arXiv subjects

E. Vazquez-Semadeni

Publications and source records attributed to E. Vazquez-Semadeni.

At least 19 recordsLinked to original sources

arXiv2020

Dynamical cloud formation traced by atomic and molecular gas

Context: Atomic and molecular cloud formation is a dynamical process. However, kinematic signatures of these processes are still observationally poorly constrained. Methods: Targeting the cloud-scale environment of the prototypical infrared dark cloud G28.3, we employ spectral line imaging observations of the two atomic lines HI and [CI] as well as molecular lines observations in 13CO in the 1--0 and 3--2 transitions. The analysis comprises investigations of the kinematic properties of the different tracers, estimates of the mass flow rates, velocity structure functions, a Histogram of Oriented Gradients (HOG) study as well as comparisons to simulations. Results: The central IRDC is embedded in a more diffuse envelope of cold neutral medium (CNM) traced by HI self-absorption (HISA) and molecular gas. The spectral line data as well as the HOG and structure function analysis indicate a possible kinematic decoupling of the HI from the other gas compounds. Spectral analysis and position-velocity diagrams reveal two velocity components that converge at the position of the IRDC. Estimated mass flow rates appear rather constant from the cloud edge toward the center. The velocity structure function analysis is consistent with gas flows being dominated by the formation of hierarchical structures. Conclusions: The observations and analysis are consistent with a picture where the IRDC G28 is formed at the center of two converging gas flows. While the approximately constant mass flow rates are consistent with a self-similar, gravitationally driven collapse of the cloud, external compression by, e.g., spiral arm shocks or supernovae explosions cannot be excluded yet. Future investigations should aim at differentiating the origin of such converging gas flows.

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arXiv2014

The identification of filaments on far infrared and submillimiter images. Morphology, physical conditions and relation with star formation of filamentary structure

Observations of molecular clouds reveal a complex structure, with gas and dust often arranged in filamentary rather than spherical geometries. The associations of pre- and proto- stellar cores with the filaments suggest a direct link with the process of star formation. Any study of the properties of such filaments requires a representative samples from different enviroments and so an unbiased detection method. We developed such an approach using the Hessian matrix of a surface-brightness distribution to identify filaments and determine their physical and morphological properties. After testing the method on simulated, but realistic filaments, we apply the algorithms to column-density maps computed from Herschel observations of the Galactic Plane obtained by the Hi-GAL project. We identified ~500 filaments, in the longitude range of l=216.5 to l=225.5, with lengths from ~1 pc up to ~30 pc and widths between 0.1 pc and 2.5 pc. Average column densities are between 10^20 cm^-2 and 10^22 cm^-2. Filaments include the majority of dense material with N_H_2 > 6x10^21cm^-2. We find that the pre- and proto-stellar compact sources already identified in the same region are mostly associated with filaments. However, surface densities in excess of the expected critical values for high-mass star formation are only found on the filaments, indicating that these structures are necessary to channel material into the clumps. Furthermore, we analyze the gravitational stability of filaments and discuss their relationship with star formation.

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arXiv2008

From the warm magnetized atomic medium to molecular clouds

{It has recently been proposed that giant molecular complexes form at the sites where streams of diffuse warm atomic gas collide at transonic velocities.} {We study the global statistics of molecular clouds formed by large scale colliding flows of warm neutral atomic interstellar gas under ideal MHD conditions. The flows deliver material as well as kinetic energy and trigger thermal instability leading eventually to gravitational collapse.} {We perform adaptive mesh refinement MHD simulations which, for the first time in this context, treat self-consistently cooling and self-gravity.} {The clouds formed in the simulations develop a highly inhomogeneous density and temperature structure, with cold dense filaments and clumps condensing from converging flows of warm atomic gas. In the clouds, the column density probability density distribution (PDF) peaks at $\sim 2 \times 10^{21} \psc$ and decays rapidly at higher values; the magnetic intensity correlates weakly with density from $n \sim 0.1$ to $10^4 \pcc$, and then varies roughly as $n^{1/2}$ for higher densities.} {The global statistical properties of such molecular clouds are reasonably consistent with observational determinations. Our numerical simulations suggest that molecular clouds formed by the moderately supersonic collision of warm atomic gas streams.}

