scholarly journals Dynamic role of dust in formation of molecular clouds

2020 ◽  
Vol 500 (2) ◽  
pp. 2209-2226
Author(s):  
V V Zhuravlev
Keyword(s):  
Growth Rate ◽  
Molecular Clouds ◽  
Collective Motion ◽  
Stopping Time ◽  
Minor Component ◽  
Free Fall ◽  
Small Mass ◽  
Cloud Formation ◽  
Sound Waves ◽  
Self Gravity

ABSTRACT Dust is the usual minor component of the interstellar medium. Its dynamic role in the contraction of the diffuse gas into molecular clouds is commonly assumed to be negligible because of the small mass fraction, f ≃ 0.01. However, as shown in this study, the collective motion of dust grains with respect to the gas may considerably contribute to the destabilization of the medium on scales λ ≲ λJ, where λJ is the Jeans length-scale. The linear perturbations of the uniform self-gravitating gas at rest are marginally stable at λ ≃ λJ, but as soon as the drift of grains is taken into account, they begin growing at a rate approximately equal to $(f \tau)^{1/3} t^{-1}_{\mathrm{ ff}}$, where τ is the stopping time of grains expressed in units of the free-fall time of the cloud, tff. The physical mechanism responsible for such a weak dependence of the growth rate on f is the resonance of heavy sound waves stopped by the self-gravity of gas with weak gravitational attraction caused by perturbations of the dust fraction. Once there is stationary subsonic bulk drift of the dust, the growing gas–dust perturbations at λ < λJ become waves propagating with the drift velocity projected on to the wavevector. Their growth has a resonant nature as well and the growth rate is substantially larger than that of the recently discovered resonant instability of gas–dust mixture in the absence of self-gravity. The new instabilities can facilitate gravitational contraction of cold interstellar gas into clouds and additionally produce dusty domains of sub-Jeans size at different stages of molecular cloud formation and evolution.

scholarly journals SEDIGISM: the kinematics of ATLASGAL filaments

2018 ◽  
Vol 619 ◽  
pp. A166 ◽  
Author(s):  
M. Mattern ◽  
J. Kauffmann ◽  
T. Csengeri ◽  
J. S. Urquhart ◽  
S. Leurini ◽  
...  
Keyword(s):  
Unit Length ◽  
Dust Emission ◽  
Free Fall ◽  
Outer Radius ◽  
Line Of Sight ◽  
Fall Velocity ◽  
Automated Algorithm ◽  
Power Law Exponent ◽  
Continuum Data ◽  
Self Gravity

Analyzing the kinematics of filamentary molecular clouds is a crucial step toward understanding their role in the star formation process. Therefore, we study the kinematics of 283 filament candidates in the inner Galaxy, that were previously identified in the ATLASGAL dust continuum data. The 13CO(2 – 1) and C18O(2 – 1) data of the SEDIGISM survey (Structure, Excitation, and Dynamics of the Inner Galactic Inter Stellar Medium) allows us to analyze the kinematics of these targets and to determine their physical properties at a resolution of 30′′ and 0.25 km s−1. To do so, we developed an automated algorithm to identify all velocity components along the line-of-sight correlated with the ATLASGAL dust emission, and derive size, mass, and kinematic properties for all velocity components. We find two-third of the filament candidates are coherent structures in position-position-velocity space. The remaining candidates appear to be the result of a superposition of two or three filamentary structures along the line-of-sight. At the resolution of the data, on average the filaments are in agreement with Plummer-like radial density profiles with a power-law exponent of p ≈ 1.5 ± 0.5, indicating that they are typically embedded in a molecular cloud and do not have a well-defined outer radius. Also, we find a correlation between the observed mass per unit length and the velocity dispersion of the filament of m ∝ σv2. We show that this relation can be explained by a virial balance between self-gravity and pressure. Another possible explanation could be radial collapse of the filament, where we can exclude infall motions close to the free-fall velocity.

scholarly journals Red Giant Winds as Emission Line Clouds (Broad or Narrow) in Active Galactic Nuclei

1989 ◽  
Vol 134 ◽  
pp. 310-311
Author(s):  
Demosthenes Kazanas
Keyword(s):  
Standard Model ◽  
Emission Line ◽  
Line Emission ◽  
Cloud Formation ◽  
Uniform Density ◽  
Dynamical Mechanism ◽  
The Standard Model ◽  
Accretion Rates ◽  
Red Giant ◽  
Self Gravity

