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Young Stellar Objects and Triggered Star Formation in the

Vulpecula OB association

N. Billot1, A. Noriega-Crespo2, S. Carey2, S. Guieu2, S. Shenoy2, R. Paladini2, W. Latter1

nbillot@ipac.caltech.edu

ABSTRACT

The Vulpecula OB association, VulOB1, is a region of active star formation located in theGalactic plane at 2.3 kpc from the Sun. Previous studies suggest that sequential star forma-tion is propagating along this 100 pc long molecular complex. In this paper, we use Spitzer

MIPSGAL and GLIMPSE data to reconstruct the star formation history of VulOB1, and searchfor signatures of past triggering events. We make a census of Young Stellar Objects (YSO) inVulOB1 based on IR color and magnitude criteria, and we rely on the properties and nature ofthese YSOs to trace recent episodes of massive star formation. We find 856 YSO candidates,and show that the evolutionary stage of the YSO population in VulOB1 is rather homogeneous- ruling out the scenario of propagating star formation. We estimate the current star formationefficiency to be ∼ 8%. We also report the discovery of a dozen pillar-like structures, which areconfirmed to be sites of small scale triggered star formation.

Subject headings: Infrared: ISM, Stars — ISM: HII regions — Stars: Formation, Pre-main sequence

1. Introduction

The physical mechanisms describing stellarbirth are fairly well understood for low- andintermediate-mass stars but still under debatefor their high-mass analogues (McKee & Ostriker2007). On larger scales, observations show thatstar forming mechanisms are of relatively poor ef-ficiency, as only a fraction of the gas reservoir inthe Universe is turning into stars. Typical StarFormation Efficiencies (SFE) are of the order 3-6% in the Galaxy (Evans et al. 2009), and 5%or less in other galaxies (Rownd & Young 1999).Still, in extreme environments like in the star-burst galaxy Arp 220, the SFE can reach 50%(Anantharamaiah et al. 2000) suggesting that thestar forming mechanism at work in such objectscould be of a different nature. For instance thefeedback into the interstellar medium (ISM) fromshort-lived massive stars seems to influence the

1NASA Herschel Science Center, IPAC, MS 100-22, Cal-ifornia Institute of Technology, Pasadena, CA 91125 USA

2Spitzer Science Center, IPAC, MS 220-6, California In-stitute of Technology, Pasadena, CA 91125 USA

yield of star formation in their local environment.As they evolve off the main sequence, high-massstars produce a copious amount of energy whilestill embedded in their native cocoon; such dis-ruption of a molecular cloud leads to gravitationalinstabilities and possibly to the onset of a newepisode of star formation. Hosokawa & Inutsuka(2006) showed that under certain conditions run-away triggering can take place around massiveOB stars. In other cases however, turbulence andmagnetic fields can have a negative feedback onthe local ISM and lead to the suppression of thestar forming activity (Price & Bate 2009; Stone1970). Elmegreen (1998) provides an exhaustivereview of the theoretical framework to study thedynamical triggers of star formation.

In this context, we use the data from the SpitzerLegacy surveys MIPSGAL (Carey et al. 2009) andGLIMPSE (Benjamin et al. 2003) to study thestar forming activity currently taking place inthe Vulpecula OB association (hereafter VulOB1).VulOB1 hosts nearly one hundred OB stars andthree bright HII regions known as Sharpless ob-jects 86, 87 and 88 (Sharpless 1959). According

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to Ehlerova et al. (2001) and Turner (1986), thestar forming activity occurring in VulOB1 mighthave been triggered by a common external source,and star formation might be propagating from oneHII region to another through the expansion of asupernova shock front. The aim of this paper isto study the triggered star formation on scales aslarge as ∼100 pc in VulOB1, which could helpus understand star formation mechanisms withinother Giant Molecular Clouds in the Galaxy andbeyond.

We use the method developed by Gutermuth et al.(2008) to obtain a census of Young Stellar Ob-jects (hereafter YSOs) in VulOB1 based on in-frared color and magnitude criteria, and we relyon the properties and nature of these YSOs totrace episodes of star formation. The MIPSGALand GLIMPSE sensitivity makes our search forYSOs in VulOB1 biased towards massive objectsthat reveal the most recent star forming activity.

In Section 2 we present the dataset used in thepaper. In Section 3 we give a comprehensive de-scription of VulOB1 encompassing the three HII

regions as well as the dozen pillar-like structureswe discovered in this region. Section 4 describesthe identification process of Young Stellar Objectsbased on their IR-excess emission. We present anddiscuss our results in Section 5, and we give ourconclusions in Section 6.

2. The dataset

2.1. Spitzer Observations and Point Source

Catalogs

The Spitzer Space Telescope (Werner et al.2004) observed over 270 square degrees of the innerGalactic plane in six wavelength bands as part oftwo legacy programs: the Galactic Legacy InfraredMid-Plane Survey Extraordinaire (GLIMPSE;E. Churchwell PI; Benjamin et al. 2003) andthe MIPS GALactic plane survey (MIPSGAL;S. Carey PI; Carey et al. 2009). The Vulpecularegion was covered by GLIMPSE I (PID 188)and MIPSGAL I (PID 20597). It was imagedwith the IRAC camera (Fazio et al. 2004) at 3.6,4.5, 5.8 and 8.0µm, and with the MIPS camera(Rieke et al. 2004) at 24 and 70µm. The angularresolution of Spitzer is 2′′, 6′′ and 18′′ at 8, 24 and70µm, respectively.

The MOPEX package (Makovoz et al. 2006)

was used to build mosaics of the sky about1 square degree wide from individual Basic Cali-brated Data (BCD) frames. Details of the post-processing and BCD pipeline modifications aredescribed in Meade et al. (2007) for GLIMPSEand in Mizuno et al. (2008) for MIPSGAL. Thelarge Vulpecula mosaics presented in this paperare also built with MOPEX using the 1 square de-gree plates available through the Infrared ScienceArchive1 (IRSA).

The search for YSO candidates in Vulpecularelies on the point source catalogs (PSC) gener-ated from the GLIMPSE and MIPSGAL surveys.For GLIMPSE, point sources are extracted usinga modified version of the Point-Spread Function(PSF) fitting program DAOPHOT (Stetson 1987).The GLIMPSE PSC we used is the enhanceddata product v2.0 available on the IRSA web-site. For MIPSGAL, sources are extracted fromthe 24µm maps using a Pixel-Response Functionfitting method (Shenoy et al. in preparation) andthe APEX software developed at the Spitzer Sci-

ence Center2. This catalog is then merged withthe GLIMPSE PSC to include IRAC and 2MASS3

fluxes if they are available. 70µm sources are ex-tracted from reprocessed 70µm maps (Paladini etal. in preparation) using the StarFinder software(Diolaiti et al. 2000), and matched to the MIPS-GAL PSC with a search radius of 9′′.

2.2. Ancillary data

We complement our dataset with visible, sub-millimeter, millimeter and radio data to obtain abroader view of the Vulpecula complex, and to in-vestigate the morphology, star formation historyand possible relationship between the HII regions.We make use of the VLA Galactic Plane Survey(VGPS) data, which consists of HI line and 21-cm continuum emission data with a resolution of1′×1′×1.56 km s−1 and 1′ respectively (Stil et al.2006). These data are analysed in details in Sec-

1http://irsa.ipac.caltech.edu/data/SPITZER/GLIMPSE/andhttp://irsa.ipac.caltech.edu/data/SPITZER/MIPSGAL/

2http://ssc.spitzer.caltech.edu/postbcd/apex.html3The Two Micron All Sky Survey (2MASS) is a joint projectof the University of Massachusetts and the Infrared Pro-cessing and Analysis Center/California Institute of Tech-nology, funded by the National Aeronautics and Space Ad-ministration and the National Science Foundation.

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tion 3.1. We also use the Virginia Tech Spectral-line Survey (VTSS) which provides arcminute-resolution images of the 6563 A Hα recombina-tion line of atomic Hydrogen in the Vulpecula re-gion (Dennison et al. 1998). S. Bontemps (pri-vate communication, see Schneider et al. (2006)for details) also provided our team with a 1.2′-resolution extinction map of VulOB1 generatedfrom stellar counts and colors derived from the2MASS point source catalog (see Figure 1). Thevisual extinction in Vulpecula ranges from 3 to20 with a distribution peaking at Av∼5. We alsouse the CO survey of the Galaxy published inDame et al. (2001). However the relatively lowspatial resolution of ∼7.5′ over VulOB1 preventeda relevant detailed study of the CO emission. Wefind nonetheless a very good correlation betweenthe CO emission and the visual extinction thatboth trace the cold and dusty molecular mate-rial (cf Figure 1). In addition, we used the sub-millimeter data obtained during the second flightof the Balloon-borne Large Aperture Submillime-ter Telescope (BLAST, Pascale et al. 2008), wherea 4 square degree region was mapped around Sh2-86 at 250, 350 and 500µm (Chapin et al. 2008).The angular resolution of the BLAST maps are40′′, 50′′ and 60′′ at 250, 350 and 500µm, re-spectively. And finally, we exploit the recently re-leased 1.1mm data of the Galactic Plane Surveycarried out with Caltech Submillimeter Observa-tory/Bolocam (Aguirre et al. in preparation) andproviding an angular resolution of 31′′.

