Young Stellar Object Candidates Research Articles

YOUNG STELLAR OBJECTS AND TRIGGERED STAR FORMATION IN THE VULPECULA OB ASSOCIATION

The Vulpecula OB association, VulOB1, is a region of active star formation located in the Galactic plane at 2.3 kpc from the Sun. Previous studies suggest that sequential star formation 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 search for signatures of past triggering events. We make a census of Young Stellar Objects (YSO) in VulOB1 based on IR color and magnitude criteria, and we rely on the properties and nature of these 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 formation efficiency to be ~8 %. We also report the discovery of a dozen pillar-like structures, which are confirmed to be sites of small scale triggered star formation.

VLT-MAD observations of the core of 30 Doradus

We present H- and Ks-band imaging of three fields at the centre of 30 Doradus in the Large Magellanic Cloud, obtained as part of the Science Demonstration programme with the Multi-conjugate Adaptive optics Demonstrator (MAD) at the Very Large Telescope. Strehl ratios of 15-30% were achieved in the Ks-band, yielding near-infrared images of this dense and complex region at unprecedented angular resolution at these wavelengths. The MAD data are used to construct a near-infrared luminosity profile for R136, the cluster at the core of 30 Dor. Using cluster profiles of the form used by Elson et al., we find the surface brightness can be fit by a relatively shallow power-law function (gamma~1.5-1.7) over the full extent of the MAD data, which extends to a radius of ~40" (~10pc). We do not see compelling evidence for a break in the luminosity profile as seen in optical data in the literature, arguing that cluster asymmetries are the dominant source, although extinction effects and stars from nearby triggered star-formation likely also contribute. These results highlight the need to consider cluster asymmetries and multiple spatial components in interpretation of the luminosity profiles of distant unresolved clusters. We also investigate seven candidate young stellar objects reported by Gruendl & Chu from Spitzer observations, six of which have apparent counterparts in the MAD images. The most interesting of these (GC09: 053839.24-690552.3) appears related to a striking bow-shock--like feature, orientated away from both R136 and the Wolf-Rayet star Brey 75, at distances of 19.5" and 8" (4.7 and 1.9pc in projection), respectively.

ASSOCIATION OF INFRARED DARK CLOUD CORES WITH YSOS: STARLESS OR STARRED IRDC CORES

In this paper we examined theassociation of InfraRed Dark Cloud (IRDC) with YSOs and the geometric properties of the IRDC cores. For this studya total of 13,650 IRDC were collected mainly from the catalogs of the IRDC published from other studies andpartially from our catalog of IRDC containing new 789 IRDC core candidates. The YSO candidates were searched for usingthe GLIMPSE, MSX, and IRAS point sources by the shape of their SED or using activity of water or methanol maser. The associationof the IRDC with these YSOs was checked by their line- of-sight coincidence within the dimension of the IRDC core.This work found that a total of 4,110 IRDC have YSO candidates while 9,540 IRDC have no indication of the existenceof YSOs. Considering the 12,200 IRDC within the GLIMPSE survey region for which the YSO candidates were determinedwith better sensitivity, we found that 4,098 IRDC (34%) have at least one YSO candidate and 1,072 among themseem to have embedded YSOs, while the rest 8,102 (66%) have no YSO candidate. Therefore, the ratio of [N(IRDC core with protostars)]/[N(IRDCcore without YSO)] for 12,200 IRDC is about 0.13. Taking into account this ratio and typical lifetime of high-massembedded YSOs, we suggest that the IRDC would spend about 104 ~ 105 years to form high-mass stars.However, we should note that the GLIMPSE point sources have a minimum detectable luminosity of about 1.2 Lq at a typical IRDC cores distanceof ~4 kpc. Therefore, the ratio given here should be a lower limit and the estimated lifetime of starless IRDC canbe an upper limit. The physical parameters of the IRDC somewhat vary depending on how many YSO candidates the IRDCcores contain. The IRDC with more YSOs tend to be larger, more elongated, and have better darkness contrast than theIRDC with fewer or no YSOs.