It is proposed that the clouds thought responsible for the line emission in AGN are not of uniform density but stratified. Such a stratification may be a result either of their self-gravity or of gas dynamics associated with each cloud (e.g. winds). To fix ideas we assume the latter possibility, we examine the consequences and compare with the observations and the phenomenology of emission line clouds.(For the general properties of these clouds see review by Netzer in this volume). Given the successes of the standard model the reader may wonder why is there any need for revisions. The reasons are given in detail elsewhere (see also Scoville and Norman this volume). In brief these are the drag of clouds though the confining medium, the excessive accretion rates implied by the standard model, the response of the line radiation to changes in the continuum, and the lack of a dynamical mechanism for cloud formation. Finally, in the “standard” picture the narrow line clouds are a distinct population with separate dynamics and origin.

scholarly journals Filament Formation in Molecular Clouds as a Scale-Free Process

2015 ◽  
Vol 11 (A29B) ◽  
pp. 709-710
Author(s):  
Enrique Vázquez-Semadeni ◽  
Gilberto Gómez
Keyword(s):  
Hierarchical Structure ◽  
Molecular Clouds ◽  
Large Scale ◽  
Filament Formation ◽  
Molecular Line ◽  
Scale Free ◽  
The Core ◽  
Free Process ◽  
Self Gravity

AbstractWe discuss the formation of filaments in molecular clouds (MCs) as the result of large-scale collapse in the clouds. We first give arguments suggesting that self-gravity dominates the nonthermal motions, and then briefly describe the resulting structure, similar to that found in molecular-line and dust observations of the filaments in the clouds. The filaments exhibit a hierarchical structure in both density and velocity, suggesting a scale-free nature, similar to that of the cosmic web, resulting from the domination of self-gravity from the MC down to the core scale.

scholarly journals Thawing the frozen-in approximation: implications for self-gravity in deeply plunging tidal disruption events

2019 ◽  
Vol 485 (1) ◽  
pp. L146-L150 ◽  
Author(s):  
Elad Steinberg ◽  
Eric R Coughlin ◽  
Nicholas C Stone ◽  
Brian D Metzger
Keyword(s):  
Free Fall ◽  
Tidal Disruption ◽  
Massive Black Hole ◽  
High Penetration ◽  
Order Of Magnitude ◽  
Sphere Radius ◽  
Hydrodynamical Simulations ◽  
Self Gravity ◽  
Hydrostatic Balance ◽  
Mesh Technique

ABSTRACT The tidal destruction of a star by a massive black hole, known as a tidal disruption event (TDE), is commonly modelled using the ‘frozen-in’ approximation. Under this approximation, the star maintains exact hydrostatic balance prior to entering the tidal sphere (radius rt), after which point its internal pressure and self-gravity become instantaneously negligible and the debris undergoes ballistic free fall. We present a suite of hydrodynamical simulations of TDEs with high penetration factors β ≡ rt/rp = 5−7, where rp is the pericentre of the stellar centre of mass, calculated using a Voronoi-based moving-mesh technique. We show that basic assumptions of the frozen-in model, such as the neglect of self-gravity inside rt, are violated. Indeed, roughly equal fractions of the final energy spread accumulate exiting and entering the tidal sphere, though the frozen-in prediction is correct at the order-of-magnitude level. We also show that an $\mathcal {O}(1)$ fraction of the debris mass remains transversely confined by self-gravity even for large β which has implications for the radio emission from the unbound debris and, potentially, for the circularization efficiency of the bound streams.

scholarly journals Deterministic Self-Propagating Star Formation

1989 ◽  
Vol 120 ◽  
pp. 518-523
Author(s):  
Jan Palouš
Keyword(s):  
Star Formation ◽  
Simple Model ◽  
Molecular Cloud ◽  
Molecular Clouds ◽  
Large Scale ◽  
Galactic Evolution ◽  
Cloud Formation ◽  
Rotating Disks ◽  
Atomic Gas ◽  
High Column

