Population Of Young Stellar Objects Research Articles

Climbing the Cliffs: Classifying Young Stellar Objects in the Cosmic Cliffs JWST Data Using a Probabilistic Random Forest

Among the first observations released to the public from the JWST was a section of the star-forming region NGC 3324 known colloquially as the “Cosmic Cliffs.” We build a photometric catalog of the region and test the ability of using the probabilistic random forest machine-learning method to identify its young stellar objects (YSOs). We find 450 candidate YSOs (cYSOs) out of 19,497 total objects within the field, 413 of which are cYSOs not found in previous works. These classifications are verified with several different metrics, including recall and precision. Using the obtained probabilities of objects being YSOs, we employ a Monte Carlo approach to determine the surface density of cYSOs in the Cosmic Cliffs, which we find to be largely coincident with column densities derived from Herschel data, up to a column density of 1.37 × 1022 cm−2. The newly determined number and spatial distribution of YSOs in the Cosmic Cliffs demonstrate that JWST is far more capable of detecting YSOs in dusty regions than Spitzer. Comparisons of the observed colors and brightness of faint cYSOs with those of pre-main-sequence models suggest JWST has detected a significant population of substellar YSOs in the Cosmic Cliffs. The size of this population further suggests previous estimates of star formation efficiencies in molecular clouds have been systematically low.

A JWST/MIRI and NIRCam Analysis of the Young Stellar Object Population in the Spitzer I Region of NGC 6822

We present an imaging survey of the Spitzer I star-forming region in NGC 6822 conducted with the NIRCam and MIRI instruments on board JWST. Located at a distance of 490 kpc, NGC 6822 is the nearest non-interacting low-metallicity (∼0.2 Z ⊙) dwarf galaxy. It hosts some of the brightest known H ii regions in the local universe, including recently discovered sites of highly embedded active star formation. Of these, Spitzer I is the youngest and most active, and houses 90 color-selected candidate young stellar objects (YSOs) identified from Spitzer Space Telescope observations. We revisit the YSO population of Spitzer I with these new JWST observations. By analyzing color–magnitude diagrams constructed with NIRCam and MIRI data, we establish color selection criteria and construct spectral energy distributions to identify candidate YSOs and characterize the full population of young stars, from the most embedded phase to the more evolved stages. In this way, we have identified 140 YSOs in Spitzer I. Comparing to previous Spitzer studies of the NGC 6822 YSO population, we find that the YSOs we identify are fainter and less massive, indicating that the improved resolution of JWST allows us to resolve previously blended sources into multiple objects.

Enhanced YSO population in Serpens

Serpens molecular cloud is one of the most active sites of ongoing star formation at a distance of about 300 pc, and hence, is very well-suited for studies of young low-mass stars and sub-stellar objects. In this paper, for the Serpens star-forming region, we found potential members of the Young Stellar Objects (YSOs) population from the Gaia DR3 data and study their kinematics and distribution. We compiled a catalog of 656 YSOs from available catalogs ranging from X-ray to the infrared. We use this as a reference set and cross-match it to find 87 Gaia DR3 member stars to produce a control sample with revised parameters. We queried the DR3 catalog with these parameters and found 1196 stars. We then applied three different density-based machine learning algorithms (DBSCAN, OPTICS and HDBSCAN) to this sample and found potential YSOs. The three clustering algorithms identified a common set of 822 YSO members from Gaia DR3 in this region. We also classified these objects using 2MASS and WISE data to study their distribution and the progress of star-formation in Serpens.

Evidence of high-mass star formation through multiscale mass accretion in hub-filament-system clouds

ABSTRACT We present a statistical study of a sample of 17 hub-filament-system (HFS) clouds of high-mass star formation using high-angular resolution (∼1–2 arcsec) ALMA 1.3 and 3 mm continuum data. The sample includes eight infrared (IR)-dark and nine IR-bright types, which correspond to an evolutionary sequence from the IR-dark to IR-bright stage. The central massive clumps and their associated most massive cores are observed to follow a trend of increasing mass (M) and mass surface density (Σ) with evolution from the IR-dark to IR-bright stage. In addition, a mass-segregated cluster of young stellar objects (YSOs) are revealed in both IR-dark and IR-bright HFSs with massive YSOs located in the hub and the population of low-mass YSOs distributed over larger areas. Moreover, outflow feedback in all HFSs are found to escape preferentially through the inter-filamentary diffuse cavities, suggesting that outflows would render a limited effect on the disruption of the HFSs and ongoing high-mass star formation therein. From the above observations, we suggest that high-mass star formation in the HFSs can be described by a multiscale mass accretion/transfer scenario, from hub-composing filaments through clumps down to cores, which can naturally lead to a mass-segregated cluster of stars.

