arXiv subjects

Javier Ballesteros-Paredes

Publications and source records attributed to Javier Ballesteros-Paredes.

At least 55 records · Page 3Linked to original sources

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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arXiv2010

Gravity or Turbulence? The velocity dispersion-size relation

We discuss the nature of the velocity dispersion vs. size relation for molecular clouds. In particular, we add to previous observational results showing that the velocity dispersions in molecular clouds and cores are not purely functions of spatial scale but involve surface gas densities as well. We emphasize that hydrodynamic turbulence is required to produce the first condensations in the progenitor medium. However, as the cloud is forming, it also becomes bound, and gravitational accelerations dominate the motions. Energy conservation in this case implies $|E_g| \sim E_k$, in agreement with observational data, and providing an interpretation for two recent observational results: the scatter in the $δv-R$ plane, and the dependence of the velocity dispersion on the surface density ${δv^2/ R} \propto Σ$. We argue that the observational data are consistent with molecular clouds in a state of hierarchical gravitational collapse, i.e., developing local centers of collapse throughout the whole cloud while the cloud itself is collapsing, and making equilibrium unnecessary at all stages prior to the formation of actual stars. Finally, we discuss how this mechanism need not be in conflict with the observed star formation rate.

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arXiv2009

Tracers of stellar mass-loss. I. Optical and near-IR colours and surface brightness fluctuations

We present optical and IR integrated colours and SBF magnitudes, computed from stellar population synthesis models that include emission from the dusty envelopes surrounding TP-AGB stars undergoing mass-loss. We explore the effects of varying the mass-loss rate by one order of magnitude around the fiducial value, modifying accordingly both the stellar parameters and the output spectra of the TP-AGB stars plus their dusty envelopes. The models are single burst, and range in age from a few Myr to 14 Gyr, and in metallicity between $Z$ = 0.0001 and $Z$ = 0.07; they combine new calculations for the evolution of stars in the TP-AGB phase, with star plus envelope SEDs produced with the radiative transfer code DUSTY. We compare these models to optical and near-IR data of single AGB stars and Magellanic star clusters. This comparison validates the current understanding of the role of mass-loss in determining stellar parameters and spectra in the TP-AGB. However, neither broad-band colours nor SBF measurements in the optical or the near-IR can discern global changes in the mass-loss rate of a stellar population. We predict that mid-IR SBF measurements can pick out such changes, and actually resolve whether a relation between metallicity and mass-loss exists.

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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

Gravitational Collapse and Filament Formation: Comparison with the Pipe Nebula

Recent models of molecular cloud formation and evolution suggest that such clouds are dynamic and generally exhibit gravitational collapse. We present a simple analytic model of global collapse onto a filament and compare this with our numerical simulations of the flow-driven formation of an isolated molecular cloud to illustrate the supersonic motions and infall ram pressures expected in models of gravity-driven cloud evolution. We apply our results to observations of the Pipe Nebula, an especially suitable object for our purposes as its low star formation activity implies insignifcant perturbations from stellar feedback. We show that our collapsing cloud model can explain the magnitude of the velocity dispersions seen in the $^{13}$CO filamentary structure by Onishi et al. and the ram pressures required by Lada et al. to confine the lower-mass cores in the Pipe nebula. We further conjecture that higher-resolution simulations will show small velocity dispersions in the densest core gas, as observed, but which are infall motions and not supporting turbulence. Our results point out the inevitability of ram pressures as boundary conditions for molecular cloud filaments, and the possibility that especially lower-mass cores still can be accreting mass at significant rates, as suggested by observations.

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arXiv2009

Tidal foces as a regulator of star formation in Taurus

Only a few molecular clouds in the Solar Neighborhood exhibit the formation of only low-mass stars. Traditionally, these clouds have been assumed to be supported against more vigorous collapse by magnetic fields. The existence of strong magnetic fields in molecular clouds, however, poses serious problems for the formation of stars and of the clouds themselves. In this {\em Letter}, we review the three-dimensional structure and kinematics of Taurus --the archetype of a region forming only low-mass stars-- as well as its orientation within the Milky way. We conclude that the particularly low star-formation efficiency in Taurus may naturally be explained by tidal forces from the Galaxy, with no need for magnetic regulation or stellar feedback.

