The discovery is reported of the second strongest source of mm and submm wavelength CO line emission, towards M8, the Lagoon Nebula in Sagittarius. The ~31 M⊙ molecular core has dimensions ~0.2 x  0.3pc and is centred on the O7V star Herschel 36 (H36), near the Hourglass Nebula in the core of M8. Emission from the CO line wings extends to the north and south of the Hourglass, although a lack of near-IR H2 emission indicates that outflow activity is much less prominent than in many active star-formation regions, and suggests that the CO line wings may trace the expanding edge of a cavity around H36. The molecular line data are compared with new near-IR narrow-band, continuum-subtracted images in He I, H2, and H,+ (Brγ) lines and archival HST emission-line images in Hα, [O III], and [S II]. The optical and near-IR data are found to be broadly consistent with previous photo-ionisation models of the Hourglass, which is excited by H 36. However, there are variations in the He I/Brγ line ratio which are difficult to explain

Huldtgren, M.

Matthews, H. E.

McCaughrean, Mark J.

McCutcheon, W. H.

Tothill, N. F. H.

White, Glenn J.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsIntense molecular emission from the Lagoon nebula,M8Journal ItemHow to cite:White, Glenn J.; Tothill, N. F. H.; Matthews, H. E.; McCutcheon, W. H.; Huldtgren, M. and McCaughrean, MarkJ. (1997). Intense molecular emission from the Lagoon nebula, M8. Astronomy & Astrophysics, 323(2) pp. 529–533.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordLink(s) to article on publisher’s website:http://aa.springer.de/papers/7323002/2300529.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukAstron. Astrophys. 323, 529–533 (1997) ASTRONOMYANDASTROPHYSICSResearch NoteIntense molecular emission from the Lagoon nebula, M8Glenn J. White1, N.F.H. Tothill1, H.E. Matthews2, W.H. McCutcheon3, M. Huldtgren4, and Mark J. McCaughrean5,61 Department of Physics, Queen Mary & Westfield College, University of London, Mile End Road, London E1 4NS, UK2 Joint Astronomy Centre, 660 N A‘o¯ho¯ku Place, University Park, Hilo, Hawaii 96720, USA3 Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada4 Stockholm Observatory, S-133 36 Saltsjo¨baden, Sweden5 Max-Planck-Institut fu¨r Astronomie, Ko¨nigstuhl, D-69117 Heidelberg, Germany6 Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel 69, D-53121 Bonn, GermanyReceived 21 January 1997 / Accepted 21 March 1997Abstract. The discovery is reported of the second strongestsourceofmmand submmwavelengthCO line emission, towardsM8, the Lagoon Nebula in Sagittarius. The∼31M molecularcore has dimensions∼0.2×0.3 pc and is centred on theO7V starHerschel 36 (H 36), near the Hourglass Nebula in the core ofM8. Emission from the CO line wings extends to the north andsouth of the Hourglass, although a lack of near-IR H2 emissionindicates that outflow activity is much less prominent than inmany active star-formation regions, and suggests that the COline wings may trace the expanding edge of a cavity aroundH 36. The molecular line data are compared with new near-IR narrow-band, continuum-subtracted images in He I, H2, andH+ (Brγ) lines and archival HST emission-line images in Hα,[O III], and [S II]. The optical and near-IR data are found tobe broadly consistent with previous photo-ionisation modelsof the Hourglass, which is excited by H 36. However, there arevariations in theHe I/Brγ line ratiowhich are difficult to explain.Key words: ISM: individual objects: M8– ISM: molecules –radio lines: ISM1. IntroductionM8, the Lagoon Nebula (NGC 6523), is one of the most promi-nent H II regions in the Galaxy. It has been studied over a widerange of wavelengths, as summarised by Lada et al. (1976), El-liot et al. (1984), Woodward et al. (1986) and Stecklum et al.