Star formation in two irradiated globules around Cygnus OB2

Context. The ultraviolet irradiation and the action of stellar winds of newly formed massive stars on their parental molecular environment often produces isolated or small clouds, some of which become sites of star formation themselves. Aims. We investigate the young stellar populations associated with DR 18 and ECX 6-21, which are two isolated globules irradiated by the O-type stars of the Cygnus OB2 association. Both are HII regions containing obvious tracers of recent and ongoing star formation. We also study smaller isolated molecular structures in their surroundings. Methods. We combined near-infrared broad- and narrow-band imaging with broad-band imaging in the visible and with archive images obtained with the Spitzer Space Telescope. We used the joint photometry to select young stellar objects (YSOs), simultaneously estimating their intrinsic properties and classifying them according to the characteristics of their infrared excess. We also present low-resolution visible spectroscopy of selected sources. Results. We reproduce previous findings of an extended population of YSOs around both globules, dominated by the more evolved classes, associated with the general Cygnus OB2 population. Both globules contain their own embedded populations, with a higher fraction of the less-evolved classes. Masses and temperatures are estimated under the assumption of a common age of 1 Myr, which has been found to appropriately represent the general Cygnus OB2 YSO population but is most probably an overestimate for both globules, especially ECX 6-21. The early-B star responsible for the erosion of DR 18 is found to be part of a small aggregate of intermediate-mass stars still embedded in the cloud, which probably contains a second site of recent star formation, also with intermediate-mass stars. We confirm the two main star forming sites embedded in ECX 6-21 described in previous works, with the southern site being more evolved than the northern site. We also discuss the small globule ECX 6-21-W (=G79.8 + 1.2), and propose that its non thermal radio spectrum is due to synchrotron emission from an embedded jet, whose existence is suggested by our observations. Conclusions. The extreme youth of some of the YSOs suggests that star formation in both globules started after they became externally irradiated. The populations of both globules are not found to be particularly rich, but they contain stars with estimated masses similar or above that of the Sun in numbers that hint at some differences with respect to the star formation process taking place in more quiescent regions where low-mass stars dominate, which deeper observations may confirm.

A survey for variable young stars with small telescopes – V. Analysis of TX Ori, V505 Ori, and V510 Ori, the HST ULLYSES targets in the σ Ori cluster

ABSTRACT Investigations of the formation of young stellar objects (YSOs) and planets require the detailed analysis of individual sources as well as statistical analysis of a larger number of objects. The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) project provides such a unique opportunity by establishing a UV spectroscopic library of young high- and low-mass stars in the local universe. Here, we analyse optical photometry of the three ULLYSES targets (TX Ori, V505 Ori, V510 Ori) and other YSOs in the σ Ori cluster taken at the time of the HST observations to provide a reference for those spectra. We identify three populations of YSOs along the line of sight to σ Ori, separated in parallax and proper motion space. The ULLYSES targets show typical YSO behaviour with pronounced variability and mass accretion rates of the order of 10−8 M⊙ yr−1. Optical colours do not agree with standard interstellar reddening and suggest a significant contribution of scattered light. They are also amongst the most variable and strongest accretors in the cluster. V505 Ori shows variability with a 7 d period, indicating an inner disc warp at the co-rotation radius. Uncovering the exact nature of the ULLYSES targets will require improved detailed modelling of the HST spectra in the context of the available photometry, including scattered light contributions as well as non-standard reddening.

