Cluster Of Young Stellar Objects Research Articles

Stellar Membership of RCW 34 HII Star-Forming Region

The Vel OB1 association, located at a distance of 1.5-1.9 kpc, is one of the most actively studied star formation regions in the southern hemisphere. Our study focuses on an “Arclike” structure within the Vel OB1 association, which may have been formed from a shock wave caused by a supernova explosion about 2-2.5 Myr ago. Optical Gaia data revealed probable stellar members only in the southern part of the Arclike structure, where the RCW34 HII (IRAS 08546-4254) region is located. In contrast, the northeastern part of the Arclike structure, in the vicinity of the IRAS 08563-4225 source, hosts more embedded stellar population. Using near-infrared astrometric and photometric data from the 2MASS survey, we identified well-defined clusters of young stellar objects in the vicinity of both IRAS 08546-4254 and IRAS 08563-4225. The members of the cluster associated with IRAS 08563-4225 exhibit stronger infrared excess.

Identification of the young stellar clusters in the G345.5+1.5 Star-Forming Region

Star-forming regions, their formation and evolution are a subject of active study. The subject of our research is the G345.5+1.5 region, which is composed of two dusty ring-like structures referred to as G345.45+1.5 and G345.10+1.35. The region is located at a distance of 1.8 kpc and classified as HII regions. According to previous studies, G345.45+1.5 could have been created by a supernova explosion represented by the 36.6 cm source J165920-400424 in its center. Despite the considerable interest, only a few works are devoted to this region, and there is very little information about its stellar population. Our work focuses specifically on the stellar population in this region. Using 2MASS astrometric and photometric data, we identified three well-defined clusters of young stellar objects (YSOs) in the vicinity of the IRAS 16561-4006, 16571-4029, and 16545-4012 sources.

The Galactic Center Lobe as an H ii Region

The Galactic center lobe (GCL) is an object ∼1° across that is located north of the Galactic center. In the mid-infrared (MIR) the GCL appears as two 8.0 μm filaments between which there is strong 24 μm and radio continuum emission. Due to its morphology and location in the sky, previous authors have argued that the GCL is located in the Galactic center region, created by outflows from star formation or by activity of the central black hole Sagittarius A*. In an associated paper, low-frequency radio emission indicates that the GCL must instead lie foreground to the Galactic center. If the GCL is foreground to the Galactic center, it is likely to be a type of object common throughout the Galactic disk; we here investigate whether its properties are similar to those of Galactic H ii regions. We find that the GCL’s MIR morphology, MIR flux densities, dust temperatures, and radio recombination line properties as traced by the Green Bank Telescope Diffuse Ionized Gas Survey are consistent with those of known Galactic H ii regions, although the derived electron temperature is low. We search for the ionizing source(s) of the possible H ii region and identify a stellar cluster candidate (Camargo #1092/Ryu & Lee #532) and a cluster of young stellar objects (the SPICY cluster G359.3+0.3) whose members have Gaia parallaxes distances of 1.7 ± 0.4 kpc. Taken together, the results of our companion paper and those shown here suggest that the GCL has properties consistent with those of an H ii region located ∼2 kpc from the Sun.

JWST Mid-infrared Spectroscopy Resolves Gas, Dust, and Ice in Young Stellar Objects in the Large Magellanic Cloud