AbstractThe evolution of large scale expanding structures in differentially rotating disks is studied. High column densities in some places may eventually lead to molecular cloud formation and initiate also star-formation. After some time, multi-structured arms evolve, where regions of intensive star-formation are separated from each other by regions of atomic gas or molecular clouds. This is due to the deterministic nature and to the coherence of this process. A simple model of galactic evolution is introduced and the different behaviour of Sa, Sb, and Sc galaxies is shown.

scholarly journals Theory of Feedback in Clusters and Molecular Cloud Turbulence

2010 ◽  
Vol 6 (S270) ◽  
pp. 275-282 ◽  
Author(s):  
Enrique Vázquez-Semadeni
Keyword(s):  
Numerical Simulations ◽  
Molecular Cloud ◽  
Molecular Clouds ◽  
Free Fall ◽  
High Mass ◽  
Driving Mechanism ◽  
Spectral Features ◽  
Fall Rate ◽  
Gaseous Environment ◽  
Analytical Work

AbstractI review recent numerical and analytical work on the feedback from both low- and high-mass cluster stars into their gaseous environment. The main conclusions are that i) outflow driving appears capable of maintaing the turbulence in parsec-sized clumps and retarding their collapse from the free-fall rate, although there exist regions within molecular clouds, and even some examples of whole clouds, which are not actively forming stars, yet are just as turbulent, so that a more universal turbulence-driving mechanism is needed; ii) outflow-driven turbulence exhibits specific spectral features that can be tested observationally; iii) feedback plays an important role in reducing the SFR; iv) nevertheless, numerical simulations suggest feedback cannot completely prevent a net contracting motion of clouds and clumps. Therefore, an appealing source for driving the turbulence everywhere in GMCs is the accretion from the environment, at all scales. In this case, feedback's most important role may be to prevent a fraction of the gas nearest to newly formed stars from actually reaching them, thus reducing the SFE.

scholarly journals The Supershell-Molecular Cloud Connection in the Milky Way and Beyond

2012 ◽  
Vol 8 (S292) ◽  
pp. 83-86
Author(s):  
J. R. Dawson ◽  
N. M. McClure-Griffiths ◽  
Y. Fukui ◽  
J. Dickey ◽  
T. Wong ◽  
...  
Keyword(s):  
Molecular Cloud ◽  
Molecular Clouds ◽  
Large Scale ◽  
Milky Way ◽  
Observational Evidence ◽  
Cloud Formation ◽  
Stellar Feedback ◽  
The Relationship ◽  
Global Contribution

AbstractThe role of large-scale stellar feedback in the formation of molecular clouds has been investigated observationally by examining the relationship between Hi and 12CO(J = 1−0) in supershells. Detailed parsec-resolution case studies of two Milky Way supershells demonstrate an enhanced level of molecularisation over both objects, and hence provide the first quantitative observational evidence of increased molecular cloud production in volumes of space affected by supershell activity. Recent results on supergiant shells in the LMC suggest that while they do indeed help to organise the ISM into over-dense structures, their global contribution to molecular cloud formation is of the order of only ∼ 10%.

scholarly journals Dynamically Driven Inflow onto the Galactic Center and its Effect upon Molecular Clouds

2021 ◽  
Vol 922 (1) ◽  
pp. 79
Author(s):  
H Perry Hatchfield ◽  
Mattia C. Sormani ◽  
Robin G. Tress ◽  
Cara Battersby ◽  
Rowan J. Smith ◽  
...  
Keyword(s):  
Molecular Clouds ◽  
Galactic Center ◽  
Critical Role ◽  
Average Density ◽  
Gas Flows ◽  
Hydrodynamic Simulations ◽  
Tracer Particles ◽  
Formation And Evolution ◽  
Self Gravity ◽  
Total Inflow

Abstract The Galactic bar plays a critical role in the evolution of the Milky Way’s Central Molecular Zone (CMZ), driving gas toward the Galactic Center via gas flows known as dust lanes. To explore the interaction between the CMZ and the dust lanes, we run hydrodynamic simulations in arepo, modeling the potential of the Milky Way’s bar in the absence of gas self-gravity and star formation physics, and we study the flows of mass using Monte Carlo tracer particles. We estimate the efficiency of the inflow via the dust lanes, finding that only about a third (30% ± 12%) of the dust lanes’ mass initially accretes onto the CMZ, while the rest overshoots and accretes later. Given observational estimates of the amount of gas within the Milky Way’s dust lanes, this suggests that the true total inflow rate onto the CMZ is 0.8 ± 0.6 M ⊙ yr−1. Clouds in this simulated CMZ have sudden peaks in their average density near the apocenter, where they undergo violent collisions with inflowing material. While these clouds tend to counter-rotate due to shear, co-rotating clouds occasionally occur due to the injection of momentum from collisions with inflowing material (∼52% are strongly counter-rotating, and ∼7% are strongly co-rotating of the 44 cloud sample). We investigate the formation and evolution of these clouds, finding that they are fed by many discrete inflow events, providing a consistent source of gas to CMZ clouds even as they collapse and form stars.

