With the aim of studying the physical properties of Galactic IR bubbles and to explore their impact in massive star formation, we present a study of the IR bubble S169, associated with the massive star forming region IRAS 12326-6245. We used CO (2-1),^13^CO (2-1), C^18^O (2-1), HCN (3-2), and HCO^+^ (3-2) line data obtained with the APEX telescope using the on-the-fly full sampling technique to study the properties of the molecular gas in the nebula and the IRAS source. To analyze the properties and distribution of the dust, we made use of images obtained from the IRAC-GLIMPSE, Herschel, and ATLASGAL archives. The properties of the ionized gas in the nebula were studied using radio continuum and H{alpha} images obtained from the SUMSS survey and SuperCOSMOS database, respectively. In our search for stellar and protostellar objects in the region, we used point source calalogs obtained from the MSX, WISE, GLIMPSE, 2MASS, AAVSO, ASCC-2.5V3, and GAIA databases. The new APEX observations allowed us to identify three molecular components, each one associated with different regions of the nebula, namely: at -39km/s (component A), -25km/s (component B), and -17km/s (component C). Component A is shown to be the most dense and clumpy. Six molecular condensations (MC1 to MC6) were identified in this component, with MC3 (the densest and more massive one) being the molecular counterpart of IRAS 12326-6245. For this source, we estimated an H_2_ column density up to 8x10^23^cm^-2^. An LTE analysis of the high density tracer lines HCO^+^ (3-2) and HCN (3-2) on this source, assuming 50 and 150K, respectively, indicates column densities of N(HCO^+^)=(5.2+/-0.1)x10^13^cm^-2^ and N(HCN)=(1.9+/-0.5)x10^14^cm^-2^. To explain the morphology and velocity of components A, B, and C, we propose a simple model consisting of a partially complete semisphere-like structure expanding at ~12km/s. The introduction of this model has led to a discussion about the distance to both S169 and IRAS 12326-6245, which was estimated to be ~2kpc. Several candidate YSOs were identified, projected mostly onto the molecular condensations MC3, MC4, and MC5, which indicates that the star-formation process is very active at the borders of the nebula. A comparison between observable and modeled parameters was not enough to discern whether the collect-and-collapse mechanism is acting at the edge of S169. However, other processes such as radiative-driven implosion or even a combination of both mechanisms, namely, collect-and-collapse and radiative-driven implosion, could be acting simultaneously in the region.
We studied the environment of the dust bubble N10 in molecular emission. Infrared bubbles, first detected by the GLIMPSE survey at 8.0{mu}m, are ideal regions to investigate the effect of the expansion of the HII region on its surroundings and the eventual triggering of star formation at its borders. In this work, we present a multi-wavelength study of N10. This bubble is especially interesting because infrared studies of the young stellar content suggest a scenario of ongoing star formation, possibly triggered on the edge of the HII region. We carried out observations of ^12^CO(1-0) and ^13^CO(1-0) emission at PMO 13.7m toward N10. We also analyzed the IR and sub-millimeter emission on this region and compare those different tracers to obtain a detailed view of the interaction between the expanding HII region and the molecular gas. We also estimated the parameters of the denser cold dust condensation and the ionized gas inside the shell. Bright CO emission was detected and two molecular clumps were identified from which we have derived physical parameters. We also estimate the parameters for the densest cold dust condensation and for the ionized gas inside the shell. The comparison between the dynamical age of this region and the fragmentation timescale favors the "Radiation-Driven Implosion" mechanism of star formation. N10 is a case of particular interest with gas structures in a narrow frontier between the HII region and surrounding molecular material, and with a range of ages of YSOs situated in the region, indicating triggered star formation.
We have carried out a multiwavelength study of the infrared dust bubble N6 to extensively investigate the molecular environs and star-forming activities therein. 99 young stellar objects (YSOs) have been identified based on their infrared colors. A group of YSOs reside inside the ring, indicating active star formation in N6. Although no confirmative features of triggered star formation are detected, the bubble and the enclosed HII region have profoundly reconstructed the natal cloud and altered the dynamics therein.
