arXiv subjects

Enrique Vazquez-Semadeni

Publications and source records attributed to Enrique Vazquez-Semadeni.

At least 37 records · Page 2Linked to original sources

arXiv2012

Physical Processes of Interstellar Turbulence

I discuss the role of self-gravity and radiative heating and cooling in shaping the nature of the turbulence in the interstellar medium (ISM) of our galaxy. The heating and cooling cause it to be highly compressible, and, in some regimes of density and temperature, to become thermally unstable, tending to spontaneously segregate into warm/diffuse and cold/dense phases. On the other hand, turbulence is an inherently mixing process, tending to replenish the density and temperature ranges that would be forbidden under thermal processes alone. The turbulence in the ionized ISM appears to be transonic (i.e, with Mach numbers $\Ms \sim 1$), and thus to behave essentially incompressibly. However, in the neutral medium, thermal instability causes the sound speed of the gas to fluctuate by up to factors of $\sim 30$, and thus the flow can be highly supersonic with respect to the dense/cold gas, although numerical simulations suggest that this behavior corresponds more to the ensemble of cold clumps than to the clumps' internal velocity dispersion. Finally, coherent large-scale compressions in the warm neutral medium (induced by, say, the passage of spiral arms or by supernova shock waves) can produce large, dense molecular clouds that are subject to their own self-gravity, and begin to contract gravitationally. Because they are populated by nonlinear density fluctuations, whose local free-fall times are significantly smaller than that of the whole cloud, the fluctuations terminate their collapse earlier, giving rise to a regime of hierarchical gravitational fragmentation, with small-scale collapses occurring within larger-scale ones. Thus, the "turbulence" in molecular clouds may be dominated by a gravitationally contracting component at all scales.

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arXiv2011

Gravity or turbulence? II. Evolving column density PDFs in molecular clouds

It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function (Npdf). Instead, clouds without star formation seem to possess a lognormal distribution, while clouds with active star formation develope a power-law tail at high column densities. The lognormal behavior of the Npdf has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power-law behavior occurs when the cloud is dominated by gravity. In the present contribution we use thermally bi-stable numerical simulations of cloud formation and evolution to show that, indeed, these two regimes can be understood in terms of the formation and evolution of molecular clouds: a very narrow lognormal regime appears when the cloud is being assembled. However, as the global gravitational contraction occurs, the initial density fluctuations are enhanced, resulting, first, in a wider lognormal Npdf, and later, in a power-law Npdf. We thus suggest that the observed Npdf of molecular clouds are a manifestation of their global gravitationally contracting state. We also show that, contrary to recent suggestions, the exact value of the power-law slope is not unique, as it depends on the projection in which the cloud is being observed.

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arXiv2011

Molecular Cloud Evolution IV: Magnetic Fields, Ambipolar Diffusion, and the Star Formation Efficiency

We investigate the formation and evolution of giant molecular clouds (GMCs) by the collision of convergent warm neutral medium (WNM) streams in the interstellar medium, in the presence of magnetic fields and ambipolar diffusion (AD), focusing on the evolution of the star formation rate (SFR) and efficiency (SFE), as well as of the mass-to-magnetic-flux ratio (M2FR) in the forming clouds. We find that: 1) Clouds formed by supercritical inflow streams proceed directly to collapse, while clouds formed by subcritical streams first contract and then re-expand, oscillating on the scale of tens of Myr. 2) Our suite of simulations with initial magnetic field strength of 2, 3, and 4 $μ\G$ show that only supercritical or marginal critical streams lead to reasonable star forming rates. 3) The GMC's M2FR is a generally increasing function of time, whose growth rate depends on the details of how mass is added to the GMC from the WNM. 4) The M2FR is a highly fluctuating function of position in the clouds. 5) In our simulations, the SFE approaches stationarity, because mass is added to the GMC at a similar rate at which it converts mass to stars. In such an approximately stationary regime, the SFE provides a proxy of the supercritical mass fraction in the cloud. 6) We observe the occurrence of buoyancy of the low-M2FR regions within the gravitationally-contracting GMCs, so that the latter naturally segregate into a high-density, high-M2FR "core" and a low-density, low-M2FR "envelope", without the intervention of AD. (Abridged)

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arXiv2010

Theory of Feedback in Clusters and Molecular Cloud Turbulence

I review recent numerical and analytical work on the feedback from both low- and high-mass cluster stars into their gasoeus 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 star formation rate; iv) nevertheless, numerical simulations suggest that 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 star formation efficiency.

