Molecular Cloud Evolution. II. From Cloud Formation to the Early Stages of Star Formation in Decaying Conditions

Vázquez‐Semadeni, Enrique; Gómez, Gilberto C.; Jappsen, Anne‐Katharina; Ballesteros-Paredes, Javier; González, R. F.; Klessen, Ralf S.

doi:10.1086/510771

Cited by 348 publications

(497 citation statements)

References 101 publications

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“…Our numerical simulation was performed using the Gadget-2 code (Springel et al 2001), with 296 3 ≈ 2.6 × 10 7 SPH particles, and including prescriptions for sink particles taken from Jappsen et al (2005) and for heating and cooling from Vázquez-Semadeni et al (2007). The initial density and temperature of the simulation were set at 3 cm −3 and 730 K, respectively, representing the mean ISM conditions at a spiral arm.…”

Section: The Numerical Simulationmentioning

confidence: 99%

“…Indeed, numerical simulations of converging E-mail: jonathanheiner@gmail.com flows in the warm neutral medium (WNM) including selfgravity but no stellar feedback (Vázquez-Semadeni et al 2007Hennebelle et al 2008;Heitsch & Hartmann 2008;Heitsch et al 2009;Banerjee et al 2009a) show in general that, once a dense cloud is formed by this mechanism, it quickly becomes gravitationally unstable, and begins to undergo gravitational collapse. An important feature of this collapse is that it begins in gas that should be primarily atomic, with molecule formation occurring as a consequence of the gravitational contraction, as initially proposed on theoretical grounds by Franco & Cox (1986) and Hartmann et al (2001).…”

Section: Introductionmentioning

confidence: 99%

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Molecular cloud formation as seen in synthetic H i and molecular gas observations

Heiner¹,

Vázquez‐Semadeni²,

Ballesteros-Paredes³

2015

Mon. Not. R. Astron. Soc.

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We present synthetic Hi and CO observations of a numerical simulation of decaying turbulence in the thermally bistable neutral medium. We first present the simulation, which produces a clumpy medium, with clouds initially consisting of clustered clumps. Self-gravity causes these clump clusters to merge and form more homogeneous dense clouds. We apply a simple radiative transfer algorithm, throwing rays in many directions from each cell, and defining every cell with A v > 1 as molecular. We then produce maps of Hi, CO-free molecular gas, and CO, and investigate the following aspects: i) The spatial distribution of the warm, cold, and molecular gas, finding the well-known layered structure, with molecular gas being surrounded by cold Hi and this in turn being surrounded by warm Hi. ii) The velocity of the various components, finding that the atomic gas is generally flowing towards the molecular gas, and that this motion is reflected in the frequently observed bimodal shape of the Hi profiles. This conclusion is, however, tentative, because we do not include feedback that may produce Hi gas receding from molecular regions. iii) The production of Hi self-absorption (HISA) profiles, and the correlation of HISA with molecular gas. In particular, we test the suggestion of using the second derivative of the brightness temperature Hi profile to trace HISA and molecular gas, finding significant limitations. On a scale of several parsecs, some agreement is obtained between this technique and actual HISA, as well as a correlation between HISA and the molecular gas column density. This correlation, however, quickly deteriorates towards sub-parsec scales. iv) The column density PDFs of the actual Hi gas and those recovered from the Hi line profiles, finding that the latter have a cutoff at column densities where the gas becomes optically thick, thus missing the contribution from the HISA-producing gas. We also find that the powerlaw tail typical of gravitational contraction is only observed in the molecular gas, and that, before the power-law tail develops in the total gas density PDF, no CO is yet present, reinforcing the notion that gravitational contraction is needed to produce this component.

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Section: The Numerical Simulationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Molecular cloud formation as seen in synthetic H i and molecular gas observations

Heiner¹,

Vázquez‐Semadeni²,

Ballesteros-Paredes³

2015

Mon. Not. R. Astron. Soc.

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“…In this work, as a first step, we have therefore assumed that Γ = 2 × 10 −26 erg s −1 (independent of density or temperature). For the low-temperature cooling ( 10 4 K), we have followed the detailed prescription of Koyama & Inutsuka (2000), fitted by Koyama & Inutsuka (2002), corrected according to Vázquez-Semadini et al (2007), namely Λ(T ) Γ = 10 7 exp −1.184 × 10 5 T + 1000…”

Section: Heating and Cooling Processesmentioning

confidence: 99%

Magnetohydrodynamical simulation of the formation of clumps and filaments in quiescent diffuse medium by thermal instability

Wareing

Pittard

Falle

et al. 2016

Mon. Not. R. Astron. Soc.

