arXiv subjects

Ralph E. Pudritz

Publications and source records attributed to Ralph E. Pudritz.

At least 109 records · Page 6Linked to original sources

arXiv2005

Accretion-powered Stellar Winds as a Solution to the Stellar Angular Momentum Problem

We compare the angular momentum extracted by a wind from a pre-main-sequence star to the torques arising from the interaction between the star and its Keplerian accretion disk. We find that the wind alone can counteract the spin-up torque from mass accretion, solving the mystery of why accreting pre-main-sequence stars are observed to spin at less than 10% of break-up speed, provided that the mass outflow rate in the stellar winds is ~10% of the accretion rate. We suggest that such massive winds will be driven by some fraction $ε$ of the accretion power. For observationally constrained typical parameters of classical T-Tauri stars, $ε$ needs to be between a few and a few tens of percent. In this scenario, efficient braking of the star will terminate simultaneously with accretion, as is usually assumed to explain the rotation velocities of stars in young clusters.

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arXiv2005

Controlling the collimation and rotation of hydromagnetic disk winds

(Abriged) We present a comprehensive set of axisymmetric, time-dependent simulations of jets from Keplerian disks whose mass loading as a function of disk radius is systematically changed. For a reasonable model for the density structure and injection speed of the underlying accretion disk, mass loading is determined by the radial structure of the disk's magnetic field structure. We vary this structure by using four different magnetic field configurations, ranging from the "potential" configuration (Ouyed&Pudritz 1997), to the increasingly more steeply falling Blandford&Payne (1982) and Pelletier&Pudritz (1992) models, and ending with a quite steeply raked configuration that bears similarities to the Shu X-wind model. We find that the radial distribution of the mass load has a profound effect on both the rotational profile of the underlying jet as well as the degree of collimation of its outflow velocity and magnetic field lines. We show analytically, and confirm by our simulations, that the collimation of a jet depends on its radial current distribution, which in turn is prescribed by the mass load. Models with steeply descending mass loads have strong toroidal fields, and these collimate to cylinders (this includes the Ouyed-Pudritz and Blandford-Payne outflows). On the other hand, the more gradually descending mass load profiles (the PP92 and monopolar distributions) have weaker toroidal fields, and these result in wide-angle outflows with parabolic collimation. We also present detailed structural information about jets such as their radial profiles of jet density, toroidal magnetic field, and poloidal jet speed, as well as an analysis of the bulk energetics of our different simulations.

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arXiv2005

The Disc-Jet Connection

A large body of theoretical and computational work shows that jets - modelled as magnetized disk winds - exert an external torque on their underlying disks that can efficiently remove angular momentum and act as major drivers of disk accretion. These predictions have recently been confirmed in direct HST measurements of the jet rotation and angular momentum transport in low mass protostellar systems. We review the theory of disc winds and show that their physics is universal and scales to jets from both low and high mass star forming regions. This explains the observed properties of outflows in massive star forming regions, before the central massive star generates an ultracompact HII region. We also discuss the recent numerical studies on the formation of massive accretion disks and outflows through gravitational collapse, including our own work on 3D Adaptive Mesh simulations (using the FLASH code) of the hydromagnetic collapse of an initial rotating, and cooling Bonner-Ebert sphere. Magnetized collapse gives rise to outflows. Our own simulations show that both a jet-like disk wind on sub AU scales, and a larger scale molecular outflow occur (Banerjee and Pudritz 2005).

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arXiv2004

Dead Zones and the Origin of Planetary Masses

Protoplanets accrete material from their natal protostellar disks until they are sufficiently massive to open a gap in the face of the disk's viscosity that arises from the magneto-rotational instability (MRI). By computing the ionization structure within observationally well-constrained disk models, we demonstrate that poorly ionized, low viscosity "dead zones" stretch out to 12 AU within typical disks. We find that planets of terrestrial mass robustly form within the dead zones while massive Jovian planets form beyond. Dead zones will also halt the rapid migration of planets into their central stars. Finally, we argue that the gravitational scattering of low mass planets formed in the dead zone, to larger radii by a rapidly accreting Jupiter beyond, can explain the distribution of planetary masses in our solar system.