VLT/NACO near-infrared imaging and spectroscopy of N88A in the SMC

We present near-infrared imaging and spectroscopic high spatial resolution observations of the SMC region N88 containing the bright, excited, extincted and compact H II region N88A of size ~ 1 pc. To investigate its stellar content and reddening, N88 was observed using spectroscopy and imagery in the JHKs- and L'-band at a spatial resolution of ~ 0.1-0.3", using the VLT UT4 equipped with the NAOS adaptive optics system. In order to attempt to establish if the origin of the infra-red (IR) excess is due to bright nebulosity, circumstellar material and/or local dust, we used Ks vs J-K colour-magnitude (CM) and JHK colour-colour (CC) diagrams, as well as L' imagery.Our IR-data reveal in the N88 area an IR-excess fraction of geq 30 per cent of the detected stars,as well as an unprecedently detailed morphology of N88A. It consists of an embedded cluster of ~3.5" (~ 1 pc) in diameter, of at least thirteen resolved stars superposed with an unusual bright continuum centered on a very bright star. The four brightest stars in this cluster lie red-ward of H-K geq 0.45 mag, and could be classified as young stellar object (YSO) candidates. Four other probable YSO candidates are also detected in N88 along a south-north bow-shaped thin H2 filament at ~ 7" east of the young central bright star. At 0.2" east of this star, a heavily embedded core is detected that could be a massive class I protostar candidate. The 2.12 mu H2 image of N88A resembles a shell of diameter ~ 3" ~ 0.9 pc) centered on the bright star. The line ratios of H2 2-1 S(1) and 1-0 S(0) relative to 1-0 S(1), as well as the presence of high v lines, are indicative of photodissociation regions, rather than shocks.

A multiwavelength study of the star forming region IRAS 18544+0112

This work aims at investigating the molecular and infrared components in the massive young stellar object (MYSO) candidate IRAS 18544+0112. The purpose is to determine the nature and the origin of this infrared source. To analyze the molecular gas towards IRAS 18544+0112, we have carried out observations in a 90" x 90" region around l = 34.69, b = -0.65, using the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2, 13CO J=3-2, HCO+ J=4-3 and CS J=7-6 lines with an angular resolution of 22". The infrared emission in the area has been analyzed using 2MASS and Spitzer public data. From the molecular analysis, we find self-absorbed 12CO J=3-2 profiles, which are typical in star forming regions, but we do not find any evidence of outflow activity. Moreover, we do not detect either HCO+ J=4-3 or CS J=7-6 in the region, which are species normally enhanced in molecular outflows and high density envelopes. The 12CO J=3-2 emission profile suggests the presence of expanding gas in the region. The Spitzer images reveal that the infrared source has a conspicuous extended emission bright at 8 um with an evident shell-like morphology of ~ 1.5 arcmin in size (~ 1.4 pc at the proposed distance of 3 kpc) that encircles the 24 um emission. The non-detection of ionized gas related to IRAS 18544+0112, together with the fact that it is still embedded in a molecular clump suggest that IRAS 18544+0112, has not reached the UCHII region stage yet. Based on near infrared photometry we search for YSO candidates in the region and propos that 2MASS 18565878+0116233 is the infrared point source associated with IRAS 18544+0112. Finally, we suggest that the expansion of a larger nearby HII region, G034.8-0.7, might be related to the formation of IRAS 18544+0112.

