Evidence Of Triggered Star Formation Research Articles

Investigation of rocket effect in bright-rimmed clouds using Gaia EDR3

ABSTRACT Bright-rimmed clouds (BRCs) are excellent laboratories to explore the radiation-driven implosion mode of star formation because they show evidence of triggered star formation. In our previous study, BRC 18 has been found to accelerate away from the direction of the ionizing H ii region because of the well-known ‘Rocket Effect’. Based on the assumption that both BRC 18 and the candidate young stellar objects (YSOs) are kinematically coupled and using the latest Gaia EDR3 measurements, we found that the relative proper motions of the candidate YSOs exhibit a tendency of moving away from the ionizing source. Using BRC 18 as a prototype, we made our further analysis for 21 more BRCs, a majority of which showed a similar trend. For most of the BRCs, the median angle of the relative proper motion of the candidate YSOs is similar to the angle of on-sky direction from the ionizing source to the central IRAS source of the BRC. Based on the Pearson’s and Spearman’s correlation coefficients, we found a strong correlation between these two angles, which is further supported by the Kolmogorov–Smirnov (K−S) test on them. The strong correlation between these two angles supports the ‘Rocket Effect’ in the BRCs on the sky plane.

Three generations of stars: a possible case of triggered star formation

ABSTRACT Evidence for triggered star formation linking three generations of stars is difficult to assemble, as it requires convincingly associating evolved massive stars with H ii regions that, in turn, would need to present signs of active star formation. We present observational evidence for triggered star formation relating three generations of stars in the neighbourhood of the star LS II +26 8. We carried out new spectroscopic observations of LS II +26 8, revealing that it is a B0 III-type star. We note that LS II +26 8 is located exactly at the geometric centre of a semi-shell-like H ii region complex. The most conspicuous component of this complex is the H ii region Sh2-90, which is probably triggering a new generation of stars. The distances to LS II +26 8 and to Sh2-90 are in agreement (between 2.6 and 3 kpc). Analysis of the interstellar medium on a larger spatial scale shows that the H ii region complex lies on the north-western border of an extended H2 shell. The radius of this molecular shell is about 13 pc, which is in agreement with what an O9 V star (the probable initial spectral type of LS II +26 8 as inferred from evolutive tracks) can generate through its winds in the molecular environment. In conclusion, the spatial and temporal correspondences derived in our analysis enable us to propose a probable triggered star formation scenario initiated by the evolved massive star LS II +26 8 during its main-sequence stage, followed by stars exciting the H ii region complex formed in the molecular shell, and culminating in the birth of young stellar objects around Sh2-90.

Near-infrared spectroscopy of the massive stellar population of W51: evidence for multi-seeded star formation

Context. The interplay between the formation of stars, stellar feedback and cloud properties strongly influences the star formation history of giant molecular clouds. The formation of massive stars leads to a variety of stellar clusters, ranging from low stellar density OB associations to dense, gravitationally bound starburst clusters. Aims. We aimed at identifying the massive stellar content and reconstructing the star formation history of the W51 giant molecular cloud. Methods. We performed near-infrared imaging and K-band spectroscopy of the massive stars in W51. We analysed the stellar populations using colour-magnitude and colour-colour diagrams and compared the properties of the spectroscopically identified stars with stellar evolution models. Results. We derive the ages of the different sub-clusters in W51 and, based on our spectroscopy derive an age for W51 of 3 Myr or less. The age of the P Cygni star LS1 and the presence of two still forming proto-clusters suggests that the star formation history of W51 is more complex than a single burst. Conclusions. We did not find evidence for triggered star formation and we concluded that the star formation in W51 is multi seeded. We finally concluded that W51 is an OB association where different sub-clusters form over a time span of at least 3–5 Myr.