(1995). The excitation conditions in the central region of M8are dominated by the radiation from recently formed OB starsinteracting strongly with the surrounding gas, especially theSend offprint requests to: Prof Glenn WhiteO7V star H 36, which excites the well-known Hourglass Neb-ula. In this paper, new molecular line maps and narrow-bandnear-IR images of the H 36 region are reported, and comparedwith a broad-band 2µm image and archival HST emission-lineimages.2. The ObservationsObservations of CO and isotopomeric J = 2 − 1 , J = 3 − 2 ,J = 4−3 rotational transitions, and the 3P1 –3P0 atomic carbonfine structure line were made with standard facility receiverson the 15 metre James Clerk Maxwell Telescope (JCMT) inHawaii. Maps in the various lines were made on grids at halfbeamwidth spacings, with spectral velocity resolutions ∼ 0.3km s−1 , and were calibrated in units of corrected antenna tem-perature Tr∗ (=Ta∗ /ηfss ). The absolute calibration uncertaintiesat the frequencies of the CO lines were J = 2 − 1 : ≤ 12 %,J = 3 − 2 : ≤ 14% and J = 4 − 3 : ≤ 20%. The calibrationscale was confirmed by observing JCMT spectral line calibra-tion standards, whose intensities appeared nominal. Details ofthe observations are listed in Table 1. At the distance of M8(1.5 kpc - Georgelin and Georgelin 1976), 10′′ corresponds toa linear size of 0.07 pc.Near-IR narrow-band (∆λ/λ ∼ 1%) observations of theHe I 21P–21S (λ2.058µm), H2 v=1–0 S(1) (λ2.122µm), andH+ Brγ (λ2.166µm) lines were made during July 1996 usingIRAC-2 at the ESO/MPG 2.2 metre telescope on La Silla. Thecamera was used in its 0.5′′ /pixel mode, covering a 2×2′ fieldof view; the seeing during these observations was 1.5′′ FWHM.In each filter, five dithered 30 second exposures were madeand mosaiced together. The absolute calibration accuracy ofthe narrow-band images is ∼0.5 magnitudes RMS. A broad-band K′ frame taken using IRIS on the 3.9 m Anglo-AustralianTelescope (with 0.8′′ /pixel and 2′′ FWHM seeing) was used to530 G.J. White et al.: (RN) Intense molecular emission from the Lagoon nebula, M8Table 1. JCMT telescope parameters - April 1996Line Transition Freq GHz HPBW ηmb ηfssC18O J = 2− 1 219.560 22′′ 0.69 0.8013CO J = 2− 1 220.398 21′′ 0.69 0.80CO J = 2− 1 230.538 19′′ 0.69 0.80C18O J = 3− 2 329.331 15′′ 0.58 0.7013CO J = 3− 2 330.588 15′′ 0.58 0.70CO J = 3− 2 345.796 14′′ 0.58 0.70CO J = 4− 3 461.041 11′′ 0.53 0.70C I 3P1 –3P0 492.160 10′′ 0.49 0.67assess the continuum emission in the line filters. The emission-line images were smoothed to the same point spread functionas the continuum image; aperture photometry was then used tomeasure stellar fluxes in both; finally, the continuum image wasscaled and subtracted from the narrow-band images.3. Molecular and CI line dataThe CO lines at the position of H 36 are unusually intense, withpeak Tr∗ values ∼ 100 K, corresponding to main beam bright-ness temperatures (see Table 1), Tmb = 115 K (CO J = 2− 1 ),120 K (CO J = 3 − 2 ) 125 K (CO J = 4 − 3 ), 33 K (13COJ = 2− 1 ), 40 K (13CO J = 3− 2 ), 6.8 K (C18O J = 2− 1 )and 9.2 K (C18O J = 3− 2 ). Several CO J = 3− 2 spectra areshown in Fig. 1. This is the second most intense CO sourceobserved with a single dish antenna; it is remarkable that ithas remained unreported for so long.The region centred on H 36 was mapped in various lines,as shown in Fig. 2 (see next page). Inspection of parts of thespectra away from the lines reported in this paper suggests thatthe source does not contain the large numbers of strong linesseen, for example, towards the Orion Hot Core.The integrated CO J = 3− 2 emission peaks at H 36 (peakvelocity 10.0 km s−1 ), and ∼ 40′′ to the NW (10.7 km s−1 ).A faint extension to the CO emission extending from H36 tothe SW at 11.2 km s−1 , is associated with a region of extinc-tion seen in Fig. 2j–l. Four peaks can be identified in the higherangular resolution CO J = 4 − 3 map; the most intense at(−9,10) lies close to the IR sources detected by Woodward etal. (1986); the second most intense peak centred 6′′ W of H36– extends ∼ 10′′ eastward towards