A SEARCH FOR INFALL EVIDENCE IN EGOs. I. THE NORTHERN SAMPLE

We report the first systematic survey of molecular lines (including HCO+ (1-0) and 12CO, 13CO, C18O (1-0) lines at 3 mm band) towards a new sample of 88 massive young stellar object (MYSO) candidates associated with ongoing outflows (known as extended green objects or EGOs) identified from the Spitzer GLIMPSE survey in the northern hemisphere with the PMO-13.7 m radio telescope. By analyzing the asymmetries of the optically thick line HCO+ for 69 of 72 EGOs with HCO+ detection, we found 29 sources with blue asymmetric profiles and 19 sources with red asymmetric profiles. This results in a blue excess of 0.14, seen as a signature of collapsing cores in the observed EGO sample. The relatively small blue excess measured in our full sample due to that the observed EGOs are mostly dominated by outflows and at an earlier evolutionary phase associated with IRDCs and 6.7 GHz methanol masers. The physical properties of clouds surrounding EGOs derived from CO lines are similar to those of massive clumps wherein the massive star forming cores associated with EGOs possibly embedded. The infall velocities and mass infall rates derived for 20 infall candidates are also consistent with the typical values found in MYSOs. Thus our observations further support the speculation of Cyganowski et al. (2008) that EGOs trace a population with ongoing outflow activity and active rapid accretion stage of massive protostellar evolution from a statistical view, although there maybe have limitations due to single-pointing survey with a large beam.

MASSIVE STAR FORMATION IN NGC 2074

Spitzer observations of the Large Magellanic Cloud (LMC) have revealed a large population of young stellar objects (YSOs), but complementary high-resolution images in the optical or near-IR wavelengths are still needed to resolve the multiplicity and immediate environments of the YSOs. The Hubble Space Telescope imaged the star-forming region NGC 2074 in the LMC during its 100,000th orbit, providing an opportunity to more closely examine the YSOs and their environments in this region. We have studied the 10 YSO candidates identified from Spitzer observations, confirming their nature and determining their physical parameters by modeling their spectral energy distributions. The majority of the YSOs and central stars of ultracompact H II regions in NGC 2074 have masses consistent with spectral types of early B to late O. The co-existence of massive early-type O stars and the less massive YSOs indicates that their formation may have started at a similar time, a few 10{sup 5} yr ago. NGC 2074 provides an opportunity to study the evolution of massive stars at their infancy.

AHUBBLE SPACE TELESCOPEVIEW OF THE INTERSTELLAR ENVIRONMENTS OF YOUNG STELLAR OBJECTS IN THE LARGE MAGELLANIC CLOUD

We have used archival HST H$\alpha$ images to study the immediate environments of massive and intermediate-mass young stellar object (YSO) candidates in the Large Magellanic Cloud (LMC). The sample of YSO candidates, taken from Gruendl & Chu (2009), was selected based on Spitzer IRAC and MIPS observations of the entire LMC and complementary ground-based optical and near-infrared observations. We found HST H$\alpha$ images for 99 YSO candidates in the LMC, of which 82 appear to be genuine YSOs. More than 95% of the YSOs are found to be associated with molecular clouds. YSOs are seen in three different kinds of environments in the H$\alpha$ images: in dark clouds, inside or on the tip of bright-rimmed dust pillars, and in small H II regions. Comparisons of spectral energy distributions for YSOs in these three different kinds of environments suggest that YSOs in dark clouds are the youngest, YSOs with small H II regions are the most evolved, and YSOs in bright-rimmed dust pillars span a range of intermediate evolutionary stages. This rough evolutionary sequence is substantiated by the presence of silicate absorption features in the Spitzer IRS spectra of some YSOs in dark clouds and in bright-rimmed dust pillars, but not those of YSOs in small H II regions. We present a discussion on triggered star formation for YSOs in bright-rimmed dust pillars or in dark clouds adjacent to H II regions. As many as 50% of the YSOs are resolved into multiple sources in high-resolution HST images. This illustrates the importance of using high-resolution images to probe the true nature and physical properties of YSOs in the LMC.