In this work, we present spectra of 11 young stellar objects (YSOs) taken with the Mid-Infrared Instrument / Medium Resolution Spectroscopy (MRS) instrument on board the James Webb Space Telescope (JWST). The YSOs are located in the N79 region of the Large Magellanic Cloud (LMC), an active star-forming region with hundreds of Spitzer- and Herschel-identified YSOs and host to super star cluster (SSC) candidate H72.97-69.39. The three giant molecular clouds in N79 (East, West, and South) have varying star formation rates and stellar populations. MRS follow-up observations of four Spitzer-identified YSOs in N79 East, West, and South have revealed that what seemed to be a single, massive YSO is actually a cluster of YSOs. We discuss the emission and absorption lines of six YSOs that have complete or almost-complete spectral coverage from 4.9–27.9 μm. YSO Y3, located in N79 East, is the youngest source in this study and likely to be less than 10,000 yr old, as inferred from the prominent CH4, NH3, CH3OH, CH3OCHO, and CO2 ice absorption features. The most luminous source is the central ionizing YSO of SSC H72.97-69.39, Y4, which has dozens of fine-structure and H2 emission lines. Unlike the other YSOs in this work, Y4 has no polyaromatic hydrocarbon emission lines, due to the intense ionizing radiation destroying these large carbon-chain molecules. The mass accretion rates based on the H i (7-6) line luminosities of YSOs Y1, Y2, Y4, and Y9 range between 1.22 × 10−4–1.89 × 10−2 M ⊙ yr−1. For the first time in the mid-infrared, we are able to resolve individual high-mass protostars forming in small clusters in an extragalactic environment like the LMC.

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.

Spectroscopic Confirmation of a Population of Isolated, Intermediate-mass Young Stellar Objects

Wide-field searches for young stellar objects (YSOs) can place useful constraints on the prevalence of clustered versus distributed star formation. The Spitzer/IRAC Candidate YSO (SPICY) catalog is one of the largest compilations of such objects (∼120,000 candidates in the Galactic midplane). Many SPICY candidates are spatially clustered, but, perhaps surprisingly, approximately half the candidates appear spatially distributed. To better characterize this unexpected population and confirm its nature, we obtained Palomar/DBSP spectroscopy for 26 of the optically bright (G < 15 mag) “isolated” YSO candidates. We confirm the YSO classifications of all 26 sources based on their positions on the Hertzsprung–Russell diagram, H and Ca ii line emission from over half the sample, and robust detection of infrared excesses. This implies a contamination rate of <10% for SPICY stars that meet our optical selection criteria. Spectral types range from B4 to K3, with A-type stars being the most common. Spectral energy distributions, diffuse interstellar bands, and Galactic extinction maps indicate moderate-to-high extinction. Stellar masses range from ∼1 to 7 M ⊙, and the estimated accretion rates, ranging from 3 × 10−8 to 3 × 10−7 M ⊙ yr−1, are typical for YSOs in this mass range. The 3D spatial distribution of these stars, based on Gaia astrometry, reveals that the “isolated” YSOs are not evenly distributed in the Solar neighborhood but are concentrated in kiloparsec-scale dusty Galactic structures that also contain the majority of the SPICY YSO clusters. Thus, the processes that produce large Galactic star-forming structures may yield nearly as many distributed as clustered YSOs.

Hierarchical structure of YSO clusters in the W40 and Serpens South region: group extraction and comparison with fractal clusters

ABSTRACT Young stellar clusters are believed to inherit the spatial distribution like hierarchical structures of their natal molecular cloud during their formation. However, the change of the structures between the cloud and the young clusters is not well constrained observationally. We select the W40–Serpens South region (∼7 × 9 pc2) of the Aquila Rift as a testbed and investigate hierarchical properties of spatial distribution of young stellar objects (YSOs) in this region. We develop a minimum spanning tree (MST) based method to group stars into several levels by successively cutting down edges longer than an algorithmically determined critical value. A total of 832 YSOs are divided into 5 levels with 23 groups. For describing the hierarchical properties in a controlled way, we construct a set of synthetic source distributions at various fractal dimensions, and apply the same method to explore their group characters. By comparing the Q parameter and the surface density profiles of the observed and the synthetic data, we find that the YSO observation matches spatial patterns from multifractal dimensions. In the periphery region where the molecular clouds are more diffuse, the YSO structure is close to a fractal dimension of 2.0. While in the core regions, the fractal dimensions are close to 1.6 and 1.4 for the W40 and the Serpens South regions, respectively. Therefore, the YSOs may inherit the fractal pattern of the dense part of the molecular clouds, but such pattern dissipates slowly in several Myr.

