© ESO, 2013

1. Introduction

The influence of high-mass stars on the interstellar medium is tremendous. During their process of formation, they are sources of powerful, bipolar outflows (e.g., Beuther et al. 2002), their strong ultraviolet and far-ultraviolet radiation fields give rise to bright Hii and photon dominated regions (PDRs) and during their whole lifetime powerful stellar winds interact with the surroundings. Finally, their short life ends in a violent supernova explosion, injecting heavy elements into the interstellar medium and possibly triggering further star formation with the accompanying shocks. These are also the type of regions that dominate far-infrared observations of starburst galaxies.

Most studies of massive star formation focus on emission peaks at infrared or submillimetre wavelengths, which correspond to peaks in the temperature and/or mass distribution. The aim of our work is to characterise the effects of clustered star formation and feedback of massive stars on the surrounding medium. We have made APEX maps of three cluster-forming regions (G327.3–0.6, NGC6334 and W51) in mid-J¹²CO ((6–5) and (7–6)) and ¹³CO transitions ((6–5) and (8–7)) in order to have a direct measure of the excitation of the warm extended inter-clump gas between dense cores in the cluster (see for example Blitz & Stark 1986; Stutzki & Güsten 1990). Our sample of sources was chosen among six nearby cluster-forming clouds mapped in water and in the ¹³CO(10–9) transition as part of the Water in Star-Forming Regions with Herschel (WISH; van Dishoeck et al. 2011) guaranteed time key program (GT-KP) for the Herschel Space Observatory (Pilbratt et al. 2010).

In this paper, we present the ¹²CO and ¹³CO maps of the star-forming region G327.3–0.6 at a distance of 3.3 kpc (Urquhart et al. 2011, based on H i absorption). G327.3–0.6 is well suited to study cluster-forming clouds because of its relatively close distance and because several sources in different evolutionary phases coexist in a small region, as found by Wyrowski et al. (2006). Our maps (with a linear extension of ~3 pc × 4 pc) cover the Hii region G327.3–0.5 (Goss & Shaver 1970) associated with a luminous PDR, and an infrared dark cloud (IRDC; Wyrowski et al. 2006) hosting the bright hot core G327.3–0.6 (Gibb et al. 2000) and the extended green object (EGO) candidate G327.30–0.58 (Cyganowski et al. 2008). Extended green objects are identified through their extended 4.5 μm emission in the Spitzer IRAC2 band, which is believed to trace outflows from massive young stellar objects (YSOs; Cyganowski et al. 2008).

This paper is organised as follows: in Sect. 2 we present the APEX and Herschel¹ observations of G327.3–0.6, in Sect. 3 we discuss the morphology and kinematics of the ¹²CO and ¹³CO emission, in Sect. 4 we investigate the physical conditions of the emitting gas. Finally, in Sect. 5 we discuss our results and compare to similar observations performed towards low- and high-mass star forming regions. Our results are summarised in Sect. 6.

2. Observations

2.1. APEX telescope

The CHAMP⁺ (Kasemann et al. 2006; Güsten et al. 2008) dual colour heterodyne array receiver of 7 pixels per frequency channel on the APEX telescope² was used in September 2008 to simultaneously map the star-forming region G327.3–0.6 in the ¹²CO (6–5) and (7–6) lines and, in a second coverage, the ¹³CO (6–5) and (8–7) transitions. The region from the hot core in G327.3–006 to the H ii region G327.3–0.5 (Fig. 1) was covered with on-the-fly maps of 200′′ × 240′′ spaced by 4′′ in declination and right ascension.

We used the fast Fourier transform spectrometer (FFTS, Klein et al. 2006) as backend with two units of fixed bandwidth of 1.5 GHz and 8192 channels per pixel. We used the two IF groups of the FFTS with an offset of  ± 460 MHz between them. The original resolution of the dataset is 0.3 km s^-1; the spectra were smoothed to 1 km s^-1 for a better signal-to-noise ratio.