Young open clusters in the Galactic star forming region NGC 6357

NGC6357 is an active star forming region with very young massive open clusters (OC). These clusters contain some of the most massive stars in the Galaxy and strongly interact with nearby giant molecular clouds (GMC). We study the young stellar populations of the region and of the OC Pismis24, focusing on their relationship with the nearby GMCs. We seek evidence of triggered star formation propagating from the clusters. We used new deep JHKs photometry, along with unpublished deep IRAC/Spitzer MIR photometry, complemented with optical HST/WFPC2 high spatial resolution photometry and X-ray Chandra observations, to constrain age, initial mass function, and star formation modes in progress. We carefully examine and discuss all sources of bias (saturation, confusion, different sensitivities, extinction). NGC6357 hosts three large young stellar clusters, of which Pismis24 is the most prominent. We found that Pismis24 is a very young (~1-3 Myr) OC with a Salpeter-like IMF and a few thousand members. A comparison between optical and IR photometry indicates that the fraction of members with a NIR excess (i. e., with a circumstellar disk) is in the range 0.3-0.6, consistent with its photometrically derived age. We also find that Pismis24 is likely subdivided into a few different sub-clusters, one of which contains almost all the massive members. There are indications of current star formation triggered by these massive stars, but clear age trends could not be derived (although the fraction of stars with a NIR excess does increase towards the HII region associated with the cluster). The gas out of which Pismis24 formed must have been distributed in dense clumps within a cloud of less dense gas ~1 pc in radius. Our findings provide some new insight into how young stellar populations and massive stars emerge, and evolve in the first few Myr after birth, from a giant molecular cloud complex.

The environment and star formation of H ii region Sh2-163: a multi-wavelength study

To investigate the environment of H ii region Sh2-163 and search for evidence of triggered star formation in this region, we performed a multi-wavelength study of this H ii region. Most of our data were taken from large-scale surveys: 2MASS, CGPS, MSX and SCUBA. We also made CO molecular line observations, using the 13.7-m telescope. The ionized region of Sh2-163 is detected by both the optical and radio continuum observations. Sh2-163 is partially bordered by an arc-like photodissociation region (PDR), which is coincident with the strongest optical and radio emissions, indicating interactions between the H ii region and the surrounding interstellar medium. Two molecular clouds were discovered on the border of the PDR. The morphology of these two clouds suggests they are compressed by the expansion of Sh2-163. In cloud A, we found two molecular clumps. And it seems star formation in clump A2 is much more active than in clump A1. In cloud B, we found new outflow activities and massive star(s) are forming inside. Using 2MASS photometry, we tried to search for embedded young stellar object (YSO) candidates in this region. The very good agreement between CO emission, infrared shell and YSOs suggest that it is probably a star formation region triggered by the expansion of Sh2-163. We also found the most likely massive protostar related to IRAS 23314+6033.

The catalogues and mid-infrared environment of interstellar OH masers

Data for a number of OH maser lines have been collected from surveys. The positions are compared with recent mid-infrared (MIR) surveys such as the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) and Wide-field Infrared Survey Explorer (WISE) surveys, restricting the comparison to point sources. The colours and intensities of the IR sources are compared. There are many 18-cm OH masers, but far fewer in lines arising from higher energy levels. We also make a comparison with 5-cm Class II methanol masers. We have divided the results into three subsamples: those associated with OH masers only, those associated with OH masers and Class II methanol masers and those associated with Class II methanol masers only. There are no obvious differences in the colour-colour or colour-magnitude results for the GLIMPSE point sources. However, according to the results from the WISE 22-mu m survey, the sources associated with OH masers are brighter than those associated with methanol masers. We interpret the presence of OH and methanol masers as marking the locations of regions where stars are forming. The OH masers are located on the borders of sharp features found in the IR. These are referred to as bubbles. If the OH masers mark the positions of protostars, the result provides indirect evidence for triggered star formation caused by the expansion of the bubbles.