3. The Vulpecula OB association

VulOB1 is an active star forming region lo-cated in the Galactic plane at a longitude l∼60◦.In this article we focus on the region centered at(l, b) = (60.2,−0.2) and covering about 6.6 squaredegrees (see Figures 1 and 2). Garmany & Stencel(1992) describe VulOB1 as an oval region about3.5◦ by 1.5◦ hosting the star cluster NGC6823.Nearly one hundred hot massive stars are foundin the direction of Vulpecula from the catalogof OB stars compiled by Reed (2003). Some ofthese stars have already been associated with andare responsible for the ionization of three brightHII regions in VulOB1, namely Sh2-86, 87 and88 (Sharpless 1959). Figure 2 presents the mid-infrared morphology of VulOB1 and the locationof the OB stars.

The VGPS 21-cm continuum emission cov-ering VulOB1 reveals two bright circular com-pact sources (∼1.3′ in diameter) coincident withSh2-87 and Sh2-88, and an extended dimmer re-gion (27′×12′) reminiscent of the infrared mor-phology of Sh2-86. The supernova remnantSNR G59.5+0.1 (Taylor et al. 1992) is also clearlyvisible in the radio continuum data at (l, b) =(59.59, 0.1), its measured diameter is 15′ (cf Sec-tion 5.4.1). Our analysis of the VGPS HI linedata indicates that the three HII regions areneighbors (cf Section 3.1), as was previouslynoted by Turner (1986), Ehlerova et al. (2001)and Cappa et al. (2002). Distance determinationsfor the three Sharpless objects and their excit-ing stars range from 1.5 to 3.2 kpc (Fich & Blitz1984; Barsony 1989; Guetter 1992; Brand & Blitz1993; Massey et al. 1995; Deharveng et al. 2000;Hoyle et al. 2003; Kharchenko et al. 2005; Bica et al.2008). Following the arguments from Chapin et al.(2008), we adopt a common distance of 2.3 kpcfor the three HII regions.

In the catalog of Reed (2003) we find about30 OB stars in the direction of Sh2-86 and Sh2-87, in what we identify as the pillars region (thesestars might be responsible for the existence ofpillar-structures, see Section 3.2). We check theirpossible association with VulOB1 by computingtheir distance moduli based on the MV -Spectraltype relation and the intrinsic colors given byLandolt-Bornstein (1982). We assume a ratio oftotal-to-selective absorption RV = 3.1 typical ofthe diffuse ISM, and we associate a visual extinc-tion to each star based on our extinction mapof VulOB1. Out of the 16 stars for which spec-tral type and magnitude information are avail-able in the Reed catalog, 12 fall between 1.3 and2.3 kpc with a mean distance from the Sun of1.91 ± 0.4 kpc. We find similar distances usingintrinsic colors and absolute magnitudes given byJohnson (1958) and Wegner (2006) respectively.Although the uncertainties are significant, whichis mostly due to the large uncertainty on the valuesof the absolute magnitudes, our distance estimatefor these stars is consistent with an associationwith the Vulpecula complex.

3.1. HII regions

Sh2-86 is an extended HII region about 40′ wide(Sharpless 1959) located in the southern part

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Fig. 1.— CO contours derived from the velocity-integrated CO map of Dame et al. (2001), integrated from20 to 40 km s−1, overlaid on a visual extinction map of VulOB1 (Av ranges from 3 to 20 mag on a linearscale). The Sharpless Objects appear as higher extinction regions.

of Vulpecula (Figures 1, 2 and 11 in the ap-pendices). It is excited by the open clusterNGC 6823, which is part of the OB associationVulOB1 (Massey et al. 1995; Bica et al. 2008).Pigulski et al. (2000) estimate the age of the starcluster at 3 ± 1 Myr old. In the atomic gas dis-tribution derived from the VGPS data, Sh2-86appears as an oblong hole at velocities between26 and 31 km s−1. We find that the dimensionof the shell is approximately 60′ along the Galac-tic plane and 25′ across it. In the optical, thesouth-eastern part of Sh2-86 is very contrasted. Itexhibits silhouetted pillar-like structures pointingtowards NGC 6823, emission and reflection nebu-lae as well as filamentary structures. The infraredemission is also contrasted in this region tracingthe complex interplay between the UV radiationand the surrounding dusty interstellar medium.Chapin et al. (2008) also report the detection of49 compact sub-millimeter sources associated withSh2-86 using the Balloon-borne Large ApertureSubmillimeter Telescope (BLAST). The presenceof these clumps, with masses ranging from 14 to700 M⊙, indicates the capability of Sh2-86 to form

massive stars.

Sh2-87 and Sh2-88 are also HII regions initiallydiscovered by Sharpless (1959). These regions areactive sites of star formation as indicated by thepresence of H2O maser line emissions and bipolaroutflows (Barsony 1989; Deharveng et al. 2000).Bolocam data show bright sources at the loca-tion of Sh2-87 and Sh2-88 indicating the pres-ence of cold core candidates. Our analysis of theVGPS data reveals that the morphology of the HI

shell around Sh2-87 at velocities between 22 and26 km s−1 is very similar to that of the 24µm emis-sion. The HII region appears as a hole in the HI

distribution as the hydrogen is ionized by an em-bedded B0 star (Felli & Harten 1981). A compactsource seen in absorption sits at the center of Sh2-87, its position is coincident with the location ofthe peak emission at 24µm (l, b) = (60.88,−0.13).Barsony (1989) and Xue & Wu (2008) give a de-tailed description of this HII region.

Sh2-88 is a diffuse nebula of diameter ∼20′ lo-cated at (l, b) = (61.45, 0.34), which is excited bythe O8 star BD +25◦3952 (Cappa et al. 2002).The star formation activity taking place in Sh2-

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88 occurs in a couple of nebular knots identifiedby Lortet-Zuckermann (1974), Sh2-88A and Sh2-88B, located 15′ southeast of Sh2-88. These knotsare responsible for the bright 24µm emission vis-ible in Figure 2 at (l, b) = (61.47, 0.1). Sh2-88Bis actually the brightest of the three Sharpless ob-jects at mid-IR wavelengths. Figure 10 in the ap-pendices shows a composite image of Sh-88B inIRAC bands. It consists of a compact cometaryHII region and an ultracompact (UC) HII region.Deharveng et al. (2000) present a detailed analy-sis of Sh2-88B and its stellar content. At radiowavelength, the association of Sh2-88 with a HI

shell is somewhat more difficult than in the othertwo cases as the edges of the shell are not as welldefined. We detect however a faint hole at veloci-ties between 22 and 26 km s−1 as well as a compactsource seen in absorption in the center of Sh2-88B.

3.2. Pillar structures

We have discovered several pillar-like struc-tures on either side of the Galactic equator be-tween Sh2-86 and Sh2-87 (59◦.5 < l < 61◦).These objects are similar to the archetypal pil-lars of creation found in M16 (Hester et al. 1996;Urquhart et al. 2003). Such pillars are usually as-sociated with recent episodes of star formation asthe winds and radiation emanating from youngmassive stars are responsible for sculpting theseelongated elephant trunks out of the surroundingmolecular material. An accepted mechanism forthe formation of the pillars is the slow photoevap-oration of a pre-existing molecular clump shad-owing a tail of more diffuse gas, but other mech-anisms have been proposed based on hydrody-namical instabilities for instance. Spitzer (1954),Bertoldi (1989), Lefloch & Lazareff (1994) andCarlqvist et al. (2003) present analytical mod-els for the formation of pillar-like structures,and Miao et al. (2006), Mizuta et al. (2006) andGritschneder et al. (2009) carry out numericalsimulations of the formation and evolution of suchobjects. According to these studies, the pressureat the surface of the pillars due to the strong ex-ternal radiation appears to trigger the formationof new stars, which is confirmed by recent observa-tions (e.g. Sugitani et al. 2002; Reach et al. 2004;Bowler et al. 2009; Reach et al. 2009)

Figure 3 presents the 24µm image of the pil-lars found in Vulpecula, and gives the nomencla-

ture to identify individual objects. The three pil-lars VulP12-13-14 were already known prior tothese observations (Chapin et al. 2008); they arelocated northeast of NCG 6823 and are obviouslyassociated with NGC 6823 as they point towardsthe star cluster. However the 11 pillars VulP1

to VulP11 have never been reported in the lit-erature before this study. Their association withVulOB1 is not as straightforward as in the caseof NGC 6823 since no obvious source could beidentified as their sculptor. The OB stars men-tioned previously are certainly good candidates,for instance the star HD 186746 (spectral typeB8Ia) is located right above the tip of the pillarVulP1 in projection (Figures 2 and 3). Neverthe-less, 10 of the discovered pillars, out of 11, seemto point towards the same faint diffuse nebulos-ity located at (l, b) = (60.37,−0.04), which is notcoincident with any known OB stars. We arguethat the chance for ten randomly distributed pil-lars to point towards the same object is negligi-ble4, thus the discovered pillars VulP1 -VulP10 aremost certainly associated with each other. Thecentral nebulosity might host the object respon-sible for the formation of the pillars. In addi-tion, a diffuse faint circularly symmetric structurecentered on the same nebulosity is discernible inthe background diffuse emission. Bright objectsat 24µm also seem to delineate this faint struc-ture at a radius of ∼ 15′ centered at (l, b) =(60.37,−0.04). These morphological features sug-gest that a single event in the past may havemolded the pillars region.

