A&A 615, A158 (2018) Astronomy https://doi.org/10.1051/0004-6361/201732437 & © ESO 2018 Astrophysics

Unveiling the remarkable photodissociation region of Messier 8? M. Tiwari1,??, K. M. Menten1, F. Wyrowski1, J. P. Pérez-Beaupuits1,2, H. Wiesemeyer1, R. Güsten1, B. Klein1, and C. Henkel1,3

1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel, 53121 Bonn, Germany e-mail: [email protected] 2 European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla 7630355, Santiago, Chile 3 Astronomy Department, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia

Received 8 December 2017 / Accepted 25 March 2018

ABSTRACT

Aims. Messier 8 (M8) is one of the brightest HII regions in the sky. We collected an extensive dataset comprising multiple sub- millimeter spectral lines from neutral and ionized carbon and from CO. Based on this dataset, we aim to understand the morphology of M8 and that of its associated photodissociation region (PDR) and to carry out a quantitative analysis of the physical conditions of these regions such as kinetic temperatures and volume densities. Methods. We used the Stratospheric Observatory For Infrared Astronomy (SOFIA), the Atacama Pathfinder Experiment (APEX) 12 m, and the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescopes to perform a comprehensive imaging survey of the emission from the fine structure lines of [C II] and [C I] and multiple rotational transitions of carbon monoxide (CO) isotopologs within 1.3 1.3 pc around the dominant Herschel 36 (Her 36) system, which is composed of at least three massive stars. To further explore the× morphology of the region, we compared archival infrared, optical, and radio images of the nebula with our newly obtained fine structure line and CO data, and in particular with the velocity information these data provide. We performed a quantitative analysis, using both LTE and non-LTE methods to determine the abundances of some of the observed species, kinetic temperatures, and volume densities. Results. Bright CO, [C II] and [C I] emission have been found toward the HII region and the PDR in M8. Our analysis places the bulk of the molecular material in the background of the nebulosity illuminated by the bright stellar systems Her 36 and 9 Sagitarii. Since the emission from all observed atomic and molecular tracers peaks at or close to the position of Her 36, we conclude that the star is still physically close to its natal dense cloud core and heats it. A veil of warm gas moves away from Her 36 toward the Sun and its associated dust contributes to the foreground extinction in the region. One of the most prominent star forming regions in M8, the 4 6 –3 Hourglass Nebula, is particularly bright due to cracks in this veil close to Her 36. We obtain H2 densities ranging from 10 –10 cm and kinetic temperatures of 100–150 K in the bright PDR caused by Her 36 using radiative transfer modeling of various∼ transitions of CO isotopologs. Key words. ISM: general – ISM: individual objects: M8 – submillimeter: ISM – HII regions – ISM: clouds

1. Introduction molecular clouds, cooling is dominated by the transitions of CO, observable at (sub)millimeter and FIR wavelengths. Modeling The influence of bright stars on their surrounding interstellar the relative intensity distributions of multiple lines from various medium (ISM) is immense. Their strong ultraviolet and far- molecular and atomic species allows us to derive the physical ultraviolet (FUV) fields give rise to bright HII regions and conditions in PDRs. photodissociation regions (PDRs). These are the best grounds Messier 8 (M8) is located in the Sagittarius-Carina arm, near to study the effect of UV and FUV photons on the heating and + our line of sight toward the Galactic center. It is located at a chemistry of ISM. The fine structure lines of C and O, observ- distance 1.25 kpc (10 corresponds to 0.36 pc) from the Sun able at far-infrared (FIR) wavelengths, are mainly responsible (Damiani∼ et al. 2004; Arias et al. 2006) with an error of 0.1 kpc for the cooling in these regions (Tielens & Hollenbach 1985b), (Tothill et al. 2008) and is about 34 12 pc in diameter.∼ M8 which allow us to deduce the amount and sometimes the source is associated with the open young stellar× cluster NGC 6530, the of heating as well. The fine structure line of C+ at 158 µm is one + HII region NGC 6523/33, and large quantities of molecular gas of the brightest lines in PDRs and traces the transition from H (Tothill et al. 2008). to H and H2 as C has an ionization potential of 11.3 eV (e.g., h m s + The open cluster NGC 6530 (centered at RA 18 04 24 , Pabst et al. 2017). In PDRs, a C layer extends to a depth of Dec 24◦2101200(J2000)) is a relatively young cluster (formed A 2–4, after which C+ recombines to C probing the interface − v ∼ about 2–4 Myrs ago, Chen et al. 2007) and contains to CO (Hollenbach & Tielens 1999). Deeper into the associated several bright O-type stars. The brightest among them is Her 36 (Woolf 1961) at RA 18h03m40.3,s Dec 24 22 43 (J2000). It is ? The reduced datacubes (FITS files) are only available at the CDS − ◦ 0 00 via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via resolved into three main components: a close massive binary http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/615/A158 consisting of an O9 V and a B0.5 V star and a more distant ?? Member of the International Max Planck Research School (IMPRS) companion O7.5 V star (Arias et al. 2010; Sanchez-Bermudez for Astronomy Astrophysics at the Universities of Bonn Cologne. et al. 2014). Her 36 is responsible for ionizing the gas in

Article published by EDP Sciences A158, page 1 of 17 A&A 615, A158 (2018) M. Tiwari et al.: Unveiling the remarkable photodissociation region of Messier 8

9 Sgr Her 36

1.5 pc

Fig.Fig. 1. LeftLeft panel: panel: Lagoon Lagoon Nebula, Nebula, Messier Messier object object 8 (M8), 8 (M8), or NGC or 6523NGC in 6523 the constellation in the constellation of Sagittarius, of Sagittarius, as seen by asthe seen Kitt Peak by the 4 m Kitt Mayall Peak 4 m Mayalltelescope telescope in 1973. in North 1973. isNorth up, eastis up, to east left. to Credits left. Credits to National to National Optical Optical Astronomy Astronomy Observatory Observatory/Association/Association of Universities of Universities for Research for Research in in Astronomy/NationalAstronomy/National Science Foundation Copyright Copyright WIYN WIYN Consortium, Consortium, Inc., Inc., all all rights rights reserved. reserved. TheRight right panel panel: WISE shows 3.4 theµ WISEm image 3.4 ofµm the image M8 region aroundof the M8 Her region 36 (denoted around Herwith 36 a star)(denoted investigated with a star) in thisinvestigated paper. The in this contour paper. levels The contour are 10% levelsto 100 are% 10%in steps to 100% of 10% in steps of the of 10% peak of emission the peak 4000 emission 4000 data number. The 1.5 pc indicated in the lower left correspond to 40. ∼ data number.∼ The 1.5 pc indicated in the lower left correspond to 40.

1 thea resolution western of half 2.44 of MHz, the HII corresponding region of to NGC 0.4 km 6523 s− includingfor up- thebinary center atwith (30 an0 , orbit300) of was9 chosen yr duration, as reference, consisting similar of to an the O3.5 V 1 − ∼ theGREAT bright and Hourglass 0.6 km s− Nebulafor the L1 (Woolf channel. 1961 We; Lada subtracted et al. spec- 1976; SOFIAprimary observations. and an O5-5.5 The pointing V secondary accuracy (Rauw (< 3 et00) al. was 2012 main-). South- Woodwardtral baselines et of al. first 1986 order). Lada and subsequently et al.(1976) compared produced data the cubes optical tainedeast by of pointing the cluster at bright core sources (NGC such 6530), as RAFGL5254 another cluster, and R M8E, andwith millimeter-wave beam/3 sampling. observations For the beam of sizes, the see M8 Table region 1. and sug- Doralthough every 1 – optically 1.5 hrs. A invisible, forward iseffi associatedciency Feff with= 0.95 two was massive used star gested the molecular cloud is located behind the HII region of forforming all receivers, regions and (theTothill beam et coupling al. 2008 effi).ciencies A superpositionBeff = 0.62, of four 0.69, 0.63, 0.43, and 0.32 were used for the PI230, FLASH+340, the2.2. nebula, APEX data similar to the Orion-KL nebula. An ultracompact HII regions+ seems to+ be responsible for+ the ionization of the gas HII region, G5.97–1.17 is also very close to Her 36 at (RA FLASHin M8:460, the CHAMP Hourglass660, Nebula and CHAMP illuminated810 by receivers, Her 36,re- the core h m s 12 13 spectively. 18Observations03 40.5 , ofDec low-24 and◦22 mid-044.3J 00(J2000))CO and (MasquéCO transitions et al. 2014 were). of NGC 6523 illuminated by Her 36, the remaining parts of performedA multiband with the near-IR− Atacama image Pathfinder of Her 36 Experiment and its surroundings (APEX) NGC 6523 and NGC 6533 illuminated by 9 Sgr (O4V) (Tothill presented12 m submillimeter in Goto et telescope al.(2006 (Güsten) shows et the al. 2006) IR source during Her 2015 36 et al. 2008), and M8E illuminated by HD 165052 (Lynds & Oneil June - August and 2016 July, September, and October. As shown 2.3. IRAM 30 m data SE lying 0.2500 SE of Her 36 and it is completely obscured. It 1982; Woodward et al. 1986). isin inferred Table 1, to we be used an the early-type following B receivers: star with PI230 a visual to map extinction the Although M812 has been13 studied extensively in the X-ray, low-J 12CO and 13CO transitions, FLASH+ in the 345 and 460 Observations of low-J CO, CO, and hydrogen recombination AV > 60 mag that is deeply embedded in dense, warm+ dust lineoptical, observations and IR were regimes performed (Stecklum with the et IRAM al. 1995 30 m; Damiani telescope et al. andGHz is bands powering to map the the ultracompact mid-J CO transitions, HII region and G5.97–1.17. CHAMP in The 2004; Arias et al. 2006; Goto et al. 2006; Damiani et al. low- and high-frequency subarrays to map the higher frequency in August 2016. We observed the whole 3 mm range using the morphology of H2 and CO J = 3 2 emission around Her EMIR2017 receivers), only few (Carter studies et al. have 2012). been We performed simultaneously at millimeter mapped and mid-J CO transitions. → 36 (White et al. 1997; Burton 2002) is in accordance with the a regionsubmillimeter of 240 x wavelengths.240 , which is White similar et to al. the(1997 size) of reported most other the dis- We used the PI230 receiver to map C18O J = 2 1 at 00 00 Hubble maps previously observed with SOFIA and APEX in the 12CO 219.560 GHzSpace , 13 TelescopeCO J = 2 (HST)1 at 220.398 jet-like GHz, feature and 12 detectionsCO→ J = covery of the second strongest source of millimeter and sub- and 13CO J = 1 0 transitions (Table 1) and the hydrogen re- extending2 1 at 230.538 0.500 southeast GHz. FLASH of→ Her+ 36was (Stecklum used in the et 345 al. 1995 GHz), band which millimeter wavelength CO line emission in our Galaxy toward combination lines→ H40α to H43α at 99.023 GHz, 92.034 GHz, suggestto→ map the there12CO mightJ = 3 be a2 molecular transition at outflow 345.795 in GHz. the core FLASH of+ M8 Her 36 in M8 (White et al. 1997). Lada et al.(1976) com- 85.688 GHz, and 79.912 GHz, respectively. Molecular high den- (wasBurton also 2002 used). in X-ray the 460→ emission GHz band from to Her map 36 the and13CO diffuseJ = 4 X-ray pared optical and millimeter-wave observations to sketch the sity tracers, which were also detected in our wide spectral band emission3 transition from at 440.765 the Hourglass GHz, C region,18O J = which4 3 is at the 439.088 brightest GHz,→ part morphology of M8 where the core surrounding Her 36, the of the optically3 visible3 nebula located →15 east from Her 36 observation,hourglass will nebula be analyzed with its in structure a subsequent and thepaper. eastern part of M8 and [C I] P1 P0 fine structure line∼ at 492.16000 GHz. The (CHAMPRauw et+ al.receiver 2002→ ), was suggest used the to map presence12CO ofJ a= bubble6 5 at of 691.473 hot gas of areAll described. maps shown Tothill in Fig. et 4 al. were(2002 observed) presented in OTF submillimeter- total power and sizeGHz 0.4 and pc13 thatCO J is= produced6 5 at by 661.067 the interaction GHz in the of→ low-frequency the stellar wind modemillimeter-wavelengths centered on Her 36. Each maps subscan of the lastedJ = 2 25 s1 andand theJ in-= 3 2 12 → → ofsubarray Her 36 complemented with the denser→ by part12CO of theJ = molecular7 6 at 806.651 cloud in GHz the and back- tegrationtransitions time on of theCO off-source tracing referencethe molecular position gas was and 5dust s. The around 13 → ground.CO J = Anomalously8 7 at 881.272 broad GHz diffuse in the interstellar high-frequency bands subarray. (DIBs) at offsetHer position 36. relative to the center at (300, 300) was similar to → − 5780.5,All maps5797.1, shown 6196.0, in Fig. and 36613.6 were observedÅ along with in OTF CH total+ and power CH are that usedWe for report the SOFIA a comprehensive and APEX mapping survey andof the the 1.5pointing1.5 pc h m s × foundmode incentered absorption on R.A. along 18 the03 line40 .3, of Dec. sight to24 Her◦22’43 3600 (Dahlstrom(J2000), accuracy(40 4 (0<)3 region00) was around maintained Her by 36 pointing (as shown at the by bright the blue cali- square − × etwhich al. 2013 corresponds). CH+ toand the CHposition are of radiatively Her 36. The excited maps obtained by strong bratorin Fig. 1757-2401) at FIR, every millimeter- 1 – 1.5 hrs. and A submillimeter forward efficiency wavelengthsFeff = to FIRfrom emission the observations from the carried adjacent out in IR July, source September, Her 36 and SE Oc-(Goto 0.95probe and thea beam physical coupling conditions efficiency andB imageeff = 0.69 the morphology were adopted of this tober 2016 have a size of 24000 x 24000. Maps that were ob- for the EMIR receivers. These values were taken from the latest et al. 2006) and the broadening of DIBs is attributed to radia- exceptional PDR. We present4 for the first time extended maps of tivetained pumping from the of observations closely spaced carried high- outJ rotational in 2015 are levels compara- of small (2015)this commissioning region in the 158 reportµm. fine structure line of C+, high-J tran- polartively carrier smaller molecules in size. We (Dahlstrom integrated 0.7 et al. s per 2013 dump; Oka for et all al. maps 2014; sitions of 12CO emission observed with the GREAT1 receiver Yorkand enough et al. 2014 coverages). We were performed performed (sub)millimeter to reach the observations rms noise levels as mentioned in Table 1. The offset position relative to 4 www.iram.es/IRAMES/mainWIKI/IRAM30mEfficiencies related to these species that will be discussed in a future paper. 1 GREAT is a development by the MPI für Radioastronomie and The eastern half of the HII region is illuminated by the KOSMA/Universität zu Köln, in cooperationArticle number, with the page MPI 3 für of 18 Sonnen- 9 Sgr stellar system, as shown in Fig.1. 9 Sgr is a well-known systemforschung and the DLR Institut für Planetenforschung.