The observations were performed under good weather conditions with a precipitable water vapour level between 0.5 and 0.7 mm. Typical single side band system temperatures during the observations were around 1600 K and 5200 K, for the low and high frequency channel respectively. The conversion from antenna temperature units to brightness temperatures was done assuming a forward efficiency of 0.95 for all channels, and a main beam efficiency of 0.48 for the ¹²CO and ¹³CO (6–5) observations, 0.45 for the ¹²CO (7–6) data, and 0.44 for ¹³CO(8–7), as measured on Jupiter in September 2008³. The pointing was checked on the continuum of the hot core G327.3–0.6 (). The maps were produced with the XY_MAP task of CLASS90⁴, which convolves the data with a Gaussian of one third of the beam: the final angular resolution is 94 for the low frequency data, 81 for the high frequency.

2.2. Herschel space observatory

The ¹³CO (10–9) line (see Table 1) was mapped (size = 210″ × 270″) with the HIFI instrument (de Graauw et al. 2010) towards G327.3-0.6 on February, 18th, 2011 (observing day – OD) 645, observing identification number (OBSID) 1342214421. The centre of the map is . The observations are part of the WISH GT-KP (van Dishoeck et al. 2011). Data were taken simultaneously in H and V polarisations using both the acousto-optical Wide-Band Spectrometer (WBS) with 1.1 MHz resolution and the correlator-based high-resolution spectrometer (HRS) with 250 kHz nominal resolution. In this paper we present only the WBS data. We used the on-the-fly mapping mode with Nyquist sampling. HIFI receivers are double sideband with a sideband ratio close to unity. The double side band system temperatures and total integration times are respectively 384 K and 3482 s. The rms noise level at 1 km s^-1 spectral resolution is ~0.1 K. Calibration of the raw data onto T_A scale was performed by the in-orbit system (Roelfsema et al. 2012); conversion to T_mb was done with a beam efficiency of 0.74 and a forward efficiency of 0.96. The flux scale accuracy is estimated to be around 15% for band 3. Data calibration was performed in the Herschel Interactive Processing Environment (HIPE, Ott 2010) version 6.0. Further analysis was done within the CLASS90 package. After inspection, data from the two polarisations were averaged together.

The original angular resolution of the data is 190. The final maps were produced with the XY_MAP task of CLASS90 and have an angular resolution of 211.

3. Observational results

3.1. Morphology

Figure 1 shows the ¹²CO(6–5) integrated intensity map overlaid on the composite image of the IRAC Spitzer 3.6, 4.5 and 8.0 μm bands of the region. The ¹²CO emission traces the distribution of the 8.0 μm emission, but it is also associated with the infrared dark cloud found on the east of the hot core. The EGO candidate G327.30–0.58 identified by Cyganowski et al. (2008) and clearly visible in Fig. 1, is also detected in the ¹²CO data as secondary peak of emission (Fig. 2). The map of the integrated intensities of the ¹²CO(7–6) line is also presented in Fig. 2 together with the integrated intensity of the ¹²CO(3–2) line from Wyrowski et al. (2006). Figure 3 shows the distribution of the ¹³CO(6–5), (8–7) and (10–9) emissions. The accuracy of the relative pointings was checked on the hot core G327.3–0.6. For this purpose, we derived integrated intensity maps of lines detected only towards this position, which are close in frequency to the ¹²CO(6–5), ¹³CO(6–5) and ¹³CO(8–7) transitions, and therefore were observed simultaneously to the current dataset. From these data, we infer a position for the hot core in agreement with interferometric measurements at 3 mm (Wyrowski et al. 2008, and Table 2) within ~15.

All observed ¹²CO transitions trace the Hii region G327.3–0.5 as well as the infrared dark cloud which hosts the hot core G327.3–0.6. Moreover, the ¹²CO(6–5), (7–6) and ¹³CO(6–5) lines show extended emission along a ridge running approximately N-S that matches very well with the distribution of the CO(3–2) transition. The hourglass shape hole to the west of the Hii region G327.3–0.5 where the ¹²CO(3–2) emission is strongly reduced (see Wyrowski et al. 2006) is seen also in the ¹²CO(6–5) and (7–6) lines which, although much weaker than in the rest of the map, are still detected at this position.

All transitions peak towards the Hii region G327.3–0.5 where the main isotopologue lines have intensities up to 60–65 K. The integrated intensities of the CO isotopologues show a distribution along a ring-like structure around the peak of the cm continuum emission (Goss & Shaver 1970). The centre of the ring also coincides with the massive young stellar object number 87 identified in the near-infrared by Moisés et al. (2011). Since the ring is detected also in high-J transitions of ¹³CO, it is plausible that this morphology is true and not due to optical depth effects. This structure likely coincides with the limb brightening of the hot surface of a PDR around G327.3–0.5 and could trace an expanding shell. We will investigate this scenario in Sect. 3.2.

