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ABSTRACT: Stars form in dense cores of molecular clouds that are observed to be
significantly magnetized. In the simplest case of a laminar (non-turbulent)
core with the magnetic field aligned with the rotation axis, both analytic
considerations and numerical simulations have shown that the formation of a
large, $10^2\au$-scale, rotationally supported protostellar disk is suppressed
by magnetic braking in the ideal MHD limit for a realistic level of core
magnetization. This theoretical difficulty in forming protostellar disks is
termed "magnetic braking catastrophe". A possible resolution to this problem,
proposed by \citeauthor{HennebelleCiardi2009} and \citeauthor{Joos+2012}, is
that misalignment between the magnetic field and rotation axis may weaken the
magnetic braking enough to enable disk formation. We evaluate this possibility
quantitatively through numerical simulations. We confirm the basic result of
\citeauthor{Joos+2012} that the misalignment is indeed conducive to disk
formation. In relatively weakly magnetized cores with dimensionless
mass-to-flux ratio $\gtrsim 5$, it enabled the formation of rotationally
supported disks that would otherwise be suppressed if the magnetic field and
rotation axis are aligned. For more strongly magnetized cores, disk formation
remains suppressed, however, even for the maximum tilt angle of $90\degree$. If
dense cores are as strongly magnetized as indicated by OH Zeeman observations
(with a mean dimensionless mass-to-flux ratio $\sim 2$), it would be difficult
for the misalignment alone to enable disk formation in the majority of them. We
conclude that, while beneficial to disk formation, especially for the
relatively weak field case, the misalignment does not completely solve the
problem of catastrophic magnetic braking in general.
01/2013;
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ABSTRACT: The majority of stars reside in multiple systems, especially binaries. The
formation and early evolution of binaries is a longstanding problem in star
formation that is not fully understood. In particular, how the magnetic field
observed in star-forming cores shapes the binary characteristics remains
relatively unexplored. We demonstrate numerically, using the ENZO-MHD code,
that a magnetic field of the observed strength can drastically change two of
the basic quantities of a binary system: the orbital separation and mass ratio
of the two components. Our calculations focus on the protostellar mass
accretion phase, after a pair of stellar 'seeds' have already formed. We find
that, in dense cores magnetized to a realistic level, the angular momentum of
the gas accreted by the protobinary is greatly reduced by magnetic braking.
Accretion of strongly braked material shrinks the protobinary separation by a
large factor compared to the non-magnetic case. The magnetic braking also
changes the evolution of the mass ratio of unequal-mass protobinaries by
producing gas of low specific angular momentum that accretes preferentially
onto the primary rather than the secondary. This is in contrast with the
preferential mass accretion onto the secondary previously found for
protobinaries accreting from an unmagnetized envelope, which tends to drive the
mass ratio towards unity. In addition, the magnetic field greatly modifies the
morphology and dynamics of the protobinary accretion flow. It suppresses the
circumstellar and circumbinary disks that feed the protobinary in the
non-magnetic case; the binary is fed instead by a fast collapsing pseudodisk
whose rotation is strongly braked. The magnetic braking-driven inward migration
of binaries from their birth locations may be constrained by high-resolution
observations of the orbital distribution of deeply embedded protobinaries,
especially with ALMA.
10/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Stars form predominantly in clusters inside dense clumps of turbulent,
magnetized molecular clouds. The typical size and mass of the cluster-forming
clumps are \sim 1 pc and \sim 10^2 - 10^3 M_\odot, respectively. Here, we
discuss some recent progress on theoretical and observational studies of
clustered star formation in such parsec-scale clumps with emphasis on the role
of protostellar outflow feedback. Recent simulations indicate that protostellar
outflow feedback can maintain supersonic turbulence in a cluster-forming clump,
and the clump can keep a virial equilibrium long after the initial turbulence
has decayed away. In the clumps, star formation proceeds relatively slowly; it
continues for at least several global free-fall times of the parent dense clump
(t_{ff}\sim a few x 10^5 yr). The most massive star in the clump is formed at
the bottom of the clump gravitational potential well at later times through the
filamentary mass accretion streams that are broken up by the outflows from
low-mass cluster members. Observations of molecular outflows in nearby
cluster-forming clumps appear to support the outflow-regulated cluster
formation model.
