© ESO, 2013

1. Introduction

The H₂CO distribution in the Galaxy has been noted by Davies & Few (1979). They found that the H₂CO distribution is similar to that of H ii and CO. Downes et al. (1980) surveyed 262 Galactic radio sources using the H₂CO absorption line at 4.830 GHz, and H110α recombination line at 4.874 GHz, and found that H₂CO is associated with most of the H ii regions. It is a good probe of the star formation region. H₂CO absorption is only seen in absorption against the background continuum, and it gives different constraints to mm and sub-mm spectral lines, which are seen both in front of and behind the H ii region. It provides a unique probe of the physical conditions for the foreground cloud. Comparative surveys of H₂CO absorption and CO emission in the galactic center have been reported by Scoville et al. (1972, 1973) and Solomon et al. (1972). A good correlation is generally found between H₂CO and CO. Cohen et al. (1983) generally found good agreement on a large scale but a connection that does not have enough detail. Recently, Rodríguez et al. (2006, 2007) and Zhang et al. (2012) compared the H₂CO absorption and CO emission profiles towards the Galactic anticenter, and the following five regions, L1204/S140, W49, W3, DR21/W75, and NGC 2024/NGC 2023. They found a crude correlation between these two molecular tracers on a large scale. Generally, the ¹²CO (1–0) emission line is optically thick, and the H₂CO (1₁₀–1₁₁) absorption line is optically thin, so the two lines have many different properties in the dense region. The ¹³CO (1–0) emission line is optically thin, and it can trace dense region (n(H₂)  >  10³ cm^-3). It is therefore similar to H₂CO.

Liszt & Lucas (1995) compared N(H₂CO) with N(HCO⁺) and N(¹³CO) towards compact extragalactic mm-wave continuum sources, and found that H₂CO has a rapid increase with N(HCO⁺), but not as rapid as that of N(¹³CO). N(¹³CO) is strongly affected by fractionation (Liszt & Lucas 1998), so that the H₂CO–¹³CO comparison is perhaps interesting, while it is not directly used in deciphering the general chemistry. Cohen et al. (1983) compared the H₂CO (beam  ~  10′) with ¹³CO (beam ~  8′) maps in the Orion molecular cloud and found that the agreement between H₂CO and ¹³CO is considerably better, but with different relative intensities and with minor differences in the detailed morphology. Therefore, making a point-by-point comparison will be interesting when the ¹³CO profiles become available. In this paper, we report new CO, H₂CO, 6 cm continuum and H110α observations in four Galactic HII regions of MON R2, S156, DR17/L906, and M17/M18. We are interested in making a comparative study of the H₂CO, ¹²CO, and ¹³CO lines.

2. Observations

2.1. Formaldehyde, H110α recombination, and continuum

From September 2010 to August 2011, we observed the H₂CO line, the H110α line, and the 6 cm continuum with the Nanshan 25 m radio telescope of Xinjiang Astronomical Observatory. The 25 m radio telescope has an HPBW (half power beam width) of 10′ at 4829.6594 MHz. A 6 cm low noise receiver was used. The system temperature was about 23 K during observations. Digital autocorrelation spectrometers were used with 4096 channels and 80 MHz bandwidth, and a corresponding velocity resolution of 1.206 km s^-1. The line detection limit is about 23 mK with 10 min integration time. To get the higher signal/noise ratio, each point’s total integration time ranged from 30 min to several hours. The continuum at 4.8 GHz were processed with a bandwidth of about 400 MHz, and the error was approximately 5%. The DPFU (degrees per flux unit) value was 0.116 K Jy^-1. The pointing and tracking accuracy was better than 15′′, and the beam efficiency was 65%. The observation was performed in the so-called ON/OFF mode. A diode noise source was used to calibrate the spectrum and the flux error was 15%.

2.2. Carbon monoxide

From 15 to 26 May 2011, the ¹²CO and ¹³CO observations of the four regions were carried out with the 13.7 m millimeter wave telescope of Purple Mountain Observatory in Delingha. The 3 mm cryogenically cooled 9 – beam SIS (Superconductor Insulator Superconductor) receiver was used in double sideband mode, and the system temperatures ranged from 105 to 140 K during the observations. Using the fast Fourier transform spectrometer, the ¹²CO velocity resolution was 0.16 km s^-1, while the ¹³CO velocity resolution was 0.17 km s^-1. The rms was about 0.1 K at 1 min integration time, and the line detection limit about 0.15 K. The three CO lines were observed simultaneously. The HPBW was 60′′ at 110 GHz. The grid spacing of mapping observations was 30′′, and the pointing accuracy was better than 10′′. The average integration time of every point was one minute. The source was mapped using the on-the-fly mode of observation. The standard source W51 was checked roughly every two hours.