The hot core G327.3–0.6 shows up as a secondary peak in the integrated intensity maps of the ¹³CO transitions, while the main CO isotopologue peaks to its north-west, probably because of optical depth effects. Strong self-absorption profiles are indeed detected in all ¹²CO lines towards the hot core (see Sect. 3.2). The submillimetre source SMM6 (seen in the continuum emission at 450 μm by Minier et al. 2009) is detected as a peak of emission in all integrated intensity maps of ¹²CO and in ¹³CO(6–5), although at the edge of the mapped region. The other submillimetre sources are also marked in Figs. 2. The EGO candidate G327.30–0.58 is also detected in the ¹³CO(6–5) map (Fig. 3). The ¹³CO(6–5) traces the whole IRDC and not only the active site of star formation where the EGO is detected. The continuum emission due to dust (seen for example at 870 μm in Fig. 3) follows the distribution of the ¹³CO lines.

In Fig. 4 we show the ratio of the integrated intensity of the ¹²CO(6–5) transition (convolved to the 18′′ resolution of the ¹²CO(3–2) data) to that of the ¹²CO(3–2) line. This ratio ranges between 0.3 and 1.8; it has values slightly larger than one towards the Hii region (1.2 at its centre), while it is about unity towards the hot core. The peak is found south-west of the Hii region G327.3–0.5, where both lines are detected with a high confidence level. However, these results could be biased by the strong self absorption in both ¹²CO lines (see Sect. 3.2). For this reason, we computed the ratio between the two transitions in four velocity ranges to cross-check the results of Fig. 4. The inferred values, however, do not change significantly.

3.2. Line profiles and velocity field

The widespread ¹²CO(6–5) emission shows line profiles with a typical width of  ~8 km s^-1 in the gas between G327.3–0.5 and the infrared dark cloud. Broader profiles are detected in the infrared dark cloud and in the northern part of the Hii region G327.3–0.5. Figure 5 shows the distribution of the line width of the ¹²CO(6–5) transition: the ¹²CO(6–5) line width follows an arc-like structure that connects the Hii region G327.3–0.5 to the infrared dark cloud where the hot core is. Interestingly, the same morphology is seen in the LABOCA map of the region (Schuller et al. 2009). Line widths are similar for all ¹²CO lines, while they are consistently narrower in the ¹³CO transitions.

Representative spectra of all ¹²CO transitions analysed in this study are presented in Fig. 6 towards the hot core, the IRDC position ((30′′, 30′′) from the centre of the APEX maps, see Sect. 4.2 and Figs. 2, 3) and the peak of the cm continuum emission in G327.3–0.5. Spectra of the ¹³CO transitions are shown in Fig. 7. Red- and blue-shifted wings are detected in the ¹²CO lines in a velocity range between −71 and −24 km s^-1 (in ¹²CO(6–5)) towards G327.3–0.6 probably due to outflow motions. However, no sign of bipolar outflows is found when inspecting the integrated intensity maps of the blue- and red-shifted wings nor in position-velocity diagrams (Fig. 8). Moreover, very similar broad lines are detected along the whole extent of the infrared dark cloud, as shown in the top panel of Fig. 8. At the IRDC position, the wings in ¹²CO(6–5) range from −60 to −31 km s^-1. All main isotopologue transitions analysed in this paper are affected by self-absorption (see Fig. 6 for reference spectra towards the hot core, the IRDC position and the Hii region); moreover, even the ¹³CO(6–5) line shows weak evidence of self-absorbed profile towards the hot core. Figure 9 shows the ¹²CO(7–6) spectra overlaid on the continuum emission at 870 μm: the self-reversed profile is spread over a large area and seems to follow the thermal dust continuum emission.

Finally, the velocity field of the ¹²CO transitions may help us to understand the nature of the ring detected towards the Hii region G327.3–0.5. We therefore used the task KSHELL built in the visualisation software package KARMA (Gooch 1996). KSHELL computes an average brightness temperature on annuli about a user defined centre. A spherically symmetric expanding shell will look like a half ellipse in a (r − v) diagram with the axis in the v direction twice the expansion velocity. Figure 10 shows the resulting (r − v) diagram obtained with the ¹²CO(6–5) data cube using the peak of the cm continuum emission as centre. The emission does not follow a perfect spherical shell. This is likely due to inhomogeneities in the distribution of the gas, as already seen in Fig. 11 where the distribution of the optical depth of ¹³CO is not symmetric.

