the intermediate-mass embedded cluster gm 24 revisited ... - IOPscience

The Astronomical Journal, 137:4127–4139, 2009 May  C 2009.

doi:10.1088/0004-6256/137/5/4127

The American Astronomical Society. All rights reserved. Printed in the U.S.A.

THE INTERMEDIATE-MASS EMBEDDED CLUSTER GM 24 REVISITED: NEW INFRARED AND RADIO OBSERVATIONS∗ 5 ´ Mauricio Tapia1 , Luis F. Rodr´ıguez2 , Paolo Persi3 , Miguel Roth4 , and Mercedes Gomez 1

2

Instituto de Astronom´ıa, Universidad Nacional Aut´onoma de M´exico, Apartado Postal 877, 22830 Ensenada, B.C., Mexico Centro de Radioastronom´ıa y Astrof´ısica, Universidad Nacional Aut´onoma de M´exico, Apdo. Postal 372, Morelia, Michoacan 58089, Mexico 3 INAF-Istituto Astrofisica Spaziale e Fisica Cosmica Roma, Via Fosso del Cavaliere 100, 00133 Roma, Italy 4 Las Campanas Observatory, Carnegie Institution of Washington, Casilla 601, La Serena, Chile 5 Observatorio Astron´ omico de C´ordoba, Laprida 854, 5000 C´ordoba, Argentina Received 2008 October 31; accepted 2009 February 7; published 2009 March 30

ABSTRACT New and archived high-resolution infrared (IR; 1–20 μm) and radio-continuum images of the isolated embedded cluster and associated compact H ii region, GM 24, are presented and measured photometrically. The nucleus of the complex is Irs 3, or IRAS 17136-3617, located at the densest part of the molecular cloud. This object is composed of at least three compact near-IR sources (A, B, and C) that are the most luminous and massive young stellar components and provide most of the ionizing energy to the cometary-shaped radio H ii region. The 3.6 cm radio map shows a complex structure with an extended emission peak and two very compact components very close to Irs 3A. Large inhomogeneities in the dust density within the nebula cause considerably different morphologies in the observed emission of hydrogen recombination lines, namely Paβ, Brγ , and Brα. No H2 line emission at 2.12 μm was detected. The embedded IR cluster is found to contain more than 100 members within a radius of around 40 , which corresponds to 0.39 pc. The total stellar mass is estimated to be 250 M . The extinction to the nearby edge of the cluster is determined to be AV = 13, though a number of sources, including Irs 3, are reddened by AV > 50. A fraction of near-IR sources, mainly in the periphery of the cluster, are main-sequence A–B-type stars, while a large fraction (∼50%) of the detected members show significant IR excesses, including several Class I young stellar objects with luminosities ranging from a few solar luminosities near our sensitivity limit, to 1.5 × 105 L , the derived luminosity of Irs 3. Key words: H ii regions – ISM: individual (GM 24) – open clusters and associations: general – stars: formation

spatial resolution in order to permit the study of the apparent luminosity function of the cluster. At the longer wavelengths, only very low resolution information had been available. A new archive Very Large Array (VLA) 3.6 cm image is also used to further study the properties of the compact H ii region associated with GM 24. The following section describes the new and archived observations analyzed in this work and lists the basic results. The overall view of the complex and the statistical considerations describing the embedded cluster are discussed in Section 3. The properties of the ionized gas are described in Section 4, while individual discussions of the most conspicuous individual objects is presented in Section 5. The conclusions are summarized in Section 6.

1. INTRODUCTION The red and compact nebulous object GM 1-24 (Gyulbudaghian & Magakian 1977) is immersed in the lowbrightness H ii region RCW 126 (Rodgers et al. 1960). It has been established to be associated with a warm and dense CO cloud (Torrelles et al. 1983; Tapia et al. 1985; G´omez et al. 1990) that has an embedded young stellar cluster of high and intermediate masses in its central region (Tapia et al. 1991). Its most luminous young stellar object (YSO) is Irs 3, which dominates the mid- and far-infrared (IR) emission of the complex source IRAS 17136−3617. In turn, this embedded (AV ∼ 50) object is the main ionization engine of a compact H ii region (Roth et al. 1988; G´omez et al. 1993). The near-IR cluster has been cataloged DBSB 122 by Dutra et al. (2003), though we will refer to the complex, collectively, as GM 24. The adopted kinematic distance of 2.0 kpc to this region has been derived from the measured radial velocities of optical, IR, and radio hydrogen recombination lines as well as from the CO line, and is in agreement with the combined physical parameters determined for the H ii region and the molecular cloud. A detailed up-to-date review of the region has been written by Tapia & Persi (2008). The purpose of this work is to derive more accurately the properties of the embedded young stellar population of the cluster by means of new sensitive and high-resolution nearand mid-IR images. Previous IR observations (Tapia et al. 1985, 1991) were not sufficiently deep and did not have the necessary

2. OBSERVATIONS AND RESULTS 2.1. New Infrared Observations A set of deep broadband JHKs images of a 114 × 114 field containing GM 24 has been obtained with the near-IR camera PANIC (Martini et al. 2004) attached to the 6.5 m Clay/Magellan Telescope at Las Campanas Observatory (LCO) on the night of 2003 May 17. PANIC is based on a Hawaii HgCdTe 1024 × 1024 detector array with optics that provides an image scale of 0. 125 pixel−1 . The measured FWHM of the point-spread function (PSF) is between 0. 7 and 0. 8. For each filter, nine dithered frames spaced 6 were taken, with total onsource integration times of 540 s, 810 s, and 540 s in J, H, and Ks , respectively. Standard sky-subtraction and flat-field correction procedures were applied. The resulting color-composite JHKs image is shown in Figure 1. PSF-fitting photometry

∗ This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile, and with the 2.1 m telescope at the Observatorio Astron´omico Nacional at San Pedro M´artir.

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Figure 1. JHKs color-composite image of GM 24 constructed from observations with the Clay/Magellan Telescope. The image is centered at (J2000) α = 17h 17m 02.s 1, δ = −36◦ 21 08. 7. The white circle, of radius 40 , marks the size of the embedded cluster determined as explained in Section 3. The cross marks the position of H2 O maser (G´omez et al. 1993). North is up; east is to the left.

was performed for the unresolved sources using DAOPHOT (Stetson 1987) within IRAF in the standard way. Flux calibration was performed using a set of standard stars from the LCO list (Persson et al. 1998). For the extended objects, photometry was performed with appropriate aperture and sky settings determined individually. Even though the on-chip integration times were kept short, the sources with Ks < 10.5, H < 10.3, and J < 10.1, became saturated. For those few cases, the adopted magnitudes were taken from the Two Micron All Sky Survey (2MASS) Point-Source Catalog (PSC; Strutskie et al. 2006), as within 2% no transformations are required between Las Campanas and 2MASS JHKs photometries (Carpenter 2001). A total of 638 sources were measured in Ks , of which 296 were measured in both H and Ks and only 119 in the three broadband filters. The limiting magnitude was around 16.7 in the three bands. The blocked-impurity-band (BIB) detector-based mid-IR camera (C´amara Infrarroja Doble (CID); Salas et al. 2003) of the Mexican Observatorio Astron´omico Nacional at San Pedro M´artir (SPM) attached to the 2.1 m telescope was used on 2000 September 13 and 2003 October 2 to obtain mid-IR images of GM 24 through narrowband filters centered at 8.9, 9.9, 12.7, and 18.7 μm (Salas et al. 2006) with total on-source integration times of 1500 s, 2300 s, 840 s, and 3000 s, respectively. Sky subtraction was achieved through the standard chop-nod technique with a throw of 25 in the north–south direction. The