3. Results

3.1. Data reduction and exhibition

Data reduction for H₂CO, H110α, ¹²CO, and ¹³CO lines were done using CLASS and GREG, which are parts of GILDAS¹. To enhance comparison with the observation, we smoothed the ¹²CO and ¹³CO observations to 10′, and resampled them on the H₂CO observing grid. Sources observed are shown in Table B.1. The H110α data are reported in Table B.2. The parameters of the H₂CO and the 6 cm continuum are reported in Table B.3, while ¹²CO and ¹³CO data are reported in Table B.4. The H₂CO, H110α, ¹²CO, and ¹³CO line spectra are shown in Fig. B.1. Line integral intensities of H₂CO, ¹²CO, and ¹³CO for four sources are shown in Figs. 1, A.1, and A.3. Continuum and H110α distributions are shown in Fig. A.5.

Fig. 1 Contours and color-scale maps of integrated area toward a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The contour and color-scale map respectively represent the integrated intensities of the H2CO and the mid-infrared 8.28 μm MSX emission in S156, DR17/L906, and M17/M18 regions. For the MON R2 region, the color-scale map represents the IRAS 12 μm data. a) For the MON R2 region: contour levels are –0.17 to –0.46 in steps of –0.06 K km s-1. b) For the S156 region: contour levels are –0.09 to –0.25 in steps of –0.03 K km s-1. c) For the DR17/L906 region: contour levels are –0.12 to –0.32 in steps of –0.04 K km s-1. d) For the M17/M18 region: contour levels are –0.42 to –1.11 in steps of –0.14 K km s-1.

The H₂CO (1₁₀–1₁₁) apparent peak optical depth was determined using a simply standard radiative transfer result, τ_app = −ln [1 + T_L/(T_c + 2.73 − T_ex)], where T_L is the antenna temperature in K, T_c is the continuum brightness temperature in K, and T_ex is the excitation temperature of the 1₁₀–1₁₁ transition of H₂CO. The excitation temperature T_ex values are in the range of 1.5 to 2.0 K (Heiles 1973; Vanden Bout et al. 1983; Young et al. 2004). Here we use a mean value of T_ex = 1.7 K. The column density of H₂CO at rotational state 1₁₁ was obtained from the apparent peak optical depth using N(H₂CO) = 9.4 × 10¹³·τ_app·ΔV (Pipenbrink & Wendker 1988), where ΔV is the FWHM in km s^-1. The difference between our assumed value of Tex and the one used in deriving the constant in the above expression (2.0 K) results in an offset of less than 0.1 dex in Fig. 5. And the H₂ column density was obtained using the relation N(H₂CO)/N(H₂) = 1.25 ×  10^-9 (Few & Booth 1979; Scoville & Solomon 1973; Evans et al. 1975). The local thermodynamic equilibrium (LTE) electron temperature ${\mathit{T}}_{\mathrm{e}}^{\mathrm{\ast }}$ was estimated following Brown et al. (1978) and Pipenbrink & Wendker (1988). The optical depths of ¹³CO and the column densities of both ¹³CO and H₂ were estimated using Sato (1994) calculations, on the assumption that the cloud is in LTE.

3.2. Description of sources

MON R2. – The size of MON R2 observed is about 60′ × 90′. The H110α line was not detected. Maps of the integrated intensities of the H₂CO, ¹²CO, and ¹³CO line velocities range from 0 to 20 km s^-1. There is a velocity gradient of several km s^-1 for H₂CO, ¹²CO, and ¹³CO across the cloud. The spectrum of intensity peaks of H₂CO shows two velocity components at 7.6 and 10.52 km s^-1. The velocity component at 10.52 km s^-1 agrees with the intensity peaks of the ¹²CO and ¹³CO lines.

S156. – About 50′ × 70′ of this region have been observed. We did not detect the H110α line. The line widths in the central part of the cloud are  ~3.8 km s^-1, more than twice the width normally measured in the dark clouds. This suggests that the cloud could be affected by the H ii region. Maps of integrated intensities of H₂CO, ¹²CO, and ¹³CO line velocities range from –55 to –45 km s^-1. There is a velocity gradient of several km s^-1 for H₂CO, ¹²CO, and ¹³CO across the cloud. The velocity of the H₂CO intensity peak is –50.28 km s^-1, which agrees with those of the ¹²CO and ¹³CO emission lines.