Multi-wavelength study of triggered star formation around 25 H II regions

We investigate 25 H II regions that show bubble morphology in 13CO(1–0) and infrared data, to search for quantitative evidence of triggered star formation by processes described by the collect and collapse (CC) and radiatively driven implosion (RDI) models. These H II regions display the morphology of a complete or partial bubble at 8 μm, and are all associated with the molecular clouds that surround them. We found that the electron temperature ranges from 5627 K to 6839 K in these H II regions, and the average electron temperature is 6083 K. The age of these H II regions is from 3.0 × 105 yr to 1.7 × 106 yr, and the mean age is 7.7 × 105 yr. Based on the morphology of the associated molecular clouds, we divide these H II regions into three groups, which may support CC and RDI models. We select 23 young IRAS sources which have an infrared luminosity of > 103L⊙ in 19 H II regions. In addition, we identify some young stellar objects (including Class I sources), which are only concentrated in H II regions G29.007+0.076, G44.339–0.827 and G47.028+0.232. The poly-cyclic aromatic hydrocarbon emissions of the three H II regions all show a cometary globule. Comparing the age of each H II region with the characteristic timescales for star formation, we suggest that the three H II regions can trigger clustered star formation by an RDI process. In addition, we detect seven molecular outflows in the five H II regions for the first time. These outflow sources may be triggered by the corresponding H II regions.

A bright-rimmed cloud sculpted by the H ii region Sh2-48

To characterize a bright-rimmed cloud embedded in the HII region Sh2-48 searching for evidence of triggered star formation. We carried out observations towards a region of 2'x2' centered at RA=18h 22m 11.39s, dec.=-14deg 35m 24.81s (J2000) using the Atacama Submillimeter Telescope Experiment (ASTE; Chile) 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 about 22". We also present radio continuum observations at 5 GHz carried out with the Jansky Very Large Array (JVLA; EEUU) interferometer with a synthetized beam of 7"x5". The analysis of our molecular observations reveals the presence of a relatively dense clump with n(H_2)~3x10^3 cm^-3, located in projection onto the interior of the HII region Sh2-48. The emission distribution of the four observed molecular transitions has, at VLSR~38 kms^-1, morphological anti-correlation with the bright-rimmed cloud as seen in the optical emission. From the new radio continuum observations we identify a thin layer of ionized gas located at the border of the clump which is facing to the ionizing star. The ionized gas has an electron density of about 73 cm^-3 which supports the hypothesis that the clump is being photoionized by the nearby star, BD-14 5014. From the evaluation of the pressure balance between the ionized and molecular gas, we conclude that the clump would be in a pre-pressure balance state with the shocks being driven into the surface layer. Two YSO candidates (class I), are placed slightly beyond the bright rim suggesting that their formation could have been triggered via the radiation-driven implosion process.

Star formation associated with the infrared dust bubble N68

We investigated the environment of the infrared dust bubble N68 and searched for evidence of triggered star formation in its surroundings. We performed a multiwavelength study of the nebula with data taken from several large-scale surveys: GLIMPSE, MIPSGAL, IRAS, NVSS, GRS and JCMT. We analyzed the spectral profile and the distribution of the molecular gas (13CO J = 1 − 0 and J = 3 − 2), and the dust in the environment of N68. The position-velocity diagram clearly shows that N68 may be expanding outward. We used two three-color images of the mid-infrared emission to explore the physical environment, and one color-color diagram to investigate the distribution of young stellar objects (YSOs). We found that the 24 μm emission is surrounded by the 8.0 μm emission. Morphologically, the 1.4 GHz continuum strongly correlates with the 24 μm emission, and the 13CO J = 1 − 0 and J = 3 − 2 emissions correlate well with the 8.0 μm emission. We investigated two compact cores located in the shell of N68. The spectral intensity ratios of 13CO J = 3 − 2 to J = 1 − 0 range from 5 to 0.3. In addition, YSOs, masers, IRAS and UC HII regions are distributed in the shell of the bubble. The active region may be triggered by the expansion of the bubble N68.