Table 1 Mid-IR Fluxes Derived with CID Central Wavelength (μm) 8.9 9.9 12.7 18.9

Irs 3

Irs 27

Flux (Jy)

Aperture ( )

Flux (Jy)

Aperture ( )

60.9 55.4 199.1 1760

12 12 12 12

5.5 3.8 29.6 90

7 7 7 7

final reduced images, reproduced in Figure 2, have a scale of 0. 55 pixel−1 with a fully covered total area of 55 ×55 . Flux calibration was done following Salas et al. (2006). Detection limits were (3σ ) around 1.0 Jy at 8.9 and 9.9 μm, 2.0 Jy at 12.7 μm, and 50 Jy at 18.7 μm. The emission longward of 8 μm from GM 24-Irs 3 was found to be extended and the total measured fluxes are given in Table 1, together with the aperture size. This table also gives the mid-IR photometry for the unresolved source Irs 27. Additionally, two sources were detected (3σ level) at 8.9 and 12.5 μm, with the following flux densities measured with a 7 aperture: 1.0 and 2.5 Jy at 8.9 and 12.7 μm, respectively, for Irs 17, while the peak emission of the diffuse “northern bar” Irs 39 (see Section 5.4) yielded 4.3 Jy at 8.9 μm and 5.3 Jy at 12.7 μm.

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corresponds to a total integration time of 0.5 s and, as a result of such short integration time, it has a superb spatial resolution (FWHM ∼ 0. 3). A composite red–green–blue image made of Brα, Brγ , and Paβ frames, respectively, is shown in Figure 4, and is compared with Hα and VLA radio-continuum emission contours. The radically different morphologies of the detected emission in each of these lines and free–free radio-continuum will be discussed in Section 4. No bright H2 line emission at 2.12 μm was detected in the field. For detailed comparisons of the central region, we also made use of old JHK deep images of a small area (20 × 20 ) around Irs 3 taken with the 4.0 m telescope at Cerro Tololo Inter-American Observatory (CTIO) on 1988 October and presented by Tapia et al. (1991). On all IR images, we adopted the astrometric reference system based on the 2MASS PSC of the region. We determined the image plate solution on all reduced images using a grid of up to 16 common stars located roughly symmetrically around GM 24. The measured absolute coordinates of all sources within 1 of the embedded cluster on all frames agreed to less than 0. 2. Uncertainties in the relative positions in each frame are lower than 0. 1. 2.3. Archive Radio Observations

Figure 2. Mosaic of the four images at 8.9, 9.9, 12.7, and 18.7 μm taken with CID at San Pedro M´artir. The common center of the field is at (J2000) α = 17h 17m 02.s 3, δ = −36◦ 21 04 and the field of view of each panel is 35 × 40 . The brightest, extended source is GM 24-Irs 3, the fainter source to the southeast is Irs 27, and the very faint one to the northeast is Irs 17. North is up; east is to the left.

2.2. Archive Infrared Observations Flux-calibrated images of the GM 24 region from the GLIMPSE Survey taken at 3.6, 4.5, 5.8, and 8 μm with IRAC onboard the Spitzer Space Telescope were retrieved from the image archive. The observations were performed in 2005 September. Unfortunately, the 8 μm image could not be used because it was heavily saturated. From all other images, we extracted stellar photometry with a circular aperture of radius 4 pixels with sky-subtraction defined by an annulus of 5 and 8 radii, except in several cases near the center of the cluster where the geometry of the source and/or background conditions required a different setup. In such cases, this was individually set. Appropriate aperture corrections and zero-magnitude fluxes were adopted according to the Data Handbook 3.0. Considering the variable background conditions in a large fraction of the cluster, we estimate the photometric uncertainties to be about 0.1 mag. The limiting magnitudes were 11.2, 11.0, and 7.0 in IRAC channels 1, 2, and 3, respectively. Figure 3 shows the color-combined 3.6, 4.5, and 5.8 μm image. From the ESO Science Archive, we retrieved uncalibrated narrowband images taken with the New Technology Telescope (NTT) and SOFI (1999 July) through filters centered in the Paβ line and in the neighboring continuum at 1.28 μm, as well as in H2 at 2.12 μm, Brγ at 2.17 μm and in the continuum at 2.09 μm with total integration times of 6 s for the filters centered on the hydrogen lines and 8 s for the rest. The image scale was 0. 29 pixel−1 . We also analyzed a set of four narrowband images taken through the filter centered in Brα at 4.07 μm with the VLT-U1 (ANTU) and ISAAC (2002 July) under good seeing conditions. These images were “snapshots” taken with very short integration times. The final combined VLT image

To obtain a high angular resolution radio image of the region, we calibrated VLA unpublished archive observations taken at 3.6 cm in the A configuration on 2003 July 5. The VLA is a facility of the NRAO.6 These observations were taken under the project AB1094, using 1331+305 as amplitude calibrator (with an adopted flux density of 5.21 Jy), and a source model provided by NRAO was used for its calibration. The phase calibrator was 1744−312, for which a bootstrapped flux density of 0.773 ± 0.001 Jy was obtained. These data were obtained in the standard VLA continuum mode and were edited and calibrated by us using the software package Astronomical Image Processing System (AIPS) of NRAO. We also recalibrated the 2 cm VLA data obtained in 1986 October 4 in the B–C hybrid configuration and discussed in detail in Roth et al. (1988). In Figure 4, the 3.6 cm contours are drawn in cyan, while the outer 2 cm contours are magenta. 3. THE EMBEDDED CLUSTER As part of a large-scale near-IR Survey using the 2MASS database, Dutra et al. (2003) located the embedded cluster GM 24 (named DBSB 122 in their catalog) centered at α = 17h 17m 02s , δ = −36◦ 20 58 , and with a radius of 33 . Unaware of this work, we performed independent Ks -band 2MASS source counts in a circular area of radius 0.◦ 3 surrounding GM 24 to obtain the Ks -band distribution of nearly 15,000 sources measured in this photometric band. This should represent the field population in the direction l = 350.◦ 50 and b = 0.◦ 95, nearly 10◦ away from the Galactic center. The existence of a cluster of bright sources (Ks < 12.5) at a position practically coincident with IRAS 17136−3617, the nucleus of GM 24, was confirmed to be evident from these 2MASS source counts, but for sources close to the sensitivity limit of the survey (Ks ∼ 14.5), this effect gets blurred by strong source confusion effects caused by the rather large effective aperture of the 2MASS Survey. Clearly, the 2MASS database provides important allsky statistical information on clustering of near-IR sources (see 6

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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Figure 3. Spitzer/IRAC color-composite image of GM 24 from the GLIMPSE Survey. IRAC channel 1 (3.6 μm) image is in blue, channel 2 (4.5 μm) in green, and channel 3 (5.8 μm) is in red. The center is at (J2000): α = 17h 17m 01.s 9, δ = −36◦ 21 03 , and the field of view is 91 × 65 . The cross marks the position of the H2 O maser (G´omez et al. 1993). North is up; east is to the left.