4. Physical conditions of the warm gas

4.1. LTE analysis

From the line ratio of the ¹²CO(6–5) to ¹³CO(6–5) transitions we can derive the optical depth of the ¹²CO(6–5) line emission, which can be then used to infer the excitation temperature of the line and the column density of ¹²CO in the region.

The line intensity in a given velocity channel of a given transition is (1)where η is the beam filling factor (assumed to be 1 in the following analysis), F_ν = hν/k × [exp(hν/kT) − 1] ^-1, T_cbg = 2.7 K, and τ_ν is the optical depth. Under the local thermodynamic equilibrium (LTE) assumption, T_ex is assumed to be equal to the kinetic temperature of the gas and equal for all transitions. In the following analysis, we study the peak intensities of the ¹²CO(6–5) and ¹³CO(6–5) lines, and include only the cosmic background as background radiation and neglect, for example, any contribution from infrared dust emission since we do not have any map of the distribution of the dust temperature. This most likely affects only our estimates at the hot core position and possibly towards the Hii region G327.3–0.5 where SABOCA continuum emission at 350 μm is also detected (Wyrowski et al., in prep.). For an appropriate analysis of the emission from the hot core, see Rolffs et al. (2011).

Assuming that the ¹²CO(6–5) emission is optically thick and that the ¹²CO(6–5) and ¹³CO(6–5) lines have the same excitation temperatures, the optical depth of the ¹³CO(6–5) transition, , is (2)The optical depth of the ¹²CO(6–5) transition can then be obtained by multiplying for the abundance of ¹²CO relative to ¹³CO, X_{¹²CO/¹³CO} ~ 60 (Wilson & Rood 1994). From the optical depth of the ¹²CO(6–5) line, one can also derive its excitation temperature using Eq. (1). Figure 11 shows the distribution of the optical depth of the ¹³CO(6–5) line and of the excitation temperature of ¹²CO(6–5). The ¹³CO(6–5) emission is moderately optically thick (0.6–0.7) at the Hii region and at the infrared dark cloud, while it reaches values of  ~1.2 at the hot core position and in a small part of ring around the Hii region. The map distribution of the excitation temperature of the ¹²CO(6–5) line is shown in the bottom panel of Fig. 11. The map is dominated by the Hii region, where T_ex reaches values of 80 K in the ring around the Hii region and then decreases with increasing distance from it. The hot core and the rest of the infrared dark cloud have values around 30–35 K. The excitation temperature increases to the south west of the hot core, in a region where there is also 8 μm emission, and to the north-east of the Hii along a layer of gas also visible in the ¹²CO(6–5) integrated intensity map (see Fig. 2), but more prominent in the T_ex map and in the 8 μm emission map (see Fig. 1).

From the optical depth and the excitation temperature of the ¹²CO(6–5) line, we derived the H₂ column density assuming a relative abundance of ¹²CO relative to H₂ of 2.7 × 10^-4 (Lacy et al. 1994). Results are shown in Fig. 12. The largest column density is found towards the hot core (~3 × 10²² cm^-2 in the 94 beam of the ¹³CO(6–5) data) and decreases along the infrared dark cloud with a distribution similar to that of the 870 μm continuum emission. Three peaks around 10²² cm^-2 are found in the Hii region.

We cross-checked our results by computing the H₂ column density under the assumption that the ¹³CO emission is optically thin. The results are consistent with those presented in Fig. 12; the largest differences (of the order of 30%) are found towards those positions where the optical depth of the ¹³CO(6–5) transition (Fig. 11) exceeds  ~0.7. The assumption of optically thin emission for ¹³CO may be particularly useful for the inter-clump medium (arbitrarily defined as the region in the map where the ¹³CO lines are not detected on individual spectra), towards which we infer H₂ column densities of  ~2 × 10²¹ cm^-2 corresponding to a ¹³CO column density of  ~10¹⁶ cm^-2 (see also Sect. 5.1).