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arXiv2008

On the gravitational content of molecular clouds and their cores

(Abridged) The gravitational term for clouds and cores entering in the virial theorem is usually assumed to be equal to the gravitational energy, since the contribution to the gravitational force from the mass distribution outside the volume of integration is assumed to be negligible. Such approximation may not be valid in the presence of an important external net potential. In the present work we analyze the effect of an external gravitational field on the gravitational budget of a density structure. Our cases under analysis are (a) a giant molecular cloud (GMC) with different aspect ratios embedded within a galactic net potential, and (b) a molecular cloud core embedded within the gravitational potential of its parent molecular cloud. We find that for roundish GMCs, the tidal tearing due to the shear in the plane of the galaxy is compensated by the tidal compression in the z direction. The influence of the external effective potential on the total gravitational budget of these clouds is relatively small, although not necessarily negligible. However, for more filamentary GMCs, the external effective potential can be dominant and can even overwhelm self-gravity, regardless of whether its main effect on the cloud is to disrupt it or compress it. This may explain the presence of some GMCs with few or no signs of massive star formation, such as the Taurus or the Maddalena's clouds. In the case of dense cores embedded in their parent molecular cloud, we found that the gravitational content due to the external field may be more important than the gravitational energy of the cores themselves. This effect works in the same direction as the gravitational energy, i.e., favoring the collapse of cores. We speculate on the implications of these results for star formation models.

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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

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

Formation and Collapse of Quiescent Cloud Cores Induced by Dynamic Compressions

(Abridged) We present numerical hydrodynamical simulations of the formation, evolution and gravitational collapse of isothermal molecular cloud cores. A compressive wave is set up in a constant sub-Jeans density distribution of radius r = 1 pc. As the wave travels through the simulation grid, a shock-bounded spherical shell is formed. The inner shock of this shell reaches and bounces off the center, leaving behind a central core with an initially almost uniform density distribution, surrounded by an envelope consisting of the material in the shock-bounded shell, with a power-law density profile that at late times approaches a logarithmic slope of -2 even in non-collapsing cases. The resulting density structure resembles a quiescent core of radius < 0.1 pc, with a Bonnor-Ebert-like (BE-like) profile, although it has significant dynamical differences: it is initially non-self-gravitating and confined by the ram pressure of the infalling material, and consequently, growing continuously in mass and size. With the appropriate parameters, the core mass eventually reaches an effective Jeans mass, at which time the core begins to collapse. Thus, there is necessarily a time delay between the appearance of the core and the onset of its collapse, but this is not due to the dissipation of its internal turbulence as it is often believed. These results suggest that pre-stellar cores may approximate Bonnor-Ebert structures which are however of variable mass and may or may not experience gravitational collapse, in qualitative agreement with the large observed frequency of cores with BE-like profiles.

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arXiv2007

Statistics of Core Lifetimes in Numerical Simulations of Turbulent, Magnetically Supercritical Molecular Clouds

We present measurements of the mean dense core lifetimes in numerical simulations of magnetically supercritical, turbulent, isothermal molecular clouds, in order to compare with observational determinations. "Prestellar" lifetimes (given as a function of the mean density within the cores, which in turn is determined by the density threshold n_thr used to define them) are consistent with observationally reported values, ranging from a few to several free-fall times. We also present estimates of the fraction of cores in the "prestellar", "stellar'', and "failed" (those cores that redisperse back into the environment) stages as a function of n_thr. The number ratios are measured indirectly in the simulations due to their resolution limitations. Our approach contains one free parameter, the lifetime of a protostellar object t_yso (Class 0 + Class I stages), which is outside the realm of the simulations. Assuming a value t_yso = 0.46 Myr, we obtain number ratios of starless to stellar cores ranging from 4-5 at n_thr = 1.5 x 10^4 cm^-3 to 1 at n_thr = 1.2 x 10^5 cm^-3, again in good agreement with observational determinations. We also find that the mass in the failed cores is comparable to that in stellar cores at n_thr = 1.5 x 10^4 cm^-3, but becomes negligible at n_thr = 1.2 x 10^5 cm^-3, in agreement with recent observational suggestions that at the latter densities the cores are in general gravitationally dominated. We conclude by noting that the timescale for core contraction and collapse is virtually the same in the subcritical, ambipolar diffusion-mediated model of star formation, in the model of star formation in turbulent supercritical clouds, and in a model intermediate between the previous two, for currently accepted values of the clouds' magnetic criticality.