Figure 4. Color-composite image of GM 24 in the hydrogen lines Brα (4.05 μm, red), Brγ (2.17 μm, green), and Paβ (1.28 μm, blue). Superimposed over this image are the contours of the VLA radio-continuum emission at 3.6 cm (cyan) and 2.0 cm (magenta). For the latter (from Roth et al. 1988), only the lower contours are drawn. The white contours are from an Hα CTIO CCD image from Tapia et al. (1991). Note the correspondence between the 3.6 cm and the Brα peaks. The yellow square marks the position of the only H2 O maser detected in the region (G´omez et al. 1993). The center of the large image is at (J2000) α = 17h 17m 02.s 3, δ = −36◦ 21 00 and the field of view is 53 × 40 . The inset is a low-brightness representation of the 8 × 8 area centered in Irs 3.

Dutra et al. 2003 and references therein) but is unreliable for detailed studies of regions of high projected source density, such as the field studied here. Very red (H − Ks > 0.8) source counts were performed on our new deep PANIC images in order to determine the center

and size of the region containing the young embedded cluster. Assuming a circular projected geometry of the embedded cluster, we found that the coordinates of its center are (J2000) α = 17h 17m 01.s 2, δ = −36◦ 20 59 and the radius at which the red-source projected density became comparable to that of the

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Figure 5. Upper panel: Ks magnitude distributions of the sources measured in this band with the Clay/Magellan Telescope in the 110 × 110 field centered in GM 24. The continuous line is for sources inside the 40 circle delimiting the embedded cluster, and the dashed line is for sources outside this circle. The dotted is the distribution of field stars within 3◦ of GM 24 (see Figure 6). All distributions have been normalized to the area of the 40 circle. The up and down arrows indicate the sensitivity limits of the Clay and 2MASS Surveys, respectively. Lower panel: the distribution of sources inside the 40 circle minus that outside. This should indicate the 2.2 μm distribution of members of the embedded cluster.

surrounding field was found to be 40 , or 0.39 pc. This circle of area 1.4 arcmin2 should define the boundaries of the embedded cluster (see Figure 1). Naturally, we divided our PANIC source population between the one located within or inside the cluster boundaries and that outside. We determined the apparent Ks -band luminosity distributions of the inside and outside sections of our field normalized to an area of 1.4 arcmin2 and the results are shown in the upper panel of Figure 5. Also drawn in this figure is the normalized luminosity distribution of the Ks -band sources cataloged in the 2MASS Survey in a field of radius 0.◦ 3 around GM 24. This distribution is similar to that of the PANIC outside region for Ks < 13.2 but, while the 2MASS source count distribution keeps increasing until it reaches the survey’s sensitivity limit (Ks = 14), it is clear that the source densities are much smaller than the corresponding ones in the PANIC outside region. Doubtless, this is caused by source confusion due to the 2MASS poor spatial resolution (∼2 ). In any case, the immense majority of the stellar population detected by 2MASS outside the cluster limits have, as expected, colors that indicate late-type field stars. This is evident in the JHKs two-color diagram (Figure 6) of the nearly 7500 2MASS sources in a field of radius 18 that excludes the central 40 around GM 24. In this sample, 9% have colors of lightly reddened (AV < 5) foreground stars and almost 90% have colors of moderately to highly reddened background late-type stars. About 1% of the sources in this sample appear to display significant near-IR (Ks band) excesses, and these presumably are from evolved stars with circumstellar envelopes. The distribution of the H − Ks color of these sources

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Figure 6. J − H vs. H −Ks diagram of around 15,000 2MASS sources in a field centered in GM 24 of radius 3◦ and excluding the inner 38 . The continuous line on the lower left represents the unreddened main sequence and the dashed line, the reddening vector. As expected for field stars, the immense majority of these stars have colors of reddened photospheres.

is roughly Gaussian with a peak at around H − Ks = 0.83 and width  0.8. Consider now the PANIC outside population: the observed color distribution peaks at the same value of H − Ks as that of the 2MASS sample, but has an excess of around 22% of sources with H − Ks > 1.2, probably real members of GM 24 that lie fortuitously out of bounds. Consistent with this, the fraction of sources with significant near-IR excess in this sample is only 15%. Taking into account the previous statistical considerations, it is reasonable to assume that the PANIC outside population is a good representation of the field surrounding GM 24. On the other hand, the inside region population includes the majority (72%) of sources with significant near-IR excess and this fraction increases to nearly 90% for those with large excesses. Subtracting the inside–outside Ks -band source distributions on the PANIC frames, displayed in the lower panel of Figure 5, should, therefore, give us a good representation of the embedded cluster population. It is noteworthy that this distribution reaches zero at about 1 mag brighter than the limiting magnitude of the observations, which was determined to be around Ks = 16.5 for the whole region, though the presence of considerable diffuse emission of ionized gas (discussed in Section 3) and scattered radiation in the central parts of the cluster may cause the detection limit in this region to be significantly brighter. The source count resulted in around 90 members of the cluster detected in Ks and their color distribution shows two distinct

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Vol. 137 Table 2 Likely Members of GM 24 with No Spitzer/IRAC Photometry

ID

Figure 7. J − H vs. H − Ks diagram for the sources detected in these three photometric bands in the field of the Clay/Magellan Telescope centered in GM 24. The filled circles are sources within the circle of radius 40 delimiting the cluster and the open circles, those outside (see Section 3). The solid line marks the locus of main-sequence stars (Bessel & Brett 1988). The dotted line marks the region occupied by unreddened T Tauri stars with disks (Meyer et al. 1997; L´opez-Chico & Salas 2007). The parallel dashed lines represent the standard reddening vector (Riecke & Lebofsky 1985) for late-type, early-type, and active T Tauri stars for AV = 20. The cross on the lower right represents the maximum photometric errors in both colors.

peaks at H − Ks = 1.25 and H − Ks = 1.85 with a wide high-valued tail which extends up to H − Ks = 3.3 as well as a single source7 with H − Ks = 4.7. Given the fact that the sample is limited by our H-band sensitivity, a number of faint very red and unresolved cluster members, are unaccounted for in this study. Therefore, we estimate that the number of YSOs GM 24 is considerably higher than 100. Table 2 lists the coordinates and near-IR photometry of the sources that have high probability of being cluster members, either because of the presence of considerable excess emission at λ > 2 μm or because the location on the upper color–magnitude, Ks versus H − Ks , diagram indicates so, as will be explained later. Table 3 includes information on 32 additional sources which were bright enough at λ > 3 μm to be measured photometrically on our Spitzer/IRAC images. Most of these are members of GM 24, though there are five bona fide field stars. To avoid confusion, the adopted nomenclature follows and extends that of Tapia et al. (1985, 1991). Figure 7 shows the J − H versus H − Ks diagram of the 119 near-IR sources in the PANIC field, of which 67 lie inside 7 Surprisingly, this source has JHK , [3.6] and [4.5] colors which clearly s indicate a highly extincted (AV = 60) late-type supergiant located behind the molecular cloud and unrelated to GM 24.

