General polytropic hydrodynamic cylinder under self-gravity

2020 ◽  
Author(s):  
Yu-Qing Lou ◽  
Ming Lin
Keyword(s):  
Free Fall ◽  
Similarity Solutions ◽  
Polytropic Index ◽  
Newtonian Gravity ◽  
Global Features ◽  
Stellar Objects ◽  
Asymptotic Behaviors ◽  
Asymptotic Dynamic ◽  
Self Gravity ◽  
Global Similarity

Abstract For filamentary clouds on various scales obeying general polytropic (GP) equation of state, their hydrodynamic collapses, expansions and shocks are investigated. Our cylindrical model is axisymmetric, infinitely long with axial uniformity and involves Newtonian gravity. For such GP cylinders, we explore various analytical and numerical similarity solutions. Based on a singular hydrostatic solution, we derive a quasi-static asymptotic dynamic solution approaching the axis. There, we also derive the asymptotic cylindrical free-fall solution for polytropic index γ ≤ 1 and show the absence of such solutions for γ > 1. We find new asymptotic solutions for expanding cylindrical central voids with no matter inside, and examine the asymptotic expansion solutions to higher orders far from the axis. We classify the sonic critical curve (SCC) into three (or five) types and analyse their properties. The asymptotic behaviors of the SCC towards the axis and infinity are examined. Examples are shown for solutions crossing the SCC twice with the global features of cylindrical envelope expansion or contraction with core collapses. We numerically construct new types of global similarity solutions with or without outgoing shocks. For γ > 1, a shock is necessary to connect the inner and outer parts. The collapse and fragmentation of massive filaments or strings may give clues and implications to the formations of chains of stellar objects, chains of black holes, chains of galaxies or even chains of galaxy clusters in proper astrophysical and cosmological contexts.

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 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.

DOES PRESSURE ACCENTUATE GENERAL RELATIVISTIC GRAVITATIONAL COLLAPSE AND FORMATION OF TRAPPED SURFACES?

2013 ◽  
Vol 22 (05) ◽  
pp. 1350021 ◽  
Author(s):  
ABHAS MITRA
Keyword(s):  
General Relativity ◽  
Gravitational Collapse ◽  
Fluid Pressure ◽  
Free Fall ◽  
Newtonian Gravity ◽  
Tangential Pressure ◽  
Trapped Surfaces ◽  
Imperfect Fluid ◽  
Newtonian Case ◽  
General Relativistic

It is widely believed that though pressure resists gravitational collapse in Newtonian gravity, it aids the same in general relativity (GR) so that GR collapse should eventually be similar to the monotonous free fall case. But we show that, even in the context of radiationless adiabatic collapse of a perfect fluid, pressure tends to resist GR collapse in a manner which is more pronounced than the corresponding Newtonian case and formation of trapped surfaces is inhibited. In fact there are many works which show such collapse to rebound or become oscillatory implying a tug of war between attractive gravity and repulsive pressure gradient. Furthermore, for an imperfect fluid, the resistive effect of pressure could be significant due to likely dramatic increase of tangential pressure beyond the "photon sphere." Indeed, with inclusion of tangential pressure, in principle, there can be static objects with surface gravitational redshift z → ∞. Therefore, pressure can certainly oppose gravitational contraction in GR in a significant manner in contradiction to the idea of Roger Penrose that GR continued collapse must be unstoppable.

scholarly journals Formation of Young Star Clusters

2005 ◽  
Vol 13 ◽  
pp. 358-362
Author(s):  
Bruce Elmegreen
Keyword(s):  
Star Formation ◽  
Energy Dissipation ◽  
Star Clusters ◽  
Young Star ◽  
Formation Processes ◽  
Stellar Objects ◽  
Scale Free ◽  
Self Gravity ◽  
Young Star Clusters ◽  
Mass Functions

AbstractTurbulence, self-gravity, and cooling convert most of the interstellar medium into cloudy structures that form stars. Turbulence compresses the gas into clouds directly and it moves pre-existing clouds around passively when there are multiple phases of temperature. Self-gravity also partitions the gas into clouds, forming giant regular complexes in spiral arms and in resonance rings and contributing to the scale-free motions generated by turbulence. Dense clusters form in the most strongly self-gravitating cores of these clouds, often triggered by compression from local stars. Pre-star formation processes inside clusters are not well observed, but the high formation rates and high densities of pre-stellar objects, and their power law mass functions suggest that turbulence, self-gravity, and energy dissipation are involved there too.

scholarly journals A timeline for massive star-forming regions via combined observation of o-H2D+ and N2D+

2019 ◽  
Vol 621 ◽  
pp. L7 ◽  
Author(s):  
A. Giannetti ◽  
S. Bovino ◽  
P. Caselli ◽  
S. Leurini ◽  
D. R. G. Schleicher ◽  
...  
Keyword(s):  
Free Fall ◽  
Young Stellar Objects ◽  
High Mass ◽  
Continuum Emission ◽  
State Transitions ◽  
Temperature Sensitive ◽  
Evolutionary Stage ◽  
Stellar Objects ◽  
Star Forming ◽  
Star Forming Regions

