HIGH-MASS STAR FORMATION IN NORMAL LATE-TYPE GALAXIES: OBSERVATIONAL CONSTRAINTS TO THE INITIAL MASS FUNCTION

Abstract A supersonic cloud–cloud collision produces a shock-compressed layer which leads to formation of high-mass stars via gravitational instability. We carried out a detailed analysis of the layer by using the numerical simulations of magneto-hydrodynamics which deal with colliding molecular flows at a relative velocity of 20 km s−1 (Inoue & Fukui 2013, ApJ, 774, L31). Maximum density in the layer increases from 1000 cm−3 to more than 105 cm−3 within 0.3 Myr by compression, and the turbulence and the magnetic field in the layer are amplified by a factor of ∼5, increasing the mass accretion rate by two orders of magnitude to more than 10−4 $ M_{\odot } $ yr−1. The layer becomes highly filamentary due to gas flows along the magnetic field lines, and dense cores are formed in the filaments. The massive dense cores have size and mass of 0.03–0.08 pc and 8–$ 50\, M_{\odot } $ and they are usually gravitationally unstable. The mass function of the dense cores is significantly top-heavy as compared with the universal initial mass function, indicating that the cloud–cloud collision preferentially triggers the formation of O and early B stars. We argue that the cloud–cloud collision is a versatile mechanism which creates a variety of stellar clusters from a single O star like RCW 120 and M 20 to tens of O stars of a super star cluster like RCW 38 and a mini-starburst W 43. The core mass function predicted by the present model is consistent with the massive dense cores obtained by recent ALMA observations in RCW 38 (Torii et al. 2021, PASJ, in press) and W 43 (Motte et al. 2018, Nature Astron., 2, 478). Considering the increasing evidence for collision-triggered high-mass star formation, we argue that cloud–cloud collision is a major mechanism of high-mass star formation.

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Formation of Stellar Clusters and the Importance of Thermodynamics for Fragmentation

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308015196 ◽

2007 ◽

Vol 3 (S246) ◽

pp. 3-12

Author(s):

Ralf S. Klessen ◽

Paul C. Clark ◽

Simon C. O. Glover

Keyword(s):

Star Formation ◽

Dynamic Properties ◽

Mass Function ◽

Mass Scale ◽

Initial Mass Function ◽

Solar Neighborhood ◽

High Mass ◽

Mass Star ◽

Heating Regime ◽

Effective Equation

AbstractWe discuss results from numerical simulations of star cluster formation in the turbulent interstellar medium (ISM). The thermodynamic behavior of the star-forming gas plays a crucial role in fragmentation and determines the stellar mass function as well as the dynamic properties of the nascent stellar cluster. This holds for star formation in molecular clouds in the solar neighborhood as well as for the formation of the very first stars in the early universe. The thermodynamic state of the ISM is a result of the balance between heating and cooling processes, which in turn are determined by atomic and molecular physics and by chemical abundances. Features in the effective equation of state of the gas, such as a transition from a cooling to a heating regime, define a characteristic mass scale for fragmentation and so set the peak of the initial mass function of stars (IMF). As it is based on fundamental physical quantities and constants, this is an attractive approach to explain the apparent universality of the IMF in the solar neighborhood as well as the transition from purely primordial high-mass star formation to the more normal low-mass mode observed today.

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Dense cores and star formation in the giant molecular cloud Vela C

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935047 ◽

2019 ◽

Vol 628 ◽

pp. A110 ◽

Cited By ~ 2

Author(s):

F. Massi ◽

A. Weiss ◽

D. Elia ◽

T. Csengeri ◽

E. Schisano ◽

...

Keyword(s):