348 23 389 18 94 472 339 53 24 489 266 491 251 12 8 14 22 16 11N 29 21 441 448 458 11S 496 15 195 199 533 614 41 593 396 514 555 201 553 200 482 250 459 126 240 60 304 370 129 434 427 483 509 36

α (J2000) δ 17:17:06.70 17:17:02.20 17:17:02.34 17:17:03.25 17:16:59.86 17:17:03.13 17:16:59.69 17:17:02.70 17:17:02.53 17:17:05.66 17:17:02.92 17:17:03.03 17:16:59.97 17:17:01.78 17:17:02.40 17:17:00.77 17:17:02.09 17:17:00.96 17:17:01.99 17:17:00.35 17:17:01.07 17:17:01.88 17:17:02.04 17:17:01.79 17:17:01.94 17:17:01.00 17:17:02.69 17:17:00.00 17:17:02.10 17:17:05.11 17:17:00.27 17:17:02.24 17:17:06.46 17:17:05.63 17:17:06.40 17:17:02.67 17:17:04.71 17:16:58.81 17:17:06.55 17:17:05.41 17:17:03.58 17:17:00.79 17:17:02.93 17:17:03.30 17:17:03.01 17:17:01.22 17:17:03.88 17:16:59.58 17:17:04.20 17:17:02.74 17:16:59.36 17:17:05.72 17:17:02.78

−36:21:06.1 −36:21:07.0 −36:20:59.8 −36:20:45.4 −36:21:48.8 −36:20:48.5 −36:21:06.6 −36:21:55.5 −36:21:05.6 −36:20:45.1 −36:21:16.6 −36:20:44.7 −36:21:18.6 −36:20:56.9 −36:21:02.7 −36:20:58.5 −36:21:09.4 −36:20:56.7 −36:21:04.0 −36:21:01.9 −36:21:03.3 −36:20:53.5 −36:20:51.9 −36:20:50.8 −36:21:05.1 −36:20:43.3 −36:20:58.5 −36:21:28.2 −36:21:27.7 −36:20:35.2 −36:20:16.1 −36:21:57.9 −36:20:23.9 −36:20:58.8 −36:20:39.7 −36:20:31.7 −36:21:27.5 −36:20:31.7 −36:21:27.7 −36:20:46.5 −36:21:18.8 −36:20:50.6 −36:21:42.1 −36:21:21.2 −36:21:54.6 −36:21:11.2 −36:21:03.1 −36:21:41.9 −36:20:54.2 −36:20:55.3 −36:20:46.2 −36:20:40.8 −36:21:58.6

Ks

J−H

H − Ks

Notes

11.40 11.54 11.71 12.09 12.09 12.21 12.34 12.37 12.45 12.56 12.60 12.90 12.95 10.66 11.10 11.64 11.79 11.88 12.16 12.52 12.61 12.76 12.77 12.81 12.82 12.92 13.16 13.54 13.64 12.08 12.17 12.22 12.42 12.83 13.04 13.02 13.66 13.79 13.27 13.33 13.36 13.38 13.41 13.41 13.49 13.65 13.93 13.97 14.00 14.15 14.19 14.29 14.40

3.16 ... 1.96 ... 2.66 ... ... ... ... ... ... ... ... 1.26 1.52 1.24 1.05 ... 1.61 2.14 1.96 1.57 1.30 1.40 2.21 1.53 1.56 1.11 1.39 1.68 1.25 1.21 1.06 1.32 1.53 1.33 1.04 1.36 ... ... ... 2.04 ... ... ... ... ... ... ... ... ... ... ...

1.81 1.96 1.30 2.32 1.45 2.63 1.64 2.25 1.60 1.55 2.37 3.10 2.21 0.89 0.75 0.71 1.05 0.81 0.81 1.01 0.92 0.87 0.77 0.72 1.03 0.84 1.10 0.81 0.81 0.78 0.66 0.61 0.71 1.02 0.95 0.86 0.81 1.04 2.47 1.65 1.82 1.36 2.00 1.83 2.17 2.00 2.50 2.21 2.42 2.16 2.13 2.18 2.04

rm rm,† rm rm rm brm rm rm rm rm rm rm rm ms ms ms ms or rm,† ms ms f or ms? f or ms? ms ms ms f or ms? ms ms,† ms ms f or ms? ms ms ms f or ms ms ms ms ms pm pm pm pm pm pm pm pm pm pm pm pm pm pm pm

Notes. brm: bright red member, excess emission at λ > 2 μm; rm: red member; pm: probl. member; ms: on cluster main sequence; f: field star; † : discussed in Section 4).

(filled circles) and 52 outside (open circles) of the cluster boundaries. The fraction of sources with significant Ks -band excess defined as those located 0.15 mag or more to the right of the reddening vector for any photosphere, is 26%. In contrast, the corresponding fraction seen in the neighboring field is, as expected, much lower, 7%. Note that because this sample

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Table 3 Sources Measured with Spitzer/IRAC ID 1 2 3 4 5 58 31 64 25 69 35 55 17 27 62 66 65 9 19 42 20 56 54 51 37 50 36 60 47 57 34 39

α (J2000) δ 17:17:00.52 17:17:00.65 17:17:02.31 17:17:01.60 17:17:05.01 17:16:58.10 17:16:59.60 17:16:59.04 17:17:01.73 17:17:01.76 17:17:05.48 17:16:58.99 17:17:02.94 17:17:02.49 17:17:00.06 17:16:59.77 17:17:04.43 17:17:01.70 17:17:03.50 17:17:05.89 17:17:01.66 17:17:02.46 17:17:01.94 17:17:02.60 17:16:59.56 17:17:02.39 17:16:57.88 17:17:04.90 17:17:04.34 17:17:04.49 17:17:04.1 17:17:02.7

−36:20:44.0 −36:21:08.6 −36:21:08.8 −36:20:57.8 −36:21:04.3 −36:21:20.6 −36:21:08.7 −36:21:11.4 −36:20:48.5 −36:21:47.5 −36:21:00.6 −36:20:17.4 −36:20:49.0 −36:21:14.3 −36:21:36.3 −36:21:51.5 −36:21:23.2 −36:21:08.9 −36:20:44.8 −36:20:54.0 −36:21:11.0 −36:20:16.4 −36:20:23.8 −36:20:36.1 −36:20:59.1 −36:20:38.7 −36:21:00.1 −36:21:44.8 −36:20:43.5 −36:20:39.9 −36:21:03 −36:20:56

Ks

J−H

H − Ks

[3.6]

[4.5]

[5.8]

Notes

7.92 8.31 8.78 9.22 9.50 10.41 10.61 10.65 10.69 10.78 10.91 11.24 11.32 11.39 11.41 11.60 11.63 11.68 12.13 12.20 12.21 12.30 12.50 12.59 12.63 12.94 12.98 13.15 13.86 13.92 ... ...

2.61 3.18 2.91 1.76 4.49 3.65 2.19 2.53 2.40 1.08 1.26 1.57 ... ... 2.06 1.25 1.59 ... 3.04 2.11 2.14 2.38 2.19 2.49 ... ... ... ... ... ... ... ...

1.83 2.08 1.95 1.26 2.26 1.78 1.33 1.82 1.74 0.94 0.90 0.93 3.34 3.28 1.53 0.76 0.86 2.43 1.72 1.36 1.38 1.60 1.56 1.40 4.64 1.91 3.30 2.54 2.79 2.55 ... ...