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arXiv2007

Diffuse interstellar medium and the formation of molecular clouds

(Abridged) The formation of molecular clouds (MCs) from the diffuse interstellar gas appears to be a necessary step for star formation, as young stars invariably occur within them. However, the mechanisms controlling the formation of MCs remain controversial. In this contribution, we focus on their formation in compressive flows driven by interstellar turbulence and large-scale gravitational instability. Turbulent compression driven by supernovae appears insufficient to explain the bulk of cloud and star formation. Rather, gravity must be important at all scales, driving the compressive flows that form both clouds and cores. Cooling and thermal instability allow the formation of dense gas out of moderate, transonic compressions in the warm diffuse gas, and drive turbulence into the dense clouds. MCs may be produced by an overshoot beyond the thermal-pressure equilibrium between the cold and warm phases of atomic gas, caused by some combination of the ram pressure of compression and the self-gravity of the compressed gas. In this case, properties of the clouds such as their mass, mass-to-magnetic flux ratio, and total kinetic and gravitational energies are in general time-variable quantities. MCs may never enter a quasi-equilibrium or virial equilibrium state but rather continuously collapse to stars.

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arXiv2007

Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions

We study the formation of giant dense cloud complexes and of stars within them by means of SPH numerical simulations of the mildly supersonic collision of gas streams (``inflows'') in the warm neutral medium (WNM). The resulting compressions cause cooling and turbulence generation in the gas, forming a cloud that then becomes self-gravitating and undergoes global collapse. Simultaneously, the turbulent, nonlinear density fluctuations induce fast, local collapse events. The simulations show that: a) The clouds are not in a state of equilibrium. Instead, they undergo secular evolution. Initially, their mass and gravitational energy |Eg| increase steadily, while the turbulent energy Ek reaches a plateau. b) When |Eg| becomes comparable to Ek, global collapse begins, causing a simultaneous increase in both that maintains a near-equipartition condition |Eg| ~ 2 Ek. c) Longer inflow durations delay the onset of global and local collapse, by maintaining a higher turbulent velocity dispersion in the cloud over longer times. d) The star formation rate is large from the beginning, without any period of slow and accelerating star formation. e) The column densities of the local star-forming clumps are very similar to reported values of the column density required for molecule formation, suggesting that locally molecular gas and star formation occur nearly simultaneously. The MC formation mechanism discussed here naturally explains the apparent ``virialized'' state of MCs and the ubiquitous presence of HI halos around them. Within their assumptions, our simulations support the scenario of rapid star formation after MCs are formed, although long (>~ 15 Myr) accumulation periods do occur during which the clouds build up their gravitational energy, and which are expected to be spent in the atomic phase.

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arXiv2006

Molecular Cloud Turbulence and Star Formation

We review the properties of turbulent molecular clouds (MCs), focusing on the physical processes that influence star formation (SF). MC formation appears to occur during large-scale compression of the diffuse ISM driven by supernovae, magnetorotational instability, or gravitational instability in galactic disks of stars and gas. The compressions generate turbulence that can accelerate molecule production and produce the observed morphology. We then review the properties of MC turbulence, including density enhancements observed as clumps and cores, magnetic field structure, driving scales, the relation to observed scaling relations, and the interaction with gas thermodynamics. We argue that MC cores are dynamical, not quasistatic, objects with relatively short lifetimes not exceeding a few megayears. We review their morphology, magnetic fields, density and velocity profiles, and virial budget. Next, we discuss how MC turbulence controls SF. On global scales turbulence prevents monolithic collapse of the clouds; on small scales it promotes local collapse. We discuss its effects on the SF efficiency, and critically examine the possible relation between the clump mass distribution and the initial mass function, and then turn to the redistribution of angular momentum during collapse and how it determines the multiplicity of stellar systems. Finally, we discuss the importance of dynamical interactions between protostars in dense clusters, and the effect of the ionization and winds from those protostars on the surrounding cloud. We conclude that the interaction of self-gravity and turbulence controls MC formation and behavior, as well as the core and star formation processes within them.

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arXiv2004

The Lifetimes of Molecular Cloud Cores: What is the Role of the Magnetic Field?

We discuss the lifetimes and evolution of dense cores formed as turbulent density fluctuations in magnetized, isothermal molecular clouds. We consider numerical simulations in which we measure the cores' magnetic criticality and Jeans stability in relation to the magnetic criticality of their ``parent clouds'' (the numerical boxes). In subcritical boxes, dense cores do not form, and collapse does not occur. In supercritical boxes, some cores collapse, being part of larger clumps that are supercritical from the start, and whose central, densest regions (the cores) are initially subcritical, but rapidly become supercritical, presumably by accretion along field lines. Numerical artifacts are ruled out. The time scales for cores to go from subcritical to supercritical and then collapse are a few times the core free-fall time, $\tfc$. Our results suggest that cores are out-of-equilibrium, transient structures, rather than quasi-magnetostatic configurations.

