arXiv subjects

Zhi-Yun Li

Publications and source records attributed to Zhi-Yun Li.

At least 271 records · Page 15Linked to original sources

arXiv2008

Magnetic Braking and Protostellar Disk Formation: Ambipolar Diffusion

It is established that the formation of rotationally supported disks during the main accretion phase of star formation is suppressed by a moderately strong magnetic field in the ideal MHD limit. Non-ideal MHD effects are expected to weaken the magnetic braking, perhaps allowing the disk to reappear. We concentrate on one such effect, ambipolar diffusion, which enables the field lines to slip relative to the bulk neutral matter. We find that the slippage does not sufficiently weaken the braking to allow rotationally supported disks to form for realistic levels of cloud magnetization and cosmic ray ionization rate; in some cases, the magnetic braking is even enhanced. Only in dense cores with both exceptionally weak fields and unreasonably low ionization rate do such disks start to form in our simulations. We conclude that additional processes, such as Ohmic dissipation or Hall effect, are needed to enable disk formation. Alternatively, the disk may form at late times when the massive envelope that anchors the magnetic brake is dissipated, perhaps by a protostellar wind.

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arXiv2008

The Molecular Ridge Close to 30 Doradus in the Large Magellanic Cloud

With the ATNF Mopra telescope we are performing a survey in the 12CO(1-0) line to map the molecular gas in the Large Magellanic Cloud (LMC). For some regions we also obtained interferometric maps of the high density gas tracers HCO+ and HCN with the Australia Telescope Compact Array (ATCA). Here we discuss the properties of the elongated molecular complex that stretches about 2 kpc southward from 30 Doradus. Our data suggests that the complex, which we refer to as the ``molecular ridge,'' is not a coherent feature but consists of many smaller clumps that share the same formation history. Likely molecular cloud formation triggers are shocks and shearing forces that are present in the surrounding south-eastern HI overdensity region, a region influenced by strong ram pressure and tidal forces. The molecular ridge is at the western edge of the the overdensity region where a bifurcated velocity structure transitions into a single disk velocity component. We find that the 12CO(1-0) and HI emission peaks in the molecular ridge are typically near each other but never coincide. A likely explanation is the conversion of warmer, low-opacity HI to colder, high-opacity HI from which H2 subsequently forms. On smaller scales, we find that very dense molecular gas, as traced by interferometric HCO+ and HCN maps, is associated with star formation along shocked filaments and with rims of expanding shell-like structures, both created by feedback from massive stars.

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arXiv2008

Magnetically Regulated Star Formation in 3D: The Case of Taurus Molecular Cloud Complex

We carry out three-dimensional MHD simulations of star formation in turbulent, magnetized clouds, including ambipolar diffusion and feedback from protostellar outflows. The calculations focus on relatively diffuse clouds threaded by a strong magnetic field capable of resisting severe tangling by turbulent motions and retarding global gravitational contraction in the cross-field direction. They are motivated by observations of the Taurus molecular cloud complex (and, to a lesser extent, Pipe Nebula), which shows an ordered large-scale magnetic field, as well as elongated condensations that are generally perpendicular to the large-scale field. We find that stars form in earnest in such clouds when enough material has settled gravitationally along the field lines that the mass-to-flux ratios of the condensations approach the critical value. Only a small fraction (of order 1% or less) of the nearly magnetically-critical, condensed material is turned into stars per local free-fall time, however. The slow star formation takes place in condensations that are moderately supersonic; it is regulated primarily by magnetic fields, rather than turbulence. The quiescent condensations are surrounded by diffuse halos that are much more turbulent, as observed in the Taurus complex. Strong support for magnetic regulation of star formation in this complex comes from the extremely slow conversion of the already condensed, relatively quiescent C$^{18}$O gas into stars, at a rate two orders of magnitude below the maximum, free-fall value. We analyze the properties of dense cores, including their mass spectrum, which resembles the stellar initial mass function.

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arXiv2008

The Formation of Distributed and Clustered Stars in Molecular Clouds

During the last two decades, the focus of star formation research has shifted from understanding the collapse of a single dense core into a star to studying the formation hundreds to thousands of stars in molecular clouds. In this chapter, we overview recent observational and theoretical progress toward understanding star formation on the scale of molecular clouds and complexes, i.e the macrophysics of star formation. We begin with an overview of recent surveys of young stellar objects (YSOs) in molecular clouds and embedded clusters, and we outline an emerging picture of cluster formation. We then discuss the role of turbulence to both support clouds and create dense, gravitationally unstable structures, with an emphasis on the role of magnetic fields (in the case of distributed stars) and feedback (in the case of clusters) to slow turbulent decay and mediate the rate and density of star formation. The discussion is followed by an overview of how gravity and turbulence may produce observed scaling laws for the properties of molecular clouds, stars and star clusters, and how the observed, low star formation rate may result from self regulated star formation. We end with some concluding remarks, including a number of questions to be addressed by future observations and simulations.

