arXiv subjects

Shantanu Basu

Publications and source records attributed to Shantanu Basu.

At least 91 records · Page 5Linked to original sources

arXiv2010

Magnetic Diffusion in Star Formation

Magnetic diffusion plays a vital role in star formation. We trace its influence from interstellar cloud scales down to star-disk scales. On both scales, we find that magnetic diffusion can be significantly enhanced by the buildup of strong gradients in magnetic field structure. Large scale nonlinear flows can create compressed cloud layers within which ambipolar diffusion occurs rapidly. However, in the flux-freezing limit that may be applicable to photoionized molecular cloud envelopes, supersonic motions can persist for long times if driven by an externally generated magnetic field that corresponds to a subcritical mass-to-flux ratio. In the case of protostellar accretion, rapid magnetic diffusion (through Ohmic dissipation with additional support from ambipolar diffusion) near the protostar causes dramatic magnetic flux loss. By doing so, it also allows the formation of a centrifugal disk, thereby avoiding the magnetic braking catastrophe.

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arXiv2010

Averting the magnetic braking catastrophe on small scales: disk formation due to Ohmic dissipation

We perform axisymmetric resistive MHD calculations that demonstrate that centrifugal disks can indeed form around Class 0 objects despite magnetic braking. We follow the evolution of a prestellar core all the way to near-stellar densities and stellar radii. Under flux-freezing, the core is braked and disk formation is inhibited, while Ohmic dissipation renders magnetic braking ineffective within the first core. In agreement with observations that do not show evidence for large disks around Class 0 objects, the resultant disk forms in close proximity to the second core and has a radius of only $\approx 10~R_{\odot}$ early on. Disk formation does not require enhanced resistivity. We speculate that the disks can grow to the sizes observed around Class II stars over time under the influence of both Ohmic dissipation and ambipolar diffusion, as well as internal angular momentum redistribution.

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arXiv2010

The burst mode of accretion and disk fragmentation in the early embedded stages of star formation

We revisit our original papers on the burst mode of accretion by incorporating a detailed energy balance equation into a thin-disk model for the formation and evolution of circumstellar disks around low-mass protostars.Our model includes the effect of radiative cooling, viscous and shock heating, and heating due to stellar and background irradiation. Following the collapse from the prestellar phase allows us to model the early embedded phase of disk formation and evolution. During this time, the disk is susceptible to fragmentation, depending upon the properties of the initial prestellar core. Globally, we find that higher initial core angular momentum and mass content favors more fragmentation, but higher levels of background radiation can moderate the tendency to fragment. A higher rate of mass infall onto the disk than that onto the star is a necessary but not sufficient condition for disk fragmentation. More locally, both the Toomre Q-parameter needs to be below a critical value _and_ the local cooling time needs to be shorter than a few times the local dynamical time. Fragments that form during the early embedded phase tend to be driven into the inner disk regions, and likely trigger mass accretion and luminosity bursts that are similar in magnitude to FU-Orionis-type or EX-Lupi-like events. Disk accretion is shown to be an intrinsically variable process, thanks to disk fragmentation, nonaxisymmetric structure, and the effect of gravitational torques. The additional effect of a generic α-type viscosity acts to reduce burst frequency and accretion variability, and is likely to not be viable for values of αsignificantly greater than 0.01.

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arXiv2010

Long-lived Magnetic-Tension-Driven Modes in a Molecular Cloud

We calculate and analyze the longevity of magnetohydrodynamic (MHD) wave modes that occur in the plane of a magnetic thin sheet. Initial turbulent conditions applied to a magnetically subcritical cloud are shown to lead to relatively rapid energy decay if ambipolar diffusion is introduced at a level corresponding to partial ionization primarily by cosmic rays. However, in the flux-freezing limit, as may be applicable to photoionized molecular cloud envelopes, the turbulence persists at "nonlinear" levels in comparison with the isothermal sound speed $\cs$, with one-dimensional rms material motions in the range of $\approx 2\,\cs -5\,\cs$ for cloud sizes in the range of $\approx 2\,\pc - 16\,\pc$. These fluctuations persist indefinitely, maintaining a significant portion of the initial turbulent kinetic energy. We find the analytic explanation for these persistent fluctuations. They are magnetic-tension-driven modes associated with the interaction of the sheet with the external magnetic field. The phase speed of such modes is quite large, allowing residual motions to persist without dissipation in the flux-freezing limit, even as they are nonlinear with respect to the sound speed. We speculate that long-lived large-scale MHD modes such as these may provide the key to understanding observed supersonic motions in molecular clouds.