We analyze the radio data in the pillars regionto look for a possible association between thesestructures and the OB association. Out of the15 pillars identified in Figure 3, only 3 are detectedin the VGPS HI line data. The pillar VulP10

presents a deep absorption feature at 27-33 km s−1

as well as smaller features centered at 8, 13 and20 km s−1. These HI features are marginally re-solved into two close compact sources located atthe base of the pillar. Since these features arequite different from the IR morphology of VulP10,we cannot exclude a fortuitous association with acoincident radio source seen in projection.

VulP4 and VulP5 appear to have similar char-

4Assuming we measure the direction of the pillars with anaccuracy of ∼ 2◦, the probability is lower than 10−11.

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Fig. 2.— The top panel shows a composite image of VulOB1 (3.6, 8.0 and 24 µm are color-coded as blue,green and red respectively). This image was assembled by R. Hurt and T. Robitaille based on GLIMPSE andMIPSGAL data. The bottom panel represents Hα contours from the VTSS overlaid on the MIPSGAL 24 µmimage. The Hα emission associated with Sh2-88 peaks at (l, b) = (61.45, 0.30) while the bright mid-IRemission located 15′ south-east of the diffuse nebula is the knot identified in the text as Sh2-88B. Greencrosses indicate the location of the 88 OB stars present in this 3.7◦×1.8◦ region.

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acteristics, they present comparable morphologiesin the HI line data and in the Spitzer bands. Theyboth emerge from the molecular cloud to be ex-posed to the ionizing radiation. These pillars arelocated at the edge of HI shells, and their silhou-ette is clearly identified at velocities 21-23 km s−1,which places them at roughly the same distanceas the three Sharpless objects for which VLSR =22− 31 km s−1.

VGPS continuum emission shows a bright two-lobed compact source located at the base of thepillar VulP10, consistent with the two blobs ob-served in the HI line data. No other pillars aredetected in the 21-cm continuum.

Tables 3 and 4 in the appendices summarizethe geometrical and morphological information ex-tracted from the Spitzer observations for individ-ual pillars. Of particular interest are the bright(∼ 2 Jy) compact sources found at 70µm in thecore of the pillars VulP1 and VulP3 (see Fig-ure 4). These embedded sources are faint at24µm (∼ 1 mJy) and have no counterparts inIRAC and 2MASS bands. Even though star for-mation is expected to occur preferentially at thetip of such structures, these red sources are likelysites of massive star formation. Longer wave-length5 observations would be necessary to iden-tify the nature of these sources. Figure 4 shows thepillars VulP5 and VulP6 and their associated redsources. YSO candidates are located at the tipof each pillar, and also in the pedestal of VulP5along some Infrared Dark Clouds (IRDC). Sub-millimeter BLAST sources are also located at thetip of the pillars and along the same IRDCs. TwoBolocam sources are also located along one of theIRDCs.

4. The IR-excess emission of Young Stel-

lar Objects

Stars form from the gravitational collapse andfragmentation of giant molecular clouds in theISM. The contraction of the cold gas leads tothe formation of a dense rotating core radiatingits thermal energy in the millimeter and far-IRregime. As the bulk of the initial cloud massis falling towards the center of the core, a pro-

5The BLAST data do not cover these pillars, and the 1.1 mmBolocam data, which is available for these pillars, is not asgood a diagnostic as the sub-millimeter data.

tostar emerges with a gaseous and dusty accre-tion disk rotating around it. The peak emis-sion of such a young stellar object shifts towardsshorter wavelengths revealing a bright source inthe mid-IR. The circumstellar disk is then dissi-pated via accretion, planet formation or evapora-tion. Further reading on star formation mecha-nisms can be found in e.g. Terebey et al. (1984);Adams et al. (1987); Andre et al. (1993, 2000);McKee & Ostriker (2007).

Recent works have shown that the wavelengthcoverage of Spitzer instruments is well suitedfor observing the mid-IR excess emission em-anating from the warm circumstellar materialaround Young Stellar Objects (e.g. Allen et al.2004; Harvey et al. 2007; Koenig et al. 2008;Guieu et al. 2009). Based on the mid-IR ex-tinction law (Lutz 1999; Indebetouw et al. 2005;Flaherty 2007), Gutermuth et al. (2008) further-more argue that the most reliable IRAC-basedcriterion for identifying YSO candidates is the[4.5]-[5.8] color. Indeed the flattening of the ex-tinction curve observed between the IRAC bands2 and 4 (4.5 to 8µm) reduces the degeneracy be-tween selective interstellar extinction and intrinsicIR excess.

Our approach to identify YSOs in VulOB1 is toexploit the method developed by Gutermuth et al.(2008) based on Spitzer colors and magnitude cutsto identify sources with IR-excess emission. Weapply this method to the Point Source Catalogs(PSC) compiled by the GLIMPSE and MIPS-GAL teams (Section 2.1) to take full advantageof the good photometric quality and the high re-liability of these data. The direct consequenceof this approach is that no extended sources willbe considered in our study even though they ex-hibit significant IR-excess emission. To mitigatethis caveat, we compare the angular resolution ofSpitzer with the typical angular size of circum-stellar envelops and disks seen at 2.3 kpc from theSun. Assuming the telescope optic is diffractionlimited6 down to 5.5µm, the resolution element at3.6 and 24µm corresponds to 3500 and 16000 AUat 2.3 kpc respectively. The spatial extend of theYoung Stellar Objects we are looking for does notexceed these limits. For instance, Vicente & Alves

6From the Spitzer Observer’s Manual available athttp://ssc.spitzer.caltech.edu.

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Fig. 3.— Identification of pillar structures in VulOB1 on a MIPSGAL24µm image. The grey scale isstretched to increase the contrast of the pillars. VulP1 to VulP10 seem to point toward the same direction.Extended concentric structures are noticeable around the central position up to a radius of ∼ 10 pc. VulP12to VulP14 are associated with the young star cluster NGC 6823.

(2005) measured the size distribution of circum-stellar disks in the Trapezium cluster and founddisk sizes ranging from 100 to 400 AU, an orderof magnitude smaller than the resolution elementat 3.6µm. Although protostellar envelops havea larger extend, i.e. a few thousand AU accord-ing to Bonnell et al. (1996) and Jørgensen et al.(2002), and assuming they can only be detectedat long wavelengths where the resolution elementis larger, we expect the number of resolved YSOsin VulOB1 to be relatively small. However a frac-tion of the YSOs in their early phases of evolutionmight be resolved by Spitzer, even at 24µm, andthose large protostars would be excluded from ouranalysis.

4.1. Initial catalog

We first compile a catalog of IRAC pointsources based on the highly reliable Point SourceCatalog available from the IRSA website as aGLIMPSE I v2.0 enhanced data product7. Allsources found in this catalog with galactic coordi-nates in the range 58.6 ◦ < l < 62.2 ◦ and −1.0 ◦ <

7http://www.astro.wisc.edu/glimpse/glimpse1 dataprod v2.0.pdf

b < 0.8 ◦ are considered (∼ 3.28 × 105 sources).Fluxes are converted to magnitudes using the ze-ropoints given in Table 1. To ensure good pho-tometry and avoid flux stealing between adjacentsources as noted by Robitaille et al. (2008), werequire that valid point sources have magnitudeerrors lower than 0.2mag, and that their closesource flag8 (csf) is zero. We also apply specificcuts on IRAC magnitudes to achieve high detec-tion reliability, the magnitude limits we adopt are14.2, 14.1, 11.9 and 10.8 mag at 3.6, 4.5, 5.8 and8.0 µm respectively. According to the GLIMPSE IAssurance Quality Document9, these limits ensurea detection reliability above 98% in all four IRACbands. The reliability of the faintest sources how-ever might vary across the VulOB1 region as itdepends on the background brightness structureand the local source density.