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arXiv2007

Magnetic Braking and Protostellar Disk Formation: The Ideal MHD Limit

Magnetic fields are usually considered dynamically important in star formation when the dimensionless mass-to-flux ratio is close to, or less than, unity (lambda<~1). We show that, in disk formation, the requirement is far less stringent. This conclusion is drawn from a set of 2D (axisymmetric) simulations of the collapse of rotating, magnetized, singular isothermal cores. We find that a weak field corresponding to 1ambda~100 can begin to disrupt the rotationally supported disk through magnetic braking, by creating regions of rapid, supersonic collapse in the disk. These regions are separated by one or more centrifugal barriers, where the rapid infall is temporarily halted. The number of centrifugal barriers increases with lambda. When lambda>~100, they merge together to form a more or less contiguous, rotationally supported disk. Even though the magnetic field in such a case is extremely weak on the scale of dense cores, it is amplified by collapse and differential rotation, to the extent that its pressure dominates the thermal pressure in both the disk and its surrounding region. For relatively strongly magnetized cores with lambda<~10, the disk formation is suppressed completely, as found previously. A new feature is that the mass accretion is highly episodic, due to reconnection of the accumulated magnetic field lines. For rotationally supported disks to appear during the protostellar mass accretion phase of star formation in dense cores with realistic field strengths, the powerful magnetic brake must be weakened, perhaps through nonideal MHD effects and/or protostellar winds. We discuss the possibility of observing a generic product of the magnetic braking, an extended circumstellar region that is supported by a combination of toroidal magnetic field and rotation - a "magnetogyrosphere".

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arXiv2007

Protostellar Turbulence Driven by Collimated Outflows

The majority of stars are thought to form in clusters. Cluster formation in dense clumps of molecular clouds is strongly influenced, perhaps controlled, by supersonic turbulence. We have previously shown that the turbulence in regions of active cluster formation is quickly transformed by the forming stars through protostellar outflows, and that the outflow-driven protostellar turbulence is the environment in which most of the cluster members form. Here, we take initial steps in quantifying the global properties of the protostellar turbulence through 3D MHD simulations. We find that collimated outflows are more efficient in driving turbulence than spherical outflows that carry the same amounts of momentum. Gravity plays an important role in shaping the turbulence, generating infall motions in the cluster forming region that balance the outward motions driven by outflows. The resulting quasi-equilibrium state is maintained through a slow rate of star formation, with a fraction of the total mass converted into stars per free fall time as low as a few percent. Magnetic fields are dynamically important even in magnetically supercritical clumps, provided that their initial strengths are not far below the critical value for static cloud support. We find that the mass weighted PDF of the volume density is often, although not always, approximately lognormal. The PDFs of the column density deviate more strongly from lognormal distributions. There is a prominent break in the velocity power spectrum of the protostellar turbulence, which may provide a way to distinguish it from other types of turbulence.

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arXiv2006

Magnetocentrifugal Winds in 3D: Nonaxisymmetric Steady State

Outflows can be loaded and accelerated to high speeds along rapidly rotating, open magnetic field lines by centrifugal forces. Whether such magnetocentrifugally driven winds are stable is a longstanding theoretical problem. As a step towards addressing this problem, we perform the first large-scale 3D MHD simulations that extend to a distance $\sim 10^2$ times beyond the launching region, starting from steady 2D (axisymmetric) solutions. In an attempt to drive the wind unstable, we increase the mass loading on one half of the launching surface by a factor of $\sqrt{10}$, and reduce it by the same factor on the other half. The evolution of the perturbed wind is followed numerically. We find no evidence for any rapidly growing instability that could disrupt the wind during the launching and initial phase of propagation, even when the magnetic field of the magnetocentrifugal wind is toroidally dominated all the way to the launching surface. The strongly perturbed wind settles into a new steady state, with a highly asymmetric mass distribution. The distribution of magnetic field strength is, in contrast, much more symmetric. We discuss possible reasons for the apparent stability, including stabilization by an axial poloidal magnetic field, which is required to bend field lines away from the vertical direction and produce a magnetocentrifugal wind in the first place.