the B4 V star KS2 (Wood-ward et al. 1990); the third peak at (+29, −2) lies close to an11 µmsource IRS2 (Dyck 1977), and the fourth at (−32, −31)traces the previously mentioned foreground extinction.The velocity structure close to H 36 can be seen in the 13COJ = 3−2 map in Fig. 3. Between 7 and 9 km s−1 , the emissioncomes from the NW of H 36, and from 8.5 and 11 km s−1 theemission peaks∼ 10′′ E and W of H 36, straddling the positionof the star (see the 10.25 km s−1 channel). Emission from thiscentral region then divides into further peaks∼ 20′′ NWofH 36in the 10.75 km s−1map. A less intense peak is also seen atFig. 1. CO spectra close to H 36 (offsets shown in arcseconds). The(0,0) position for this and the maps is that of H 36: α1950 = 18h 00m36.3s, δ1950 = -24◦22′ 53′′ . By comparison, the peak CO J = 3 − 2line intensity of the most intense known source, Orion A, is ∼ 130 K,under similar calibration conditions.(−20,−40), at ∼ 11.25 km s−1 , where the optical obscurationis seen on the HST images.As shown in Fig. 1, emission in the CO line wings extendsover a range of ∼ 20 km s−1 . The spatial distribution of thewings is complex; the red-shifted gas lies predominantly to thenorth, and blue-shifted gas dominates close to H 36 and theHourglass Nebula. Emission in high velocity line wings is ofteninterpreted as being a tracer of outflowing gas, but the lackof H2 emission (see later) may suggest instead that some of thepredominantly blue-shifted wing emission traces material at theedge of the cavity surrounding H 36. However, we cannot ruleout an outflow interpretation at this stage, particularly in viewof the near-IR jet reported by Stecklum et al. (1995).The integrated C18O J = 3 − 2 map peaks at (5,5),close to KS2, and has a size of ∼ 30×20′′ , oriented SE-NW. The peak column density N(C18O) is 2.1 1016 cm−2 ,or N(H2) = 1.3 1023 cm−2 (for [C18O]/[H2] = 1.6 10−7). Asan independent estimate, an LVG code was used to simulta-neously fit the intensities of the CO and isotopomeric datalisted earlier, gave a best estimate of n(H2)= 7 103 cm−3 andNcol /dv/dr = 2 1018 cm−2 km−1 s pc. From the C18O J = 3−2map, we estimate dv/dr >∼ 14 km s−1 pc−1, so the LVG upperlimit for N(C18O) = 6 1016 cm−2 . Assuming the core has sim-ilar dimensions along the line of sight to that seen in the planeof the sky, and that it has a constant volume density, its mass is∼ 31 M .G.J. White et al.: (RN) Intense molecular emission from the Lagoon nebula, M8 531Fig. 2a–l. Maps and images of the Hourglass region. The exciting source H 36 is located at 0,0 in each frame. a integrated CO J = 3 − 2emission (2 − 23 km s−1 ). b integrated CO J = 4 − 3 . c high velocity blue (0 - 5.5 km s−1 ) and red (15 - 20.5 km s−1 ) emission, with theHourglass Nebula indicated by the shaded region close to the centre. d 13CO J = 3 − 2 integrated emission. e C18O J = 3 − 2 integratedemission. f CI integrated emission. g continuum-subtracted Brγ , the residual stellar images in this and Fig. 2h result from imperfect continuumsubtraction, and are at the level of about 1% of the peak flux of H 36. h continuum-subtracted He I. i broad-band AAT K′ (used as the continuumto prepare g and h). j HST Archive Hα . k HST Archive [O III]. l HST [S II]. The first white contour and the steps are respectively a (230, 30K km s−1 ), b (225, 25 K km s−1 ), c (blue: lowest contour at 5 K km s−1 , steps at 5 K km s−1 , red: lowest contour at 2.5 K km s−1 , steps at2.5 K km s−1 ), d (90, 10 K km s−1 ), e (14, 2 K km s−1 ), f (25, 2.5 K km s−1 ). The near-IR and optical images are shown with logarithmicscaling, except g and h which have linear scales.532 G.J. White et al.: (RN) Intense molecular emission from the Lagoon nebula, M8Fig. 3. Channel maps of the 13CO J = 3 − 2 emission in 0.5km s−1 channel widths. The (0,0) position is α1950 = 18h 00m 36.3s,δ1950 = -24◦22′ 53′′ , and the first white contour is at 11 K km s−1 andthe interval is 2 K