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arXiv2006

Remarks on Rapid vs. Slow Star Formation

We discuss problems with some observational estimates indicating long protostellar core lifetimes and large stellar age spreads in molecular clouds. We also point out some additional observational constraints which suggest that protostellar cores do not have long lifetimes before collapsing. For external galaxies, we argue that the widths of spiral arms does not imply a long star-formation process, since the formation of massive stars will disrupt molecular clouds, move material around, compress it in other regions which produce new star-forming clouds. Thus, it seems unavoidable that this cyclical process will result in an extended period of enhanced star formation, which does not represent the survival time of any individual molecular cloud. We argue that the rapid star formation indicated observationally is also easier to understand theoretically than the traditional scenario of slow quasi-static contraction with ambipolar diffusion.

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arXiv2006

Six Myths on the Virial Theorem for Interstellar Clouds

It has been paid little or no attention to the implications that turbulent fragmentation has on the validity of at least six common assumptions on the Virial Theorem (VT), which are: (i) the only role of turbulent motions within a cloud is to provide support against collapse, (ii) the surface terms are negligible compared to the volumetric ones, (iii) the gravitational term is a binding source for the clouds, (iv) the sign of the second-time derivative of the moment of inertia determines whether the cloud is contracting or expanding, (v) interstellar clouds are in Virial Equilibrium (VE), and (vi) Larson's (1981) relations are the observational proof that clouds are in VE. Interstellar clouds cannot fulfill these assumptions, however, because turbulent fragmentation will induce flux of mass, moment and energy between the clouds and their environment, and will favor local collapse while may disrupt the clouds within a dynamical timescale. It is argued that, although the observational and numerical evidence suggests that interstellar clouds are not in VE, the so-called ``Virial Mass'' estimations, which actually should be called ``energy-equipartition mass'' estimations, are good order-of magnitude estimations of the actual mass of the clouds just because observational surveys will tend to detect interstellar clouds appearing to be close to energy equipartition. However, since clouds are actually out of VE, as suggested by asymmetrical line profiles, they should be transient entities. These results are compatible with observationally-based estimations for rapid star formation. , and call into question the models for the star formation efficiency based on clouds being in VE.

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arXiv2005

The Mass Spectra of Cores in Turbulent Molecular Clouds and Implications for the Initial Mass Function

We investigate the core mass distribution (CMD) resulting from numerical models of turbulent fragmentation of molecular clouds. In particular we study its dependence on the sonic root-mean-square Mach number $\Ms$. We analyze simulations with $\Ms$ ranging from 1 to 15 to show that, as $\Ms$ increases, the number of cores increases as well while their average mass decreases. This stems from the fact that high-Mach number flows produce many and strong shocks on intermediate to small spatial scales, leading to a highly-fragmented density structure. We also show that the CMD from purely turbulent fragmentation does not follow a single power-law, but it may be described by a function that changes continuously its shape, probably more similar to a log-normal function. The CMD in supersonic turbulent flows does not have a universal slope, and as consequence, cast some doubt on attempts to directly relate the CMD to a universal Initial Mass Function.

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arXiv2005

Star Formation Efficiency in Driven, Supercritical, Turbulent Clouds

We present measurements of the star formation efficiency (SFE) in 3D numerical simulations of driven turbulence in supercritical, ideal-MHD, and non-magnetic regimes, characterized by their mean normalized mass-to-flux ratio $μ$, all with 64 Jeans masses and similar rms Mach numbers ($\sim 10$). In most cases, the moderately supercritical runs with $μ= 2.8$ have significantly lower SFEs than the non-magnetic cases, being comparable to observational estimates for whole molecular clouds ($\lesssim$ 5% over 4 Myr). Also, as the mean field is increased, the number of collapsed objects decreases, and the median mass of the collapsed objects increases. However, the largest collapsed-object masses systematically occur in the weak-field case $μ= 8.8$. The high-density tails of the density histograms in the simulations are depressed as the mean magnetic field strength is increased. This suggests that the smaller numbers and larger masses of the collapsed objects in the magnetic cases may be due to a greater scarcity and lower mean densities (implying larger Jeans masses) of the collapse candidates. In this scenario, the effect of a weak field is to reduce the probability of a core reaching its thermal Jeans mass, even if it is supercritical. We thus suggest that the SFE may be monotonically reduced as the field strength increases from zero to subcritical values, rather than there being a discontinuous transition between the sub- and supercritical regimes, and that a crucial question to address is whether the turbulence in molecular clouds is driven or decaying, with current observational and theoretical evidence favoring (albeit inconclusively) the driven regime.