Fig. 2 Position-velocity diagrams of H2CO for a) MON R2 and b) S156, regions along Dec for Δα = 0, for c) DR17/L906 region from offset (20, –30) to (–10, 20), and d) M17/M18 region from offset ( − 30, − 40) to (0, 30). For the MON R2, S156, and DR17/L906 regions: contour levels are –0.03 to –0.08 in steps of –0.01 K. For the M17/M18 region: contour levels are –0.03 to –0.18 in steps of –0.03 K.

DR17/L906. – The size of our observed region is about 40′  ×  60′. The strong H110α emission was detected in the DR17 region with an average velocity of 11.4 km s^-1. Maps of the integrated intensities of the H₂CO, ¹²CO and ¹³CO line velocities range from –10 to 20 km s^-1. The H₂CO average spectrum shows two velocity components at 6.3 and 14.9 km s^-1. The velocity component at 6.3 km s^-1 of the H₂CO agrees with those of the DR17 H ii region and the continuum emission. This is nearly half of the velocity of the H110α recombination line. The ¹²CO and ¹³CO average spectrum shows three velocity components at –1.6, 6.0, and 13.8 km s^-1. The ¹²CO and ¹³CO velocity components at 13.8 km s^-1 and the H₂CO velocity component at 14.9 km s^-1 are associated with the L906 region.

M17/M18. – About 70′  ×  80′ region have been observed. It covers the M17 region and north of M18. Nine H110α recombination lines were detected in the M17 region with an average velocity of 16.4 km s^-1. Maps of the integrated intensities of the H₂CO, ¹²CO and ¹³CO line velocities range from 10 to 50 km s^-1. The H₂CO average spectrum shows three velocity components at 18.3, 22.4, and 37.6 km s^-1. The ¹²CO and ¹³CO average spectrum shows four velocity components at 20.0, 28.2, 38.3, and 57.8 km s^-1. The H₂CO velocity component 18.3 km s^-1 agrees with the velocity component at 20.0 km s^-1 of the ¹²CO and ¹³CO, which also agrees with the H ii region in M17. The H₂CO cloud distribution, line temperatures, and narrow line width comparing with values observed in the dark cloud shows that the cloud has been negatively influenced by the H ii region for the velocity component at 22.4 km s^-1. The H₂CO velocity component at 37.6 km s^-1 agrees with the 38.3 km s^-1 value for the ¹²CO and ¹³CO lines.

Fig. 3 Correlation between the H2CO line flux and 12CO, 13CO line flux at corresponding points in MON R2, S156, DR17/L906, and M17/M18. The solid line is the linear fit for H2CO and 12CO flux data, and the dashed line is the linear fit for H2CO and 13CO.

4. Discussion

We mapped four regions for the H₂CO, ¹²CO, and ¹³CO (Figs. 1, A.1, and A.3). The maps show that the distribution of H₂CO is similar to ¹²CO and ¹³CO on a scale of 2  ~  10 pc. The position-velocity diagrams toward four regions (Figs. 2, A.2, and A.4) also show that the H₂CO velocity agrees well with ¹²CO and ¹³CO. This suggests that H₂CO is directly related to the CO, and not merely located along the same line of sight. These indicate that in large-scale H₂CO can trace warm regions like ¹²CO and ¹³CO. The H₂CO is better correlated with the ¹³CO than with the ¹²CO especially in the Mon R2, S156, L906, and M17/M18 regions. The DR17 region shows a prominent feature that the ¹²CO and ¹³CO are not as strong as H₂CO. Toward four regions, the ¹²CO, ¹³CO, and H₂CO cloud distribution ranges gradually decrease with these three molecules. The H₂CO maps agree well with the IR maps toward the Mon R2 and DR17 regions. Towards the M17 region, the H₂CO intensity peak is at about 10′ (~4 pc) offset from the 8.28 μm MSX peak. The H₂CO absorption probably has no relation with the MSX emission in the S156 region.

The relation between H₂CO fluxes and those of ¹²CO and ¹³CO are shown in Fig. 3. The best-fitted straight lines are log  (F_¹²CO) = (0.54 ± 0.07)log  (F_H2CO) + (1.65 ± 0.07) (K km s^-1) and log  (F_¹³CO) = (0.83 ± 0.07)log  (F_H2CO) + (1.09 ± 0.07) (K km s^-1). The equations show that F_H2CO is linearly well correlated with ${\mathit{F}}_{{\mathrm{12}}^{}\mathrm{CO}}$ and ${\mathit{F}}_{{\mathrm{13}}^{}\mathrm{CO}}$; the correlation coefficients are 0.58 and 0.73. The good relation between F_H2CO and ${\mathit{F}}_{{\mathrm{13}}^{}\mathrm{CO}}$ is an indication that the H₂CO and ¹³CO lines are located in similar environments.

