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Appendix A: Results of the APEX observations

W33 Main1

The detected transitions of W33 Main1 are common to all six sources and are listed in Sect. 5 (Fig. 3). The upper energy levels E_u of the observed transitions range from 27 to 141 K. The average central velocity is 36.4 km s^-1 and the average line width is 3.2 km s^-1. The detection of high-energy transitions of H₂CO at 82 and 141 K and CH₃OH at 64 and 75 K hints to the existence of a heating source within or near W33 Main1. The moment 0 maps of these lines (Fig. A.1) show only extended emission at the position of the continuum peak, while the peak of the line emission is located close to the edge of the maps, which point to an external rather than an internal heating source. The moment 0 maps of the two low-temperature transitions of H₂CO at 35 and 46 K, and the N₂H⁺ and the CS transitions show strong compact emission, which peaks within ~6″ of the continuum peak, indicating that the interior of W33 Main1 is still cold (Fig. A.1).

W33 A1 and W33 B1

In W33 A1 and W33 B1 (Fig. 3), we observe the same transitions (CH₃OH(9-−8_0,8) and CH₃OH(6_1,5−5_1,4), in addition to the spectral lines detected in W33 Main1). The upper energy levels E_u of all detected spectral lines range between 27 and 141 K. The average central velocity is 33.3 km s^-1 and 36.7 km s^-1 for W33 B1 and W33 A1, respectively. The spectral lines have an average line width of 5.4 km s^-1 and 3.9 km s^-1 in W33 B1 and W33 A1, respectively. In general, the detected spectral lines tend to be stronger and broader in W33 B1 compared to W33 A1. The peaks of the compact emission of the four strongest lines, as seen in the moment 0 maps, are located within ~6″ of the continuum peaks in both sources (Figs. A.2 and A.3). In W33 B1, the emission of the high-excitation transitions of H₂CO and CH₃OH is also compact and peaks close to the continuum peak (Fig. A.3). This suggests that a heating source is present in W33 B1. The moment 0 maps of these high-temperature lines in W33 A1 show more extended emission and less isolated peaks (Fig. A.2), which indicates that W33 A1 already contains a heating source but is probably less developed than W33 B1.

W33 B

Besides the spectral lines detected in W33 B1 and W33 A1, we observe emission from H₂CS(8_1,7−7_1,6), CH₃OH(6_2,4−5_2,3), CH₃OH(3_2,1−4_1,4), C³³S(6−5), and OCS(24−23) in W33 B (Fig. 4). The spectral lines have upper energy levels E_u between 27 and 175 K. The average central velocity and line width of W33 B are 55.4 km s^-1 and 5.3 km s^-1, respectively. Although the radial velocity of W33 B differs by ~20 km s^-1 from the radial velocities of the other clumps in W33, Immer et al. (2013) have shown that this clump is located at a similar distance as the other clumps. However, it is still unclear what the reason for this large radial velocity difference is. While the integrated emission of CS and the two low-excitation transitions of H₂CO is compact and peaks close to the continuum peak, the emission of N₂H⁺ is

extended in south-east direction and peaks ~12″ east of the continuum peak (Fig. A.4). The moment 0 map of H₂CO(4_2,2−3_2,1) shows extended emission in the north-south direction. The peak of the emission is offset by ~12″ in north-west direction from the continuum peak (Fig. A.4). The integrated emission of C³³S(6−5) is extended to the west, but its maximum is located close to the continuum peak (Fig. A.4). The emission of the remaining lines is mostly compact and peaks within ~6″ of the continuum peak (Fig. A.4). The stronger lines and the detection of transitions at higher excitation energies compared to W33 B1 and W33 A1 suggests that W33 B is even more evolved than W33 B1 and W33 A1.

W33 A

In addition to the lines found in W33 B, we also detect emission of OCS(23−22), HC₃N(31−30), HC₃N(32−31), and CH₃CCH(17₀−16₀) in W33 A (Fig. 4). However, we do not observe emission of C³³S(6−5) or CH₃OH(3_2,1−4_1,4). The upper energy levels of the detected lines are between 27 and 231 K. The average central velocity of all transitions is 37.6 km s^-1, which is close to the systemic velocity of 38.5 km s^-1, determined by Galván-Madrid et al. (2010) from their SMA observations of W33 A. The average line width is 5.4 km s^-1. The integrated emission of all lines in W33 A is compact and their maxima are located close to the continuum peak within ~6″−12″ (Fig. A.5). The object W33 A is probably more evolved than W33 B.

