© ESO 2018

1 Introduction

The influence of bright stars on their surrounding interstellar medium (ISM) is immense. Their strong ultraviolet and far-ultraviolet (FUV) fields give rise to bright HII regions and photodissociation regions (PDRs). These are the best grounds to study the effect of UV and FUV photons on the heating and chemistry of ISM. The fine structure lines of C⁺ and O, observable at far-infrared (FIR) wavelengths, are mainly responsible for the cooling in these regions (Tielens & Hollenbach 1985b), which allow us to deduce the amount and sometimes the source of heating as well. The fine structure line of C⁺ at 158 μm is one of the brightest lines in PDRs and traces the transition from H⁺ to H and H₂ as C has an ionization potential of 11.3 eV (e.g., Pabst et al. 2017). In PDRs, a C⁺ layer extends to a depth of Av ~ 2–4, after which C⁺ recombines to C probing the interface to CO (Hollenbach & Tielens 1999). Deeper into the associated molecular clouds, cooling is dominated by the transitions of CO, observable at (sub)millimeter and FIR wavelengths. Modeling the relative intensity distributions of multiple lines from various molecular and atomic species allows us to derive the physical conditions in PDRs.

Messier 8 (M8) is located in the Sagittarius-Carina arm, near our line of sight toward the Galactic center. It is located at a distance ~1.25 kpc (1′ corresponds to 0.36 pc) from the Sun (Damiani et al. 2004; Arias et al. 2006) with an error of ~0.1 kpc (Tothill et al. 2008) and is about 34 × 12 pc in diameter. M8 is associated with the open young stellar cluster NGC 6530, the HII region NGC 6523/33, and large quantities ofmolecular gas (Tothill et al. 2008).

The open cluster NGC 6530 (centered at RA 18^h04^m24^s, Dec − 24°21′12′′(J2000)) is a relatively young cluster (formed about 2–4 Myrs ago, Chen et al. 2007) and contains several bright O-type stars. The brightest among them is Her 36 (Woolf 1961) at RA 18^h03^m40^s.3, Dec − 24°22′43′′(J2000). It is resolved into three main components: a close massive binary consisting of an O9 V and a B0.5 V star and a more distant companion O7.5 V star (Arias et al. 2010; Sanchez-Bermudez et al. 2014). Her 36 is responsible for ionizing the gas in the western half of the HII region of NGC 6523 including the bright Hourglass Nebula (Woolf 1961; Lada et al. 1976; Woodward et al. 1986). Lada et al. (1976) compared the optical and millimeter-wave observations of the M8 region and suggested the molecular cloud is located behind the HII region of the nebula, similar to the Orion-KL nebula. An ultracompact HII region, G5.97–1.17 is also very close to Her 36 at (RA 18^h03^m40.5^s, Dec − 24°22′44.3′′(J2000)) (Masqué et al. 2014).

A multiband near-IR image of Her 36 and its surroundings presented in Goto et al. (2006) shows the IR source Her 36 SE lying 0.25′′ SE of Her 36 and it is completely obscured. It is inferred to be an early-type B star with a visual extinction A_V > 60 mag that is deeply embedded in dense, warm dust and is powering the ultracompact HII region G5.97–1.17. The morphology of H2 and CO J = 3 → 2 emission around Her 36 (White et al. 1997; Burton 2002) is in accordance with the Hubble Space Telescope (HST) jet-like feature detections extending 0.5′′ southeast of Her 36 (Stecklum et al. 1995), which suggest there might be a molecular outflow in the core of M8 (Burton 2002). X-ray emission from Her 36 and diffuse X-ray emission from the Hourglass region, which is the brightest part of the optically visible nebula located ~15′′ east from Her 36 (Rauw et al. 2002), suggest the presence of a bubble of hot gas of size 0.4 pc that is produced by the interaction of the stellar wind of Her 36 with the denser part of the molecular cloud in the background. Anomalously broad diffuse interstellar bands (DIBs) at 5780.5, 5797.1, 6196.0, and 6613.6 Å along with CH⁺ and CH are found in absorption along the line of sight to Her 36 (Dahlstrom et al. 2013). CH⁺ and CH are radiatively excited by strong FIR emission from the adjacent IR source Her 36 SE (Goto et al. 2006) and the broadening of DIBs is attributed to radiative pumping of closely spaced high-J rotational levels of small polar carrier molecules (Dahlstrom et al. 2013; Oka et al. 2014; York et al. 2014). We performed (sub)millimeter observations related to these species that will be discussed in a future paper.

The eastern half of the HII region is illuminated by the 9 Sgr stellar system, as shown in Fig. 1. 9 Sgr is a well-known binary with an orbit of ~9 yr duration, consisting of an O3.5 V primary andan O5-5.5 V secondary (Rauw et al. 2012). Southeast of the cluster core (NGC 6530), another cluster, M8E, although optically invisible, is associated with two massive star forming regions (Tothill et al. 2008). A superposition of four HII regions seems to be responsible for the ionization of the gas in M8: the Hourglass Nebula illuminated by Her 36, the core of NGC 6523 illuminated by Her 36, the remaining parts of NGC 6523 and NGC 6533 illuminated by 9 Sgr (O4V) (Tothill et al. 2008), and M8E illuminated by HD 165052 (Lynds & Oneil 1982; Woodward et al. 1986).

Although M8 has been studied extensively in the X-ray, optical, and IR regimes (Stecklum et al. 1995; Damiani et al. 2004; Arias et al. 2006; Goto et al. 2006; Damiani et al. 2017), only few studies have been performed at millimeter and submillimeter wavelengths. White et al. (1997) reported the discovery of the second strongest source of millimeter and sub- millimeter wavelength CO line emission in our Galaxy toward Her 36 in M8 (White et al. 1997). Lada et al. (1976) compared optical and millimeter-wave observations to sketch the morphology of M8 where the core surrounding Her 36, the hourglass nebula with its structure and the eastern part of M8 are described. Tothill et al. (2002) presented submillimeter- and millimeter-wavelengths maps of the J = 2 → 1 and J = 3 → 2 transitions of ¹²CO tracing the molecular gas and dust around Her 36.

We report a comprehensive survey of the 1.5 × 1.5 pc (4′ × 4′) region around Her 36 (as shown by the blue square in Fig. 1) at FIR, millimeter- and submillimeter wavelengths to probe the physical conditions and image the morphology of this exceptional PDR. We present for the first time extended maps of this region in the 158 μm fine structure line of C⁺, high-J transitions of ¹²CO emission observed with the GREAT¹ receiver on board SOFIA observatory, the mid-J transitions of ¹²CO and ¹³CO using the PI230, FLASH⁺, and CHAMP⁺ receivers of the APEX² telescope, and low-J transitions of ¹²CO and ¹³CO using the EMIR receiver of the IRAM³ 30 m telescope.

Fig. 1 Left panel: Lagoon Nebula, Messier object 8 (M8), or NGC 6523 in the constellation of Sagittarius, as seen by the Kitt Peak 4 m Mayall telescope in 1973. North is up, east to left. Credits to National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation Copyright WIYN Consortium, Inc., all rights reserved. Right panel: WISE 3.4 μm image of the M8 region around Her 36 (denoted with a star) investigated in this paper. The contour levels are 10% to 100% in steps of 10% of the peak emission ~4000 data number. The 1.5 pc indicated in the lower left correspond to 4′.

2 Observations

2.1 SOFIA/GREAT data

The high-J CO and [C II] 158 μm observations summarized in Table 1 were conducted with the L1 channel of the German Receiver for Astronomy at Terahertz frequencies (GREAT; Heyminck et al. 2012) and the upGREAT LFA arrays (Risacher et al. 2016) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA; Young et al. 2012). The data was acquired during observatory flight #297 on 2016 May 14 at 14.2 km altitude and under a median water vapor column of 11 μm. The upGREAT was employed with 14 pixels (seven pixels for each polarization, with a hexagonal layout). The spectral analysis was performed by means of fast Fourier transform spectrometers (Klein et al. 2012), in a mode providing 4.0 GHz bandwidth with 2¹⁴ spectral channels.