6.33 6.48 6.34 7.95 8.07 9.36 8.53 8.94 8.49 9.26 10.19 10.94 7.24 7.80 9.99 10.34 10.56 8.14 9.39 10.64 11.40 10.65 11.11 11.35 9.93 10.61 10.86 10.29 11.54 12.40 9.04 7.46

5.88 6.13 5.13 7.71 8.03 9.25 8.28 8.02 8.06 8.84 10.28 10.85 5.90 5.87 9.42 10.15 10.96 7.74 9.42 10.32 10.30 10.35 10.96 10.88 9.72 9.52 11.16 10.31 10.92 12.30 8.67 6.98

4.57 5.11 3.51 6.63 7.76 ... 5.08 6.17 ... ... ... ... 4.58 3.59 ... ... ... 6.60 ... ... ... ... ... ... ... ... ... ... ... ... 5.75 4.48

brm,# brm,# ABC,brm,# brm,# f f n5 brm,# rm,ex ms ms ms brm,bp,# brm,# rm,ex ms ms brm,# rm,ex pm brm,#,n4 rm rm rm f rm,ex f rm,ex rm f n3 n3,‡

Notes. n5: nebulous at λ > 5 μm; n4: nebulous at λ > 4 μm; n3: nebulous at λ > 3 μm and no JHKs ; ‡: peak emission on nebulous “bar;” bp: bipolar, magnitudes of E lobe included in table, W lobe has Ks = 13.01 and H −Ks = 2.73; brm: bright red member, excess emission at λ > 2 μm; rm: red member; pm: probable member; f: reddened background star; #: SED plotted and discussed in Section 4; ABC: Irs 3 is a triple system, all components are included in IRAC beam. Individual magnitudes are: A: Ks = 9.54, H − Ks = 1.76, J − H = 2.35; B: Ks = 10.00, H − Ks = 2.21, J − H = 3.2; C: Ks = 10.62, H − Ks = 2.17, J − H < 15.7. Their coordinates (J2000) are: A: 17:17:02.29, −36:21:08.2; B: 17:17:02.33, −36:21:09.2; C: 17:17:02.30, −36:21:09.6.

is limited by the J magnitude, there is a strong bias against the reddest GM 24 sources and is, therefore, incomplete. The Ks versus H − Ks diagram, shown in Figure 8, includes a more complete sample of the near-IR sources in GM 24 (167 sources inside r = 40 ) and in its immediate surroundings (153 sources). The location occupied by the unreddened zero-age main-sequence (ZAMS) stars at a distance of 2 kpc is drawn (broken line) in this diagram. A relevant feature of this plot is the location of a number of sources that lie on or very close to a reddened (AV  13) main sequence or, for stars of masses lower than 7 M , to an isochrone of very young age, i.e., 105 years (Palla & Stahler 1999; continuous lines in Figure 8). Statistically, one would expect that most of these bright sources (with Ks < 13.0, referred to “ms”) are early- to late-B stars that belong to the cluster and are less than one million years old. Two important properties of this group are explained below. First, these young stars are physically located at the nearest edge or the progenital cloud, as these are the less extincted of the cluster. Consistent with this, we find that 10 out of the 16 “ms” sources brighter than 12.5 at 2.2 μm are located in the close periphery of the cluster, i.e., just outside the circle which include all very red members

of GM 24. It must be noted that only two sources brighter than Ks = 13.0, clearly foreground stars, lie to the left of the cluster’s reddened main sequence. The second common feature is that these sources do not display appreciable IR excesses, as their positions on the two-color diagrams (Figures 7 and 9) confirm. The probable older ages of these stars compared to the more massive ones located closer to the center of the cluster, suggest, as yet unexplained, a star formation sequence that started in the outskirts and propagated into the center of the cloud. The rest of the population of probable cluster members can be classified by their position on the color–magnitude diagram and the presence or absence of IR excesses as determined from the various two-color diagrams. First, consider the brightest and more highly reddened embedded members of GM 24 that are located in the Ks versus H − Ks diagram (Figure 8) above the reddening vector for B2-type ZAMS stars. These conform the group of the brightest and reddest (brm) sources of the sample. All of them are bright enough at λ > 3 μm to be measurable on the Spitzer/IRAC 3.6 and 4.5 μm frames and most (10), also at 5.8 μm. The coordinates, JHKs , and [3.6], [4.5], and [5.8] magnitudes are listed in Table 3, together with fainter sources that had measurable fluxes (above 5σ ) in IRAC channels 1

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O6

B1

B3

A0

F0 G0 K0

Figure 9. H −Ks vs. Ks −[3.6] diagram for the sources detected in H −Ks and IRAC 3.6 μm band. Symbols are as in Figure 7. The solid line marks the locus of main-sequence stars and the dashed lines represent the standard reddening vector (Tapia 1981) of length corresponding to AV = 50. The cross on the lower right represents the maximum photometric errors in both colors. Figure 8. Ks vs. H − Ks diagram of all sources detected in these bands toward GM 24. Symbols are as in Figure 7. The long-dashed line marks the locus of the unreddened main sequence at d = 2.0 kpc. The straight solid line represents the main sequence at the same distance but extincted by AV = 13 and the curved solid line is its associated 105 year pre-main-sequence isochrone as computed by Palla & Stahler (1999). The short-dashed lines represent the standard reddening vector for AV = 40 for B1 and A0 main-sequence stars, while the arrow represents the average slope of near-IR excesses caused by low-mass YSO disks as determined by L´opez-Chico & Salas (2007) based on models by D’Alessio et al. (2005).

and 2. Figures 9 and 10 show the corresponding H − Ks versus Ks −[3.6] and [3.6]−[4.5] versus [4.5]−[5.8] diagrams. These plots are more efficient in discriminating the population of the younger stellar objects, as the IR emission from disks or other circumstellar structures are better detected at λ > 3 μm than at shorter wavelengths (e.g., Haisch et al. 2001). The spectral energy distributions (SEDs) of the most peculiar objects are shown in Figures 11 and 12 and will be discussed in Section 4. Note that five of the sources in this sample have 1–5.8 μm SEDs (Figure 12) that are indicative of highly reddened background field stars. It is interesting that all but one of these background sources lie outside the cluster limits. Also, that the two sources with the reddest H − Ks color indices turned out to be in the background, i.e., located behind the molecular cloud. Note also that there is a fair number of fainter sources that have near-IR colors indicative of early B-type photospheres reddened by AV > 12, thus consistent with being part of the cluster and these are also considered likely members (rm) of GM 24, as are the objects showing significant Ks -band excesses (ex). All these are included in Tables 2 and 3. Given the sensitivity of the present near-IR Survey and the mean cluster extinction, we estimate that most of its embedded members with M  2.5 M and a small fraction of the less massive ones have been detected and classified in this work.

Figure 10. [3.6]−[4.5] vs. [4.5]−[5.8] diagram of GM 24 from the Spitzer/IRAC images. Symbols are as in Figure 7, except the small crosses, which are the colors of nebulous mid-IR emission (see Sections 3 and 5.4). The large cross on the lower right represents the typical photometric errors in both colors. The dashed line square marks the area occupied by Type II YSO (Hartmann et al. 2005).

