A study of the massive star forming region M8 using Spitzer IRAC images
aa r X i v : . [ a s t r o - ph . GA ] M a y Mon. Not. R. Astron. Soc. , 1– ?? (2009) Printed 31 October 2018 (MN L A TEX style file v2.2)
A study of the massive star forming region M8 using
Spitzer
IRAC images
Lokesh Kumar Dewangan ⋆ , & B.G. Anandarao † Astronomy & Astrophysics Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.
ABSTRACT
Spitzer
IRAC images (3.6, 4.5, 5.8 and 8.0 µ m) and photometry of the star formingregion M8 are presented. IRAC photometry reveals ongoing star formation in the M8complex, with 64 Class 0/I and 168 Class II sources identified in several locations inthe vicinity of sub-mm gas cores/clumps. Nearly 60% of these YSOs occur in about 7small clusters. The spatial surface density of the clustered YSOs is determined to beabout 10-20 YSOs/pc . Fresh star formation by the process of “collect and collapse”might have been triggered by the expanding HII regions and winds from massive stars.IRAC ratio images are generated and studied in order to identify possible diagnosticemission regions in M8. The image of 4.5 µ m/8.0 µ m reveals Br α counterpart of theoptical Hourglass HII region, while the ratio 8.0 µ m/4.5 µ m indicates PAH emission ina cavity-like structure to the east of the Hourglass. The ratio maps of 3.6 µ m/4.5 µ m,5.8 µ m/4.5 µ m and 8.0 µ m/4.5 µ m seem to identify PAH emission regions in the sharpridges and filamentary structures seen East to West and NE to SW in M8 complex. Key words: stars: formation – stars: pre-main-sequence – stars: winds and outflows– infrared: ISM – ISM: HII Regions – ISM: Individual: M8
Messier 8 (M8), the Lagoon Nebula (or NGC 6523) is awell known galactic HII region (comprehensively reviewedin Tothill et al. (2008), and the references therein) situatedat a distance of 1.25 kpc (Arias et al. 2006) in Sagittarius-Carina spiral arm of the Galaxy. The core of M8 containsa spectacular blister-type HII region, called the Hourglassnebula, ionized by the O7.5 V star Herschel 36 (Her 36)(Woolf 1961). The HII region is embedded within a giantmolecular cloud that extends eastwards to the young starcluster NGC 6530 of age 2 × yrs (Lada et al. 1976).While Her 36 is responsible for the Hourglass and the ionisedbubble surrounding it, the other early type stars in M8complex, 9 Sgr (O4 V(f)) and HD165052(O6.5V+O7.5V)are believed to account for the ionised regions east ofthe central core/bubble (Goudis 1976; Lada et al. 1976;Lynds & O’Neil 1982; Woodward et al. 1986). Allen (1986)discovered a few near-IR sources in the vicinity of Her 36,designated by Woodward et al. (1990) as KS 1 to KS 5.From a high resolution near-IR study, Arias et al. (2006,2007) found the existence of a very young star cluster aroundHer 36, having an age of ∼ yrs. Barba & Arias (2007)discovered a number of HH objects in M8, which confirms ⋆ [email protected] † [email protected] by implication, the existence of very young stars undergoingthe accretion phase of formation. Narrow-band imaging bythe Hubble Space Telescope (HST) revealed the presence ofproplyds in the neighbourhood of Her 36 (Stecklum et al.1998). The core of M8 is detected by the Mid-course SpaceExperiment (MSX) in mid-infrared as a luminous extendedsource (Crowther & Conti 2003). Color composite map ofM8 in mid-infrared bands of Spitzer
Infrared Array Cam-era (IRAC) shows a ridge extending in east-west direc-tion to the south-east of the Hourglass (Tothill et al. 2008).There appear also a number of filamentary structures ex-tending in the NE-SW direction to the east of Her 36 orthe Hourglass. Among the new compact star forming re-gions in the M8 complex, M8E stands out with its compactHII region powered by an early B type star (M8E-Radio)(Lada et al. 1976; Wright et al. 1977; Brand & Zealey 1978;Mitchell et al. 1991; Linz et al. 2009). White et al. (1997)discovered very intense CO line emission in mm and sub-mm wavelength regions from the central core of M8. Later,from a larger survey in mm and sub-mm continuum andCO lines, Tothill et al. (2002) found bright rims and darklanes stretching in the east-west direction. White et al.(1997) found from CO (J=3–2) line observations a loosebipolar structure extending NW-SE from Her 36; whileStecklum et al. (1995) found the presence of a jet-like objectvery close to Her 36. These observations provide evidence foroutflow activity around Her 36 region. As for the spectral di- c (cid:13) L.K. Dewangan & B.G. Anandarao agnostics in the infrared region, Woodward et al. (1986) de-tected Br γ (2.17 µ m), Br α (4.05 µ m) as well as Pf ǫ (3.03 µ m)and the 3.28 µ m polycyclic aromatic hydrocarbons (PAH)emission to the west of Her 36. Burton (2002) observed H µ m near the Hourglass/Her 36 regionattributed primarily to shock-excited molecular gas; but UVexcitation can not be ruled out.The M8 region seems to be quite complex and very in-teresting, owing to the presence of stellar winds and expand-ing HII region bubbles from massive stars, which can triggerfresh star formation by sweeping up and compressing the lo-cal dense interstellar matter. The afore-mentioned near-IRsurveys studied the regions around Her 36 and NGC 6530,leaving the ridge regions far-east and south-east of Her 36relatively under-explored in near-IR (except 2MASS survey)and mid-IR regions (except MSX survey). In view of this, wewish to examine in detail the extended region of M8 in thenear-/mid-infrared images provided by Spitzer , which are sofar not looked into.
Spitzer
IRAC provides an opportunity with an unprece-dented high spatial resolution in thermal infrared wave-length regime that is very useful in identifying embeddedsources in massive star forming regions. IRAC has four wave-length bands (with λ eff / ∆ λ , 3.55/0.75, 4.49/1.0, 5.73/1.43and 7.87/2.91 µ m) which include molecular emissions suchas those from H and PAH molecules, as well as recombi-nation lines from hydrogen. The aims of the present studyare to identify the embedded sources using the IRAC bandsin order to classify the different stages of their evolution; touse ratio maps of the four IRAC bands in order to identifypossible H , PAH or H emission regions, following the sug-gestions of Smith & Rosen (2005) and Povich et al. (2007).In Section 2, we describe the data used for the presentstudy and the analysis tasks utilised. Section 3 presents theresults and discussion on Spitzer
IRAC photometry of em-bedded sources associated with M8 complex. In this section,we also present the results and discussion on the ratio maps.In Section 4, we give the conclusions. SPITZER
IRAC DATA ON MESSIER 8 ANDDATA REDUCTION
The
Spitzer
Space Telescope IRAC observation of M8 wereobtained by Spitzer Science Center (SSC) on 16 Septem-ber 2005 and Basic Calibrated Data (BCDs) images wereprocessed by SSC using software version S14.0.0 for all fourbands (see Fazio et al. (2004), for details on the IRAC in-strument). The observations relevant for M8 region weretaken in High Dynamic Range (HDR) mode with 12s in-tegration time in all filters. These observations were a partof the project entitled, “Spitzer Follow-up of HST Obser-vations of Star Formation in H II Regions” (Program id20726; PI: Jeff Hester). The IRAC archival images of M8were obtained by us on 8 April 2009 from the Spitzer publicarchive, using ‘leopard’ software. The BCD images were pro-cessed for ‘jailbar’ removal, saturation and ‘muxbleed’ cor-rection before making the final mosaic using Mopex and IDLsoftwares (Makovoz & Marleau 2005). A pixel ratio (definedas the ratio of the area formed by the original pixel scale,1.22 arcsec/px, to that of the mosaiced pixel scale) of 2 wasadopted for making the mosaic (which gives a mosaic pixel scale of 0.86 arcsec/pixel) . Using these procedures, a totalnumber of 320 BCD images of 5.2 × were mo-saiced to make a final image of 42.5 × in eachof the four bands. Aperture photometry was performed onthe mosaic with 2.8 pixel aperture and sky annuli of 2.8 and8.5 pixels using APPHOT task in IRAF package. The zeropoints for these apertures (including aperture corrections)are, 17.08, 17.30, 16.70 and 15.88 mag for the 3.6, 4.5, 5.8,8.0 µ m bands, here onwards called as Ch1, Ch2, Ch3 andCh4 respectively (see Reach et al. (2005)). The photometricuncertainties vary between 0.01 to 0.25 for the four channels,with Ch3 and Ch4 on the higher side.We have looked into the 2MASS archives(Skrutskie et al. 2006) as well as published literaturefor JHK photometric data on sources identified from IRACand succeeded in extracting for nearly half of them. From2MASS archives, we have considered only the data withtags of A, B or C (or a S/N of >
5) in all the JHK bands.
We divide the results and discussion into two subsections:one in which we discuss the IRAC photometry and the pre-main-sequence sources or the young stellar objects (YSOs)detected and their possible formation scenario; and the sec-ond in which we describe the ratio maps produced fromIRAC images and discuss possible interpretations and theirimplications.