In the first setup we simultaneously mapped the ¹²CO J = 11 → 10 transition at 1267.014.486 GHz and the [C II] ²P_3∕2 → ²P_1∕2 fine structure line at 1900.537 GHz. In a second setup the ¹²CO J = 13 → 12 and J = 16 → 15 transitions at 1496.923 GHz and 1841.345 GHz were mapped, respectively. Typical single-sideband system temperatures ranged between 1600 and 1800 K for the lower frequency L1 channel and between 2080 and 2260 K for the higher frequency upGREAT array with atmospheric transmissions of 0.90–0.94 and 0.85–0.88, respectively. All maps shown in Fig. 2 were observed in on-the-fly (OTF) total power mode with a sampling of 6″ centered on RA 18^h03^m40. ^s33, Dec − 24° 22’42.′′ 7 (J2000), the Her 36 location. We integrated 0.4 s per record for the CO (11 → 10)∕[CII] setup, and 0.8 s for the CO (13 → 12)∕(16 → 15) setup. The originally chosen reference position at (Δα, Δδ) = (+500″, −500″) (relative to the map center) was found to be contaminated in both transitions of the first setup and was therefore changed in favor of a second, clean reference at offset (+30′, −30′), while the pointing accuracy of <3′′ was maintained.

The raw data was calibrated with the program kalibrate (Guan et al. 2012), which is part of the KOSMA software package. The resulting antenna temperatures were converted to main beam brightness temperatures using a forward efficiency F_eff = 0.97 and beam efficiencies of 0.66 for the L1 channel of GREAT, and 0.58 to 0.68 for the upGREAT pixels, with a median value of 0.65. In order to optimize the signal-to-noise ratio per channel, the spectra were smoothed to a resolution of 2.44 MHz, corresponding to 0.4 km s⁻¹ for upGREAT and 0.6 km s⁻¹ for the L1 channel. We subtracted spectral baselines of first order and subsequently produced data cubes with beam/3 sampling. For the beam sizes, see Table 1.

Fig. 2 Color maps of the integrated intensity of the [C II] 158 μm and J = 11 → 10, J = 13 → 12, and J = 16 → 15 transitions of 12CO toward Her 36, which is the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′, denoted with an asterisk. The contour levels are 10% (>3 × rms, given in Table 1) to 100% in steps of 10% of the corresponding peak emission given in Table 1. All maps are plotted using original beam sizes shown in the lower left of each map.

2.2 APEX data

Observations of low- and mid-J ¹²CO and ¹³CO transitions were performed with APEX 12 m submillimeter telescope (Güsten et al. 2006) during 2015 June–August and 2016 July, September, and October. As shown in Table 1, we used the following receivers: PI230 to map the low-J ¹²CO and ¹³CO transitions, FLASH⁺ in the 345 and 460 GHz bands to map the mid-J CO transitions, and CHAMP⁺ in low- and high-frequency subarrays to map the higher frequency mid-J CO transitions.

We used the PI230 receiver to map C¹⁸O J = 2 → 1 at 219.560 GHz, ¹³CO J = 2 → 1 at 220.398 GHz, and ¹²CO J = 2 → 1 at 230.538 GHz. FLASH⁺ was used in the 345 GHz band to map the ¹²CO J = 3 → 2 transition at 345.795 GHz. FLASH⁺ was also used in the 460 GHz band to map the ¹³CO J = 4 → 3 transition at440.765 GHz, C¹⁸O J = 4 → 3 at 439.088 GHz, and [C I] ³P₁ → ³P₀ fine structure line at 492.160 GHz. The CHAMP⁺ receiver was used to map ¹²CO J = 6 → 5 at 691.473 GHz and ¹³CO J = 6 → 5 at 661.067 GHz in the low-frequency subarray complemented by ¹²CO J = 7 → 6 at 806.651 GHz and ¹³CO J = 8 → 7 at 881.272 GHz in the high-frequency subarray.

All maps shown in Fig. 3 were observed in OTF total power mode centered on RA 18^h03^m40^s.3, Dec − 24°22′43′′(J2000), which corresponds to the position of Her 36. The maps obtained from the observations carried out in July, September, and October 2016 have a size of 240′′ × 240′′. Maps that were obtained from the observations carried out in 2015 are comparatively smaller in size. We integrated 0.7 s per dump for all maps and enough coverages were performed to reach the rms noise levels as mentioned in Table 1. The offset position relative to the center at (30′, −30′) was chosen as reference, similar to the SOFIA observations. The pointing accuracy (<3′′) was maintained by pointing at bright sources such as RAFGL5254 and R Dor every 1–1.5 h. A forward efficiency F_eff = 0.95 was used for all receivers, and the beam coupling efficiencies B_eff = 0.62, 0.69, 0.63, 0.43, and 0.32 were used for the PI230, FLASH⁺340, FLASH⁺460, CHAMP⁺660, and CHAMP⁺810 receivers, respectively.

2.3 IRAM 30 m data

Observations of low-J ¹²CO, ¹³CO, and hydrogen recombination line observations were performed with the IRAM 30 m telescope in August 2016. We observed the whole 3 mm range using the EMIR receivers (Carter et al. 2012). We simultaneously mapped a region of 240′′ × 240′′, which is similar to the size of most other maps previously observed with SOFIA and APEX in the ¹²CO and ¹³CO J =1 → 0 transitions (Table 1) and the hydrogen recombination lines H40α to H43α at 99.023 GHz,92.034 GHz, 85.688 GHz, and 79.912 GHz, respectively. Molecular high density tracers, which were also detected in our wide spectral band observation, will be analyzed in a subsequent paper.

All maps shown in Fig. 4 were observed in OTF total power mode centered on Her 36. Each subscan lasted 25 s and the integration time on the off-source reference position was 5 s. The offset position relative to the center at (30′, −30′) was similar to that used for the SOFIA and APEX mapping and the pointing accuracy (<3′′) was maintained by pointing at the bright calibrator 1757-240 every 1–1.5 h. A forward efficiency F_eff = 0.95 and a beam coupling efficiency B_eff = 0.69 wereadopted for the EMIR receivers. These values were taken from the latest (2015) commissioning report⁴.

All data reduction was performed using the CLASS and MIRA programs that are a part of the GILDAS⁵ softwarepackage and all the observations are summarized in Table 1.

3 Results

3.1 Peak intensities of the molecular line emission

The maxima of the distributions of the velocity integrated intensities of the emission in the [C I] and [C II] lines and different transitions of ¹²CO, ¹³CO, and C¹⁸O are presented in Table 1. Figure 2 shows velocity integrated intensity maps of the ¹²CO J = 11 → 10, 13 → 12, and 16 → 15 transitions. The emission in all the lines has a similar spatial distribution and peaks are found at about the same offset position (Δ α =5.0′′, Δ δ =5.0′′) northwest of Her 36.

Figures 3 and 4 show velocity integrated intensity maps of low- and mid-J transitions of ¹²CO, ¹³CO, and C¹⁸O, i.e., the J = 1 → 0, 2 → 1, 3 → 2, 6 → 5, 7 → 6 transitions of ¹²CO, the J = 1 → 0, 2 → 1, 4 → 3, and 6 → 5 transitions of ¹³CO, and the J = 1 → 0 transition of C¹⁸O. The intensities of the low-J transitions peak close to Her 36 (Δ α =0.0′′, Δ δ =0.0′′) for ¹²CO mid-J transitions; the peaks shift toward the northwest of Her 36 with offsets of (Δ α = −13.0′′), Δ δ =8.0′′). It seems like there is a systematic shift in the peak emission of CO transitions with low-J peaking near Her 36, mid-J peaking toward the northwest, while high-J lines peak again closer to Her 36. Nevertheless, all maps show at least a small offset toward the northwest and the emission from CO transitions becomes more and more compact with increasing J.