Massive young stellar objects in the Local Group irregular galaxy NGC 6822 identified using machine learning

ABSTRACT We present a supervised machine learning methodology to classify stellar populations in the Local Group dwarf-irregular galaxy NGC 6822. Near-IR colours (J − H, H − K, and J − K), K-band magnitudes and far-IR surface brightness (at 70 and 160 $\mu$m) measured from Spitzer and Herschel images are the features used to train a Probabilistic Random Forest (PRF) classifier. Point-sources are classified into eight target classes: young stellar objects (YSOs), oxygen- and carbon-rich asymptotic giant branch stars, red giant branch and red supergiant stars, active galactic nuclei, massive main-sequence stars, and Galactic foreground stars. The PRF identifies sources with an accuracy of ∼ 90 per cent across all target classes rising to ∼96 per cent for YSOs. We confirm the nature of 125 out of 277 literature YSO candidates with sufficient feature information, and identify 199 new YSOs and candidates. Whilst these are mostly located in known star-forming regions, we have also identified new star formation sites. These YSOs have mass estimates between ∼15 and 50 M⊙, representing the most massive YSO population in NGC 6822. Another 82 out of 277 literature candidates are definitively classified as non-YSOs by the PRF analysis. We characterize the star formation environment by comparing the spatial distribution of YSOs to those of gas and dust using archival images. We also explore the potential of using (unsupervised) t-distributed stochastic neighbour embedding maps for the identification of the same stellar population classified by the PRF.

Mass segregation and sequential star formation in NGC 2264 revealed by Herschel

Context. The mass segregation of stellar clusters could be primordial rather than dynamical. Despite the abundance of studies of mass segregation for stellar clusters, those for stellar progenitors are still scarce, so the question concerning the origin and evolution of mass segregation is still open. Aims. Our goal is to characterize the structure of the NGC 2264 molecular cloud and compare the populations of clumps and young stellar objects (YSOs) in this region whose rich YSO population has shown evidence of sequential star formation. Methods. We separated the Herschel column density map of NGC 2264 into three subregions and compared their cloud power spectra using a multiscale segmentation technique. We extracted compact cloud fragments from the column density image, measured their basic properties, and studied their spatial and mass distributions. Results. In the whole NGC 2264 cloud, we identified a population of 256 clumps with typical sizes of ~0.1 pc and masses ranging from 0.08 M⊙ to 53 M⊙. Although clumps have been detected all over the cloud, most of the massive, bound clumps are concentrated in the central subregion of NGC 2264. The local surface density and the mass segregation ratio indicate a strong degree of mass segregation for the 15 most massive clumps, with a median Σ6 three times that of the whole clumps population and ΛMSR ≃ 8. We show that this cluster of massive clumps is forming within a high-density cloud ridge, which is formed and probably still fed by the high concentration of gas observed on larger scales in the central subregion. The time sequence obtained from the combined study of the clump and YSO populations in NGC 2264 suggests that the star formation started in the northern subregion, that it is now actively developing at the center, and will soon start in the southern subregion. Conclusions. Taken together, the cloud structure and the clump and YSO populations in NGC 2264 argue for a dynamical scenario of star formation. The cloud could first undergo global collapse, driving most clumps to centrally concentrated ridges. After their main accretion phase, some YSOs, and probably the most massive, would stay clustered while others would be dispersed from their birth sites. We propose that the mass segregation observed in some star clusters is inherited from that of clumps, originating from the mass assembly phase of molecular clouds.

A VLBA Survey of Radio Stars in the Orion Nebula Cluster. I. The Nonthermal Radio Population

Abstract We present first results of a four-epoch VLBA survey for nonthermal emission toward all 556 compact radio sources previously identified in a deep VLA survey of the Orion Nebula Cluster (ONC). We identify VLBA counterparts toward an unprecedented 123 sources. Of these, 41 do not have X-ray counterparts, of which 34 also do not display near-infrared counterparts. Since these cannot be explained by extragalactic background sources, this suggests a component of the ONC population of young stellar objects that are too deeply embedded for even X-rays to be detectable. We find pervasive variability and detect even most of the highest-S/N sources in only one out of four epochs. Neither a negative spectral index nor extreme variability in the VLA data is a good predictor of a VLBA detection.