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arXiv2005

Cluster Formation in Protostellar Outflow-Driven Turbulence

Most, perhaps all, stars go through a phase of vigorous outflow during formation. We examine, through 3D MHD simulation, the effects of protostellar outflows on cluster formation. We find that the initial turbulence in the cluster-forming region is quickly replaced by motions generated by outflows. The protostellar outflow-driven turbulence (``protostellar turbulence'' for short) can keep the region close to a virial equilibrium long after the initial turbulence has decayed away. We argue that there exist two types of turbulence in star-forming clouds: a primordial (or ``interstellar'') turbulence and a protostellar turbulence, with the former transformed into the latter mostly in embedded clusters such as NGC 1333. Since the majority of stars are thought to form in clusters, an implication is that the stellar initial mass function is determined to a large extent by the stars themselves, through outflows which individually limit the mass accretion onto forming stars and collectively shape the environments (density structure and velocity field) in which most cluster members form. We speculate that massive cluster-forming clumps supported by protostellar turbulence gradually evolve towards a highly centrally condensed ``pivotal'' state, culminating in rapid formation of massive stars in the densest part through accretion.

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arXiv2005

The Structure of Magnetocentrifugal Winds I. Steady Mass Loading

We present the results of a series of time-dependent numerical simulations of cold, magnetocentrifugally launched winds from accretion disks. Our simulations span four and half decades of mass loading; in the context of a disk with a launching region from $0.1\AU$ to $1.0\AU$ around a $1\solarmass$ star and a field strength of about $20\gauss$ at the inner disk edge, this amounts to mass loss rates of $1\times 10^{-9}$ -- $3\times 10^{-5}\solarmassyear$ from each side of the disk. We find that the degree of collimation of the wind increases with mass loading; however even the ``lightest'' wind simulated is significantly collimated compared with the force-free magnetic configuration of the same magnetic flux distribution. The implication is that for flows from young stellar objects a radial field approximation is inappropriate. Surprisingly, the terminal velocity of the wind and the magnetic lever arm are still well-described by the analytical solutions for a radial field geometry. We also find that the isodensity contours and Alfvén surface are very nearly self-similar in mass loading. The wind becomes unsteady above some critical mass loading rate. For a small enough injection speed, we are able to obtain the first examples of a class of heavily-loaded magnetocentrifugal winds with magnetic fields completely dominated by the toroidal component all the way to the launching surface.

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arXiv2005

Quiescent Cores and the Efficiency of Turbulence-Accelerated, Magnetically Regulated Star Formation

The efficiency of star formation, defined as the ratio of the stellar to total (gas and stellar) mass, is observed to vary from a few percent in regions of dispersed star formation to about a third in cluster-forming cores. This difference may reflect the relative importance of magnetic fields and turbulence in controlling star formation. We investigate the interplay between supersonic turbulence and magnetic fields using numerical simulations, in a sheet-like geometry. We demonstrate that star formation with an efficiency of a few percent can occur over several gravitational collapse times in moderately magnetically subcritical clouds that are supersonically turbulent. The turbulence accelerates star formation by reducing the time for dense core formation. The dense cores produced are predominantly quiescent, with subsonic internal motions. These cores tend to be moderately supercritical. They have lifetimes long compared with their local gravitational collapse time. Some of the cores collapse to form stars, while others disperse away without star formation. In turbulent clouds that are marginally magnetically supercritical, the star formation efficiency is higher, but can still be consistent with the values inferred for nearby embedded clusters. If not regulated by magnetic fields at all, star formation in a multi-Jeans mass cloud endowed with a strong initial turbulence proceeds rapidly, with the majority of cloud mass converted into stars in a gravitational collapse time. The efficiency is formally higher than the values inferred for nearby cluster-forming cores, indicating that magnetic fields are dynamically important even for cluster formation.

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arXiv2004

Magnetically Regulated Star Formation in Turbulent Clouds

We investigate numerically the combined effects of supersonic turbulence, strong magnetic fields and ambipolar diffusion on cloud evolution leading to star formation. We find that, in clouds that are initially magnetically subcritical, supersonic turbulence can speed up star formation, through enhanced ambipolar diffusion in shocks. The speedup overcomes a major objection to the standard scenario of low-mass star formation involving ambipolar diffusion, since the diffusion time scale at the average density of a molecular cloud is typically longer than the cloud life time. At the same time, the strong magnetic field can prevent the large-scale supersonic turbulence from converting most of the cloud mass into stars in one (short) turbulence crossing time, and thus alleviate the high efficiency problem associated with the turbulence-controlled picture for low-mass star formation. We propose that relatively rapid but inefficient star formation results from supersonic collisions of somewhat subcritical gas in strongly magnetized, turbulent clouds. The salient features of this shock-accelerated, ambipolar diffusion-regulated scenario are demonstrated with numerical experiments.