About 60% of the sources located inside the cluster boundaries, relative to the total expected cluster population, show nearand/or mid-IR excesses, suggesting extreme youth (e.g., Haisch et al. 2001). The small fraction of these that show peculiar IR properties will be discussed individually in Section 5. We have presented in Tables 2 and 3 individual photometric properties of more than 80 members and probable members of

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Figure 11. SEDs of Irs 3 from 1.2 to 1200 μm (filled points, asterisks, and cross), of Irs 27 from 1.6 to 18.9 μm (open symbols) and of Irs 17 from 1.6 to 12.7 μm (starred symbols). Squares and circles are from ground-based observations, triangles are from Spitzer/IRAC images, pentagons from the IRAS PSC, asterisks are from the MSX Catalog, and the large star is a 1.2 mm measurement by Fa´undez et al. (2004). The lines are the best fits to the data points of star+disk+envelope models by Robitaille et al. (2007) to Irs 3 (continuous line), 27 (short-dashed line), and Irs 17 (dotted line) with the parameters listed in Table 4. For clarity, the fluxes of Irs 17 have been divided by 5. The crosses are radio measurements of the compact region associated with Irs 3 by Tapia et al. (1985), Roth et al. (1988), G´omez et al. (1993), and Walsh et al. (1998). The long-dashed line represents thermal Bremsstrahlung.

the GM 24 cluster. It is expected that a significant number of fainter sources in our sample are also associated with GM 24 but cannot be distinguished from field stars with the present data. It is, thus, reasonable to conclude that GM 24 contains more than 100 stars with Ks magnitudes brighter than our

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sensitivity limit. Adding up the individual masses assigned to embedded sources (Section 5) and extrapolating to lower masses (luminosities), assuming for GM 24 an initial mass function (IMF) similar to that observed in the Orion Trapezium Cluster (Muench et al. 2002), we estimate a value for the lower limit of the total stellar mass in GM 24 of ∼250 M . Combined with the value of ∼1500 M determined for the molecular gas mass of the central clump by G´omez et al. (1990) from their C18 O measurements, we conclude that the star formation efficiency [SFE = Mstars /(Mgas + Mstars )] in this region has been 15%. Finally, we note that more than 90% of the bright red members of the cluster are located within the circular (r = 40 ) boundary, but this fraction is much lower (around 60%) for the fainter stars on the cluster main sequence (ms) and also probable fainter members (pm). This confirms that the younger and most massive stars in an embedded cluster (of ages less than 106 years) occupy a smaller volume at its center, compared to the rest of the more evolved and less massive members. 4. THE IONIZED GAS 4.1. Radio-Continuum Emission The morphology of the extended 3.6 cm emission from GM 24 is that of a cometary H ii region, with a sharp edge to the south and a smoother tapering to the north. This morphology has been discussed by Roth et al. (1988). What is remarkable about the new high angular resolution 3.6 cm image (see Figure 13) is that it reveals the presence of two bright, compact radio sources embedded in the small region that also contains the IR sources A, B, and C, of Irs 3. These two radio components have flux densities of order 100 mJy (about 1/30 of the radiocontinuum emission from the whole region) and deconvolved dimensions of order 0. 6–0. 8 (∼ 2 × 1016 cm), placing them in

Figure 12. SEDs of some of the reddest and brightest sources measured on the Clay/Magellan and Spitzer/IRAC images. The SEDs of most sources in this diagram indicate they are young members of GM 24. Only Irs 5, Irs 18, Irs 36, and Irs 37 have colors of highly reddened late-type stars, most probably unrelated to the cluster. For clarity, the plots have been shifted vertically a logarithmic value indicated by the corresponding label.

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Figure 13. Contour plot of the 3.6 cm continuum emission obtained with the VLA in the A configuration with the location and identification of compact near-IR sources (Tables 2 and 3) in the immediate vicinity indicated. The three components of Irs 3, ABC, are labeled in the external square. The broken-line circles mark the size of the emission of Irs 3 at 5.8 μm (small circle) and at 18.7 μm (large circle). The synthesized beam of the 3.6 cm observations has a half-maximum contour of 0. 60 × 0. 30 at a position angle of −1◦ . The contour interval is 8% of the peak emission.

the classification of hypercompact H ii regions (see Kurtz 2002). With their flux densities and angular dimensions, these compact components have brightness temperatures of order 4000 K, suggesting that at 3.6 cm they are in the transition from optically thick to optically thin. At the distance of GM 24 (2.0 kpc), each component would require a B0 ZAMS star to maintain its photoionization. In the case of the eastern component, that within the position uncertainties could be coincident with Irs 3A, we believe that the best explanation is that it is a hypercompact H ii region ionized by Irs 3A since, as will be discussed in Section 5.1, we conclude that this object is a star with the required spectral type. We have, however, no good explanation for the western compact component. No star seems to be associated with it. One possibility is that we are dealing with an extremely obscured object. Another is that this component is a neutral gaseous condensation, being ionized externally by the other stars in the region. These radio “proplyd-like” objects have been observed in a few regions (M¨ucke et al. 2002; Moffat et al. 2002; Rodr´ıguez et al. 2003). This hypothesis can be tested by radio observations of even higher angular resolution, that could reveal the crescent-shaped structure with its cusp pointing toward the exciting star that characterizes these sources. 4.2. Visible and Infrared Hydrogen Emission Lines The strikingly different morphology observed in the emission of each of the hydrogen lines is attributed to strongly inhomogeneous dust extinction within the nebula. In fact, there is a clear extinction gradient, which is highest to the south of Irs 3, where the extended radio-continuum and Brα emission distributions coincide (and AV is around 55; Tapia et al. 1991) and no Brγ (2.2 μm) is detected. The extinction decreases to the north–northwest, allowing sequentially Brα, Brγ and further away, Paβ (1.28 μm) and Hα (0.68 μm) to get brighter (Figure 4). The Brγ line emission strength has been measured in the spectrum of Irs 3, taken with a wide slit, by Tapia et al.

(1991), and later by Bik et al. (2006). It is important to remark that no molecular hydrogen line emission at 2.12 μm was detected from any location of the cluster/nebula. 5. INDIVIDUAL YOUNG STELLAR OBJECTS AND PECULIAR SOURCES 5.1. The Complex Core of GM 24: Irs 3 ABC, Irs 27, and Surroundings It has become clear that the rule among very young massive clusters is that the brightest and more massive members are located close to the center of the conglomerate, forming gravitationally attached multiple systems, with similar separations between the three, four or more components. The prototype of this kind of dynamically unstable structures is the Orion Trapezium and the youngest known examples, seen only at millimeter wavelengths, are in NGC 6334 I and NGC 6334 I(N) (Hunter et al. 2006). The core of GM 24, discovered and named Irs 3 by Tapia et al. (1985), seems to be another example of a massive multiple system in the core of a large molecular cloud which recently gave origin to an embedded cluster. As displayed in Figure 4 and schematically presented in Figure 13, it consists of a triple system, with the following separations and position angles between the near-IR components A, B, and C (see Table 3 for precise individual coordinates and JHKs magnitudes): sep (AB) = 1. 11, PA (AB) = 154◦ ; sep (BC) = 0. 62, PA (BC) = 234◦ . Most of the luminosity of this young complex (IRAS 17136−3617) is radiated in the mid- and far-IR and its 1.2 μm to 1.2 mm SED is shown in Figure 11, which also displays the radio-continuum thermal Bremsstrahlung fluxes of the compact H ii region ionized by UV photons from this stellar system. Irs 3 is unresolved on any of the IRAC (3.6–5.8 μm) or CID (8.9–18.9 μm) diffraction-limited images (resolutions between 1. 4 and 2. 0), and the centroids of the peak emission at all these wavelengths lie close to the midpoint between components A, B, and C. In contrast with the compact (unresolved) appearance

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Table 4 Best-Fit Parameters from Robitaille et al.’s (2007) Models to Irs 3, 27, and 17 Parameters Stellar mass (M ) Stellar temperature (K) Envelope accretion rate (M yr−1 ) Envelope outer radius (AU) Disk mass (M ) Inclination (◦ ) AV Total luminosity (L )