A158, page 2 of 17 M. Tiwari et al.: Unveiling the remarkable photodissociation region of Messier 8

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 h m → s → → Her 36, which is the central position (∆α = 0, ∆δ = 0) at RA(J2000) = 18 03 40.3 and Dec(J2000) = 24◦2204300, denoted with an asterisk. The contour levels are 10% (>3 rms, given in Table1) to 100% in steps of 10% of the corresponding peak− emission given in Table1. All maps are plotted using original beam× sizes shown in the lower left of each map. on board SOFIA observatory, the mid-J transitions of 12CO and 2. Observations 13 + + CO using the PI230, FLASH , and CHAMP receivers of the 2.1. SOFIA/GREAT data APEX2 telescope, and low-J transitions of 12CO and 13CO using 3 the EMIR receiver of the IRAM 30 m telescope. The high-J CO and [C II] 158 µm observations summarized

2 in Table1 were conducted with the L1 channel of the This publication is based on data acquired with the Atacama German Receiver for Astronomy at Terahertz frequencies Pathfinder EXperiment (APEX). APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern (GREAT; Heyminck et al. 2012) and the upGREAT LFA arrays Observatory, and the Onsala Space Observatory. (Risacher et al. 2016) on board the Stratospheric Observatory 3 Based on observations carried out with the IRAM 30 m telescope. for Infrared Astronomy (SOFIA; Young et al. 2012). The data IRAM is supported by INSU/CNRS (France), the MPG (Germany), and was acquired during observatory flight #297 on 2016 May 14 IGN (Spain). at 14.2 km altitude and under a median water vapor column

A158, page 3 of 17 A&A 615, A158 (2018) of 11 µm. The upGREAT was employed with 14 pixels (seven October 2016 have a size of 24000 24000. Maps that were pixels for each polarization, with a hexagonal layout). The spec- obtained from the observations carried× out in 2015 are compara- tral analysis was performed by means of fast Fourier transform tively smaller in size. We integrated 0.7 s per dump for all maps spectrometers (Klein et al. 2012), in a mode providing 4.0 GHz and enough coverages were performed to reach the rms noise bandwidth with 214 spectral channels. levels as mentioned in Table1. The offset position relative to 12 In the first setup we simultaneously mapped the CO the center at (300, 300) was chosen as reference, similar to the − J = 11 10 transition at 1267.014.486 GHz and the [C II] SOFIA observations. The pointing accuracy (<300) was main- 2 →2 P3/2 P1/2 fine structure line at 1900.537 GHz. In a tained by pointing at bright sources such as RAFGL5254 and → 12 second setup the CO J = 13 12 and J = 16 15 transitions R Dor every 1–1.5 h. A forward efficiency Feff = 0.95 was at 1496.923 GHz and 1841.345→ GHz were mapped,→ respec- used for all receivers, and the beam coupling efficiencies tively. Typical single-sideband system temperatures ranged Beff = 0.62, 0.69, 0.63, 0.43, and 0.32 were used for the PI230, between 1600 and 1800 K for the lower frequency L1 chan- FLASH+340, FLASH+460, CHAMP+660, and CHAMP+810 nel and between 2080 and 2260 K for the higher frequency receivers, respectively. upGREAT array with atmospheric transmissions of 0.90–0.94 and 0.85–0.88, respectively. All maps shown in Fig.2 were 2.3. IRAM 30 m data observed in on-the-fly (OTF) total power mode with a sam- h m s 12 13 pling of 600 centered on RA 18 03 40.33, Dec 24◦2204200. 7 Observations of low-J CO, CO, and hydrogen recombina- (J2000), the Her 36 location. We integrated 0.4− s per record tion line observations were performed with the IRAM 30 m for the CO (11 10)/[CII] setup, and 0.8 s for the CO telescope in August 2016. We observed the whole 3 mm range (13 12)/(16 15)→ setup. The originally chosen reference using the EMIR receivers (Carter et al. 2012). We simultane- → → ously mapped a region of 240 240 , which is similar to position at (∆α, ∆δ) = (+50000, 50000) (relative to the map cen- 00 × 00 ter) was found to be contaminated− in both transitions of the first the size of most other maps previously observed with SOFIA and APEX in the 12CO and 13CO J = 1 0 transitions setup and was therefore changed in favor of a second, clean → reference at offset (+30 , 30 ), while the pointing accuracy of (Table1) and the hydrogen recombination lines H40 α to H43α at 0 − 0 <300 was maintained. 99.023 GHz, 92.034 GHz, 85.688 GHz, and 79.912 GHz, respec- The raw data was calibrated with the program kalibrate tively. Molecular high density tracers, which were also detected (Guan et al. 2012), which is part of the KOSMA software pack- in our wide spectral band observation, will be analyzed in a subsequent paper. age. The resulting antenna temperatures T ∗ were converted to A All maps shown in Fig.4 were observed in OTF total power main beam brightness temperatures T = T ∗ F ff/B ff using a mb A · e e mode centered on Her 36. Each subscan lasted 25 s and the inte- forward efficiency Feff = 0.97 and beam efficiencies of 0.66 for the L1 channel of GREAT, and 0.58 to 0.68 for the upGREAT gration time on the off-source reference position was 5 s. The offset position relative to the center at (30 , 30 ) was similar to pixels, with a median value of 0.65. In order to optimize the 0 − 0 signal-to-noise ratio per channel, the spectra were smoothed that used for the SOFIA and APEX mapping and the pointing 1 accuracy (<300) was maintained by pointing at the bright cali- to a resolution of 2.44 MHz, corresponding to 0.4 km s− for 1 brator 1757-240 every 1–1.5 h. A forward efficiency Feff = 0.95 upGREAT and 0.6 km s− for the L1 channel. We subtracted spectral baselines of first order and subsequently produced data and a beam coupling efficiency Beff = 0.69 were adopted for the cubes with beam/3 sampling. For the beam sizes, see Table1. EMIR receivers. These values were taken from the latest (2015) commissioning report4. 2.2. APEX data All data reduction was performed using the CLASS and MIRA programs that are a part of the GILDAS5 software Observations of low- and mid-J 12CO and 13CO transitions were package and all the observations are summarized in Table1. performed with APEX 12 m submillimeter telescope (Güsten et al. 2006) during 2015 June–August and 2016 July, Septem- 3. Results ber, and October. As shown in Table1, we used the following receivers: PI230 to map the low-J 12CO and 13CO transitions, 3.1. Peak intensities of the molecular line emission + FLASH in the 345 and 460 GHz bands to map the mid-J CO The maxima of the distributions of the velocity integrated inten- transitions, and CHAMP+ in low- and high-frequency subarrays sities of the emission in the [C I] and [C II] lines and different to map the higher frequency mid-J CO transitions. 12 13 18 18 transitions of CO, CO, and C O are presented in Table1. We used the PI230 receiver to map C O J = 2 1 Figure2 shows velocity integrated intensity maps of the 12CO at 219.560 GHz, 13CO J = 2 1 at 220.398 GHz,→ and 12 → + J = 11 10, 13 12, and 16 15 transitions. The emission CO J = 2 1 at 230.538 GHz. FLASH was used in the in all the→ lines has→ a similar spatial→ distribution and peaks are 345 GHz band→ to map the 12CO J = 3 2 transition at + → found at about the same offset position (∆α = 5.000, ∆δ = 5.000) 345.795 GHz. FLASH was also used in the 460 GHz northwest of Her 36. band to map the 13CO J = 4 3 transition at 440.765 GHz, 18 → 3 3 Figures3 and4 show velocity integrated intensity maps of C O J = 4 3 at 439.088 GHz, and [C I] P1 P0 fine 12 13 18 → + → low- and mid-J transitions of CO, CO, and C O, i.e., the structure line at 492.160 GHz. The CHAMP receiver was 12 12 13 J = 1 0, 2 1, 3 2, 6 5, 7 6 transitions of CO, used to map CO J = 6 5 at 691.473 GHz and CO J = → → → → → 13 → the J = 1 0, 2 1, 4 3, and 6 5 transitions of CO, and 6 5 at 661.067 GHz in the low-frequency subarray comple- → → → 18 → → 12 13 the J = 1 0 transition of C O. The intensities of the low-J mented by CO J = 7 6 at 806.651 GHz and CO J = 8 7 → 12 → → transitions peak close to Her 36 (∆α = 0.000, ∆δ = 0.000) for CO at 881.272 GHz in the high-frequency subarray. mid-J transitions; the peaks shift toward the northwest of Her 36 All maps shown in Fig.3 were observed in OTF total power h m s with offsets of (∆α = 13.000, ∆δ = 8.000). It seems like there mode centered on RA 18 03 40.3, Dec 24◦2204300(J2000), − which corresponds to the position of Her 36.− The maps obtained 4 www.iram.es/IRAMES/mainWiki/IRAM30mEfficiencies from the observations carried out in July, September, and 5 www.iram.fr/IRAMFR/GILDAS/

A158, page 4 of 17 M. Tiwari et al.: Unveiling the remarkable photodissociation region of Messier 8 M. Tiwari et al.: Unveiling the remarkable photodissociation region of Messier 8

Fig. 3. Colour maps of the the integrated integrated intensity intensity of of the theJJ==22 1,1,JJ ==33 2,2, JJ == 66 5,, andand JJ == 7 6 transitions of of 1212CO,CO, the the JJ == 22 1,1, 13 13 →→ 18 →→18 → → → J ==44 3,3and, andJ J= =6 6 5 transitions5 transitions of ofCO,CO, the theJ = J2 = 21 line1 of line C ofO CandO [C andI] 1 [C I]0 1toward0 toward Her 36. Her This 36. corresponds This corresponds to the central to the position central →→ →→ h m s h m s→ → → → 12 18 12 18 (position∆α = 0, (∆∆δα== 0)0, at∆ RA(J2000)δ = 0) at R.A.(J2000) = 18 03 40.3= 18and03 Dec(J2000)40.3 and Dec.(J2000) = 24◦22043=00, denoted24◦22043 with00, denoted an asterisk. with The an asterisk. contour Thelevels contour of C levelsO and of [CCI] areO − − 3andrms [C I] in are steps 3 ofrms 2 inrms, steps while of 2 thoserms, of otherwhile molecules those of other are from molecules 10% (> are3 fromrms, 10% given (> in3 Tablerms,1) togiven 100% in inTable steps 1) of to 10% 100% of inthe steps corresponding of 10% of peakthe× corresponding emission given× peak× in Table emission1. All given× maps in are Table plotted 1. All using maps original are plotted beam using sizes× original shown in beam the lower sizes× shownleft of each in the map. lower left of each map.