W33 Main

Besides the transitions that we detect in W33 A and W33 B, we observe the higher-excitation transitions of CH₃CCH (CH₃CCH(17₁−16₁), CH₃CCH(17₂−16₂), CH₃CCH(17₃−16₃), CH₃CCH(17₄−16₄)), and H₂¹³CO(4_1,3−3_1,2) in W33 Main (Fig. 4). The transition with the highest upper energy level E_u = 241 K is detected in this source. The average central velocity and line width of W33 Main are 35.6 km s^-1 and 6.0 km s^-1, respectively. Except for the N₂H⁺, CS, and OCS transitions, the emission of all spectral lines peaks close to the continuum peak (Fig. A.6). The N₂H⁺ emission is strongest at the northwestern edge of the map. At the center of W33 Main, the N₂H⁺ emission is much weaker (Fig. A.6). This shows that N₂H⁺ is not a good tracer of the dust continuum in evolved sources anymore (Reiter et al. 2011). The CS emission is spread from the center of the map to the north and peaks about ~24″ from the continuum peak to the north (Fig. A.6). The emission of C³³S, CH₃CCH (except CH₃CCH(17₄−16₄)), CH₃OH (except CH₃OH(3_2,1−4_1,4) and CH₃OH(6_2,4−5_2,3)), H₂CO, and H₂CS is also extended to the north but peaks close to the continuum peak (Fig. A.6). The OCS emission is extended and the peak close to the center of the map is not very pronounced (Fig. A.6). The extended emission hints to the existence of another source in the north of the map. Since CS and N₂H⁺ trace cold gas, the line peak offsets of the CS and N₂H⁺ emission from the continuum peak indicate that the gas towards the center of the W33 Main clump is not cold anymore. We conclude that W33 Main is not in an early stage of star formation anymore.

Appendix B: Results of the SMA line observations

W33 Main1

Besides the two CO lines ¹²CO and C¹⁸O, we only observe emission of SO(6₅−5₄) in the spectrum of W33 Main1 (Fig. 8). ¹³CO is not detected at the position of the dust continuum peak, but extended emission of ¹³CO is observed in the W33 Main1 mosaic (Fig. B.1). The upper energy levels E_u of the observed transitions are 16−35 K. The average central velocity and the average line width are 35.9 km s^-1 and 4.0 km s^-1, which are similar to the results of the APEX observations.

The blueshifted and redshifted ¹²CO(2−1) emission of W33 Main1 shows several peaks at and around the dust continuum peak (Fig. 7a). The distribution of the emission does not look ordered and might be excited by turbulences in this source.

The moment 0 map of the SO transition shows compact emission, whose peak is offset by ~2″ from the dust continuum peak (Fig. B.1). The moment 1 map of the SO transition shows a velocity gradient over the central part of the source (Fig. B.6).

The lack of spectral lines besides the CO and SO transitions either indicates low temperatures in the dust core or the emission of more complex molecules is not compact enough to be detected with the SMA. Both hint to a very early evolutionary age of W33 Main1. We conclude that W33 Main1 is probably in an early protostellar phase before the protostar strongly influences the surrounding material, and strong emission of primary molecules like H₂CO and CH₃OH is detected.

W33 B1

In the spectrum of W33 B1, we detect the three transitions of H₂CO, CH₃OH(4_2,2−3_1,2), SO(6₅−5₄), and ¹²CO (Fig. 8). Extended emission of ¹³CO and C¹⁸O is observed in the W33 B1 map but not at the center of the core (Fig. B.3). The detected transitions have upper energy levels between 17 and 68 K, indicating that this source is still relatively cold. The average central velocity and the average line width of W33 B1 are 32.7 km s^-1 and 5.0 km s^-1, respectively. The ¹²CO(2−1) emission shows a preferred direction in W33 B1 (north-west to south-east). Most of the emission is detected close to the dust continuum peak (Fig. 7c).

The moment 0 maps of CH₃OH and H₂CO show compact emission, which peaks close to the dust continuum peak (Fig. B.3). The emission of SO is also compact but peaks at the edge of the core (Fig. B.3). The moment 1 maps of H₂CO(3_0,3−2_0,2) and CH₃OH(4_2,2−3_1,2) show similar velocity gradients across the source (Fig. B.6).