We also computed column densities and rotational temperatures in the region using the rotational diagram technique applied to the ¹³CO data. We did not include the ¹²CO lines in the analysis because of their complex line profiles and high optical depths. Given the optical depth previously derived for the ¹³CO(6–5) line, we did not apply any correction due to optical depth effects to the ¹³CO data. Results are consistent with the estimates based on Eq. (1). The main differences between the two analyses are found towards the hot core, where the rotational temperature is higher than the excitation derived with Eq. (1) (T_rot ~ 70 K and T_ex ~ 32 K) and the H₂ column density lower (10²² cm^-2 versus 3 × 10²² cm^-2 obtained with the first method). The differences between the two methods are likely influenced by the self-absorption profile detected in the ¹²CO(6–5) line which results in lower line intensities towards the regions of large column densities. In particular, while the first approach overestimates the optical depths (and hence column density) because of the self absorption, the column density derived with the rotational diagram analysis is likely underestimated towards the hot core because of the optically thin emission assumption, and should be corrected by a factor τ_LTE/(1−exp(τ_LTE)−) ~ 1.7.

Finally, we computed the ratio between the amount of warm gas (traced by ¹²CO and ¹³CO(6–5) and shown in Fig. 12) and the total amount of gas (traced by the continuum emission at 870 μm) as derived assuming a dust temperature equal to the excitation temperature of ¹²CO(6–5) and a dust opacity of 0.0182 cm² g¹ (Kauffmann et al. 2008). The results are shown in Fig. 13: the warm gas is only a small percentage (~10%) of the total gas in the infrared dark cloud, while it reaches values up to ~35% of the total gas in the ring surrounding the Hii region.

4.2. ¹³CO ladder

Since the G327.3–0.6 region was mapped in three different transitions of the ¹³CO molecule, we can perform a multi-line analysis towards selected positions and infer the parameter of the gas. The advantage of ¹³CO compared to the main isotopologue is in the lower opacities of the lines and in the less complex line profiles. For this analysis, we used the RADEX program (van der Tak et al. 2007) with expanding sphere geometry. The molecular dataset comes from the LAMDA database (Schöier et al. 2005) and includes collisional rates adapted from Yang et al. (2010). We ran models with temperatures from 20 to 200 K, densities in the range 10⁴−5 × 10⁷ cm^-3, and ¹³CO column densities between 10¹⁴ and 10¹⁹ cm^-2.

All data were smoothed to the resolution of the ¹³CO(10–9) map. We selected three positions for the analysis: the hot core, the IRDC position ((30′′, 30′′) from the centre of the APEX maps), and the centre of the Hii region. The IRDC position was selected to be a position associated with high column density in the infrared dark cloud (see Figs. 2, 3 and 12) but without IR emission. However, it is only 10″ to the north of the EGO candidate (Cyganowski et al. 2008, see Figs. 2, 3), and therefore, given the beam of the observations, contamination from the embedded YSO may still be possible. Table 3 reports the measured line parameters of the ¹³CO transitions obtained with Gaussian fits; based on these values, we adopt line widths of 6, 3 and 7, for the hot core, the IRDC and the Hii respectively, in agreement with values reported by San José-García et al. (2012) for the ¹³CO(10–9) line towards a sample of intermediate- and high-mass sources. The spectra are shown in Fig. 7.

The results of the RADEX analysis are listed in Table 4 and shown in Fig. 14. For the IRDC position, the ¹³CO(10–9) line intensity is not well fitted by our one-temperature model. This likely reflects the fact that the ¹³CO(10–9) spectrum is dominated by the embedded source while the other two lines sample colder gas in the envelope. We considered a 20% calibration error for the ¹³CO(6–5) and (8–7) observations and a 15% error for the ¹³CO(10–9) data. Wyrowski et al. (2006) mapped the region with the APEX telescope in the C¹⁸O(3–2) line. Therefore, since no observations were performed in the ¹³CO(3–2) transition, we included the C¹⁸O(3–2) data in Fig. 14. Note however, that the C¹⁸O(3–2) fluxes are not included in the fitting procedure, but that they are simply used to cross-check results. Since the C¹⁸O(3–2) flux corresponds to a lower limit to the flux of ¹³CO(3–2) line, we also plotted the C¹⁸O(3–2) flux corrected for the abundance ratio of ¹³CO to C¹⁸O, X_{¹³CO/C¹⁸O} ~ 8 (Wilson & Rood 1994). This value is likely an upper limit to the flux of the ¹³CO(3–2) line due to opacity effects.