Statistical histograms of the Gaussian fitting widths of H₂CO, ¹²CO, and ¹³CO (Fig. 4a) shows that the H₂CO line width range is from 1.2 to 8.5 km s^-1, and the average line width  ⟨ ΔV(H₂CO) ⟩  is 2.5 km s^-1. All the H₂CO line widths observed exceed the thermal line widths 0.3 km s^-1. The thermal line widths for ¹²CO and ¹³CO is about 0.2 km s^-1, which is lower than all the ¹²CO and ¹³CO line widths observed. In addition, the widest hyperfine structure components of the H₂CO line are separated by about 0.8 km s^-1 (Young et al. 2004; Troscompt et al. 2009). This is less than our velocity resolution. The contribution of hyperfine structure broadening and thermal broadening to the measured H₂CO line widths is therefore likely to be small to moderate. Ninety percent of H₂CO line widths are distributed in the range of ΔV(H₂CO) < 4 km s^-1, and the distribution range of ΔV(H₂CO) is less than for ¹²CO and ¹³CO. The main distributions and average line widths of H₂CO and ¹³CO are similar. There are no obvious correlations between the line widths of H₂CO, ¹²CO, and ¹³CO. The frequency distribution of the peak optical depths of H₂CO and ¹³CO spectra (Fig. 4b) shows that nearly all the H₂CO optical depths are lower than those of ¹³CO. The average optical depth of H₂CO is  ⟨ τ(H₂CO) ⟩ ~ 0.037, which can be compared to the value 0.055 quoted by Downes et al. (1980) for 262 galactic radio sources. The fairly low optical depth indicates that the H₂CO spectra is optically thin in almost all the regions observed.

The correlation between H₂CO and ¹³CO peak column density (Fig. 5) shows that the H₂CO peak column density range is 1.1 × 10¹²–6.3 × 10¹³ cm^-2, which is similar to the value range quoted by Federman et al. (1990) for the dense interstellar clouds. The column density corresponds to the MON R2, S156, L906, and M17 H₂CO intensity peak positions, and parts of M18 northern region are distributed in a box of the range N(H₂CO) < 1.7 × 10¹³ cm^-2 and N(¹³CO) > 4.7 × 10¹⁵ cm^-2, which shows a lack of H₂CO. With the exception of this box, the remaining data points show a strong correlation, and the best fit slope is N(H₂CO)/N(¹³CO) = (4.1 ± 0.2) × 10^-3. The N(H₂CO)/N(¹³CO) ratio against offset position towards four regions (Fig. 6) shows that the N(H₂CO)/N(¹³CO) ratios increase from the center to the edge regions of the molecular cloud where the N(H₂CO)/N(¹³CO) ratio increases about two to three times.

5. Conclusions

We observed and mapped large areas in four regions of MON R2, S156, DR17/L906, and M17/M18 using the H₂CO (1₁₀–1₁₁) absorption, H110α recombination, 6 cm continuum, ¹²CO (1–0), and ¹³CO (1–0) emissions. The H₂CO distributions are similar to ¹²CO and ¹³CO distributions in four regions, with the ¹³CO distribution better correlated with the H₂CO distributions than the ¹²CO distribution. The H₂CO and ¹³CO tracers systematically provide consistent views of the dense regions, in which their maps have similar shapes, sizes, peak positions, and molecular spectra, presenting similar central velocities and line widths. Such good agreement indicates that the H₂CO absorption and the ¹³CO emission lines both arise in similar regions. The H₂CO and ¹³CO column density ratio N(H₂CO)/N(¹³CO) changes with different n(H₂) density regions in the molecular cloud. From the center to the edges of the molecular cloud, the N(H₂CO)/N(¹³CO) ratio increases about two to three times.

Fig. 4 a) Histogram showing the measured widths for H2CO, 12CO, and 13CO. b) Histogram showing the histogram of the peak optical depths of H2CO and 13CO distribution. Parts of H2CO data are selected from Zhang et al. (2012).