THE SPITZER SURVEY OF INTERSTELLAR CLOUDS IN THE GOULD BELT. II. THE CEPHEUS FLARE OBSERVED WITH IRAC AND MIPS

We present Spitzer Infrared Array Camera (IRAC; ~2 deg^2) and Multiband Imaging Photometer for Spitzer (MIPS; ~8 deg^2) observations of the Cepheus Flare, which is associated with the Gould Belt, at an approximate distance of ~300 pc. Around 6500 sources are detected in all four IRAC bands, of which ~900 have MIPS 24 μm detections. We identify 133 young stellar object (YSO) candidates using color-magnitude diagram techniques, and a large number of the YSO candidates are associated with the NGC 7023 reflection nebula. Cross-identifications were made with the Guide Star Catalog II and the IRAS Faint Source Catalog, and spectral energy distributions (SEDs) were constructed. SED modeling was conducted to estimate the degree of infrared excess. It was found that a large majority of disks were optically thick accreting disks, suggesting that there has been little disk evolution in these sources. Nearest-neighbor clustering analysis identified four small protostellar groups (L1228, L1228N, L1251A, and L1251B) with 5-15 members each and the larger NGC 7023 association with 32 YSO members. The star-formation efficiency for cores with clusters of protostars and for those without clusters was found to be ~8% and ~1%, respectively. The cores L1155, L1241, and L1247 are confirmed to be starless down to our luminosity limit of L _(bol) = 0.06 L ⊙.

EMBEDDED YOUNG STELLAR OBJECT CANDIDATES IN THE ACTIVE STAR-FORMING COMPLEX W51: MASS FUNCTION AND SPATIAL DISTRIBUTION

We present 737 candidate Young Stellar Objects (YSOs) near the W51 Giant Molecular Cloud (GMC) over an area of 1.25 deg x 1.00 deg selected from Spitzer Space Telescope data. We use spectral energy distribution (SED) fits to identify YSOs and distinguish them from main-sequence or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses > 5 Msun (corresponding to mid-to early-B main-sequence spectral types) 46 of which are located in the central active star forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 HII region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 Msun and 18 Msun. The derived slopes of the MF are -1.26 and -2.36 in the active star-forming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

Unveiling the molecular environment of the ring nebula RCW 78

We present a study of the ionized, neutral atomic, and molecular gas associated with the ring nebula RCW 78 around the WR star HD 117688 (= WR 55). We based our study on CO observations carried out with the SEST and NANTEN telescopes. We report the detection of molecular gas with velocities in the range -56 to -33 km/s. The CO emission is mainly connected to the western section, with a total molecular mass of 1.3 x 10^5 solar masses. The analysis of the HI gas distribution reveals the HI envelope of the molecular cloud, while the radio continuum emission shows a ring-like structure, which is the radio counterpart of the optical nebula. The gas distribution is compatible with the western section of RCW 78 having originated in the photodissociation and ionization of the molecular gas by HD 117688, and with the action of the stellar winds of the WR star. A number of infrared point sources classified as YSO candidates showed that stellar formation activity is present in the molecular gas linked to the nebula. The fact that the expansion of the bubble have triggered star formation in this region can not be discarded.

A Hubble Space Telescope/NICMOS view of the prototypical giant Hii region NGC604 in M33

We present the first high-spatial-resolution near-infrared (NIR) imaging of NGC604, obtained with the NICMOS camera onboard the Hubble Space Telescope (HST). These NICMOS broad-band images reveal new NIR point sources, clusters, and diffuse structures. We find an excellent spatial correlation between the 8.4 GHz radio continuum and the 2.2 μm nebular emission. Moreover, massive young stellar object candidates appear aligned with these radio peaks, reinforcing the idea that those areas are star-forming regions. Three different scaled OB associations are recognized in the NICMOS images. The brightest NIR sources in our images have properties that suggest that they are red supergiant stars, of which one was previously known. This preliminary analysis of the NICMOS images shows the complexity of the stellar content of the NGC604 nebula.

