The Spitzer Gould Belt Survey of Large Nearby Interstellar Clouds: Discovery of a Dense Embedded Cluster in the Serpens-Aquila Rift R
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The Astrophysical Journal, 673: L151–L154, 2008 February 1 ᭧ 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE SPITZER GOULD BELT SURVEY OF LARGE NEARBY INTERSTELLAR CLOUDS: DISCOVERY OF A DENSE EMBEDDED CLUSTER IN THE SERPENS-AQUILA RIFT R. A. Gutermuth,1 T. L. Bourke,1 L. E. Allen,1 P. C. Myers,1 S. T. Megeath,2 B. C. Matthews,3 J. K. Jørgensen,4 J. Di Francesco,3 D. Ward-Thompson,5 T. L. Huard,1 T. Y. Brooke,6 M. M. Dunham,7 L. A. Cieza,7 P. M. Harvey,7 and N. L. Chapman8 Received 2007 November 21; accepted 2007 December 14; published 2008 January 8 ABSTRACT We report the discovery of a nearby, embedded cluster of young stellar objects, associated filamentary infrared dark cloud, and 4.5 mm shock emission knots from outflows detected in Spitzer IRAC mid-infrared imaging of the Serpens-Aquila Rift obtained as part of the Spitzer Gould Belt Legacy Survey. We also present radial velocity measurements of the region from molecular line observations obtained with the Submillimeter Array (SMA) that suggest the cluster is comoving with the Serpens Main embedded cluster3Њ to the north. We therefore assign it the same distance, 260 pc. The core of the new cluster, which we call Serpens South, is composed of an unusually large fraction of protostars (77%) at high mean surface density (1430 pcϪ2 ) and short median nearest neighbor spacing (3700 AU). We perform basic cluster structure characterization using nearest neighbor surface density mapping of the YSOs and compare our findings to other known clusters with equivalent analyses available in the literature. Subject headings: ISM: individual (IRAS 18275Ϫ0203, IRAS 18274Ϫ0205) — stars: formation — stars: low-mass, brown dwarfs Ј 1. INTRODUCTION et al. 1985), 20 to the east. We found structured Ks nebulosity at the site in 2MASS Atlas images, but few sources. The aim of the Spitzer Gould Belt (GB; L. Allen et al., in The region is flanked by two Infrared Astronomy Satellite preparation) and c2d (Evans et al. 2003) Legacy Surveys is to (IRAS) sources (IRAS 18275Ϫ0203 and IRAS 18274Ϫ0205), build a complete mid-infrared record of star formation in all and Midcourse Space Experiment (MSX) coverage of this re- of the large molecular clouds within 500 pc, largely dominated gion reveals only a few additional point sources. No deep X-ray by the Gould Belt itself. This is one of several surveys of the images of this region are available. Gould Belt being performed at many wavelengths from the near-IR (Two Micron All Sky Survey [2MASS]; Skrutskie et al. 2006) through the far-infrared (Herschel; Andre´ & Saraceno 2005) to the submillimeter (Submillimeter Common User Bo- 2. OBSERVATIONS lometer Array 2 [SCUBA-2]; Ward-Thompson et al. 2007). In combination, the resulting database offers a nearly complete We obtained 3.6, 4.5, 5.8, and 8.0 mm imaging of 5.1 square and unbiased view of all nearby star formation in large mo- degrees of the Serpens-Aquila Rift with IRAC (Fazio et al. 2004) lecular clouds. This penetrating examination of regions that on board Spitzer (Werner et al. 2004) as part of the GB Legacy have often received little attention in the literature is likely to Survey (PID: 30574) on 2006 October 27. In this Letter, we Ј # Ј reveal new regions of star formation. focus on the 14 10 field of view centered on Serpens South p h m s Ϫ Њ Ј Љ The Spitzer Infrared Array Camera (IRAC) imaging pre- (R.A., decl. [J2000.0] 18 30 03 , 02 01 58.2 ). High Dy- sented here reveals a dense cluster of sources with excess in- namic Range mode was used, resulting in 0.4 and 10.4 s inte- frared emission and associated 4.5 mm–bright structured neb- grations for each of four dithered images at each mosaic po- sition. Mosaics were constructed at 1.2Љ pixelϪ1 with Basic ulosity (typically shocked H2 emission from outflows; see Smith et al. 2006) within a filamentary infrared dark cloud seen Calibrated Data (BCD) products from the Spitzer Science Cen- in absorption against bright diffuse 5.8 and 8.0 mm emission. ter’s data pipeline version 14.4. All artifact treatment, mosaic This emission is most likely PAH-feature emission (Morris et construction, point-source detection and aperture photometry, al. 2004) excited by the young, high-mass stars in W40 (Smith and photometric classification of sources presented here have been performed using custom Interactive Data Language (IDL) routines (Gutermuth et al. 2007). We adopted aperture radii 