scholarly journals Physical properties and scaling relations of molecular clouds: the impact of star formation

2020 ◽  
Vol 500 (3) ◽  
pp. 3552-3568
Author(s):  
Kearn Grisdale
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Milky Way ◽  
Free Fall ◽  
Scaling Relations ◽  
Formation Method ◽  
Hydrodynamical Simulations ◽  
Formation Efficiency ◽  
The Impact ◽  
Very High

ABSTRACT Using hydrodynamical simulations of a Milky Way-like galaxy, reaching 4.6 pc resolution, we study how the choice of star formation criteria impacts both galactic and giant molecular cloud (GMC) scales. We find that using a turbulent, self-gravitating star formation criteria leads to an increase in the fraction of gas with densities between 10 and $10^{4}{\, \rm {cm^{-3}}}$ when compared with a simulation using a molecular star formation method, despite both having nearly identical gaseous and stellar morphologies. Furthermore, we find that the site of star formation is effected with the the former tending to only produce stars in regions of very high density (${\gt}10^{4}{\, \rm {cm^{-3}}}$) gas, while the latter forms stars along the entire length of its spiral arms. The properties of GMCs are impacted by the choice of star formation criteria with the former method producing larger clouds. Despite the differences, we find that the relationships between clouds properties, such as the Larson relations, remain unaffected. Finally, the scatter in the measured star formation efficiency per free-fall time of GMCs remains present with both methods and is thus set by other factors.

scholarly journals Cloud formation in the atomic and molecular phase: H I self absorption (HISA) towards a giant molecular filament

2020 ◽  
Vol 634 ◽  
pp. A139 ◽  
Author(s):  
Y. Wang ◽  
S. Bihr ◽  
H. Beuther ◽  
M. R. Rugel ◽  
J. D. Soler ◽  
...  
Keyword(s):  
Interstellar Medium ◽  
Power Law ◽  
Molecular Clouds ◽  
Column Density ◽  
Cloud Formation ◽  
Neutral Medium ◽  
Molecular Component ◽  
Density Peaks ◽  
Log Normal ◽  
High Column

Molecular clouds form from the atomic phase of the interstellar medium. However, characterizing the transition between the atomic and the molecular interstellar medium (ISM) is a complex observational task. Here we address cloud formation processes by combining H I self absorption (HISA) with molecular line data. Column density probability density functions (N-PDFs) are a common tool for examining molecular clouds. One scenario proposed by numerical simulations is that the N-PDF evolves from a log-normal shape at early times to a power-law-like shape at later times. To date, investigations of N-PDFs have been mostly limited to the molecular component of the cloud. In this paper, we study the cold atomic component of the giant molecular filament GMF38.1-32.4a (GMF38a, distance = 3.4 kpc, length ~ 230 pc), calculate its N-PDFs, and study its kinematics. We identify an extended HISA feature, which is partly correlated with the 13CO emission. The peak velocities of the HISA and 13CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s−1 is found on the western side. The sonic Mach number we derive from the linewidth measurements shows that a large fraction of the HISA, which is ascribed to the cold neutral medium (CNM), is at subsonic and transonic velocities. The column density of the CNM part is on the order of 1020 to 1021 cm−2. The column density of molecular hydrogen, traced by 13CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H I emission (the warm and cold neutral medium), and 13CO (molecular component) are well described by log-normal functions, which is in agreement with turbulent motions being the main driver of cloud dynamics. The N-PDF of the molecular component also shows a power law in the high column-density region, indicating self-gravity. We suggest that we are witnessing two different evolutionary stages within the filament. The eastern subregion seems to be forming a molecular cloud out of the atomic gas, whereas the western subregion already shows high column density peaks, active star formation, and evidence of related feedback processes.

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