Figures 2 and 3 show velocity integrated intensity maps of the [C II] ²P3/2 →² P1/2 and [C I] ³P1 → ³P0 transitions. [C I] peaks at Her 36 and is very bright toward the northwest of Her 36. [C II] peaks at an offset of (Δ α =30.6′′, Δ δ = −1.6′′), which is toward the east of Her 36 and the emission extends even further. This extended emission comes from the part of the HII region that is illuminated by the stellar system 9 Sgr (Tothill et al. 2008).

Figure 4 shows a velocity integrated intensity map of an average of the H40α to H43α hydrogen recombination lines. We have taken the average to obtain a better signal-to-noise ratio. The distribution or the radio recombination line emission agrees well with the 5 GHz continuum Very Large Array (VLA) interferometric map in Fig. 4 of Woodward et al. (1986) and the peak of the Hα lines is at (Δ α =10.0′′, Δ δ =9.0′′), close to the center of the Hourglass Nebula, which indicates the presence of hot ionized gas in the east of Her 36.

Fig. 3 Colour maps of the integrated intensity of the J = 2 → 1, J = 3 →2, J = 6 →5, and J = 7 → 6 transitions of 12CO, the J = 2 → 1, J = 4 →3, and J = 6 → 5 transitions of 13CO, the J = 2 → 1 line of C18O and [C I] 1 → 0 toward Her 36. This corresponds to the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′, denoted with an asterisk. The contour levels of 12C18O and [C I] are 3 × rms in steps of 2 × rms, while those of other molecules are from 10% (>3 × rms, given in Table 1) to 100% in steps of 10% of the corresponding peak emission given in Table 1. All maps are plotted using original beam sizes shown in the lower left of each map.

Fig. 4 Color maps of the integrated intensity (left to right) of the J = 1 → 0 transition of 12CO, 13CO and average of Hα 40, 41, 42, and 43 lines toward Her 36, which is the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′, denoted with an asterisk. The contour levels are 10% (>3 × rms, given in Table 1) to 100% in steps of 10% of the corresponding peak emission given in Table 1. All maps are plotted using original beam sizes shown in the lower left of each map.

3.2 Correlation between ¹²CO, ¹³CO, [C I], and [C II]

As can be seen from the velocity integrated intensity maps, emission from [C II] is spread out the most as compared to [C I], ¹²CO, and ¹³CO. In order to visualize the correlation between these species, scatter plots of [C II] vs. [C I], [C II] vs. ¹²CO J = 6 → 5, and [C I] vs. ¹³CO J = 2 → 1 are shown in Fig. 5. We choseCO 6 → 5 owing to itsassociation with the warm PDR due to its higher upper level energy compared to low-J CO transitions, while we chose ¹³CO 2 → 1 in particular, as its critical density is comparable to that of [C I]. [C II] is correlated the least with ¹²CO J = 6 → 5. The Pearson correlation coefficient is r = 0.471. Two branches appear to bud out in the upper left and lower right of this correlation. The upper left, where the [C II] emission intensifies for a slowly strengthening ¹²CO J = 6 → 5 emission, corresponds to the northeast of Her 36 where [C II] is more extended. The lower right, where ¹²CO J = 6 → 5 emission intensifies at a faster rate than [C II], corresponds to the southwest of Her 36, where ¹²CO J = 6 → 5 is much more prominent. The correlation of [C II] with [C I] has a correlation coefficient of r = 0.473 and again shows two different branches corresponding to different regions. The upper left, where [C II] emission gets brighter for an almost constant [C I] emission, corresponds to the northeast of Her 36. The branch in the lower right, similar to the situation shown in Fig. 5b, corresponds to the southwest of Her 36. In contrast to these correlations, [C I] is well correlated with ¹³CO J = 2 → 1 with r = 0.908. This resembles the case M17 SW, for which a correlation coefficient of [C I] with ¹³CO J = 2 → 1 was reported to be 0.942 (Pérez-Beaupuits et al. 2015c).

Fig. 5 Scatter plots and correlation coefficients r between the velocity integrated intensity of (a) [C II] and [C I] (b) [C II] and the J = 6 → 5 transition of 12CO, and (c) [C I] and the J = 2 → 1 transition of 13CO. All data points were extracted from velocity integrated intensity maps convolved to the same beam size of 31′′.

3.3 Channel maps

In order to investigate the differences in the distribution of ionized and atomic carbon, channel maps of the ²P_3∕2 →²P_1∕2 transition of [C II] are compared tothose of the [C I] ³P₁ →³P₀ transition. Figure 6 shows that the emission from [C II] (lower panels) is more spread out as compared to that from [C I] (upper panels). In the velocity range from 2 to 6 km s⁻¹ and 15 to 17 km s⁻¹ there is no emission from [C I] while there is emission from [C II] close to Her 36 and toward the east of it, respectively. In the range from 7 to 9 km s⁻¹ both [C II] and [C I] emission are found toward the west. These structures extend further toward the northeast for higher velocities in the range of 12 to 15 km s⁻¹. This is verysimilar to the case of M17 SW, where the [C II] channel map shows a strong spatial association with [C I] and CO channel maps only at intermediate 10 to 24 km s⁻¹ velocities. While at lower (<10 km s⁻¹) and higher (>24 km s⁻¹) velocity channels, [C II] emission is mostly not associated with the other tracers of dense and diffuse gas (Pérez-Beaupuits et al. 2015c). Notably, our [C II] channel maps show a clumpy structure at an offset of (Δ α =60.0′′, Δ δ =27.0′′) which is missing in the [C I] maps; this complements the argument that the east of Her 36 is comprised of hot gas and strong UV fields capable of ionizing carbon, i.e., it is part of an HII region. This is consistent with the Hα and 5 GHz continuum VLA interferometric maps presented by Woodward et al. (1986) in their Figs. 1 and 4, which also have their peak intensities east from Her 36.

Fig. 6 Velocity channel maps of the 3P1 → 3P0 transition of [C I] (upper 16 panels) and the 2P3∕2 → 2P1∕2 transition of [C II] (lower 16 panels) in a range of 2–17 km s−1 with a channel width of 1 km s−1 toward Her 36, which is the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′, denoted with a black asterisk. The contour levels of [C I] are 3 × rms in steps of 2 × rms, while those of [C II] are from 10% (>3 × rms) to 100% in steps of 10% of the corresponding peak emission. All maps are plotted using original beam sizes shown in the lower left of both panels.

3.4 Ancillary data

For a multiwavelength view of M8 and in order to relate our observations to the dense and cold molecular cloud and the hot ionized gas in M8, we compared our data with observations obtained at other wavelength ranges. The surveys chosen for this comparison are as follows: firstly, we extracted data from the 870 μm APEX Telescope Large Area Survey of the Galaxy (ATLASGAL; Schuller et al. 2009) performed with the APEX 12 m telescope using the Large APEX BOlometer CAmera (LABOCA). The dust continuum emission probes dense and cold clumps in the ISM of our Galaxy. Figure 7a shows our [C II] velocity integrated intensity map overlaid with the ATLASGAL dust continuum image that peaks at Her 36 and also traces the cold molecular cloud in the northwest. The dust emission morphology is similar to the ¹²CO, ¹³CO, and C¹⁸O distribution. Secondly, we used data from the National Radio Astronomy Observatory (NRAO)/Very Large Array (VLA) Archive Survey (NVAS⁶). Figure 7b shows the [C II] velocity integrated intensity map overlaid with the 1.3 cm radio continuum image⁷, which peaksvery close to Her 36 and traces free–free emission from the HII region NGC 6523/33. This compact HII region, which is also traced by the H recombination lines with the IRAM 30 m telescope, is also shown in Fig. 4 (right panel). Thirdly, the Wide-field Infrared Survey Explorer (WISE) imaged the sky at four mid-infrared wavelengths. Figure 7c and d compare the [C II] velocity integrated intensity map with WISE 3.4 μm (band 1) and 4.6 μm (band 2) continuum images, which peak closer to the [C II] peak. Overall, the mid-infrared emission that originates from hot dust shows the best agreement with the morphology seen in the [C II] image; both probe hot material from HII regions and warm surfaces of PDRs.

