Observations of star formation triggered by H <scp>ii</scp> regions

Deharveng, L.; Zavagno, A.

doi:10.1017/s1743921311000457

Cited by 5 publications

(2 citation statements)

References 27 publications

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“…This prediction differs from the observed stellar content of OB associations (Garmany, 1994;Massey, 2003), which show a fully populated stellar IMF within their sub-clusters (de Geus, 1992;Preibisch & Zinnecker, 1999;Brice ño et al, 2007). Nevertheless, observations of HII bubbles do suggest a causal relation between the first generation of stars located in the bubble and the second generation of embedded clusters located at its rim (Deharveng & Zavagno, 2011).…”

Section: The Formation Of Unbound Systemsmentioning

confidence: 98%

Unbound Young Stellar Systems: Star Formation on the Loose

Gouliermis¹

2018

PASP

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Unbound young stellar systems, the loose ensembles of physically related young bright stars, trace the typical regions of recent star formation in galaxies. Their morphologies vary from small few pc-size associations of newly formed stars to enormous few kpc-size complexes composed of stars few 100 Myr old. These stellar conglomerations are located within the disks and along the spiral arms and rings of star-forming disk galaxies, and they are the active star-forming centers of dwarf and starburst galaxies. Being associated with star-forming regions of various sizes, these stellar structures trace the regions where stars form at various length-and timescales, from compact clusters to whole galactic disks. Stellar associations, the prototypical unbound young systems, and their larger counterparts, stellar aggregates, and stellar complexes, have been the focus of several studies for quite a few decades, with special interest on their demographics, classification, and structural morphology. The compiled surveys of these loose young stellar systems demonstrate that the clear distinction of these systems into welldefined classes is not as straightforward as for stellar clusters, due to their low densities, asymmetric shapes and variety in structural parameters. These surveys also illustrate that unbound stellar structures follow a clear hierarchical pattern in the clustering of their stars across various scales. Stellar associations are characterized by significant sub-structure with bound stellar clusters being their most compact parts, while associations themselves are the brighter denser parts of larger stellar aggregates and stellar complexes, which are members of larger superstructures up to the scale of a whole star-forming galaxy. This structural pattern, which is usually characterized as self-similar or fractal, appears to be identical to that of star-forming giant molecular clouds and interstellar gas, driven mainly by turbulence cascade. In this short review, I make a concise compilation of our understanding of unbound young stellar systems across various environments in the local universe, as it is developed during the last 60 years. I present a factual assessment of the clustering behavior of star formation, as revealed from the assembling pattern of stars across loose stellar structures and its relation to the interstellar medium and the environmental conditions. I also provide a consistent account of the processes that possibly play important role in the formation of unbound stellar systems, compiled from both theoretical and observational investigations on the field.

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Section: The Formation Of Unbound Systemsmentioning

confidence: 98%

Unbound Young Stellar Systems: Star Formation on the Loose

Gouliermis¹

2018

PASP

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“…Thus, sequential star groups and stellar populations in a turbulent cloud cannot easily be observationally recognised unless very accurate age measurements are possible. Nevertheless, the observational evidence for ongoing triggered star formation near massive stars has been growing recently (for an updated list, see Deharveng & Zavagno 2011). Numerical simulations as well (e. g., Dale et al 2012b) have shown that triggered star formation does occur in giant molecular clouds as a result of ionising feedback (and molecular outflows) from massive stars, although this feedback also tends to result in a globally lower star-formation efficiency.…”

Section: Introductionmentioning

confidence: 99%

Young open clusters in the Galactic star forming region NGC 6357

Massi

Giannetti

Carlo

et al. 2014

A&A

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Context. NGC 6357 is an active star forming region with very young massive open clusters. These clusters contain some of the most massive stars in the Galaxy and strongly interact with nearby giant molecular clouds. Aims. We study the young stellar populations of the region and of the open cluster Pismis 24, focusing on their relationship with the nearby giant molecular clouds. We seek evidence of triggered star formation "propagating" from the clusters. Methods. We used new deep JHK s photometry, along with unpublished deep Spitzer/IRAC mid-infrared 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). Results. NGC 6357 hosts three large young stellar clusters, of which Pismis 24 is the most prominent. We found that Pismis 24 is a very young (∼1-3 Myr) open cluster with a Salpeter-like initial mass function and a few thousand members. A comparison between optical and infrared photometry indicates that the fraction of members with a near-infrared 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 Pismis 24 is likely subdivided into a few different subclusters, 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 near-infrared excess does increase towards the Hii region associated with the cluster). The gas out of which Pismis 24 formed must have been distributed in dense clumps within a cloud of less dense gas ∼1 pc in radius. Conclusions. 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.

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Interstellar MHD Turbulence and Star Formation

Vázquez‐Semadeni

2014

Astrophysics and Space Science Library

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This chapter reviews the nature of turbulence in the Galactic interstellar medium (ISM) and its connections to the star formation (SF) process. The ISM is turbulent, magnetized, self-gravitating, and is subject to heating and cooling processes that control its thermodynamic behavior, causing it to behave approximately isobarically, in spite of spanning several orders of magnitude in density and temperature. The turbulence in the warm and hot ionized components of the ISM appears to be trans-or subsonic, and thus to behave nearly incompressibly. However, the neutral warm and cold components are highly compressible, as a consequence of both thermal instability (TI) in the atomic gas and of moderately-to-strongly supersonic motions in the roughly isothermal cold atomic and molecular components. Within this context, we discuss: i) the production and statistical distribution of turbulent density fluctuations in both isothermal and polytropic media; ii) the nature of the clumps produced by TI, noting that, contrary to classical ideas, they in general accrete mass from their environment in spite of exhibiting sharp discontinuities at their boundaries; iii) the density-magnetic field correlation (and, at low densities, lack thereof) in turbulent density fluctuations, as a consequence of the superposition of the different wave modes in the turbulent flow; iv) the evolution of the mass-tomagnetic flux ratio (MFR) in density fluctuations as they are built up by dynamic compressions; v) the formation of cold, dense clouds aided by TI, in both the hydrodynamic (HD) and the magnetohydrodynamic (MHD) cases; vi) the expectation that star-forming molecular clouds are likely to be undergoing global gravitational contraction, rather than being near equilibrium, as generally believed, and vii) the regulation of the star formation rate (SFR) in such gravitationally contracting clouds by stellar feedback which, rather than keeping the clouds from collapsing, evaporates and disperses them while they collapse.

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Observations of star formation triggered by H ii regions

Abstract: Abstract. Observations show that expanding H ii regions may trigger star formation. We discuss several aspects of this type of star formation, and try to estimate its prevalence. We show how LMC H ii regions may help us to understand what we see in our Galaxy.

Cited by 5 publications

References 27 publications

Unbound Young Stellar Systems: Star Formation on the Loose

Unbound Young Stellar Systems: Star Formation on the Loose

Young open clusters in the Galactic star forming region NGC 6357

Interstellar MHD Turbulence and Star Formation

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