Sh 2-301: A Blistered H ii Region Undergoing Star Formation

Abstract We present a multiwavelength study of the H ii region Sh 2-301 (S301) using deep optical data, near-infrared data, radio continuum data, and other archival data at longer wavelengths. A cluster of young stellar objects (YSOs) is identified in the northeast (NE) direction of S301. The Hα and radio continuum images trace the distribution of the ionized gas surrounding a massive star, ALS 207, and the S301 H ii region is bounded by an arc-like structure of gas and dust emission in the southeastern direction. The northwestern part of S301 seems to be devoid of gas and dust emission, while the presence of molecular material between the NE cluster and the central massive star ALS 207 is found. The distribution of warm dust emission, ionized gas, and neutral hydrogen together suggests a blistered morphology of the S301 H ii region powered by ALS 207, which appears to be located near the edge of the cloud. The location of the NE cluster embedded in the cold molecular cloud is found opposite to the blistered morphology. There is a noticeable age difference investigated between the massive star and the NE cluster. This age difference, pressure calculation, photodissociation regions, and the distribution of YSOs favor the positive feedback of the massive star ALS 207 in S301. On a wider scale of S301, the H ii region and the young stellar cluster are depicted toward the central region of a hub-filamentary system, which is evident in the infrared images.

Clustered star formation at early evolutionary stages

Context. The process of high-mass star formation during the earliest evolutionary stages and the change over time of the physical and chemical properties of individual fragmented cores are still not fully understood. Aims. We aim to characterize the physical and chemical properties of fragmented cores during the earliest evolutionary stages in the very young star-forming regions ISOSS J22478+6357 and ISOSS J23053+5953. Methods. NOrthern Extended Millimeter Array 1.3 mm data are used in combination with archival mid- and far-infrared Spitzer and Herschel telescope observations to construct and fit the spectral energy distributions of individual fragmented cores. The radial density profiles are inferred from the 1.3 mm continuum visibility profiles, and the radial temperature profiles are estimated from H2CO rotation temperature maps. Molecular column densities are derived with the line fitting tool XCLASS. The physical and chemical properties are combined by applying the physical-chemical model MUlti Stage ChemicaL codE in order to constrain the chemical timescales of a few line-rich cores. The morphology and spatial correlations of the molecular emission are analyzed using the histogram of oriented gradients (HOG) method. Results. The mid-infrared data show that both regions contain a cluster of young stellar objects. Bipolar molecular outflows are observed in the CO 2−1 transition toward the strong millimeter (mm) cores, indicating protostellar activity. We find strong molecular emission of SO, SiO, H2CO, and CH3OH in locations that are not associated with the mm cores. These shocked knots can be associated either with the bipolar outflows or, in the case of ISOSS J23053+5953, with a colliding flow that creates a large shocked region between the mm cores. The mean chemical timescale of the cores is lower (~20 000 yr) compared to that of the sources of the more evolved CORE sample (~60 000 yr). With the HOG method, we find that the spatial emission of species that trace the extended emission and of shock-tracing molecules are well correlated within transitions of these groups. Conclusions. Clustered star formation is observed toward both regions. Comparing the mean results of the density and temperature power-law index with the results of the original CORE sample of more evolved regions, it appears that neither change significantly from the earliest evolutionary stages to the hot molecular core stage. However, we find that the 1.3 mm flux, kinetic temperature, H2 column density, and core mass of the cores increase in time, which can be traced both in the M/L ratio and the chemical timescale, τchem.