Fig. 7 A [C II] velocity integrated intensity map overlaid with contours of (a) ATLASGAL870 μm continuumemission. Important offset positions discussed in the text are indicated with triangles of different colors: black represents the secondary ATLASGAL peak deep inside the molecular cloud at (Δ α = −53.0′′, Δ δ =23.0′′), red represents the emission peak of mid-J transitions of 12CO and 13CO at (Δα = −13.0′′, Δ δ =8.0′′), blue represents the peak of the [C II] 158 μm emission at (Δα =30.0′′, Δ δ = −2.0′′), and light blue represents the clump to the east of Her 36 observed in the channel maps of [C II] at (Δ α =60.0′′, Δ δ =27.0′′). The white arrows point along the molecular cloud in the west to the HII regions in the east of Her 36; (b) VLA 1.3 cm free-free continuum emission; (c) WISE 3.4 μm, and (d) WISE 4.6 μm mid-infrared continuum. Her 36 is the central position denoted with an asterisk. The contour levels are 5% to 95% in steps of 10% of the peak emission for (a) and 10% to 100% in steps of 10% of the peak emission for (b), (c), and (d).

3.5 Spectra of ¹²CO, ¹³CO, [C I] and [C II] emission lines at different offsets

Figure 8 shows a comparison between the spectra of ¹²CO, ¹³CO, [C I], and [C II] emission lines at different offsets relative to Her 36. Line parameters of Gaussian fits to profiles are reported in Table 2. In several cases the profiles show evidence of two velocity components that were fit separately. The ¹²CO J = 6 → 5 and ¹³CO J = 1 → 0 transitions are representative of the general appearance of all ¹²CO and ¹³CO line profiles discussed in this paper. The different offsets were chosen along a curved line from the molecular cloud in the west to the east of Her 36 (see Fig. 7a): the secondary C⁺ peak, which corresponds to the clump observed in the channel map of [C II] at (Δ α =60.0′′, Δ δ =27.0′′); the C⁺ peak, which is the emission peak of the ²P_3∕2 → ²P_1∕2 transition of [C II] at (Δ α =30.0′′, Δ δ = −2.0′′); Her 36 is located at (Δ α =0.0′′, Δ δ =0.0′′); the mid-J CO peak, which is the mid-J transition emission peak of ¹²CO and ¹³CO at (Δ α = −13.0′′, Δ δ =8.0′′); and the secondary ATLASGAL peak arises from deep into the molecular cloud to the west traced by ATLASGAL at (Δ α = −53.0′′, Δ δ =23.0′′).

A lower velocity (2–6 km s⁻¹) component is spectrally resolved at several positions. The higher velocity component emission lines have blue-shifted wings in the molecular cloud in the west, while the emission is red-shifted toward the [C II] peak toward the east compared to their emission peaking at 9 km s⁻¹ toward our reference position, Her 36. Furthermore, the peak of the lines shifts from the east to the west to lower velocities. The ¹²CO and ¹³CO line profiles are similar in being most intense with broadest line widths at the mid-J transition emission peak of ¹²CO and ¹³CO (Δ α = −13.0′′, Δ δ =8.0′′) and at Her 36 itself, while getting less intense with narrower line widths at the [C II] peak. The [C I] line profile gets most intense with broadest line width toward Her 36 itself with almost no emission from the clumpy structure in the HII region at an offset of (Δ α =60.0′′, Δ δ =27.0′′). As can also be seen from the comparison of various molecular transitions at Her 36 in Fig. 9, CO and [C I] are not associated with [C II] at lower and higher velocities (see also Figs. 5 and 6). This is very similar to M17 SW as reported by Pérez-Beaupuits et al. (2015c), where [C II] is not associated with other gas tracers at lower and higher velocities.

Fig. 8 Line profiles at different offsets (′′) relative to Her 36 are shown in different colors at five positions mentioned in the upper left plot. For the detailed positions, see Sect. 3.5. Upper panels: J = 6 → 5 12CO and J = 1 → 0 13CO spectra; lower panels: 3P1 → 3P0 and 2 P3∕2 → 2P1∕2transitions of [C I] and [C II]. All spectra were extracted from their original beam sizes as mentioned in Table 1.

Fig. 9 Line profiles toward Her 36 for [C I] 3P1 → 3P0, 13CO J = 6 → 5, 12CO J = 6 → 5, and [C II] 2P3∕2 → 2P1∕2. All spectra are extracted from maps that were convolved to the same beam size of 15′′.

4 Analysis

In this section we determine the temperature and density in the PDR of M8 with several complementary methods. We start by using the data for the J = 6 →5 transition of CO, which has the highest angular resolution, to estimate excitation temperatures and column densities throughout the PDR as probed by the mid-J CO emission.

4.1 Excitation temperature and column density estimates

A detailed description of spectral line radiative transfer relevant here can be found in Draine (2011) and Mangum & Shirley (2016). For a constant excitation temperature T_ex, we can integrate the radiative transfer equation to obtain the observable Rayleigh–Jeans equivalent temperature (Eq. (1) in Peng et al. 2012). The background radiation temperature comprises the cosmic background radiation of 2.73 K and the radiation from warm dust. The latter was calculated using the results from the Spectral and Photometric Imaging Receiver (SPIRE) of the European Space Agency (ESA) Herschel Space observatory. Data obtained in the second band at 350 μm (close to the ¹²CO 7 → 6 line) wavelength was used. The maximum intensity in the analyzed region around Her 36 is about 1200 MJy sr⁻¹ near the dense molecular cloud, which corresponds to a Rayleigh–Jeans equivalent brightness temperature of about 0.06 K from dust. Thus, the total contribution from dust and background can be neglected as it contributes ≤1% to the resulting .

Assuming that the excitation temperature for ¹²CO and ¹³CO is the same and ¹²CO is optically thick, (1)

The excitation temperature of the ¹²CO J = 6 → 5 transition can be estimated by further assuming a beam filling factor of unity, i.e., = TMB. This equation is written as (2)

where TMB is the main beam brightness temperature in K that is estimated from the peak temperature map of ¹²CO J = 6 → 5 transition. The resulting T_ex distribution is shown in Fig. 10a. Formally, we show lower limits to the excitation temperature due to the assumption of a beam filling factor of unity. It is highest immediately in the northwest of Her 36 and decreases with distance from the star.

Using the computed Tex and the main beam brightness temperature TMB estimated from the peak temperature map of ¹³CO J = 6 → 5, the total column density (Fig. 10b) of ¹³CO can be calculated over the complete velocity range of the source from (3)

where T_ex is in K and T_MB dv is in K km s⁻¹. Figure 10b shows the resulting ¹³CO total column density with a peak value of ~5 × 10¹⁶ cm^–2 northwest ofHer 36. This results in a H2 column density N(H₂) of ~3.7 × 10²² cm⁻² adopting an isotopic abundance ratio [¹²CO/¹³CO] of ~63 (Milam et al. 2005) and a CO abundance ratio [¹²CO/H2] of ~8.5 × 10⁻⁵ (Tielens 2010). The mass of the warm CO gas can be computed by integrating the column density over the whole clump in a region of 3.12 arcmin², which results in a mass of ~467 M⊙. Complementary to this, the cold gas mass in a region of 5.1 arcmin² has an estimated value of 10³ M⊙, calculated from a flux of ~133 Jy at 870 μm measured with ATLASGAL (Schuller et al. 2009) assuming an absorption coefficient of k_ν = 1.85 g cm² and a temperature of 23 K (Urquhart et al. 2018), and not including potential uncertainties in the choice of these values. However, these mass estimations have an error of ~26%, which accounts for errors of ~16% from the distance to the star (Tothill et al. 2008) and ~20% from the calibration.

Fig. 10 For the J = 6 → 5 transition (a) shows the excitation temperature of 12CO, which is assumed to be equal to that of 13CO and (b) shows the column density of 13CO. The asterisk represents Her 36, which is the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′. The contour levels are 10% to 100% in steps of 10% of the corresponding peak emissions. All maps are plotted using original beam sizes shown in the lower left of each map.