Star Formation ◽

Molecular Cloud ◽

Galactic Plane ◽

Mass Function ◽

Initial Mass Function ◽

High Mass ◽

Mass Star ◽

Ancillary Data ◽

Core Mass ◽

Giant Molecular Cloud

Context. The Vela Molecular Ridge is one of the nearest (700 pc) giant molecular cloud (GMC) complexes hosting intermediate-mass (up to early B, late O stars) star formation, and is located in the outer Galaxy, inside the Galactic plane. Vela C is one of the GMCs making up the Vela Molecular Ridge, and exhibits both sub-regions of robust and sub-regions of more quiescent star formation activity, with both low- and intermediate(high)-mass star formation in progress. Aims. We aim to study the individual and global properties of dense dust cores in Vela C, and aim to search for spatial variations in these properties which could be related to different environmental properties and/or evolutionary stages in the various sub-regions of Vela C. Methods. We mapped the submillimetre (345 GHz) emission from vela C with LABOCA (beam size ~19′′2, spatial resolution ~0.07 pc at 700 pc) at the APEX telescope. We used the clump-finding algorithm CuTEx to identify the compact submillimetre sources. We also used SIMBA (250 GHz) observations, and Herschel and WISE ancillary data. The association with WISE red sources allowed the protostellar and starless cores to be separated, whereas the Herschel dataset allowed the dust temperature to be derived for a fraction of cores. The protostellar and starless core mass functions (CMFs) were constructed following two different approaches, achieving a mass completeness limit of 3.7 M⊙. Results. We retrieved 549 submillimetre cores, 316 of which are starless and mostly gravitationally bound (therefore prestellar in nature). Both the protostellar and the starless CMFs are consistent with the shape of a Salpeter initial mass function in the high-mass part of the distribution. Clustering of cores at scales of 1–6 pc is also found, hinting at fractionation of magnetised, turbulent gas.

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Unifying low- and high-mass star formation through density-amplified hubs of filaments

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038232 ◽

2020 ◽

Vol 642 ◽

pp. A87 ◽

Cited By ~ 1

Author(s):

M. S. N. Kumar ◽

P. Palmeirim ◽

D. Arzoumanian ◽

S. I. Inutsuka

Keyword(s):

Star Formation ◽

Massive Stars ◽

Radial Profile ◽

Beam Width ◽

Mass Function ◽

Initial Mass Function ◽

High Mass ◽

Mass Star ◽

Giant Molecular Clouds ◽

Stellar Sources

Context. Star formation takes place in giant molecular clouds, resulting in mass-segregated young stellar clusters composed of Sun-like stars, brown dwarfs, and massive O-type(50–100 M⊙) stars. Aims. We aim to identify candidate hub-filament systems (HFSs) in the Milky Way and examine their role in the formation of the highest mass stars and star clusters. Methods. The Herschel survey HiGAL has catalogued about 105 clumps. Of these, approximately 35 000 targets are detected at the 3σ level in a minimum of four bands. Using the DisPerSE algorithm we detect filamentary skeletons on 10′ × 10′ cut-outs of the SPIRE 250 μm images (18′′ beam width) of the targets. Any filament with a total length of at least 55′′ (3 × 18′′) and at least 18′′ inside the clump was considered to form a junction at the clump. A hub is defined as a junction of three or more filaments. Column density maps were masked by the filament skeletons and averaged for HFS and non-HFS samples to compute the radial profile along the filaments into the clumps. Results. Approximately 3700 (11%) are candidate HFSs, of which about 2150 (60%) are pre-stellar and 1400 (40%) are proto-stellar. The filaments constituting the HFSs have a mean length of ~10–20 pc, a mass of ~5 × 104 M⊙, and line masses (M∕L) of ~2 × 103 M⊙ pc−1. All clumps with L > 104 L⊙ and L > 105 L⊙ at distances within 2 and 5 kpc respectively are located in the hubs of HFSs. The column densities of hubs are found to be enhanced by a factor of approximately two (pre-stellar sources) up to about ten (proto-stellar sources). Conclusions. All high-mass stars preferentially form in the density-enhanced hubs of HFSs. This amplification can drive the observed longitudinal flows along filaments providing further mass accretion. Radiation pressure and feedback can escape into the inter-filamentary voids. We propose a “filaments to clusters” unified paradigm for star formation, with the following salient features: (a) low-intermediate-mass stars form slowly (106 yr) in the filaments and massive stars form quickly (105 yr) in the hub, (b) the initial mass function is the sum of stars continuously created in the HFS with all massive stars formed in the hub, (c) feedback dissipation and mass segregation arise naturally due to HFS properties, and explain the (d) age spreads within bound clusters and the formation of isolated OB associations.