The higher-energy transitions of H₂CO and CH₃OH hint to the presence of a heating source in W33 B1. However, since we do not see the line-richness of hot cores in this core, we conclude that W33 B1 is a young protostellar core.

W33 A1

In addition to the lines detected in W33 B1, we observe emission of DCN(3−2) and C¹⁸O at the peak of W33 A1 (Fig. 8). Again, extended emission of ¹³CO is observed in the map but not at the position of the continuum peak (Fig. B.2). The upper energy levels of the detected transitions are between 16 and 68 K. The average central velocity of W33 A1 is 35.5 km s^-1. The average line width is 3.7 km s^-1.

Two streams of ¹²CO(2−1) emission are detected in W33 A1 that are almost perpendicular to each other (Fig. 7b). Blueshifted emission is observed north of the dust continuum peak, while redshifted emission is detected west of the continuum peak.

The emission of DCN is compact and peaks close to the dust continuum peak (Fig. B.2). The moment 0 map of SO shows two peaks at the edges of the dust core with the main peak of the SO emission being offset from the dust continuum peak by ~2″ (Fig. B.2). In the moment 1 map, a small velocity gradient is visible over both SO emission peaks (Fig. B.6). The integrated emission of H₂CO and CH₃OH is compact and peaks within 1″ of the dust continuum peak (Fig. B.2). The low-energy transitions of H₂CO and CH₃OH both show a velocity gradient over the source (Fig. B.6). For the same reasons as for W33 B1, we conclude that W33 A1 is a young protostellar core.

W33 B

The object W33 B is one of the most line-rich sources of the six W33 cores. We observed 37 transitions of 14 molecules with the SMA (Fig. 9). Emission of the CO isotopologue C¹⁸O is not observed towards the center of the molecular core but is detected as extended emission in the map (Fig. B.4). The ionised gas tracer H30α is not detected in W33 B, indicating that an UC H ii region is not yet present. The strongest line in the spectrum is ¹²CO with a peak brightness temperature of ~6 K. The average central velocity is 55.6 km s^-1. The average line width is 6.8 km s^-1.

In W33 B, we detect an outflow in the ¹²CO emission in almost north-south direction. Another stream of ¹²CO emission at blueshifted velocities points in the south-west direction and probably marks another outflow of which we only see the blueshifted side (Fig. 7d).

The integrated emission of most of the detected lines is concentrated within the boundaries of the dust emission and only barely resolved (Fig. B.4). The lines that show more extended emission are ¹³CS, CH₃OH(4_2,2−3_1,2), H₂CO(3_0,3−2_0,2), SO(6₅−5₄), as well as the CO lines, which are spread over the whole field of view (Fig. B.4). Velocity gradients are detected in the moment 1 maps of the H₂CO and CH₃OH transitions as well as the OCS(19−18) and HC₃N(24−23) transitions (Fig. B.6).

The detection of complex molecules like CH₃CN, HNCO, HC₃N, CH₃OCHO, or CH₃OCH₃ indicates that W33 B is in the hot core stage.

W33 A

For comparison with our sample of W33 sources, we obtained the spectrum of W33 A by integrating the emission over one synthesised beam at the position of the stronger continuum peak (MM1 in Galván-Madrid et al. 2010). The spectra of the two sidebands are shown in Fig. 9. In the covered frequency range, we detect the same transitions as in W33 B. In addition, high-energy CH₃OH transitions at E_u = 579 K and emission of more complex molecules (CH₃OCHO, CH₃OCH₃) at higher excitation energies are observed. All transitions have upper energy levels E_u between 16 and 579 K. Unfortunately, the frequency of the radio recombination line (RRL) H30α is located outside the frequency range of the W33 A spectra. The variety of detected molecules (including complex molecules) and the high temperature needed for the excitation of the high-energy transitions supports the identification of W33 A as a hot core. Since W33 A shows emission of complex molecules at higher excitation energies, we conclude that W33 A is probably more evolved than W33 B.