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We have used the AMR hydrodynamic code, MG, to perform idealised 3D MHD simulations of the formation of clumpy and filamentary structure in a thermally unstable medium without turbulence. A stationary thermally unstable spherical diffuse atomic cloud with uniform density in pressure equilibrium with low density surroundings was seeded with random density variations and allowed to evolve. A range of magnetic field strengths threading the cloud have been explored, from β = 0.1 to β = 1.0 to the zero magnetic field case (β = ∞), where β is the ratio of thermal pressure to magnetic pressure. Once the density inhomogeneities had developed to the point where gravity started to become important, self-gravity was introduced to the simulation. With no magnetic field, clouds and clumps form within the cloud with aspect ratios of around unity, whereas in the presence of a relatively strong field (β = 0.1) these become filaments, then evolve into interconnected corrugated sheets that are predominantly perpendicular to the magnetic field. With magnetic and thermal pressure equality (β = 1.0), filaments, clouds and clumps are formed. At any particular instant, the projection of the 3D structure onto a plane parallel to the magnetic field, i.e. a line of sight perpendicular to the magnetic field, resembles the appearance of filamentary molecular clouds. The filament densities, widths, velocity dispersions and temperatures resemble those observed in molecular clouds. In contrast, in the strong field case β = 0.1, projection of the 3D structure along a line of sight parallel to the magnetic field reveals a remarkably uniform structure.

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“…It has been suggested, fragmentation of shocked bodies though rapid, is more likely to produce unbound fragments, confined by external pressure and may merge to form larger clumps. These fragments are unlike those that form under the assumption of isothermality in which case they are likely to be relatively short-lived on account of their propensity towards re-expansion due to insufficient confining pressure (see e.g., Vázquez-Semadeni et al 2007;Heitsch et al 2008 b;Anathpindika 2009). …”

Section: Anathpindikamentioning

confidence: 99%

“…Gas particles were allowed to cool by employing a parametric cooling function( ), defined by Equation (3) below and plotted in Figure 2. The curve can be readily identified as the thermal equilibrium curve for gas in the interstellar medium (ISM) (e.g., Wolfire et al 1995 (Koyama & Inutsuka 2002;Vázquez-Semadeni et al 2007). …”

Section: Numerical Algorithmmentioning

confidence: 99%

Formation of Prestellar Cores via Non-Isothermal Gas Fragmentation

Anathpindika

2015

Publ. Astron. Soc. Aust.

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Sheet-like clouds are common in turbulent gas and perhaps form via collisions between turbulent gas flows. Having examined the evolution of an isothermal shocked slab in an earlier contribution, in this work we follow the evolution of a sheet-like cloud confined by (thermal) pressure and gas in it is allowed to cool. The extant purpose of this endeavour is to study the early phases of core-formation. The observed evolution of this cloud supports the conjecture that molecular clouds themselves are three-phase media (comprising viz. a stable cold and warm medium, and a third thermally unstable medium), though it appears, clouds may evolve in this manner irrespective of whether they are gravitationally bound. We report, this sheet fragments initially due to the growth of the thermal instability (TI) and some fragments are elongated, filament-like. Subsequently, relatively large fragments become gravitationally unstable and sub-fragment into smaller cores. The formation of cores appears to be a three stage process: first, growth of the TI leads to rapid fragmentation of the slab; second, relatively small fragments acquire mass via gas-accretion and/or merger and third, sufficiently massive fragments become susceptible to the gravitational instability and sub-fragment to form smaller cores. We investigate typical properties of clumps (and smaller cores) resulting from this fragmentation process. Findings of this work support the suggestion that the weak velocity field usually observed in dense clumps and smaller cores is likely seeded by the growth of dynamic instabilities. Simulations were performed using the smooth particle hydrodynamics algorithm.

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Molecular Cloud Evolution. II. From Cloud Formation to the Early Stages of Star Formation in Decaying Conditions

Cited by 348 publications

References 101 publications

Molecular cloud formation as seen in synthetic H i and molecular gas observations

Molecular cloud formation as seen in synthetic H i and molecular gas observations

Magnetohydrodynamical simulation of the formation of clumps and filaments in quiescent diffuse medium by thermal instability

Formation of Prestellar Cores via Non-Isothermal Gas Fragmentation

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