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arXiv2003

Star formation and turbulent dissipation in models of disk galaxy evolution

The kinetic energy dissipation rate in the turbulent ISM of disk galaxies is a key ingredient in galaxy evolution models since it determines the effectiveness of large-scale star formation (SF) feedback. Using magneto-hydro-dynamic simulations, we find that the ISM dissipates efficiently the turbulent kinetic energy injected by sources of stellar nature. Thus, the SF process may be self-regulated by an energy balance only at the level of the disk ISM. The use of the self-regulation SF mechanism in galaxy evolutionary models, where disks form inside growing Cold Dark Matter halos, allows to predict the SF history of disk galaxies, including the Milky Way and the solar neighborhood, as well as the contribution of the whole population of disk galaxies to the cosmic SF history. The results are encouraging.

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arXiv2003

A Holistic Scenario of Turbulent Molecular Cloud Evolution and Control of the Star Formation Efficiency. First Tests

We compile a holistic scenario for molecular cloud (MC) evolution and control of the star formation efficiency (SFE), and present a first set of numerical tests of it. A {\it lossy} compressible cascade can generate density fluctuations and further turbulence at small scales from large-scale motions, implying that the turbulence in MCs may originate from the compressions that form them. Below a {\it sonic} scale $\ls$, turbulence cannot induce any further subfragmentation, nor be a dominant support agent against gravity. Since progressively smaller density peaks contain progressively smaller fractions of the mass, we expect the SFE to decrease with decreasing $\ls$, at least when the cloud is globally supported by turbulence. Our numerical experiments confirm this prediction. We also find that the collapsed mass fraction in the simulations always saturates below 100% efficiency. This may be due to the decreased mean density of the leftover interclump medium, which in real clouds (not confined to a box) should then be more easily dispersed, marking the ``death'' of the cloud. We identify two different functional dependences (``modes'') of the SFE on $\ls$, which roughly correspond to globally supported and unsupported cases. Globally supported runs with most of the turbulent energy at the largest scales have similar SFEs to those of unsupported runs, providing numerical evidence of the dual role of turbulence, whereby large-scale turbulent modes induce collapse at smaller scales. We tentatively suggest that these modes may correspond to the clustered and isolated modes of star formation, although here they are seen to form part of a continuum rather than being separate modes. Finally, we compare with previous proposals that the relevant parameter is the energy injection scale.

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arXiv2002

The nonlinear development of the thermal instability in the atomic ISM and its interaction with random fluctuations

(Abridged) We discuss the nonlinear development of the isobaric mode of thermal instability (TI) in the context of the atomic interstellar medium (ISM), in order to assess the ability of TI to establish a well-segregated multi-phase structure in the turbulent ISM. The key parameter is the ratio of the cooling time to the dynamical crossing time $η$. The isobaric density perturbations of sizes $\gtrsim 15$ pc in media with mean density $\sim 1$ cm$^{-3}$ develop inflow motions with Mach number larger than 0.5 and a shock that propagates outwards from the condensation and brings the surrounding medium out of thermal equilibrium. The time for the dynamical transient state to subside ranges from 4 to 30 Myr for initial density perturbations of 20% and sizes 3 to 75 pc. By the time the condensations have formed, a substantial fraction of the mass is still traversing the unstable range and, therefore, clouds formed by TI should be bounded by accreting gas traversing the unstable range, rather than by sharp transitions to the stable warm phase. In the presence of random velocity forcing the condensation process can be suppressed for arbitrarily long times if the forcing causes a moderate rms Mach number ($\gtrsim 0.3$) and extends to small enough scales that $η>1$. We suggest that these mechanisms may be at the origin of the relatively large amounts of gas mass in the unstable regime. We remark that in the (stable) warm diffuse medium, $η>1$ for velocity perturbations of scales up to several pc. The flow's response to them is thus nearly adiabatic, and relatively weakly compressible, consistent with observations that suggest a nearly Kolmogorov power spectrum in this medium.

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arXiv2001

The Temperature Distribution in Turbulent Interstellar Gas

We discuss the temperature distribution in a two-dimensional, thermally unstable numerical simulation of the warm and cold gas in the Galactic disk, including the magnetic field, self-gravity, the Coriolis force, stellar energy injection and a realistic cooling function. We find that ~50% of the turbulent gas mass has temperatures in what would be the thermally unstable range if thermal instability were to be considered alone. This appears to be a consequence of there being many other forces at play than just thermal pressure. We also point out that a bimodal temperature pdf is a simple consequence of the form of the interstellar cooling function and is not necessarily a signature of discontinuous phase transitions.