12 ispeak a systematic close to Her shift 36 in ( the∆α peak= 0.0 emission00, ∆δ = of0.0 CO00) transitionsfor CO mid-J with whichFigs. is toward 2 and 3 the show east velocity of Her 36 integrated and the emission intensity extends maps of even the 2 2 low-transitions;J peaking the near peaks Her shift 36, towardmid-J peaking the northwest toward of the Her northwest, 36 with [Cfurther.II] P This extendedP and emission [C I] 3P comes3P fromtransitions. the part [C ofI] the peaks HII 3/2 → 1/2 1 → 0 whileoffsets high- of (∆Jαlines= 13 peak.0 , again∆δ = closer8.0 ). to It Her seems 36. like Nevertheless, there is a sys- all atregion Her 36 that and is illuminatedis very bright by toward the stellar the northwest system 9 Sgr of Her (Tothill 36. [C et al.II] − 00 00 mapstematic show shift at in least the peaka small emission offset toward of CO transitionsthe northwest with and low- theJ peaks2008). at an offset of (∆α = 30.600, ∆δ = 1.600), which is toward emissionpeaking near from Her CO 36, transitions mid-J peaking becomes toward more the and northwest, more compact while the eastFigure of4Her shows 36 a and velocity the emission integrated extends intensity− even map further. of an aver- This withhigh- increasingJ lines peakJ. again closer to Her 36. Nevertheless, all maps extendedage of the emission H40α to H43 comesα hydrogen from the recombination part of the HII lines. region We that have is showFigures at least2 and a small3 show offset velocity toward integrated the northwest intensity and maps the emis- of illuminatedtaken the average by the to stellar obtain system a better 9 signal-to-noiseSgr (Tothill et al. ratio. 2008). The dis- 2 2 3 3 thesion [C fromII] COP3/2 transitionsP1/2 becomesand [C I] moreP1 andP more0 transitions. compact [C withI] tribution or the radio recombination line emission agrees well → → peaksincreasing at HerJ. 36 and is very bright toward the northwest of withFig. the 4 5 shows GHz continuum a velocity Very integrated Large intensity Array (VLA) map ofinterfero- an av- Her 36. [C II] peaks at an offset of (∆α = 30.600, ∆δ = 1.600), eragemetric of map the in H40 Fig.α 4to of H43 Woodwardα hydrogen et al. recombination(1986) and the lines. peak We of − have taken the average to obtain a better signal-to-noise ratio. A158, page 5 of 17 Article number, page 5 of 18 A&AA&A proofs: 615,manuscript A158 (2018) no. main

Fig. 4. Color maps of the integrated intensity (left(left toright) right) of of the theJJ ==11 00transition of 1212CO, 1313COCO and and average of of H Hαα40,40, 41, 41, 42, and 43 lines → h m h sm s towardtoward Her Her 36, 36, which which is is the the central central position position (∆ (α∆=α 0,= ∆0,δ∆=δ 0)= at0) RA(J2000) at R.A.(J2000) = 18 03= 1840.303 and40.3 Dec(J2000)and Dec.(J2000) = 24◦22= 0432400◦,22 denoted04300, denoted with an asterisk. with an − Theasterisk. contour The levels contour are levels 10% ( are>3 10%rms, (> given3 rms, in Table given1) in to Table 100% 1) in to steps 100% of in 10% steps of of the 10% corresponding of the corresponding peak emission peak− given emission in Table given1. in All Table maps 1. × areAll plotted maps are using plotted original using beam original sizes beam shown sizes× in theshown lower in leftthelower of each left map. of each map.

Table 1. Line parameters of observed transitions.

1 1 Transition Frequency (GHz) ηmb θmb(00) Peak line flux (K km s− ) rms (K km s− ) Telescope Telescope 12 12CO J = 1 0 115.271 0.73 22.5 610.2 1.1 IRAM 30m/EMIR J = 1 → 0 115.271 0.73 22.5 610.2 1.1 IRAM 30 m/EMIR J = 2 → 1 230.538 0.65 28.7 355.5 0.4 APEX APEX/PI230/PI230 → + J = 3 → 2 345.796 0.73 19.2 210.2 0.8 APEX APEX/FLASH/FLASH + → + J = 6 → 5 691.473 0.43 9.6 580.1 4.0 APEX APEX/CHAMP/CHAMP + → + J = 7 → 6 806.652 0.34 8.2 673.4 19.0 APEX APEX/CHAMP/CHAMP + J = 11 → 10 1267.014 0.68 22.9 130.9 3.5 SOFIA/GREAT J = 11 → 10 1267.014 0.68 22.9 130.9 3.5 SOFIA/GREAT J = 13 → 12 1496.923 0.68 19.1 155.3 2.4 SOFIASOFIA/GREAT/GREAT J = 16 → 15 1841.346 0.70 14.8 46.2 1.8 SOFIA/GREAT J = 16 → 15 1841.346 0.70 14.8 46.2 1.8 SOFIA/GREAT → 13 13CO J = 1 0 110.201 0.73 23.5 64.9 0.4 IRAM 30m/EMIR J = 1 → 0 110.201 0.73 23.5 64.9 0.4 IRAM 30 m/EMIR J = 2 → 1 220.399 0.65 30.1 92.6 0.6 APEX APEX/PI230/PI230 → + J = 4 → 3 440.765 0.59 15.0 198.1 2.5 APEX APEX/FLASH/FLASH + J = 6 → 5 661.067 0.45 10.0 158.8 3.2 APEX/CHAMP++ J = 6 → 5 661.067 0.45 10.0 158.8 3.2 APEX/CHAMP → 18 C18O J = 2 1 219.561 0.65 30.2 12.7 0.6 APEX/PI230 J = 2 → 1 219.561 0.65 30.2 12.7 0.6 APEX/PI230 → 12 12C 3P 3P 492.160 0.59 13.5 34.0 1.8 APEX/FLASH++ P1 → P0 492.160 0.59 13.5 34.0 1.8 APEX/FLASH → 12 + 12C+ 2 2 2P3/2 2P1/2 1900.53 0.70 14.8 728.5 2.1 SOFIASOFIA/GREAT/GREAT 3/2 → 1/2 Hα H40 – 43α 80–90 0.73 2626 11.2 0.2 IRAM 3030m m/EMIR/EMIR ≈≈

12 13 13 I II theThe H distributionα lines is at or ( the∆α radio= 10 recombination.000, ∆δ = 9.000 line), close emission to the agrees cen- 3.1.1.and [C CorrelationI] vs. CO betweenJ = 2 1CO, are shownCO, [C in] Fig.and5.[C We] chose terwell of with the Hourglass the 5 GHz Nebula, continuum which Very indicates Large Array the presence (VLA) of inter- hot CO 6 5 owing to its association→ with the warm PDR due ionizedferometric gas map in the in east Fig. of 4 of Her Woodward 36. et al. (1986) and the peak Asto its can higher→ be seen upper from level the velocity energy integrated compared intensity to low-J maps,CO transi- emis- of the Hα lines is at (∆α = 10.000, ∆δ = 9.000), close to the cen- siontions, from while [C weII] chose is spread13CO out 2 the1 most in particular, as compared as its to critical [C I], 12 13 → ter of the Hourglass Nebula,12 which13 indicates the presence of hot densityCO, and is comparableCO. In order to that to visualize of [C I]. the [C correlationII] is correlated between the 3.2. Correlation between CO, CO, [C I], and [C II] ionized gas in the east of Her 36. theseleast with species,12CO scatterJ = 6 plots5. Theof [C PearsonII] versus correlation [C I], [C coefficientII] versus 12 13 As can be seen from the velocity integrated intensity maps, emis- isCOr =J 0.471.= 6 Two5, branchesand→ [C I]appear versus toCO budJ out= 2 in the1 are upper shown left 12 → → sion from [C II] is spread out the most as compared to [C I], CO, inand Fig. lower 5. We right chose of this CO correlation. 6 5 owing The to upper its association left, where with the 13 → 12 and CO. In order to visualize the correlation between these the[C II warm] emission PDR due intensifies to its higher for a upper slowly level strengthening energy comparedCO J to= 12 low-J CO transitions, while we chose 13CO 2 1 in particu- species, scatter plots of [C II] vs. [C I], [C II] vs. CO J = 6 5, 6 5 emission, corresponds to the northeast of→ Her 36 where → lar,→ as its critical density is comparable to that of [C I]. [C II] is A158, page 6 of 17 Article number, page 6 of 18 M.M. Tiwari Tiwari et et al.: al.: Unveiling the remarkable photodissociation region of Messier 8

Fig. 5. Scatter plots and correlation coefficientscoefficients r between the velocity integrated intensity of (a)(a) [C IIII]] and and [C [C II]]( (b)b) [C [C IIII]] and and the the JJ == 66 5 12 13 → transitiontransition of of CO, and ((c)c) [C I] and the J == 22 1 transition of CO. All data points were extracted from velocity integrated intensity maps → convolved to to the the same same beam beam size size of of 31 310000..