Fig 1 shows the IRAC Ch4 (8 µ m) image of ∼ × of M8 region with Her 36 situated near the cen-tre. Following earlier workers, M8 may be divided into afew distinct regions for convenience (see Fig 1): the Her 36region comprising of the bubble-like structure around Her36 with the massive stars 9 Sgr and HD 164816 formingthe eastern/north-eastern bounds; the central ridge that in-cludes filamentary structures seen in the NE-SW direction;the massive star HD 164906 and the cluster NGC 6530; theeast-west ridge region consisting of the finger-like filamen-tary structures starting from far-east to the south of Her 36;and the compact young cluster region M8E.Using the [3.6]-[4.5] vs [5.8]-[8.0] colour-colour dia-grams, Allen et al. (2004) and Megeath et al. (2004) formu-lated division criteria for various pre-main-sequence classessuch as Classes 0/I, I, II and III. These criteria have sincebeen refined by several authors (e.g., Harvey et al. (2006,2007); Gutermuth et al. (2008, 2009); Evans et al. (2009)and the references therein), in order to account for pos-sible contaminations from broad-line AGNs, PAH-emittinggalaxies, shocked emission blobs/knots and unresolved PAH-emission-contaminated apertures, which may lead to wrong-ful identifications of YSOs. We used the updated criteriagiven clearly in the Appendix A of Gutermuth et al. (2009)to delineate the PMS sources. The total number of pointsources identified that are common to all the four IRAC see http://ssc.spitzer.caltech.edu/postbcd/doc/mosaiker.pdfc (cid:13) , 1– ?? pitzer IRAC study of M8 Table 1.
Spitzer
IRAC 4-channel photometry (in mag) of the Class 0/I YSOs identified in M8 (see text for details); the numbers in thelast column refer to: 1. 2MASS; 2. Arias et al. (2006); 3. Arias et al. (2007)Object RA [2000] Dec [2000] Ch1 Ch2 Ch3 Ch4 α IRAC
H J-H H-K NIR Ref.1 18:02:25.56 -24:32:41.7 12.63 11.88 10.87 9.76 0.512 18:02:29.28 -24:08:15.8 5.79 5.12 4.41 3.45 -0.15 9.46 3.16 1.96 13 18:02:42.07 -24:19:13.2 12.19 10.63 9.54 8.62 1.204 18:02:42.97 -24:23:25.5 11.28 10.13 9.28 8.33 0.515 18:02:49.71 -24:22:26.6 11.36 9.90 9.13 8.82 -0.016 18:02:52.85 -24:20:46.6 11.52 10.27 9.54 8.85 0.157 18:02:53.21 -24:20:17.3 8.91 8.31 7.82 5.94 0.49 9.79 0.16 0.19 18 18:03:06.89 -24:20:57.6 11.08 10.29 9.52 8.24 0.409 18:03:06.99 -24:21:20.1 8.78 7.87 6.86 5.94 0.4610 18:03:11.64 -24:11:57.0 6.63 5.57 4.28 3.41 0.9411 18:03:12.28 -24:33:30.5 8.12 7.13 6.16 5.88 -0.2212 18:03:25.59 -24:21:08.3 10.88 10.21 9.42 8.55 -0.1313 18:03:27.38 -24:21:04.0 11.81 9.98 9.08 8.63 0.6914 18:03:29.22 -24:21:49.9 10.34 9.89 9.05 7.80 0.13 13.27 1.44 1.08 115 18:03:30.84 -24:20:03.1 11.73 9.94 8.63 7.36 2.1216 18:03:35.58 -24:22:04.6 11.86 10.75 9.51 7.37 2.3117 18:03:36.21 -24:18:13.7 9.87 8.67 7.91 7.11 0.2718 18:03:36.28 -24:17:51.1 8.75 7.66 6.68 5.53 0.8519 18:03:37.05 -24:22:31.6 10.34 9.36 8.25 7.06 0.96 13.87 1.13 0.98 1,220 18:03:37.41 -24:13:56.9 4.64 4.04 3.09 2.43 -0.21 6.83 1.95 1.28 121 18:03:37.74 -24:25:30.3 11.24 10.18 9.47 8.17 0.6122 18:03:38.31 -24:33:59.8 11.71 10.59 9.67 8.56 0.7523 18:03:38.65 -24:22:24.3 9.00 7.90 6.96 5.22 1.45 12.52 1.01 0.92 1,2,324(KS 1) 18:03:40.37 -24:22:38.6 6.19 5.29 4.46 2.51 1.34 10.61 1.63 1.08 225(Her 36) 18:03:40.43 -24:22:43.6 5.42 4.72 3.86 2.27 0.79 7.45 0.49 0.54 1,226(KS 4) 18:03:41.50 -24:22:44.3 9.36 8.58 — — — 12.99 2.17 1.46 227 18:03:44.95 -24:16:08.7 11.13 10.66 9.88 8.58 0.1328 18:03:45.16 -24:23:25.0 8.88 8.20 7.54 6.24 0.18 11.36 1.30 0.92 2,329 18:03:47.05 -24:25:37.4 8.45 7.86 6.95 6.08 -0.05 11.81 1.74 1.37 130 18:03:47.37 -24:18:44.4 8.73 8.16 7.29 6.33 -0.0331 18:03:47.47 -24:25:34.5 8.73 7.87 6.99 6.13 0.1632 18:03:48.44 -24:26:32.0 9.77 8.64 7.75 7.29 0.0033 18:03:48.47 -24:25:58.6 10.47 9.09 8.40 7.66 0.2834 18:03:50.26 -24:22:23.4 10.03 9.54 8.95 7.80 -0.28 11.96 0.85 0.56 135 18:03:54.23 -24:25:33.3 10.15 9.40 8.28 6.88 0.98 13.27 1.72 1.04 136 18:04:05.11 -24:16:42.3 10.80 10.04 9.56 8.43 -0.19 12.59 0.83 0.32 137 18:04:08.81 -24:27:27.5 10.36 9.54 8.90 8.11 -0.2838 18:04:10.55 -24:26:56.1 9.00 8.32 7.84 6.59 -0.13 11.10 0.99 0.62 1,339 18:04:11.03 -24:27:20.5 11.66 9.48 8.11 7.10 2.2940 18:04:11.07 -24:21:31.5 6.56 5.72 4.83 3.96 0.1741 18:04:11.08 -24:26:54.2 10.07 8.46 7.35 6.39 1.3342 18:04:17.06 -24:28:10.9 11.32 10.62 9.87 8.72 0.15 13.67 1.13 0.65 143 18:04:20.09 -24:29:14.7 5.56 4.33 2.95 2.77 0.4644 18:04:20.74 -24:28:22.1 10.06 9.35 8.57 7.50 0.11 13.03 1.87 1.20 145 18:04:20.95 -24:21:07.9 11.40 10.78 10.44 9.01 -0.19 13.10 1.06 0.56 146 18:04:21.24 -24:28:03.5 9.86 9.45 8.60 7.39 0.07 12.49 1.31 0.88 1,347 18:04:21.63 -24:11:26.3 9.33 8.68 7.92 7.08 -0.23 12.61 2.35 1.51 148 18:04:21.70 -24:21:14.6 11.47 10.98 10.29 9.05 -0.0449 18:04:23.98 -24:21:27.0 9.19 8.79 8.30 6.58 0.12 9.25 0.07 0.03 150 18:04:24.31 -24:20:59.7 9.45 9.17 8.49 6.82 0.20 9.83 0.04 0.01 151 18:04:26.52 -24:29:00.3 9.72 9.30 8.59 7.36 -0.09 11.82 1.04 0.50 152 18:04:28.09 -24:22:41.8 11.25 10.82 10.10 8.89 -0.1053 18:04:28.90 -24:14:02.6 8.46 8.05 7.30 6.06 -0.04 10.19 0.42 0.32 154 18:04:30.74 -24:28:45.6 6.75 6.02 5.34 4.27 -0.01 10.52 1.99 1.49 155 18:04:35.78 -24:28:35.6 9.03 8.44 7.71 6.74 -0.1956 18:04:40.67 -24:12:16.9 11.06 10.66 9.96 8.78 -0.19 13.01 1.06 0.52 157 18:04:44.19 -24:15:25.1 10.67 10.25 9.57 8.32 -0.11 12.01 0.91 0.37 158 18:04:47.09 -24:27:55.4 9.05 8.08 7.36 6.51 0.0459 18:04:48.42 -24:27:53.8 10.99 10.66 9.92 8.64 -0.0960 18:04:50.37 -24:14:25.8 5.02 4.27 3.29 2.31 0.32c (cid:13) , 1– ?? L.K. Dewangan & B.G. Anandarao
Table 1 – continued Object RA [2000] Dec [2000] Ch1 Ch2 Ch3 Ch4 α IRAC
H J-H H-K NIR Ref.61 18:04:50.62 -24:25:42.2 8.73 8.11 7.67 5.87 0.36 10.16 0.29 0.43 362 18:04:51.13 -24:26:33.7 9.48 9.03 8.52 6.93 0.05 12.68 1.95 1.13 163 18:04:56.77 -24:27:16.4 10.30 9.24 8.44 7.56 0.2764 18:04:58.82 -24:26:24.1 9.54 8.47 7.82 6.89 0.1365 18:05:00.47 -24:13:26.8 9.51 8.88 8.25 6.94 0.08 10.89 0.62 0.37 1
Table 2.