This intermediate catalog is then mergedwith the MIPSGAL 24µm Point Source Cata-

8Following the convention of the GLIMPSE I v2.0 enhanceddata products, csf=0 implies that no sources in the ArchiveCatalog are within 3′ of the source.

9http://www.astro.wisc.edu/glimpse/GQA-master.pdf

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Fig. 4.— Spitzer 3-color images of some pillars discovered in VulOB1. YSO candidates are marked as whitesmall squares and circles, large blue boxes show BLAST sources and green ellipses indicate Bolocam sources.Top panel : Five pillars identified as VulP11 and VulP1 to VulP4 in Figure 3 (from left to right). Blue,green and red are 8, 24 and 70µm images, respectively. VulP1 and VulP2 exhibit bright 70µm sources intheir core. Bottom panel : VulP5 and VulP6. Blue, green and red are respectively 4.5, 8.0 and 24µm images.InfraRed Dark Clouds (IRDCs) are visible in the pedestal of the pillar VulP5. YSO candidates are alignedalong IRDCs.

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log (Shenoy et al. in preparation) with the con-straint that the maximum separation between twomatching sources is 3′′. Once again to ensure agood photometry, we require that the magnitudeof 24µm sources and their associated errors areconstrained. These constraints however are ap-plied at a later stage of the selection procedure(see Section 4.2).

Out of nearly 3.3 × 105 sources present in theGLIMPSE catalog within our search area, only ∼

1.47 × 105 bypassed the initial magnitude cuts.Most of the exclusions were due to noisy detectionsin bands 3 and 4 of IRAC.

In the end, the initial catalog from which wesearch for YSO candidates contains point sourceswith well measured photometry and with at leastone detection in IRAC bands, plus the associated2MASS and MIPSGAL 24µm fluxes if they exist.

4.2. Mid-IR selection of YSO candidates

We start our search for YSO candidates in theinitial catalog by removing extragalactic contam-

inants that might be misidentified as YSOs. Us-ing the selection criteria presented in the appen-dices of Gutermuth et al. (2008), which are basedon a complementary analysis of the Bootes fieldIRAC data (Stern et al. 2005), we identify and re-ject 6 sources dominated by Polycyclic AromaticHydrocarbons (PAH) feature emission, likely star-forming galaxies or weak-line active galactic nuclei(AGN). No sources matching the colors and mag-nitudes criteria for broad-line AGNs were found.Considering a search area of 6.5 square degrees, wefind a density of extragalactic sources in Vulpeculaslightly lower than one per square degree. Thisis comparable to the density of ∼0.5 galaxy persquare degrees found in the zone of avoidance byMarleau et al. (2008) using GLIMPSE and MIPS-GAL data. We further filter the catalog accordingto Gutermuth et al. criteria and reject 8 sourcesthat have a large 4.5µm excess emission consistentwith molecular hydrogen line emission found inregions where high-velocity outflows interact withthe cold molecular cloud. The remaining sourcesare presumably of stellar origin.

All sources with the following color constraintsare considered likely YSOs:

[4.5]− [8.0] > 0.5,

[3.6]− [5.8] > 0.35,

[3.6]− [5.8] ≤ 3.5× ([4.5]− [8.0])− 1.25,

Out of the ∼ 2.38× 104 sources possessing all fourIRAC magnitudes, we identified 820 YSO candi-dates based on their IR-excess emission. We fur-ther classify the selected objects according to theirinfrared spectral index αIR = ∂ log(λFλ)/∂ log(λ)as defined by Lada (1987). We specifically com-pute the spectral index αIRAC as the slope ofthe Spectral Energy Distribution (SED) measuredfrom 3.6 to 8.0µm. Objects with αIRAC > −0.3have a flat-ish or rising SED indicating the pres-ence of a cold dusty envelop infalling onto a centralprotostar, these are designated class 0/I YSOs.Objects with −0.3 > αIRAC > −1.6 are classi-fied as class II YSOs, these are pre-main-sequencestars with warm optically thick dusty disks or-biting around them. We define class III objectsas having −1.6 > αIRAC > −2.56: these starshave cleared most of their circumstellar environ-ment, their near-IR emission is mostly photo-spheric but they may present some excess emis-sion above 20µm. Such objects are dubbed ane-

mic disks by Lada et al. (2006). Finally, objectswith αIRAC < −2.56 are stars with photosphericemission only, this spectral index corresponds tothe slope of the stellar photosphere SED in theRayleigh-Jeans domain.

Note that this YSO classification is based on theclassification scheme proposed by Greene et al.(1994), except that the Flat Class (0.3 > α >−0.3) is included into the class 0/I which repre-sents the population of protostars with infallingenvelopes. Calvet et al. (1994) indeed showed thatFlat Class YSOs could be interpreted as infallingenvelopes. Figure 5 presents a color-color diagrambased on IRAC data only, the different symbolsindicate the classes associated with each YSO can-didate identified in our analysis.

The next step is to exploit the extra informa-tion contained at longer wavelengths to confirm,or reclassify, the YSO candidates identified solelyfrom their IRAC colors. We first check that theSEDs of all YSO candidates continue to rise from 8to 24µm. We also look for objects previously clas-sified as photospheric sources that exhibit bright24µm fluxes ([5.8]− [24] > 1.5). These sources arethought to be transition disks, i.e. class II YSOswith significant dust clearing from their innerdisks (Calvet et al. 2002; D’Alessio et al. 2005),

10

Table 1

Zeropoints used for Flux-to-Magnitude Conversions.

2MASS IRAC MIPS

Band J H K [3.6] [4.5] [5.8] [8.0] [24] [70]

Zeropoint [Jy] 1594 1024 666.7 280.9 179.7 115.0 64.13 7.17 0.778

Note.—2MASS zeropoints are from Cohen et al. (2003), IRAC from Reach et al. (2005) andMIPS from Rieke et al. (2008).

these objects are of particular importance for un-derstanding the evolution of circumstellar disksaround young stars. We also check for protostarmisclassifications due to extreme visual extinctionlevels. Gutermuth et al. (2008) suggests that if aprotostar that has MIPS detection does not meetthe criterion [5.8]− [24] > 4 (if they possess [5.8]photometry) or [4.5]− [24] > 4, then it is likely ahighly reddened class II YSO. Finally, we requirethat any sources lacking detections in some IRACbands yet having bright 24µm fluxes ([24] < 7 and[X ]− [24] > 4.5 mag, where [X ] is the photometryfor any IRAC detection available in our catalog)have to be added to the list of likely YSOs, and tobe classified as highly embedded protostars. Weapply the 24µm-based color constraints on YSOcandidates only if their MIPS detection reliabilityis > 95 % (S. Shenoy, private communication);thus we require that, for the MIPS photometry tobe relevant for the YSO selection, the 24µm mag-nitude is [24] < 8.6 mag and that its associatederror is σ[24] < 0.2 mag.

We tested the ability of the method at find-ing genuine YSOs on a well studied star form-ing region, RCW79 (Zavagno et al. 2006), andit proved to be adequately reliable and efficient(Zavagno, private communication). Furthermore,this method has been successfully applied tostudy embedded stellar clusters in NGC 1333(Gutermuth et al. 2008), the star formation activ-ity in the HII region W5 (Koenig et al. 2008) or inthe giant molecular cloud G216-2.5 (Megeath et al.2009). Note that other methods, also based onSpitzer colors and magnitudes, have been devel-oped to find YSO candidates (e.g. Hartmann et al.

2005; Harvey et al. 2007; Robitaille et al. 2008;Chavarrıa et al. 2008).

4.3. Final census of YSO candidates

We finally find 856 YSO candidates in VulOB1:239 are likely protostars with infalling envelops(class 0/I), 464 are disk-bearing stars (class II),and 153 are class III objects with very little cir-cumstellar material. Among the class I objects,15 YSO candidates are likely deeply embeddedprotostars, and 89 have a spectral index consistentwith the Flat Class. Among the class II objects,85 are highly reddened class II YSOs that weremisclassified as protostars based on IRAC criteriaonly. A further 21 sources are classified as likelytransition disk class II objects.

Except for a few cases, all YSO candidates havedetections in all four IRAC bands, about half ofthem have 2MASS detections in all three JHKbands, 75% have 24µm counterparts (it reaches100% for class 0/I objects) and 8% have 70µm de-tections. Table 2 gives the coordinates and fluxesof all YSO candidates. Table 5 in the appendicesgives the coordinates, fluxes and type of the con-

taminants identified in VulOB1.

To test the robustness of our analysis relative tothe interstellar extinction, we have used the visualextinction map of VulOB1 and the formulae pro-vided by Flaherty (2007) to deredden IRAC fluxes.We run the exact same procedure as describedabove, and we find that the YSO census is onlymarginally different from the uncorrected case (ex-cept for the class III population that is reduced by35%). This justifies the choice of [5.8]-[8.0] as anextinction-free indicator for finding YSOs.

