software is distributed by the National Optical Astronomy Observatory.