4.2 Rotational diagrams of ¹³CO

With observations of CO lines with different J, rotational diagrams can be used to study the excitation of the CO emitting gas. In a rotationaldiagram or Boltzmann plot the natural logarithm of the column density N_u ∕g_u of different lines is plotted against theirupper energies E_up∕k. Here g_u is the degeneracy of the upper energy level, (≡ 2J + 1), and k is the Boltzmann constant. For a single temperature and optically thin emission these data points fall onto a straight line. Deviations from a straight line indicate then either optical depth effects or temperature gradients in the clouds. A complete derivation of rotational diagrams for a local thermodynamic equilibrium (LTE) case can be found in Goldsmith & Langer (1999).

Firstly, by assuming ¹³CO to be optically thin, i.e., the optical depth correction term, C_τ is unity in Eq. (24) of Goldsmith & Langer (1999), we plot versus E_u∕K as shown in Fig. 11 in black for the five ¹³CO lines observed toward Her 36. A curvature in a rotational diagram can be due to optical depth effects, therefore we estimate the expected optical depths for the computed column density from Eq. (25) of Goldsmith & Langer (1999) and apply the optical depth corrections C_τ that lead to the corrected diagram as shown in red in Fig. 11. The new temperatures and column densities are then calculated as shown in Table 3. Further iterations would lead to corrections smaller than the error bars. After the optical depth correction the curvature in the rotational diagram remains and indicates temperature gradients in the gas, as expected in a PDR.

The J = 1 → 0 and J =2 → 1 transitions of ¹³CO seem to originate from colder gas, while the J = 4 → 3 and J = 6 → 5 transitions probe hotter gas, which agrees with the analysis carried out for J = 6 → 5 in Sect. 4.1. The J = 8 → 7 transition appears to originate from the hottest gas.

Fig. 11 Rotational diagrams of J = 1 → 0, J = 2 → 1, J = 4 → 3, J = 6 → 5, and J = 8 → 7 transitions of 13CO at Her 36. Shown in black is the rotational diagram when 13CO is assumed to be optically thin. In red the rotational diagram is shown including the optical depth correction factors. Rotation temperatures obtained from different slopes are indicated. Values of the velocity integrated intensities for different transitions were extracted from maps convolved to the same resolution of 31′′. The error bars were calculated from the maximum noise level of the integrated intensities of individual transitions and from calibration uncertainties of 20%.

4.3 RADEX modeling

We used the non-LTE radiative transfer program RADEX (van der Tak et al. 2007) to verify the calculations carried out in Sects. 4.1 and 4.2, which assumed LTE to determine the temperatures and densities. The RADEX program uses the escape probability approximation for a homogeneous medium and takes into account optical depth effects. We chose a uniform sphere geometry. The Leiden Molecular and Atomic Database⁸ (LAMDA; Schöier et al. 2005) provides rates coefficients for collisions of CO and H2 used in the modeling. ¹²CO and ¹³CO transitions were modeled taking their line width from the average spectra of our data, i.e., 4 and 3 km s^–1, respectively.As input parameters we computed grids in temperature and volume density with a background temperature of 2.73 K, kinetic temperatures in the range of 50–250 K and H2 densities in the range of 10⁴–10⁸ cm⁻³. For a linear molecule line ratios from different J depend on both temperature and density. To break this degeneracy, we computed with RADEX not only the line ratio of two ¹³CO transitions but also the temperature of the ¹²CO line. The latter is optically thick, probes the excitation temperature(cf. Sect. 4.1), and can therefore be used to break the degeneracy between temperature and density.

In our first approach to determine the dominant kinetic temperatures and densities in M8 near Her 36, we selected from the CO maps data points for column densities of ¹²CO in three ranges as follows: (a) 8 × 10¹⁷–1.8 × 10¹⁸ cm^–2, (b) 1.8 × 10¹⁸–3.5 × 10¹⁸ cm^–2, and (c) 3.5 × 10¹⁸–5.1 × 10¹⁸ cm^–2. These ranges were estimated from the LTE calculations carried out in Sects. 4.1 and 4.2. Another criterium for selection of these data points aims at selecting the most conspicuous regions inside the maps. The molecular cloud region consists of all data points in an area of 15′′ × 15′′ around the mid-J CO peak, the UC HII region/ Her 36 encompasses all data points in an area of 20′′ × 20′′ around Her 36, and the eastern HII region has data points in an area of 20′′ × 20′′ around the [C II] peak.

Figure 12 shows the J = 4 → 3 and J = 6 → 5 line ratios of ¹³CO vs. the J = 6 → 5 ¹²CO line temperature for both the measured data points in the maps and the results of the RADEX computation of the density/temperature grid. The H₂ number densities in most of the regions are in the range of ~10⁴–10⁶ cm⁻³. High densities are obtained for the molecular cloud and the eastern HII region, while the region consisting of the bright stellar system Her 36 and the ultracompact HII region has densities in the range of 10⁴ –10⁵ cm⁻³. The kinetic temperatures from the modeling results are in a range of 100–250 K for 10⁴ cm⁻³ and in a range of 50–150 K for 10⁵ –10⁶ cm⁻³.

In order to include all CO lines toward several positions in the RADEX analysis, their intensities as a function of J are compared to RADEX results in Fig. 13. To fit the modeling results to the observed data points we varied the column densities of ¹²CO and ¹³CO. While for ¹²CO a column density of 4 × 10¹⁸ cm⁻² was chosen, for ¹³CO a column density of 8 × 10¹⁶ cm⁻² could make the modeling results fit the data points. This ¹³CO column density exceeds the value obtained by the LTE calculations and corresponds to a ¹² CO/¹³CO ratio of 50. While this is lower than the assumed value of 63 in Sect. 4.1, it is still within the typical scatter of this ratio in the ISM (Milam et al. 2005). The ¹²CO and ¹³CO observed data is compared to the RADEX model at the peak of the mid-J CO transitions in the molecular cloud, at Her 36 and at the emission peak of [C II]. Panels a and b of Fig. 13 show results with the kinetic temperature varied from 50–250 K and keeping the H2 density fixed at 10⁵ cm^–3, while panels c and d show results in which the H2 density is varied from 10⁴–10⁸ cm⁻³ while keeping the kinetic temperature fixed at 120 K.

It can beseen that no single kinetic temperature or H2 density can fit all the observed data points. This suggests solutions with kinetic temperatures in the range of 100–150 K and H2 densities to be in the range of 10⁴–10⁶ cm^–3. Such a spread in the ambient temperature was also implied by the rotational diagram analysis. Furthermore, these values are similar to the temperature and density ranges found in OMC 1 (Peng et al. 2012).

Fig. 12 For 3 different 12CO column densities, RADEX modeling results are shown for 12CO J = 6 → 5 peak main-beam brightness temperatures and corresponding 13CO J = 4 → 3/13CO J = 6 → 5 ratios. The blue lines denote the RADEX modeling results for different kinetic temperatures at 3 different volume densities, and color points represent data from near the HII region, PDR, and molecular cloud (see Sect. 4.3). Each point denotes a 12CO J = 6 → 5 temperature and 13CO J = 4 → 3/13CO J = 6 → 5 temperature ratio obtained from a single pixel. Data points are extracted from peak temperature maps convolved to the same resolution of 16′′. (a) The data points plotted are obtained for a 12CO column density range of 8 × 1017 –1.8 × 1018 cm−2 and for modeling the chosen input 12CO column density is 1 × 1018 cm−2. (b) The data points plotted are obtained for a 12CO column density range of 1.8 × 1018 –3.5 × 1018 cm−2 and for the modeling the input 12CO column density is 2 × 1018 cm−2. (c) The data points plotted are obtained for a 12CO column density range of 3.5 × 1018 –5.1 × 1018 cm−2 and for the modeling input the 12CO column density is 4 × 1018 cm−2.