11

Fig. 5.— Left panel : IRAC color-color diagram of point sources found in the Vulpecula region. Stars withphotospheric emission are marked as black dots in the figure, they have null colors in IRAC bands so theyaggregate around point (0,0). Color- and symbol-coded YSO candidates protrude from the bulk of the stellarsources towards the positive color quadrant due to their IR-excess emission. The reddening vector is derivedfrom the extinction law of Flaherty (2007). Right panel : Color-Magnitude diagram of point sources basedon MIPS and IRAC bands. (A color version of this figure is available in the online journal.)

Table 2

List of YSO candidates and their 2MASS, IRAC and MIPS photometry.

Galactic 2MASS IRAC MIPS

GLIMPSE source name Glon Glat J H Ks [3.6] [4.5] [5.8] [8.0] [24] [70] Class

SSTGLMC G060.3261-00.6407 60.3261 -0.6407 ... ... ... 8.68 8.19 7.74 7.07 4.55 ... IISSTGLMC G060.1815-00.5632 60.1816 -0.5632 12.71 10.61 9.45 8.31 8.22 7.89 7.42 7.16 ... IIISSTGLMC G060.3304-00.6042 60.3304 -0.6042 ... ... ... 12.99 11.40 10.71 10.14 4.78 -5.65 0/ISSTGLMC G060.3331-00.6065 60.3331 -0.6065 16.04 14.38 13.41 11.79 10.83 10.12 9.09 5.89 ... 0/ISSTGLMC G060.2117-00.6700 60.2118 -0.6700 14.69 13.61 13.05 12.30 11.62 11.24 9.95 ... ... IISSTGLMC G060.2898-00.6427 60.2898 -0.6427 ... ... ... 13.14 12.17 11.44 10.74 6.98 ... 0/ISSTGLMC G060.2868-00.6351 60.2868 -0.6351 ... ... ... 12.90 11.92 11.17 10.32 7.65 ... 0/ISSTGLMC G060.1319-00.5355 60.1320 -0.5355 10.84 8.99 8.01 7.30 7.24 6.94 6.70 5.44 ... IIISSTGLMC G060.2180-00.6362 60.2180 -0.6362 10.68 8.79 7.86 7.24 7.28 6.85 6.55 5.27 ... IIISSTGLMC G060.1975-00.6052 60.1975 -0.6052 ... ... ... 13.63 12.32 11.28 10.38 7.24 ... 0/I

Note.—Galactic coordinates are in units of degrees (◦). Table 2 is published in its entirety in the electronics edition of theAstrophysical Journal. A portion is shown here for guidance regarding its form and content.

12

4.4. Reliability and completeness

In the present study, we make the deliberatechoice to favor the highest possible detection re-liability, at the expense of a modest completenessfigure. This choice was largely motivated by theneed to automate the search for YSOs over such alarge area of the sky.

As mentioned in Section 4.1, we achieve a de-tection reliability better than 98% for the set ofIRAC magnitude cuts chosen for populating ourinitial catalog. However the actual detection re-liability depends to a certain extent on the levelof the background diffuse emission. An adequateassessment of the completeness of our YSOs cat-alog would require to carry out a large spectro-scopic survey of the red objects in VulOB1 inorder to identify genuine YSOs, and then com-pare the results to our catalog. This is impracticalthough, given the large number of (faint) objectsin VulOB1.

Considering the stringent criteria used to reacha reliability over 98%, we expect a noticeablenumber of YSOs to be excluded from our initialpoint source catalog. For instance, any saturatedsources, that are either bright sources on faintbackground or faint sources on bright background,are not considered in our analysis. There are veryfew saturated data in IRAC images so that theYSOs selection is basically unaffected by this ef-fect. Still, we expect that a fraction of YSO candi-dates will not have 24µm counterparts because ofsaturation issues (especially around Sh2-88B andSh2-87 where the background is high). Extendedsources are also excluded based on the selectionprocedure used in this work (see Section 4), buttheir actual number is expected to be fairly smallat 2.3 kpc. Point sources with too large a magni-tude error are not considered either. We set thelimit for bad photometry to be σmag > 0.2 mag.IRAC bands 3 and 4 being the noisiest bands, theselection criterion on σmag mostly impacts [5.6]and [8.0] detections.

The completeness of our YSO catalog is alsodependent on the evolutionary stage of the ob-ject. For instance, the wavelength band 2-24µm iswell suited for detecting and identifying disksaround young stars (class II), but longer wave-lengths are more appropriate for observing early-stage objects (class 0/I). We believe that our sam-

ple of IRAC-selected YSO candidates may lack asignificant fraction of these protostars. For in-stance, we looked for a possible correlation be-tween the spectral index αIRAC and the visualextinction of the YSO candidates expecting tofind a population of protostars with high valuesof αIRAC, i.e. the youngest objects, to be themost extinct/embedded objects. However we didnot find such correlation in our sample which sug-gests that we might be missing the most embed-ded phases of star formation based on IRAC andMIPSGAL 24µm data only. Similarly, the popu-lation of class III YSOs which lacks near-IR excessemission is strongly underestimated in our study,even if they are genuine YSOs possessing Hα or X-ray emission. For all these reasons, it is very diffi-cult to assess the completeness of our YSO catalogwith confidence. We use the evolutionary modelsof Baraffe et al. (1998) to compute the mass of a2 Myr old star at 2.3 kpc with no IR-excess emis-sion having a 3.6µm magnitude of 14 mag; andwe find a mass of ∼1 M⊙. Considering that the8µm channel of IRAC is the less sensitive chan-nel (limit magnitude of 10.8 mag), and that mostYSOs have significant IR-excess emission, we esti-mate our YSO catalog to be complete down to afew solar masses.

Lastly, we have to account for the possiblemisidentification of YSO candidates with evolvedstars. The circumstellar environment of Plane-tary Nebulae (PNe) or Asymptotic Giant Branch(AGB) stars are rich in warm dust grains emittingin the near- mid-IR such that these objects occupythe same locus in color-color diagrams as YSOs(Hora et al. 2008; Srinivasan et al. 2009). Follow-ing the analysis of Robitaille et al. (2008), we es-timate the fraction of evolved stars in our cata-log of YSO candidates to be about 25%, of whichmost are AGB stars. We will argue in Section 5.2that AGB stars could be further excluded fromthe sample of YSO candidates based on clusteringcriteria.

5. Results and discussion

5.1. Star Formation Efficiency

We estimate the current star formation effi-ciency in Vulpecula by comparing the mass ofthe gas reservoir, Mcloud, with the mass that hasturned into stars during the last few million years,

13

MYSO. The Star Formation Efficiency (SFE) isthen derived as follows:

SFE =MYSO

MYSO +Mcloud(1)

We compute the mass of the cloud using our ex-tinction map of Vulpecula and the formula relat-ing the column density and the visual extinctionNH/AV = 1.37 × 1021 cm−2.mag−1 assuming avalue of RV = 5.5 typical for molecular clouds assuggested in Evans et al. (2009). We use a contourof AV = 7mag to delineate the active star formingregion on the AV map of Vulpecula. We find anaverage mass surface density of 16.2 M⊙.pc

−2. Weintegrate the surface density over the 2.7×103 pc2

enclosing the AV > 7 region, and we find a cloudmass Mcloud = 4.5× 104 M⊙.

Since our YSO sample represents only the high-end of the Initial Mass Function (IMF), as we es-timated in Section 4.4, we need to account for themissing mass of the total YSO population in or-der to derive a relevant MYSO. We use the IMFof Kroupa (2001) to compute the number of starswith masses ranging from 0.01 to 50 M⊙. We nor-malize the IMF assuming 510 stars have masses>1 M⊙ within the AV > 7 region, and we findthat VulOB1 should contain 2× 104 YSOs with atotal mass of MYSO = 4200 M⊙. We thus obtain aSFE of ∼8%, similar to other star forming regionsin nearby molecular clouds (Evans et al. 2009).

5.2. Spatial distribution of YSO candi-

dates

The spatial distribution of YSO candidates inVulOB1 is presented in Figure 6. YSO candi-dates are not distributed randomly in the field. Werather find several coherent structures, or groupsof YSO candidates, which in most cases are rem-iniscent of the mid-infrared morphology of thecomplex. For instance the highest densities ofYSO candidates are located close to, or withinthe three HII regions. Other YSOs appear to lineup along InfraRed Dark Clouds (IRDC), e.g. at(l, b) = (59.98, 0.06) at the pedestal of the pillarVulP5 (see Figure 4); or along bright contrastedstructures of the extended 24µm emission, e.g. at(l, b) = (61.82, 0.33) or (l, b) = (59.80, 0.64). YSOcandidates have also been identified at the tip ofmost newly discovered pillar structures (see Ta-ble 4 and Sections 3.2 and 5.4.2).