08/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Magnetic flux redistribution lies at the heart of the problem of star
formation in dense cores of molecular clouds that are magnetized to a realistic
level. If all of the magnetic flux of a typical core were to be dragged into
the central star, the stellar field strength would be orders of magnitude
higher than the observed values. This well-known "magnetic flux problem" can in
principle be resolved through non-ideal MHD effects. Two dimensional
(axisymmetric) calculations have shown that ambipolar diffusion, in particular,
can transport magnetic flux outward relative to matter, allowing material to
enter the central object without dragging the field lines along. We show
through simulations that such axisymmetric protostellar accretion flows are
unstable in three dimensions to magnetic interchange instability in the
azimuthal direction. The instability is driven by the magnetic flux
redistributed from the matter that enters the central object. It typically
starts to develop during the transition from the prestellar phase of star
formation to the protostellar mass accretion phase. In the latter phase, the
magnetic flux is transported outward mainly through advection, by strongly
magnetized low-density regions that expand against the collapsing inflow. The
tussle between the gravity-driven infall and magnetically driven expansion
leads to a filamentary inner accretion flow, more disordered than previously
pictured. The efficient outward transport of magnetic flux by advection lowers
the field strength at small radii, making the magnetic braking less efficient
and the formation of rotationally supported disks easier in principle. However,
we find no evidence for such disks in any of our rotating collapse simulations.
We conclude that the inner protostellar accretion flow is shaped to a large
extent by this magnetic interchange instability. How disks form in such an
environment is unclear.
05/2012;
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[hide abstract]
ABSTRACT: We investigate the physical properties of dense cores formed in turbulent,
magnetized, parsec-scale clumps of molecular clouds, using three-dimensional
numerical simulations that include protostellar outflow feedback. The dense
cores are identified in the simulated density data cube through a clumpfind
algorithm. We find that the core velocity dispersion does not show any clear
dependence on the core size, in contrast to Larson's linewidth-size relation,
but consistent with recent observations. In the absence of a magnetic field,
the majority of the cores have supersonic velocity dispersions. A
moderately-strong magnetic field reduces the dispersion to a subsonic or at
most transonic value typically. Most of the cores are out of virial
equilibrium, with the external pressure dominating the self-gravity. The
implication is that the core evolution is largely controlled by the
outflow-driven turbulence. Even an initially-weak magnetic field can retard
star formation significantly, because the field is amplified by the
outflow-driven turbulence to an equipartition strength, with the distorted
field component dominating the uniform one. In contrast, for a
moderately-strong field, the uniform component remains dominant. Such a
difference in the magnetic structure is evident in our simulated polarization
maps of dust thermal emission; it provides a handle on the field strength.
Recent polarization measurements show that the field lines in cluster-forming
clumps are spatially well-ordered. It is indicative of a moderately-strong,
dynamically important, field which, in combination with outflow feedback, can
keep the rate of star formation in embedded clusters at the
observationally-inferred, relatively-slow rate of several percent per free-fall
time.
07/2011;
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[show abstract]
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ABSTRACT: Dense, star-forming, cores of molecular clouds are observed to be
significantly magnetized. A realistic magnetic field of moderate strength has
been shown to suppress, through catastrophic magnetic braking, the formation of
a rotationally supported disk during the protostellar accretion phase of
low-mass star formation in the ideal MHD limit. We address, through 2D
(axisymmetric) simulations, the question of whether realistic levels of
nonideal effects, computed with a simplified chemical network including dust
grains, can weaken the magnetic braking enough to enable a rotationally
supported disk to form. We find that ambipolar diffusion, the dominant nonideal
MHD effect over most of the density range relevant to disk formation, does not
enable disk formation, at least in 2D. The reason is that ambipolar diffusion
allows the magnetic flux that would be dragged into the central stellar object
in the ideal MHD limit to pile up instead in a small circumstellar region,
where the magnetic field strength (and thus the braking efficiency) is greatly
enhanced. We also find that, on the scale of tens of AU or more, a realistic
level of Ohmic dissipation does not weaken the magnetic braking enough for a
rotationally supported disk to form, either by itself or in combination with
ambipolar diffusion. The Hall effect, the least explored of these three
nonideal MHD effects, can spin up the material close to the central object to a
significant, supersonic rotation speed, even when the core is initially
non-rotating, although the spun-up material remains too sub-Keplerian to form a
rotationally supported disk. The problem of catastrophic magnetic braking that
prevents disk formation in dense cores magnetized to realistic levels remains
unresolved. Possible resolutions of this problem are discussed.