An example of the χ² distribution projected on to the T − n plane is shown in Fig. 15 for the Hii position, where the reduced χ² at the best fit position is 3. Figure 15 shows the typical inverse n − T relationship often seen in χ² distributions and due to the fact that density and temperature are, in the case of the CO molecule, not independent parameters (see Appendix C of van der Tak et al. 2007).

We note here that the detection of the ¹³CO(10–9) line breaks the degeneracy between density and temperature typical of ¹²CO analyses (e.g., Kramer et al. 2004; van der Tak et al. 2007) and help to give stronger constraints: indeed, with the exception of the hot core, at least the temperature of the gas is well determined at all positions.

The results of the RADEX analysis confirm that the LTE assumption used in Sect. 4.1 is reasonable since the inferred densities are much larger than the critical densities (a few 10⁴ cm^-3 for all three analysed transitions). Assuming X_{¹²CO/¹³CO} ~ 60 and an abundance of ¹²CO relative to H₂ of 2.7 × 10^-4, the derived ¹³CO column densities listed in Table 4 correspond to H₂ column densities of some 10²² cm^-2 for the hot core and the IRDC position, and of 2 × 10²² cm^-2 for the Hii position. The derived column densities and temperatures are in agreement with those derived in Sect. 4.1 through rotational diagrams of the ¹³CO emission. The ¹³CO(6–5) optical depths are also in agreement with those estimated in Sect. 4.1 through the ¹³CO and ¹²CO(6–5) line ratio, with the exception of the hot core position: τ_LTE ~ 1.2 and τ_RADEX ~ 0.3 for the hot core, 0.6 and 0.5 for the Hii region, and 0.6 and 0.7 for the IRDC position.

We finally stress that the results obtained with the LTE analysis (Sect. 4.1) and with the RADEX code (this section) are based on the assumptions that 1) all lines have a beam filling factor of one, and 2) that the emitting gas is homogeneous, whereas self-absorption profiles in the ¹²CO lines indicate an excitation gradient along the line of sights. For optically thin lines (¹³CO in the current case), a beam filling factor less than one (but equal for all transitions) would mostly affect the column density and result in larger values of N; for optically thick lines, a smaller value of η would imply larger values of density and/or temperature.

The uncertainties on the derived parameters due to the assumption of a homogeneous medium are of less immediate interpretation, and more complex radiative transfer codes (e.g., Hogerheijde & van der Tak 2000) should be used to reproduce the observed line velocity profiles.

5. Discussion

5.1. Total ¹²CO and ¹³CO emission

Figure 16 shows the ¹²CO and ¹³CO ladders obtained by averaging the emission of the different observed transitions, smoothed to the resolution of the HIFI data, over four regions: the total map, the Hii region G327.3–0.5, the IRDC hosting the hot core (the selected region does include the hot core), and finally the inter-clump gas, which was defined as the region in the map where the ¹³CO lines are not detected on individual spectra. This region has an equivalent radius of 50′′(corresponding to  ~0.8 pc at the distance of the source). Examples of ¹²CO line profiles towards the inter-clump gas are shown in Fig. 17. The C¹⁸O(3–2) cannot be used in this analysis because the observations cover a much smaller region than that mapped in ¹³CO. The total mass of the mapped region can be computed using the excitation temperature and H₂ column density distributions shown in Figs. 11 and 12. This corresponds to ~700 M_⊙.

