Molecular Clouds and Star Formation in the Southern H II Regions

1999 ◽  
Vol 51 (6) ◽  
pp. 791-818 ◽  
Author(s):  
Reiko Yamaguchi ◽  
Hiro Saito ◽  
Norikazu Mizuno ◽  
Yoshihiro Mine ◽  
Akira Mizuno ◽  
...  
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Formation Rate ◽  
Point Sources ◽  
Young Stellar Objects ◽  
H Ii Regions ◽  
Stellar Objects ◽  
Order Of Magnitude ◽  
Formation Efficiency ◽  
Active Formation

Abstract We have carried out extensive 13CO(J = 1−0) observations toward 23 southern H II regions associated with bright-rimmed clouds. In total, 95 molecular clouds have been identified to be associated with the H II regions. Among the 95, 57 clouds \ are found to be associated with 204 IRAS point sources which are candidates for young stellar objects. There is a significant increase of star-formation efficiency on the side facing to the H II regions; the luminosity-to-mass ratio, defined as the ratio of the stellar luminosity to the molecular cloud mass, is higher by an order of magnitude on the near side of the H II regions than that on the far side. This indicates that molecular gas facing to the H II regions is more actively forming massive s\ tars whose luminosity is ≳103L⊙. In addition, the number density of the IRAS point sources increases by a factor of 2 on the near side of the H II regions compared with on the far side. These results strongly suggest that the active formation of massive stars on the near side of the H II regions is due to the effects of the H II regions, such as the compression of molecular material by the ionization/shock fronts. For the whole Galaxy, we estimate that the present star-formation rate under such effects is at least 0.2−0.4 M⊙ yr-1, corresponding to a few 10% by mass.

scholarly journals The Single-Cloud Star Formation Relation

2020 ◽  
Author(s):  
Riwaj Pokhrel
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Dynamic Range ◽  
Formation Rate ◽  
Free Fall ◽  
High Dynamic Range ◽  
Young Stellar Objects ◽  
Interstellar Gas ◽  
Stellar Objects ◽  
Fall Time

Abstract One of the most important and well-established empirical results in astronomy is the so-called Kennicutt-Schmidt (KS) relation between the density of interstellar gas and the rate at which that gas forms stars. While a tight correlation between these quantities has long been measured at galactic scales, the difficulty of measuring star formation rates and gas densities over a large dynamic range at sub-galactic scales has thus far precluded a definitive determination of whether the same relationship holds within individual star-forming clouds. In this article we use a new, high-accuracy catalogue of young stellar objects from Spitzer combined with new, high-dynamic range maps of twelve nearby ($<$1.5 kpc) molecular clouds from Herschel to re-examine the KS relation within individual molecular clouds. We find a tight, linear correlation between clouds' star formation rate per unit area and their gas surface density normalised by the gas free-fall time. The measured relation extends over more than two orders of magnitude within each cloud, and is nearly identical in each of the twelve clouds, implying a constant star formation efficiency per free-fall time eff≈0.026$. The finding of a universal correlation within individual molecular clouds, including clouds that contain no massive stars or massive stellar feedback, favours models in which star formation is regulated by local processes such as turbulence or protostellar outflows, and disfavours models in which star formation is regulated primarily by galaxy properties or supernova feedback on galactic scales.

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

2020 ◽  
Vol 496 (1) ◽  
pp. 870-874
Author(s):  
M B Areal ◽  
A Buccino ◽  
S Paron ◽  
C Fariña ◽  
M E Ortega
Keyword(s):  
Star Formation ◽  
Young Stellar Objects ◽  
H Ii Regions ◽  
Stellar Objects ◽  
Three Generations ◽  
The North ◽  
Western Border ◽  
H Ii Region ◽  
New Generation ◽  
North Western

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.

scholarly journals Molecular environs and triggered star formation around the large Galactic infrared bubble N 24

2019 ◽  
Vol 487 (2) ◽  
pp. 1517-1528 ◽  
Author(s):  
Xu Li ◽  
Jarken Esimbek ◽  
Jianjun Zhou ◽  
W A Baan ◽  
Weiguang Ji ◽  
...  
Keyword(s):  
Star Formation ◽  
Massive Star ◽  
Young Stellar Objects ◽  
Collapse Mechanism ◽  
Kinetic Temperature ◽  
H Ii Regions ◽  
Dark Cloud ◽  
Stellar Objects ◽  
Multi Wavelength ◽  
Mass Size