Context. In cold and dense gas prior to the formation of young stellar objects, heavy molecular species (including CO) are accreted onto dust grains. Under these conditions H3+ and its deuterated isotopologues become more abundant, enhancing the deuterium fraction of molecules such as N2H+ that are formed via ion-neutral reactions. Because this process is extremely temperature sensitive, the abundance of these species is likely linked to the evolutionary stage of the source. Aims. We investigate how the abundances of o-H2D+ and N2D+ vary with evolution in high-mass clumps. Methods. We observed with APEX the ground-state transitions of o-H2D+ near 372 GHz, and N2D+(3–2) near 231 GHz for three massive clumps in different evolutionary stages. The sources were selected within the G351.77–0.51 complex to minimise the variation of initial chemical conditions, and to remove distance effects. We modelled their dust continuum emission to estimate their physical properties, and also modelled their spectra under the assumption of local thermodynamic equilibrium to calculate beam-averaged abundances. Results. We find an anticorrelation between the abundance of o-H2D+ and that of N2D+, with the former decreasing and the latter increasing with evolution. With the new observations we are also able to provide a qualitative upper limit to the age of the youngest clump of about 105 yr, comparable to its current free-fall time. Conclusions. We can explain the evolution of the two tracers with simple considerations on the chemical formation paths, depletion of heavy elements, and evaporation from the grains. We therefore propose that the joint observation and the relative abundance of o-H2D+ and N2D+ can act as an efficient tracer of the evolutionary stages of the star-formation process.

scholarly journals Cloud fragmentation cascades and feedback: on reconciling an unfettered inertial range with a low star formation rate

2020 ◽  
Vol 493 (1) ◽  
pp. 815-820
Author(s):  
Eric G Blackman
Keyword(s):  
Star Formation ◽  
Molecular Cloud ◽  
Dynamic Range ◽  
Star Formation Rate ◽  
Formation Rate ◽  
Free Fall ◽  
Young Stellar Objects ◽  
Stellar Objects ◽  
Star Forming ◽  
Cloud Complex

ABSTRACT Molecular cloud complexes exhibit both (i) an unfettered Larson-type spectrum over much of their dynamic range, whilst (ii) still producing a much lower star formation rate than were this cascade to remain unfettered all the way down to star-forming scales. Here we explain the compatibility of these attributes with minimalist considerations of a mass-conserving fragmentation cascade, combined with estimates of stellar feedback. Of importance is that the amount of feedback needed to abate fragmentation and truncate the complex decreases with decreasing scale. The scale at which the feedback momentum matches the free-fall momentum marks a transition scale below most of the cascade is truncated and the molecular cloud complex dissipated. For a 106 M⊙ giant molecular cloud (GMC) complex starting with radius of ∼50 pc, the combined feedback from young stellar objects, supernovae, radiation, and stellar winds for a GMC cloud complex can truncate the cascade within an outer free-fall time but only after the cascade reaches parsec scales.

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.

Self-gravity in magnetized accretion discs as a result of a dynamo mechanism with outflows

2020 ◽  
Vol 493 (2) ◽  
pp. 2101-2110
Author(s):  
S Karimzadeh ◽  
A R Khesali ◽  
A Khosravi
Keyword(s):  
Magnetic Field ◽  
Vertical Direction ◽  
Galactic Nuclei ◽  
Young Stellar Objects ◽  
Stellar Objects ◽  
Magnetic Diffusivity ◽  
Dynamo Mechanism ◽  
Self Gravity ◽  
The Magnetic Field ◽  
Effective Contribution

ABSTRACT We investigate the stationary model of a geometrically thin, magnetized accretion disc, which has a dipole-symmetry magnetic field that is produced by an α−ω dynamo and can emanate winds from the disc’s surfaces. Although self-gravity has an important role in the evolution of astrophysical systems, it has been disregarded in many cases, because the equations become more complicated when the mass distribution of the disc is included in the total gravitational potential. In this paper, we consider the effects of self-gravity on the above-mentioned model. It is shown that in the presence of vertical self-gravity, while the magnetic diffusivity decreases, the magnetic field bends and the inflow speed increases. Also, in the inner parts of the disc, mass flux resulting from the wind has a positive value compared with the non-self-gravitating solution, in which all accreted materials are lost. These results can be used for the discs of active galactic nuclei, in which self-gravity is only important in the vertical direction. However, for other types, such as the discs surrounding young stellar objects, self-gravity can be considered in both vertical and radial directions. Here, our analysis of fully self-gravitating discs has revealed that, in this case, the inflow speed depends on the radius. In the model we study, it is also found that the outflows have no effective contribution to the removal of angular momentum for certain radii r ≥ 6R, as is > 60°. However, the system cannot be stabilized by viscous dissipation.

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