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arXiv2010

Formation and survivability of giant planets on wide orbits

Motivated by the recent discovery of massive planets on wide orbits, we present a mechanism for the formation of such planets via disk fragmentation in the embedded phase of star formation. In this phase, the forming disk intensively accretes matter from the natal cloud core and undergoes several fragmentation episodes. However, most fragments are either destroyed or driven into the innermost regions (and probably onto the star) due to angular momentum exchange with spiral arms, leading to multiple FU-Ori-like bursts and disk expansion. Fragments that are sufficiently massive and form in the late embedded phase (when the disk conditions are less extreme) may open a gap and evolve into giant planets on typical orbits of several tens to several hundreds of AU. For this mechanism to work, the natal cloud core must have sufficient mass and angular momentum to trigger the burst mode and also form extended disks of the order of several hundreds of AU. When mass loading from the natal cloud core diminishes and the main fragmentation phase ends, such extended disks undergo a transient episode of contraction and density increase, during which they may give birth to a last and survivable set of giant planets on wide and relatively stable orbits.

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arXiv2009

The Bimodality of Accretion In T Tauri Stars and Brown Dwarfs

We present numerical solutions of the collapse of prestellar cores that lead to the formation and evolution of circumstellar disks. The disk evolution is then followed for up to three million years. A variety of models of different initial masses and rotation rates allows us to study disk accretion around brown dwarfs and low-mass T Tauri stars, with central object mass $M_st < 0.2 Msun$, as well as intermediate and upper-mass T Tauri stars (0.2 Msun < M_st < 3.0 Msun). Our models include self-gravity and allow for nonaxisymmetric motions. In addition to the self-consistently generated gravitational torques, we introduce an effective turbulent α-viscosity with α= 0.01, which allows us particularly to model accretion in the low-mass regime where disk self-gravity is diminishing. A range of models with observationally-motivated values of the initial ratio of rotational to gravitational energy yields a correlation between mass accretion rate \dot{M} and M_st that is relatively steep, as observed. Additionally, our modeling reveals evidence for a bimodality in the \dot{M}--M_st correlation, with a steeper slope at lower masses and a shallower slope at intermediate and upper masses, as also implied by observations. We show that the neglect of disk self-gravity leads to a much steeper \dot{M}--M_st relation for intermediate and upper-mass T Tauri stars. This demonstrates that an accurate treatment of global self-gravity is essential to understanding observations of circumstellar disks.

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arXiv2009

The Interplay of Turbulence & Magnetic Fields in Star-Forming Regions: Simulations and Observations

We analyze a suite of thin sheet magnetohydrodynamical simulations based on the formulation of Basu, Ciolek, Dapp & Wurster. These simulations allow us to examine the observational consequences to a star-forming region of varying the input level of turbulence (between thermal and a Mach number of 4) and the initial magnetic field strength corresponding to a range of mass to flux ratios between subcritical (mu_0=0.5) and supercritical (mu_0=10). The input turbulence is allowed to decay over the duration of the simulation. We compare the measured observable quantities with those found from surveying the Perseus molecular cloud. We find that only the most turbulent of simulations (high Mach number and weak magnetic field) have sufficient large-scale velocity dispersion (at ~1 pc) to match that observed across extinction regions in Perseus. Generally, the simulated core (~0.02 pc) and line of sight velocity dispersions provide a decent match to observations. The motion between the simulated core and its local environment, however, is far too large in simulations with high large-scale velocity dispersion.

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arXiv2009

An analytic column density profile to fit prestellar cores

We present a new analytical three-parameter formula to fit observed column density profiles of prestellar cores. It represents a line-of-sight integral through a spherically symmetric or disc-like isothermal cloud. The underlying model resembles the Bonnor-Ebert model in that it features a flat central region leading into a power-law decline \propto r^{-2} in density, and a well-defined outer radius. However, we do not assume that the cloud is in equilibrium, and can instead make qualitative statements about its dynamical state (expansion, equilibrium, collapse) using the size of the flat region as a proxy. Instead of having temperature as a fitting parameter, our model includes it as input, and thus avoids possible inconsistencies. It is significantly easier to fit to observational data than the Bonnor-Ebert sphere. We apply this model to L1689B and B68. We show that L1689B cannot be in equilibrium but instead appears to be collapsing, while our model verifies that B68 is not far from being a hydrostatic object.