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arXiv1999

Compressible MHD Turbulence: Implications for Molecular Cloud and Star Formation

We review recent results from numerical simulations and related models of MHD turbulence in the interstellar medium (ISM) and in molecular clouds. We discuss the implications of turbulence for the processes of cloud formation and evolution, and the determination of clouds' physical properties. Numerical simulations of the turbulent ISM to date have included magnetic fields, self-gravity, parameterized heating and cooling, modeled star formation and other turbulent inputs. The structures which form reproduce well observed velocity-size scaling properties, while predicting the non-existence of a general density-size scaling law. Criteria for the formation of gravitationally-bound structures by turbulent compression are summarized. For flows with equations of state $P\propto ρ^γ$, the statistics of the density field depend on the exponent $γ$. Numerical simulations of both forced and decaying MHD compressible turbulence have shown that the decay rate is comparable to the non-magnetic case. For virialized clouds, the turbulent decay time is shorter than the gravitational free-fall time, so wholesale cloud collapse is only prevented by ongoing turbulent inputs and/or a strong mean magnetic field. Finally, perspectives for future work in this field are briefly discussed.

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arXiv1997

On the Density Probability Function of Galactic Gas. I. Numerical Simulations and the Significance of the Polytropic Index

We investigate the form of the one-point probability distribution function (pdf) for the density field of the interstellar medium using numerical simulations that successively reduce the number of physical processes included. Two-dimensional simulations of self-gravitating supersonic MHD and hydrodynamic turbulence, and of decaying Burgers turbulence, produce in all cases filamentary density structures and a power-law density pdf with logarithmic slope around -1.7. This suggests that the functional form of the pdf and the general filamentary morphology are the signature of the nonlinear advection operator. These results do not support previous claims that the pdf is lognormal. A series of 1D simulations of forced supersonic polytropic turbulence is used to resolve the discrepancy. They suggest that the pdf is lognormal only for effective polytropic indices $γ=1$ (or nearly lognormal for $γ\not=1$ if the Mach number is sufficiently small), while power laws develop at high densities if $γ<1$. We evaluate the polytropic index for conditions relevant to the cool interstellar medium using published cooling functions and different heating sources, finding that a lognormal pdf may occur at densities between 10$^3$ and at least 10$^4$ cm$^{-3}$. Several applications are examined. First, we question a recent derivation of the IMF from the density pdf by Padoan, Nordlund & Jones because a) the pdf does not contain spatial information, and b) their derivation produces the most massive stars in the voids of the density distribution. Second, we illustrate how a distribution of ambient densities can alter the predicted form of the size distribution of expanding shells. Finally, a brief comparison is made with the density pdfs found in cosmological simulations.

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arXiv1997

Does Turbulent Pressure Behave as a Logatrope?

We present numerical simulations of an isothermal turbulent gas undergoing gravitational collapse, aimed at testing for ``logatropic'' behavior of the form $P_t \sim \log ρ$, where $P_t$ is the ``turbulent pressure'' and $ρ$ is the density. To this end, we monitor the evolution of the turbulent velocity dispersion $σ$ as the density increases during the collapse. A logatropic behavior would require that $σ\propto ρ^{-1/2}$, a result which, however, is not verified in the simulations. Instead, the velocity dispersion increases with density, implying a polytropic behavior of $P_t$. This behavior is found both in purely hydrodynamic as well as hydromagnetic runs. For purely hydrodynamic and rapidly-collapsing magnetic cases, the velocity dispersion increases roughly as $σ\propto ρ^{1/2}$, implying $P_t\sim ρ^2$, where $P_t$ is the turbulent pressure. For slowly-collapsing magnetic cases the behavior is close to $σ\propto ρ^{1/4}$, which implies $P_t \sim ρ^{3/2}$. We thus suggest that the logatropic ``equation of state'' may represent only the statistically most probable state of an ensemble of clouds in equilibrium between self-gravity and kinetic support, but does not adequately represent the behavior of the ``turbulent pressure'' within a cloud undergoing a dynamic compression due to gravitational collapse. Finally, we discuss the importance of the underlying physical model for the clouds (in equilibrium vs. dynamic) on the results obtained.