If we assume the IMF parameters presentedin Section 5.1, we estimate that the average sur-face density of YSO candidates is 7.4 YSO.pc−2.This value is consistent with the typical surfacedensity found in other star forming regions, e.g.13.0 YSO.pc−2 in Serpens (Harvey et al. 2007) or3.3 YSO.pc−2 in Lupus (Merın et al. 2008). Webuild a surface density map of YSO candidatesfollowing the definition of Chavarrıa et al. (2008).At each point of a 5′′ grid, we compute the surfacedensity of YSOs as

σ =N

πr2N

,

where rN is the distance to the N = 5 nearestneighbor. This image is then convolved with agaussian of FWHM = 1′ to obtain a smooth con-tour map of the surface density. Figure 6 showsthe surface density contours over a 70µm im-age of VulOB1. It clearly delineates the mid-IR-bright HII regions, plus other groups of YSOcandidates associated with fainter compact mid-IR sources, e.g. at (l, b) = (60.60,−0.70). Thesurface density peaks at ∼50 YSO.pc−2 at theposition (l, b) = (59.80, 0.064) which is coinci-dent with the BLAST source V30, also known asIRAS 19410+2336. This source is a candidate Hy-per Compact HII region according to Chapin et al.(2008), and about 15 of our YSO candidates ap-pear to be grouped around it (see Section 5.4.2).

In addition to this population of clustered YSOcandidates, there is a population of distributed IR-excess sources. Robitaille et al. (2008) argue thata fraction of these isolated red objects could beAGB stars mistaken for YSO candidates in our se-lection process (they occupy the same locus in theC-C diagram, cf Section 4). However Koenig et al.(2008) argue that the isolated YSO candidatescould be genuine YSOs that formed from isolatedevents or that were ejected10 from their birthplacedue to gravitational interactions with other mem-bers of their native star cluster.

We now focus our analysis on the popula-tion of clustered YSO candidates since they aremore reliable tracers of triggered star formation

10Assuming a relative velocity of 3 km s−1 between a youngstar and its parental cloud, a YSO could travel 6 pc inless than 2 Myr. This explains how genuine YSOs cancontribute to the distributed population in star formingregions.

14

events. Various methods can be used to discrim-inate between distributed/clustered populations.For instance the two-point correlation functionwas used by Karr & Martin (2003) on W5 andby Indebetouw et al. (2007) on M16, while theminimum spanning tree technique was used onW5 by Koenig et al. (2008) and on NGC 1333 byGutermuth et al. (2008). Both methods providevaluable qualitative results but dynamical mea-surements are absolutely necessary to assign adefinitive membership to a given YSO. Since suchdata are not available for VulOB1 we opt for thesimplest filtering, namely the nearest neighbor, togain insight into the clustering properties of ourYSO sample. In practice, we consider all YSOcandidates and keep only those that have theirnearest neighbor YSO within 3′ (2 pc at 2.3 kpc).The result of this selection is a much more clus-tered distribution of YSO candidates that tracesmid-IR morphology even more closely (192 YSOcandidates are marked as isolated objects).

15

Fig. 6.— Distribution of YSO candidates in VulOB1 displayed over the 70µm negative image from MIPSGAL. Red circles representenvelops/Class I YSO candidates, green horizontal rectangles represents disks/Class II objects, and blue vertical rectangles are opticallythin disks/Class III objects. Black contours represent the surface density of YSO candidates in VulOB1 as described in the text. Theirdistribution is well correlated with the mid-IR morphology. They are found at the tip of most pillars and around HII regions. The largeand small dashed circles around (l, b) = (59.58, 0.12) indicate the diameter of the supernova remnant SNR G59.5+0.1 as published inTaylor et al. (1992) and Green (2006) respectively. The orientation and scale of the image are also indicated.

16

We also look for trends in the environment ofYSO candidates compared to field stars. The toppanel of Figure 7 shows that YSO candidates sitpreferentially on regions of bright 24µm back-ground emission compared to the more evolvedstars. Since the 24µm emission is mostly due tothe thermal emission of small dust grains excitedby UV radiation, the trend for YSO candidatesto sit on bright mid-IR background implies thatthe identified YSO candidates are largely associ-ated with photon dominated regions (PDR) andHII regions. This is consistent with known sce-narios of triggered star formation mechanisms andwith previous studies of star forming regions (e.g.Koenig et al. 2008). We also compare the visualextinction associated with the YSO population tothe entire population of point sources found in ourinitial catalog (see bottom panel of Figure 7). Wefind that YSO candidates are mostly located inregions of high extinction compared to field stars.This is again consistent with the idea that infantstars still live in the dense and dusty cloud fromwhich they were born. Remarkably the environ-mental trends mentioned above are even more pro-nounced for the population of clustered YSO can-didates (see Figure 7).

Note that Chapin et al. (2008) identified a fewBLAST sources in the direction of VulOB1 thatare actually located at 8.5 kpc from the Sun (radialvelocity around −5 km s−1), i.e. in the Perseusarm. A dozen of our YSO candidates seem to beassociated with these distant BLAST sources sothat these might belong to the Perseus arm.

5.3. Spectral Energy Distribution

We have built Spectral Energy Distributions(SEDs) for all of our YSO candidates. Figure 8presents a small SED sample with the Class des-ignation associated with each object.

We used the SED fitter tool developed byRobitaille et al. (2007) to extract the physical pa-rameters of our YSO candidates. However, thelimited wavelength coverage (near- to mid-IR)resulted in strong degeneracies over the outputparameters, and we were unable to gain reliableinformation on the physical properties of our YSOcandidates.

We looked for possible associations in theBLAST field to extend the wavelength coverage

Fig. 7.— Histograms of 24µm background bright-ness (upper panel) and visual extinction (lowerpanel) associated with each point sources of ourinitial catalog. YSO candidates appear to sit pref-erentially on high 24µm background as well as inhigh extinction, i.e. embedded, regions. This iseven more pronounced for clustered YSO candi-dates. (A color version of this figure is availablein the online journal.)

17

to the sub-millimeter, but in most cases severalYSO candidates fell within the BLAST beamrendering the association ambiguous. For theYSO candidates that could be uniquely associ-ated with a BLAST source, Chapin et al. (2008)provide physical parameters derived from SEDfitting. They find clump masses ranging from 40to 400 M⊙ and temperatures from 19 to 28 K.We also use Molinari et al. (2008a) diagnostic di-agrams based on MIPS [24-70] and BLAST [250-500] colors to identify the most massive YSOs inVulOB1. Molinari et al. show that the high-massanalogues of the low- or intermediate-mass Class 0objects have distinctive MIPS colors. From oursample of YSO candidates, we find 6 objects with[24− 70] > 3, which is indicative of an SED peak-ing longwards of 70µm presumably due to a mas-sive infalling envelop. Three of these red MIPSsources are located next to the brightest BLASTsource V30 and are barely discernible on the map,two have no BLAST counterparts, and one mighthave a BLAST counterpart but the association isambiguous as another YSO candidate falls withinthe BLAST beam.

Finally we searched the IRSA for Bolocamsources, and we found 24 sources in VulOB1.Among them we find isolated sources not asso-ciated with any YSO candidates, these could beYSOs in their earliest evolutionary phases, i.e.starless cores emitting in the millimeter only. Theother Bolocam sources are usually located close tothe peaks of the YSO surface density and are asso-ciated with groups of YSO candidates. Note thatalmost all Bolocam sources have BLAST counter-parts over the Vulpecula BLAST field.

5.4. Cases of triggered star formation?

5.4.1. The case of SNR G59.5+0.1

Shock waves generated by supernovae can po-tentially trigger episodes of star formation atthe interface between the SNR and the ISM(Melioli et al. 2006). The supernova remnantSNR G59.5+0.1 was first detected by Taylor et al.(1992) in the direction of VulOB1 at the posi-tion (l, b) = (59.58, 0.12). Taylor et al. describethis object as a shell-type SNR with non-thermalspectral index (α < −0.4) and a diameter of 15′.They also mention the close proximity and pos-sible association of the SNR with the HII region

Sh2-86. We investigate the possibility of such anassociation and potential signs of triggered starformation.