12 13 I II coefficient of r = 0.473 and again shows two different branches Table 2. CO, CO, [C I],] and and [C [CII]] line line parameters. parameters. 12CO J = 6 5 emission, corresponds to the northeast of Her corresponding→ to different regions. The upper left, where12 [C II] 36 where [C II] is more extended. The lower right, where CO emission gets brighter for an almost constant [C I] emission, cor- aa 11 ∆ 11 b b J = 6 5 emission intensifies at a faster rate than [C II], cor- OffsetOffset ( (0000)) VV(km(km s s−− )) ∆VV(km(km s s−− )) TTpeakpeak(K)(K) responds to the northeast of Her 36. The branch in the lower responds→ to the southwest of Her 36, where 12CO J = 6 5 is 1212CO J = 6 5 right, similar to the situation shown in Fig.5b, corresponds→ to CO J = 6 5 much more prominent. The correlation of [C II] with [C I] has a → the southwest of Her 36. In contrast to these correlations, [C I] is ( 40, 35) 10.33 (0.06) 3.62 (0.16) 77.24 correlation coefficient of r = 0.473 and again shows two differ- (−40, 35) 10.33 (0.06) 3.62 (0.16) 77.24 well correlated with 13CO J = 2 1 with r = 0.908. This resem- − 5.925.92 (0.10) (0.10) 3.30 3.30 (0.26) (0.26) 58.41 58.41 ent branches corresponding to di→fferent regions. The upper left, (( 13, 8) bles the case M17 SW, for which a correlation coefficient of [C I] − 10.43 (0.04) 2.92 (0.10) 134.43 where [C II] emission gets brighter for an almost constant [C I] − 10.43 (0.04) 2.92 (0.10) 134.43 with 13CO J = 2 1 was reported to be 0.942 (Pérez-Beaupuits 6.396.39 (0.03) (0.03) 2.42 2.42 (0.07) (0.07) 45.67 45.67 emission, corresponds→ to the northeast of Her 36. The branch (0,(0, 0) 0) et al. 2015c). 10.4210.42 (0.02) (0.02) 3.58 3.58 (0.04) (0.04) 97.94 97.94 in the lower right, similar to the situation shown in Fig. 5 (b), (30,(30, 2) 10.28 10.28 (0.09) (0.09) 4.73 4.73 (0.20) (0.20) 62.60 62.60 corresponds to the southwest of Her 36. In contrast to these cor- − 13 (60,(60, 27)− 27) 11.49 11.49 (0.07) (0.07) 2.51 2.51 (0.17) (0.17) 53.77 53.77 relations,3.3. Channel [C I] maps is well correlated with CO J = 2 1 with r = → 1313CO J = 1 0 0.908. This resembles the case M17 SW, for which a correlation CO J = 1 0 In order to investigate the13 differences in the distribution of ion- → coefficient of [C I] with CO J = 2 1 was reported2 to be2 0.942 ( 40, 35) 8.47 (0.04) 3.00 (0.11) 8.26 ized and atomic carbon, channel→ maps of the P3/2 P1/2 (−40, 35) 8.47 (0.04) 3.00 (0.11) 8.26 (Pérez-Beaupuits et al. 2015c). 3 → 3 (− 13, 8) 8.76 (0.03) 2.58 (0.08) 12.61 transition of [C II] are compared to those of the [C I] P1 P0 (−13, 8) 8.76 (0.03) 2.58 (0.08) 12.61 → −(0,(0, 0) 0) 8.91 8.91 (0.01) (0.01) 2.68 2.68 (0.03) (0.03) 15.16 15.16 transition. Figure6 shows that the emission from [C II] (lower (30, 2) 9.37 (0.03) 2.48 (0.09) 11.15 panels) is more spread out as compared to that from [C I] (upper (30, −2) 9.37 (0.03) 2.48 (0.09) 11.15 1 (60, 27)− 10.48 (0.02) 1.45 (0.05) 8.46 3.2.panels). Channel In the maps velocity range from 2 to 6 km s− and 15 to (60, 27) 10.48 (0.02) 1.45 (0.05) 8.46 1 12 3 3 17 km s− there is no emission from [C I] while there is emission 12C 3P1 3 P0 C P1 → P0 from [C II] close to Her 36 and toward the east of it, respectively. → In order to investigate the differences1 in the distribution of (( 40, 35) 9.40 9.40 (0.63) (0.63) 5.91 5.91 (2.16) (2.16) 2.84 2.84 In the range from 7 to 9 km s− both [C II] and2 [C I]2 emis- − ionized and atomic carbon, channel maps of the P3/2 P1/2 (( 13, 8) 9.47 9.47 (0.25) (0.25) 3.63 3.63 (0.63) (0.63) 5.62 5.62 sion are found toward the west. These structures extend3 → further3 − transition of [C II] are compared to those of the [C I] P P −(0,(0, 0) 0) 9.89 9.89 (0.18) (0.18) 3.86 3.86 (0.47) (0.47) 7.65 7.65 toward the northeast for higher velocities in the range1 of→ 12 to0 transition.1 Fig. 6 shows that the emission from [C II] (lower (30,(30, 2) 10.05 10.05 (0.30) (0.30) 4.01 4.01 (0.64) (0.64) 5.45 5.45 15 km s− . This is very similar to the case of M17 SW, where − panels) is more spread out as compared to that from [C I] (upper (60,(60, 27) 27) 11.80 11.80 (0.09) (0.09) 0.68 0.68 (0.35) (0.35) 5.85 5.85 the [C II] channel map shows a strong spatial1 association1 with 12 + 2 2 panels). In the velocity range from 2 km s− to 6 km s− and1 12C + 2P3/2 2 P1/2 [C I] and1 CO channel maps1 only at intermediate 10 to 24 km s− C P3/2 P1/2 15 km s− to 17 km s− there is no1 emission from [C I] while1 → velocities. While at lower (<10 km s− ) and higher (>24 km s− ) 5.405.40 (0.14) (0.14) 2.84 2.84 (0.30) (0.30) 33.45 33.45 there is emission from [C II] close to Her 36 and toward the east ( 40, 35) velocity channels, [C II] emission is mostly not associated1 with ( 40, 35) 9.76 (0.11) 3.80 (0.31) 45.05 of it, respectively. In the range from 7 to 9 km s both [C II] − 9.76 (0.11) 3.80 (0.31) 45.05 the other tracers of dense and diffuse gas (Pérez-Beaupuits− et al. 4.924.92 (0.10) (0.10) 3.18 3.18 (0.25) (0.25) 43.04 43.04 and [C I] emission are found toward the west. These structures (( 13, 8) 2015c). Notably, our [C II] channel maps show a clumpy struc- − 9.719.71 (0.04) (0.04) 2.89 2.89 (0.10) (0.10) 110.50 110.50 extend further toward the northeast for higher velocities in the − ture at an offset of1 (∆α = 60.000, ∆1 δ = 27.000) which is missing 5.12 (0.23) 3.31 (0.57) 24.97 range of 12 km s− to 15 km s− . This is very similar to the (0, 0) 5.12 (0.23) 3.31 (0.57) 24.97 in the [C I] maps; this complements the argument that the east (0, 0) 9.94 (0.06) 4.11 (0.14) 114.49 case of M17 SW, where the [C II] channel map shows a strong 9.94 (0.06) 4.11 (0.14) 114.49 of Her 36 is comprised of hot gas and strong UV fields capable 3.883.88 (0.16) (0.16) 4.01 4.01 (0.36) (0.36) 37.66 37.66 spatial association with [C I] and CO channel maps only at (30,(30, 2) of ionizing carbon, i.e.,1 it is part of1 an HII region. This is con- − 9.729.72 (0.07) (0.07) 4.86 4.86 (0.16) (0.16) 100.10 100.10 intermediate 10 km s− to 24 km s− velocities. While at lower − sistent with1 the Hα and 5 GHz continuum1 VLA interferometric (60, 27) 10.22 (0.04) 3.34 (0.09) 112.83 (<10 km s ) and higher (>24 km s ) velocity channels, [C II] (60, 27) 10.22 (0.04) 3.34 (0.09) 112.83 maps presented− by Woodward et al.(1986− ) in their Figs. 1 and 4, emission is mostly not associated with the other tracers of a which also have their peak intensities east from Her 36. The(a) reference position is that of Her 36. (b) dense and diffuse gas (Pérez-Beaupuits et al. 2015c). Notably, Notes.b InThe units reference of main position beam brightness is that of Hertemperature. 36. In units of main beam brightness temperature. our [C II] channel maps show a clumpy structure at an offset of3.4. (∆ Ancillaryα = 60. data000, ∆δ = 27.000) which is missing in the [C I] 12 maps; this complements the argument that the east of Her 36 is [C II] is more extended. The lower right, where CO J = 6 5 For a multiwavelength view of M8 and in order to relate our correlated the least with 12CO J = 6 5. The Pearson correla-→ comprised of hot gas and strong UV fields capable of ionizing emission intensifies at a faster rate than [C II], corresponds to observations to the dense and cold molecular cloud and the tion coefficient is r = 0.471. Two branches→ appear to bud out in carbon, i.e., it is part of an HII region. This is consistent with the the southwest of Her 36, where 12CO J = 6 5 is much more hot ionized gas in M8, we compared our data with observa- the upper left and lower right of this correlation.→ The upper left, Hα and 5 GHz continuum VLA interferometric maps presented prominent. The correlation of [C II] with [C I] has a correlation tions obtained at other wavelength ranges. The surveys chosen where the [C II] emission intensifies for a slowly strengthening by Woodward et al. (1986) in their figs. 1 and 4, which also have A158, page 7 of 17 Article number, page 7 of 18 A&AA&A proofs: 615,manuscript A158 (2018) no. main

3 3 2 2 Fig. 6. VelocityVelocity channel channel maps maps of of the the P3P1 P30Ptransitiontransition of of [C [CI](I]upper (upper 16 16 panels panels)) and and the theP32/P2 P12/2Ptransitiontransition of [C ofII [C](lowerII] (lower 16 panels 16 panels)) in a Fig. 6. 1 1→ 0 1 3→/2 1/2 h m s 1 → 1 →∆α ∆δ rangein a range of 2–17 of 2km – s17− kmwith s− a channelwith a channel width of width 1 km of s− 1toward km s− Hertoward 36, which Her 36, is the which central is the position central ( position= 0, (∆α= 0)= 0, at RA(J2000)∆δ = 0) at R.A.(J2000) = 18 03 40.3= h m s I II and18 03 Dec(J2000)40.3 and = Dec.(J2000)24◦2204300=, denoted24◦220 with4300, a denoted black asterisk. with a black The contour asterisk. levels The contour of [C ] levels are 3 ofrms [C I] in are steps 3 ofrms 2 inrms, steps while of 2 thoserms, of while [C those] 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 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 lowershown left in theof both lower panels. left of both× panels. A158, page 8 of 17 Article number, page 8 of 18 M. Tiwari et al.: Unveiling the remarkable photodissociation region of Messier 8

Fig. 7. A [C II] velocity integrated intensity map overlaid with contours of (a) ATLASGAL 870 µm continuum emission. Important offset positions discussed in the text are indicated with triangles of different colors: black represents the secondary ATLASGAL peak deep inside the molecular 12 13 cloud at (∆α = 53.000, ∆δ = 23.000), red represents the emission peak of mid-J transitions of CO and CO at (∆α = 13.000, ∆δ = 8.000), blue − − represents the peak of the [C II] 158 µm emission at (∆α = 30.000, ∆δ = 2.000), and light blue represents the clump to the east of Her 36 observed − in the channel maps of [C II] at (∆α = 60.000, ∆δ = 27.000). 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). for this comparison are as follows: firstly, we extracted data also traces the cold molecular cloud in the northwest. The dust from the 870 µm APEX Telescope Large Area Survey of the emission morphology is similar to the 12CO, 13CO, and C18O Galaxy (ATLASGAL; Schuller et al. 2009) performed with distribution. Secondly, we used data from the National Radio the APEX 12 m telescope using the Large APEX BOlometer Astronomy Observatory (NRAO)/Very Large Array (VLA) 6 CAmera (LABOCA). The dust continuum emission probes Archive Survey (NVAS ). Figure7b shows the [C II] veloc- dense and cold clumps in the ISM of our Galaxy. Figure7a ity integrated intensity map overlaid with the 1.3 cm radio shows our [C II] velocity integrated intensity map overlaid with the ATLASGAL dust continuum image that peaks at Her 36 and 6 http://archive.nrao.edu/nvas/

A158, page 9 of 17 A&AA&A proofs: 615,manuscript A158 (2018) no. main A&A proofs: manuscript no. main

Fig. 8. Line profiles at differentdifferent offsetsoffsets (00) relative to Her 36 are shown in didifferentfferent colors at five positions mentioned in the upper left left plot.plot. For the Fig. 8. Line profiles at different offsets (00) relative to Her1212 36 are shown in different1313 colors at five positions3 mentioned3 33 in the22 upper2 left plot. For the detailed positions, see Sect. 3.4.3.5. Upper panels: panels: J J= =6 6 55 12COCO and and J J= =1 1 0 13CO spectra; lowerlower panels: panels:3PP11 3P00 andand 2PP33//22 2P1/2 transitionstransitions detailed positions, see Sect. 3.4. Upper panels: J = 6 →→5 CO and J = 1 →→0 CO spectra; lower panels: P1 →→ P0 and P3/2 → P1/2 transitions of [C II]] and and [C [C IIII].]˜. All All spectra spectra were were extracted extracted from from their their→ original original beam beam sizes sizes→ as as mentioned mentioned in in Table Table1 1.. → → of [C I] and [C II]˜. All spectra were extracted from their original beam sizes as mentioned in Table 1.

