2.3 Kinematic results

We derived an expansion velocity of $ V_{\rm HWHM}=20.2\pm1.3$ kms$ ^{-1}$ from the half width at half maximum (HWHM) for the [NII] $ \lambda\lambda$6548,6584 and [SII] $ \lambda\lambda$6716,6731 emission-line profiles integrated over the main shell ( $ 6\hbox {$^{\prime \prime }$}\times 7\hbox {$^{\prime \prime }$}$). The local standard of rest (LSR) systemic velocity of the whole nebula was estimated to be at $ 122.9\pm 12$kms$ ^{-1}$, which is in agreement with $ V_{\rm LSR}=133.1 \pm 13.3$kms$ ^{-1}$ measured by Durand et al. (1998). The LSR velocity is defined as the line of sight radial velocity, transferred to the local standard of rest by correcting for the motions of the Earth and Sun.

Figure 2 shows spatially resolved flux and velocity maps of M2-42 extracted from the [NII] $ \lambda $6584 emission line across the WiFeS field. The observed radial velocity map was transferred to the LSR radial velocity. The white/black contour lines in the figures depict the 2-D distribution of the H$ \alpha $ emission obtained from the SHS, which can aid us in distinguishing the nebular border. As seen in Fig.2, the kinematic map depicts an elliptical structure with a pair of collimated bipolar outflows, which is easily noticeable in the channel maps (see Fig. 3) and discussed below.

Figure 3 presents the flux intensity maps of the [NII] $ \lambda $6584 emission lines on a logarithmic scale observed in a sequence of 6 velocity channels with a resolution of $ \sim 20$ kms$ ^{-1}$, which can be used to identify different morphological components of the nebula. We subtracted the systemic velocity $ v_{\rm sys}=123$ kms$ ^{-1}$ from the central velocity value given at the top of each channel. The stellar continuum map was also subtracted from the flux intensity maps. While there is a dense torus in the center, a pair of collimated bipolar outflows can be also identified in the velocity channels. The torus has a radius of $ 3\hbox{$^{\prime\prime}$}\pm 1 \hbox{$^{\prime\prime}$}$. This torus is clearly evident in the VVV J, H, and K$ _{\rm s}$ color combined image of M 2-42 presented in Figure. 1 (bottom panel). We notice that the bipolar outflows are highly asymmetric, the SW jet apparently having a bow shock structure. While the NE jet reaches a distance of $ 12\hbox{$^{\prime\prime}$}\pm 2\hbox{$^{\prime\prime}$}$ from the nebular center, the distance of the SW jet from the central star is about 25 percent shorter than the NE jet. Interaction with the interstellar medium (ISM) can lead to the formation of asymmetric bipolar outflows (see e.g. Wareing et al., 2007). Both the jet components have similar brightness in the velocity channels. Brightness discontinuities are seen in the channels, where the bipolar outflows emerge from the main shell.

Figure: Velocity slices of M2-42 along the [NII] $ \lambda $6584 emission-line profiles. The slices have a $ \sim 20$ kms$ ^{-1}$ width, the central velocity is given at the top of each slice, and the LSR systemic velocity is $ v_{\rm sys}=123$ kms$ ^{-1}$. The color bars show flux measurements in logarithm of $ 10^{-15}$ ergs$ {}^{-1}$cm$ {}^{-2}$spaxel$ {}^{-1}$. Velocity channels are in kms$ {}^{-1}$. The contours in the channel maps are the narrow-band H$ \alpha $ emission in arbitrary unit obtained from the SHS. North is up and east is toward the left-hand side.

Ashkbiz Danehkar