TESTING TRIGGERED STAR FORMATION IN SIX H II REGIONS

We investigated six H II regions with infrared, bright rimmed bubble or cometary morphology, in search of quantitative evidence for triggered star formation, both collect and collapse and radiatively driven implosion. We identified and classified 458 Young Stellar Objects (YSOs) in and around the H II regions. YSOs were determined by fitting a collection of radiative transfer model spectral energy distributions (SEDs) to infrared photometry for a large sample of point sources. We determined areas where there exist enhanced populations of relatively unevolved YSOs on the bright rims of these regions, suggesting that star formation has been triggered there. We further investigated the physical properties of the regions by using radio continuum emission as a proxy for ionizing flux powering the H II regions, and 13CO (1-0) observations to measure masses and gravitational stability of molecular clumps. We used an analytical model of collect and collapse triggered star formation, as well as a simulation of radiatively driven implosion, and thus we compare the observed properties of the molecular gas with those predicted in the triggering scenarios. Notably, those regions in our sample that show evidence of cometary, or "blister," morphology are more likely to show evidence of triggering. Keywords: HII regions -- ISM: bubbles -- Stars: formation -- Stars: protostars

Near-Infrared Survey of Bright Rimmed Clouds

Abstract We conducted wide-field near-infrared imaging observations of 32 bright rimmed clouds (BRCs). Given a detection limit of 17.7 mag at the $ K$-band, we identified 2099 objects as young stellar object (YSO) candidates that displayed near-infrared excesses. Their masses, estimated from luminosities, range from 0.006 $ M_{\odot}$ to 2.7 $ M_{\odot}$ . The candidates are not uniformly distributed. For 21 BRCs, more than half of the associated YSO candidates are located inside the cloud. We found clear evidence of triggered star formation. The number of YSO candidates is not correlated with the core mass of the molecular clouds. Instead, the YSO number increases with increasing UV photon flux from the exciting star illuminating the cloud surface. UV radiation activates star formation in the BRCs.

Triggered Star Formation from Bubbles S51, N68, and N131

AbstractWe investigated the environment of the infrared dust bubbles S51 (Zhang & Wang 2012a), N68 (Zhang & Wang 2012b), and N131 (Zhang et al. 2012) from catalogue of Churchwell et al. (2006), and searched for evidence of triggered star formation.

Multiwavelength study of the infrared dust bubble S51

We investigate the environment of the infrared dust bubble S51 and search for evidence of triggered star formation in its surroundings. We performed a multiwavelength study of the region around S51 with data taken from large-scale surveys: 2MASS, GLIMPSE, MIPSGAL, IRAS, and MALT90. We analyzed the spectral profile and the distribution of the molecular gas (13CO, C18O, HCN, HNC, HCO+, C2H, N2H+, and HC3N), and dust in the environment of S51. We used mid-infrared emission three-color image to explore the physical environment and GLIMPSE color-color diagram [5.8]-[8.0] versus [3.6]-[4.5] to search for young stellar objects and identify the ionizing star candidates. From a comparison of the morphology of the molecular gas and the Spitzer 8.0 \mu m emission, we conclude that the dust bubble is interacting with CO at a kinematic distance of 3.4 kpc. The bubble S51 structure, carried with shell and front side, is exhibited with 13CO and C18O emission. Both outflow and inflow may exist in sources in the shell of bubble S51. We discover a small bubble G332.646-0.606 (R_in = 26", r_in = 15", R_out = 35" and r_out = 25") located at the northwest border of S51. A water maser, a methanol maser and IRAS 16158-5055 are located at the junction of the two bubbles. Several young stellar objects are distributed along an arc-shaped structure near the S51 shell. They may represent a second generation of stars whose formation was triggered by the bubble expanding into the molecular gas.

The HII region G35.673-00.847: another case of triggered star formation?