µ 12 13 and3.5. 4.6 Spectraµm (band of CO, 2) continuumCO, [C I] images,and [C II which] emission peak lines closer at to the [C II] peak. Overall, the mid-infrared emission that originates the [CdifferentII] peak. offsets Overall, the mid-infrared emission that originates from hot dust shows the best agreement with the morphology from hot dust shows the best agreement with the morphology12 seenFigure in8 theshows [C II a] image; comparison both probe between hot the material spectra from of HIICO, re- seen13 in the [C II] image; both probe hot material from HII re- gionsCO, and [C I warm], and surfaces [C II] emission of PDRs. lines at different offsets rela- gionstive to and Her warm 36. Line surfaces parameters of PDRs. of Gaussian fits to profiles are reported in Table2. In several cases the profiles show evidence 12 13 12 3.4.of two Spectra velocity of 12 componentsCO, 13CO, that[C I] wereand [C fitII separately.] emission The linesCO at 3.4. Spectra of CO, CO, [C I] and [C II] emission lines at J = 6 5 and 13CO J = 1 0 transitions are representative of different offsets the general→ appearance of all→ 12CO and 13CO line profiles dis- 12 13 Fig.cussed 8 shows in this a comparisonpaper. The differentbetween the offsets spectra were of chosen12CO, 13 alongCO, [Ca curvedI] and [ClineII] from emission the molecular lines at di cloudfferent in off thesets west relative to the to Hereast [C I] and [C II] emission lines at different o+ffsets relative to Her 36.of Her Line 36 parameters (see Fig.7a): of the Gaussian secondary fits to C profilespeak, which are reported corre- 3 3 13 insponds Table to 2. the In clump several observed cases the in the profiles channel show map evidence of [C II] of at Fig. 9. Line profiles toward Her 36 for [C I] 3P1 33P0, 13 CO J = 6 5, in Table 2. In several cases the profiles show evidence of Fig.12 9. Line profiles toward2 Her 36 for2 [C I] P1 → P00,, CO J = 6→ 55,, + 12 1212CO J = 6 5, and [C II] 2P3/2 22P1/2. All spectra→ are extracted→ from two(∆α = velocity60.000, components∆δ = 27.000); that the Cwerepeak, fit separately. which is the The emission12CO CO JJ = 66→ 5, and [C II] P3/2 → PP11//22.. All All spectra spectra are are extracted extracted from from two velocity2 components13 2 that were fit separately. The CO maps that were→ convolved to the→ same beam size of 1500. Jpeak= 6 of the5 andP3/132 CO PJ1/=2 1transition0 transitions of [C II] are at representative (∆α = 30.000, maps that were convolved convolved to to the the same same beam beam size size of of 15 150000.. J = 6 → 5 and CO→ J = 1 → 012 transitions13 are representative of∆δ the= → general2.000); Herappearance 36 is located of→ all 12 atCO (∆α and= 130.CO000, ∆ lineδ = profiles0.000); discussedthe mid-− J inCO this peak, paper. which The di isfferent the mid- offsetsJ transition were chosen emission along dust continuum image that peaks at Her 36 and also traces the discussed12 in this paper.13 The different offsets were chosen along continuumdust continuum image image7, which that peaks peaks very at Her close 36 to and Her also 36 and traces traces the apeak curved of CO line and fromCO the at molecular (∆α = 13 cloud.000, ∆ inδ = the8.0 west00); and to the cold molecular cloud in the northwest. The dust emission mor- a curved line from the molecular− cloud in the+ west to the free–freecold molecular emission cloud from in the the HII northwest. region NGC The 6523/33. dust emission This com- mor- eastsecondary of Her ATLASGAL 36 (see Fig. peak 7 (a)): arises the from secondary deep into C+ thepeak, molecular which phology is similar to the 12CO, 13CO, and C18O distribution. east of Her 36 (see Fig. 7 (a)): the secondary C peak, which pactphology HII isregion, similar which to the is12 alsoCO, traced13CO, by and the C H18O recombination distribution. correspondscloud to the to west the clump traced observed by ATLASGAL in the channel at (∆α map= of53 [C.0II00], Secondly, we used data from the National Radio Astronomy corresponds to the clump observed+ in the channel map of− [C II] linesSecondly, with wethe usedIRAM data 30 fromm telescope, the National is also Radio shown Astronomy in Fig.4 at∆δ (∆=α23=.06000)..000, ∆δ = 27.000); the C+ peak, which is the emission / at (∆α = 60.02 00, ∆δ = 227.000); the C 1peak, which is the emission Observatory (NRAO) Very Large Array (VLA) Archive Survey peakA of lower the 2P velocity/ 2P (2–6/ transition km s ) of component [C II] at (∆ isα spectrally= 30.000, (rightObservatory panel).6 (NRAO) Thirdly,/ theVery Wide-field Large Array Infrared (VLA) Survey Archive Explorer Survey peak of the P3 2 P1 2 transition− of [C II] at (∆α = 30.0 , (NVAS6). Fig. 7 (b) shows the [C II] velocity integrated intensity ∆δ = 2.0 ); Her3/2 → 36 is1 located/2 at (∆α = 0.0 , ∆δ = 0.0 ); the00 (WISE)(NVAS ). imaged Fig. 7 (b) the shows sky the at [C fourII] velocity mid-infrared integrated wavelengths. intensity7 ∆resolvedδ = . at00 several positions. The∆ higherα = . velocity00 ∆δ = component. 00 map overlaid with the 1.3 cm radio continuum image,7 which −2 000); Her 36 is located at ( 0 000, 0 000); the Figuremap overlaid7c and d with compare the 1.3 the cm [C radioII] velocity continuum integrated image, intensitywhich mid-emissionJ−CO lines peak, have which blue-shifted is the mid- wingsJ transition in the molecular emission cloud peak of in peaks very close to Her 36 and traces free-free emission from the 12mid-J CO13 peak, which is the mid-J transition emission peak of mappeaks with very WISE close to 3.4 Herµm 36 (band and traces 1) and free-free 4.6 µm emission (band 2) from contin- the 12theCO west, and while13CO the at (∆ emissionα = 13 is.0 red-shifted00, ∆δ = 8.000 toward); and the the [CsecondaryII] peak HII region NGC6523/33. This compact HII region, which is also CO and CO at (∆α = −13.000, ∆δ = 8.000); and the secondary1 uumHII region images, NGC6523 which peak/33. This closer compact to the HII[C II region,] peak. which Overall, is also the ATLASGALtoward the east peak compared arises− from to their deep emission into the peaking molecular at 9 cloud km s to− traced by the H recombination lines with the IRAM 30 m tele- ATLASGAL peak arises from deep into the molecular cloud to mid-infraredtraced by the emission H recombination that originates lines with from the hot IRAM dust 30shows m tele- the thetoward west our traced reference by ATLASGAL position, Her at ( 36.∆α Furthermore,= 53.000, ∆δ the= 23 peak.000). of scope, is also shown in Fig. 4 (right panel). Thirdly, the Wide- the west traced by ATLASGAL at (∆α = −53.000, ∆δ = 23.000). bestscope, agreement is also shown with thein Fig. morphology 4 (right panel). seen in Thirdly, the [C theII] image; Wide- the lines shifts from the east to the west to− lower velocities. The field Infrared Survey Explorer (WISE) imaged the sky at four 12 13 1 1 bothfield probe Infrared hot Survey material Explorer from HII (WISE) regions imaged and warm the skysurfaces at four of COA and lowerCO velocity line profiles (2 km are s−1 similar– 6 km in sbeing−1) component most intense is mid-infrared wavelengths. Fig. 7 (c) and (d) compare the [C II] A lower velocity (2 km s− – 6 km s− ) component is PDRs.mid-infrared wavelengths. Fig. 7 (c) and (d) compare the [C II] spectrallywith broadest resolved line widths at several at the positions. mid-J transition The higher emission velocity peak velocity integrated intensity map with WISE 3.4 µm (band 1) spectrally12 resolved13 at several positions. The higher velocity velocity integrated intensity map with WISE 3.4 µm (band 1) componentof CO and emissionCO ( lines∆α = have13 blue-shifted.000, ∆δ = 8 wings.000) and in the at Hermolec- 36 7 component emission lines have− blue-shifted wings in the molec- 6 NRAO/VLAhttp://archive.nrao.edu Archive Survey,/nvas/ (c) 2005–2007 AUI/NRAO. ularitself, cloud while in getting the west, less while intense the with emission narrower is red-shifted line widths toward at the 6 http://archive.nrao.edu/nvas/ ular cloud in the west, while the emission is red-shifted toward 7 NRAO/VLA Archive Survey, (c) 2005-2007 AUI/NRAO the [C II] peak toward the east compared to their emission 7 NRAO/VLA Archive Survey, (c) 2005-2007 AUI/NRAO the [C II] peak toward the east compared to their emission A158, page 10 of 17 Article number, page 10 of 18 Article number, page 10 of 18 M. Tiwari et al.: Unveiling the remarkable photodissociation region of Messier 8

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 (∆α = 0, ∆δ = 0) at RA(J2000) = 18h03m40.3s and Dec(J2000) = 24◦2204300. 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.

[C II] peak. The [C I] line profile gets most intense with broadest dust. Thus, the total contribution from dust and background can line width toward Her 36 itself with almost no emission from the be neglected as it contributes 1% to the resulting T . ≤ R∗ clumpy structure in the HII region at an offset of (∆α = 60.000, Assuming that the excitation temperature for 12CO and 13CO ∆δ = 27.000). As can also be seen from the comparison of vari- is the same and 12CO is optically thick, ous molecular transitions at Her 36 in Fig.9, CO and [C I] are 12 not associated with [C II] at lower and higher velocities (see also T ∗ ( CO) 1 R = . (1) Figs.5 and6). This is very similar to M17 SW as reported by 13 τ(13CO) T ∗ ( CO) 1 e− Pérez-Beaupuits et al.(2015c), where [C II] is not associated with R − other gas tracers at lower and higher velocities. The excitation temperature of the 12CO J = 6 5 transition can be estimated by further assuming a beam filling→ factor of 4. Analysis unity, i.e., TR∗ = T MB. This equation is written as

In this section we determine the temperature and density in the 33.2 1 = . + − , PDR of M8 with several complementary methods. We start by Tex 33 2 ln 1 12 K (2) TMB( CO) using the data for the J = 6 5 transition of CO, which has the    highest angular resolution,→ to estimate excitation temperatures where T MB is the main beam brightness temperature in K and column densities throughout the PDR as probed by the mid- that is estimated from the peak temperature map of 12CO J = J CO emission. 6 5 transition. The resulting Tex distribution is shown in Fig.→ 10a. Formally, we show lower limits to the excitation tem- 4.1. Excitation temperature and column density estimates perature due to the assumption of a beam filling factor of unity. It is highest immediately in the northwest of Her 36 and decreases A detailed description of spectral line radiative transfer rele- with distance from the star. vant here can be found in Draine(2011) and Mangum & Shirley Using the computed T ex and the main beam brightness tem- (2016). For a constant excitation temperature Tex, we can inte- 13 perature T MB estimated from the peak temperature map of CO grate the radiative transfer equation to obtain the observable J = 6 5, the total column density (Fig. 10b) of 13CO can be Rayleigh–Jeans equivalent temperature T (Eq. (1) in Peng R∗ calculated→ over the complete velocity range of the source from et al. 2012). The background radiation temperature comprises the cosmic background radiation of 2.73 K and the radiation from 13 12 116.2 warm dust. The latter was calculated using the results from the N( CO) = 1.06 10 (Tex + 0.88) exp × T Spectral and Photometric Imaging Receiver (SPIRE) of the ex ! European Space Agency (ESA) Herschel Space observatory. 13 2 T ( CO)dv cm− , (3) Data obtained in the second band at 350 µm (close to the MB 12CO 7 6 line) wavelength was used. The maximum intensity Z → 1 1 in the analyzed region around Her 36 is about 1200 MJy sr− near where Tex is in K and TMB dv is in K km s− . Figure 10b shows the dense molecular cloud, which corresponds to a Rayleigh– the resulting 13CO total column density with a peak value of Jeans equivalent brightness temperature of about 0.06 K from 5 1016 cm–2 northwest of Her 36. This results in a H column ∼ × 2 A158, page 11 of 17 A&A proofs: manuscript no. main

J = 6 5, the total column density (Fig. 10 (b)) of 13CO can be calculated→ over the complete velocity range of the source from

13 12 116.2 13 2 N( CO) = 1.06 10 (T +0.88) exp T ( CO)dv cm− , × ex T MB ex ! Z (3)

1 where Tex is in K and TMB dv is in K kms− . Fig. 10 (b) shows the resulting 13CO total column density with a peak value of 16 -2 ∼ 5 10 cm northwest of Her 36. This results in a H2 col- × 22 2 umn density N(H2) of 3.7 10 cm− adopting an isotopic abundance ratio [12CO/∼13CO]× of 63 (Milam et al. 2005) and a 12 ∼ 5 CO abundance ratio [ CO/H2] of 8.5 10− (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 arcmin 2 has an esti- mated value of 103 M , calculated from a flux of 133 Jy at 870 µm measured with ATLASGAL (Schuller et al. 2009)∼ assuming 2 an absorption coefficient of kν = 1.85 g cm and a temperature of 23 K (Urquhart et al. 2018), and not including potential un- certainties in the choice of these values. However, these mass estimations have an error of 26%, which accounts for errors ∼ A&A 615, A158of (2018)16% from the distance to the star (Tothill et al. 2008) and 20%∼ from the calibration. ∼ Table 3. Physical parameters calculated from rotational diagrams.