Abstract A multi-wavelength analysis of the large Galactic infrared bubble N 24 is presented in this paper in order to investigate the molecular and star-formation environment around expanding H ii regions. Using archival data from Herschel and ATLASGAL, the distribution and physical properties of the dust over the entire bubble are studied. Using the Clumpfind2d algorithm, 23 dense clumps are identified, with sizes and masses in the range 0.65–1.73 pc and 600–16 300 M⊙, respectively. To analyse the molecular environment in N 24, observations of NH3 (1,1) and (2,2) were carried out using the Nanshan 26-m radio telescope. Analysis of the kinetic temperature and gravitational stability of these clumps suggests gravitational collapse in several of them. The mass–size distributions of the clumps and the presence of massive young protostars indicate that the shell of N 24 is a region of ongoing massive-star formation. The compatibility of the dynamical and fragmentation timescales and the overabundance of young stellar objects and clumps on the rim suggest that the ‘collect-and-collapse’ mechanism is in play at the boundary of the bubble, but the existence of the infrared dark cloud at the edge of bubble indicates that a ‘radiation-driven implosion’ mechanism may also have played a role there.

scholarly journals SK 1: A possible case of triggered star formation in perseus

2006 ◽  
Vol 2 (S237) ◽  
pp. 217-221
Author(s):  
Miriam Rengel ◽  
Klaus Hodapp ◽  
Jochen Eislöffel
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Thermal Emission ◽  
Young Stellar Objects ◽  
Wide Field ◽  
Dust Shell ◽  
Stellar Objects ◽  
Star Forming ◽  
Star Forming Regions ◽  
Suitable Target

AbstractAccording to a triggered star formation scenario (e.g. Martin-Pintado & Cernicharo 1987) outflows powered by young stellar objects shape the molecular clouds, can dig cavities, and trigger new star formation. NGC 1333 is an active site of low- and intermediate star formation in Perseus and is a suggested site of self-regulated star formation (Norman & Silk 1980). Therefore it is a suitable target for a study of triggered star formation (e.g. Sandell & Knee 2001, SK1). On the other hand, continuum sub-mm observations of star forming regions can detect dust thermal emission of embedded sources (which drive outflows), and further detailed structures.Within the framework of our wide-field mapping of star formation regions in the Perseus and Orion molecular clouds using SCUBA at 850 and 450 μm, we mapped NCG 1333 with an area of around 14′× 21′. The maps show more structure than the previous maps of the region observed in sub-mm. We have unveiled the known embedded SK 1 source (in the dust shell of the SSV 13 ridge) and detailed structure of the region, among some other young protostars.In agreement with the SK 1 observations, our map of the region shows lumpy filaments and shells/cavities that seem to be created by outflows. The measured mass of SK 1 (~0.07 M) is much less than its virial mass (~0.2-1 M). Our observations support the idea of SK 1 as an event triggered by outflow-driven shells in NGC 1333 (induced by an increase in gas pressure and density due to radiation pressure from the stellar winds that have presumably created the dust shell). This kind of evidences provides a more thorough understanding of the star formation regulation processes.

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

2021 ◽  
Vol 507 (4) ◽  
pp. 5106-5131
Author(s):  
David A Kinson ◽  
Joana M Oliveira ◽  
Jacco Th van Loon
Keyword(s):  
Machine Learning ◽  
Star Formation ◽  
Local Group ◽  
Point Sources ◽  
Asymptotic Giant Branch ◽  
Supervised Machine Learning ◽  
Young Stellar Objects ◽  
Irregular Galaxy ◽  
Stellar Objects ◽  
Ngc 6822

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.

scholarly journals Dense molecular clouds and associated star formation in the Magellanic Clouds

2006 ◽  
Vol 2 (S237) ◽  
pp. 40-46
Author(s):  
Mónica Rubio
Keyword(s):  
Star Formation ◽  
Uv Radiation ◽  
Molecular Clouds ◽  
Massive Star ◽  
Dust Emission ◽  
Young Stellar Objects ◽  
Continuum Emission ◽  
Giant Molecular Clouds ◽  
Stellar Objects ◽  
Ir Sources

AbstractMultiwavelengths studies of massive star formation regions in the LMC and SMC reveal that a second generation of stars is being formed in dense molecular clouds located in the surroundings of the massive clusters. These dense molecular clouds have survived the action of massive star UV radiation fields and winds and they appear as compact dense H2 knots in regions of weak CO emission. Alternatively, we have found that large molecular clouds, probably remnants of the parental giant molecular clouds where the first generation of stars were formed, are suffering the interaction of the winds and UV radiation field in their surfaces in the direction of the central massive cluster or massive stars. These molecular regions show 1.2 mm continuum emission form cold dust and they show embedded IR sources as determined from deep ground base JHKs imaging. The distribution of young IR sources as determined from their Mid IR colors obtained by SPITZER concentrate in the maxima of CO and dust emission. IR spectroscopy of the embedded sources with high IR excess confirm their nature as massive young stellar objects (MYSO's). Our results are suggestive of contagious star formation where triggering and induced star formation could be taking place.