W33 Main

Compared to W33 B and W33 A, the spectrum of W33 Main shows significantly fewer spectral lines (Fig. 9). The dust continuum peak of W33 Main-Central is almost devoid of H₂CO emission (Fig. B.5). In the spectrum, we detect H₂CO(3_0,3−2_0,2) and H₂CO(3_2,2−2_2,1). However, we observe emission of the 68 K H₂CO transitions (3_2,2−2_2,1) and (3_2,1−2_2,0) in the western part of W33 Main-Central and at the peak positions of W33 Main-West and W33 Main-North (Fig. B.5). Besides two transitions of CH₃OH (CH₃OH(4_2,2−3_1,2), CH₃OH(8-−7_0,7)), we detect SO, the CO isotopologues ¹²CO, ¹³CO, and C¹⁸O, HC₃N, and the RRL H30α in the spectrum. The detected lines have upper energy levels from 16 to 131 K. The two strongest lines are ¹²CO and C¹⁸O with peak brightness temperatures of ~8 and ~6.5 K, respectively. The average central velocity and the average line width of W33 Main are 37.1 km s^-1 and 4.3 km s^-1.

Figure 7e shows the outflowing gas in the C¹⁸O emission. While there seem to be two redshifted streams of C¹⁸O emission, we only see one strong peak of blueshifted emission that is located close to W33 Main-Central. In Fig. B.8, we show a comparison of the integrated emission of the ¹³CO and C¹⁸O transitions from the SMA data only and from the combination of the SMA and IRAM30 m data. In Fig. B.9, the ¹³CO spectra from the SMA data and the IRAM30 m+SMA data, which are integrated over one synthesised beam at the continuum peak of W33 Main are plotted. While the spectrum of the combined data shows a broad Gaussian with a blue and a red “shoulder”, the SMA spectrum mimics a P-Cygni profile. These two figures present the importance of zero-spacing information, especially for widespread line emission, and show the strong filtering of large scale emission and the importance of side lobes in spectral features.

The object W33 Main is the only source in our SMA sample that shows emission of the RRL H30α and the shock tracer SiO (Fig. B.5). The integrated emission of the RRL has the same

shape as the dust continuum emission of W33 Main-Central which suggests that part of the continuum emission comes from free-free radiation (Fig. B.5, see Sect. 4). The detection of the RRL supports the identification of W33 Main as a more evolved object where ionised emission of H ii region(s) is observed. The integrated emission of CH₃OH(4_2,2−3_1,2) and SO(6₅−5₄) also peaks at the center of W33 Main-Central (Fig. B.5). However, the HC₃N transition peaks in the western part of W33 Main-Central, which is offset from the main dust peak by ~0.1 pc (Fig. B.5). Emission of the SiO(5−4) transition is observed at the western and southern edges of W33 Main-Central (Fig. B.5). Velocity gradients are seen in the moment 1 maps of the CH₃OH and HC₃N transitions (Fig. B.6).

The object W33 Main-North is bright in ¹³CS, H₂CO(3_0,3−2_0,2), SO(6₅−5₄), and SiO(5−4) (Fig. B.5). Weaker emission of CH₃OH(4_2,2−3_1,2) is also observed at this position. The strongest lines in W33 Main-West are H₂CO(3_0,3−2_0,2) and CH₃OH(4_2,2−3_1,2) (Fig. B.5). In addition, weaker emission of ¹³CS, SO(6₅−5₄), and SiO(5−4) is detected. At the position of W33 Main-South, we only observed diffuse line emission (Fig. B.5).

In the sources W33 Main1, W33 A1, W33 B1, and W33 Main, we detect ¹²CO(2−1) and ¹³CO(2−1) emission at velocities of ~60 km s^-1. This emission is offset from the dust emission peak by several arcseconds in all sources. This shows that the velocity component of W33 B (v_sys = 56 km s^-1) is not unique in the W33 complex but also observed towards the other sources in the low density gas tracers (see also Goldsmith & Mao 1983; Urquhart et al. 2008; Chen et al. 2010).

Appendix C: Weeds modelling

APEX observations

A good fit of the 46 K H₂CO line in W33 Main1 is achieved with a temperature of 40 K and a column density of 3.4 × 10¹³ cm^-2. The synthetic spectrum slightly underestimates the 35 and 141 K transitions and slightly overestimates the two 82 K transitions.

In W33 A1, the synthetic spectrum, based on a kinetic temperature of 40 K and a column density of 4.1 × 10¹³ cm^-2, gives a good fit to the 46 K transition. As in W33 Main1, the emission of the 35 K and the 141 K lines is a bit underestimated and the emission of the 82 K lines a bit overestimated. A slightly better fit for all transitions is achieved if we fit the two components with 25 K and 2.0 × 10¹³ cm^-2 and 55 K and 2.5 × 10¹³ cm^-2.