Optically thin 13CO Correction factor included a 13 -2 a 13 -2 Trot (K) N( CO) (cm ) Trot (K) N( CO) (cm ) 8.3 1.3 3.6 1016 8.6 1.5 6.2 1016 ± × ± × 41 3.7 3.6 1016 30 4.8 5.4 1016 ± × ± × 90 21.6 1.9 1016 39 4.9 4.6 1016 ± × ± × 84 10 2.1 1016 ± × Transition τ Transition τ J = 1 0 1.68 J = 1 0 1.92 J = 2 → 1 1.78 J = 2 → 1 2.20 J = 4 → 3 1.25 J = 4 → 3 0.74 J = 6 → 5 1.12 J = 6 → 5 0.28 J = 8 → 7 1.04 J = 8 → 7 0.10 → → Fig.Fig. 11. 11.RotationalRotational diagrams diagrams of JJ == 11 0,0,JJ==22 1,1,JJ==44 3,3, →→ 13 →→ →→ Notes. (a)Calculated for different gradients with their errors in the plot J J==6 6 5,5, and andJJ==88 77transitions transitions of of CO at at Her Her 36. 36. Shown Shown in in →→ →→ 13 as indicated in Fig. 11. blackblack is is the the rotational rotational diagram diagram when CO is is assumed assumed to to be be optically optically thin.thin. In In red red the the rotational rotational diagram diagram is shown including including the the optical optical depth depth correctioncorrection factors. factors. Rotation Rotation temperatures temperatures obtained from from di differentfferent slopes slopes 22 2 ff density N(H2) of 3.7 10 cm− adopting an isotopic abun- areare indicated. indicated. Values Values of of the the velocity velocity integrated intensities intensities for for di differenterent dance ratio [12CO/∼13CO]× of 63 (Milam et al. 2005) and a CO transitionstransitions were were extracted extracted from from maps maps convolved to to the the same same resolution resolution Fig. 12. For 3 different 12CO column densities, RADEX modeling re- of 3100. The error bars were calculated from the maximum noise level of 12 12 ∼ 5 of 3100. The error bars were calculated from the maximum noise level of sults are shown for CO J = 6 5 peak main-beam brightness tem- abundance ratio [ CO/H2] of 8.5 10− (Tielens 2010). The 13 → 13 ∼ × thethe integrated integrated intensities intensities of of individual individual transitions transitions and and from from calibration calibration peratures and corresponding CO J = 4 3/ CO J = 6 5 ratios. mass of the warm CO gas can be computed by integrating the uncertainties of 20% → → 2 uncertainties of 20%. The blue lines denote the RADEX modeling results for different kinetic column density over the whole clump in a region of 3.12 arcmin , . temperatures at 3 different volume densities, and color points represent which results in a mass of 467 M . Complementary to this, the data from near the HII region, PDR, and molecular cloud (see Sect. 4.3). ∼ 2 cold gas mass in a region of 5.1 arcmin has an estimated value and indicates temperature gradients in the gas, as expected in a Each point denotes a 12CO J = 6 5 temperature and 13CO J = 4 3 PDR. 3/13CO J = 6 5 temperature ratio→ obtained from a single pixel. Data→ of 10 M , calculated from a flux of 133 Jy at 870 µm measured 13 13 ∼ 4.2.The RotationalJ = 1 diagrams0 and J = of2 CO1 transitions of CO seem to points are extracted→ from peak temperature maps convolved to the same with ATLASGAL (Schuller et al. 2009) assuming an absorp- → → 12 2 originate from colder gas, while the J = 4 3 and J = 6 5 resolution of 1600. (a) The data points plotted are obtained for a CO tion coefficient of kν = 1.85 g cm and a temperature of 23 K With observations of CO lines with different→ J, rotational dia-→ 17 18 2 transitions probe hotter gas, which agrees with the analysis car- column density range of 8 10 – 1.8 10 cm− and for modeling (Urquhart et al. 2018), and not including potential uncertainties grams can be used to study the excitation of the CO emitting gas. 12 × × 18 2 ried out for J = 6 5 in Sect. 4.1. The J = 8 7 transition the chosen input CO column density is 1 10 cm− . (b) The data in the choice of these values. However, these mass estimations In a rotational diagram→ or Boltzmann plot the natural→ logarithm points plotted are obtained for a 12CO column× density range of 1.8 appears to originate from the hottest gas. 18 18 2 12 × have an error of 26%, which accounts for errors of 16% from of the column density Nu/gu of different lines is plotted against 10 – 3.5 10 cm− and for the modeling the input CO column ∼ ∼ × 18 2 the distance to the star (Tothill et al. 2008) and 20% from the their upper energies Eup/k. Here gu is the degeneracy of the up- density is 2 10 cm− . (c) The data points plotted are obtained for a ∼ 12 × 18 18 2 calibration. 4.3.per RADEX energy level, modeling ( 2J + 1), and k is the Boltzmann constant. CO column density range of 3.5 10 – 5.1 10 cm− and for the 12 × × 18 2 ≡ modeling input the CO column density is 4 10 cm− . × WeArticle used number, the non-LTEpage 12 of 18 radiative transfer program RADEX 4.2. Rotational diagrams of 13CO (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 With observations of CO lines with different J, rotational dia- temperatures and densities. The RADEX program uses the grams can be used to study the excitation of the CO emitting gas. escape probability approximation for a homogeneous medium In a rotational diagram or Boltzmann plot the natural logarithm and takes into account optical depth effects. We chose a uniform of the column density Nu/gu of different lines is plotted against sphere geometry. The Leiden Molecular and Atomic Database8 their upper energies Eup/k. Here gu is the degeneracy of the (LAMDA; Schöier et al. 2005) provides rates coefficients for upper energy level, ( 2J + 1), and k is the Boltzmann constant. 12 13 ≡ collisions of CO and H2 used in the modeling. CO and CO For a single temperature and optically thin emission these data transitions were modeled taking their line width from the aver- points fall onto a straight line. Deviations from a straight line age spectra of our data, i.e., 4 and 3 km s–1, respectively. As indicate then either optical depth effects or temperature gradi- input parameters we computed grids in temperature and vol- ents in the clouds. A complete derivation of rotational diagrams ume density with a background temperature of 2.73 K, kinetic for a local thermodynamic equilibrium (LTE) case can be found temperatures in the range of 50–250 K and H2 densities in the in Goldsmith & Langer(1999). 4 8 3 13 range of 10 –10 cm− . For a linear molecule line ratios from Firstly, by assuming CO to be optically thin, i.e., the different J depend on both temperature and density. To break this optical depth correction term, Cτ is unity in Eq. (24) of thin degeneracy, we computed with RADEX not only the line ratio of Goldsmith & Langer(1999), we plot ln Nu /gu versus Eu/K 13 12 13 two CO transitions but also the temperature of the CO line. as shown in Fig. 11 in black for the five CO lines observed The latter is optically thick, probes the excitation temperature toward Her 36. A curvature in a rotational diagram can be due (cf. Sect. 4.1), and can therefore be used to break the degeneracy to optical depth effects, therefore we estimate the expected between temperature and density. optical depths for the computed column density from Eq. (25) In our first approach to determine the dominant kinetic of Goldsmith & Langer(1999) and apply the optical depth temperatures and densities in M8 near Her 36, we selected from corrections Cτ that lead to the corrected diagram as shown in the CO maps data points for column densities of 12CO in three red in Fig. 11. The new temperatures and column densities are ranges as follows: (a) 8 1017–1.8 1018 cm–2, (b) 1.8 1018– then calculated as shown in Table3. Further iterations would 3.5 1018 cm–2, and (c)× 3.5 10×18–5.1 1018 cm–2×. These lead to corrections smaller than the error bars. After the optical × × × depth correction the curvature in the rotational diagram remains 8 http://www.strw.leidenuniv.nl/~moldata/

A158, page 12 of 17 M. Tiwari et al.: Unveiling the remarkable photodissociation region of Messier 8 A&A proofs: manuscript no. main