Modeling the Accretion Disk around the High-mass Protostar GGD 27-MM1

Abstract Recent high angular resolution (≃40 mas) ALMA observations at 1.14 mm resolve a compact (R ≃ 200 au), flattened dust structure perpendicular to the HH 80–81 jet emanating from the GGD 27-MM1 high-mass protostar, making it a robust candidate for a true accretion disk. The jet–disk system (HH 80–81/GGD 27-MM1) resembles those found in association with low- and intermediate-mass protostars. We present radiative transfer models that fit the 1.14 mm ALMA dust image of this disk, which allow us to obtain its physical parameters and predict its density and temperature structure. Our results indicate that this accretion disk is compact (R disk ≃ 170 au) and massive (≃5 M ⊙), at about 20% of the stellar mass of ≃20 M ⊙. We estimate the total dynamical mass of the star–disk system from the molecular line emission, finding a range between 21 and 30 M ⊙, which is consistent with our model. We fit the density and temperature structures found by our model with power-law functions. These results suggest that accretion disks around massive stars are more massive and hotter than their low-mass siblings, but they still are quite stable. We also compare the temperature distribution in the GGD 27–MM1 disk with that found in low- and intermediate-mass stars and discuss possible implications for the water snow line. We have also carried out a study of the distance based on Gaia DR2 data and the population of young stellar objects in this region and from the extinction maps. We conclude that the source distance is within 1.2 and 1.4 kpc, closer than what was derived in previous studies (1.7 kpc).

Identification of Young Stellar Object candidates in the Gaia DR2 x AllWISE catalogue with machine learning methods

ABSTRACT The second Gaia Data Release (DR2) contains astrometric and photometric data for more than 1.6 billion objects with mean Gaia G magnitude <20.7, including many Young Stellar Objects (YSOs) in different evolutionary stages. In order to explore the YSO population of the Milky Way, we combined the Gaia DR2 data base with Wide-field Infrared Survey Explorer (WISE) and Planck measurements and made an all-sky probabilistic catalogue of YSOs using machine learning techniques, such as Support Vector Machines, Random Forests, or Neural Networks. Our input catalogue contains 103 million objects from the DR2xAllWISE cross-match table. We classified each object into four main classes: YSOs, extragalactic objects, main-sequence stars, and evolved stars. At a 90 per cent probability threshold, we identified 1 129 295 YSO candidates. To demonstrate the quality and potential of our YSO catalogue, here we present two applications of it. (1) We explore the 3D structure of the Orion A star-forming complex and show that the spatial distribution of the YSOs classified by our procedure is in agreement with recent results from the literature. (2) We use our catalogue to classify published Gaia Science Alerts. As Gaia measures the sources at multiple epochs, it can efficiently discover transient events, including sudden brightness changes of YSOs caused by dynamic processes of their circumstellar disc. However, in many cases the physical nature of the published alert sources are not known. A cross-check with our new catalogue shows that about 30 per cent more of the published Gaia alerts can most likely be attributed to YSO activity. The catalogue can be also useful to identify YSOs among future Gaia alerts.

Low-mass star formation and subclustering in the H II regions RCW 32, 33, and 27 of the Vela Molecular Ridge

Context. Most stars are born in clusters, and recent results suggest that star formation (SF) preferentially occurs in subclusters. Studying the morphology and SF history of young clusters is crucial for understanding early cluster formation processes. Aims. We aim to identify the embedded population of young stellar objects (YSOs) down to the low-mass stars in the M-type regime in the three H II regions RCW 33, RCW 32, and RCW 27, which are located in the northwestern region of the Vela Molecular Ridge. Our aim is to characterize their properties, such as morphology and extent of the clusters in the three H II regions, derive stellar ages, and determine the connection of the SF history with the environment. Methods. Through public photometric surveys such as Gaia, VPHAS+, 2MASS, and Spitzer/GLIMPSE, we identify YSOs with classical techniques aimed at detecting IR, Hα, and UV excesses as signatures of circumstellar disks and accretion. In addition, we implement a method for distinguishing main-sequence (MS) stars and giants in the M-type regime by comparing the reddening derived in several optical/IR color-color diagrams, assuming suitable theoretical models. Since this diagnostic is sensitive to stellar gravity, the procedure allows us to also identify pre-MS (PMS) stars. Results. Using the classical membership criteria, we find that a large population of YSOs shows signatures of circumstellar disks with or without accretion. In addition, with the new technique of M-type star selection, we find a rich population of young M-type stars whose spatial distribution strongly correlates with the more massive population. We find evidence of three young clusters, with different morphology, for which we estimate the individual distances using TGAS Gaia data of the brighter subsample. In addition, we identify field stars falling in the same region by securely classifying them as giants and foreground MS stars. Conclusions. We identify the embedded population of YSOs down to about 0.1 M⊙ that is associated with the three H II regions RCW 33, RCW 32, and RCW 27 and the three clusters Vela T2, Cr 197, and Vela T1, respectively. All the three clusters are located at a similar distance, but they have very different morphologies. Our results suggest a decreasing SF rate in Vela T2 and triggered SF in Cr 197 and Vela T1.