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arXiv2009

Magnetically-Regulated Fragmentation Induced by Nonlinear Flows and Ambipolar Diffusion

We present a parameter study of simulations of fragmentation regulated by gravity, magnetic fields, ambipolar diffusion, and nonlinear flows. The thin-sheet approximation is employed with periodic lateral boundary conditions, and the nonlinear flow field ("turbulence") is allowed to freely decay. In agreement with previous results in the literature, our results show that the onset of runaway collapse (formation of the first star) in subcritical clouds is significantly accelerated by nonlinear flows in which a large-scale wave mode dominates the power spectrum. In addition, we find that a power spectrum with equal energy on all scales also accelerates collapse, but by a lesser amount. For a highly super-Alfvenic initial velocity field with most power on the largest scales, the collapse occurs promptly during the initial compression wave. However, for trans-Alfvenic perturbations, a subcritical magnetic field causes a rebound from the initial compression, and the system undergoes several oscillations before runaway collapse occurs. Models that undergo prompt collapse have highly supersonic infall motions at the core boundaries. Cores in magnetically subcritical models with trans-Alfvenic initial perturbations also pick up significant systematic speeds by inheriting motions associated with magnetically-driven oscillations. Core mass distributions are much broader than in models with small-amplitude initial perturbations, although the disturbed structure of cores that form due to nonlinear flows does not guarantee subsequent monolithic collapse. Our simulations also demonstrate that significant power can (if present initially) be maintained with negligible dissipation in large-scale compressive modes of a magnetic thin sheet, in the limit of perfect flux freezing.

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arXiv2008

Secular evolution of viscous and self-gravitating circumstellar discs

We add the effect of turbulent viscosity via the α-prescription to models of the self-consistent formation and evolution of protostellar discs. Our models are non-axisymmetric and carried out using the thin-disc approximation. Self-gravity plays an important role in the early evolution of a disc, and the later evolution is determined by the relative importance of gravitational and viscous torques. In the absence of viscous torques, a protostellar disc evolves into a self-regulated state with disk-averaged Toomre parameter Q \sim 1.5-2.0, non-axisymmetric structure diminishing with time, and maximum disc-to-star mass ratio ξ= 0.14. We estimate an effective viscosity parameter α_eff associated with gravitational torques at the inner boundary of our simulation to be in the range 10^{-4}-10^{-3} during the late evolution. Addition of viscous torques with a low value α= 10^{-4} has little effect on the evolution, structure, and accretion properties of the disc, and the self-regulated state is largely preserved. A sequence of increasing values of αresults in the discs becoming more axisymmetric in structure, being more gravitationally stable, having greater accretion rates, larger sizes, shorter lifetimes, and lower disc-to-star mass ratios. For α=10^{-2}, the model is viscous-dominated and the self-regulated state largely disappears by late times. (Abridged)

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arXiv2008

Nonlinear Evolution of Gravitational Fragmentation Regulated by Magnetic Fields and Ambipolar Diffusion

We present results from an extensive set of simulations of gravitational fragmentation in the presence of magnetic fields and ambipolar diffusion. The average fragmentation spacing in the nonlinear phase of evolution is in excellent agreement with the prediction of linear perturbation theory. The time scale for nonlinear growth and runaway of the first core is $\approx 10$ times the calculated growth time $\taugm$ of the eigenmode with minimum growth time, when starting from a uniform background state with small-amplitude white-noise perturbations. Subcritical and transcritical models typically evolve on a significantly longer time scale than the supercritical models. Infall motions in the nonlinear fully-developed contracting cores are subsonic on the core scale in subcritical and transcritical clouds, but are somewhat supersonic in supercritical clouds. Core mass distributions are sharply peaked with a steep decline to large masses, consistent with the existence of a preferred mass scale for each unique set of dimensionless free parameters. However, a sum total of results for various initial mass-to-flux ratios yields a broad distribution reminiscent of observed core mass distributions. Based on our results, we conclude that fragmentation spacings, magnitude of infall motions, core shapes, and, especially, the curvature of magnetic field morphology, may serve as indirect observational means of determining a cloud's ambient mass-to-flux ratio.