Spitzer
IRAC 4-channel photometry (in mag) of the Class II YSOs identified in M8 (see text for details); the numbers in thelast column refer to: 1. 2MASS; 2. Arias et al. (2006); 3. Arias et al. (2007)Object RA [2000] Dec [2000] Ch1 Ch2 Ch3 Ch4 α IRAC
H J-H H-K NIR Ref.1 18:02:23.78 -24:08:49.4 10.76 10.41 10.04 9.26 -1.13 12.29 0.94 0.42 12 18:02:24.95 -24:36:08.4 7.92 7.57 7.12 6.86 -1.59 9.59 2.31 1.02 13 18:02:25.98 -24:27:31.8 6.83 6.25 5.59 4.74 -0.42 11.46 3.65 2.14 14 18:02:27.52 -24:22:52.9 8.12 7.53 6.69 6.49 -0.885 18:02:27.65 -24:11:38.0 11.53 11.20 10.43 9.70 -0.666 18:02:29.49 -24:24:53.7 10.58 10.05 9.81 8.97 -1.07 12.71 1.08 0.69 17 18:02:36.33 -24:21:07.9 8.10 7.83 7.23 6.94 -1.43 11.15 3.46 1.78 18 18:02:37.34 -24:16:24.4 10.40 9.98 9.55 8.86 -1.08 12.47 0.92 0.56 19 18:02:41.45 -24:10:35.3 8.26 7.91 7.36 7.20 -1.56 10.85 3.13 1.48 110 18:02:41.65 -24:33:55.0 7.92 7.55 6.94 6.11 -0.73 9.60 2.07 0.93 111 18:02:42.67 -24:17:18.9 6.34 6.06 5.42 4.88 -1.0912 18:02:43.91 -24:15:23.1 10.71 10.26 9.85 9.03 -0.93 12.55 0.77 0.56 113 18:02:45.65 -24:13:49.7 8.47 7.92 7.53 7.24 -1.44 10.85 2.89 1.47 114 18:02:48.85 -24:21:08.8 9.56 9.24 8.87 7.88 -0.92 12.34 0.84 0.62 115 18:02:49.10 -24:28:15.8 7.32 6.91 6.47 6.13 -1.46 10.59 2.99 1.62 116 18:02:49.15 -24:08:52.3 6.50 6.04 5.73 4.96 -1.12 9.75 2.88 1.59 117 18:02:49.33 -24:11:19.0 5.80 5.34 5.00 4.63 -1.51 9.30 2.55 1.47 118 18:02:50.81 -24:17:56.6 9.95 9.29 9.03 8.29 -1.02 12.50 1.43 0.95 119 18:02:50.95 -24:22:20.1 10.65 10.21 9.83 8.82 -0.7620 18:02:51.10 -24:19:23.4 9.59 9.20 8.84 8.02 -1.05 12.64 1.32 0.95 121 18:02:51.12 -24:16:57.2 5.29 4.20 3.38 3.18 -0.4322 18:02:51.15 -24:18:07.9 10.45 9.78 9.50 8.74 -0.96 12.95 0.96 0.71 123 18:02:51.39 -24:17:13.3 9.48 9.15 8.59 8.26 -1.3824 18:02:52.47 -24:18:44.7 8.56 8.14 7.64 6.78 -0.79 10.72 0.90 0.75 125 18:02:53.83 -24:20:19.8 9.22 8.68 8.60 7.92 -1.4526 18:02:53.96 -24:20:11.3 9.97 9.51 9.16 8.41 -1.09 12.61 1.29 1.00 127 18:02:54.30 -24:20:56.5 8.85 8.35 8.01 7.20 -0.99 11.23 0.92 0.77 128 18:02:54.72 -24:19:56.9 9.57 9.14 8.75 8.47 -1.5729 18:02:56.79 -24:23:34.8 10.17 9.89 9.62 8.98 -1.49 12.32 2.42 1.10 130 18:02:56.88 -24:35:40.7 8.08 7.79 7.28 7.04 -1.59 10.20 2.84 1.29 131 18:03:01.50 -24:27:47.1 10.53 10.13 9.47 8.48 -0.45 12.93 1.70 0.90 132 18:03:05.14 -24:31:54.2 5.57 5.16 4.61 3.81 -0.8033 18:03:06.56 -24:32:13.1 5.60 4.96 4.44 3.56 -0.53 8.46 2.91 1.63 134 18:03:09.96 -24:33:50.4 8.10 7.50 6.97 6.67 -1.19 10.79 2.63 1.38 135 18:03:10.31 -24:26:59.5 6.49 5.89 5.43 4.59 -0.69 10.50 2.93 1.92 136 18:03:17.69 -24:20:53.2 10.74 10.25 9.77 8.93 -0.77 12.20 1.08 0.45 137 18:03:18.30 -24:25:58.7 8.19 7.44 6.70 6.20 -0.53 10.30 2.58 1.45 138 18:03:20.03 -24:20:20.4 10.45 10.21 9.81 9.05 -1.21 12.45 2.12 0.99 139 18:03:20.53 -24:30:29.8 5.27 5.09 4.69 4.18 -1.54 8.31 2.35 1.44 140 18:03:23.06 -24:23:41.8 7.03 6.61 6.22 5.82 -1.45 11.43 2.97 1.82 141 18:03:23.09 -24:21:33.1 9.80 9.53 8.88 8.65 -1.4242 18:03:24.06 -24:21:23.3 8.58 7.94 7.17 6.95 -0.9143 18:03:25.06 -24:19:02.2 8.07 7.72 7.07 6.93 -1.44 10.34 3.06 1.45 144 18:03:25.12 -24:21:28.4 11.06 10.06 9.49 8.85 -0.3645 18:03:26.71 -24:22:11.3 8.91 8.68 8.27 7.22 -0.8846 18:03:34.15 -24:24:59.7 8.17 7.83 7.19 7.07 -1.50 10.50 2.96 1.44 147 18:03:34.74 -24:18:53.7 6.44 6.20 5.68 5.32 -1.50 10.57 3.94 2.21 148 18:03:35.86 -24:09:11.9 9.61 8.85 8.82 7.90 -1.04 11.63 0.91 0.64 149 18:03:36.29 -24:09:48.2 6.11 5.74 5.31 4.38 -0.85 10.88 2.82 1.78 150 18:03:36.68 -24:10:32.6 8.59 8.39 8.00 7.52 -1.58 10.50 2.04 0.86 1c (cid:13) , 1–, 1–
H J-H H-K NIR Ref.1 18:02:23.78 -24:08:49.4 10.76 10.41 10.04 9.26 -1.13 12.29 0.94 0.42 12 18:02:24.95 -24:36:08.4 7.92 7.57 7.12 6.86 -1.59 9.59 2.31 1.02 13 18:02:25.98 -24:27:31.8 6.83 6.25 5.59 4.74 -0.42 11.46 3.65 2.14 14 18:02:27.52 -24:22:52.9 8.12 7.53 6.69 6.49 -0.885 18:02:27.65 -24:11:38.0 11.53 11.20 10.43 9.70 -0.666 18:02:29.49 -24:24:53.7 10.58 10.05 9.81 8.97 -1.07 12.71 1.08 0.69 17 18:02:36.33 -24:21:07.9 8.10 7.83 7.23 6.94 -1.43 11.15 3.46 1.78 18 18:02:37.34 -24:16:24.4 10.40 9.98 9.55 8.86 -1.08 12.47 0.92 0.56 19 18:02:41.45 -24:10:35.3 8.26 7.91 7.36 7.20 -1.56 10.85 3.13 1.48 110 18:02:41.65 -24:33:55.0 7.92 7.55 6.94 6.11 -0.73 9.60 2.07 0.93 111 18:02:42.67 -24:17:18.9 6.34 6.06 5.42 4.88 -1.0912 18:02:43.91 -24:15:23.1 10.71 10.26 9.85 9.03 -0.93 12.55 0.77 0.56 113 18:02:45.65 -24:13:49.7 8.47 7.92 7.53 7.24 -1.44 10.85 2.89 1.47 114 18:02:48.85 -24:21:08.8 9.56 9.24 8.87 7.88 -0.92 12.34 0.84 0.62 115 18:02:49.10 -24:28:15.8 7.32 6.91 6.47 6.13 -1.46 10.59 2.99 1.62 116 18:02:49.15 -24:08:52.3 6.50 6.04 5.73 4.96 -1.12 9.75 2.88 1.59 117 18:02:49.33 -24:11:19.0 5.80 5.34 5.00 4.63 -1.51 9.30 2.55 1.47 118 18:02:50.81 -24:17:56.6 9.95 9.29 9.03 8.29 -1.02 12.50 1.43 0.95 119 18:02:50.95 -24:22:20.1 10.65 10.21 9.83 8.82 -0.7620 18:02:51.10 -24:19:23.4 9.59 9.20 8.84 8.02 -1.05 12.64 1.32 0.95 121 18:02:51.12 -24:16:57.2 5.29 4.20 3.38 3.18 -0.4322 18:02:51.15 -24:18:07.9 10.45 9.78 9.50 8.74 -0.96 12.95 0.96 0.71 123 18:02:51.39 -24:17:13.3 9.48 9.15 8.59 8.26 -1.3824 18:02:52.47 -24:18:44.7 8.56 8.14 7.64 6.78 -0.79 10.72 0.90 0.75 125 18:02:53.83 -24:20:19.8 9.22 8.68 8.60 7.92 -1.4526 18:02:53.96 -24:20:11.3 9.97 9.51 9.16 8.41 -1.09 12.61 1.29 1.00 127 18:02:54.30 -24:20:56.5 8.85 8.35 8.01 7.20 -0.99 11.23 0.92 0.77 128 18:02:54.72 -24:19:56.9 9.57 9.14 8.75 8.47 -1.5729 18:02:56.79 -24:23:34.8 10.17 9.89 9.62 8.98 -1.49 12.32 2.42 1.10 130 18:02:56.88 -24:35:40.7 8.08 7.79 7.28 7.04 -1.59 10.20 2.84 1.29 131 18:03:01.50 -24:27:47.1 10.53 10.13 9.47 8.48 -0.45 12.93 1.70 0.90 132 18:03:05.14 -24:31:54.2 5.57 5.16 4.61 3.81 -0.8033 18:03:06.56 -24:32:13.1 5.60 4.96 4.44 3.56 -0.53 8.46 2.91 1.63 134 18:03:09.96 -24:33:50.4 8.10 7.50 6.97 6.67 -1.19 10.79 2.63 1.38 135 18:03:10.31 -24:26:59.5 6.49 5.89 5.43 4.59 -0.69 10.50 2.93 1.92 136 18:03:17.69 -24:20:53.2 10.74 10.25 9.77 8.93 -0.77 12.20 1.08 0.45 137 18:03:18.30 -24:25:58.7 8.19 7.44 6.70 6.20 -0.53 10.30 2.58 1.45 138 18:03:20.03 -24:20:20.4 10.45 10.21 9.81 9.05 -1.21 12.45 2.12 0.99 139 18:03:20.53 -24:30:29.8 5.27 5.09 4.69 4.18 -1.54 8.31 2.35 1.44 140 18:03:23.06 -24:23:41.8 7.03 6.61 6.22 5.82 -1.45 11.43 2.97 1.82 141 18:03:23.09 -24:21:33.1 9.80 9.53 8.88 8.65 -1.4242 18:03:24.06 -24:21:23.3 8.58 7.94 7.17 6.95 -0.9143 18:03:25.06 -24:19:02.2 8.07 7.72 7.07 6.93 -1.44 10.34 3.06 1.45 144 18:03:25.12 -24:21:28.4 11.06 10.06 9.49 8.85 -0.3645 18:03:26.71 -24:22:11.3 8.91 8.68 8.27 7.22 -0.8846 18:03:34.15 -24:24:59.7 8.17 7.83 7.19 7.07 -1.50 10.50 2.96 1.44 147 18:03:34.74 -24:18:53.7 6.44 6.20 5.68 5.32 -1.50 10.57 3.94 2.21 148 18:03:35.86 -24:09:11.9 9.61 8.85 8.82 7.90 -1.04 11.63 0.91 0.64 149 18:03:36.29 -24:09:48.2 6.11 5.74 5.31 4.38 -0.85 10.88 2.82 1.78 150 18:03:36.68 -24:10:32.6 8.59 8.39 8.00 7.52 -1.58 10.50 2.04 0.86 1c (cid:13) , 1–, 1– ?? pitzer IRAC study of M8 Table 2 – continued Object RA [2000] Dec [2000] Ch1 Ch2 Ch3 Ch4 α IRAC