km s−1 . The positions of several infrared sourcesdiscussed in this paper are indicated with filled circles.The integrated CI emission peaks at (+10, -6), just south ofH 36 (where it has a value of 42 K km s−1 ), and at (30, -3)(39 K km s−1 ), close to the third most intense CO peak and the11 µmsource reported by Dyck (1977). There is no prominentemission at H 36 or at the intense CO peak to its north. Thisfollows the trend noted previously by White & Padman (1991)and White & Sandell (1995) for CI and CO peaks to be offsetfrom each other, which can be understood if the CI traces warmsurface layers of dense neutral clumps.Assuming thatTex (CI)∼Tex (CO), the column density at H 36, N(CI) = 7 1017 cm−2 , andthe abundance ratio [CI]/[CO] = 0.07, typical of ratios found indense molecular cloud cores (White & Sandell 1995).4. Infrared and optical dataThe continuum-subtracted H2 image showed no evidence fordiffuse line emission from the Hourglass region to ≤ 3.4 10−4erg s−1 cm−2 sr−1. The correspondingH2 columndensity upperlimit is 9.9 1017 cm−2 (following Gautier et al. 1976, Brand etal. 1988); more than 20 times less than the H2 seen reportedtowards Peak 1 in the Orion Nebula (Beckwith et al. 1978).The relative intensities of the He I and Brγ lines vary bya factor of ∼ 3 across the Hourglass Nebula: He I emission isprominent in the southern lobe, while Brγ emission is strongerin the northern lobe and near H 36. In a classical Stro¨mgrensphere this ratio should remain constant, except near the edge ofthe HII region. The observed variation cannot be a consequenceof extinction; a change in the intensity ratio of only 20 %wouldrequire Av ∼ 20 magnitudes, whereas Woodward et al. (1986)Fig. 4. Ratios of line intensities calculated using CLOUDY 90.02 asdescribed in the textestimate that Av is only ∼ a few magnitudes at most towardsthe optically-visible Hourglass.The variation in the He I/Brγ ratio is more likely to bedue to changes in excitation than extinction. The He I line isenhanced by collisional depopulation at a critical density ∼103 cm−3 (Doyon et al. 1992), similar to that of the Hour-glass region (4.4 103 cm−3 ). It therefore seems plausible thatthe HeI/Brγ ratio variation could result from density variationswithin the HII region. To test this, the photoionisation codeCloudy 90.02 (Ferland 1996) was used to calculate the emit-ted intensity ratios of various lines from a spherical HII regionilluminated by a 35,000K star (i.e., the O7V H36) with a Ku-rucz model atmosphere, as a function of hydrogen density, nH2 ,as shown in Fig. 4.The He I/Brγ intensity ratio varies markedly over the rangeof densities of interest: but nH2 would need to change by afactor of∼10 to reproduce the observed line ratio change. Thissame density variation would also predict changes in the ratiosof [O III]/Hα and [S II]/Hα of factors of 2 and 3–4 respectively,changes which are not found in the HST data.Woodward et al. (1986) published radio observations ofM8;at 5 GHz where the nebula is optically thin, the flux density Sν∝ n2e. From their radio map there is no brightness enhancementapparent in the southern part of the Hourglass; hence the densitycannot differ much from the average. It is therefore unlikely thatthe change in the He I/Brγ ratio is a consequence of electrondensity contrasts in the H II region. Other mechanisms must besought to explain how this ratio varies. Possible explanations areabundance effects and the role of dust in the radiative transferprocesses in the H II region. Further simulations suggest that thechange of line intensity with abundance is roughly linear overthe density range of interest; an enhancement of He I by ∼ 3would require at least a doubling of the He abundance, whichis difficult to argue for without further observational evidence.G.J. White et al.: (RN) Intense molecular emission from the Lagoon nebula, M8 5335. ConclusionsThe detection of the second strongest