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arXiv2008

Three-dimensional Simulation of Magnetized Cloud Fragmentation Induced by Nonlinear Flows and Ambipolar Diffusion

We demonstrate that the formation of collapsing cores in subcritical clouds is accelerated by nonlinear flows, by performing three-dimensional non-ideal MHD simulations. An initial random supersonic (and trans-Alfvenic) turbulent-like flow is input into a self-gravitating gas layer that is threaded by a uniform magnetic field (perpendicular to the layer) such that the initial mass-to-flux ratio is subcritical. Magnetic ambipolar diffusion occurs very rapidly initially due to the sharp gradients introduced by the turbulent flow. It subsequently occurs more slowly in the traditional near-quasistatic manner, but in regions of greater mean density than present in the initial state. The overall timescale for runaway growth of the first core(s) is several times, 10^6 yr, even though previous studies have found a timescale of several times, 10^7 yr when starting with linear perturbations and similar physical parameters. Large-scale supersonic flows exist in the cloud and provide an observationally testable distinguishing characteristic from core formation due to linear initial perturbations. However, the nonlinear flows have decayed sufficiently that the relative infall motions onto the first core are subsonic, as in the case of starting from linear initial perturbations. The ion infall motions are very similar to those of neutrals; however, they lag the neutral infall in directions perpendicular to the mean magnetic field direction and lead the neutral infall in the direction parallel to the mean magnetic field.

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arXiv2008

Mass accretion rates in self-regulated disks of T Tauri stars

We have studied numerically the evolution of protostellar disks around intermediate and upper mass T Tauri stars (0.25 M_sun < M_st < 3.0 M_sun) that have formed self-consistently from the collapse of molecular cloud cores. In the T Tauri phase, disks settle into a self-regulated state, with low-amplitude nonaxisymmetric density perturbations persisting for at least several million years. Our main finding is that the global effect of gravitational torques due to these perturbations is to produce disk accretion rates that are of the correct magnitude to explain observed accretion onto T Tauri stars. Our models yield a correlation between accretion rate M_dot and stellar mass M_st that has a best fit M_dot \propto M_st^{1.7}, in good agreement with recent observations. We also predict a near-linear correlation between the disk accretion rate and the disk mass.

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arXiv2008

The Formation and Evolution of Prestellar Cores

Improving our understanding of the initial conditions and earliest stages of star formation is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. We review the properties of low-mass dense cores as derived from recent millimeter/submillimeter observations of nearby molecular clouds and discuss them in the context of various contemporary scenarios for cloud core formation and evolution. None of the extreme scenarios can explain all observations. Pure laminar ambipolar diffusion has relatively long growth times for typical ionization levels and has difficulty satisfying core lifetime constraints. Purely hydrodynamic pictures have trouble accounting for the inefficiency of core formation and the detailed velocity structure of individual cores. A possible favorable scenario is a mixed model involving gravitational fragmentation of turbulent molecular clouds close to magnetic criticality. The evolution of the magnetic field and angular momentum in individual cloud cores after the onset of gravitational collapse is also discussed. In particular, we stress the importance of radiation-magnetohydrodynamical processes and resistive MHD effects during the protostellar phase. We also emphasize the role of the formation of the short-lived first (protostellar) core in providing a chance for sub-fragmentation into binary systems and triggering MHD outflows. Future submillimeter facilities such as Herschel and ALMA will soon provide major new observational constraints in this field. On the theoretical side, an important challenge for the future will be to link the formation of molecular clouds and prestellar cores in a coherent picture.

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arXiv2007

Three-dimensional simulations of molecular cloud fragmentation regulated by magnetic fields and ambipolar diffusion

We employ the first fully three-dimensional simulation to study the role of magnetic fields and ion-neutral friction in regulating gravitationally-driven fragmentation of molecular clouds. The cores in an initially subcritical cloud develop gradually over an ambipolar diffusion time while the cores in an initially supercritical cloud develop in a dynamical time. The infall speeds on to cores are subsonic in the case of an initially subcritical cloud, while an extended (\ga 0.1 pc) region of supersonic infall exists in the case of an initially supercritical cloud. These results are consistent with previous two-dimensional simulations. We also found that a snapshot of the relation between density (rho) and the strength of the magnetic field (B) at different spatial points of the cloud coincides with the evolutionary track of an individual core. When the density becomes large, both relations tend to B \propto ρ^{0.5}.