Radio and infrared study of southern H II regions G346.056−0.021 and G346.077−0.056

Aim. We present a multiwavelength study of two southern Galactic H II regions G346.056−0.021 and G346.077−0.056 which are located at a distance of 10.9 kpc. The distribution of ionized gas, cold and warm dust, and the stellar population associated with the two H II regions are studied in detail using measurements at near-infrared, mid-infrared, far-infrared, submillimeter and radio wavelengths. Methods. The radio continuum maps at 1280 and 610 MHz were obtained using the Giant Metrewave Radio Telescope to probe the ionized gas. The dust temperature, column density, and dust emissivity maps were generated using modified blackbody fits in the far-infrared wavelength range 160–500 μm. Various near- and mid-infrared color and magnitude criteria were adopted to identify candidate ionizing star(s) and the population of young stellar objects in the associated field. Results. The radio maps reveal the presence of diffuse ionized emission displaying distinct cometary morphologies. The 1280 MHz flux densities translate to zero age main sequence spectral types in the range O7.5V–O7V and O8.5V–O8V for the ionizing stars of G346.056−0.021 and G346.077−0.056, respectively. A few promising candidate ionizing star(s) are identified using near-infrared photometric data. The column density map shows the presence of a large, dense dust clump enveloping G346.077−0.056. The dust temperature map shows peaks towards the two H II regions. The submillimeter image shows the presence of two additional clumps, one being associated with G346.056−0.021. The masses of the clumps are estimated to range between ~1400 and 15250 M⊙. Based on simple analytic calculations and the correlation seen between the ionized gas distribution and the local density structure, the observed cometary morphology in the radio maps is better explained invoking the champagne-flow model.

A Comparative Observational Study of YSO Classification in Four Small Star-forming H ii Regions

Abstract We have developed a new young stellar object (YSO) identification and classification technique using mid-infrared Wide-field Infrared Survey Explorer (WISE) data. We compare this new technique with previous WISE YSO detection and classification methods that used either infrared colors or spectral energy distribution slopes. In this study, we also use the new technique to detect and examine the YSO population associated with four small H ii regions: KR 7, KR 81, KR 120, and KR 140. The relatively simple structure of these regions allows us to effectively use both spatial and temporal constraints to identify YSOs that are potential products of triggered star formation. We are also able to identify regions of active star formation around these H ii regions that are clearly not influenced by the H ii region expansion, and thus demonstrate that star formation is on-going on megayear timescales in some of these molecular clouds.

The Star-formation Law in Galactic High-mass Star-forming Molecular Clouds

We study the star formation (SF) law in 12 Galactic molecular clouds with ongoing high-mass star formation (HMSF) activity, as traced by the presence of a bright IRAS source and other HMSF tracers. We define the molecular cloud (MC) associated to each IRAS source using 13CO line emission, and count the young stellar objects (YSOs) within these clouds using GLIMPSE and MIPSGAL 24 micron Spitzer databases.The masses for high luminosity YSOs (Lbol>10~Lsun) are determined individually using Pre Main Sequence evolutionary tracks and the evolutionary stages of the sources, whereas a mean mass of 0.5 Msun was adopted to determine the masses in the low luminosity YSO population. The star formation rate surface density (sigsfr) corresponding to a gas surface density (siggas) in each MC is obtained by counting the number of the YSOs within successive contours of 13CO line emission. We find a break in the relation between sigsfr and siggas, with the relation being power-law (sigsfr ~ siggas^N) with the index N varying between 1.4 and 3.6 above the break. The siggas at the break is between 150-360 Msun/pc^2 for the sample clouds, which compares well with the threshold gas density found in recent studies of Galactic star-forming regions. Our clouds treated as a whole lie between the Kennicutt (1998) relation and the linear relation for Galactic and extra-galactic dense star-forming regions. We find a tendency for the high-mass YSOs to be found preferentially in dense regions at densities higher than 1200 Msun/pc^2 (~0.25 g/cm^2).