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arXiv2010

Molecular Cloud Evolution

I describe the scenario of molecular cloud (MC) evolution that has emerged over the past decade or so. MCs can start out as cold atomic clouds formed by compressive motions in the warm neutral medium (WNM) of galaxies. Such motions can be driven by large-scale instabilities, or by local turbulence. The compressions induce a phase transition to the cold neutral medium (CNM) to form growing cold atomic clouds, which in their early stages may constitute thin CNM sheets. Several dynamical instabilities soon destabilize a cloud, rendering it turbulent. For solar neighborhood conditions, a cloud is coincidentally expected to become molecular, magnetically supercritical, and gravitationally dominated at roughly the same column density, $N \sim 1.5 \times 10^21 \psc \approx 10 \Msun$ pc$^{-2}$. At this point, the cloud begins to contract gravitationally. However, before its global collapse is completed ($\sim 10^7$ yr later), the nonlinear density fluctuations within the cloud, which have shorter local free-fall times, collapse first and begin forming stars, a few Myr after the global contraction started. Large-scale fluctuations of lower mean densities collapse later, so the formation of massive star-forming regions is expected to occur late in the evolution of a large cloud complex, while scattered low-mass regions are expected to form earlier. Eventually, the local star formation episodes are terminated by stellar feedback, which disperses the local dense gas, although more work is necessary to clarify the details and characteristic scales of this process.

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arXiv2010

Low-mass galaxy formation in cosmological AMR simulations: the effects of varying the sub-grid physics parameters

We present numerical simulations aimed at exploring the effects of varying the sub-grid physics parameters on the evolution and the properties of the galaxy formed in a low-mass dark matter halo (~7 times 10^10 Msun/h at redshift z=0). The simulations are run within a cosmological setting with a nominal resolution of 218 pc comoving and are stopped at z = 0.43. In all of our simulations, an extended old/intermediate-age stellar halo and a more compact younger stellar disk are formed. We found that a non negligible fraction of the halo stars are formed in situ in a spheroidal distribution. Changes in the sub-grid physics parameters affect significantly and in a complex way the evolution and properties of the galaxy: (i) Lower threshold densities nsf produce larger stellar effective radii Re, less peaked circular velocity curves V_c(R), and greater amounts of low-density and hot gas in the disk mid-plane; (ii) When stellar feedback is modeled by temporarily switching off radiative cooling in the star forming regions, Re increases (by a factor of ~ 2 in our particular model), the circular velocity curve becomes flatter, and a complex multi-phase gaseous disk structure develops; (iii) A more efficient local conversion of gas mass to stars, measured by a stellar particle mass distribution biased toward larger values, increases the strength of the feedback energy injection -driving outflows and inducing burstier SF histories; iv) If feedback is too strong, gas loss by galactic outflows -which are easier to produce in low-mass galaxies- interrupts SF, whose history becomes episodic. The simulations exhibit two important shortcomings: the baryon fractions are higher, and the specific SF rates are much smaller, than observationally inferred values for redshifts ~ 0.4-1.