As part of a systematic study that we are performing with the aim to increase the observational evidence of triggered star formation in the surroundings of HII regions, we analyze the ISM around the HII region G35.673-00.847, a poorly studied source. Using data from large-scale surveys: Two Micron All Sky Survey, Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE), MIPSGAL, Galactic Ring Survey (GRS), VLA Galactic Plane Survey (VGPS), and NRAO VLA Sky Survey (NVSS) we performed a multiwavelength study of G35.673-00.847 and its surroundings. The mid IR emission, shows that G35.673-00.847 has an almost semi-ring like shape with a cut towards the galactic west. The radius of this semi-ring is about 1.5' (~1.6 pc, at the distance of ~3.7 kpc). The distance was estimated from an HI absorption study and from the analysis of the molecular gas. Indeed, we find a molecular shell composed by several clumps distributed around the HII region, suggesting that its expansion is collecting the surrounding material. We find several YSO candidates over the molecular shell. Finally, comparing the HII region dynamical age and the fragmentation time of the molecular shell, we discard the collect and collapse as the mechanism responsible for the YSOs formation, suggesting other processes such as radiative driven implosion and/or small-scale Jeans gravitational instabilities.

The environment of the infrared dust bubble N65: a multiwavelength study

<i>Aims. <i/>We investigate the environment of the infrared dust bubble N65 and search for evidence of triggered star formation in its surroundings. <i>Methods. <i/>We performed a multiwavelength study of the region around N65 with data taken from large-scale surveys: Two Micron All Sky Survey, GLIMPSE, MIPSGAL, SCUBA, and GRS. We analyzed the distribution of the molecular gas and dust in the environment of N65 and performed infrared photometry and spectral analysis of point sources to search for young stellar objects and identify the ionizing star candidates. <i>Results. <i/>We found a molecular cloud that appears to be fragmented into smaller clumps along the N65 PDR. This indicates that the so-called collect and collapse process may be occurring. Several young stellar objects are distributed among the molecular clumps. They may represent a second generation of stars whose formation was triggered by the bubble expanding into the molecular gas. We identified O-type stars inside N65, which are the most reliable ionizing star candidates.

STAR FORMATION IN THE COMETARY GLOBULE ORI I-2

We investigate the young stellar population in and near the cometary globule Ori,I-2. The analysis is based on deep Nordic Optical Telescope R-band and H-alpha images, JCMT SCUBA 450 and 850 micron images combined with near-infrared 2MASS photometry and mid-infrared archival Spitzer images obtained with the IRAC (3.6, 4.5, 5.8 and 8 micron), and MIPS (24 and 70 micron) instruments. We identify a total of 125 sources within the 5'x5' region imaged by IRAC. Of these sources 87 are detected in the R-band image and 51 are detected in the 2MASS survey. The detailed physical properties of the sources are explored using a combination of near/mid-infrared color-color diagrams, greybody fitting of SEDs and an online SED fitting tool that uses a library of 2D radiation transfer based accretion models of young stellar objects with disks. Ori I-2 shows clear evidence of triggered star formation with four young low luminosity pre-main sequence stars embedded in the globule. At least two, possibly as many as four, additional low-mass PMS objects, were discovered in the field which are probably part of the young sigma-Orionis cluster. Among the PMS stars which have formed in the globule, MIR-54 is a young, deeply embedded Class 0/I object, MIR-51 and 52 are young Class II sources, while MIR-89 is a more evolved, heavily extincted Class II object with its apparent colors mimicking a Class 0/I object. The Class II source, MIR-52, which is strong a H-alpha emission line star appears to drive an outflow approximately aligned with the outflow from MIR-54, and because of the proximity of the two outflows, either star could contribute. MIR-89 appears to excite a low excitation HH object, HH 992, discovered for the first time in this study.