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arXiv2004

The spin of accreting stars: dependence on magnetic coupling to the disc

We formulate a general, steady-state model for the torque on a magnetized star from a surrounding accretion disc. For the first time, we include the opening of dipolar magnetic field lines due to the differential rotation between the star and disc, so the magnetic topology then depends on the strength of the magnetic coupling to the disc. This coupling is determined by the effective slip rate of magnetic field lines that penetrate the diffusive disc. Stronger coupling (i.e., lower slip rate) leads to a more open topology and thus to a weaker magnetic torque on the star from the disc. In the expected strong coupling regime, we find that the spin-down torque on the star is more than an order of magnitude smaller than calculated by previous models. We also use our general approach to examine the equilibrium (`disc-locked') state, in which the net torque on the star is zero. In this state, we show that the stellar spin rate is roughly an order of magnitude faster than predicted by previous models. This challenges the idea that slowly-rotating, accreting protostars are disc locked. Furthermore, when the field is sufficiently open (e.g., for mass accretion rates > 5 x 10^{-9} M_sun / yr, for typical accreting protostars), the star will receive no magnetic spin-down torque from the disc at all. We therefore conclude that protostars must experience a spin-down torque from a source that has not yet been considered in the star-disc torque models--possibly from a stellar wind along the open field lines.

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arXiv2004

The Formation and Evolution of Protostellar Disks; 3D AMR Hydro-Simulations of Collapsing, Rotating Bonnor-Ebert-Spheres

We present a detailed study of the collapse of molecular cloud cores using high resolution 3D adaptive mesh refinement (AMR) numerical simulations. In this first in a series of investigations our initial conditions consists of spherical molecular core obeying the hydrostatic Bonnor-Ebert-Profile with varying degrees of initial rotation. Our simulations cover both the formation of massive disks in which massive stars form as well as low mass disks. We use a customized version of the FLASH code whose AMR technique allows us to follow the formation of a protstellar disk and protostellar core(s) through more than ten orders in density increase while continuously resolving the local Jeans length (i.e. obeying the Truelove criterion, Truelove et al. (1997)). Our numerical simulations also incorporate the energy loss due to molecular line emission in order to obtain a more realistic picture of protostellar core and disk formation. Out initial states model system of mass 168 M_sol and 2.1 M_sol that will form high and low mass stars, respectively. We follow many features such as the development complex shock structures, and the possible fragmentation of the disk. We find that slowly rotating cores (Omega t_ff = 0.1) produce disks in which a strong bar develops but which does not fragment. Faster initial rotation rates (Omega t_ff = 0.2) result in the formation of a ring which may fragment into two star-forming cores. The size of the rings found in our simulated disks agree with the observations of similar systems.

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arXiv2004

The Formation of Star Clusters I: 3D Simulations of Hydrodynamic Turbulence

We present the results of a series of numerical simulations of compressible, self-gravitating hydrodynamic turbulence of cluster-forming clumps in molecular clouds. We examine the role that turbulence has in the formation of gravitationally bound cores, studying the dynamical state, internal structure and bulk properties of these cores. Complex structure in turbulent clumps is formed provided that the damping time of the turbulence, $t_{damp}$ is longer than the gravitational free-fall time $t_{ff}$ in a region. We find a variety of density and infall velocity structures among the cores in the simulation, including cores that resemble the Larson-Penston collapse of an isothermal sphere ($ρ\propto r^{-2}$) as well as cores that resemble the McLaughlin-Pudritz collapse of logatropic spheres ($ρ\propto r^{-1}$). The specific angular momentum profiles range between $j \propto r^{1} - r^{2}$. The masses of the bound cores that form are well-fit by the turbulent mass spectrum of Padoan and Nordlund (2002), while the specific angular momentum distribution can be fit by a broken power law. While our hydrodynamic simulations reproduce many of the observed properties of cores, we find an upper limit for the star formation efficiency (SFE) in clusters of 40-50 per cent.

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arXiv2004

Does Disk Locking Solve the Stellar Angular Momentum Problem?

We critically examine the theory of disk locking, which assumes that the angular momentum deposited on an accreting protostar is exactly removed by torques carried along magnetic field lines connecting the star to the disk. In this letter, we consider that the differential rotation between the star and disk naturally leads to an opening (i.e., disconnecting) of the magnetic field between the two. We find that this significantly reduces the spin-down torque on the star by the disk. Thus, disk-locking cannot account for the slow rotation ($\sim$10% of breakup speed) observed in several systems and for which the model was originally developed.