H J-H H-K NIR Ref.51 18:03:36.84 -24:24:15.1 10.23 9.71 9.39 8.20 -0.58 12.26 0.89 0.37 152 18:03:37.32 -24:22:46.9 9.01 8.44 7.99 6.88 -0.44 11.47 1.34 0.66 253 18:03:38.49 -24:22:31.7 8.81 8.11 7.57 6.63 -0.38 10.80 1.22 0.80 1,254 18:03:38.81 -24:08:58.8 5.43 5.08 4.59 3.86 -1.02 8.58 2.92 1.55 155 18:03:39.05 -24:28:10.8 8.16 7.74 7.09 6.81 -1.23 9.97 2.41 1.11 156 18:03:39.38 -24:25:24.2 9.54 9.06 8.54 7.38 -0.37 11.85 1.14 0.69 157 18:03:40.23 -24:22:03.8 9.54 9.15 8.74 7.84 -0.90 11.66 1.31 0.63 158 18:03:40.24 -24:29:08.2 5.05 4.51 3.84 2.93 -0.38 8.93 2.76 1.92 159 18:03:40.33 -24:25:05.2 10.39 9.78 9.54 8.58 -0.86 12.25 1.04 0.54 160 18:03:40.74 -24:23:16.3 8.67 8.29 7.83 6.87 -0.77 11.24 1.10 0.80 1,2,361 18:03:41.05 -24:25:45.6 8.54 8.21 7.73 7.23 -1.3062 18:03:42.26 -24:23:22.4 9.68 9.10 8.70 7.76 -0.69 12.48 1.40 0.94 1,263 18:03:42.89 -24:16:26.4 8.17 7.79 7.21 7.03 -1.47 9.83 2.26 1.07 164 18:03:43.09 -24:21:29.6 8.49 8.29 7.87 7.08 -1.20 10.84 2.75 1.25 165 18:03:43.29 -24:28:07.1 8.14 7.90 7.19 7.09 -1.52 10.13 2.62 1.29 166 18:03:45.07 -24:22:05.6 8.61 8.45 8.15 6.37 -0.30 9.13 0.11 0.10 1,267 18:03:47.93 -24:18:02.1 9.05 8.45 8.00 7.37 -0.94 12.65 1.94 1.43 168 18:03:49.38 -24:26:14.6 8.47 8.04 7.42 7.27 -1.4069 18:03:49.61 -24:22:09.3 10.85 10.36 9.99 8.85 -0.59 12.65 0.99 0.42 170 18:03:50.73 -24:20:13.3 9.81 9.57 9.31 8.44 -1.2971 18:03:50.79 -24:21:10.9 6.46 6.14 5.61 4.60 -0.68 9.21 0.86 0.85 1,372 18:03:51.65 -24:28:26.7 9.51 9.21 8.91 8.12 -1.25 11.58 0.80 0.64 173 18:03:53.05 -24:14:51.6 8.17 7.59 7.17 6.89 -1.3874 18:03:57.82 -24:20:51.3 10.21 9.67 9.43 8.52 -0.9775 18:03:57.84 -24:25:34.9 6.09 5.54 5.09 4.25 -0.76 8.84 0.94 0.83 176 18:03:58.29 -24:16:49.3 9.95 9.26 8.95 8.10 -0.81 12.24 1.02 0.71 1,377 18:03:58.54 -24:24:58.8 9.66 9.31 8.90 8.18 -1.14 11.24 0.87 0.41 178 18:03:59.27 -24:23:08.2 9.01 8.79 8.59 7.76 -1.44 11.16 1.24 0.93 179 18:04:01.13 -24:22:35.6 10.49 10.30 9.93 8.96 -1.06 12.33 2.19 0.99 180 18:04:03.10 -24:25:19.5 9.53 9.17 8.70 7.60 -0.63 11.72 0.66 0.58 181 18:04:04.65 -24:08:49.8 8.03 7.69 7.13 6.92 -1.51 9.89 2.31 1.12 182 18:04:07.89 -24:26:06.3 10.69 10.27 9.69 8.55 -0.3783 18:04:08.11 -24:20:55.6 10.90 10.39 10.04 9.04 -0.75 13.03 1.00 0.47 184 18:04:08.47 -24:20:49.5 10.76 10.44 10.08 8.93 -0.76 12.39 0.93 0.38 185 18:04:09.94 -24:25:32.7 9.72 9.27 9.02 8.20 -1.15 11.15 0.94 0.38 186 18:04:10.19 -24:25:49.6 9.09 8.62 8.20 7.27 -0.78 11.80 1.17 0.69 187 18:04:11.57 -24:28:42.1 12.10 11.85 11.11 10.95 -1.4088 18:04:11.99 -24:26:28.1 10.18 9.57 9.07 8.18 -0.57 11.81 0.96 0.43 189 18:04:12.48 -24:11:51.6 10.83 10.10 9.70 8.67 -0.44 12.83 1.00 0.73 190 18:04:12.52 -24:35:47.1 8.15 7.83 7.43 7.07 -1.58 9.39 2.02 0.81 191 18:04:13.10 -24:26:13.3 10.88 10.43 9.90 8.80 -0.46 12.38 0.99 0.43 192 18:04:15.75 -24:19:01.7 10.07 9.80 9.64 8.80 -1.43 11.71 0.80 0.41 1,393 18:04:15.77 -24:25:15.8 10.71 10.17 9.79 9.09 -1.0294 18:04:15.91 -24:18:46.2 10.05 9.35 9.12 8.48 -1.13 12.44 1.11 0.81 1,395 18:04:16.06 -24:27:57.4 10.05 9.70 9.34 8.79 -1.4096 18:04:16.14 -24:19:52.5 9.41 8.86 8.44 7.45 -0.63 11.45 0.92 0.62 197 18:04:16.41 -24:24:38.8 9.67 9.17 8.79 7.74 -0.67 11.88 1.10 0.78 1,398 18:04:16.93 -24:24:14.9 9.16 8.75 8.38 7.84 -1.33 10.96 0.70 0.38 199 18:04:17.42 -24:19:09.8 8.83 8.42 8.00 7.53 -1.34 11.37 1.57 0.98 1,3100 18:04:17.89 -24:17:46.8 10.39 9.84 9.60 8.44 -0.69 12.02 0.94 0.57 1101 18:04:19.09 -24:27:58.8 10.28 9.70 9.17 8.18 -0.46 12.43 1.20 0.73 1102 18:04:19.31 -24:22:54.9 9.95 9.60 9.33 8.70 -1.42 11.50 0.94 0.47 1,3103 18:04:19.58 -24:24:04.7 9.00 8.77 8.37 7.81 -1.44 11.79 1.48 1.16 1104 18:04:19.87 -24:28:23.7 9.45 8.86 8.46 7.73 -0.91105 18:04:20.07 -24:22:48.2 10.58 10.26 9.66 8.51 -0.44 12.09 0.89 0.83 3106 18:04:20.26 -24:20:24.8 10.14 9.79 9.41 8.65 -1.14 12.32 1.04 0.73 1107 18:04:20.34 -24:24:34.6 10.75 9.93 9.51 9.07 -0.96108 18:04:20.52 -24:23:04.1 9.96 9.48 8.97 8.12 -0.73109 18:04:20.61 -24:23:01.1 10.24 9.89 9.45 8.47 -0.81110 18:04:20.82 -24:23:22.5 10.33 9.92 9.68 8.88 -1.22 12.46 1.05 0.83 1c (cid:13) , 1– ?? L.K. Dewangan & B.G. Anandarao
Table 2 – continued Object RA [2000] Dec [2000] Ch1 Ch2 Ch3 Ch4 α IRAC
H J-H H-K NIR Ref.111 18:04:21.01 -24:13:41.8 5.42 5.01 4.51 3.69 -0.85 8.69 2.32 1.39 1112 18:04:21.11 -24:23:25.5 10.08 9.81 9.42 8.66 -1.20113 18:04:21.12 -24:20:47.7 10.46 10.24 9.84 8.79 -0.91114 18:04:21.19 -24:24:22.4 10.39 9.96 9.55 8.89 -1.13115 18:04:21.47 -24:23:19.1 9.46 9.16 8.62 7.94 -1.05116 18:04:21.69 -24:23:19.7 9.75 9.27 8.63 7.75 -0.51117 18:04:21.82 -24:22:15.6 10.84 10.58 10.09 9.03 -0.74 13.02 0.91 0.52 1118 18:04:21.84 -24:16:26.2 9.83 9.50 9.27 8.23 -1.05 11.67 0.89 0.49 1119 18:04:21.86 -24:11:37.5 10.57 10.24 9.80 8.53 -0.51 11.77 0.51 0.22 1120 18:04:22.77 -24:22:09.7 8.53 8.09 7.65 6.87 -0.95 8.86 0.22 0.30 1121 18:04:23.05 -24:24:15.2 10.12 9.88 9.27 8.94 -1.40122 18:04:23.54 -24:22:47.6 9.69 9.15 8.76 8.46 -1.45123 18:04:24.23 -24:16:25.2 9.95 9.53 9.04 8.29 -0.94 12.71 1.47 1.01 1124 18:04:26.15 -24:22:45.1 6.73 6.24 5.77 5.03 -0.90 10.23 1.65 1.27 1125 18:04:26.75 -24:22:42.0 11.17 10.85 10.35 9.08 -0.43 13.08 1.09 0.62 1126 18:04:26.84 -24:23:23.5 9.07 8.71 8.44 7.71 -1.30 11.30 0.88 0.66 1127 18:04:27.09 -24:21:06.8 10.84 10.37 10.16 9.07 -0.89 12.36 0.95 0.51 1128 18:04:27.27 -24:20:57.0 9.41 9.06 8.82 8.23 -1.52 12.59 1.47 1.03 1129 18:04:27.36 -24:14:27.3 9.84 9.38 8.74 8.05 -0.75 12.69 1.31 1.09 1130 18:04:27.48 -24:07:31.1 8.03 7.76 7.19 6.85 -1.42131 18:04:28.03 -24:21:43.2 8.03 7.57 7.16 6.04 -0.59 7.97 -0.04 0.07 1132 18:04:28.25 -24:25:48.0 10.14 9.76 9.39 8.62 -1.11133 18:04:28.66 -24:20:20.0 5.77 5.47 5.16 4.44 -1.33 8.19 2.12 1.18 1134 18:04:29.67 -24:25:19.4 7.09 6.90 6.26 6.03 -1.52 10.36 2.07 1.31 1135 18:04:29.93 -24:14:29.8 10.46 10.08 9.76 9.05 -1.24 12.67 1.13 0.63 1136 18:04:30.59 -24:26:06.9 8.16 7.69 7.25 6.85 -1.33 10.29 1.94 1.00 1137 18:04:31.07 -24:29:09.9 10.59 10.41 9.89 9.04 -1.00138 18:04:32.35 -24:19:28.1 10.07 9.53 9.07 8.02 -0.52 12.48 0.96 0.59 1139 18:04:32.39 -24:27:55.2 10.33 9.95 9.61 8.82 -1.13 13.36 1.62 1.04 1140 18:04:33.22 -24:27:18.0 9.68 9.02 8.77 8.17 -1.18 11.56 1.15 0.77 1141 18:04:33.58 -24:21:54.8 8.70 8.45 7.81 6.89 -0.70 10.42 0.97 0.61 1142 18:04:34.61 -24:09:02.0 5.30 4.66 4.23 3.43 -0.75 9.29 2.80 1.74 1143 18:04:34.94 -24:22:51.9 10.70 10.27 9.92 8.84 -0.75 12.99 0.99 0.58 1144 