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arXiv2003

Does Magnetic Levitation or Suspension Define the Masses of Forming Stars?

We investigate whether magnetic tension can define the masses of forming stars by holding up the subcritical envelope of a molecular cloud that suffers gravitational collapse of its supercritical core. We perform an equilibrium analysis of the initial and final states assuming perfect field freezing, no rotation, isothermality, and a completely flattened configuration. The sheet geometry allows us to separate the magnetic tension into a levitation associated with the split monopole formed by the trapped flux of the central star and a suspension associated with curved field lines that thread the static pseudodisk and envelope of material external to the star. We find solutions where the eigenvalue for the stellar mass is a fixed multiple of the initial core mass of the cloud. We verify the analytically derived result by an explicit numerical simulation of a closely related 3-D axisymmetric system. However, with field freezing, the implied surface magnetic fields much exceed measured values for young stars. If the pinch by the central split monopole were to be eliminated by magnetic reconnection, then magnetic suspension alone cannot keep the subcritical envelope (i.e., the entire model cloud) from falling onto the star. We argue that this answer has general validity, even if the initial state lacked any kind of symmetry, possessed rotation, and had a substantial level of turbulence. These findings strongly support a picture for the halt of infall that invokes dynamic levitation by YSO winds and jets, but the breakdown of ideal magnetohydrodynamics is required to allow the appearance in the problem of a rapidly rotating, centrifugally supported disk. We use these results to calculate the initial mass function and star formation efficiency for the distributed and clustered modes of star formation.

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arXiv2003

The Diversity of Gamma-Ray Bursts and the Surroundings of Massive Stars

The finding of a Type Ic supernova connected with GRB 030329 showed a massive star origin for this burst, supporting evidence for this association in previous bursts with lightcurve bumps at the appropriate time for a supernova. Here, we explore the possibility that all long bursts have massive star progenitors, interacting with either the freely expanding wind of the progenitor or the shocked wind. We present models for the afterglows of GRB 020405 and GRB 021211, which are a challenge to wind interaction models. Considering sources for which wind interaction models are acceptable, a range of wind densities is required, from values typical of Galactic Wolf-Rayet stars to values ~100 times smaller. The reason for the low densities is unclear, but may involve low progenitor masses and/or low metallicities. If mass is a factor, a low density event should be associated with a low mass supernova. The interpretation of bursts apparently interacting with constant density media as interaction with a shocked wind requires both a range of mass loss densities and a range of external pressures. The highest pressures, p/k > 10^8 cm^{-3} K, may be due to an extreme starburst environment, which would imply that the burst is superposed on an active star forming region. Although the range of observed events can be accomodated by the shocked wind theory, special circumstances are necessary to bring this about. Finally, we consider the high velocity, high ionization absorption features observed in some afterglow spectra. If the features are circumstellar, the presence of the burst in a starburst region may be important for the formation of clumps near the burst.

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arXiv2003

Magneto-Centrifugal Launching of Jets from Accretion Disks. II: Inner Disk-Driven Winds

We follow numerically the time evolution of axisymmetric outflows driven magneto-centrifugally from the inner portion of accretion disks, from their launching surface to large, observable distances. Special attention is paid to the collimation of part of the outflow into a dense, narrow jet around the rotation axis, after a steady state has been reached. For parameters typical of T Tauri stars, we define a fiducial ``jet'' as outlined by the contour of constant density at 10^4 cm^{-3}. We find that the jet, so defined, appears nearly cylindrical well above the disk, in agreement with previous asymptotic analyses. Closer to the equatorial plane, the density contour can either bulge outwards or pinch inwards, depending on the conditions at the launching surface, particularly the mass flux distribution. We find that even though a dense, jet-like feature is always formed around the axis, there is no guarantee that the high-density axial jet would dominate the more tenuous, wide-angle part of the wind. Specifically, on the 100 AU scale, resolvable by HST and ground-based adaptive optics for nearby T Tauri winds, the fraction of the wind mass flux enclosed by the fiducial jet can vary substantially, again depending on the launching conditions. We show two examples in which the fraction is ~20% and ~45%. These dependences may provide a way to constrain the conditions at the launching surface, which are poorly known at present.

