We aim to measure the mass associated with each molecular structures described in the previous section. We defined spatial boundaries enclosing these structures and independently studied the spectral data corresponding to each subregion of the field. The spatial boxes defined for the cloudlet (A), ring-like structure (B), shocked clump (C), and shocked knot (D) are shown in Fig. 6, and the average spectra obtained in these boxes are presented in Figs. 8 and 7 for every line from CO and its isotopologs, which are available in our IRAM 30 m and APEX data cubes. The choice of the boundaries is based on our morphological classification, but we carefully checked that the brightest spectral features are coherent across the different boxes that we defined (coordinates of these boxes are given in Table A.1). We performed that selection manually, as the size of our sample is not large enough to apply statistical methods (e.g., clustering, see Bron et al. 2018). Based on the analysis of the emission of 12CO, 13CO and C18O lines, our description of these spectral features is the following:

1.Cloudlet: toward box A (Fig. 7, left-panel), the line profile of 12CO and 13CO lines are similarly double peaked and best modeled by the sum of two Gaussian functions centered on the systemic velocities vLSR = −5.7 ± 0.3 km s−1 (associated with the cloudlet) and vLSR = −3.3 ± 0.1 km s−1 (associated with the ambient cloud).
2.Ring-like structure: toward box B (Fig. 7, right-panel), the 12CO and 13CO lines are double peaked as well. The use of two Gaussian functions to model the line profile yields the systemic velocities vLSR = −5.6 ± 0.2 km s−1 (associated with the ring-like structure) and vLSR = −3.3 ± 0.1 km s−1 (associated with the ambient cloud). The Gaussian decomposition is very similar to that of the cloudlet, suggesting that the apparent ring-like structure might be incidental despite its remarkable features in the first moment map (Fig. 5).
3.Shocked clump: considering the geometry of the SNR and the locally perpendicular direction of propagation of the SNR shockwave (van Dishoeck et al. 1993), the high-velocity emission arises from at least two shock waves, if not a collection of transverse shocks propagating along the molecular shell. In other words, the projection along the line of sight of several distinct shocked knots with distinct systematic velocities could contribute to the broadening of the 12CO lines. We measure vs ≃ 27 km s−1 and vs ≃ 21 km s−1 for the blueshifted and redshifted transverse shocks, respectively. Except for the J = 1–0 spectrum for which the emission of the ambient gas contributes to the average spectra, all spectra of 12CO lines exhibit a significant absorption feature around the vLSR of IC443G, suggesting that there is strong self-absorption of the emission lines. Evidence of line absorption is found in the velocity range −6 km s−1 vLSR  −2 km s−1, which is where we detect the spatially extended features associated with the NW-SE complex of molecular gas described by Lee et al. (2012). Hence, it is possible that the foreground cold molecular cloud is at the origin of the absorption of the 12CO J = 1–0 and J = 2–1 lines. A faint and thin emission line is detected around v = 6.5 km s−1 both in the 12CO and 13CO spectra. This signal is associated with the NE–SW complex of molecular gas described in Sect. 3.2.
4.Shocked knot: the shock signature of this line is distinct from the shocked clump. As hinted by the moments map (Fig. 5), its fainter high-velocity wings are displaced toward negative velocities. A self-absorption feature is also observed in this structure. Between v = −5.5 km s−1 and v = −2 km s−1 a bright andthin feature traces the ambient gas shown on the channel maps in Fig. 2 (second row from bottom; first, second and third panels from left).
Citation: Bron et al. 2018
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