The link between gas and stars in the S254–S258 star-forming region

ABSTRACT The paper aims to study relation between the distributions of the young stellar objects (YSOs) of different ages and the gas-dust constituents of the S254–S258 star formation complex. This is necessary to study the time evolution of the YSO distribution with respect to the gas and dust compounds that are responsible for the birth of the young stars. For this purpose, we use correlation analysis between different gas, dust, and YSO tracers. We compared the large-scale CO, HCO+, near-IR extinction, and far-IR Herschel maps with the density of YSOs of the different evolutionary classes. The direct correlation analysis between these maps was used together with the wavelet-based spatial correlation analysis. This analysis reveals a much tighter correlation of the gas-dust tracers with the distribution of class I YSOs than with that of class II YSOs. We argue that class I YSOs that were initially born in the central bright cluster S255-IR (both N and S parts) during their evolution to class II stage (∼2 Myr) had enough time to travel through the whole S254–S258 star formation region. Given that the region contains several isolated YSO clusters, the evolutionary link between these clusters and the bright central S255-IR (N and S) cluster can be considered. Despite the complexity of the YSO cluster formation in the non-uniform medium, the clusters of class II YSOs in the S254-258 star formation region can contain objects born in the different locations of the complex.

Herbig–Haro flows around the BBWo 192E (GM 1–23) nebula

ABSTRACT Looking for evidence of recent star formation, we have studied a small comet-shaped reflection nebula, known as BBWo 192E (GM 1–23), which is located in the dark cloud SL 4 in the Vela Molecular Ridge cloud C, and a young infrared cluster embedded into the nebula. We obtained the images of BBWo 192E in Hα and [S ii] lines and in a Sloan Digital Sky Survey i ′ filter with the Blanco telescope at the Cerro Tololo Inter-American Observatory in order to discover new Herbig–Haro (HH) flows. We used the Two-Micron All-Sky Survey and the Wide-field Infrared Survey Explorer to search for additional member stars of the cluster. We also studied the proper motions and parallaxes of the cluster members using GAIA Data Release 2. Five new groups containing at least nine HH objects, tracing several distinct outflows, were revealed. A previously unreported reflection nebula and a number of probable outflow sources were found in the infrared range. The proper motions allowed us to select eight probable member stars in the visual range. Their parallaxes correspond to a mean distance 800 ± 100 pc for this cluster. The bolometric luminosities of the brightest cluster members are 1010 L⊙ (IRAS 08513−4201, the strong source in the centre of the cluster) and 2–6 L⊙ for the five other stars. The existence of optical HH flows around the infrared cluster of young stellar objects suggests that star formation in this cloud is ongoing around the more massive Herbig Ae/Be star. Considering its morphology and other features, this star-forming region is similar to the zone of star formation near CPM 19.

The Cluster-forming Site AFGL 5157: Colliding Filamentary Clouds and Star Formation

Abstract We observationally investigate star formation process occurring in AFGL 5157 (area ∼13.5 pc × 13.5 pc) using a multi-wavelength approach. Embedded filaments are seen in the Herschel column density map, and one of them is identified as an elongated filamentary feature (FF) (length ∼8.3 pc; mass ∼1170 M ⊙). Five Herschel clumps (M clump ∼45–300 M ⊙) are traced in the central part of the FF, where an extended temperature structure (T d ∼13.5–26.5 K) is observed. In the direction of the central part of the FF, the warmer region at T d ∼20–26.5 K spatially coincides with a mid-infrared shell surrounding a previously known evolved infrared cluster. Diffuse Hα emission is traced inside the infrared shell, suggesting the presence of massive stars in the evolved cluster. Based on the surface density analysis of young stellar objects (YSOs), embedded clusters of YSOs are traced toward the central part of the FF, and are distributed around the infrared shell. Previously detected H2O masers, H2 knots, massive protostar candidates, and an H ii region are also seen toward the embedded clusters. Using the 12CO and 13CO line data, the central part of the FF is observed at the overlapping zones of two filamentary molecular clouds (length ∼12.5 pc) around −20 and −17 km s−1, which are also connected in velocity. Our observational results suggest that the formation of massive stars appears to be triggered by a collision of two filamentary molecular clouds, which might have also influenced the birth of YSOs in AFGL 5157.

