Description of source "G352.630-1.067" from Chen et al. (2019)

Figure 1 shows the integrated intensity images for the three class I methanol transitions detected toward G352.630-1.067 in the current SMA observations, the 42−31 E (218.4 GHz), 8−1−70 E (229.7 GHz) and 52−41 E (266.8 GHz) lines. All the three methanol transitions are detected toward this source for the first time. It can clearly be seen that all three transitions are detected toward the phase tracking center and thus are associated with the region where the class II 6.7 GHz methanol maser arises. Hereafter, we refer to this central molecular emission region as “M1.” The methanol emission from M1 has a compact structure, suggesting that it originates from a small region and is not resolved by the current SMA observations.

Therefore, an upper limit on the angular size of M1 is ∼3″ (corresponding to a linear size of ∼2000 au for a distance of 0.7 kpc). In addition to the methanol emission associated with the M1 region, we detect emission from two other sites in the three transitions. One of these is to the east (labeled as M2), and the other to the west of M1 (labeled as M3). These two regions are offset by 10–17 arcsec (7000–12,000 au or ∼0.05 pc) from M1. Figure 1 shows that the emission from M2 is extended compared to the synthesized beam size of the 218.4 and 229.7 GHz transitions, but it is compact for the 266.8 GHz transition. In contrast, M3 is relatively compact for all three methanol transitions where emission is detected. The fitting results for the beam-deconvolved sizes of the three components assuming Gaussian distributions (listed in Table 1) support this.

The spectra for each of the detected methanol transitions is shown in the bottom panels of Figure 1. The spectra are constructed from a combination of summing the three spectral features toward the peak positions of the three components. A single Gaussian can be fitted to the methanol spectrum for each component in each transition, with the exception of the 52−41 E (266.8 GHz) transition toward M3, where two Gaussian components were required (see below). The fitted parameters are given in Table 1. For M1, the three maser transitions show a similar line profile with peak velocities at ∼−3 km s−1 and line widths of ∼5 km s−1. The peak brightness of the transitions detected toward M1 ranges between 1.6 K beam −1 (229.7 GHz) and 2.3 K beam −1 (266.8 GHz).

For the M2 emission, all three methanol transitions show similar profiles, with a narrow line width (∼1.5 km s−1) at peak velocity of ∼1 km s−1. The strongest emission toward M2 is detected from the 229.7 GHz transition, which has a peak brightness of 6.0 K beam−1. The other two transitions (218.4 and 266.8 GHz) have a similar peak brightness of 1.7 K beam−1. For M3, all three detected methanol transitions show a broad profile (∼10 km s−1). In addition, the 266.8 GHz emission also shows a narrow spectral feature (line width of 1.7 km s −1 at peak velocity of −2 km s−1) overlaid on the broad spectral emission. Because of this we have used two Gaussian profiles to fit this transition toward M3. To enable a direct comparison of the methanol spectra from the three class I methanol transitions toward M2 and M3, we have plotted their spectra toward each of the components in the right panel of Figure 2. From Figure 1, we can also see that there is another methanol component in the 42−31 E (218.4 GHz) transition toward the southeast of M1, with an offset of 10″. This component does not show emission from any of the other methanol transitions.

For comparison, in the right panel of Figure 1 we present an image of the integrated emission and spectrum of the thermal methanol emission from the 61–51 A+ (287.6 GHz) transition detected in the current SMA observations. As for the 287.6 GHz transition, all the other thermal methanol transitions were only detected toward M1. No emission from any of the thermal methanol transitions is detected toward M2 and M3, suggesting that their physical conditions are very different from M1 and supporting the hypothesis that the methanol emission toward M2 and M3 is due to masing (see discussion in Section 4.1).