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Simulating star clusters across cosmic time – I. Initial mass function, star formation rates, and efficiencies

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2239 ◽

2019 ◽

Vol 489 (2) ◽

pp. 1880-1898 ◽

Cited By ~ 6

Author(s):

Chong-Chong He ◽

Massimo Ricotti ◽

Sam Geen

Keyword(s):

Star Formation ◽

Power Law ◽

High Redshift ◽

Star Clusters ◽

Cosmic Time ◽

Mass Function ◽

Initial Mass Function ◽

High Mass ◽

Initial Mass ◽

Solar Metallicity

ABSTRACT We present radiation-magneto-hydrodynamic simulations of star formation in self-gravitating, turbulent molecular clouds, modelling the formation of individual massive stars, including their UV radiation feedback. The set of simulations have cloud masses between mgas = 103 M⊙ and 3 × 105 M⊙ and gas densities typical of clouds in the local Universe ($\overline{n}_{\rm gas} \sim 1.8\times 10^2$ cm−3) and 10× and 100× denser, expected to exist in high-redshift galaxies. The main results are as follows. (i) The observed Salpeter power-law slope and normalization of the stellar initial mass function at the high-mass end can be reproduced if we assume that each star-forming gas clump (sink particle) fragments into stars producing on average a maximum stellar mass about $40{{\ \rm per\ cent}}$ of the mass of the sink particle, while the remaining $60{{\ \rm per\ cent}}$ is distributed into smaller mass stars. Assuming that the sinks fragment according to a power-law mass function flatter than Salpeter, with log-slope 0.8, satisfy this empirical prescription. (ii) The star formation law that best describes our set of simulation is ${\rm d}\rho _*/{\rm d}t \propto \rho _{\rm gas}^{1.5}$ if $\overline{n}_{\rm gas}\lt n_{\rm cri}\approx 10^3$ cm−3, and ${\rm d}\rho _*/{\rm d}t \propto \rho _{\rm gas}^{2.5}$ otherwise. The duration of the star formation episode is roughly six cloud’s sound crossing times (with cs = 10 km s−1). (iii) The total star formation efficiency in the cloud is $f_*=2{{\ \rm per\ cent}} (m_{\rm gas}/10^4~\mathrm{M}_\odot)^{0.4}(1+\overline{n}_{\rm gas}/n_{\rm cri})^{0.91}$, for gas at solar metallicity, while for metallicity Z < 0.1 Z⊙, based on our limited sample, f* is reduced by a factor of ∼5. (iv) The most compact and massive clouds appear to form globular cluster progenitors, in the sense that star clusters remain gravitationally bound after the gas has been expelled.

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TIMESCALES FOR LOW-MASS STAR FORMATION IN EXTRAGALACTIC ENVIRONMENTS: IMPLICATIONS FOR THE STELLAR INITIAL MASS FUNCTION

The Astrophysical Journal ◽

10.1088/0004-637x/692/1/283 ◽

2009 ◽

Vol 692 (1) ◽

pp. 283-289 ◽

Cited By ~ 9

Author(s):

M. Banerji ◽

S. Viti ◽

D. A. Williams ◽

J. M. C. Rawlings

Keyword(s):

Star Formation ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Mass Star ◽

Stellar Initial Mass Function ◽

Low Mass

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Star Formation in Cooling Flows

Australian Journal of Physics ◽

10.1071/ph920501 ◽

1992 ◽

Vol 45 (4) ◽

pp. 501

Author(s):

PEJ Nulsen

Keyword(s):

Star Formation ◽

Mass Function ◽

Good Evidence ◽

Initial Mass Function ◽

High Mass ◽

Mass Star ◽

Cooling Flows ◽

Cooling Flow ◽

The Masses ◽

Intracluster Gas

There is good evidence that the intracluster gas near the centres of many clusters of galaxies is cooling over at least a decade in temperature. This results in an inflow, which is approximately steady where the cooling time of the gas is shorter than the age of the flow. X-ray observations indicate that the cooling gas must have substantial inhomogeneities. Non-linear development of thermal instability causes cooled gas clouds to be deposited throughout the region of the steady cooling flow. The masses of these clouds will be small, typically a lot less than a Jeans mass when they first form. It is argued that the difficulty of forming large clouds in the bulk of a cooling flow inhibits high mass star formation there. There is evidence of some 'normal' star formation near to the centres of many cooling flows where conditions are more favourable for the formation of giant clouds. The initial mass function is strongly affected by the star formation environment in cooling flows.

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