The synthetic spectrum, computed from the RTD results of W33 B1, is a good fit for the transitions with E_u above 80 K but underestimates the emission of the 35 K and 46 K lines. We conclude that a low-temperature component is missing in the construction of the synthetic spectrum. Thus, a new synthetic spectrum was produced, based on two components with 30 K and 2.0 × 10¹³ cm^-2 and 60 K and 2.5 × 10¹³ cm^-2. The synthetic spectrum yields a good fit for the 46 K and 141 K lines but underestimates the emission of the 35 K transition and slightly overestimates the emission of the 82 K line.

In W33 B, we produce a synthetic spectrum with two components of 30 K and 2.3 × 10¹³ cm^-2 and 100 K and 5.9 × 10¹³ cm^-2. This spectrum reproduces the transitions of the upper sideband well but underestimates the emission of the 46 K line. The synthetic spectrum for W33 A, constructed from two components of 40 K and 9.0 × 10¹³ cm^-2 and 100 K and 1.6 × 10¹⁴ cm^-2, yields a good fit for the high-energy and the 35 K transitions but overestimates the emission of the 46 K line by about 25%.

Two components with 50 K and 2.0 × 10¹⁴ cm^-2 and 100 K and 4.2 × 10¹⁴ cm^-2 give a synthetic spectrum for W33 Main, which fits all transitions except the 46 K very well. However, the emission of the 46 K line is overestimated by about 33%. In W33 Main, we also produced synthetic spectra for CH₃OH and CH₃CCH. The RTD results of CH₃CCH give a synthetic spectrum that underestimates the emission of all detected transitions. A better fit is achieved using a kinetic temperature of 59 K and a column density of 3.4 × 10¹⁵ cm^-2 for the construction of the synthetic spectrum. The observed and the synthetic spectrum of CH₃CCH are shown in Fig. C.1. For CH₃OH, we find a fairly good fit of the transitions for one component with 40 K and 1.3 × 10¹⁵ cm^-2. However, the synthetic spectrum underestimates the emission of CH₃OH(9-−8_0,8).

SMA observations

We tried to compute synthetic spectra from one component for the H₂CO transitions in W33 A1, W33 B1, and W33 B but it was not possible to fit all three transitions well. The synthetic spectra show that the peak line ratio between H₂CO(3_2,1−2_2,0) and H₂CO(3_2,2−2_2,1) should be

close to 1, and the peak of the H₂CO(3_0,3−2_0,2) line is larger than the peaks of the other two transitions. The peak line ratios H₂CO(3_2,1−2_2,0)/H₂CO(3_2,2−2_2,1) in W33 A1 and W33 B1 are 0.6 and 2.0. In W33 B, the line ratio is close to 1, but the peak of H₂CO(3_0,3−2_0,2) is smaller than the peaks of the other two transitions. This indicates that the assumption of a single temperature relating the level populations is not justified for H₂CO in all three sources.

We constructed a synthetic spectrum for HNCO in W33 B for one component with the RTD results. The synthetic spectrum underestimates the emission of all transitions. A good fit of the observed spectrum is achieved with 280 K and 2.5 × 10¹⁵ cm^-2. However, the emission of the transition at 220.58 GHz is slightly overestimated, while the emission of the transition at 218.98 GHz is slightly underestimated. Through comparison with the synthetic spectrum, we found two additional transitions of HNCO at 219.66 GHz (HNCO(10_3,8−9_3,7), HNCO(10_3,7−9_3,6)), which are blended and a 2σ detection in our data.

The synthetic spectrum of the CH₃CN emission in W33 B, which was compiled from the RTD results, is only a good fit for the CH₃CN(12₃−11₃) transition. A better fitting synthetic spectrum is constructed from a kinetic temperature of 280 K and a column density of 1.3 × 10¹⁵ cm^-2. However, the emission of the CH₃CN(12₃−11₃) transition is now overestimated. The comparison of synthetic and observed spectrum yields the identification of another transition of the CH₃CN ladder (CH₃CN(12₇−11₇)), which is a 2σ detection in our data. The RTD results of CH₃OH in W33 B yield a synthetic spectrum, which underestimates the emission of all CH₃OH transitions. The synthetic spectrum shows that at least two components are necessary to fit the observed spectrum. However, we do not find two combinations of temperatures and column densities that fit all transitions well.

© ESO, 2014