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arXiv2010

Molecular Cloud Evolution III. Accretion vs. stellar feedback

We numerically investigate the effect of feedback from the ionizing radiation heating from massive stars on the evolution of giant molecular clouds (GMCs) and their star formation efficiency (SFE). We find that the star-forming regions within the GMCs are invariably formed by gravitational contraction. After an initial period of contraction, the collapsing clouds begin forming stars, whose feedback evaporates part of the clouds' mass, opposing the continuing accretion from the infalling gas. The competition of accretion against dense gas consumption by star formation (SF) and evaporation by the feedback, regulates the clouds' mass and energy balance, as well as their SFE. We find that, in the presence of feedback, the clouds attain levels of the SFE that are consistent at all times with observational determinations for regions of comparable SF rates (SFRs). However, we observe that the dense gas mass is larger in general in the presence of feedback, while the total (dense gas + stars) is nearly insensitive to the presence of feedback, suggesting that the total mass is determined by the accretion, while the feedback inhibits mainly the conversion of dense gas to stars. The factor by which the SFE is reduced upon the inclusion of feedback is a decreasing function of the cloud's mass, for clouds of size ~ 10 pc. This naturally explains the larger observed SFEs of massive-star forming regions. We also find that the clouds may attain a pseudo-virialized state, with a value of the virial mass very similar to the actual cloud mass. However, this state differs from true virialization in that the clouds are the center of a large-scale collapse, continuously accreting mass, rather than being equilibrium entities.

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arXiv2009

Dependence of the Star Formation Efficiency on the Parameters of Molecular Cloud Formation Simulations

We investigate the response of the star formation efficiency (SFE) to the main parameters of simulations of molecular cloud formation by the collision of warm diffuse medium (WNM) cylindrical streams, neglecting stellar feedback and magnetic fields. The parameters we vary are the Mach number of the inflow velocity of the streams, Msinf, the rms Mach number of the initial background turbulence in the WNM, and the total mass contained in the colliding gas streams, Minf. Because the SFE is a function of time, we define two estimators for it, the "absolute" SFE, measured at t = 25 Myr into the simulation's evolution (sfeabs), and the "relative" SFE, measured 5 Myr after the onset of star formation in each simulation (sferel). The latter is close to the "star formation rate per free-fall time" for gas at n = 100 cm^-3. We find that both estimators decrease with increasing Minf, although by no more than a factor of 2 as Msinf increases from 1.25 to 3.5. Increasing levels of background turbulence similarly reduce the SFE, because the turbulence disrupts the coherence of the colliding streams, fragmenting the cloud, and producing small-scale clumps scattered through the numerical box, which have low SFEs. Finally, the SFE is very sensitive to the mass of the inflows, with sferel decreasing from ~0.4 to ~0.04 as the the virial parameter in the colliding streams increases from ~0.15 to ~1.5. This trend is in partial agreement with the prediction by Krumholz & McKee (2005), since the latter lies within the same range as the observed efficiencies, but with a significantly shallower slope. We conclude that the observed variability of the SFE is a highly sensitive function of the parameters of the cloud formation process, and may be the cause of significant scatter in observational determinations.

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arXiv2009

High- and Low-Mass Star Forming Regions from Hierarchical Gravitational Fragmentation. High local Star Formation Rates with Low Global Efficiencies

We investigate the properties of "star forming regions" in a previously published numerical simulation of molecular cloud formation out of compressive motions in the warm neutral atomic interstellar medium, neglecting magnetic fields and stellar feedback. In this simulation, the velocity dispersions at all scales are caused primarily by infall motions rather than by random turbulence. We study the properties (density, total gas+stars mass, stellar mass, velocity dispersion, and star formation rate) of the cloud hosting the first local, isolated "star formation" event in the simulation and compare them with those of the cloud formed by a later central, global collapse event. We suggest that the small-scale, isolated collapse may be representative of low- to intermediate-mass star-forming regions, while the large-scale, massive one may be representative of massive star forming regions. We also find that the statistical distributions of physical properties of the dense cores in the region of massive collapse compare very well with those from a recent survey of the massive star forming region in the Cygnus X molecular cloud. The star formation efficiency per free-fall time (SFE_ff) of the high-mass SF clump is low, ~0.04. This occurs because the clump is accreting mass at a high rate, not because its specific SFR (SSFR) is low. This implies that a low value of the SFE_ff does not necessarily imply a low SSFR, but may rather indicate a large gas accretion rate. We suggest that a globally low SSFR at the GMC level can be attained even if local star forming sites have much larger values of the SSFR if star formation is a spatially intermittent process, so that most of the mass in a GMC is not participating of the SF process at any given time.