Star Formation in the Sh 2-53 Region Influenced by Accreting Molecular Filaments

Abstract We present a multiwavelength analysis of a ∼30′ × 30′ area around the Sh 2-53 region (hereafter S53 complex), which is associated with at least three H ii regions, two mid-infrared bubbles (N21 and N22), and infrared dark clouds. The 13CO line data trace the molecular content of the S53 complex in a velocity range of 36–60 km s−1 and show the presence of at least three molecular components within the selected area along this direction. Using the observed radio continuum flux of the H ii regions, the derived spectral types of the ionizing sources agree well with the previously reported results. The S53 complex harbors clusters of young stellar objects (YSOs) that are identified using the photometric 2–24 μm magnitudes. It also hosts several massive condensations ( ) that are traced in the Herschel column density map. The complex is found at the junction of at least five molecular filaments, and the flow of gas toward the junction is evident in the velocity space of the 13CO data. Together, the S53 complex is embedded in a very similar “hub–filament” system to those reported in Myers, and the active star formation is evident toward the central “hub” inferred by the presence of the clustering of YSOs.

The Molecular Cloud S242: Physical Environment and Star-formation Activities

Abstract We present a multi-wavelength study to probe the star-formation (SF) processes on a larger scale ( ) around the S242 site. The S242 molecular cloud is depicted in a velocity range from −3.25 to 4.55 km s−1 and has a spatially elongated appearance. Based on the virial analysis, the cloud is prone to gravitational collapse. The cloud harbors an elongated filamentary structure (EFS; length ∼25 pc), which is evident in the Herschel column density map, and the EFS has an observed mass per unit length of ∼200 pc−1, exceeding the critical value of ∼16 pc−1 (at T = 10 K). The EFS contains a chain of Herschel clumps (M clump ∼ 150–1020 ), revealing the evidence of fragmentation along its length. The most massive clumps are observed at both the EFS ends, while the S242 H ii region is located at one EFS end. Based on the radio continuum maps at 1.28 and 1.4 GHz, the S242 H ii region is ionized by a B0.5V–B0V type star and has a dynamical age of ∼0.5 Myr. The photometric 1–5 μm data analysis of point-like sources traces young stellar objects (YSOs) toward the EFS and the clusters of YSOs are exclusively found at both the EFS ends, revealing the SF activities. Considering the spatial presence of massive clumps and YSO clusters at both the EFS ends, the observed results are consistent with the prediction of an SF scenario of the end-dominated collapse driven by the higher acceleration of gas.

Hub-filament System in IRAS 05480+2545: Young Stellar Cluster and 6.7 GHz Methanol Maser

Abstract To probe the star formation (SF) process, we present a multi-wavelength study of IRAS 05480+2545 (hereafter I05480+2545). Analysis of Herschel data reveals a massive clump (M clump ∼ 1875 peak N(H2) ∼ 4.8 × 1022 cm−2; A V ∼ 51 mag) containing the 6.7 GHz methanol maser and I05480+2545, which is also depicted in a temperature range of 18–26 K. Several noticeable parsec-scale filaments are detected in the Herschel 250 μm image and seem to be radially directed to the massive clump. It resembles more of a “hub-filament” system. Deeply embedded young stellar objects (YSOs) have been identified using the 1–5 μm photometric data, and a significant fraction of YSOs and their clustering are spatially found toward the massive clump, revealing the intense SF activities. An infrared counterpart (IRc) of the maser is investigated in the Spitzer 3.6–4.5 μm images. The IRc does not appear as a point-like source and is most likely associated with the molecular outflow. Based on the 1.4 GHz and Hα continuum images, the ionized emission is absent toward the IRc, indicating that the massive clump harbors an early phase of a massive protostar before the onset of an ultracompact H ii region. Together, the I05480+2545 is embedded in a very similar “hub-filament” system to those seen in the Rosette Molecular Cloud. The outcome of the present work indicates the role of filaments in the formation of the massive star-forming clump and cluster of YSOs, which might help channel material to the central hub configuration and the clump/core.