18:04:36.50 -24:19:13.7 10.23 9.73 9.38 8.96 -1.40145 18:04:38.90 -24:19:28.2 8.24 7.87 7.32 7.11 -1.49146 18:04:39.31 -24:32:24.4 5.64 5.25 4.61 3.77 -0.65 10.00 2.60 1.61 1147 18:04:39.46 -24:27:09.2 10.46 9.99 9.57 8.65 -0.79 12.26 1.10 0.53 1148 18:04:39.86 -24:23:05.2 9.55 9.05 8.53 7.57 -0.58 12.28 1.22 0.83 1149 18:04:40.90 -24:17:10.8 9.29 9.01 8.71 8.15 -1.54 11.70 1.05 0.70 1,3150 18:04:41.27 -24:15:44.9 5.14 4.58 4.14 3.83 -1.35 7.66 2.63 1.41 1151 18:04:41.63 -24:26:31.8 8.11 7.49 7.02 6.71 -1.24 10.05 2.07 1.11 1152 18:04:43.53 -24:27:38.7 8.40 7.76 7.22 6.46 -0.64 10.78 1.21 0.76 3153 18:04:43.65 -24:27:59.1 10.26 9.86 9.63 9.00 -1.44 13.01 1.47 0.94 1154 18:04:44.06 -24:19:39.7 10.48 10.02 9.75 8.99 -1.18 12.07 0.84 0.43 1155 18:04:44.46 -24:10:17.5 10.15 9.85 9.64 8.85 -1.38156 18:04:46.41 -24:26:08.0 10.42 10.05 9.52 8.52 -0.65 12.54 1.13 0.53 1157 18:04:48.05 -24:27:24.6 7.20 6.59 6.16 5.74 -1.18158 18:04:48.56 -24:26:40.7 8.89 8.46 8.04 7.29 -1.02 11.02 1.58 0.95 1,3159 18:04:50.23 -24:27:59.4 10.40 10.09 9.36 8.82 -0.93160 18:04:51.53 -24:24:17.4 10.06 9.45 9.00 8.08 -0.61 12.28 1.21 0.61 1161 18:04:51.57 -24:26:10.9 10.72 10.02 9.96 9.00 -1.01162 18:04:51.63 -24:25:15.8 10.37 9.88 9.40 8.91 -1.16 12.02 0.97 0.47 1163 18:04:52.64 -24:27:30.8 10.72 10.42 9.67 8.99 -0.76 13.28 1.16 0.70 1164 18:04:53.48 -24:26:08.7 8.45 7.91 7.21 6.66 -0.74165 18:04:54.08 -24:26:23.7 7.05 6.33 5.96 5.45 -1.06166 18:04:55.00 -24:27:18.1 9.15 8.77 8.29 7.42 -0.84 12.59 2.04 1.33 1167 18:04:56.18 -24:09:00.9 8.16 7.78 7.55 6.98 -1.52 10.06 2.02 0.99 1168 18:04:57.35 -24:20:48.4 4.49 4.30 3.58 3.28 -1.33 c (cid:13) , 1–, 1–
H J-H H-K NIR Ref.111 18:04:21.01 -24:13:41.8 5.42 5.01 4.51 3.69 -0.85 8.69 2.32 1.39 1112 18:04:21.11 -24:23:25.5 10.08 9.81 9.42 8.66 -1.20113 18:04:21.12 -24:20:47.7 10.46 10.24 9.84 8.79 -0.91114 18:04:21.19 -24:24:22.4 10.39 9.96 9.55 8.89 -1.13115 18:04:21.47 -24:23:19.1 9.46 9.16 8.62 7.94 -1.05116 18:04:21.69 -24:23:19.7 9.75 9.27 8.63 7.75 -0.51117 18:04:21.82 -24:22:15.6 10.84 10.58 10.09 9.03 -0.74 13.02 0.91 0.52 1118 18:04:21.84 -24:16:26.2 9.83 9.50 9.27 8.23 -1.05 11.67 0.89 0.49 1119 18:04:21.86 -24:11:37.5 10.57 10.24 9.80 8.53 -0.51 11.77 0.51 0.22 1120 18:04:22.77 -24:22:09.7 8.53 8.09 7.65 6.87 -0.95 8.86 0.22 0.30 1121 18:04:23.05 -24:24:15.2 10.12 9.88 9.27 8.94 -1.40122 18:04:23.54 -24:22:47.6 9.69 9.15 8.76 8.46 -1.45123 18:04:24.23 -24:16:25.2 9.95 9.53 9.04 8.29 -0.94 12.71 1.47 1.01 1124 18:04:26.15 -24:22:45.1 6.73 6.24 5.77 5.03 -0.90 10.23 1.65 1.27 1125 18:04:26.75 -24:22:42.0 11.17 10.85 10.35 9.08 -0.43 13.08 1.09 0.62 1126 18:04:26.84 -24:23:23.5 9.07 8.71 8.44 7.71 -1.30 11.30 0.88 0.66 1127 18:04:27.09 -24:21:06.8 10.84 10.37 10.16 9.07 -0.89 12.36 0.95 0.51 1128 18:04:27.27 -24:20:57.0 9.41 9.06 8.82 8.23 -1.52 12.59 1.47 1.03 1129 18:04:27.36 -24:14:27.3 9.84 9.38 8.74 8.05 -0.75 12.69 1.31 1.09 1130 18:04:27.48 -24:07:31.1 8.03 7.76 7.19 6.85 -1.42131 18:04:28.03 -24:21:43.2 8.03 7.57 7.16 6.04 -0.59 7.97 -0.04 0.07 1132 18:04:28.25 -24:25:48.0 10.14 9.76 9.39 8.62 -1.11133 18:04:28.66 -24:20:20.0 5.77 5.47 5.16 4.44 -1.33 8.19 2.12 1.18 1134 18:04:29.67 -24:25:19.4 7.09 6.90 6.26 6.03 -1.52 10.36 2.07 1.31 1135 18:04:29.93 -24:14:29.8 10.46 10.08 9.76 9.05 -1.24 12.67 1.13 0.63 1136 18:04:30.59 -24:26:06.9 8.16 7.69 7.25 6.85 -1.33 10.29 1.94 1.00 1137 18:04:31.07 -24:29:09.9 10.59 10.41 9.89 9.04 -1.00138 18:04:32.35 -24:19:28.1 10.07 9.53 9.07 8.02 -0.52 12.48 0.96 0.59 1139 18:04:32.39 -24:27:55.2 10.33 9.95 9.61 8.82 -1.13 13.36 1.62 1.04 1140 18:04:33.22 -24:27:18.0 9.68 9.02 8.77 8.17 -1.18 11.56 1.15 0.77 1141 18:04:33.58 -24:21:54.8 8.70 8.45 7.81 6.89 -0.70 10.42 0.97 0.61 1142 18:04:34.61 -24:09:02.0 5.30 4.66 4.23 3.43 -0.75 9.29 2.80 1.74 1143 18:04:34.94 -24:22:51.9 10.70 10.27 9.92 8.84 -0.75 12.99 0.99 0.58 1144 18:04:36.50 -24:19:13.7 10.23 9.73 9.38 8.96 -1.40145 18:04:38.90 -24:19:28.2 8.24 7.87 7.32 7.11 -1.49146 18:04:39.31 -24:32:24.4 5.64 5.25 4.61 3.77 -0.65 10.00 2.60 1.61 1147 18:04:39.46 -24:27:09.2 10.46 9.99 9.57 8.65 -0.79 12.26 1.10 0.53 1148 18:04:39.86 -24:23:05.2 9.55 9.05 8.53 7.57 -0.58 12.28 1.22 0.83 1149 18:04:40.90 -24:17:10.8 9.29 9.01 8.71 8.15 -1.54 11.70 1.05 0.70 1,3150 18:04:41.27 -24:15:44.9 5.14 4.58 4.14 3.83 -1.35 7.66 2.63 1.41 1151 18:04:41.63 -24:26:31.8 8.11 7.49 7.02 6.71 -1.24 10.05 2.07 1.11 1152 18:04:43.53 -24:27:38.7 8.40 7.76 7.22 6.46 -0.64 10.78 1.21 0.76 3153 18:04:43.65 -24:27:59.1 10.26 9.86 9.63 9.00 -1.44 13.01 1.47 0.94 1154 18:04:44.06 -24:19:39.7 10.48 10.02 9.75 8.99 -1.18 12.07 0.84 0.43 1155 18:04:44.46 -24:10:17.5 10.15 9.85 9.64 8.85 -1.38156 18:04:46.41 -24:26:08.0 10.42 10.05 9.52 8.52 -0.65 12.54 1.13 0.53 1157 18:04:48.05 -24:27:24.6 7.20 6.59 6.16 5.74 -1.18158 18:04:48.56 -24:26:40.7 8.89 8.46 8.04 7.29 -1.02 11.02 1.58 0.95 1,3159 18:04:50.23 -24:27:59.4 10.40 10.09 9.36 8.82 -0.93160 18:04:51.53 -24:24:17.4 10.06 9.45 9.00 8.08 -0.61 12.28 1.21 0.61 1161 18:04:51.57 -24:26:10.9 10.72 10.02 9.96 9.00 -1.01162 18:04:51.63 -24:25:15.8 10.37 9.88 9.40 8.91 -1.16 12.02 0.97 0.47 1163 18:04:52.64 -24:27:30.8 10.72 10.42 9.67 8.99 -0.76 13.28 1.16 0.70 1164 18:04:53.48 -24:26:08.7 8.45 7.91 7.21 6.66 -0.74165 18:04:54.08 -24:26:23.7 7.05 6.33 5.96 5.45 -1.06166 18:04:55.00 -24:27:18.1 9.15 8.77 8.29 7.42 -0.84 12.59 2.04 1.33 1167 18:04:56.18 -24:09:00.9 8.16 7.78 7.55 6.98 -1.52 10.06 2.02 0.99 1168 18:04:57.35 -24:20:48.4 4.49 4.30 3.58 3.28 -1.33 c (cid:13) , 1–, 1– ?? pitzer IRAC study of M8 bands is 3376; of these, 235 sources are found to be con-taminations (1 PAH galaxy, 6 shocked emissions, 228 PAHaperture-contaminations), while there are 327 YSOs.After removing the contaminants, we used the crite-ria based on the spectral index, α λ (= dlog( λ F λ )/dlog( λ )),to classify the YSOs (numbering 327 as shown above) intodifferent evolutionary classes (see e.g., Green et al. (1994);Smith (2004); Lada et al. (2006)). We followed Billot et al.(2010) in the classification of Class 0/I as sources whose α IRAC is > -0.3; Class II as those having -0.3 > α IRAC > -1.6; and Class III as those having -1.6 > α IRAC > -2.6(termed as sources with faint or anemic disks by Lada et al.(2006)). In applying these classifications to M8, we havenot considered the flat-spectrum sources (e.g., Green et al.(1994)) as a separate class but included them in the Class0/I, the sources with in-falling envelopes (see Billot et al.(2010)). The sources with α IRAC < -2.6 are taken as starswith purely photospheric emissions. With the α IRAC classi-fication, we obtain 64 Class 0/I, 168 Class II sources and 95Class III sources. The rest of the sources, numbering 2814,are purely photospheric sources.We then verified the selected sample of YSOs for possi-ble interstellar extinction/reddening bias (see Muench et al.