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cam- Љ Љ bridge, MA 02138; [email protected]. and inner and outer background annulus radii of 2.4 , 2.4 , and 2 Ritter Observatory, Department of Physics and Astronomy, University of 7.2Љ, respectively. Point-source flux calibration is discussed in Toledo, Toledo, OH 43606. detail in Reach et al. (2005). Field averaged 90% differential 3 Herzberg Institute of Astrophysics, National Research Council of Canada, completeness magnitude limits are 14.5, 14.2, 13.2, and 12.0 Victoria, BC V9E 2E7, Canada. for the 14Ј # 10Ј submosaics at 3.6, 4.5, 5.8, and 8.0 mm, 4 Argelander-Institut fu¨r Astronomie, University of Bonn, Auf dem Hu¨gel 71, 53121 Bonn, Germany. respectively (Gutermuth et al. 2005). The sensitivity of these 5 Department of Physics and Astronomy, University of Cardiff, P.O. Box data is equivalent to that of the IRAC data presented by Winston 913, Cardiff CF2 3YB, Wales, UK. et al. (2007) for the Serpens Main cluster, which will be dis- 6 Spitzer Science Center, MC 220-6, California Institute of Technology, Pas- cussed in § 4. IRAC photometry catalogs are band-merged adena, CA 91125. Љ 7 Department of Astronomy, University of Texas, Austin, TX 78712. together with a radial matching tolerance of 1 , and then merged 8 Department of Astronomy, University of Maryland, College Park, MD with the 2MASS Point Source Catalog (PSC) at a tolerance of 20742. 1Љ. L151 L152 GUTERMUTH ET AL. Vol. 673 Fig. 1.—At left, a color-composite image of the IRAC mosaics of Serpens South; 3.6, 4.5, and 8.0 mm images are mapped to blue, green, and red, respectively. At right, the spatial distribution of YSOs overlaid on the gray-scale 8.0 mm image. Red circles are Class I protostars, and green diamonds are Class II stars with disks. The white circle marks the dense core of the cluster. 2.1. Submillimeter Array with Gaussian FWHM widths of ∼2 km sϪ1 . Thus, its molecular 9 line emission can be separated from that associated with W40, Observations with the SMA (Ho et al. 2004) were performed which occurs in the LSR range of 2–6 km sϪ1 (Zeilik & Lada on 2007 January 30. The array was tuned to cover the frequency 1978; Vallee et al. 1992; Zhu et al. 2006), and is spatially range 219.4–221.4 GHz (LSB) and 229.4–231.4 GHz (USB), compact. The SMA data clearly show that the velocities of which includes the lines of 12CO, 13CO, and C18 O 2–1, with a Ϫ Ϫ IRAS 18275Ϫ0203 and the nearby embedded source are the resolution of ∼0.5 km s1 channel1 . Three positions were ob- Ϫ same as that of the Serpens Main cluster, and not of the W40 -pc (Strai 37 ע served for 30 minutes each: IRAS 18275 0203 (R.A., decl. cluster. We therefore assign the distance of 260 [J2000.0] p 18h30m05.8s, Ϫ02Њ01Ј45Љ) and a nearby embedded p h m s Ϫ Њ Ј Љ zˇys et al. 1996) to Serpens South. This is consistent with more source (R.A., decl. [J2000.0] 18 30 01.3 , 02 01 48 ; la- recent results for the entire Serpens-Aquila Rift (Straizˇysetal. beled “Embedded” in Fig. 1), both in Serpens South, and a 2003). position near the 12CO 1–0 peak of W40, as determined from the maps of Grabelsky et al. (1987) at R.A., decl. (J2000.0) p 18h31m20.7s, Ϫ02Њ01Ј07Љ. Ganymede was observed for pass- 3.2. Identification, Membership, and Spatial Distribution band calibration. No flux or complex gain calibration was per- of YSOs formed. More detailed SMA results will be discussed in a future We identify likely YSOs from their excess infrared emission paper. over that expected from a typical photosphere and high infrared flux compared to extragalactic contaminants (Gutermuth et al. 3. RESULTS 2007). These candidates are classified as Class I (protostar) or Class II (pre–main-sequence star with disk) YSOs10 by the their 3.1. Distance value of aIRAC, the linear least-squares fit to their spectral energy Serpens South appears to be associated with a filamentary distribution (loglFl vs. log l) through the four IRAC band- cloud seen in absorption against PAH emission from W40 passes (Lada et al. 2006). In this way, we have identified 54 1 Ϫ pc; Radhak- Class I and flat-spectrum “protostars” (aIRAC 0.3; Greene 150 ע Fig. 1). Thus the distance to W40 (550) Ϫ ≤ ≤ Ϫ rishnan et al. 1972; Smith et al. 1985) is an upper limit to the et al. 1994) and 37 Class II ( 1.6 aIRAC 0.3) YSOs, a distance to Serpens South. The Serpens Main cluster has local total of 91 (59% protostars), within the 14Ј # 10Ј (1.1 pc # standard of rest (LSR) velocities in the range 6–11 km sϪ1 , 0.8 pc) field of view presented in Figure 1. Half of the sources when observed in the isotopologues of CO (White et al.