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arXiv2009

Clump morphology and evolution in MHD simulations of molecular cloud formation

Abridged: We study the properties of clumps formed in three-dimensional weakly magnetized magneto-hydrodynamic simulations of converging flows in the thermally bistable, warm neutral medium (WNM). We find that: (1) Similarly to the situation in the classical two-phase medium, cold, dense clumps form through dynamically-triggered thermal instability in the compressed layer between the convergent flows, and are often characterised by a sharp density jump at their boundaries though not always. (2) However, the clumps are bounded by phase-transition fronts rather than by contact discontinuities, and thus they grow in size and mass mainly by accretion of WNM material through their boundaries. (3) The clump boundaries generally consist of thin layers of thermally unstable gas, but these layers are often widened by the turbulence, and penetrate deep into the clumps. (4) The clumps are approximately in both ram and thermal pressure balance with their surroundings, a condition which causes their internal Mach numbers to be comparable to the bulk Mach number of the colliding WNM flows. (5) The clumps typically have mean temperatures 20 < T < 50 K, corresponding to the wide range of densities they contain (20 < n < 5000 pcc) under a nearly-isothermal equation of state. (6) The turbulent ram pressure fluctuations of the WNM induce density fluctuations that then serve as seeds for local gravitational collapse within the clumps. (7) The velocity and magnetic fields tend to be aligned with each other within the clumps, although both are significantly fluctuating, suggesting that the velocity tends to stretch and align the magnetic field with it. (8) The typical mean field strength in the clumps is a few times larger than that in the WNM. (9) The magnetic field strength has a mean value of B ~ 6 mu G ...

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arXiv2009

Are There Phases in the ISM?

The interstellar medium (ISM) is subject, on one hand, to heating and cooling processes that tend to segregate it into distinct phases due to thermal instability (TI), and on the other, to turbulence-driving mechanisms that tend to produce strong nonlinear fluctuations in all the thermodynamic variables. In this regime, large-scale turbulent compressions in the stable warm neutral medium (WNM) dominate the clump-formation process rather than the linear developent of TI. Cold clumps formed by this mechanism are often bounded by sharp density and temperature discontinuities, which however are not contact discontinuities as in the classical 2-phase model, but rather "phase transition fronts", across which there is net mass and momentum flux from the WNM into the clumps. The clumps grow mainly by accretion through their boundaries, are in both thermal and ram pressure balance with their surroundings, and are internally turbulent as well, thus also having significant density fluctuations inside. The temperature and density of the cold and warm gas around the phase transition fronts fluctuate with time and location due to fluctuations in the turbulent pressure. Moreover, shock-compressed diffuse unstable gas can remain in the unstable regime for up to a few Myr before it undergoes a phase transition to the cold phase. These processes populate the classically forbidden density and temperature ranges. Since gas at all temperatures appears to be present in bi- or tri-stable turbulence, we conclude that the word "phase" applies only locally, surrounding phase transition sites in the gas. Globally, the word "phase" must relax its meaning to simply denote a certain temperature or density range.

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arXiv2008

The Nature of the Velocity Field in Molecular Clouds. I. The Non-Magnetic Case

We present three numerical simulations of randomly driven, isothermal, non-magnetic, self-gravitating turbulence with different rms Mach numbers Ms and physical sizes L, but approximately the same value of the virial parameter, alpha approx 1.2. We obtain the following results: a) We test the hypothesis that the collapsing centers originate from locally Jeans-unstable ("super-Jeans"), subsonic fragments; we find no such structures. b) We find that the fraction of small-scale super-Jeans structures is larger in the presence of self-gravity. c) The velocity divergence of subregions of the simulations exhibits a negative correlation with their mean density. d) The density probability density function (PDF) deviates from a lognormal in the presence of self-gravity. e) Turbulence alone in the large-scale simulation does not produce regions with the same size and mean density as those of the small-scale simulation. Items (b)-(e) suggest that self-gravity is not only involved in causing the collapse of Jeans-unstable density fluctuations produced by the turbulence, but also in their {it formation}. We also measure the star formation rate per free-fall time, as a function of Ms for the three runs, and compare with the predictions of recent semi-analytical models. We find marginal agreement to within the uncertainties of the measurements. However, the hypotheses of those models neglect the net negative divergence of dense regions we find in our simulations. We conclude that a) part of the observed velocity dispersion in clumps must arise from clump-scale inwards motions, and b) analytical models of clump and star formation need to take into account this dynamical connection with the external flow and the fact that, in the presence of self-gravity, the density PDF may deviate from a lognormal.