J = 6 5, the total column density (Fig. 10 (b)) of 13CO can be ranges were estimated from the LTE calculations carried out in calculated→ over the complete velocity range of the source from Sects. 4.1 and 4.2. Another criterium for selection of these data points aims at selecting the most conspicuous regions inside the 116.2 maps. The molecular cloud region consists of all data points in an 13 = . 12 + . 13 2 , N( CO) 1 06 10 (Tex 0 88) exp TMB( CO)dv cm− area of 1500 1500 around the mid-J CO peak, the UC HII region/ × Tex × ! Z Her 36 encompasses all data points in an area of 2000 2000 (3) around Her 36, and the eastern HII region has data points× in an area of 2000 2000 around the [C II] peak. 1 × where Tex is in K and TMB dv is in K kms− . Fig. 10 (b) shows Figure 12 shows the J = 4 3 and J = 6 5 line ratios the resulting 13CO total column density with a peak value of of 13CO vs. the J = 6 5 12→CO line temperature→ for both 16 -2 ∼ → 5 10 cm northwest of Her 36. This results in a H2 col- the measured data points in the maps and the results of the × 22 2 umn density N(H2) of 3.7 10 cm− adopting an isotopic RADEX computation of the density/temperature grid. The H2 12 ∼13 × abundance ratio [ CO/ CO] of 63 (Milam et al. 2005) and a number densities in most of the regions are in the range of CO abundance ratio [12CO/H ] of∼ 8.5 10 5 (Tielens 2010). 4 6 3 2 ∼ × − 10 –10 cm− . High densities are obtained for the molecular The mass of the warm CO gas can be computed by integrating cloud∼ and the eastern HII region, while the region consisting of the column density over the whole clump in a region of 3.12 the bright stellar system Her 36 and the ultracompact HII region arcmin2, which results in a mass of 467 M . Complementary 4 5 3 has densities in the range of 10 –10 cm− . The kinetic tempera- to this, the cold gas mass in a region∼ of 5.1 arcmin 2 has an esti- 3 tures from the modeling results are in a range of 100–250 K for mated value of 10 M , calculated from a flux of 133 Jy at 870 104 cm 3 and in a range of 50–150 K for 105–106 cm 3. µ ∼ − − m measured with ATLASGAL (Schuller et al. 2009) assuming In order to include all CO lines toward several positions in an absorption coefficient of k = 1.85 g cm2 and a temperature ν the RADEX analysis, their intensities as a function of J are of 23 K (Urquhart et al. 2018), and not including potential un- certainties in the choice of these values. However, these mass compared to RADEX results in Fig. 13. To fit the modeling estimations have an error of 26%, which accounts for errors results to the observed data points we varied the column den- ∼ sities of 12CO and 13CO. While for 12CO a column density of of 16% from the distance to the star (Tothill et al. 2008) and 18 2 13 20%∼ from the calibration. 4 10 cm− was chosen, for CO a column density of × 16 2 ∼ 8 10 cm− 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 12CO/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 temper- ature varied from 50–250 K and keeping the H2 density fixed 5 –3 at 10 cm , while panels c and d show results in which the H2 4 8 3 density is varied from 10 –10 cm− while keeping the kinetic temperature fixed at 120 K. It can be seen 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 Fig. 11. Rotational diagrams of J = 1 0, J = 2 1, J = 4 3, temperature was also implied by the rotational diagram analy- J = 6 5, and J = 8 7 transitions→ of 13CO at→ Her 36. Shown→ in sis. Furthermore, these values are similar to the temperature and → → 13 black is the rotational diagram when CO is assumed to be optically density ranges found in OMC 1 (Peng et al. 2012). 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 4.4. CO, [C I] and [C II] luminosities transitions were extracted from maps convolved to the same resolution Fig.Fig. 12. 12. ForFor 3 di differentfferent 1212COCO column column densities, densities, RADEX RADEX modeling modeling re- of 3100. The error bars were calculated from the maximum noise level of resultssults are are shown shown for for12CO12COJ =J6 = 65 peak5 peak main-beam main-beam brightness brightness tem- We obtained the total luminosities of the CO spectral line energy the integrated intensities of individual transitions and from calibration peratures and corresponding 13CO→13J →= 4 3/13CO J13= 6 5 ratios. distributions (SLED) and of the [C I] 609 µm and [C II] lines temperatures and corresponding CO J→= 4 3/ CO→J = 6 5 uncertainties of 20% ratios.The blue The lines blue denote lines the denote RADEX the modeling RADEX results modeling→ for di resultsfferent kinetic for→ dif- over the total observed region seen in the maps in Figs.2–4, as . ferenttemperatures kinetic temperatures at 3 different volume at 3 different densities, volume and color densities, points represent and color derived by Solomon et al.(1997), Carilli & Walter(2013). We data from near the HII region, PDR, and molecular cloud (see Sect. 4.3). points represent data12 from near the HII region, PDR,13 and molec- scaled the luminosity for Galactic sources, i.e., Each point denotes a CO J = 6 5 temperature and12 CO J = 4 ular13 cloud (see Sect. 4.3). Each→ point denotes a CO J = 6 → 5 3/ CO J = 6 135 temperature ratio13 obtained from a single pixel. Data→ 9 2 13 temperature and→ CO J = 4 3/ CO J = 6 5 temperature ratio L = 1.04 10− S ∆VνD , (4) 4.2. Rotational diagrams of CO points are extracted from peak→ temperature maps→ convolved to the same × L obtained from a single pixel. Data points are extracted from12 peak resolution of 1600. (a) The data points plotted are obtained for a CO With observations of CO lines with different J, rotational dia- temperature maps convolved to17 the same resolution18 2 of 1600.(a) The where L is the line luminosity in L , S ∆V is the velocity inte- column density range of 8 10 – 1.8 1210 cm− and for modeling grams can be used to study the excitation of the CO emitting gas. data points plotted12 are obtained× for a× CO column18 2 density range 1 ν the chosen17 input CO18 column2 density is 1 10 cm− . (b) The data grated flux in Jy km s− , is the transition frequency in GHz, In a rotational diagram or Boltzmann plot the natural logarithm of 8 10 –1.8 10 cm− and12 for modeling× the chosen input 12points× plotted are× obtained for a18 CO column2 density range of 1.8 and DL is the distance in kpc. A total CO luminosity of LCO = 18 18 2 12 × of the column density Nu/gu of different lines is plotted against CO column density is 1 10 cm− .(b) The data points plotted 10 – 3.5 10 cm12 − and× for the modeling the input CO18 column 9.5 L was calculated for the observed transitions and by are obtained× for a18 CO2 column density range of 1.8 10 –3.5 their upper energies Eup/k. Here gu is the degeneracy of the up- density is 2 10 cm− . (c) The data points plotted are obtained for a 1218 2 × 18 12 18 ×2 × accounting for the luminosities of the missing transitions. A per energy level, ( 2J + 1), and k is the Boltzmann constant. 10 COcm column− and density for the range modeling of 3.5 the10 input– 5.1 CO10 columncm− and density for the is 18 2 12 × × 18 2 12 [C I] 609 µm line luminosity of L = 0.11 L was obtained, ≡ 2 modeling10 cm input− .(c the) TheCO data column points density plotted is are 4 obtained10 cm− for. a CO col- [C I] × 18 18 × 2 Article number, page 12 of 18 umn density range of 3.5 10 –5.1 10 cm− and for the modeling which is a lower limit to the total [C I] luminosity since the [C I] 12 × × 18 2 input the CO column density is 4 10 cm− . 370 µm line was not observed. The estimated [C II] luminosity is × A158, page 13 of 17 A&A 615, A158 (2018)

12 13 Fig. 13. Results obtained from RADEX modeling for CO and CO at the mid-J CO peak (∆α = 13.000, ∆δ = 8.000), at Her 36 (∆α = 000, − ∆δ = 000) and at the [C II] peak (∆α = 30.000, ∆δ = 2.000). Our observed data points are in green, red, and blue and are extracted from peak − 12 18 2 16 temperature maps convolved to the same resolution of 3100. The column densities used in the modeling are for CO 4 10 cm− and 8 10 2 13 × × cm− for CO. 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 5 –3 4 8 –3 10 cm ; panels c and d: results obtained by varying the H2 density from 10 –10 cm in steps by a factor of 10 and keeping the kinetic temperature fixed at 120 K.

L = 95.8 L . Similar to the mass estimations in Her 36. With increasing velocities, the [C II] follows the dark [C II] Sect. 4.1, these luminosity estimations have an error of patches in the HST images that form a foreground veil cover- 26%. ing 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 1 5. Discussion s− . Therefore we suggest that the low velocity [C II] probes directly the gas of the veil that forms a foreground PDR illu- 5.1. Overview of the PDR and HII region around Her 36 minated by Her 36. On a fainter level, weak emission from this 9 veil is also seen in the CO maps at low velocities (5–7 km In Fig. 14, we present a F487N filtered 4865 Å HST image 1 (observation ID number: 6227, observed in the year 1995) of s− ). This foreground veil is receding away from Her 36 toward Her 36 and its surroundings overlaid with contours from low 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 [C II] channel maps. The [C II] at the lowest velocity 1 velocity blue-shifted emission of Hα, [N II] and [S II] doublets, (2 km s− ) peaks at the Hourglass Nebula slightly to the east of [O III], and absorption lines of the sodium D doublet as mea- 9 Based on observations made with NASA/ESA Hubble Space Tele- sured by Damiani et al.(2017). Assuming optically thin emission scope, and obtained from the Hubble Legacy Archive, which is a collab- from [C II] in this warm veil in the velocity range of 2–7 km 1 oration between the Space Telescope Science Institute (STScI/NASA), s− and a kinetic temperature of 500 K (Tielens & Hollenbach the Space Telescope European Coordinating Facility (ST-ECF/ESA) 1985b), we calculated the [C II] column density using Eq. (A.1) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). with a peak value of 9.6 1017 cm 2 at an offset of (∆α = 30 , ∼ × − 00 A158, page 14 of 17 M. Tiwari et al.: UnveilingA& theA proofs: remarkablemanuscript photodissociation no. main region of Messier 8

11 Fig.Fig. 14. 14.HSTHST F487N F487N (4865 (4865 Å) Å) image image of of Her Her 36 36 and and its its surroundings surroundings overlaid overlaid with with [C IIII]] channel map contours (in white) white) at at (a) (a) 2 2 km km s s−−,,( (b)b) 5 11 11 5km km s s−− ,, and and (c) (c) 7 7 km km s s−− toto show show the progression of the foreground material at lower velocities. velocities. Her Her 36 36 is is the the central central position position ( (∆∆αα== 0,0,∆∆δδ== 0)0) at at h m s RA(J2000) denoted with an asterisk at RA(J2000) = 18=03 h40.3m ands Dec(J2000) = 24=◦2204300, with the hourglass nebula representing the two R.A.(J2000) denoted with an asterisk at R.A.(J2000) 18 03 40.3 and Dec.(J2000)− –24◦2204300, with the hourglass nebula representing the brighttwo bright hotspots hotspots about about1500 to15 the00 to east. the Contour east. Contour levels levels are from are 10% 10% toto 100% 100% inin steps steps of of 10% 10% of of the the peak peak emission emission observed observed at at the the channel channel with with 10 1 1 ∼ ∼ 10km km s− s−velocity.velocity.

The J = 1 0 and J = 2 1 transitions of 13CO seem to around Her 36, and the eastern HII region has data points in an → → originate from colder gas, while the J = 4 3 and J = 6 area of 2000 2000 around the [C II] peak. 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 J = J = → → Fig. 12 shows the 4 3 and 6 5 line ratios of appears to originate from the hottest gas. 13CO versus the J = 6 5 12→CO line temperature→ for both the measured data points in→ the maps and the results of the RADEX computation of the density/temperature grid. The H number 4.3. RADEX modeling 2 densities in most of the regions are in the range of 104 – 106 3 ∼ We used the non-LTE radiative transfer program RADEX (van cm− . High densities are obtained for the molecular cloud and der Tak et al. 2007) to verify the calculations carried out in Sects. the eastern HII region, while the region consisting of the bright 4.1 and 4.2, which assumed LTE to determine the temperatures stellar system Her 36 and the ultracompact HII region has den- 4 5 3 and densities. The RADEX program uses the escape probability sities in the range of 10 – 10 cm− . The kinetic temperatures approximation for a homogeneous medium and takes into ac- from the modeling results are in a range of 100 – 250 K for 4 3 5 6 3 count optical depth effects. We chose a uniform sphere geom- 10 cm− and in a range of 50 – 150 K for 10 – 10 cm− . etry. The Leiden Molecular and Atomic Database8 (LAMDA; Schöier et al. (2005)) provides rates coefficients for collisions In order to include all CO lines toward several positions in Fig. 15. Left panel: velocity integrated intensities normalized to their value at peak vs. offset (00) from Her 36 along the green arrows shown in the of CO and H used in the modeling. 12CO and 13CO transitions the RADEX analysis, their intensities as a function of J are com- right. They follow2 a path from the second ATLASGAL peak at ( 5300, 2300) to the second [C II] peak at (6000, 2700) via Her 36 at (000, 000). Plots are 12 18 − shownwere for modeled [C II], [C takingI], J = their6 line5 CO, widthJ = from2 1 theC O, average Hα, ATLASGAL spectra pared 870 µm, to RADEXand WISE results 3.4 µm in emission; Fig. 13.right To fit panel the: modeling schematic results diagram to -1 12 ofof the our prominent data, i.e., optical 4 and features 3→ km of s M8, respectively. pertinent→ to the As discussion input parame- in this paper.the observed The cold dataand dense points molecular we varied cloud the is column in the background densities of shownCO 13 1 12 1 18 2 inters blue. we The computed foreground grids gas in of temperature the warm PDR and veil volume is receding density away with from Herand 36CO. ( 9 km While s− ) forwith lowerCO a velocities column (2–7 density km sof− ), 4 while10 thecm HII− ∼ 13 16× 2 regiona background is powered temperature by both stellar of systems, 2.73 K, Her kinetic 36 and temperatures 9 Sgr. in the was chosen, for CO a column density of 8 10 cm− could 4 8 × 13 range of 50 – 250 K and H2 densities in the range of 10 – 10 make the modeling results fit the data points. This CO column 3 ff J density exceeds the value obtained by the LTE calculations and ∆cmδ =− 10. For) from a linear Her molecule 36. Taking line C/H ratios from1.2 di10erent4, assumingdepend dominates with the second dense but colder clump at an offset of on both00 temperature and density. To break∼ this× degeneracy,− we corresponds to a 12CO/13CO ratio of 50. While this is lower than that all the carbon is in ionized form and from the column den- about 7000 from Her 36. [C I] diffuse emission is very extended 13 the assumed value of 63 in Sect. 4.1, it is still within the typical sitycomputed of H , N with(H ) RADEX= 9.4 10 not20 onlycm the2 (A line/mag) ratio (Kauffmann of two CO et tran- al. in this region. 2 2 − v 12 scatter of this ratio in the ISM (Milam et al. 2005). The 12CO 2008sitions), we but derived also the the temperature visual× extinction of theA .CO A maximum line. The extinc- latter is Taking this into consideration along with our analysis of the optically thick, probes the excitation temperaturev (cf. Sect. 4.1), and 13CO observed data is compared to the RADEX model at tion of A 4.25 is obtained at the same position where the [C II] morphology carried out in Sects.3 and4, we propose a geome- v the peak of the mid-J CO transitions in the molecular cloud, at columnand can density therefore∼ peaks be used and tothe break values the get degeneracy lower around between it. This tem- try of the region illuminated by Her 36 and 9 Sgr as presented in Her 36 and at the emission peak of [C II] . Panels (a) and (b) positionperature is and the density. same as position 13 in Fig. 5 of Woodward et al. Fig. 15 (right). We propose that the cold dense molecular cloud of Fig. 13 show results with the kinetic temperature varied from (1986In), whereour first an approachA 3.9 to was determine calculated. the dominant kinetic tem- is behind the bright stars Her 36 and 9 Sgr. Her 36 is still much v 50 – 250 K and keeping the H density fixed at 105 cm-3, while peraturesIn Fig. 15 and (left) densities we∼ show in M8 how near the Her intensity 36, we of variousselected trac- from closer to the dense part of the2 cloud in which it was born; in 12 panels (c) and (d) show results in which the H density is varied ersthe evolves CO maps along data a pathpoints from for column the direction densities of 9 of SgrCO to Her in three 36 fact the foreground veil is part of the original cloud2 accelerated 17 18 -2 18 from 104 – 108 cm 3 while keeping the kinetic temperature fixed andranges then as continuing follows: (a) to the 8 northwest10 – 1.8 along10 thecm molecular, (b) 1.8 cloud.10 toward us by the strong− radiation and wind of Her 36, show- – 3.5 1018 cm-2, and (c)× 3.5 1018×– 5.1 1018 cm-2.× These at 120 K. Toward the northeast [C II] and the mid-IR WISE emission dom- ing the expansion of the nebula, blue shifted with respect to × × × 1 inate,ranges probing were estimated the extended from HII the regions LTE calculations toward 9 carriedSgr and out the in the molecular cloud (3–7 km s− ) moving away from Her 36 1 resultingSects. 4.1 PDR. and 4.2.All ofAnother the tracers criterium peak for on selection or close of to these Her 36, data ( 9It km can s be− ) seen toward that no the single observer kinetic and temperature the west, or while H2 density the ∼ 1 showingpoints aims the tightat selecting spatial the correlation most conspicuous between the regions ultracompact inside the red-shiftedcan fit all the (11–13 observed km s− data) [C points.II] probes This thesuggests background solutions mate- with maps. The molecular cloud region consists of all data points in rialkinetic toward temperatures the northeast in the of range Her of 36. 100 The – 150 ultracompact K and H2 densi- HII HII region (as seen in the recombination line), a dense clump 4 6 -3 an area of 1500 1500 around the mid-J CO peak, the UC HII re- ties to be in the range of 10 – 10 cm . Such a spread in the in the larger scale× molecular cloud (870 µm dust and molecu- region is fueled by Her 36 around its vicinity and the gion/ Her 36 encompasses all data points in an area of 2000 2000 ambient temperature was also implied by the rotational diagram lar emission), and the bright PDR illuminated by Her 36 ([C× II] more extended diffuse HII region by 9 Sgr toward the east and WISE). To the northwest, emission from the molecular cloud (Tothillanalysis. et al.Furthermore, 2008). these values are similar to the tempera- 8 http://www.strw.leidenuniv.nl/moldata/ ture and density ranges found in OMC 1 (Peng et al. 2012). A158, page 15 of 17 Article number, page 14 of 18 A&A 615, A158 (2018)