Guseinov et al. (2003) computed the distanceto SNR G59.5+0.1 using an empirical formula thatrelates the surface brightness of a SNR (Σ) withits diameter (D). They calibrated the Σ −D re-lation against SNRs of known distances, and theyfound that SNR G59.5+0.1 lies at ∼11 kpc fromthe Sun. This would imply that SNR G59.5+0.1is not associated with the HII region Sh2-86. Westress however that Guseinov et al. computed thedistance to G59.5+0.1 based on data from theSNR catalog of Green (2006) (Σ1GHz = 18.1 ×

10−21 W.m−2.Hz−1.sr−1 and D = 5′ from obser-vations at 1720 MHz), whereas Taylor et al. re-port different values from observations at 327 and4850MHz (Σ1GHz = 0.7×10−21 W.m−2.Hz−1.sr−1

and D = 15′). We use the VGPS 21-cm datato look for traces of the SNR and settle thisdiscrepancy. We find a circular structure cen-tered at the position of the SNR with a diam-eter of 15′, which confirms the value found byTaylor et al.. Therefore we compute the distanceto SNR G59.5+0.1 using Guseinov et al.’s for-mula with Taylor et al.’s measurements insteadof Green’s; and we find that the SNR likely liesbetween 2.1 kpc and 5.3 kpc from the Sun pro-vided the given uncertainties of the Σ−D relation.This distance estimate reconciles the hypothesisthat SNR G59.5+0.1 is indeed associated withSh2-86.

Reach et al. (2006) have searched for infraredcounterparts to the known SNRs in the innergalactic plane using IRAC observations from theGLIMPSE survey, however they did not detectSNR G59.5+0.1 because of the high confusionlevel in the vicinity of Sh2-86 structured extendedemission. For the same reason, we could not de-tect the SNR on MIPSGAL 24µm images either.No morphological clues were found in the ancil-lary data mentioned in Section 2.2. Neverthe-less, the distribution of YSO candidates aroundSNR G59.5+0.1 reveals two groups of YSOs ap-parently lining up along two arcs at the northernand southern rims of SNR G59.5+0.1 (cf Figure 6).Assuming that the actual diameter of the SNR isindeed 15′, the two groups do not lie exactly onthe rim of the shell but slightly outside. We stillbelieve that these YSO overdensities could have

18

Fig. 8.— Sample of SEDs from our catalog of YSO candidates. Almost all YSO candidates have detectionin all four IRAC bands, 75% have detections at 24µm, 6% at 70µm and about half in J, H and K bands.Our classification of each YSO candidates is shown on individual plots. The axis scale is the same for eachsubplot.

arisen from the interaction of the expanding shellof SNR G59.5+0.1 with the neutral gas of Sh2-86. According to Xu et al. (2005), the age of ashell-type SNR such as SNR G59.5+0.1 with a di-ameter of 10 pc (assuming a 15′ diameter and adistance of 2.3 kpc) ranges from 103 to 104 yearsold. This timescale is comparable to the lifetime ofthe first phases of star formation, making the SNRa possible trigger for the surrounding YSO candi-dates. These clues tend to confirm the associationof SNR G59.5+0.1 with Sh2-86, but since we couldnot find unequivocal evidences for the SNR-HII re-gion association, a fortuitous spatial coincidencecannot be excluded.

5.4.2. Photoevaporation at the tip of the pillars

An interstellar cloud exposed to the ionizing ra-diation of a newly formed star is compressed byan ionization-shock front which can focus the neu-tral gas into a compact globule. This mechanismis called radiation-driven implosion (RDI) and isdescribed in Bertoldi (1989). In some cases, thisleads to the birth of a second generation of starslocated at the interface of the ionized and neutral

gas (e.g. Reach et al. 2009).

Even though we could not identify the sculptorof the pillars described in Section 3.2, we expectthe RDI mechanism to be at work in these objects.Figure 4 shows the location of our YSO candidatesin pillars VulP1 to VulP6 and VulP11. We find atleast one YSO candidate at the tip of most of thepillars (see Figure 3 and last column of Table 4).YSO candidates can also be found along the pillarswhen the gas is protruding and exposed to theionizing star (e.g. YSO on the West side of VulP3on Figure 4). BLAST also detected sources atthe tip of every pillar found within the VulpeculaBLAST field (e.g. bottom panel of Figure 4).

5.4.3. Embedded star cluster

Cr404 is a young (9 Myr old, Bica et al. 2008)star cluster located at (l, b) = (59.14,−0.11) em-bedded in Sh2-86. We find a high surface den-sity of YSO candidates associated with this clus-ter. 11 Class II-III and 3 Class I YSO candi-dates surround the mid-IR peak emission of Cr404.BLAST observations further reveal that the em-bedded cluster is coincident with the bright sub-

19

millimeter source V18, also identified as IRAS19403+2258. Chapin et al. (2008) classify thissource as a candidate Hyper Compact (HC) HII

region, i.e. the precursor of a high-mass star, andthey derived a mass of ∼150 M⊙ and a tempera-ture of ∼30 K for this object. The simultaneouspresence of a HC HII region (∼ 105 yr lifetime)and Class II YSOs (few 106 yr lifetime) around ayoung star cluster such as Cr404 is consistent withthe results of Molinari (2008b) which suggests thatthe most massive objects in a cluster are the lastones to form.

Other cases of HC HII regions associated withgroups of Spitzer -selected YSO candidates arefound in the BLAST field. For instance, theBLAST source V30, the brightest and most mas-sive clump according to Chapin et al. (2008) iscoincident with the highest density of YSO can-didates in the whole 6.6 square degree map at(l, b) = (59.80, 0.064).

5.4.4. Large scale propagating Star formation

Based on radio observations of HI supershellsand on energetics arguments, Ehlerova et al.(2001) estimate that the HI shell GS061+00+51,which encompasses Sh2-87 and Sh2-88, is 6-7times larger and 3-4 Myr older than the shellGS59.7-0.4+44 that hosts Sh2-86. Ehlerova et al.attribute these age and size differences to the de-layed expansion of HII regions, and they suggestthat star formation might be propagating fromSh2-88 to Sh2-86. Previously, Turner (1986) sug-gested that the morphology of the three HII re-gions was shaped by a single supernova shock waveassociated with the fossil HII region Lynds 792(l, b) = (60.81, 2.24). Since Lynds 792 is ap-proximately equidistant from the three HII re-gions, Turner’s scenario implies that star forma-tion would be triggered simultaneously, as opposedto sequentially in the case of Ehlerova et al. Wenow test the validity of these scenarios using thesupplemental information gathered in this work.

Figure 9 shows the evolution of the number ofYSO candidates as a function of their Galacticlongitude. The increasing number of YSO candi-dates towards Sh2-86 seems to favor the scenarioof Ehlerova et al. since the youngest star formingregion should indeed possess the largest numberof young stellar objects, assuming identical IMFsand instantaneous star formation. Nevertheless

the shape of this histogram could also be inter-preted as Sh2-86 being a more active star formingregion than the other two Sharpless objects, cre-ating more YSOs from a larger molecular reser-voir, which is unrelated to any process of propa-gating star formation. For a proper interpretationof the Figure 9 histogram, we need to compare thetypical lifetime of a YSO and the timescale neces-sary for star formation to propagate in a molecularcloud.

Nomura & Kamaya (2001) have studied self-propagating star formation using numerical sim-ulations and found that the time delay of se-quential star formation sites against the origi-nal one, ∆t, correlates with their physical sep-aration, ∆x, as ∆t ∼ 50Myr[∆x/(0.5kpc)]0.5.Efremov & Elmegreen (1998) have derived a sim-ilar expression based on observations of starclusters in the Large Magellanic Cloud (∆t ∼

26Myr[∆x/(0.5kpc)]0.4). Assuming Sh2-88 andSh2-86 are separated by ∼80 pc, we find that starformation would take ∼10-20 Myr to propagateacross VulOB1. Besides, Evans et al. (2009) es-timate the lifetime of a YSO to be of the orderof 1-3 Myr based on a large statistical sample ofYSOs identified as part of the c2d legacy survey11.If star formation was indeed propagating along theGalactic plane at the pace derived above, YSOswould only be found in a narrow slab of longi-tudes since they would age, and fade away in theIR, faster than the triggering shock needs to crossthe molecular complex. We can already rule outthis scenario based on the presence of YSOs in allthree HII regions of VulOB1 (see Figure 6). It ispossible however that star formation is propagat-ing faster than previously estimated due to thepronounced inhomogeneities of the propagationmedium (cf distribution of CO in Figure 1). If thepropagation timescale was of the same order ofthe YSOs lifetime, then YSOs would be found allacross VulOB1 with a gradient in the evolution-ary phases of the YSO population as a function oflongitude.

We use the ratio of Class II-III to Class 0-I as an indicator of the aging of the YSO pop-ulation. Figure 9 shows that this ratio doesnot present a gradient with respect to longitudes.This indicates that if star formation was once

11http://irsa.ipac.caltech.edu/data/SPITZER/C2D

20

triggered in VulOB1, then the triggering agentwould have been independent of Galactic longi-tude, and this definitely rules out the suggestionof Ehlerova et al. Nonetheless, the histogram ofFigure 9 is still consistent with Turner’s scenarioin which star formation occurs as an instantaneousburst in the three HII regions.