(2007) and the references therein). Muench et al. (2007)showed that only for very large values of A v do sources,with α IRAC ranges relevant here, get misclassified as YSOs.Such large values are seen only as intrinsic for YSOs asmay be inferred from the H-K colour. Typically, we canidentify the bias by comparing the ratio ( N ) of number ofClass II sources to that of the Class 0/I, for different val-ues of extinction and see if the ratio changes substantially(Guieu et al. 2009). In the case of M8, the visual extinctionand the reddening (defined as the ratio of total to selectiveextinction, R v = A v /E(B-V)) varies from region to region;and a value of A v = 3.2 was determined towards the Hour-glass region for standard reddening (R v = 3.1) (Arias et al.2006; Tothill et al. 2008). For the present purpose, however,we compared the ratio N , for A v values 0.0, 3.2 and 5.0. Theratio N remains at 2.63 with a Poisson error of ± v v = 5.0, we get N = 2.73 ± N obtained here for M8 is comparable with that obtainedfor the Serpens (Harvey et al. 2006) and North AmericanNebula (Guieu et al. 2009) star forming regions, indicatingthe similarity of ages of these regions.Fig 2 shows the mid-infrared colour-colour diagramconstructed from the IRAC photometry for [3.6]-[4.5] vs[5.8]-[8.0] colour. In this diagram we show the photosphericsources as black dots, and the Class 0/I, Class II and ClassIII sources as open circles, open triangles and open squares.Table 1 lists the Class 0/I sources while Table 2 gives theClass II sources. Also included in Table 1 is the source KS 4(No. 26; classified as YSO by Arias et al. (2006)) for whichIRAC has detections in Chs. 1 & 2 only. Where available, thecolours [J-H] and [H-K] along with the H magnitude fromJHK surveys are also given in the Tables with references.In order to obtain quantitatively the spatial distri-bution of YSOs, we adopted the nearest-neighbour tech-nique (see Chavarria et al. (2008); Guieu et al. (2009);Evans et al. (2009)). We have followed the suggestion ofCasertano & Hut (1985) to obtain surface density distribu-tion, without a bias towards over-estimation. We used a 5arcsec grid to compute the surface number density, defined as ρ n = ( n − /A n , where A n is the surface area definedby the radial distance r n to the n (= 5) nearest-neighbours.Fig 3 shows the YSO spatial density contours with the in-ner and outer contours representing 10 and 5 YSOs/pc re-spectively. The maximum densities are about 20 YSOs/pc .Also shown in the figure are the spatial distributions of allthe 327 YSOs (a) and the contaminants (b). One can no-tice in Fig 3 several small clusters (about 7) isolated by thetechnique used as above. These clusters are mostly confinedto the Hourglass, NGC 6530, M8E and the ridge regionsalong with two more clumps east of H36. In order to iden-tify the cluster members as against the isolated or scatteredcases, we computed the empirical cumulative distribution asa function of nearest-neighbour distance and found that thesources within an arbitrarily chosen inflection distance d c (that signifies the maximum separation between the clustermembers) of ∼ o at 1.25 kpc). By varying d c between 0.65 and 0.85 pc, the number of cluster membersdoes not change significantly. The Class 0/I and Class IIYSOs in clusters constitute about 60% of the total numberdetected and are confined mainly within the YSO densitycontours (corresponding to about 7 clusters) shown in Fig3. In comparison, only about 26 % of the Class III sources(having “anemic” disks) occur in the clusters. The ratio N mentioned before varies from cluster to cluster with an av-erage of 2.29 which is comparable to the one for the entiresample (2.63) within the Poisson errors. It may be notedhere that the surface density of YSOs derived by us is veryclose to the values obtained for star forming regions else-where (see Billot et al. (2010) and references therein).It may be noted here that since M8 ( l = 5.958; b = -1.167) is located near the mid-plane of the Galaxy, there ex-ists a possibilty of our YSO sample being still contaminatedfrom other intrinsically “red sources”, such as AGB stars.Recently Robitaille et al. (2008) prepared an extensive cat-alogue of such red sources based on the Spitzer
GLIMPSEand MIPSGAL surveys. While the best way to distinguishbetween YSOs and AGB stars is by spectroscopy, these au-thors showed that the two classes are well separated in the[8.0-24.0] colour space, YSOs being redder than AGB starsin this space (see also Whitney et al. (2008)). Since in thepresent case of M8 we do not have the 24 µ m data (fromMIPS), we have estimated the AGB contamination by usingthe criteria based on the IRAC magnitudes and colour space(Robitaille et al. 2008). Since AGB stars are unlikely to oc-cur in clusters, we have removed the clustered YSOs fromour estimates. We find that our Class 0/I and II samplesmay be contaminated by AGB stars up to about 19%, whilewe do not find any contamination for our Class III sources.As pointed out by Robitaille et al. (2008), these separationcriteria (including [8.0-24.0] colour) are “only approximateand there is likely to be contamination in both directions”.Fig 4 shows Ch3 (5.8 µ m) image overlaid by the IRACClass 0/I (open circles) and Class II (open triangles) sourcesidentified by using the α IRAC criteria (see Tables 1 and 2).Also shown (as black star symbols) in the figure are thepositions of sub-mm (850 µ m) gas clumps (taken from Ta-ble 1 of Tothill et al. (2008)). The YSO density contours(in white) are also shown in the figure. A number of theClass 0/I and Class II sources occur very close to the densefilamentary/pillar-like structures seen all along the ridge re-gion in the east-west direction as well as perpendicular to c (cid:13) , 1– ?? L.K. Dewangan & B.G. Anandarao it in the central ridge or NGC 6530 region (see Fig 1). Amajority of these sources are found to be present in thevicinity of the sub-mm gas clumps (marked by black starsymbols in Fig 4). As mentioned earlier, M8E is a youngcompact high-mass star forming region. The central sourceof this cluster was resolved into a protostar M8E-IR andM8E-Radio, a B2 type star that is responsible for the com-pact HII region (Simon et al. 1984). While IRAC bands aresaturated for M8E-IR itself, about 6 Class I or flat-spectrumand 10 Class II sources are identified by IRAC in a region of4 arcmin around M8E. These sources are not common withthose listed in Table 6 of Tothill et al. (2008). We did notfind any IRAC pre-main-sequence sources in the vicinity ofthe source IRAS 18014-2428, believed to be another youngstar forming region (corresponding to the sub-mm gas clumpcalled SE3 (Tothill et al. 2002)).Triggered star formation by the “collect and collapse”process (Elmegreen & Lada 1977) could be responsible forthe existence of the IRAC sources, possibly started by thestellar winds or expanding HII regions associated with thenearby massive stars, viz. HD 165052, M8E, HD 164806, HD164816, 9 Sgr and Her 36 (see Tothill et al. (2002)). Whilethe ionization fronts from 9 Sgr and Her 36 have expandedwell into the molecular cloud, that from M8E seems to havestarted later, as is evident from the bright narrow rim infront of M8E seen in Fig 1. Thus the small cluster in M8Eregion may be younger than others in the region. Linz et al.