Table 4. CO J = 2 1, [C II] and FIR luminosity ratios of M8 and the symmetrically around the ionization front (Stacey et al. 1993). → Orion Bar. 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 Luminosity ratio M8 Orion Bar the northwest of Her 36, which supports the proposition of a face-on geometry. 3 3 L[C II] /LFIR 10− 1.1 10− 2 × 3 L[C II] /LCO2 1 7.5 10 1.6 10 5.3. Comparison with the PDR of M17 SW − × 6 × 7 LCO2 1/LFIR 1.3 10− 6.6 10− − × × M17, the Omega nebula is also among the best nearby labora- tories to study star formation. It has an edge-on geometry in 5.2. Comparison with PDR of the Orion Bar contrast to the face-on geometry of M8. It has a bright HII region ionized by the rich cluster NGC 6618 (Povich et al. The Orion Bar is a well-known PDR and its properties 2009) and beyond this HII region lies the bright PDR in the are described in depth by Hollenbach & Tielens(1997) and southwest of M17 (M17 SW), which is responsible for the photo- Walmsley et al.(2000). It is part of the OMC-1 core at the edge electric heating of the warm gas (Pérez-Beaupuits et al. 2015a). of M42 and is illuminated by young massive stars, which form M17 SW also contains a wide ranged clumpy molecular a trapezium at the center of the Orion Nebular Cluster, mainly cloud studied widely by Pérez-Beaupuits et al.(2010) and from θ1 C, which is a O5–O7 star. Also, the PDR appears to Pérez-Beaupuits et al.(2015a). be located at the edge of the HII blister tangential to the line of In Sect. 3.2, the scatter plots related to M8 show only a 12 sight (Peng et al. 2012). A comparison of LCO2 1, L and weak correlation of [C II] with [C I] and CO. The channel maps − [C II] LFIR between M8 and the Orion Bar is presented in Table4. and line profiles at different offsets also show different mor- For M8, the LCO2 1 = 0.128 L (calculated in a similar way as in phologies of [C II] compared with those seen in [C I] and CO − 4 Sect. 4.4), L is calculated in Sect. 4.4 and LFIR 10 L is except in a small range of intermediate velocities toward Her 36. [C II] ∼ obtained from White et al.(1998). The luminosities of the Orion This suggests that [C II] and the molecular gas tracers on scales Bar are taken from Goicoechea et al.(2015). away from Her 36 do not originate from the same spatial region. 5 We calculated the FUV radiation field, G0 0.6–1.12 10 This is similar to M17 SW as reported by Pérez-Beaupuits et al. ∼ 5 × 3 12 in Habing units and density, n 0.97–1.93 10 cm− (2015a). In Sect. 4.3, the comparison between the observed CO ∼ × 13 for the PDR of M8 by adopting an electron density ne of and CO data with non-LTE RADEX modeling results show –3 2000–4000 cm , electron temperature T e of 7000–9000 K that the UC HII region or molecular gas near the Her 36 region (Woodward et al. 1986; Esteban et al. 1999). We used the val- has the highest density and kinetic temperature, while the molec- ues of stellar luminosity and number of ionizing photons for an ular gas near the eastern HII region has low density and lower O7 star from Sect. 7.2.1 of Tielens(2010). The densities match kinetic temperature. In contrast to M17 SW (Pérez-Beaupuits 12 well with the RADEX calculations carried out in Sect. 4.4. et al. 2015b), the CO SLED shapes we see in Fig. 13 toward J Interestingly, these calculated values of G0 and n also match Her 36 and mid- CO positions follow a similar trend. Thus, very well with those calculated for the Orion Bar. This leads they do not indicate large fluctuations in gas temperatures of the us to a direct comparison of the results of the PDR models molecular gas. However, similarly to the case of M17 SW, the of the Orion Bar (Tielens & Hollenbach 1985a,b; Jansen et al. higher J CO lines show significantly lower intensities at the [C II] 1995; Hogerheijde et al. 1995; Hollenbach & Tielens 1997, 1999; peak position. This is consistent with a PDR, where the [C II] Andree-Labsch et al. 2017) with the PDR of M8 since we expect peak emission is expected to arise from less dense gas than at the similar chemical and thermal conditions in both PDRs. Tielens Her 36 position. & Hollenbach(1985a) and Hollenbach & Tielens(1999) cal- culated the structure of the Orion Bar as a function of visual 6. Conclusions extinction Av. They show a typical case in which H2 does not self-shield until dust attenuation of the FUV photons creates In this paper, we presented for the first time velocity integrated an atomic surface layer. According to this, in the PDR of M8 intensity maps of J = 11 10, J = 13 12, J = 16 15 12CO, → → → the transition of atomic H to molecular H2 occurs at Av = 2, and [C II] 158 µm, observed toward Her 36 in M8 using the dual- + the carbon balance shifts from C to C and CO at Av = 4, and color Terahertz receiver GREAT on board the SOFIA telescope; except for the O in CO, all oxygen is in atomic form until very J = 2 1, J = 3 2, J = 6 5, and J = 7 6 12CO tran- → → → 13→ deep into the molecular cloud at Av = 8. The gas in the surface sitions; J = 2 1, J = 4 3, and J = 6 5 CO transitions layer is much warmer at about 500 K than the dust, which is using the CHAMP→ +, FLASH→+, and PI230 receivers→ of the APEX ∼ 12 13 at about 30–75 K. Complementary to the H2 column density telescope; and J = 1 0 transitions of CO and CO using the calculation∼ carried out in Sect. 4.1, assuming a dust tempera- EMIR receiver of IRAM→ 30 m telescope. 1 ture of 75 K and a maximum flux value of 5000 mJy beam− Combining the information obtained from Sects.3 and obtained from ATLASGAL data, allowed us to calculate the H2 5.1, we put forward the geometry of the region surrounding 22 2 column density at Her 36, N(H2) 3.75 10 cm− which is in Her 36. M8 has a face-on geometry where the cold dense reasonable agreement with that calculated∼ × in Sect. 4.1 within a molecular cloud lies in the background with Her 36 being still factor 1.25. very close to the dense core of the cloud from which it was Hogerheijde∼ et al.(1995), Walmsley et al.(2000), and born. Her 36 is powering the HII region toward the east of it Andree-Labsch et al.(2017) described the geometry of the along with 9 Sgr, while the foreground veil of a warm PDR Orion Bar where the trapezium stars illuminate the PDR, which is receding away (at lower velocities) from Her 36 toward the changes from a face-on to an edge-on orientation along the vary- observer. ing length of the line of sight. This geometry explains the [C I] Using different techniques we studied the physical conditions peak that is symmetric around the peak of the CO emission in the molecular gas associated with M8. CO rotation dia- (Tauber et al. 1994). The [C II] emission peak is also distributed grams indicate temperature gradients through the PDR. Low-J

A158, page 16 of 17 M. Tiwari et al.: Unveiling the remarkable photodissociation region of Messier 8

13CO transitions seem to originate from colder gas, while the Pabst, C. H. M., Goicoechea, J. R., Teyssier, D., et al. 2017, A&A, 606, A29 J = 8 7 transition seems to originate from the hottest gas. Peng, T.-C., Wyrowski, F., Zapata, L. A., Güsten, R., & Menten, K. M. 2012, → A&A, 538, A12 Quantitative analysis including LTE approximation methods and Pérez-Beaupuits, J. P., Güsten, R., Spaans, M., et al. 2015a, EAS Pub. Ser., 75, the non-LTE RADEX program were used to calculate the tem- 205 peratures and H2 number density in the PDR around Her 36. Pérez-Beaupuits, J. P., Güsten, R., Spaans, M., et al. 2015b, A&A, 583, A107 Kinetic temperatures ranging from 100–150 K and densities Pérez-Beaupuits, J. P., Spaans, M., Hogerheijde, M. R., et al. 2010, A&A, 510, 4 6 –3 A87 ranging from 10 –10 cm were obtained. Pérez-Beaupuits, J. P., Stutzki, J., Ossenkopf, V., et al. 2015c, A&A, 575, A9 Acknowledgements. SOFIA is jointly operated by the Universities Povich, M. S., Churchwell, E., Bieging, J. H., et al. 2009, ApJ, 696, 1278 Space Research Association, Inc. (USRA), under NASA contract Rauw, G., Nazé, Y., Gosset, E., et al. 2002, A&A, 395, 499 NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract Rauw, G., Sana, H., Spano, M., et al. 2012, A&A, 542, A95 50 OK 0901 and 50 OK 1301 to the University of Stuttgart. We are thankful Risacher, C., Güsten, R., Stutzki, J., et al. 2016, A&A, 595, A34 to the SOFIA operations team for their help and support during and after the Sanchez-Bermudez, J., Alberdi, A., Schödel, R., et al. 2014, A&A, 572, L1 observations. M. Tiwari was supported for this research by the International Schöier, F. L., van der Tak, F. F. S., van Dishoeck, E. F., & Black, J. H. 2005, Max-Planck-Research School (IMPRS) for Astronomy and Astrophysics at the A&A, 432, 369 Universities of Bonn and Cologne. We also thank Thushara Pillai for helpful Schuller, F., Menten, K. 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