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arXiv2006

Formation and Collapse of Nonaxisymmetric Protostellar Cores in Planar Magnetic Interstellar Clouds: Formulation of the Problem and Linear Analysis

We formulate the problem of the formation and collapse of nonaxisymmetric protostellar cores in weakly ionized, self-gravitating, magnetic molecular clouds. In our formulation, molecular clouds are approximated as isothermal, thin (but with finite thickness) sheets. We present the governing dynamical equations for the multifluid system of neutral gas and ions, including ambipolar diffusion, and also a self-consistent treatment of thermal pressure, gravitational, and magnetic (pressure and tension) forces. The dimensionless free parameters characterizing model clouds are discussed. The response of cloud models to linear perturbations is also examined, with particular emphasis on length and time scales for the growth of gravitational instability in magnetically subcritical and supercritical clouds. We investigate their dependence on a cloud's initial mass-to-magnetic-flux ratio (normalized to the critical value for collapse), the dimensionless initial neutral-ion collision time, and also the relative external pressure exerted on a model cloud. Among our results, we find that nearly-critical model clouds have significantly larger characteristic instability lengthscales than do more distinctly sub- or supercritical models. Another result is that the effect of a greater external pressure is to reduce the critical lengthscale for instability. Numerical simulations showing the evolution of model clouds during the linear regime of evolution are also presented, and compared to the results of the dispersion analysis. They are found to be in agreement with the dispersion results, and confirm the dependence of the characteristic length and time scales on parameters such as the initial mass-to-flux ratio and relative external pressure.

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arXiv2006

The Burst Mode of Protostellar Accretion

We present new numerical simulations in the thin-disk approximation which characterize the burst mode of protostellar accretion. The burst mode begins upon the formation of a centrifugally balanced disk around a newly formed protostar. It is comprised of prolonged quiescent periods of low accretion rate (typically $\la 10^{-7} \Msun$ yr$^{-1}$) which are punctuated by intense bursts of accretion (typically $\ga 10^{-4} \Msun$ yr$^{-1}$, with duration $\la 100$ yr) during which most of the protostellar mass is accumulated. The accretion bursts are associated with the formation of dense protostellar/protoplanetary embryos, which are later driven onto the protostar by the gravitational torques that develop in the disk. Gravitational instability in the disk, driven by continuing infall from the envelope, is shown to be an effective means of transporting angular momentum outward, and mass inward to the protostar. We show that the disk mass always remains significantly less than the central protostar mass throughout this process. The burst phenomenon is robust enough to occur for a variety of initial values of rotation rate, frozen-in (supercritical) magnetic field, and density-temperature relations. Even in cases where the bursts are nearly entirely suppressed, a moderate increase in cloud size or rotation rate can lead to vigorous burst activity. We conclude that most (if not all) protostars undergo a burst mode of evolution during their early accretion history, as inferred empirically from observations of FU Orionis variables.

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arXiv2006

Nonlinear Hydromagnetic Wave Support of a Stratified Molecular Cloud II: A Parameter Study

We use numerical simulations to study the effect of nonlinear MHD waves in a stratified, self-gravitating molecular cloud that is bounded by a hot and tenuous external medium. In a previous paper, we had shown the details of a standard model and studied the effect of varying the dimensionless amplitude. In this paper, we present the results of varying two other important free parameters: beta_0, the initial ratio of gas to magnetic pressure at the cloud midplane, and the dimensionless frequency of driving. Furthermore, we present the case of a temporally random driving force. Our results demonstrate that a very important consideration for the actual level of turbulent support against gravity is the ratio of driving wavelength lambda_0 to the the size of the initial non-turbulent cloud; maximum cloud expansion is achieved when this ratio is close to unity. The best consistency with the observational correlation of magnetic field strength, turbulent line width, and density is achieved by cloud models with beta_0 approx 1. We also calculate the spatial power spectra of the turbulent clouds, and show that significant power is developed on scales larger than the scale length H_0 of the initial cloud, even if the input wavelength of turbulence lambda_0 approx H_0. The cloud stratification and resulting increase of Alfven speed toward the cloud edge allows for a transfer of energy to wavelengths significantly larger than lambda_0. This explains why the relevant time scale for turbulent dissipation is the crossing time over the cloud scale rather than the crossing time over the driving scale.

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