The four regions have similar ¹²CO and ¹³CO ladders. In order to correct for self-absorption, integrated fluxes were obtained for each of the four regions from line fitting of the CO spectra with one Gaussian component. While this works fine for the spectra of the IRDC hosting the hot core and for the inter-clump gas, the line profiles of the total map and of Hii region are red-skewed and therefore the fluxes derived with this method are likely underestimated. For the main isotopologue, the flux of the (7–6) and (6–5) transitions is very similar, although for the IRDC + HC and the inter-clump the flux of the (7–6) line is lower than that of the (6–5) line. On the other hand, in the ¹³CO ladder the peak flux decreases with increasing energy level. For all transitions presented in this paper, the spectra are dominated in intensity by the Hii region, whose flux is of the order of 60% of the total flux for the main isotopologue lines, and ranging from ~49% of the total flux in the (6–5) line to ~85% in the (10–9) transition for ¹³CO (see Table 5). The main difference between the ¹²CO spectra and those of ¹³CO lies in the emission from the inter-clump gas: for all ¹²CO lines, the intensity is relatively strong, 10% of the flux from the whole region. On the other hand, the flux of the ¹³CO lines coming from the inter-clump gas decreases with increasing J, from ~7% of the total flux for J = 6 to ~2% for J = 10. The general behaviour of the ¹²CO and ¹³CO ladders is qualitatively compatible with PDR models from Koester et al. (1994) for high density (10⁶−10⁷ cm^-3) clouds illuminated on one side by a UV radiation field (their model B). In Fig. 16, we show the predicted CO and ¹³CO line intensities for models with a density of 10⁷ cm^-3, incident UV fields with strength 10³ and 10⁴ relative to the average interstellar field (Draine 1978), a visual extinction of 10, and a Doppler broadening of 3 and 1 km s^-1. High densities (n > 10⁶ cm^-3, in agreement with our results from Sect. 4.2) are needed to locate the peak of the CO ladders at mid-, high-J transitions (see Figs. 9–10, 12–13 of Koester et al. 1994). Stronger UV radiation fields also shift the peak of the CO ladders to higher J transitions than observed. In Fig. 16, the ¹²CO model intensities are corrected by a factor 0.2 and 0.25 to correct for different line-widths between the model and the observations, and possibly for beam dilution effects. On the other hand, the ¹³CO results of Fig. 16 are not scaled down by any factor as they are far too weak to match the observations.

Koester et al. (1994) already noticed that the predicted line intensities of mid-J¹³CO transitions in their models are much weaker than observed in star-forming regions. They proposed that mid-J¹³CO emission comes from a large number of filamentary structures, or clumps, along the line of sight. In this way, the modelled line intensity of mid-J¹³CO lines would increase significantly as the lines are optically thin, while it would not change for optically thick transitions.

Rotational diagrams of the ¹³CO emission applied to the spectra of the Hii region, of the IRDC and of the inter-clump gas infer rotational temperatures of 66 K, 47 K and 44 K, respectively, and ¹³CO column densities of 6  ×  10¹⁵ cm^-2 for the Hii region and the IRDC, and of 2  ×  10¹⁵ cm^-2 for the inter-clump gas. Since the inter-clump gas has physical parameters very similar to those of the IRDC region, but a much lower column density, we suggest that it is composed of high-density clumps with low filling factors. This is again in agreement with PDR models (e.g., Koester et al. 1994; Cubick et al. 2008) which predict strong emission at mid-J¹³CO and high-J¹²CO lines in the case of small, low mass, high density clumps.

Cubick et al. (2008) suggested that the COBE ¹²CO ladder of the Milky Way can be reproduced by a clumpy PDR model, and that the bulk of the Galactic FIR line emission comes from PDRs around the Galactic population of massive stars. Our observations seem to confirm this result, since the CO emission of the G327.3–0.6 region is dominated by the PDR around the Hii region. Our results are also consistent with the findings from Davies et al. (2011) and Mottram et al. (2011). These authors studied the properties of massive YSOs and compact Hii regions in the RMS survey (Hoare et al. 2005), and found that there is no significant population of massive YSOs above ~10⁵  L_⊙, while compact Hii regions are detected up to ~10⁶  L_⊙. Since high-J CO lines are among the most important cooling lines in PDR, they reflect the luminosity of their heating sources: if the luminosity distribution of massive stars in the Galaxy is dominated by Hii regions and not by younger massive stars, then we also expect that the CO distribution follows the same rule.

5.2. Comparisons with other star forming regions

Large-scale mapping of some low- and high-mass star forming regions was performed in several ¹²CO transitions. However, given the different critical densities of the lines, we prefer to compare our results with studies carried on with J > 3 CO lines. Excitation temperatures around 8–30 K are found in extended diffuse emission and towards the brightest positions, respectively, in low-mass star forming regions (e.g., Davis et al. 2010; Curtis et al. 2010), while massive star-forming regions are usually warmer as confirmed by our findings (see also Wilson et al. 2001; Jakob et al. 2007; Kirk et al. 2010; Wilson et al. 2011; Peng et al. 2012). The CO column densities (10¹⁷–10¹⁸ cm^-2 from the LTE analysis) found in G327 are also comparable with values obtained in other massive star-forming regions (Wilson et al. 2001; Jakob et al. 2007; White et al. 2010; Wilson et al. 2011).