scholarly journals 3D shape of Orion A with Gaia DR2 An informed view on Star Formation Rates and Efficiencies

2018 ◽  
Vol 14 (S345) ◽  
pp. 27-33
Author(s):  
Josefa E. Großschedl ◽  
João Alves ◽  
Stefan Meingast ◽  
Birgit Hasenberger
Keyword(s):  
Star Formation ◽  
Formation Rate ◽  
Young Stellar Objects ◽  
Mass Star ◽  
Orion Nebula ◽  
Stellar Objects ◽  
Star Forming ◽  
The North ◽  
Low Mass ◽  
The Rich

AbstractThe giant molecular cloud Orion A is the closest massive star-forming region to earth (d ∼ 400 pc). It contains the rich Orion Nebula Cluster (ONC) in the North, and low-mass star-forming regions (L1641, L1647) to the South. To get a better understanding of the differences in star formation activity, we perform an analysis of the gas mass distribution and star formation rate across the cloud. We find that the gas is roughly uniformly distributed, while, oddly, the ONC region produced about a factor of ten more stars compared to the rest of the cloud. For a better interpretation of this phenomenon, we use Gaia DR2 parallaxes, to analyse distances of young stellar objects, using them as proxy for cloud distances. We find that the ONC region indeed lies at about 400 pc while the low-mass star-forming parts are inclined about 70∘ from the plane of the sky reaching until ∼470 pc. With this we estimate that Orion A is an about 90 pc long filamentary cloud (about twice as long as previously assumed), with its “Head” (the ONC region) being “bent” and oriented towards the galactic mid-plane. This striking new view allows us to perform a more robust analysis of this important star-forming region in the future.

scholarly journals Photoionizing feedback in spiral arm molecular clouds

2020 ◽  
Vol 495 (2) ◽  
pp. 1672-1691
Author(s):  
Thomas J R Bending ◽  
Clare L Dobbs ◽  
Matthew R Bate
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Total Mass ◽  
Star Formation Rate ◽  
Formation Rate ◽  
Rapid Decrease ◽  
H Ii Regions ◽  
Noticeable Effect ◽  
Spiral Arm ◽  
Dense Features

ABSTRACT We present simulations of a 500 pc2 region, containing gas of mass 4 × 106 M⊙, extracted from an entire spiral galaxy simulation, scaled up in resolution, including photoionizing feedback from stars of mass &gt;18 M⊙. Our region is evolved for 10 Myr and shows clustered star formation along the arm generating ≈ 5000 cluster sink particles ≈ 5 per cent of which contain at least one of the ≈ 4000 stars of mass &gt;18 M⊙. Photoionization has a noticeable effect on the gas in the region, producing ionized cavities and leading to dense features at the edge of the H ii regions. Compared to the no-feedback case, photoionization produces a larger total mass of clouds and clumps, with around twice as many such objects, which are individually smaller and more broken up. After this we see a rapid decrease in the total mass in clouds and the number of clouds. Unlike studies of isolated clouds, our simulations follow the long-range effects of ionization, with some already dense gas, becoming compressed from multiple sides by neighbouring H ii regions. This causes star formation that is both accelerated and partially displaced throughout the spiral arm with up to 30 per cent of our cluster sink particle mass forming at distances &gt;5 pc from sites of sink formation in the absence of feedback. At later times, the star formation rate decreases to below that of the no-feedback case.

scholarly journals Theory of Protostellar Objects

1986 ◽  
Vol 89 ◽  
pp. 10-34
Author(s):  
Frank H. Shu
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Analytical Approach ◽  
Young Stellar Objects ◽  
Governing Equations ◽  
Hydrogen Atoms ◽  
Stellar Objects ◽  
Know How ◽  
T Tauri ◽  
Low Efficiency

AbstractMany problems in the theory of star formation are amenable to a complementary attack in which the analytical approach is used to reduce the governing equations to a form amenable to efficient numerical solution. This strategy has proven very useful in helping to resolve several astrophysical puzzles which arise because the bulk of star formation today is observed to occur, with relatively low efficiency, in giant molecular cloud complexes. How does a cloud of 105-106M⊙ know how to form stars of mass ~ 1 M⊙? How does the interstellar medium know, to one or two orders of magnitude, that roughly hydrogen atoms of mass mH are needed to yield thermonuclear fusion in a self-gravitating ball of gas? Why have radio astronomers not detected unambiguous evidence for the collapse motions attendant to star formation? Why has a true protostar, the “holy grail” of infrared astronomy, been so hard to find? Why do young stellar objects almost universally exhibit powerful outflows? Why is the geometry for these outflows often bipolar? Why do T Tauri stars have such active chromospheres? In this review we suggest that these puzzles all have a related resolution, in the nature of how gravitational collapse is initiated and terminated in the slowly rotating cores of molecular clouds.

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