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arXiv2008

Formation and Properties of Molecular Cloud Cores

In this paper, we review some of the properties of dense molecular cloud cores. The results presented here rely on three-dimensional numerical simulations of isothermal, magnetized, turbulent, and self-gravitating molecular clouds (MCs) in which dense core form as a consequence of the gravo-turbulent fragmentation of the clouds. In particular we discuss issues related to the mass spectrum of the cores, their lifetimes and their virial balance.

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arXiv2008

Massive Star Forming Regions: Turbulent Support or Global Collapse?

We present preliminary numerical evidence that the physical conditions in high-mass star forming regions can arise from global gravitational infall, with the velocity dispersions being caused primarily by infall motions rather than random turbulence. To this end, we study the clumps and cores appearing in the region of central collapse in a numerical simulation of the formation, evolution, and subsequent collapse of a dense cloud out of a transonic compression in the diffuse atomic ISM. The clumps have sizes $\sim 1$ pc, masses of several hundred $M_\odot$, and three-dimensional velocity dispersions $\sim 3$ km s$^{-1}$, in agreement with typical observed values for such structures. The clumps break down into massive cores of sizes $\sim 0.1$ pc, densities $\sim 10^5$, masses 2-300 $M_\odot$, with distributions of these quantities that peak at the same values as the massive core sample in a recent survey of the Cygnus X molecular cloud complex. Although preliminary, these results suggest that high-mass star forming clumps may be in a state of global gravitational collapse rather than in equilibrium supported by strong turbulence.

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arXiv2007

The Virial Balance of Clumps and Cores in Molecular Clouds

We study the instantaneous virial balance of clumps and cores (CCs) in 3D simulations of driven, MHD, isothermal molecular clouds (MCs). The models represent a range of magnetic field strengths in MCs from subcritical to non-magnetic regimes. We identify CCs at different density thresholds, and for each object, we calculate all the terms that enter the Eulerian form of the virial theorem (EVT). A CC is considered gravitationally bound when the gravitational term in the EVT is larger than the amount for the system to be virialized, which is more stringent than the condition that it be large enough to make the total volume energy negative. We also calculate, quantities commonly used in the observations to indicate the state of gravitational boundedness of CCs such as the Jeans number J_c, the mass-to magnetic flux ratio mu_c, and the virial parameter alpha_vir. Our results show that: a) CCs are dynamical out-of-equilibrium structures. b) The surface energies are of the same order than their volume counterparts c) CCs are either in the process of being compressed or dispersed by the velocity field. Yet, not all CCs that have a compressive net kinetic energy are gravitationally bound. d) There is no 1-to-1 correspondence between the state of gravitational boundedness of a CC as described by the virial analysis or as implied by the classical indicators. In general, in the virial analysis, we observe that only the inner regions of the objects are gravitationally bound, whereas J_c, alpha_vir, and mu_c estimates tend to show that they are more gravitationally bound at the lowest threshold levels and more magnetically supercritical. g) We observe, in the non-magnetic simulation, the existence of a bound core with structural and dynamical properties that resemble those of the Bok globule Barnard 68 (B68).