Star Formation Activity in the Molecular Cloud G35.20–0.74: Onset of Cloud–Cloud Collision

Abstract To probe star formation (SF) processes, we present results of an analysis of the molecular cloud G35.20−0.74 (hereafter MCG35.2) using multi-frequency observations. The MCG35.2 is depicted in a velocity range of 30–40 km s−1. An almost horseshoe-like structure embedded within the MCG35.2 is evident in the infrared and millimeter images and harbors the previously known sites, ultra-compact/hyper-compact G35.20−0.74N H ii region, Ap2-1, and Mercer 14 at its base. The site, Ap2-1, is found to be excited by a radio spectral type of B0.5V star where the distribution of 20 cm and Hα emission is surrounded by the extended molecular hydrogen emission. Using the Herschel 160–500 μm and photometric 1–24 μm data analysis, several embedded clumps and clusters of young stellar objects (YSOs) are investigated within the MCG35.2, revealing the SF activities. A majority of the YSOs clusters and massive clumps (500–4250 ) are seen toward the horseshoe-like structure. The position–velocity analysis of 13CO emission shows a blueshifted peak (at 33 km s−1) and a redshifted peak (at 37 km s−1) interconnected by lower intensity intermediate velocity emission, tracing a broad bridge feature. The presence of such a broad bridge feature suggests the onset of a collision between molecular components in the MCG35.2. A noticeable change in the H-band starlight mean polarization angles has also been observed in the MCG35.2, probably tracing the interaction between molecular components. Taken together, it seems that the cloud–cloud collision process has influenced the birth of massive stars and YSOs clusters in the MCG35.2.

MULTIWAVELENGTH STUDY OF THE STAR FORMATION IN THE S237 H ii REGION

ABSTRACT We present a detailed multiwavelength study of observations from X-ray, near-infrared, and centimeter wavelengths to probe the star formation processes in the S237 region. Multiwavelength images trace an almost sphere-like shell morphology of the region, which is filled with the 0.5–2 keV X-ray emission. The region contains two distinct environments—a bell-shaped cavity-like structure containing the peak of 1.4 GHz emission at center, and elongated filamentary features without any radio detection at edges of the sphere-like shell—where Herschel clumps are detected. Using the 1.4 GHz continuum and 12CO line data, the S237 region is found to be excited by a radio spectral type of B0.5V star and is associated with an expanding H ii region. The photoionized gas appears to be responsible for the origin of the bell-shaped structure. The majority of molecular gas is distributed toward a massive Herschel clump (M clump ∼ 260 ), which contains the filamentary features and has a noticeable velocity gradient. The photometric analysis traces the clusters of young stellar objects (YSOs) mainly toward the bell-shaped structure and the filamentary features. Considering the lower dynamical age of the H ii region (i.e., 0.2–0.8 Myr), these clusters are unlikely to be formed by the expansion of the H ii region. Our results also show the existence of a cluster of YSOs and a massive clump at the intersection of filamentary features, indicating that the collisions of these features may have triggered cluster formation, similar to those found in the Serpens South region.