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arXiv2004

The Spin History of Protostars: Disk Locking, Revisited

In this talk, we take a new look at the theory of disk locking, which assumes that an accreting protostar rids itself of accreted angular momentum through a magnetic coupling with the accretion disk. We consider that differential rotation between the star and disk twists the field lines. For large enough twist, the magnetic field lines connecting the star and disk open and disconnect. This significantly reduces the spin-down torque on the star by the disk, and so we find that disk-locking theory predicts spin periods that are much too short to account for typical observed systems.

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arXiv2003

The Origin of Jovian Planets in Protostellar Disks: The Role of Dead Zones

The final masses of Jovian planets are attained when the tidal torques that they exert on their surrounding protostellar disks are sufficient to open gaps in the face of disk viscosity, thereby shutting off any further accretion. In sufficiently well-ionized disks, the predominant form of disk viscosity originates from the Magneto-Rotational Instability (MRI) that drives hydromagnetic disk turbulence. In the region of sufficiently low ionization rate -- the so-called dead zone -- turbulence is damped and we show that lower mass planets will be formed. We considered three ionization sources (X-rays, cosmic rays, and radioactive elements) and determined the size of a dead zone for the total ionization rate by using a radiative, hydrostatic equilibrium disk model developed by Chiang et al. (2001). We studied a range of surface mass density (Sigma_{0}=10^3 - 10^5 g cm^{-2}) and X-ray energy (kT_{x}=1 - 10 keV). We also compared the ionization rate of such a disk by X-rays with cosmic rays and find that the latter dominate X-rays in ionizing protostellar disks unless the X-ray energy is very high (5 - 10 keV). Among our major conclusions are that for typical conditions, dead zones encompass a region extending out to several AU -- the region in which terrestrial planets are found in our solar system. Our results suggest that the division between low and high mass planets in exosolar planetary systems is a consequence of the presence of a dead zone in their natal protoplanetary disks. We also find that the extent of a dead zone is mainly dependent on the disk's surface mass density. Our results provide further support for the idea that Jovian planets in exosolar systems must have migrated substantially inwards from their points of origin.

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arXiv2002

Fragmentation and Collapse of Turbulent Molecular Clouds

We performed simulations of self-gravitating hydrodynamic turbulence to model the formation of filaments, clumps and cores in molecular clouds. We find that when the mass on the initial computational grid is comparable to the Jeans mass, turbulent pressure is able to prevent gravitational collapse. When the turbulence has damped away sufficiently, gravitational collapse can occur, and the resulting structure closely resembles the pre-singularity collapse of an isothermal sphere of Penston (1969). If several Jeans masses are initially placed on the grid, turbulence may not be sufficient to prevent collapse before turbulence can be significantly damped. In this case, the cores have density structures which are considerably shallower than expected for an isothermal gas, and resemble the solutions for a logatropic equation of state.

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arXiv2001

Cosmological Evolution of Supergiant Star-Forming Clouds

In an exploration of the birthplaces of globular clusters, we present a careful examination of the formation of self-gravitating gas clouds within assembling dark matter haloes in a hierarchical cosmological model. Our high-resolution smoothed particle hydrodynamical simulations are designed to determine whether or not hypothesized supergiant molecular clouds (SGMCs) form and, if they do, to determine their physical properties and mass spectra. It was suggested in earlier work that clouds with a median mass of several 10^8 M_sun are expected to assemble during the formation of a galaxy, and that globular clusters form within these SGMCs. Our simulations show that clouds with the predicted properties are indeed produced as smaller clouds collide and agglomerate within the merging dark matter haloes of our cosmological model. We find that the mass spectrum of these clouds obeys the same power-law form observed for globular clusters, molecular clouds, and their internal clumps in galaxies, and predicted for the supergiant clouds in which globular clusters may form. We follow the evolution and physical properties of gas clouds within small dark matter haloes up to z = 1, after which prolific star formation is expected to occur. Finally, we discuss how our results may lead to more physically motivated "rules" for star formation in cosmological simulations of galaxy formation.