The role of angular momentum transport in establishing the accretion rate–protostellar mass correlation

We model the mass accretion rate M˙ to stellar mass M* correlation that has been inferred from observations of intermediate to upper mass T Tauri stars—that is M˙∝M*1.3±0.3. We explain this correlation within the framework of quiescent disk evolution, in which accretion is driven largely by gravitational torques acting in the bulk of the mass and volume of the disk. Stresses within the disk arise from the action of gravitationally driven torques parameterized in our 1D model in terms of Toomre’s Q criterion. We do not model the hot inner sub-AU scale region of the disk that is likely stable according to this criterion, and appeal to other mechanisms to remove or redistribute angular momentum and allow accretion onto the star. Our model has the advantage of agreeing with large-scale angle-averaged values from more complex nonaxisymmetric calculations. The model disk transitions from an early phase (dominated by initial conditions inherited from the burst mode of accretion) into a later self-similar mode characterized by a steeper temporal decline in M˙. The models effectively reproduce the spread in mass accretion rates that have been observed for protostellar objects of 0.2 M⊙ ≤ M* ≤ 3.0 M⊙, such as those found in the ρ Ophiuchus and Taurus star forming regions. We then compare realistically sampled populations of young stellar objects produced by our model to their observational counterparts. We find these populations to be statistically coincident, which we argue is evidence for the role of gravitational torques in the late time evolution of quiescent protostellar disks.

Classifying the embedded young stellar population in Perseus and Taurus and the LOMASS database

Context. The classification of young stellar objects (YSOs) is typically done using the infrared spectral slope or bolometric temperature, but either can result in contamination of samples. More accurate methods to determine the evolutionary stage of YSOs will improve the reliability of statistics for the embedded YSO population and provide more robust stage lifetimes. Aims. We aim to separate the truly embedded YSOs from more evolved sources. Methods. Maps of HCO+ J=4-3 and C18O J=3-2 were observed with HARP on the James Clerk Maxwell Telescope (JCMT) for a sample of 56 candidate YSOs in Perseus and Taurus in order to characterize emission from high (column) density gas. These are supplemented with archival dust continuum maps observed with SCUBA on the JCMT and Herschel PACS to compare the morphology of the gas and dust in the protostellar envelopes. The spatial concentration of HCO+ J=4-3 and 850 micron dust emission are used to classify the embedded nature of YSOs. Results. Approximately 30% of Class 0+I sources in Perseus and Taurus are not Stage I, but are likely to be more evolved Stage II pre-main sequence (PMS) stars with disks. An additional 16% are confused sources with an uncertain evolutionary stage. Conclusions. Separating classifications by cloud reveals that a high percentage of the Class 0+I sources in the Perseus star forming region are truly embedded Stage I sources (71%), while the Taurus cloud hosts a majority of evolved PMS stars with disks (68%). The concentration factor method is useful to correct misidentified embedded YSOs, yielding higher accuracy for YSO population statistics and Stage timescales. Current estimates (0.54 Myr) may overpredict the Stage I lifetime on the order of 30%, resulting in timescales of 0.38 Myr for the embedded phase.

STAR FORMATION ACTIVITY IN THE LONG, FILAMENTARY INFRARED DARK CLOUD G53.2

We present star formation activity in the infrared dark cloud (IRDC) G53.2, a remarkable IRDC located at Galactic coordinates $(l,b)\sim(53^{\circ}.2,\,0^{\circ}.0)$ based on the census of young stellar object (YSO) candidates. IRDC G53.2 was previously identified as several IRDCs in mid-IR images, but it is in fact a long ($\gtrsim$45 pc) cloud, well consistent with a CO cloud at $v\sim23$ \kms\ (or at $d\sim$1.7 kpc). We present a point-source catalog of IRDC G53.2 that contains $\sim$370 sources from our photometry of the {\it Spitzer} MIPS 24 \um\ data and Galactic Legacy Infrared Mid-Plane Survey Extraordinaire Catalog. The classification of the identified sources based on their spectral index and control field analysis to remove field star contamination reveals that IRDC G53.2 is an active star-forming region with $\sim$300 YSO candidates. We compare the YSO classification based on spectral index, mid-IR colors, and the wavelength range used, which results in consistent classification, except for flat-spectrum objects, with some ambiguity between Class I and II. Comparison of the YSO population in IRDC G53.2 with those of other nearby star-forming clusters indicates that they are similar in age; on the other hand, stronger association with mid-IR stellar sources in IRDC G53.2 compared with other IRDCs indicates that IRDC G53.2 is at a later evolutionary stage among IRDCs. Spatial distribution of the YSO candidates in IRDC G53.2 shows a good correlation with $^{13}$CO column density and far-IR emission, and earlier-class objects tend to be more clustered in the regions with higher density.