ASPITZERCENSUS OF STAR FORMATION ACTIVITY IN THE PIPE NEBULA

The Pipe Nebula, a large nearby molecular cloud lacks obvious signposts of star formation in all but one of more than 130 dust extinction cores that have been identified within it. In order to quantitatively determine the current level of star formation activity in the Pipe Nebula, we analyzed 13 square degrees of sensitive mid-infrared maps of the entire cloud, obtained with the Multiband Imaging Photometer for Spitzer (MIPS) at wavelengths of 24 micron and 70 micron to search for candidate Young Stellar Objects (YSOs) in the high-extinction regions. We argue that our search is complete for class I and typical class II YSOs with luminosities of Lbol~0.2 Lo and greater. We find only 18 candidate YSOs in the high-extinction regions of the entire Pipe cloud. Twelve of these sources are previously known members of a small cluster associated with Barnard 59, the largest and most massive dense core in the cloud. With only six candidate class I and class II YSOs detected towards extinction cores outside of this cluster, our findings emphatically confirm the notion of an extremely low level of star formation activity in the Pipe Nebula. The resulting star formation efficiency for the entire cloud mass is only ~0.06 %.

HIGH- AND INTERMEDIATE-MASS YOUNG STELLAR OBJECTS IN THE LARGE MAGELLANIC CLOUD

(Abridged) Photometry of archival Spitzer observations of the Large Magellanic Cloud (LMC) are used to search for young stellar objects (YSOs). Simple mid-infrared selection criteria were used to exclude most normal and evolved stars and background galaxies. We identify a sample of 2,910 sources in the LMC that could potentially be YSOs. We then simultaneously considered images and photometry from the optical through mid-IR wavelengths to assess the source morphology, spectral energy distribution (SED), and the surrounding interstellar environment to determine the most likely nature of each source. From this examination of the initial sample, we suggest 1,172 sources are most likely YSOs and 1,075 probable background galaxies, consistent with expectations based on SWIRE survey data. Spitzer IRS observations of 269 of the brightest YSOs from our sample have confirmed that ~>95% are indeed YSOs. A comprehensive search for YSOs in the LMC has also been carried out by the SAGE team. There are three major differences between these two searches. (1) In the common region of color-magnitude space, ~850 of our 1,172 probable YSOs are missed in the SAGE YSO catalog because their conservative point source identification criteria have excluded YSOs superposed on complex diffuse emission. (2) About 20-30% of the YSOs identified by the SAGE team are sources we classify as background galaxies. (3) the SAGE YSO catalog identifies YSO in parts of color-magnitude space that we excluded and thus contains more evolved or fainter YSOs missed by our analysis. Finally, the mid-IR luminosity functions of our most likely YSO candidates in the LMC can be well described by N(L) propto L^-1, which is consistent with the Salpeter initial mass function if a mass-luminosity relation of L propto M^2.4 is adopted.

The environs of the ultracompact HII region G45.45+0.06

G45.45+0.06 is an ultra-compact HII (UCHII) region extensively studied. It is known that G45.45+0.06 is embedded in a complex of UCHII regions, but up the date, the surrounding ISM in a larger spatial scale has not been analyzed. Using data from large-scale surveys: Two Micron All Sky Survey, GLIMPSE, MIPSGAL, MAGPIS and GRS, we performed a multiwavelength study of a region about 7' x 7'in the vicinity of G45.45+0.06. We found that the UCHII complex lies in a border of a larger (diameter of ~ 3') and fainter HII region, which is located at the same distance as G45.45+0.06, ~ 8 kpc. In this work, this larger HII region was called G45L. A good morphological correlation is observed between the PDRs and the molecular gas mapped in the 13CO J=1--0 and CS J=2--1 lines, suggesting that G45L may be collecting the molecular material. From a near- and mid-IR photometric study, we found three sources, likely O-type stars, that are possibly responsible for the creation of G45L. Additionally we found several YSO candidates lying preferently in the molecular shell that surrounds G45L. Our results confirm that the region southeastern the UCHII complex where G45.45+0.06 is embedded and eastern G45L is active in star formation. We suggest that G45L has been expanding during about 2 x 10^6 yr and could have triggered the formation of the zero-age main sequence stars that are ionizing the UCHII region G45.45+0.06. However we can not discard that both HII regions are coeval.