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arXiv2000

Polarized Sub-Millimetre Emission from Filamentary Molecular Clouds

We model the sub-millimetre polarization patterns that are expected for filamentary clouds that are threaded by helical magnetic fields. We previously developed a three parameter model of such clouds (Fiege & Pudritz 2000a), which are described by a concentration parameter $C$, and two flux to mass ratios $\Gz$ and $\Gphi$ to specify the mass loading of the poloidal and toroidal field lines respectively. Our models provide a simple and purely geometric explanation for the well-known ``polarization hole'' effect, in which the sub-millimetre polarization percentage decreases toward the regions of peak intensity. This occurs because of a cancellation between contributions to the polarization from the ``backbone'' of poloidal flux along the filament's axis and its surrounding envelope, which is dominated by the toroidal field component. A systematic exploration of our parameter space allows us to classify the polarization patterns due to filaments aligned approximately perpendicular to the plane of the sky into three basic types. The polarization vectors are parallel to filaments when $\ratio\appleq 0.1$, where $\Bzs$ and $\Bphis$ are respectively the poloidal and toroidal magnetic field components at the outer surface of the filament. The polarization vectors are perpendicular to filaments when $\ratio\appgeq 0.33$. Intermediate cases result in polarization patterns that contain $90^\circ$ flips in the orientation of the polarization vectors. The flips are symmetric about the central axis for filaments oriented parallel to the plane of the sky, but more complicated asymmetric patters result from filaments that are inclined at some angle.

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arXiv1999

Prolate Cores in Filamentary Molecular Clouds

We present a new model of molecular cloud cores that originate from filamentary clouds that are threaded by helical magnetic fields. Only modest toroidal fields are required to produce elongated cores, with intrinsic axis ratios in the range 0.1 <= q <= 0.9 and mean projected axis ratios in the range 0.3 <= <= 1. Thus many of our models are in good agreement with the observed shapes of cores (Myers et al 1991, Ryden 1996), which find axis ratios distributed about the mean values \approx 0.5-0.6 and q \approx 0.4-0.5. We find that the Bonnor-Ebert critical mass is reduced by about 20% by the helical field threading our models. We also find that models are generally most elongated when the mass is significantly less than the Bonnor-Ebert critical mass for unmagnetized cores.

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arXiv1999

A New Model for Filamentary Molecular Clouds

We develop a theory for filamentary molecular clouds including the effects of ordered magnetic fields, and external pressure. We first derive a form of the virial equation appropriate for filamentary clouds. By comparing with observational results collected from the literature, we find that the fields are likely helical. Secondly, we construct numerical, MHD models of filamentary clouds that agree with the observational constraints. We find that our models produce much more realistic density profiles $r\sim r^{-1.8 to -2}$ than previous models, where the density falls off as $r^{-4}$.

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arXiv1999

Cluster Formation and the ISM

We review the physics of star formation, and its links with the state of the ISM in galaxies. Current obervations indicate that the preferred mode of star formation is clustered. Given that OB associations provide the dominant energy input into the ISM, deep links exist between the ISM and star formation. We present a multi-scale discussion of star formation, and attempt to create an integrated vision of these processes.

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arXiv1999

Disk Winds and the Accretion--Outflow Connection

We review recent observational and theoretical results on the relationship between circumstellar accretion disks and jets in young stellar objects. We then present a theoretical framework that interprets jets as accretion-powered, centrifugally driven winds from magnetized accretion disks. Recent progress in the numerical simulation of such outflows is described. We also discuss the structure of the underlying magnetized protostellar disks, emphasizing the role that large-scale, open magnetic fields can play in angular momentum transport.

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arXiv1999

Helical Fields and Filamentary Molecular Clouds II - Axisymmetric Stability and Fragmentation

In Paper I (Fiege & Pudritz, 1999), we constructed models of filamentary molecular clouds that are truncated by a realistic external pressure and contain a rather general helical magnetic field. We address the stability of our models to gravitational fragmentation and axisymmetric MHD-driven instabilities. By calculating the dominant modes of axisymmetric instability, we determine the dominant length scales and growth rates for fragmentation. We find that the role of pressure truncation is to decrease the growth rate of gravitational instabilities by decreasing the self-gravitating mass per unit length. Purely poloidal and toroidal fields also help to stabilize filamentary clouds against fragmentation. The overall effect of helical fields is to stabilize gravity-driven modes, so that the growth rates are significantly reduced below what is expected for unmagnetized clouds. However, MHD ``sausage'' instabilities are triggered in models whose toroidal flux to mass ratio exceeds the poloidal flux to mass ratio by more than a factor of $\sim 2$. We find that observed filaments appear to lie in a physical regime where the growth rates of both gravitational fragmentation and axisymmetric MHD-driven modes are at a minimum.

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