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arXiv1997

Highly Compressible MHD Turbulence and Gravitational Collapse

We investigate the properties of highly compressible turbulence and its ability to produce self-gravitating structures. The compressibility is parameterized by an effective polytropic exponent gama-eff. In the limit of small gama-eff, the density jump at shocks is shown to be of the order of e^{M^2}, and the production of vorticity by the nonlinear terms appears to be negligible. In the presence of self-gravity, we suggest that turbulence can produce bound structures for gama-eff < 2(1-1/n), where 'n' is the typical dimensionality of the turbulent compressions. We show, by means of numerical simulations, that, for sufficiently small gama-eff, small-scale turbulent density fluctuations eventually collapse even though the medium is globally stable. This result is preserved in the presence of a magnetic field for supercritical mass-to-flux ratios.

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arXiv1996

A Search for Larson-Type Relations in Numerical Simulations of the ISM. Evidence for Non-Constant Column Densities

We present results from a statistical study of clouds in two-dimensional numerical simulations of the interstellar medium. The clouds in the simulations exhibit a differential mass spectrum $dN(M)/dM \sim M^{-1.44 \pm 0.1}$ and a velocity dispersion-size relation $Δv \sim R^{0.41 \pm 0.08}$. However, the clouds do {\it not} exhibit a clear density-size relation. At a given mean density, clouds span a range of sizes from the smallest resolved scales up to a maximum given by a Larson-type relation $R_{\rm max} \sim ρ^α$, with $α= -0.81 \pm .15$, although numerical effects cannot be ruled out as responsible for the latter correlation. Clouds span a range of column densities $N$ of two orders of magnitude, supporting the suggestion that the observational density-size relation may be an artifact of survey limitations. In this case, the $Δv$--$R$ relation can be interpreted as a direct consequence of a $k^{-2}$ turbulent spectrum, characteristic of a field of shocks, verified in the simulations, rather than of virial equilibrium of clouds with a $ρ\propto R^{-1}$ law. However, we also discuss the possibility that the clouds are in balance between self-gravity and turbulence, but with a scatter of at least a factor of 10 in the $Delta v$--$R$ relation, and of 100 in the density-size relation, according to the equilibrium relation $Δv \sim (NR)^{1/2}$. Finally, we compare these results with observational data. We propose a simple model suggesting that recent results finding nearly constant column densities for dark IRAS clouds may be an artifact of a temperature gradient within the clouds induced by external radiative heating. As a consequence, we emphasize that IRAS surface brightness maps are not appropriate for measuring column densities.

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arXiv1996

Influence of Cooling-Induced Compressibility on the Structure of Turbulent Flows and Gravitational Collapse

We investigate the properties of highly compressible turbulence, the compressibility arising from a small effective polytropic exponent $γ_e$ due to cooling. In the limit of small $γ_e$, the density jump at shocks is shown to be of the order of $e^{M^2}$. Without self-gravity, the density structures arising in the moderately compressible case consist mostly of patches separated by shocks and behaving like waves, while in the highly compressible case clearly defined long-lived object-like clouds emerge. When the forcing in the momentum equation is purely compressible, the rotational energy decays monotonically in time, indicating that the vortex-stretching term is not efficient in transferring energy to rotational modes. This property may be at the origin of the low amount of rotation found in interstellar clouds. Vorticity production is found to rely heavily on the presence of additional terms in the equations. In the presence of self-gravity, we suggest that turbulence can produce bound structures for $γ_e < 2(1-1/n)$, where $n$ is the typical dimensionality of the turbulent compressions. We support this result by means of numerical simulations in which, for sufficiently small $γ_e$, small-scale turbulent density fluctuations eventually collapse even though the medium is globally stable. This result is preserved in the presence of a magnetic field for supercritical mass-to-flux ratios. At larger polytropic exponents, turbulence alone is not capable of producing bound structures, and collapse can only occur when the medium is globally unstable. This mechanism is a plausible candidate for the differentiation between primordial and present-day stellar-cluster formation and for the low efficiency of star formation.

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arXiv1996

Cloud Statistics in Numerical Simulations of the ISM

We present preliminary results on the energy budgets of clouds in two-dimensional numerical simulations of the interstellar medium. Using an automated cloud-identification algorithm, we calculate the gravitational, internal, kinetic and magnetic energies of the clouds. We find that, within a dispersion of roughly one order of magnitude, the gravitational energy in the clouds is balanced by the remaining energies. Furthermore, within the same dispersion, there appears to be equipartition between the kinetic and magnetic energies.

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