Fig. 13 Results obtained from RADEX modeling for 12CO and 13CO at the mid-J CO peak (Δα = −13.0′′, Δ δ = 8.0′′), at Her 36 (Δα = 0′′, Δ δ = 0′′) and at the [C II] peak (Δα = 30.0′′, Δ δ = −2.0′′). Our observed data points are in green, red, and blue and are extracted from peak temperature maps convolved to the same resolution of 31′′. The column densities used in the modeling are for 12CO 4 × 1018 cm−2 and 8 × 1016 cm−2 for 13CO. Panels a and b: results obtained by varying the kinetic temperature from 50–250 K in steps of 50 K and keeping the density fixed at 105 cm–3; panels c and d: results obtained by varying the H2 density from 104–108 cm–3 in steps by a factor of 10 and keeping the kinetic temperature fixed at 120 K.

4.4 CO, [C I] and [C II] luminosities

We obtained the total luminosities of the CO spectral line energy distributions (SLED) and of the [C I] 609 μm and [C II] lines over the total observed region seen in the maps in Figs. 2–4, as derived by Solomon et al. (1997), Carilli & Walter (2013). We scaled the luminosity for Galactic sources, i.e., (4)

where L is the line luminosity in L⊙, S Δ V is the velocity integrated flux in Jy km s⁻¹, ν is the transition frequency in GHz, and D_L is the distance in kpc. A total CO luminosity of L_CO = 9.5 L⊙ was calculated for the observed transitions and by accounting for the luminosities of the missing transitions. A [C I] 609 μm line luminosity of L_{[C I]} = 0.11 L⊙ was obtained, which is a lower limit to the total [C I] luminosity since the [C I] 370 μm line was not observed. The estimated [C II] luminosity is L_[CII]  = 95.8 L⊙. Similar to the mass estimations in Sect. 4.1, these luminosity estimations have an error of ~26%.

5 Discussion

5.1 Overview of the PDR and HII region around Her 36

In Fig. 14, we present a F487N filtered 4865 Å HST image⁹ (observation ID number: 6227, observed in the year 1995) of Her 36 and its surroundings overlaid with contours from low velocity [C II] channel maps. The [C II] at the lowest velocity (2 km s⁻¹) peaks at the Hourglass Nebula slightly to the east of Her 36. With increasing velocities, the [C II] follows the dark patches in the HST images that form a foreground veil covering parts of the bright nebulosity excited by Her 36. The strong correlation of [C II] and foreground absorption is particularly evident at the sharp southern edge of the veil seen at 7 km s⁻¹. Therefore we suggest that the low velocity [C II] probes directly the gas of the veil that forms a foreground PDR illuminated by Her 36. On a fainter level, weak emission from this veil is also seen in the CO maps at low velocities (5–7 km s⁻¹). This foreground veil is receding away from Her 36 toward us and to the west with a change in the line-of-sight velocity. This is consistent with both high velocity red-shifted and low velocity blue-shifted emission of Hα, [N II] and [S II] doublets, [O III], and absorption lines of the sodium D doublet as measured by Damiani et al. (2017). Assuming optically thin emission from [C II] in this warm veil in the velocity range of 2–7 km s⁻¹ and a kinetic temperature of 500 K (Tielens & Hollenbach 1985b), we calculated the [C II] column density using Eq. (A.1) with a peak value of ~9.6 × 10¹⁷ cm⁻² at an offset of (Δα = 30′′, Δ δ = 10′′) from Her 36. Taking C/H ~ 1.2 × 10⁻⁴, assuming that all the carbon is in ionized form and from the column density of H₂, N(H₂) = 9.4 × 10²⁰ cm⁻² (A_v /mag) (Kauffmann et al. 2008), we derived the visual extinction A_v. A maximum extinction of A_v ~ 4.25 is obtained at the same position where the [C II] column density peaks and the values get lower around it. This position is the same as position 13 in Fig. 5 of Woodward et al. (1986), where an A_v ~ 3.9 was calculated.

In Fig. 15 (left) we show how the intensity of various tracers evolves along a path from the direction of 9 Sgr to Her 36 and then continuing to the northwest along the molecular cloud. Toward the northeast [C II] and the mid-IR WISE emission dominate, probing the extended HII regions toward 9 Sgr and the resulting PDR. All of the tracers peak on or close to Her 36, showing the tight spatial correlation between the ultracompact HII region (as seen in the recombination line), a dense clump in the larger scale molecular cloud (870 μm dust and molecular emission), and the bright PDR illuminated by Her 36 ([C II] and WISE). To the northwest, emission from the molecular cloud dominates with the second dense but colder clump at an offset of about 70′′ from Her 36. [C I] diffuse emission is very extended in this region.

Taking this into consideration along with our analysis of the morphology carried out in Sects. 3 and 4, we propose a geometry of the region illuminated by Her 36 and 9 Sgr as presented in Fig. 15 (right). We propose that the cold dense molecular cloud is behind the bright stars Her 36 and 9 Sgr. Her 36 is still much closer to the dense part of the cloud in which it was born; in fact the foreground veil is part of the original cloud accelerated toward us by the strong radiation and wind of Her 36, showing the expansion of the nebula, blue shifted with respect to the molecular cloud (3–7 km s⁻¹) moving away from Her 36 (~9 km s⁻¹) toward the observer and the west, while the red-shifted (11–13 km s⁻¹) [C II] probes the background material toward the northeast of Her 36. The ultracompact HII region is fueled by Her 36 around its vicinity and the more extended diffuse HII region by 9 Sgr toward the east (Tothill et al. 2008).

Fig. 14 HST F487N (4865 Å) image of Her 36 and its surroundings overlaid with [C II] channel map contours (in white) at (a) 2 km s−1, (b) 5 km s−1, and (c) 7 km s−1 to show the progression of the foreground material at lower velocities. Her 36 is the central position (Δ α = 0, Δ δ = 0) at RA(J2000) denoted with an asterisk at RA(J2000) = 18h03m40.3s and Dec(J2000) = − 24°22′43′′, with the hourglass nebula representing the two bright hotspots about ~15′′ to the east. Contour levels are from 10% to 100% in steps of 10% of the peak emission observed at the channel with 10 km s−1 velocity.

Fig. 15 Left panel: velocity integrated intensities normalized to their value at peak vs. offset (′′) from Her 36 along the green arrows shown in the right. They follow a path from the second ATLASGAL peak at (−53′′, 23′′) to the second [C II] peak at (60′′, 27′′) via Her 36 at(0′′, 0′′). Plots are shown for [C II], [C I], J = 6 → 5 12 CO, J = 2 → 1 C18 O, Hα, ATLASGAL870 μm, and WISE 3.4 μm emission; right panel: schematic diagram of the prominent optical features of M8 pertinent to the discussion in this paper. The cold and dense molecular cloud is in the background shown in blue. The foreground gas of the warm PDR veil is receding away from Her 36 (~9 km s−1) with lower velocities (2–7 km s−1), while the HII region is powered by both stellar systems, Her 36 and 9 Sgr.

5.2 Comparison with PDR of the Orion Bar

The Orion Bar is a well-known PDR and its properties are described in depth by Hollenbach & Tielens (1997) and Walmsley et al. (2000). It is part of the OMC-1 core at the edge of M42 and is illuminated by young massive stars, which form a trapezium at the center of the Orion Nebular Cluster, mainly from θ¹ C, which is a O5–O7 star. Also, the PDR appears to be located at the edge of the HII blister tangential to the line of sight (Peng et al. 2012). A comparison of L_CO2−1, L_{[C II]} and L_FIR between M8 and the Orion Bar is presented in Table 4. For M8, the L_CO2−1 = 0.128 L⊙ (calculated in a similar way as in Sect. 4.4), L_{[C II]} is calculated in Sect. 4.4 and L_FIR ~ 10⁴ L⊙ is obtained from White et al. (1998). The luminosities of the Orion Bar are taken from Goicoechea et al. (2015).