The Magellanic Bridge is the nearest low-metallicity, tidally stripped environment, offering a unique high-resolution view of physical conditions in merging and forming galaxies. In this paper, we present an analysis of candidate massive young stellar objects (YSOs), i.e., in situ, current massive star formation (MSF) in the Bridge using Spitzer mid-IR and complementary optical and near-IR photometry. While we definitely find YSOs in the Bridge, the most massive are ~10 M_{sun}_<<45 M_{sun}_ found in the LMC. The intensity of MSF in the Bridge also appears to be decreasing, as the most massive YSOs are less massive than those formed in the past. To investigate environmental effects on MSF, we have compared properties of massive YSOs in the Bridge to those in the LMC. First, YSOs in the Bridge are apparently less embedded than in the LMC: 81% of Bridge YSOs show optical counterparts, compared to only 56% of LMC sources with the same range of mass, circumstellar dust mass, and line-of-sight extinction. Circumstellar envelopes are evidently more porous or clumpy in the Bridge's low-metallicity environment. Second, we have used whole samples of YSOs in the LMC and the Bridge to estimate the probability of finding YSOs at a given H I column density, N(H I). We found that the LMC has ~3xhigher probability than the Bridge for N(H I)>12x10^20^/cm^2^, but the trend reverses at lower N(H I). Investigating whether this lower efficiency relative to H I is due to less efficient molecular cloud formation or to less efficient cloud collapse, or to both, will require sensitive molecular gas observations.
We present the initial results of our investigation of the star-forming complex W49, one of the youngest and most luminous massive star-forming regions in our Galaxy. We used Spitzer/Infrared Array Camera (IRAC) data to investigate massive star formation with the primary objective of locating a representative set of protostars and the clusters of young stars that are forming around them. We present our source catalog with the mosaics from the IRAC data. In this study we used a combination of IRAC, MIPS, Two Micron All Sky Survey, and UKIRT Deep Infrared Sky Survey (UKIDSS) data to identify and classify the young stellar objects (YSOs). We identified 232 Class 0/I YSOs, 907 Class II YSOs, and 74 transition disk candidate objects using color-color and color-magnitude diagrams. In addition, to understand the evolution of star formation in W49, we analyzed the distribution of YSOs in the region to identify clusters using a minimal spanning tree method. The fraction of YSOs that belong to clusters with >=7 members is found to be 52% for a cutoff distance of 96", and the ratio of Class II/I objects is 2.1. We compared the W49 region to the G305 and G333 star-forming regions and concluded that W49 has the richest population, with seven subclusters of YSOs.
The physical mechanisms that induce the transformation of a certain mass of gas in new stars are far from being well understood. Infrared bubbles associated with H II regions have been considered to be good samples for investigating triggered star formation. In this paper we report on the investigation of the dust properties of the infrared bubble N4 around the H II region G11.898+0.747, analyzing its interaction with its surroundings and star formation histories therein, with the aim of determining the possibility of star formation triggered by the expansion of the bubble. Using Herschel PACS and SPIRE images with a wide wavelength coverage, we reveal the dust properties over the entire bubble. Meanwhile, we are able to identify six dust clumps surrounding the bubble, with a mean size of 0.50pc, temperature of about 22K, mean column density of 1.7x10^22^/cm2, mean volume density of about 4.4x10^4^/cm3, and a mean mass of 320M_{sun}_. In addition, from PAH emission seen at 8 {mu}m, free-free emission detected at 20cm, and a probability density function in special regions, we could identify clear signatures of the influence of the HII region on the surroundings. There are hints of star formation, though further investigation is required to demonstrate that N4 is the triggering source.