6. Conclusions

We have presented a thorough description ofthe Vulpecula OB association. We have comple-mented the existing observations of isolated ob-jects in VulOB1 with a global view of the wholestar forming complex from an infrared perspec-tive. We exploited Spitzer legacy surveys MIPS-GAL and GLIMPSE data to identify 856 youngstellar objects with IR-excess emission. We relyon the nature and properties of these objects tohighlight the recent activity of star formation inthe complex, and we look for evidences of triggeredstar formation.

We find two populations of YSO candidates:one population of distributed objects that likelycontains IR-bright evolved stars and some gen-uine YSOs born in isolation or ejected from theirparental cloud; and another population of clus-tered YSO candidates whose spatial distributioncorrelates very well with the mid-IR morphologyof the complex. YSO candidates surface den-sity peaks locally around the three Sharpless ob-jects, the Hyper Compact HII regions, and otherembedded star clusters like Cr404. The vigor-ous star forming activity around these energeticsources is consistent with scenarios of triggeredstar formation mechanisms. Still we cannot as-certain that these stars were born as a direct con-sequence of their extreme environment, nor thatthey would have formed in the absence of whatwe consider the triggering agents. Neverthelessour analysis allowed to rule out the scenario ofEhlerova et al. (2001) according to which star for-mation was propagating from Sh2-88 to Sh2-86.We rather find that the evolutionary stage of theYSO population across VulOB1 is homogeneous,and thus consistent with the scenario of Turner(1986).

We have further reported the discovery of adozen pillar-like structures in VulOB1, and wecomment on their morphology from the near-IR

Fig. 9.— Top panel : Distribution of class 0/Iand II/III candidates as a function of Galacticlongitude (bin size ∼15′). The number of youngstellar objects increases towards lower longitudes.Bottom panel : Evolution of the class II/III toclass 0/I ratio as a function of longitude. Thenumber of class 0/I and II/III in each bin is indi-cated on the figure. (A color version of this figureis available in the online journal.)

21

to the radio regime. We were not able to identifythe energetic source(s) responsible for the moldingof the pillars, but we argue that these objects areindeed associated with the OB association. Ourfinding of YSO candidates at the tip of most ofthe pillars is consistent with mechanisms of trig-gered star formation on small scales.

The authors would like to thank S. Bontempsfor providing the extinction map of VulOB1. Thiswork is based on observations made with theSpitzer Space Telescope, which is operated by theJet Propulsion Laboratory, California Institute ofTechnology under a contract with NASA. Thisresearch used the facilities of the Canadian As-tronomy Data Centre operated by the NationalResearch Council of Canada with the support ofthe Canadian Space Agency. The National Ra-dio Astronomy Observatory is a facility of the Na-tional Science Foundation operated under cooper-ative agreement by Associated Universities, Inc.The Virginia Tech Spectral-Line Survey (VTSS)is supported by the National Science Founda-tion. This research has made use of the SIMBADdatabase, operated at CDS, Strasbourg, France.This research has made use of the NASA/ IPACInfrared Science Archive, which is operated bythe Jet Propulsion Laboratory, California Insti-tute of Technology, under contract with the Na-tional Aeronautics and Space Administration.

Facilities: Spitzer Space Telescope

22

A. Appendix material

23

Table 3

Pillars identification, coordinates and approximate size.

Galactic Equatorial

Pillar ID l b RA (J2000) Dec (J2000) Size Position angle

VulP1 60.18 -0.31 296.36 23.88 5-8 × 0.5 250VulP2 60.03 -0.35 296.32 23.74 1 × 0.5 230VulP3 59.97 -0.31 296.25 23.70 8 × 2 260VulP4 59.81 -0.29 296.15 23.58 3 × 1 268VulP5 60.06 0.10 295.91 23.99 4 × 2 335VulP6 60.11 0.21 295.83 24.10 5 × 2-4 350VulP7 60.38 0.21 295.98 24.33 9 × 1-4 17VulP8 60.90 0.22 296.26 24.78 11 × 4 55VulP9 61.10 -0.45 297.00 24.61 4 × 1 158VulP10 60.77 -0.63 297.00 24.24 4 × 2 190VulP11 60.21 -0.45 296.52 23.84 4 × 2 280VulP12 59.60 -0.14 295.90 23.47 3 × 1 133VulP13 59.51 -0.22 295.92 23.36 3 × 0.5 160VulP14 59.49 -0.19 295.88 23.35 2 × 0.5 150

Note.—Coordinates and position angles are in units of degrees. Pillar sizes are in units ofarcminutes. Position angles are given between the long axis of the pillar and the West axis,positive values are towards the North.

24

Table 4

Morphological description of the pillar structures identified in VulOB1.

MIPS IRAC

Pillar ID [70] [24] [8.0] [5.8] [4.5] [3.6] YSOa

Bright PillarVulP1

Compact SourceStructured Structured Faint None Faint Y

VulP2 Faint Faint Faint None None None N

Bright Pillar Bright BrightVulP3

Compact Source Structured StructuredFaint Faint Faint Y

VulP4 Faint Tip Faint Faint None None None Y

Faint Pillar Bright Pillar Bright Pillar No Pillar No Pillar No PillarVulP5

Pedestal Pedestal Pedestal Faint Pedestal Faint Pedestal Faint PedestalY

VulP6 Faint Faint Faint None None None Y

Faint Bright FaintVulP7

Structured Structured StructuredNone None None Y

Faint FaintVulP8 Faint Tip

Structured StructuredNone None None Y

VulP9 Faint Bright tip Bow Shock Faint Tip None Faint N

Bright TipVulP10 Faint

Possible JetsFaint None None None Y

VulP11 Bright Tip Bright Tip Bright Tip Faint Tip Faint Tip Faint Tip N

Bright Tip Bright Tip Bright TipVulP12

Pedestal Pedestal PedestalFaint Faint Faint Y

VulP13 Faint Bright Edges Bright Edges None None None Y

Bow ShockVulP14 Bright Tip Bright Tip

Bright TipBow Shock Bow Shock Bow Shock Y

aIndicates the presence of a YSO candidate at the tip of the pillar.

Table 5

List of point sources excluded from the YSO catalog and identified as probable

contaminants based on their IRAC fluxes.

Galactic 2MASS IRAC MIPS

GLIMPSE source name Glon Glat J H Ks [3.6] [4.5] [5.8] [8.0] [24] [70] Type

SSTGLMC G059.6291-00.8301 59.6291 -0.8301 12.97 12.59 12.35 11.96 11.63 11.41 9.84 ... ... GalaxySSTGLMC G059.7319+00.2144 59.7320 0.2144 12.95 12.47 12.23 11.98 11.71 11.68 10.23 7.77 ... GalaxySSTGLMC G059.9431+00.1783 59.9432 0.1783 ... 15.06 14.19 13.02 12.40 11.68 9.76 6.19 ... GalaxySSTGLMC G060.9219-00.1041 60.9220 -0.1041 14.37 13.44 12.98 12.15 11.87 11.83 10.53 ... ... GalaxySSTGLMC G061.1282-00.7549 61.1282 -0.7549 13.60 12.59 12.32 12.11 12.14 11.77 10.63 ... ... GalaxySSTGLMC G061.6453-00.7439 61.6454 -0.7439 ... ... ... 12.19 11.91 11.79 9.53 ... ... GalaxySSTGLMC G060.0154+00.1111 60.0154 0.1111 ... ... ... 12.88 10.83 10.01 9.56 ... -4.96 ShockSSTGLMC G059.6940+00.1840 59.6940 0.1840 ... ... ... 12.80 11.62 11.14 10.69 ... -4.82 ShockSSTGLMC G059.7893+00.6298 59.7893 0.6298 ... ... ... 12.61 10.75 10.19 9.27 ... -4.81 ShockSSTGLMC G059.3780-00.2438 59.3781 -0.2438 ... ... 14.84 12.76 11.23 10.63 10.30 3.14 -4.77 ShockSSTGLMC G059.7973+00.0750 59.7973 0.0750 ... ... ... 13.22 11.04 11.11 10.75 ... ... ShockSSTGLMC G059.6366-00.1864 59.6366 -0.1864 ... ... 14.73 11.48 9.56 8.79 8.33 3.13 ... ShockSSTGLMC G058.6970+00.6316 58.6971 0.6316 ... ... ... 12.88 11.41 10.80 9.70 3.17 -4.15 ShockSSTGLMC G059.0385-00.2512 59.0386 -0.2512 ... 14.47 13.19 11.32 10.15 9.89 8.85 3.67 ... Shock

Note.—Coordinates are in units of degrees (◦).

25

Fig. 10.— Composite image of the HII region Sh88B as seen by Spitzer (blue, green and red are [3.6], [5.8]and [8.0] respectively).

26

Fig. 11.— Composite image of the HII region Sh86 as seen by Spitzer (blue, green and red are [5.8], [8.0] and [24] respectively).

27

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