AN OPTICAL SPECTROSCOPIC SURVEY OF THE SERPENS MAIN CLUSTER: EVIDENCE FOR TWO POPULATIONS?

We have completed an optical spectroscopic survey of a sample of candidate young stars in the Serpens Main starforming region selected from deep B, V, and R band images. While infrared, X-ray, and optical surveys of the cloud have identified many young stellar objects (YSOs), these surveys have been biased toward particular stages of premain sequence evolution. We have obtained over 700 moderate resolution optical spectra that, when combined with published data, have led to the identification of 63 association members based on the presence of Hα in emission, lithium absorption, X-ray emission, a mid-infrared excess, and/or reflection nebulosity. Twelve YSOs are identified based on the presence of lithium absorption alone. An additional 16 objects are classified as possible association members and their pre-main sequence nature is in need of confirmation. Spectral types along with V and R band photometry were used to derive effective temperatures and bolometric luminosities for association members to compare with theoretical tracks and isochrones for pre-main sequence stars. An average age of 2 Myr is derived for this population. When compared to simulations, there is no obvious evidence for an age spread when considering the major sources of uncertainties in the derived luminosities. However when compared to the young cluster in Ophiuchus, the association members in Serpens appear to have a larger spread in luminosities and hence ages which could be intrinsic to the region or the result of a foreground population of YSOs associated with the Aquila Rift. Modeling of the spectral energy distributions from optical through mid-infrared wavelengths has revealed three new transition disk objects, making a total of six in the cluster. Echelle spectra for a subset of these sources enabled estimates of v sin i for seven association members. Analysis of gravity-sensitive lines in the echelle and moderate resolution spectra of the association members indicate surface gravities consistent with dwarf or subgiant stars.

On the 3–5 μm variability of young variables in Vela-D through Spitzer-WISE observations

Flux variability is a common feature of Young Stellar Objects (YSOs), which is often related to intermittent events of disk accretion (EXors events in case of 3–4 magnitudes variations). Recently, thanks to the surveys carried out by the space missions Spitzer and WISE, it has become possible to perform statistical studies on the mid-IR variability on large samples of YSOs. As a follow-up of our recent statistical study on five star forming regions (Antoniucci et al., Astrophys. J. 782:51, 2014), we present the 3–5 μm variability study of the YSOs population of the Vela-D star forming region. We have compared the 3.6 μm and 4.5 μm Spitzer-IRAC fluxes of 181 YSOs in Vela-D with their WISE fluxes at 3.4 μm and 4.6 μm and selected those objects simultaneously varying in both bands. We have identified a robust sample of 34 variables. On the base of the infrared excess of the Spectral Energy Distribution (SED) and the magnitude vs. color variations, we select 5 EXors candidates, which will be systematically monitored to firmly ascertain their nature. The selected 34 variables represent ∼18 % of the YSOs detected with Spitzer and WISE, a percentage higher than that of other young star forming regions. Conversely, the percentage of candidate EXors (2.7 %) is quite similar to that measured in Perseus, Ophiuchus and Serpens, and also equals that found in Vela-D on the base of Spitzer variability (Giannini et al., Astrophys. J. 704:606, 2009). Consistently with our finding presented in Antoniucci et al. (2014), this fraction equals the probability of observing the source once in burst and once in quiescence, under the hypothesis that the time elapsed between the two events is of about 0.5–1 year. Of the 5 selected EXors candidates, 3 are Class I sources, and 2 are flat-spectrum sources, a circumstance that suggests that accretion-driven variability is a common phenomenon during the earlier phases of the protostellar evolution. In the light of the new WISE data, we also re-examine a sample of 10 variables, which we had already selected in Giannini et al. (2009). From the inspection of their light curves, we select two flat-spectrum sources as the best EXors candidates.