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arXiv2003

Binary and Multiple Star Formation in Magnetic Clouds: Bar Growth and Fragmentation

We study the non-axisymmetric evolution of magnetized clouds, using a 2D MHD code based on the physically motivated thin-disk approximation. We found that such clouds become unstable to non-axisymmetric perturbations after the supercritical cores are formed due to ambipolar diffusion. We show that for a wide range of initial cloud parameters, the $m=2$ mode grows nonlinearly into a bar during the isothermal collapse after the supercritical core formation. The supercritical core can break up into fragments during or after the isothermal phase of cloud evolution. The outcome of fragmentation depends on the initial cloud conditions, such as the magnetic field strength, rotation rate, amount of cloud mass and mass distribution. It is classified into three different types: (1) separate core formation, (2) bar fragmentation, and (3) disk fragmentation. These three types of fragmentation loosely correspond to the empirical classification of embedded binary and multiple systems of Looney, Mundy, & Welch, based on millimeter dust continuum observations. The well-studied starless core, L1544, appears to belong to the bar fragmentation type. We expect it to produce a highly elongated, opaque bar at the center in the future, which should break up into fragments of initial masses in the substellar regime.

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arXiv2003

Locating the Launching Region of T Tauri Winds: The Case of DG Tau

It is widely believed that T Tauri winds are driven magnetocentrifugally from accretion disks close to the central stars. The exact launching conditions are uncertain. We show that a general relation exists between the poloidal and toroidal velocity components of a magnetocentrifugal wind at large distances and the rotation rate of the launching surface, independent of the uncertain launching conditions. We discuss the physical basis of this relation and verify it using a set of numerically-determined large-scale wind solutions. Both velocity components are in principle measurable from spatially resolved spectra, as has been done for the extended low-velocity component (LVC) of the DG Tau wind by Bacciotti et al. For this particular source, we infer that the spatially resolved LVC originates from a region on the disk extending from $\sim 0.3$ to $\sim 4.0\AU$ from the star, which is consistent with, and a refinement over, the previous rough estimate of Bacciotti et al.

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arXiv2003

Wind-Interaction Models for the Early Afterglows of Gamma-Ray Bursts: The Case of GRB 021004

Wind-interaction models for gamma-ray burst afterglows predict that the optical emission from the reverse shock drops below that from the forward shock within 100s of seconds of the burst. The typical frequency $ν_m$ of the synchrotron emission from the forward shock passes through the optical band typically on a timescale of minutes to hours. Before the passage of $ν_m$, the optical flux evolves as $t^{-1/4}$ and after the passage, the decay steepens to $t^{-(3p-2)/4}$, where $p$ is the exponent for the assumed power-law energy distribution of nonthermal electrons and is typically $\sim 2$. The steepening in the slope of temporal decay should be readily identifiable in the early afterglow light curves. We propose that such a steepening was observed in the R-band light curve of GRB 021004 around day 0.1. Available data at several radio frequencies are consistent with this interpretation, as are the X-ray observations around day~1. The early evolution of GRB 021004 contrasts with that of GRB 990123, which can be described by emission from interaction with a constant density medium.

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arXiv2002

Self-Gravitating Magnetically-Supported Protostellar Disks and the Formation of Substellar Companions

Isolated low-mass stars are formed, in the standard picture, from the collapse of dense cores condensed out of strongly magnetized molecular clouds. The dynamically collapsing inflow traps nearly half of the critical magnetic flux needed for the core support and deposits it in a small region surrounding the protostar. It has been argued previously that the deposited flux can slow down the inflow, allowing matter to pile up and settle along field lines into a magnetically supported, circumstellar disk. Here we show that the disk typically contains $\sim 10%$ of the stellar mass, and that it could become self-gravitating under plausible conditions during the rapidly accreting, ``Class 0'' phase of star formation. Subsequent fragmentation of the self-gravitating, magnetically subcritical disk, driven by magnetic diffusion, could produce fragments of substellar masses, which collapse to form brown dwarfs and possibly massive planets. This scenario predicts substellar object formation at distances of order 100 AU from the central star, although orbital evolution is possible after formation. It may provide an explanation for the small, but growing, number of brown dwarf companions found around nearby stars by direct imaging. The relatively large formation distances make the substellar companions vulnerable to dynamic ejection, particularly in binary (multiple) systems and dense clusters. Those ejected may account for, at least in part, the isolated brown dwarfs and perhaps free-floating planetary mass objects.

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