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arXiv2004

Quiescent and coherent cores from gravoturbulent fragmentation

[abridged] We investigate the velocity structure of protostellar cores that result from non-magnetic numerical models of the gravoturbulent fragmentation of molecular cloud material. A large fraction of the cores analyzed are ``quiescent'', and more than half are identified as ``coherent''. The fact that dynamically evolving cores in highly supersonic turbulent flows can be quiescent may be understood because cores lie at the stagnation points of convergent turbulent flows, where compression is at a maximum, and relative velocity differences are at a minimum. The coherence may be due to an observational effect related to the length and concentration of the material contributing to the line. The velocity dispersion of the our cores often has its local maximum at small offsets from the column density maximum, suggesting that the core is the dense region behind a shock. Such a configuration is often found in observations of molecular cloud cores, and argues in favor of the gravoturbulent scenario of stellar birth as it is not expected in star-formation models based on magnetic mediation. Cores with collapsed objects tend to be near equipartition between their gravitational and kinetic energies, while cores without collapsed objects tend to be gravitationally unbound, suggesting that gravitational collapse occurs immediately after gravity becomes dominant. Finally, cores in simulations driven at large scales are more frequently quiescent and coherent, and have more realistic ratios of $M_{\rm vir}/M$, supporting the notion that molecular cloud turbulence is driven at large scales.

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arXiv2004

The Lifetimes and Evolution of Molecular Cloud Cores

We discuss the lifetimes and evolution of clumps and cores formed as turbulent density fluctuations in nearly isothermal molecular clouds. In the non-magnetic case, clumps are unlikely to reach a hydrostatic state, and instead are expected to either proceed directly to collapse, or else ``rebound'' towards the mean pressure and density of the parent cloud. Rebounding clumps are delayed in their re-expansion by their self-gravity. From a simple virial calculation, we find re-expansion times of a few free-fall times. In the magnetic case, we present a series of driven-turbulence, ideal-MHD isothermal numerical simulations in which we follow the evolution of clumps and cores in relation to the magnetic criticality of their ``parent clouds'' (the numerical boxes). In subcritical boxes, magnetostatic clumps do not form. A few moderately-gravitationally bound clumps form which however are dispersed by the turbulence in < 1.3 Myr. An estimate of the ambipolar diffusion (AD) time scale t_AD in these cores gives t_AD > 1.3 Myr, only slightly longer than the dynamical times. In supercritical boxes, some cores become locally supercritical and collapse in typical times ~ 1 Myr. We also observe longer-lived supercritical cores that however do not collapse because they are smaller than the local Jeans length. Fewer clumps and cores form in these simulations than in their non-magnetic counterpart. Our results suggest that a) A fraction of the cores may not form stars, and may correspond to some of the observed starless cores. b) Cores may be out-of-equilibrium structures, rather than quasi-magnetostatic ones. c) The magnetic field may help reduce the star formation efficiency by reducing the probability of core formation, rather than by significantly delaying the collapse of individual cores.

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arXiv2004

Molecular Cloud Formation Behind Shock Waves

We examine the formation of molecular gas behind shocks in atomic gas using a chemical/dynamical model, particular emphasis is given to constraints the chemistry places on the dynamical evolution. The most important result of this study is to stress the importance of shielding the molecular gas from the destructive effects of UV radiation. For shock ram pressures comparable to or exceeding typical local ISM pressures, self-shielding controls the formation time of H2 but CO formation requires shielding of the interstellar radiation field by dust grains. We find that the molecular hydrogen fractional abundance can become significant well before CO forms. The timescale for (CO) molecular cloud formation is not set by H2 formation, but rather by the timescale for accumulating a sufficient column density or extinction, A_V > 0.7. The local ratio of atomic to molecular gas (4:1), coupled with short estimates for cloud lifetimes (3-5 Myr), suggests that the timescales for accumulating molecular clouds from atomic material typically must be no longer than about 12-20 Myr. Based on the shielding requirement, this implies that the typical product of pre-shock density and velocity must be n*v > 20 cm^-3 km s^-1. Based on these results we find that flow-driven formation of molecular clouds in the local interstellar medium can occur sufficiently rapidly to account for observations. We also provide detailed predictions of atomic and molecular emission and absorption that track molecular cloud formation, with a view toward helping to verify cloud formation by shock waves. Finally, we provide an analytic solution for time-dependent H2 formation which may be of use in numerical hydrodynamic calculations.

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