source of mm and submmCO line emission is reported towards the Lagoon Nebula, M8.A molecular core with a size ∼ 0.2×0.3 pc and mass ∼ 31M is centred close to the O7 star Herschel 36. The lack ofnear-IR H2 emission suggests that shock activity is very weak,and we speculate that CO line wings may instead trace the ex-panding edge of the cavity around H 36. Near-IR narrow-bandcontinuum-subtracted images in He I, H2, and Brγ lines werecompared with the molecular line data and HST narrow bandHα , [O III], and [S II] images. Although they are broadly con-sistent with predictions of the Woodward et al. (1986) model,the variability of the HeI/Brγ ratio is unexplained. Comparisonwith HST and radio observations suggest this is not related todensity or abundance variations.Acknowledgements. We thank the JCMT and ESO TO’s and supportscientists for their help; Prof S Beckwith for allocating MPG observ-ing time on the ESO/MPG 2.2m; PPARC for travel funds; Prof GaryFerland for providing the CLOUDY code for the community-at-large.The K′ frame was archival data from the Anglo Australian Telescope,taken by Dr David Allen, and kindly made available by Dr David Ma-lin. The JCMT is operated by the JAC on behalf of PPARC, NWOand the NRC. The HST data were based on operations made with theNASA/ESA Hubble Space Telescope, obtained from the data archiveat the Space Telescope Science Institute. STScI is operated by the As-sociation of Universities for Research inAstronomy, Inc., under NASAcontract NAS5-26555.ReferencesBeckwith, S., Persson, S.E., Neugebauer, G., Becklin, E.E. 1978, ApJ223, 464.Brand, P.W.J.L., Moorhouse, A., Burton, M.G., Geballe, T.R., Bird,M., Wade, R. 1988, ApJ 334, L103.Doyon, R., Puxley, P.J., Joseph, R.D. 1992, ApJ 397, 117.Dyck, H.M. 1977, AJ 82, 129.Elliot, K.H., Goudis, C., Hippelein., H. Meaburn, J. 1984, A&A 138,451.Ferland, G.J. 1996, University of Kentucky, Department of Physics andAstronomy Internal Report.Gautier, T.N., Fink, U., Treffers, R.R. Larson, H.P. 1976, ApJ 207,L129.Georgelin, Y.M., Georgelin, Y.P. 1976, A&A 49, 57.Lada, C.J., Gull, T.R., Gottlieb, C.A., Gottlieb, E.W. 1976, ApJ 203,159Stecklum,B.,Henning, T., Eckart,A.,Howell, R.R.,Hoare,M.G. 1995,ApJ, 445, L153White, G.J., Padman, R. 1991, Nature 354, 511White, G.J., Sandell, G. 1995 A&A, 299, 179.Woodward, C.E., Pipher, J.L., Helfer, H.L., Sharpless, S., Moneti, A.,Kozikwski, D., Oliveri, M., Willner, S.P., Lacasse, M.G., Herter, T.1986, AJ 91, 870Woodward, C.E., Pipher, J.L., Helfer, H.L., Forrest, W.J. 1990, ApJ365, 252This article was processed by the author using Springer-Verlag LaTEXA&A style file L-AA version 3.

Intense molecular emission from the Lagoon nebula, M8

The discovery is reported of the second strongest source of mm and submm wavelength CO line emission, towards M8, the Lagoon Nebula in Sagittarius. The ∼31M⊙ molecular core has dimensions ∼0.2×0.3 pc and is centred on the O7V star Herschel 36 (H 36), near the Hourglass Nebula in the core of M8. Emission from the CO line wings extends to the north and south of the Hourglass, although a lack of near-IR H2 emission indicates that outflow activity is much less prominent than in many active star-formation regions, and suggests that the CO line wings may trace the expanding edge of a cavity around H 36. The molecular line data are compared with new near-IR narrow-band, continuum-subtracted images in He I, H2, and H+ (Brγ) lines and archival HST emission-line images in Hα, [O III], and [S II]. The optical and near-IR data are found to be broadly consistent with previous photo-ionisation models of the Hourglass, which is excited by H 36. However, there are variations in the He I/Brγ line ratio which are difficult to explain

Tothill, Nicholas F. H. (R17058)

Matthews, Henry E.

McCutcheon, William H.

Western Sydney ResearchDirect

Intense molecular emission from the Lagoon nebula, M8

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