(2009) modelled the spectral energy distribution of M8E-IRand concluded that it is a B0 type YSO. That the star for-mation is sequential in M8 starting from north-west regionsto southern edge regions has been shown by Damiani et al.(2004), based on Chandra X-ray survey of NGC 6530 regionand its neighbourhood in conjunction with 2MASS data andoptical surveys (see Tothill et al. (2008) for a discussion). As mentioned earlier, the IRAC bands contain a num-ber of prominent molecular lines/features. Ch1 contains H vibrational-rotational lines while Ch2-4 mostly contain purerotational lines. Ch1, 3 and 4 also contain the PAH featuresat 3.3, 6.2, 7.7 and 8.6 µ m; but Ch2 does not include anyPAH features. In addition to these molecular lines/features,IRAC bands also contain hydrogen recombination lines, no-tably the Br α line (4.05 µ m) in Ch2, which can be used totrace HII regions. Several authors have utilised the ratios ofIRAC bands to identify some of the atomic and moleculardiagnostics mentioned above (e.g., Smith & Rosen (2005);Povich et al. (2007); Neufeld & Yuan (2008)). Since it is dif-ficult to assess the contribution of different atomic or molec-ular transitions to different channels, the ratio maps are onlyindicative; until/unless supplemented by spectroscopic evi-dence.The Ch2 is more sensitive to H lines of high excitationtemperatures while the Ch4 represents rotational lines of lowexcitation temperatures (Neufeld & Yuan 2008). Likewise,the Ch2 does not have any PAH features while Ch4 has.Thus, in the ratio image of Ch2/Ch4, the brighter regionsindicate emission regions from higher excitations from H and the darker regions indicate PAH emission. This trend isreversed in the image of Ch4/Ch2 (i.e., bright regions showPAH emission and dark regions the H emission). However in HII regions, the H recombination lines (Br α and Pf β )are more significant contributors to Ch2 rather than the H lines.In order to make the ratio maps, point sources fromall the IRAC images are removed by using an extendedaperture of 12.2 arcsec and sky annulus of 12.2-24.4 arc-sec in IRAF/DAOPHOT software (Reach et al. 2005). Thenthese residual frames are subjected to median filtering with awidth of 15 pixels and smoothing by 3 × × around Her36. Contours overlaid on the ratio map represent the H α emission observed by HST (in F656N filter image extractedfrom HST public archive). The minimum and maximum val-ues of the contours are 5288 and 14100 counts respectively;six contours are drawn with interval of 1762 counts. In thevicinity of Her 36 the ratio map Ch2/Ch4 shows brightregions coinciding very nicely with the Hourglass HII re-gion (as traced out by the contours of H α ). The bright re-gions coinciding with the Hourglass can not be attributedto the molecular hydrogen lines of higher excitation tem-perature present in Ch2 in comparison with those of lowerexcitation temperature present in Ch4 (see Smith & Rosen(2005)). Further, the molecular hydrogen (1-0 S(1) at 2.12 µ m) images presented by Burton (2002) do not show sub-stantial emission around the Hourglass region (refer Figs 3and 4 of Burton (2002)). Considering the fact that the IRACCh2 also contains Hydrogen Br α line (as well as Pf β (4.65 µ m)), we may attribute the bright regions in the ratio mapof Ch2/Ch4 coinciding with Hourglass, as due to Br α (andPf β ) emission. One can notice narrow bright regions sur-rounding Her 36 (and the near-IR source KS1) in the ratiomap of Ch2/Ch4. These regions are coincident with those ofmolecular hydrogen in Burton (2002). In these regions it islikely that the molecular hydrogen transitions of high exci-tation temperature may be responsible rather than Br α . Asimilar trend is seen in the ratio image of Ch2/Ch3 also. Fig5b gives the ratio image Ch4/Ch2 in the same region as inFig 5a. In this figure, the ratio contours are overlaid on ratioimage for better clarity and insight. One can notice a bright(corresponding to dark region in Fig 5a) “cavity”-like struc-ture to the east of the Hourglass. Actually, this structureoccurs towards the east of the regions of very high columndensity reported by Arias et al. (2006) towards north andeast of the Hourglass. It is possible that the PAH moleculescan be shielded from high energy UV photons by these denseregions. But the molecules may be excited by the low energy(non-ionising) UV photons that can escape from the narrowdense regions between the Hourglass and the “cavity”. Thuswe may attribute the bright tubular structure to PAH emis-sions.It may be mentioned here that the ratio image ofCh2/Ch1 does not show the Hourglass as prominently asin Ch2/Ch4 or Ch2/Ch3. This could be because of the factthat Ch1 contains Pf γ (3.7 µ m) and Pf δ (3.3 µ m) lines whichmay be cancelling partly the recombination lines in Ch2. Infact we do clearly see the Hourglass HII region in the ra-tio images of Ch1/Ch3 and Ch1/Ch4 also. Thus the Ch1and Ch2 bring out well the H recombination emission in theHourglass. Ch4 is unable to depict the HII region in spiteof the presence of Pf α (7.5 µ m), mainly because of the factthat its spectral response is inferior to that of Ch1 (or that c (cid:13) , 1– ?? pitzer IRAC study of M8 of Ch2) (Smith & Rosen 2005). For a HII region under CaseB situation (Hummer & Storey 1987) with a kinetic tem-perature of 10 K and electron densities of 10 -10 cm − ,the combined relative intensities of Pf γ and Pf δ are a factorof ∼ α . Hence the combineddetected flux of Pf γ and Pf δ in Ch1 can exceed that of Pf α in Ch4.Elsewhere in M8 the ratio maps seem to have a differentstory to tell. Fig 6 gives the ratio maps of the ridge regionsin M8 complex in an area of 24.2 x 20.0 arcmin , to theeast/south-east of Her 36: Ch1/Ch2 (top), Ch3/Ch2 (mid-dle) and Ch4/Ch2 (bottom). The ratio images show brightrims corresponding to the filamentary structures all alongand perpendicular to the east-west ridge region. These re-gions seem to be bright in all bands, except Ch2 that isfree of PAH features. Hence it is tempting to attribute thebright regions seen in the ratio images in Fig 6 to the PAHfeatures. It is unlikely that these bright regions are a resultof molecular hydrogen emission; since they occur in all thebands though at different excitation temperatures (mostlyrotational lines) and should have cancelled each other in theratio maps. The important conclusions of this work are as follows:(i)