Irs 3

Irs 27

Irs 17

33 41,350 4.3 × 10−3 105 0 32 53 1.5 × 105

17 4500 5.3 × 10−3 2.5 × 103 0 18 47 1.2 × 104

16 32,550 0 0 7.7 × 10−6 76 58 2.7 × 104

of Irs 3 at shorter wavelengths, at λ > 8 this object appears extended and symmetric (see Figure 2), with diameters that increase with wavelength, from 5 at 8.9 μm to 11 at 18.9 μm. The IRAS, MSX, and SEST beams are much bigger and include many other near-IR sources, but our CID images clearly indicate that Irs 3 ABC by far dominates the emission at λ > 8. The radio and IR morphology of the nuclear region of GM 24 is shown schematically in Figure 13. The JHKs photometry (Table 3) of the individual components of this young multiple system shows that each one displays considerable near-IR excess emission over a highly reddened photosphere. Their positions on the IR color–magnitude diagram suggest that the three are reddened massive stars with disks or envelopes, but suffering different amounts of extinction; the southern components B and C being more highly reddened by around 20%–30% (in magnitudes) than component A (assuming similar Ks -band excesses), a fact that is consistent with the observed steep extinction gradient determined from the ionized hydrogen emission lines (see Figure 4). Nevertheless, with the available observations, it is impossible to determine their individual stellar properties. Indeed, because in the thermal regime (λ > 4 μm) the dust emission is extended, well beyond the size of the multiple system, it is reasonable to assume that the putative circumstellar envelope/disk is common to the three components. The embedded multiple system is surrounded by fainter diffuse emission, dominated by Paβ (1.28 μm), Brγ (2.17 μm), and Brα (at 4.05 μm), as seen in narrowband images (the inset of Figure 4). The SED of Irs 3 from 1.2 to 1200 μm, displayed in Figure 11, is similar to that of Class I YSOs, with a 2.2–12.7 μm spectral index of α2.2–12.7 = 2.9, and a deep 10 μm silicate absorption feature that implies a value of AV  50 (Tapia et al. 1985), although the JHKs photometry suggests a much smaller extinction. To get a more elaborate insight of the probable nature of this mid- and far-IR source, we used the tool developed by Robitaille et al. (2007) to obtain the best fit of a model of an infalling envelope with accreting disk and a central young source to the 1.2–1200 μm SED of Irs 3. This model uses a two-dimensional radiation transfer code applied under several geometries as described in detail by Whitney et al. (2003). The best solution, drawn in Figure 11, was obtained with the physical parameters of the envelope and central engine (and no disk) listed in Table 4. The fit implies a total luminosity of 1.5 × 105 L and AV  53, consistent with previous determinations. It is important to point out that the flux densities of the multiple pointlike sources measured at λ < 5 μm were impossible to fit satisfactorily with any model simultaneously with the long-wavelength data. This is due to the multiplicity and complex structure of the Irs 3 region in the near-IR, as the model assumes a single central YSO.

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The nearby Irs 27 is a fainter IR source located less than 6 to the southeast of Irs 3 (Figure 13) and has, together with Irs 17 (discussed in Section 5.2), the highest IR spectral indices in the region. Its position on the Ks versus H − Ks and H − Ks versus Ks − [3.6] diagrams implies an early spectral type and an extinction value, AV , in excess of 30 mag and large near- and mid-IR excess. The value of its spectral index is α2.2–12.7 = 3.1 (see SED in Figure 11). The source appears slightly elongated in all near-IR images, particularly in the 4.05 μm frame, and its morphology (Figure 4) can be explained with the presence of a close binary separated 0. 5 and PA 135◦ , with the emission of the NW component dominating at λ > 3.5 μm. Alternatively, the shifted bluer emission on the SE side may be due to light scattered by dust in a dense circumstellar disk, as in the case of Irs 17 (see Section 5.2). As expected, IRAC and CID images cannot resolve this system. The SED of Irs 27, shown in Figure 11, was fitted with Robitaille et al.’s (2007) model with the resulting parameters listed in Table 4. This object is, thus, a less luminous (L = 1.2 × 104 L ) YSO, similar to Irs 3. Two other embedded sources lie within a radius of 3 of Irs 3. These are Irs 22 to the west and Irs 23 to the northwest (Figure 13). The former appears to be an early-B main-sequence star with AV = 15–20 and no evidence of significant IR excess, while the latter is brighter and is embedded deep into the cloud. Irs 8 and 11N appear to be on the cluster’s main sequence with no IR excess, Irs 24 is a reddened and less luminous member with no notable IR excess. Irs 9 is a bright member with a SED suggesting a Class I YSO (Figure 12) with an IR (1–8 μm) luminosity of around 10 L . Irs 20 is medium-luminosity object with considerable IR excess. In the near-IR, it displays a nebulous faint extension of around 2 (0.02 pc) to the east while it appears embedded in a mid-IR nebulosity dominated by emission in the 5.8 and 8 μm IRAC bands, and likely caused by polycyclic aromatic hydrocarbon (PAH) features. Close to the western edge of this IR nebula, lies Irs 304, a faint red compact near-IR source, probably a cluster member and some 5 further to the SW, the presence of an H2 O maser was reported by Sakellis et al. (1984) and G´omez et al. (1993). Additionally, from its JHKs colors, we can unambiguously determine that Irs 285 is a background star while the nature of Irs 11S is unclear. 5.2. The Bipolar Irs 17 and Surroundings Around 40 to the north and 10 to the east, we found a bright and extended near-IR source, Irs 17, conspicuous for having a well-defined butterfly shape, with two extended lobes (seen red in Figure 1 and green in Figure 4) that extend some 3 (7000 AU) to either side of a compact mid-IR source (red in Figure 4) in the E–W direction, as shown in Figure 14. The very similar morphology displayed at several near-IR wavelengths through broadband and narrowband continuum filters in the range 1.6–2.3 μm indicate that both nebulous lobes around Irs 17 are originated by scattered light from the central YSO. In fact, practically no emission is seen on the narrowband filters that isolate the hydrogen lines. The western lobe is much fainter, suggesting this to be more embedded than the eastern lobe. For consistency with the long-wavelength data, the H − Ks photometry given in Table 3 for this source includes the brightest eastern lobe. The compact central source, that is significantly redder than the lobes, has a steep SED (Figure 12) with α2.2–12.7 μm  1.8, with very high extinction, also evident by the presence of strong silicate absorption at 10 μm. This is most likely caused by the presence of a dense circumstellar disk

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5.4. The Nebulous Mid-IR Objects: Irs 31, 34, and the South Nebula

Figure 14. Contour plots of the narrowband 2.17 μm (black) and broad H-band (white) emission over a Ks -band image of the region containing the bipolar nebular source Irs 17. The crosses mark the positions of pointlike detections on the narrowband 4.05 μm VLT image. The centroids of the unresolved source Irs 17 on the three IRAC images lie precisely coincident with the central stellar object dominating the 4 μm emission. The eastern and western diffuse lobes at 1.6 and 2.2 μm are caused by reflection. The unresolved sources Irs 18, 19, 472, and 491 appear to be members of the cluster but not directly associated with Irs 17.