We calculated the FUV radiation field, G0 ~ 0.6–1.12 × 10⁵ in Habing units and density, n ~ 0.97–1.93 × 10⁵ cm⁻³ for the PDR of M8 by adopting an electron density ne of 2000–4000 cm^–3, electron temperature Te of 7000–9000 K (Woodward et al. 1986; Esteban et al. 1999). We used the values of stellar luminosity and number of ionizing photons for an O7 star from Sect. 7.2.1 of Tielens (2010). The densities match well with the RADEX calculations carried out in Sect. 4.4. Interestingly, these calculated values of G0 and n also match very well with those calculated for the Orion Bar. This leads us to a direct comparison of the results of the PDR models of the Orion Bar (Tielens & Hollenbach 1985a,b; Jansen et al. 1995; Hogerheijde et al. 1995; Hollenbach & Tielens 1997, 1999; Andree-Labsch et al. 2017) with the PDR of M8 since we expect similar chemical and thermal conditions in both PDRs. Tielens & Hollenbach (1985a) and Hollenbach & Tielens (1999) calculated the structure of the Orion Bar as a function of visual extinction Av. They show atypical case in which H2 does not self-shield until dust attenuation of the FUV photons creates an atomic surface layer. According to this, in the PDR of M8 the transition of atomic H to molecular H2 occurs at Av = 2, the carbon balance shifts from C⁺ to C and CO at Av = 4, and except for the O in CO, all oxygen is in atomic form until very deep into the molecular cloud at Av = 8. The gas in the surface layer is much warmer at about ~500 K than the dust, which is at about ~30–75 K. Complementary to the H₂ column density calculation carried out in Sect. 4.1, assuming a dust temperature of 75 K and a maximum flux value of 5000 mJy beam⁻¹ obtained from ATLASGAL data, allowed us to calculate the H₂ column density at Her 36, N(H₂) ~ 3.75 × 10²² cm⁻² which is in reasonable agreement with that calculated in Sect. 4.1 within a factor ~1.25.

Hogerheijde et al. (1995), Walmsley et al. (2000), and Andree-Labsch et al. (2017) described the geometry of the Orion Bar where the trapezium stars illuminate the PDR, which changes from a face-on to an edge-on orientation along the varying length of the line of sight. This geometry explains the [C I] peak that is symmetric around the peak of the CO emission (Tauber et al. 1994). The [C II] emission peak is also distributed symmetrically around the ionization front (Stacey et al. 1993). Contrary to this, in M8, we see that [C II] peaks at the east of Her 36, [C I] peaks at Her 36, while the CO transitions peak in the northwest of Her 36, which supports the proposition of a face-on geometry.

5.3 Comparison with the PDR of M17 SW

M17, the Omega nebula is also among the best nearby laboratories to study star formation. It has an edge-on geometry in contrast to the face-on geometry of M8. It has a bright HII region ionized by the rich cluster NGC 6618 (Povich et al. 2009) and beyond this HII region lies the bright PDR in the southwest of M17 (M17 SW), which is responsible for the photoelectric heating of the warm gas (Pérez-Beaupuits et al. 2015a). M17 SW also contains a wide ranged clumpy molecular cloud studied widely by Pérez-Beaupuits et al. (2010) and Pérez-Beaupuits et al. (2015a).

In Sect. 3.2, the scatter plots related to M8 show only a weak correlation of [C II] with [C I] and ¹²CO. The channel maps and line profiles at different offsets also show different morphologies of [C II] compared with those seen in [C I] and CO except in a small range of intermediate velocities toward Her 36. This suggests that [C II] and the molecular gas tracers on scales away from Her 36 do not originate from the same spatial region. This is similar to M17 SW as reported by Pérez-Beaupuits et al. (2015a). In Sect. 4.3, the comparison between the observed ¹²CO and ¹³CO data with non-LTE RADEX modeling results show that the UC HII region or molecular gas near the Her 36 region has the highest density and kinetic temperature, while the molecular gas near the eastern HII region has low density and lower kinetic temperature. In contrast to M17 SW (Pérez-Beaupuits et al. 2015b), the ¹²CO SLED shapes we see in Fig. 13 toward Her 36 and mid-J CO positions follow a similar trend. Thus, they do not indicate large fluctuations in gas temperatures of the molecular gas. However, similarly to the case of M17 SW, the higher J CO lines show significantly lower intensities at the [C II] peak position. This is consistent with a PDR, where the [C II] peak emission is expected to arise from less dense gas than at the Her 36 position.

6 Conclusions

In this paper, we presented for the first time velocity integrated intensity maps of J = 11 → 10, J = 13 → 12, J = 16 → 15 ¹²CO, and [C II] 158 μm, observed toward Her 36 in M8 using the dual-color Terahertz receiver GREAT on board the SOFIA telescope; J = 2 → 1, J = 3 → 2, J = 6 → 5, and J = 7 → 6 ¹²CO transitions; J = 2 → 1, J = 4 → 3, and J = 6 → 5 ¹³CO transitions using the CHAMP⁺, FLASH⁺, and PI230 receivers of the APEX telescope; and J = 1 → 0 transitions of ¹²CO and ¹³CO using the EMIR receiver of IRAM 30 m telescope.

Combining the information obtained from Sects. 3 and 5.1, we put forward the geometry of the region surrounding Her 36. M8 has a face-on geometry where the cold dense molecular cloud lies in the background with Her 36 being still very close to the dense core of the cloud from which it was born. Her 36 is powering the HII region toward the east of it along with 9 Sgr, while the foreground veil of a warm PDR is receding away (at lower velocities) from Her 36 toward the observer.

Using different techniques we studied the physical conditions in the molecular gas associated with M8. CO rotation diagrams indicate temperature gradients through the PDR. Low-J ¹³CO transitions seem to originate from colder gas, while the J = 8 → 7 transition seems to originate from the hottest gas. Quantitative analysis including LTE approximation methods and the non-LTE RADEX program were used to calculate the temperatures and H2 number density in the PDR around Her 36. Kinetic temperatures ranging from 100–150 K and densities ranging from 10⁴–10⁶ cm^–3 were obtained.

Acknowledgements

SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 and 50 OK 1301 to the University of Stuttgart. We are thankful to the SOFIA operations team for their help and support during and after the observations. M. Tiwari was supported for this research by the International Max-Planck-Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. We also thank Thushara Pillai for helpful discussions. The research reported here was motivated by a discussion between Don York and Karl Menten.

Appendix A [C II] column density

For optically thin, thermalized [C II] emission and neglecting the effects from the background, the observed antenna temperature is found to be: (A.1)

where T_kin is the kinetic temperature, N([C II] ) is [C II] column density in cm⁻² and δv is the line width in km s⁻¹.

References

All Tables

All Figures

Fig. 1 Left panel: Lagoon Nebula, Messier object 8 (M8), or NGC 6523 in the constellation of Sagittarius, as seen by the Kitt Peak 4 m Mayall telescope in 1973. North is up, east to left. Credits to National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation Copyright WIYN Consortium, Inc., all rights reserved. Right panel: WISE 3.4 μm image of the M8 region around Her 36 (denoted with a star) investigated in this paper. The contour levels are 10% to 100% in steps of 10% of the peak emission ~4000 data number. The 1.5 pc indicated in the lower left correspond to 4′.

In the text

Fig. 2 Color maps of the integrated intensity of the [C II] 158 μm and J = 11 → 10, J = 13 → 12, and J = 16 → 15 transitions of 12CO toward Her 36, which is the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′, denoted with an asterisk. The contour levels are 10% (>3 × rms, given in Table 1) to 100% in steps of 10% of the corresponding peak emission given in Table 1. All maps are plotted using original beam sizes shown in the lower left of each map.

In the text

Fig. 3 Colour maps of the integrated intensity of the J = 2 → 1, J = 3 →2, J = 6 →5, and J = 7 → 6 transitions of 12CO, the J = 2 → 1, J = 4 →3, and J = 6 → 5 transitions of 13CO, the J = 2 → 1 line of C18O and [C I] 1 → 0 toward Her 36. This corresponds to the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′, denoted with an asterisk. The contour levels of 12C18O and [C I] are 3 × rms in steps of 2 × rms, while those of other molecules are from 10% (>3 × rms, given in Table 1) to 100% in steps of 10% of the corresponding peak emission given in Table 1. All maps are plotted using original beam sizes shown in the lower left of each map.