Spitzer
IRAC photometry of M8 region revealed 64Class 0/I and 168 Class II YSOs. About 60% of these arepresent in about 7 small clusters with spatial surface densi-ties of 10-20 YSOs/pc ;(ii) These sources are positioned close to the sub-mm gasclumps and the filamentary or pillar like structures presentin M8. It is possible that the formation of these sourcescould have been triggered by stellar winds or expanding HIIregions associated with the massive stars in the region;(iii) The ratio map Ch2/Ch4 reveals Br α emission cor-responding to the Hourglass HII region powered by Her 36and its inverted ratio (Ch4/Ch2) identifies PAH emission ina cavity, east of the Hourglass;(iv) The ratio maps Ch1/Ch2, Ch3/Ch2 and Ch4/Ch2indicate the presence of PAH emission in both the ridgesoriented along E-W and NE-SW directions. ACKNOWLEDGMENTS
The research work is supported by the Department of Space,Government of India at PRL. This work is based (for a largepart) on observations made with the Spitzer Space Tele-scope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology under a contract withNASA. We acknowledge the use of data from the 2MASS,which is a joint project of the University of Massachusettsand the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the NASA and the NSF.The authors sincerely appreciate the very useful commentsfrom the anonymous referee.
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Spitzer IRAC Ch4 (8.0 µ m) image of Lagoon Nebula region ( ∼ × ). The locations of well known sources areshown circled: the massive O type stars Her 36 and 9 Sgr; the early B type stars HD 164906 and HD 164816; the near-infrared sourceKS1 just north of Her 36; and the young compact massive star-forming region M8E to the extreme east. The Hourglass HII region isshown by the arrow near the core of M8. The young cluster NGC 6530 is situated just to the west of the massive star HD 164906.The arrows at the bottom show the ridge region in the east-west direction. One can also notice filamentary features in the central ridgerunning north-south near the star HD 164906. The massive star HD 165052 is towards the east of M8E but it is not covered by the IRACobservations.c (cid:13) , 1– ?? L.K. Dewangan & B.G. Anandarao
Figure 2.
Mid-IR color-color diagram using the Spitzer IRAC bands for all the sources identified within the region shown in Fig 1. Thesources KS1 and Her 36 are marked by arrows. The upward arrow on the left shows the extinction vector for A K = 5 mag, using averageextinction law from Flaherty et al. (2007). The black dots around the centre (0,0) locate the stars with only photospheric emissions.The open squares (blue), open triangles (violet) and open circles (red) represent respectively, Class III, Class II and Class 0/I sources,obtained from the α IRAC criteria. c (cid:13) , 1– ?? pitzer IRAC study of M8 (a)(b) Figure 3. a: Spitzer IRAC field of M8 showing all the 327 YSOs: open circles (red) show Class 0/I sources, open triangles (violet)show Class II and open squares (blue) show Class III sources; b: Spitzer IRAC field of M8 showing contaminants: crosses (red) showunresolved PAH aperture contaminations, filled squares (black) show shocked emission knots and the lone filled circle (blue) the PAHgalaxy contaminant. In both the figures, the contours show YSO iso-density at 5 (outer) and 10 (inner) YSOs/pc .c (cid:13) , 1– ?? L.K. Dewangan & B.G. Anandarao
Figure 4.
Spitzer IRAC Ch3 (5.8 µ m) image of M8 ( ∼ × ) superposed by IRAC Class 0/I and II sources and sub-mmgas clumps. The open circles (red) and open triangles (blue) show the IRAC Class 0/I and II sources respectively (from Tables 1 and2 of the present work); the black star symbols represent the locations of the sub-mm gas clumps (from Table 1 of Tothill et al. (2008)).The overlaid contours (white) are YSO density contours generated using a grid size of 5 arcsec: the inner contours are 10 YSOs/pc andthe outer contours represent 5 YSOs/pc . c (cid:13) , 1–, 1–
Spitzer IRAC Ch3 (5.8 µ m) image of M8 ( ∼ × ) superposed by IRAC Class 0/I and II sources and sub-mmgas clumps. The open circles (red) and open triangles (blue) show the IRAC Class 0/I and II sources respectively (from Tables 1 and2 of the present work); the black star symbols represent the locations of the sub-mm gas clumps (from Table 1 of Tothill et al. (2008)).The overlaid contours (white) are YSO density contours generated using a grid size of 5 arcsec: the inner contours are 10 YSOs/pc andthe outer contours represent 5 YSOs/pc . c (cid:13) , 1–, 1– ?? pitzer IRAC study of M8 (a)(b) Figure 5. a: Ratio image of Ch2/Ch4 of M8 (Lagoon Nebula) in log scale overlaid by HST H α (F656N filter) contours, in a region aroundHer 36 of size 110 ×
76 arcsec . The contour levels are between 5288 and 14100 counts. The bright regions indicate the prominence of Ch2over Ch4, while the dark regions have the reverse trend. The bright “3” shaped structure is more extended than the H α contours andnearly coincide with them. b: Ratio image of Ch4/Ch2 of M8 (Lagoon Nebula) in log scale. The bright regions indicate the prominenceof Ch4 over Ch2. For better insight, the ratio image is overlaid by the ratio contours: the black contours in the bright regions representratio levels between 17.4 and 22.8; and the white contours in the dark regions indicate the ratio levels of 14.3 to 8.4. The region ofpossible PAH emission is the bright tubular structure within the “cavity”, seen east of the Hourglass.c (cid:13) , 1– ?? L.K. Dewangan & B.G. Anandarao
Figure 6.
Ratio maps of Ch1/Ch2 (top), Ch3/Ch2 (middle) and Ch4/Ch2 (bottom) in log scale (in a region of size ∼ × ), showing the ridges and filamentary structures (towards east/south-east of Her 36). The dominance of Ch1, Ch3 and Ch4(bright portions) over Ch2 in the ridge region and filamentary regions can be noticed, which may be attributed to the PAH emission inthese regions. The black dots (seen mostly in Ch1/Ch2) and the white dots (seen mostly in Ch3/Ch2 and Ch4/Ch2) are the result ofthe residueing process. c (cid:13) , 1–, 1–