FIRST SPECTROSCOPIC IDENTIFICATION OF MASSIVE YOUNG STELLAR OBJECTS IN THE GALACTIC CENTER

We report the detection of several molecular gas-phase and ice absorption features in three photometrically-selected young stellar object (YSO) candidates in the central 280 pc of the Milky Way. Our spectra, obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, reveal gas-phase absorption from CO2 (15.0um), C2H2 (13.7um) and HCN (14.0um). We attribute this absorption to warm, dense gas in massive YSOs. We also detect strong and broad 15um CO2 ice absorption features, with a remarkable double-peaked structure. The prominent long-wavelength peak is due to CH3OH-rich ice grains, and is similar to those found in other known massive YSOs. Our IRS observations demonstrate the youth of these objects, and provide the first spectroscopic identification of massive YSOs in the Galactic Center.

STAR FORMATION IN THE CENTRAL 400 PC OF THE MILKY WAY: EVIDENCE FOR A POPULATION OF MASSIVE YOUNG STELLAR OBJECTS

The central kpc of the Milky Way might be expected to differ significantly from the rest of the Galaxy with regard to gasdynamics and the formation of young stellar objects (YSOs). We probe this possibility with mid-infrared observations obtained with Infrared Array Camera and Multiband Imaging Photometer on Spitzer and with Midcourse Space Experiment. We use color-color diagrams and spectral energy distribution (SED) fits to explore the nature of YSO candidates (including objects with 4.5 {mu}m excesses possibly due to molecular emission). There is an asymmetry in the distribution of the candidate YSOs, which tend to be found at negative Galactic longitudes; this behavior contrasts with that of the molecular gas, approximately 2/3 of which is at positive longitudes. The small-scale height of these objects suggests that they are within the Galactic center region and are dynamically young. They lie between two layers of infrared dark clouds and may have originated from these clouds. We identify new sites for this recent star formation by comparing the mid-IR, radio, submillimeter, and methanol maser data. The methanol masers appear to be associated with young, embedded YSOs characterized by 4.5 {mu}m excesses. We use the SEDs of these sources to estimate their physicalmore » characteristics; their masses appear to range from {approx}10 to {approx}20 M{sub sun}. Within the central 400 x 50 pc (|l| < 1.{sup 0}3 and |b| < 10') the star formation rate (SFR) based on the identification of Stage I evolutionary phase of YSO candidates is about 0.14 M{sub sun} yr{sup -1}. Given that the majority of the sources in the population of YSOs are classified as Stage I objects, we suggest that a recent burst of star formation took place within the last 10{sup 5} yr. This suggestion is also consistent with estimates of SFRs within the last {approx}10{sup 7} yr showing a peak around 10{sup 5} yr ago. Lastly, we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of the Galaxy. This implies that star formation does not appear to be dramatically affected by the extreme physical conditions in the Galactic center region.« less

SPITZER OBSERVATIONS OF THE H II REGION NGC 2467: AN ANALYSIS OF TRIGGERED STAR FORMATION

We present new Spitzer Space Telescope observations of the region NGC 2467, and use these observations to determine how the environment of an H II region affects the process of star formation. Our observations comprise IRAC (3.6, 4.5, 5.8, and 8.0 μm) and MIPS (24 μm) maps of the region, covering approximately 400 arcmin2. The images show a region of ionized gas pushing out into the surrounding molecular cloud, powered by an O6V star and two clusters of massive stars in the region. We have identified as candidate young stellar objects (YSOs) 45 sources in NGC 2467 with infrared excesses in at least two mid-infrared colors. We have constructed color-color diagrams of these sources and have quantified their spatial distribution within the region. We find that the YSOs are not randomly distributed in NGC 2467; rather, over 75% of the sources are distributed at the edge of the H II region, along ionization fronts driven by the nearby massive stars. The high fraction of YSOs in NGC 2467 that are found in proximity to gas that has been compressed by ionization fronts supports the hypothesis that a significant fraction of the star formation in NGC 2467 is triggered by the massive stars and the expansion of the H II region. At the current rate of star formation, we estimate at least 25%-50% of the total population of YSOs formed by this process.