Evidence of triggered star formation in G327.3-0.6

Aims. Expanding HII regions and propagating shocks are common in the environment of young high-mass star-forming complexes. They can compress a pre-existing molecular cloud and trigger the formation of dense cores. We investigate whether these phenomena can explain the formation of high-mass protostars within an infrared dark cloud located at the position of G327.3-0.6 in the Galactic plane, in between two large infrared bubbles and two HII regions. Methods: The region of G327.3-0.6 was imaged at 450 ? m with the CEA P-ArT\'eMiS bolometer array on the Atacama Pathfinder EXperiment telescope in Chile. APEX/LABOCA and APEX-2A, and Spitzer/IRAC and MIPS archives data were used in this study. Results: Ten massive cores were detected in the P-ArT\'eMiS image, embedded within the infrared dark cloud seen in absorption at both 8 and 24 ?m. Their luminosities and masses indicate that they form high-mass stars. The kinematical study of the region suggests that the infrared bubbles expand toward the infrared dark cloud. Conclusions: Under the influence of expanding bubbles, star formation occurs in the infrared dark areas at the border of HII regions and infrared bubbles.

SPITZERVIEW OF YOUNG MASSIVE STARS IN THE LARGE MAGELLANIC CLOUD H II COMPLEX N 44

The HII complex N44 in the Large Magellanic Cloud (LMC) provides an excellent site to perform a detailed study of star formation in a mild starburst, as it hosts three regions of star formation at different evolutionary stages and it is not as complicated and confusing as the 30 Doradus giant HII region. We have obtained Spitzer Space Telescope observations and complementary ground-based 4m uBVIJK observations of N44 to identify candidate massive young stellar objects (YSOs). We further classify the YSOs into Types I, II, and III, according to their spectral energy distributions (SEDs). In our sample of 60 YSO candidates, ~65% of them are resolved into multiple components or extended sources in high-resolution ground-based images. We have modeled the SEDs of 36 YSOs that appear single or dominant within a group. We find good fits for Types I and I/II YSOs,but Types II and II/III YSOs show deviations between their observed SEDs and models that do not include PAH emission. We have also found that some Type III YSOs have central holes in their disk components. YSO counterparts are found in four ultracompact HII regions and their stellar masses determined from SED model fits agree well with those estimated from the ionization requirements of the HII regions. The distribution of YSOs is compared with those of the underlying stellar population and interstellar gas conditions to illustrate a correlation between the current formation of O-type stars and previous formation of massive stars. Evidence of triggered star formation is also presented.

STAR FORMATION AROUND SUPERGIANT SHELLS IN THE LARGE MAGELLANIC CLOUD

We examine the recent star formation associated with four supergiant shells (SGSs) in the Large Magellanic Cloud (LMC): LMC 1, 4, 5, and 6, which have been shown to have simple expanding-shell structures. H II regions and OB associations are used to infer star formation in the last few Myr, while massive young stellar objects (YSOs) reveal the current ongoing star formation. Distributions of ionized, H I, and molecular components of the interstellar gas are compared with the sites of recent and current star formation to determine whether triggering has taken place. We find that a great majority of the current star formation has occurred in gravitationally unstable regions, and that evidence of triggered star formation is prevalent at both large and local scales.

The Bubbling Galactic Disk. II. The Inner 20o

We report 269 mid-infrared bubbles within 10° of the Galactic center from visual inspection of the Spitzer GLIMPSE II Legacy Science program images. The surface density of bubbles is ~5 deg-2 or about 3 times that detected in longitudes |l| = 10°-65°, because the inner 10° of longitude were more thoroughly searched for small bubbles. There is a gradient in the number of bubbles with longitude with an increase of about a factor of 2 from 2° to 10°; this is probably the result of several factors, including decreasing diffuse background brightness, confusion, and opacity with longitude. Bubble eccentricities are typically between 0.6 and 0.8, and >50% show evidence for blowouts, which we suggest result from local density fluctuations of the ISM and/or anisotropic stellar winds and radiation fields. The fraction of bubbles identified with H II regions and clusters is only about half that found at |l| > 10°. This is largely a result of the much smaller angular diameter of bubbles cataloged in the inner Galaxy than in the outer Galaxy. At least 12% of the bubbles have morphologies suggestive of triggered star formation. Most of the bubbles that show evidence for triggered star formation in the inner Galaxy have not formed secondary bubbles; it is postulated that this may be because they are too young to have had time for this to occur.