We have performed an unbiased search for outflows from young stars in Cygnus-X using 42deg^2^ of data from the UKIRT Widefield Infrared Survey for H_2_ (UWISH2 Survey), to identify shock-excited near-IR H_2_ emission in the 1-0 S(1) 2.122{mu}m line. We uncovered 572 outflows, of which 465 are new discoveries, increasing the number of known objects by more than 430%. This large and unbiased sample allows us to statistically determine the typical properties of outflows from young stars. We found 261 bipolar outflows, and 16% of these are parsec scale. The typical bipolar outflow is 0.45pc in length and has gaps of 0.025-0.1pc between large knots. The median luminosity in the 1-0 S(1) line is 10^-3^L_{sun}_. The bipolar flows are typically asymmetrical, with the two lobes misaligned by 5{deg}, one lobe 30% shorter than the other, and one lobe twice as bright as the other. Of the remaining outflows, 152 are single- sided and 159 are groups of extended, shock-excited H2 emission without identifiable driving sources. Half of all driving sources have sufficient WISE data to determine their evolutionary status as either protostars (80%) or classical T Tauri stars (20%). One-fifth of the driving sources are variable by more than 0.5mag in the K-band continuum over several years. Several of the newly identified outflows provide excellent targets for follow-up studies. We particularly encourage the study of the outflows and young stars identified in a bright-rimmed cloud near IRAS 20294+4255, which seems to represent a textbook example of triggered star formation.
We present high-resolution (R~18000), high signal-to-noise ratio, 2{mu}m spectra of 52 IR-selected Class I and flat-spectrum young stellar objects in the Taurus-Auriga, {rho} Ophiuchi, Serpens, Perseus, and Corona Australis dark clouds. We detect key absorption lines in 41 objects and fit synthetic spectra generated from pre-main-sequence models to deduce the effective temperatures, surface gravities, near-IR veilings, rotation velocities, and radial velocities of each of these 41 sources.
We have compiled a catalog of candidate protostars from the major astronomical journals up to the end of 1993. The Belchman-Ichikawa color criterion was used as the main test of an author's claim that a source should be deemed a candidate protostar. Names, positions (1950 and 2000), LSR velocities, information on whether there are associated outflows, and references are provided. This catalog is meant to update an earlier compilation by Wynn-Williams (1982).
During the embedded stage of star formation, bipolar molecular outflows and UV radiation from the protostar are important feedback processes. Both processes reflect the accretion onto the forming star and affect subsequent collapse or fragmentation of the cloud. Our aim is to quantify the feedback, mechanical and radiative, for a large sample of low-mass sources in a consistent manner. The outflow activity is compared to radiative feedback in the form of UV heating by the accreting protostar to search for correlations and evolutionary trends. Large-scale maps of 26 young stellar objects, which are part of the Herschel WISH key program are obtained using the CHAMP+ instrument on the Atacama Pathfinder EXperiment (^12^CO and ^13^CO 6-5; E_up_~100K), and the HARP-B instrument on the James Clerk Maxwell Telescope (^12^CO and ^13^CO 3-2; E_up_~30K). The maps have high spatial resolution, particularly the CO 6-5 maps taken with a 9" beam, resolving the morphology of the outflows. The maps are used to determine outflow parameters and the results are compared with higher-J CO lines obtained with Herschel. Envelope models are used to quantify the amount of UV-heated gas and its temperature from ^13^CO 6-5 observations. All sources in our sample show outflow activity, with the spatial extent decreasing from the Class 0 to the Class I stage. Consistent with previous studies, the outflow force, F_CO_, is larger for Class 0 sources than for Class I sources, even if their luminosities are comparable. The outflowing gas typically extends to much greater distances than the power-law envelope and therefore influences the surrounding cloud material directly. Comparison of the CO 6-5 results with HIFI H2O and PACS high-J CO lines, both tracing currently shocked gas, shows that the two components are linked, even though the transitions do not probe the same gas. The link does not extend down to CO 3-2. The conclusion is that CO 6-5 depends on the shock characteristics (density and velocity), whereas CO 3-2 is more sensitive to conditions in the surrounding environment (density). The radiative feedback is responsible for increasing the gas temperature by a factor of two, up to 30-50K, on scales of a few thousand AU, particularly along the direction of the outflow. The mass of the UV heated gas exceeds the mass contained in the entrained outflow in the inner ~3000AU and is therefore at least as important on small scales.
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