Fig. 5 Correlation between the H2CO and 13CO peak column density. Straight line is the best-fit line for the source outside the range of N(H2CO)  <  1.7 × 1013 cm-2 and N(13CO)  >  4.7 × 1015 cm-2.

Fig. 6 Derived H2CO and 13CO peak column density radio N(H2CO)/N(13CO) against position along Dec for Δα = 0 towards the MON R2, S156, and M17 regions. Towards DR17/L906 region Δα = 10.

Online material

Appendix A: The ¹²CO, ¹³CO and continuum maps

Fig. A.1 The integrated intensities of the 12CO maps of integrated area toward a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The color-scale maps are the same as Fig. 1. a) For the MON R2 region: 12CO contour levels are from 12.80 to 34.14 in steps of 4.27 K km s-1. b) For the S156 region: 12CO contour levels are from 12.63 to 33.69 in steps of 4.21 K km s-1. c) For the DR17/L906 region: 12CO contour levels are from 12.72 to 39.26 in steps of 4.91 K km s-1. d) For the M17/M18 region: 12CO contour levels are from 29.34 to 78.24 in steps of 9.78 K km s-1.

Fig. A.2 Position-velocity diagrams of 12CO for a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The positions for 12CO are the same as Fig. 2. For the MON R2, S156, and M17/M18 regions, contour levels are 3 to 8 in steps of 1 K. Contour levels are 1.3 to 6.3 in steps of 1 K for the DR17/L906 region.

Fig. A.3 The integrated intensities of the 13CO maps of integrated area toward a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The color-scale maps are the same as Fig. 1. a) For the MON R2 region: 13CO contour levels are from 2.66 to 7.10 in steps of 0.89 K km s-1. b) For the S156 region: 13CO contour levels are from 2.53 to 6.76 in steps of 0.84 K km s-1. c) For the DR17/L906 region: 13CO contour levels are from 2.64 to 7.05 in steps of 0.88 K km s-1. d) For the M17/M18 region: 13CO contour levels are from 7.83 to 20.89 in steps of 2.61 K km s-1.

Fig. A.4 Position-velocity diagrams of 13CO for a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The positions for 13CO are the same as in Fig. 2. a) For the MON R2 region: contour levels are 0.5 to 3 in steps of 0.5 K. b) For the S156 region: contour levels are 0.6 to 2.1 in steps of 0.3 K. c) For the DR17/L906 region: contour levels are 0.3 to 1.8 in steps of 0.3 K. d) For the M17/M18 region: contour levels are 0.8 to 3.8 in steps of 0.5 K.

Fig. A.5 Location of the radio continuum emission (gray scale) overlaid on the H2CO observation lines (black contour lines) and H110α emission lines (dash contour lines) toward a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. H2CO contour levels are the same as Fig. 1 for four regions. For the DR17/L906 region: H110α contour levels are 0.67, 0.89, 1.11, 1.33, 1.56, and 1.78 K km s-1; for the M17/M18 region: H110α contour levels are 2.91, 4.36, 5.81, 7.26, 8.72, 10.17, and 11.62 K km s-1. The gray bars are given in units of K.

Appendix B: Line parameters and spectra

Fig. B.1 The spectra of a) H2CO, b) 12CO, and c) 13CO lines toward MON R2 and S156 regions. And the spectra of a) H2CO, b) H110α, c) 12CO, and d) 13CO lines toward DR17/L906 and M17/M18 regions.

Acknowledgments

We thank all the staff of Nanshan Observatory and Delingha of Purple Mountain Observatory for observations. And we thank Z. B. Jiang, Z. W. Chen, and J. Y. Li for providing the CO data of the M17/M18 region. This work was funded by The National Natural Science foundation of China under grant 10778703, and partly supported by the China Ministry of Science and Technology under State Key Development Program for Basic Research (2012CB821800) and The National Natural Science foundation of China under grant 10873025.

References

All Tables

All Figures

Fig. 1 Contours and color-scale maps of integrated area toward a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The contour and color-scale map respectively represent the integrated intensities of the H2CO and the mid-infrared 8.28 μm MSX emission in S156, DR17/L906, and M17/M18 regions. For the MON R2 region, the color-scale map represents the IRAS 12 μm data. a) For the MON R2 region: contour levels are –0.17 to –0.46 in steps of –0.06 K km s-1. b) For the S156 region: contour levels are –0.09 to –0.25 in steps of –0.03 K km s-1. c) For the DR17/L906 region: contour levels are –0.12 to –0.32 in steps of –0.04 K km s-1. d) For the M17/M18 region: contour levels are –0.42 to –1.11 in steps of –0.14 K km s-1.