THE PHYSICAL ENVIRONMENT AROUND IRAS 17599–2148: INFRARED DARK CLOUD AND BIPOLAR NEBULA

ABSTRACT We present a multiscale and multiwavelength study to investigate the star formation process around IRAS 17599–2148, which is part of an elongated filamentary structure (EFS) (extension ∼21 pc) seen in the Herschel maps. Using the Herschel data analysis, at least six massive clumps (M clump ∼ 777–7024 M ⊙) are found in the EFS with a range of temperature and column density of ∼16–39 K and ∼(0.6–11) × 1022 cm−2 (A V ∼ 7–117 mag), respectively. The EFS hosts cold gas regions (i.e., infrared dark cloud) without any radio detection and a bipolar nebula (BN) linked with the H ii region IRAS 17599–2148, tracing two distinct environments inferred through the temperature distribution and ionized emission. Based on virial analysis and higher values of self-gravitating pressure, the clumps are found unstable against gravitational collapse. We find 474 young stellar objects (YSOs) in the selected region, and ∼72% of these YSOs are found in the clusters distributed mainly toward the clumps in the EFS. These YSOs might have spontaneously formed due to processes not related to the expanding H ii region. At the edges of BN, four additional clumps are also associated with YSO clusters, which appear to be influenced by the expanding H ii region. The most massive clump in the EFS contains two compact radio sources traced in the Giant Metre-wave Radio Telescope 1.28 GHz map and a massive protostar candidate, IRS 1, prior to an ultracompact H ii phase. Using the Very Large Telescope/NACO near-infrared images, IRS 1 is resolved with a jet-like feature within a 4200 au scale.

STAR FORMATION AROUND MID-INFRARED BUBBLE N37: EVIDENCE OF CLOUD–CLOUD COLLISION

ABSTRACT We have performed a multi-wavelength analysis of a mid-infrared (MIR) bubble N37 and its surrounding environment. The selected 15′ × 15′ area around the bubble contains two molecular clouds (N37 cloud; 37–43 km s−1, and C25.29+0.31; 43–48 km s−1) along the line of sight. A total of seven OB stars are identified toward the bubble N37 using photometric criteria, and two of them are spectroscopically confirmed as O9V and B0V stars. The spectro-photometric distances of these two sources confirm their physical association with the bubble. The O9V star appears to be the primary ionizing source of the region, which is also in agreement with the desired Lyman continuum flux analysis estimated from the 20 cm data. The presence of the expanding H ii region is revealed in the N37 cloud, which could be responsible for the MIR bubble. Using the 13CO line data and photometric data, several cold molecular condensations as well as clusters of young stellar objects (YSOs) are identified in the N37 cloud, revealing ongoing star formation (SF) activities. However, the analysis of ages of YSOs and the dynamical age of the H ii region do not support the origin of SF due to the influence of OB stars. The position–velocity analysis of 13CO data reveals that two molecular clouds are interconnected by a bridge-like structure, favoring the onset of a cloud–cloud collision process. The SF activities (i.e., the formation of YSO clusters and OB stars) in the N37 cloud are possibly influenced by the cloud–cloud collision.

STAR-FORMATION ACTIVITY IN THE NEIGHBORHOOD OF W–R 1503-160L STAR IN THE MID-INFRARED BUBBLE N46

ABSTRACT In order to investigate star-formation (SF) processes in extreme environments, we have carried out a multi-wavelength analysis of the mid-infrared bubble N46, which hosts a WN7 Wolf–Rayet (W–R) star. We have used 13CO line data to trace an expanding shell surrounding the W–R star containing about five condensations within the molecular cloud associated with the bubble. The W–R star is associated with a powerful stellar wind having a mechanical luminosity of ∼4 × 1037 erg s−1. A deviation of the H-band starlight mean polarization angles around the bubble has also been traced, indicating the impact of stellar wind on the surroundings. The Herschel temperature map shows a temperature range of ∼18–24 K toward the five molecular condensations. The photometric analysis reveals that these condensations are associated with the identified clusters of young stellar objects, revealing ongoing SF process. The densest among these five condensations (peak N(H2) ∼9.2 × 1022 cm−2 and A V ∼ 98 mag) is associated with a 6.7 GHz methanol maser, an infrared dark cloud, and the CO outflow, tracing active massive SF within it. At least five compact radio sources (CRSs) are physically linked with the edges of the bubble, and each of them is consistent with the radio spectral class of a B0V–B0.5V-type star. The ages of the individual infrared counterparts of three CRSs (∼1–2 Myr) and a typical age of WN7 W–R star (∼4 Myr) indicate that the SF activities around the bubble are influenced by the feedback of the W–R star.