THE EVOLUTION OF MASSIVE YOUNG STELLAR OBJECTS IN THE LARGE MAGELLANIC CLOUD. I. IDENTIFICATION AND SPECTRAL CLASSIFICATION

We present and categorize Spitzer infrared spectrometer spectra of 294 objects in the Large Magellanic Cloud (LMC) to create the largest and most complete catalog of massive young stellar object (YSO) spectra in the LMC. Target sources were identified from infrared photometry and multiwavelength images indicative of young, massive stars highly enshrouded in their natal gas and dust clouds. Several objects have been spectroscopically identified as non-YSOs and have features similar to more-evolved stars such as red supergiants, asymptotic giant branch (AGB), and post-AGB stars. Our sample primarily consists of 277 objects we identify as having spectral features indicative of embedded YSOs. The remaining sources are comprised of seven C-rich evolved sources, eight sources dominated by broad silicate emission, and one source with multiple broad emission features. Those with YSO-like spectra show a range of spectral features including polycyclic aromatic hydrocarbon emission, deep silicate absorption, fine-structure lines, and ice absorption features. Based upon the relative strengths of these features, we have classified the YSO candidates into several distinct categories using the widely used statistical procedure known as principal component analysis. We propose that these categories represent a spectrum of evolutionary stages during massive YSO formation. Using our catalog we put statistical constraints on the relative evolutionary timescale of processes involved in massive star formation. We conclude that massive pre-main-sequence stars spend a majority (possibly as high as 90%) of their massive, embedded lives emitting in the UV. Half of the sources in our study have features typical of compact H ii regions, suggesting that massive YSOs can create a detectable compact H ii region half-way through the formation time present in our sample. This study also provides a check on commonly used source-selection procedures including the use of photometry to identify YSOs. We determine that a high success rate (>95%) of identifying objects with YSO-like spectra can be achieved through careful use of infrared color–magnitude diagrams, spectral energy distributions, and image inspections.

THE EXTENDED ENVIRONMENT OF M17: A STAR FORMATION HISTORY

M17 is one of the youngest and most massive nearby star-formation regions in the Galaxy. It features a bright H II region erupting as a blister from the side of a giant molecular cloud (GMC). Combining photometry from the Spitzer GLIMPSE survey with complementary infrared (IR) surveys, we identify candidate young stellar objects (YSOs) throughout a 1.5 deg x 1 deg field that includes the M17 complex. The long sightline through the Galaxy behind M17 creates significant contamination in our YSO sample from unassociated sources with similar IR colors. Removing contaminants, we produce a highly-reliable catalog of 96 candidate YSOs with a high probability of association with the M17 complex. We fit model spectral energy distributions to these sources and constrain their physical properties. Extrapolating the mass function of 62 intermediate-mass YSOs (M >3 Msun), we estimate that >1000 stars are in the process of forming in the extended outer regions of M17. From IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim of a large shell structure ~0.5 deg in diameter (~20 pc at 2.1 kpc). We present new maps of CO and 13CO (J=2-1) emission, which show that the shell is a coherent, kinematic structure associated with M17 at v = 19 km/s. The shell is an extended bubble outlining the photodissociation region of a faint, diffuse H II region several Myr old. We provide evidence that massive star formation has been triggered by the expansion of the bubble. The formation of the massive cluster ionizing the M17 H II region itself may have been similarly triggered. We conclude that the star formation history in the extended environment of M17 has been punctuated by successive waves of massive star formation propagating through a GMC complex.