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arXiv2007

Small Ionized and Neutral Structures: A Theoretical Review

The workshop on Small Ionized and Neutral Structures in the Interstellar Medium featured many contributions on the theory of the objects which are responsible for ``Tiny Scale Atomic Structures'' (TSAS) and ``Extreme Scattering Events'' (ESE). The main demand on theory is accounting for objects that have the high densities and small sizes apparently required by the observations, but also persist over a sufficiently long time to be observable. One extensively-discussed mechanism is compressions by transonic turbulence in the warm interstellar medium, followed by thermal instabilities leading to an even more compressed state. In addressing the requirements for overpressured but persistent objects, workshop participants also discussed fundamental topics in the physics of the interstellar medium, such as the timescale for evaporation of cool dense clouds, the relevance of thermodynamically-defined phases of the ISM, the effect of magnetic fields, statistical effects, and the length and time scales introduced by interstellar processes.

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arXiv2006

Molecular Cloud Turbulence And The Star Formation Efficiency: Enlarging the Scope

We summarize recent numerical results on the control of the star formation efficiency (SFE), addressing the effects of turbulence and the magnetic field strength. In closed-box numerical simulations, the effect of the turbulent Mach number $\Ms$ depends on whether the turbulence is driven or decaying: In driven regimes, increasing $\Ms$ decreases the SFE, while in decaying regimes the converse is true. The efficiencies in non-magnetic cases for realistic Mach numbers $\Ms \sim 10$ are somewhat too high compared to observed values. Including the magnetic field can bring the SFE down to levels consistent with observations, but the intensity of the magnetic field necessary to accomplish this depends again on whether the turbulence is driven or decaying. In this kind of simulations, a lifetime of the molecular cloud (MC) needs to be assumed. Further progress requires determining the true nature of the turbulence driving and the lifetimes of the clouds. Simulations of MC formation by large-scale compressions in the warm neutral medium (WNM) show that the clouds' initial turbulence is produced by the accumulation process that forms them, and that the turbulence is driven for as long as this process lasts, producing realistic velocity dispersions and also thermal pressures in excess of the mean WNM value. In simulations including self-gravity, but neglecting the magnetic field and stellar energy feedback, the clouds never reach an equilibrium state, but rather evolve secularly, increasing their mass and gravitational energy until they engage in generalized gravitational collapse. However, local collapse events begin midways through this process, and produce enough stellar objetcs to disperse the cloud or at least halt its collapse before the latter is completed.

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arXiv2006

Molecular cloud evolution. I. Molecular cloud and thin CNM sheet formation

We discuss molecular cloud formation by large-scale supersonic compressions in the diffuse warm neutral medium (WNM). Initially, a shocked layer forms, and within it, a thin cold layer. An analytical model and high-resolution 1D simulations predict the thermodynamic conditions in the cold layer. After $\sim 1$ Myr of evolution, the layer has column density $\sim 2.5 \times 10^{19} \psc$, thickness $\sim 0.03$ pc, temperature $\sim 25$ K and pressure $\sim 6650$ K $\pcc$. These conditions are strongly reminiscent of those recently reported by Heiles and coworkers for cold neutral medium sheets. In the 1D simulations, the inflows into the sheets produce line profiles with a central line of width $\sim 0.5 \kms$ and broad wings of width $\sim 1 \kms$. 3D numerical simulations show that the cold layer develops turbulent motions and increases its thickness, until it becomes a fully three-dimensional turbulent cloud. Fully developed turbulence arises on times ranging from $\sim 7.5$ Myr for inflow Mach number $\Mr = 2.4$ to $> 80$ Myr for $\Mr = 1.03$. These numbers should be considered upper limits. The highest-density turbulent gas (HDG, $n > 100 \pcc$) is always overpressured with respect to the mean WNM pressure by factors 1.5--4, even though we do not include self-gravity. The intermediate-density gas (IDG, $10 < n [{\rm cm}^ {-3}] < 100$) has a significant pressure scatter that increases with $\Mr$, so that at $\Mr = 2.4$, a significant fraction of the IDG is at a higher pressure than the HDG. Our results suggest that the turbulence and at least part of the excess pressure in molecular clouds can be generated by the compressive process that forms the clouds themselves, and that thin CNM sheets may be formed transiently by this mechanism, when the compressions are only weakly supersonic.

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