In the text

	Fig. 2 Position-velocity diagrams of H2CO for a) MON R2 and b) S156, regions along Dec for Δα = 0, for c) DR17/L906 region from offset (20, –30) to (–10, 20), and d) M17/M18 region from offset ( − 30, − 40) to (0, 30). For the MON R2, S156, and DR17/L906 regions: contour levels are –0.03 to –0.08 in steps of –0.01 K. For the M17/M18 region: contour levels are –0.03 to –0.18 in steps of –0.03 K.
In the text

	Fig. 3 Correlation between the H2CO line flux and 12CO, 13CO line flux at corresponding points in MON R2, S156, DR17/L906, and M17/M18. The solid line is the linear fit for H2CO and 12CO flux data, and the dashed line is the linear fit for H2CO and 13CO.
In the text

	Fig. 4 a) Histogram showing the measured widths for H2CO, 12CO, and 13CO. b) Histogram showing the histogram of the peak optical depths of H2CO and 13CO distribution. Parts of H2CO data are selected from Zhang et al. (2012).
In the text

	Fig. 5 Correlation between the H2CO and 13CO peak column density. Straight line is the best-fit line for the source outside the range of N(H2CO)  <  1.7 × 1013 cm-2 and N(13CO)  >  4.7 × 1015 cm-2.
In the text

	Fig. 6 Derived H2CO and 13CO peak column density radio N(H2CO)/N(13CO) against position along Dec for Δα = 0 towards the MON R2, S156, and M17 regions. Towards DR17/L906 region Δα = 10.
In the text

Fig. A.1 The integrated intensities of the 12CO maps of integrated area toward a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The color-scale maps are the same as Fig. 1. a) For the MON R2 region: 12CO contour levels are from 12.80 to 34.14 in steps of 4.27 K km s-1. b) For the S156 region: 12CO contour levels are from 12.63 to 33.69 in steps of 4.21 K km s-1. c) For the DR17/L906 region: 12CO contour levels are from 12.72 to 39.26 in steps of 4.91 K km s-1. d) For the M17/M18 region: 12CO contour levels are from 29.34 to 78.24 in steps of 9.78 K km s-1.

In the text

	Fig. A.2 Position-velocity diagrams of 12CO for a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The positions for 12CO are the same as Fig. 2. For the MON R2, S156, and M17/M18 regions, contour levels are 3 to 8 in steps of 1 K. Contour levels are 1.3 to 6.3 in steps of 1 K for the DR17/L906 region.
In the text

Fig. A.3 The integrated intensities of the 13CO maps of integrated area toward a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The color-scale maps are the same as Fig. 1. a) For the MON R2 region: 13CO contour levels are from 2.66 to 7.10 in steps of 0.89 K km s-1. b) For the S156 region: 13CO contour levels are from 2.53 to 6.76 in steps of 0.84 K km s-1. c) For the DR17/L906 region: 13CO contour levels are from 2.64 to 7.05 in steps of 0.88 K km s-1. d) For the M17/M18 region: 13CO contour levels are from 7.83 to 20.89 in steps of 2.61 K km s-1.

In the text

Fig. A.4 Position-velocity diagrams of 13CO for a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. The positions for 13CO are the same as in Fig. 2. a) For the MON R2 region: contour levels are 0.5 to 3 in steps of 0.5 K. b) For the S156 region: contour levels are 0.6 to 2.1 in steps of 0.3 K. c) For the DR17/L906 region: contour levels are 0.3 to 1.8 in steps of 0.3 K. d) For the M17/M18 region: contour levels are 0.8 to 3.8 in steps of 0.5 K.

In the text

Fig. A.5 Location of the radio continuum emission (gray scale) overlaid on the H2CO observation lines (black contour lines) and H110α emission lines (dash contour lines) toward a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. H2CO contour levels are the same as Fig. 1 for four regions. For the DR17/L906 region: H110α contour levels are 0.67, 0.89, 1.11, 1.33, 1.56, and 1.78 K km s-1; for the M17/M18 region: H110α contour levels are 2.91, 4.36, 5.81, 7.26, 8.72, 10.17, and 11.62 K km s-1. The gray bars are given in units of K.

In the text

	Fig. B.1 The spectra of a) H2CO, b) 12CO, and c) 13CO lines toward MON R2 and S156 regions. And the spectra of a) H2CO, b) H110α, c) 12CO, and d) 13CO lines toward DR17/L906 and M17/M18 regions.
In the text