Comparison with ¹²CO large maps of other high-mass star forming regions would be important to verify whether our result that the ¹²CO distribution is dominated by the Hii region is a common feature or not. However, most studies do not cover different evolutionary phases as in our case. For OMC-1, Peng et al. (2012) confirmed that the peak of the integrated intensity of several CO isotopologue lines is close to the Orion-KL hot core (although Orion-south and the Orion Bar PDR are also very prominent). One should notice however, that Orion is roughly six times closer to the Sun than G327.3–0.6, and therefore the Orion Bar and Orion-KL would be much closer on sky (~30″) if one would place them at the distance of G327.3–0.6. Moreover, Orion-KL likely represents a special case since it hosts a very powerful outflow (e.g., Kwan & Scoville 1976; Snell et al. 1984), which could alter the distribution of ¹²CO in the region. Indeed, Marrone et al. (2004) show that broad velocity emission arises mainly from the Orion-KL region, while much of the narrower emission arises from the PDR excited by the M42 Hii region.

5.3. Self-absorption profiles

As noted in Sect. 3.2, all ¹²CO transitions analysed in this paper are affected by self-absorption, which is likely to be due to cold gas surrounding a warmer component (Phillips et al. 1981). In particular along the infrared dark cloud (Fig. 8), the ¹²CO(6–5) line has blue-skewed profiles in the north-east (towards the EGO and the IRDC positions) and the red-skewed ones towards the south-west (the hot core). Blue- and red-skewed profiles (e.g., Mardones et al. 1997) are commonly interpreted as due to rotation or outflow motions (which should produce equal numbers of red and blue profiles, and could therefore explain the profiles detected towards the EGO and IRDC position, the hot core and the Hii region, Fig. 6) or to infall (which should produce profiles which are skewed towards the blue, e.g. towards the EGO and IRDC position) or to expansion (which should produce profiles skewed towards the red and could be responsible for the ¹²CO spectra of the hot core and the Hii region). From the PV diagram of the ¹²CO(6–5) line, we do not have any evidence of global rotation towards the hot core and the red-skewed profile can be interpreted in terms of expansion or outflow motion. Similarly, from Fig. 10 we see that the emission does not follow a perfect expanding spherical shell which might imply that the rotation is on the origin of the self-absorbed profile seen towards the Hii region. Finally, the blue-skewed line profile detected toward the EGO and IRDC positions are typical of infall motion.

6. Conclusions

To study the effect of feedback from massive star forming regions in their surrounding environment, we selected the region G327.3–0.6 for large scale mapping of several mid-J¹²CO and ¹³CO lines with the APEX telescope and of the high-J¹³CO(10–9) transition with the Herschel satellite. Our results can be summarised as follows:

• 1.

  maps of all transitions are dominated by the PDR associatedwith the Hii region G327.3–0.5; mid-J¹²CO and ¹³CO emission is detected along the whole extent of the IRDC;

• 2.

  mid-J transitions show rather extended emission with typical excitation temperatures of  ~30 K and column densities of some 10²¹ cm^-2;

• 3.

  all observed transitions are detected also in the inter-clump gas when averaged over large regions. The inter-clump emission is compatible with LTE emission from a gas at 44 K, and with a ¹³CO column density of 2 × 10¹⁵ cm^-2, thus suggesting that the inter-clump is composed of high-density clumps with low filling factors;

• 4.

  the warm gas traced by ¹² and ¹³CO(6–5) is only a small percentage (~10%) of the total gas in the infrared dark cloud, while it reaches values up to  ~35% of the total gas in the ring surrounding the Hii region;

• 5.

  the ¹²CO and ¹³CO ladders are qualitatively compatible with PDR models for high density gas (n > 10⁶ cm^-3) and, in the case of ¹³CO, suggest that the emission comes from a large number of clumps;

• 6.

  the detection of the ¹³CO(10–9) line allows to give stronger constraints on the physics of the gas by breaking the degeneracy between density and temperature (typical of ¹²CO and ¹³CO transitions) in the high temperature-low density part of the T–n plane.

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Acknowledgments

The authors thank Dr. Joe Mottram for a careful review of the manuscript and an anonymous referee for useful comments and suggestions. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri- INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA). Sweden: Chalmers University of Technology – MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University – Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC.

References

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