In the text

Fig. 4 Color maps of the integrated intensity (left to right) of the J = 1 → 0 transition of 12CO, 13CO and average of Hα 40, 41, 42, and 43 lines toward Her 36, which is the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′, denoted with an asterisk. The contour levels are 10% (>3 × rms, given in Table 1) to 100% in steps of 10% of the corresponding peak emission given in Table 1. All maps are plotted using original beam sizes shown in the lower left of each map.

In the text

	Fig. 5 Scatter plots and correlation coefficients r between the velocity integrated intensity of (a) [C II] and [C I] (b) [C II] and the J = 6 → 5 transition of 12CO, and (c) [C I] and the J = 2 → 1 transition of 13CO. All data points were extracted from velocity integrated intensity maps convolved to the same beam size of 31′′.
In the text

Fig. 6 Velocity channel maps of the 3P1 → 3P0 transition of [C I] (upper 16 panels) and the 2P3∕2 → 2P1∕2 transition of [C II] (lower 16 panels) in a range of 2–17 km s−1 with a channel width of 1 km s−1 toward Her 36, which is the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′, denoted with a black asterisk. The contour levels of [C I] are 3 × rms in steps of 2 × rms, while those of [C II] are from 10% (>3 × rms) to 100% in steps of 10% of the corresponding peak emission. All maps are plotted using original beam sizes shown in the lower left of both panels.

In the text

Fig. 7 A [C II] velocity integrated intensity map overlaid with contours of (a) ATLASGAL870 μm continuumemission. Important offset positions discussed in the text are indicated with triangles of different colors: black represents the secondary ATLASGAL peak deep inside the molecular cloud at (Δ α = −53.0′′, Δ δ =23.0′′), red represents the emission peak of mid-J transitions of 12CO and 13CO at (Δα = −13.0′′, Δ δ =8.0′′), blue represents the peak of the [C II] 158 μm emission at (Δα =30.0′′, Δ δ = −2.0′′), and light blue represents the clump to the east of Her 36 observed in the channel maps of [C II] at (Δ α =60.0′′, Δ δ =27.0′′). The white arrows point along the molecular cloud in the west to the HII regions in the east of Her 36; (b) VLA 1.3 cm free-free continuum emission; (c) WISE 3.4 μm, and (d) WISE 4.6 μm mid-infrared continuum. Her 36 is the central position denoted with an asterisk. The contour levels are 5% to 95% in steps of 10% of the peak emission for (a) and 10% to 100% in steps of 10% of the peak emission for (b), (c), and (d).

In the text

Fig. 8 Line profiles at different offsets (′′) relative to Her 36 are shown in different colors at five positions mentioned in the upper left plot. For the detailed positions, see Sect. 3.5. Upper panels: J = 6 → 5 12CO and J = 1 → 0 13CO spectra; lower panels: 3P1 → 3P0 and 2 P3∕2 → 2P1∕2transitions of [C I] and [C II]. All spectra were extracted from their original beam sizes as mentioned in Table 1.

In the text

	Fig. 9 Line profiles toward Her 36 for [C I] 3P1 → 3P0, 13CO J = 6 → 5, 12CO J = 6 → 5, and [C II] 2P3∕2 → 2P1∕2. All spectra are extracted from maps that were convolved to the same beam size of 15′′.
In the text

Fig. 10 For the J = 6 → 5 transition (a) shows the excitation temperature of 12CO, which is assumed to be equal to that of 13CO and (b) shows the column density of 13CO. The asterisk represents Her 36, which is the central position (Δα = 0, Δ δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = −24°22′43′′. The contour levels are 10% to 100% in steps of 10% of the corresponding peak emissions. All maps are plotted using original beam sizes shown in the lower left of each map.

In the text

Fig. 11 Rotational diagrams of J = 1 → 0, J = 2 → 1, J = 4 → 3, J = 6 → 5, and J = 8 → 7 transitions of 13CO at Her 36. Shown in black is the rotational diagram when 13CO is assumed to be optically thin. In red the rotational diagram is shown including the optical depth correction factors. Rotation temperatures obtained from different slopes are indicated. Values of the velocity integrated intensities for different transitions were extracted from maps convolved to the same resolution of 31′′. The error bars were calculated from the maximum noise level of the integrated intensities of individual transitions and from calibration uncertainties of 20%.

In the text

Fig. 12 For 3 different 12CO column densities, RADEX modeling results are shown for 12CO J = 6 → 5 peak main-beam brightness temperatures and corresponding 13CO J = 4 → 3/13CO J = 6 → 5 ratios. The blue lines denote the RADEX modeling results for different kinetic temperatures at 3 different volume densities, and color points represent data from near the HII region, PDR, and molecular cloud (see Sect. 4.3). Each point denotes a 12CO J = 6 → 5 temperature and 13CO J = 4 → 3/13CO J = 6 → 5 temperature ratio obtained from a single pixel. Data points are extracted from peak temperature maps convolved to the same resolution of 16′′. (a) The data points plotted are obtained for a 12CO column density range of 8 × 1017 –1.8 × 1018 cm−2 and for modeling the chosen input 12CO column density is 1 × 1018 cm−2. (b) The data points plotted are obtained for a 12CO column density range of 1.8 × 1018 –3.5 × 1018 cm−2 and for the modeling the input 12CO column density is 2 × 1018 cm−2. (c) The data points plotted are obtained for a 12CO column density range of 3.5 × 1018 –5.1 × 1018 cm−2 and for the modeling input the 12CO column density is 4 × 1018 cm−2.

In the text

Fig. 13 Results obtained from RADEX modeling for 12CO and 13CO at the mid-J CO peak (Δα = −13.0′′, Δ δ = 8.0′′), at Her 36 (Δα = 0′′, Δ δ = 0′′) and at the [C II] peak (Δα = 30.0′′, Δ δ = −2.0′′). Our observed data points are in green, red, and blue and are extracted from peak temperature maps convolved to the same resolution of 31′′. The column densities used in the modeling are for 12CO 4 × 1018 cm−2 and 8 × 1016 cm−2 for 13CO. Panels a and b: results obtained by varying the kinetic temperature from 50–250 K in steps of 50 K and keeping the density fixed at 105 cm–3; panels c and d: results obtained by varying the H2 density from 104–108 cm–3 in steps by a factor of 10 and keeping the kinetic temperature fixed at 120 K.

In the text

Fig. 14 HST F487N (4865 Å) image of Her 36 and its surroundings overlaid with [C II] channel map contours (in white) at (a) 2 km s−1, (b) 5 km s−1, and (c) 7 km s−1 to show the progression of the foreground material at lower velocities. Her 36 is the central position (Δ α = 0, Δ δ = 0) at RA(J2000) denoted with an asterisk at RA(J2000) = 18h03m40.3s and Dec(J2000) = − 24°22′43′′, with the hourglass nebula representing the two bright hotspots about ~15′′ to the east. Contour levels are from 10% to 100% in steps of 10% of the peak emission observed at the channel with 10 km s−1 velocity.

In the text

Fig. 15 Left panel: velocity integrated intensities normalized to their value at peak vs. offset (′′) from Her 36 along the green arrows shown in the right. They follow a path from the second ATLASGAL peak at (−53′′, 23′′) to the second [C II] peak at (60′′, 27′′) via Her 36 at(0′′, 0′′). Plots are shown for [C II], [C I], J = 6 → 5 12 CO, J = 2 → 1 C18 O, Hα, ATLASGAL870 μm, and WISE 3.4 μm emission; right panel: schematic diagram of the prominent optical features of M8 pertinent to the discussion in this paper. The cold and dense molecular cloud is in the background shown in blue. The foreground gas of the warm PDR veil is receding away from Her 36 (~9 km s−1) with lower velocities (2–7 km s−1), while the HII region is powered by both stellar systems, Her 36 and 9 Sgr.

In the text