around the central star that is seen nearly edge-on. The whole complex source was unresolved by IRAC and CID, and the emission in thermal IR is dominated by the central source. The best fit to the SED of Irs 17 of Robitaille et al.’s (2007) model corresponds to a medium luminosity (L = 2.7 × 104 L ) YSO with a disk at a large inclination angle and large extinction (AV = 58), in accordance with the geometry of this source described before. Of the compact sources in the vicinity (Figure 14), Irs 472 appears embedded in the eastern lobe and seems to be a highly reddened (AV ∼ 30) bright (early B-type) red member of the cluster and its direct association with Irs 17 is improbable. The three red sources to the northeast, Irs 18, 19, and 491, are also likely members with no direct relation to Irs 17. 5.3. Irs 1, 2, 4, 19, 25, 50, and 64 The brightest near-IR sources in the GM 24 region are Irs 1, Irs 2 and, except for Irs 3 discussed in Section 5.1, Irs 4. The SEDs of Irs 1 and 2 are almost identical (Figure 12), showing considerable IR emission beyond 2 μm, and their flux densities start to decrease at 5.8 μm. In combination with their position in the two-color diagrams, the presence of circumstellar disks is inferred. The implied extinctions correspond to AV values between 20 and 25 mag. Assuming their total luminosities to be twice that measured in the 1.2–5.8 μm range, these would amount to 230 and 150 L , respectively. Note that, because of the above assumption, these have to be considered lower limits, as we have no information of their fluxes beyond 5.8 μm. Premain-sequence tracks by Palla & Stahler (1999), which run at approximately constant luminosity, imply masses between 3 and 4 M . Given the small number of points available to construct their SED, any attempt to fit a disk/envelope model would be unreliable. Following similar arguments, Irs 4 and Irs 64, also in their pre-main-sequence evolution with associated disks and would have luminosities somewhat larger than 40 L (2–3 M ). Irs 19, 25, and 50 have luminosities almost an order of magnitude smaller. It is safe to conclude that the data presented here for these objects indicate that they are Class I T Tauri stars.

One of the most conspicuous characteristic seen on the Spitzer/IRAC images (Figure 3) of GM 24 is the presence of several bright extended emission features, mostly dominated by emission through the 5.8 μm and 8 μm IRAC bands. In fact, the GLIMPSE Survey shows that this is an extremely common fact along the Galactic plane, particularly close to star-forming regions. The PAH bright emission bands at 6.2, 7.7, and 8.6 μm are known to dominate the observed radiation in these broad bands, though emission from these large molecules also affect considerably the flux in the shorter wavelength IRAC bands, by means of the 3.3 and 4.4 μm features. The case of Irs 31, where a compact near-IR source is embedded in the PAH-dominated mid-IR nebula seems to be similar to Irs 20, discussed in Section 5.1. The SED of Irs 31, drawn in the right panel of Figure 12, clearly displays the two components, one which dominates the near-IR emission and a second one with enhanced emission at 5.8 and 8 μm, with no further information available at longer wavelengths. Lacking more observational elements, the physical association of the stellar component of Irs 31 with the extended emission is uncertain. A similar ellipsoidal nebula dominated by radiation in the IRAC 5.8 μm and 8 μm bands is named Irs 34, near the eastern edge of the cluster. The size and colors of this source are similar to those of Irs 20 and Irs 31 but with no embedded compact near-IR source. A similar case is presented by the diffuse emission to the south of the field studied here. In fact, several large PAH-dominated structures are delineated to regions south and southeast of GM 24 with no apparent counterparts at other wavelengths. 5.5. The Northern “Bar,” Including Irs 39 and Irs 15 The northern faint extension of the radio H ii region discovered by Roth et al. (1988) is characterized by a relatively narrow “bar” of emission of length of around 15 (0.15 pc) in the east–west direction, perpendicular to the overall direction of expansion of the ionized gas. As pointed out by Tapia et al. (1991) and now clearly demonstrated in Figure 4, this elongated structure is coincident with Brγ emission, and this structure is also evident on the broadband JHKs images (Figure 1). A parallel, though not precisely coincident, bar of bright emission is seen on the IRAC images (Figure 3), particularly dominant in the 5.8 and 8 μm bands and naturally ascribed to PAH-bands emission. Although the lower spatial resolution of Spitzer precludes a good comparison with our ground-based near-IR images, it is clear that the PAH structure, relative to the hydrogen recombination lines, is displaced approximately 1 –2 (0.01–0.02 pc) to the north, away from the main ionization source, Irs 3. The similarities with the Orion bar, southeast of the Trapezium, lead us to confirm the original suggestion by Tapia et al. (1991) that the bar is the ionization front expanding northbounds into a lower density neutral medium, creating a photodissociation region. Unlike the Orion bar, we can see a distinct, unresolved emission peak (Irs 39) within the PAH bar, near its eastern edge. The IRAC colors of this peak are similar to the rest of the bar and has no near-IR counterpart. Nevertheless, approximately 1. 2 to the south of the IRAC peak, we find a compact, very red near-IR source, Irs 15. Its JHKs colors suggest a moderate 2.2 μm excess and a position in the color–magnitude diagram

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close to the 105 year-isochrone and a value of AV ∼ 16, typical of a number of pre-main-sequence medium-luminosity cluster members. There is no evidence, though, that Irs 15, Irs 39, and the rest of the ionization front are physically connected.

acknowledges the support of CONACyT, M´exico and DGAPA, UNAM. Facilities: Clay/Magellan Telescope (LCO), Spitzer Space Telescope, OAN-SPM (UNAMexico)

6. CONCLUSIONS

REFERENCES

The analyses of the new high-resolution IR and radio observations presented in this paper have produced the following conclusions.

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1. The embedded cluster GM 24 and its associated highdensity molecular cloud are at a distance of 2.0 kpc from the Sun. It has produced a young cluster of intermediate- to high-mass stars, seen deeply embedded, up to 50 mag in V, into the cloud. The radius of the cluster was found to be 40 (0.39 pc) and contains more than 100 members, some 50% of these showing IR excesses. We estimate a total stellar mass in excess of 250 M and an SFE 15%. 2. At the nucleus of the cluster, we find what appears to be a young multiple (proto)stellar system, Irs 3ABC, which has a total luminosity of 1.5 × 105 L . Its SED can be fitted by a model of a 33 M star with a dust envelope of some 105 AU in size. 3. Irs 3 ionizes a compact H ii region with a complex morphology, showing at 3.6 cm an extended (∼3 ) emission peak and two very compact (less than 0. 5) components very close to the brightest near-IR source in the system. 4. A number of embedded, peculiar, and less luminous YSOs were found within the cluster limits and their IR properties are described. 5. Faint and diffuse ionized gas emission, extending some 15 to the NW of Irs 3, was mapped in several hydrogen recombination lines: Hα, Paβ, Brγ , and Brα. Comparing these with the radio-continuum maps, a steep and patchy NW to SE extinction gradient is derived, with a maximum close to Irs 3. 6. Spitzer/IRAC images of the region show the presence of several extended (∼5 ) nebulae of emission dominated by PAH features, mainly in the 8 μm IRAC band. Narrowband 2.12 μm images showed no evidence of molecular hydrogen emission. We made use of GLIMPSE data obtained with IRAC on the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology, under NASA contract 1407. This research has made use of the NASA/IRAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This publication makes use of data products from the 2MASS, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. M.T. acknowledges financial support from PAAPIIT/UNAM grant IN102607. Additionally, L.F.R.