06/2011;
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[show abstract]
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ABSTRACT: Stars form in dense cores of magnetized molecular clouds. If the magnetic
flux threading the cores is dragged into the stars, the stellar field would be
orders of magnitude stronger than observed. This well-known "magnetic flux
problem" demands that most of the core magnetic flux be decoupled from the
matter that enters the star. We carry out the first exploration of what happens
to the decoupled magnetic flux in 3D, using an MHD version of the ENZO adaptive
mesh refinement code. The field-matter decoupling is achieved through a sink
particle treatment, which is needed to follow the protostellar accretion phase
of star formation. We find that the accumulation of the decoupled flux near the
accreting protostar leads to a magnetic pressure buildup. The high pressure is
released anisotropically, along the path of least resistance. It drives a
low-density expanding region in which the decoupled magnetic flux is expelled.
This decoupling-enabled magnetic structure has never been seen before in 3D MHD
simulations of star formation. It generates a strong asymmetry in the
protostellar accretion flow, potentially giving a kick to the star. In the
presence of an initial core rotation, the structure presents an obstacle to the
formation of a rotationally supported disk, in addition to magnetic braking, by
acting as a rigid magnetic wall that prevents the rotating gas from completing
a full orbit around the central object. We conclude that the decoupled magnetic
flux from the stellar matter can strongly affect the protostellar collapse
dynamics.
05/2011;
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[show abstract]
[hide abstract]
ABSTRACT: Stars form in dense cores of molecular clouds that are observed to be
significantly magnetized. A dynamically important magnetic field presents a
significant obstacle to the formation of protostellar disks. Recent studies
have shown that magnetic braking is strong enough to suppress the formation of
rotationally supported disks in the ideal MHD limit. Whether non-ideal MHD
effects can enable disk formation remains unsettled. We carry out a first study
on how disk formation in magnetic clouds is modified by the Hall effect, the
least explored of the three non-ideal MHD effects in star formation (the other
two being ambipolar diffusion and Ohmic dissipation). For illustrative
purposes, we consider a simplified problem of a non-self-gravitating,
magnetized envelope collapsing onto a central protostar of fixed mass. We find
that the Hall effect can spin up the inner part of the collapsing flow to
Keplerian speed, producing a rotationally supported disk. The disk is generated
through a Hall-induced magnetic torque. Disk formation occurs even when the
envelope is initially non-rotating, provided that the Hall coefficient is large
enough. When the magnetic field orientation is flipped, the direction of disk
rotation is reversed as well. The implication is that the Hall effect can in
principle produce both regularly rotating and counter-rotating disks around
protostars. We conclude that the Hall effect is an important factor to consider
in studying the angular momentum evolution of magnetized star formation in
general and disk formation in particular.
01/2011;
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Fumitaka Nakamura,
Yuhei Kamada,
Takeshi Kamazaki,
Ryohei Kawabe,
Yoshimi Kitamura,
Yoshito Shimajiri,
Takashi Tsukagoshi,
Kengo Tachihara,
Toshiya Akashi,
Kenta Azegami,
Norio Ikeda,
Yasutaka Kurono, Zhi-Yun Li,
Tomoya Miura,
Ryoichi Nishi,
and Tomofumi Umemoto
[show abstract]
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ABSTRACT: We present the results of CO (J = 3 – 2) and CO (J = 1 – 0) mapping observations toward the active cluster-forming clump, L1688, in the ρ Ophiuchi molecular cloud. From the CO (J = 3 – 2) and CO (J = 1 – 0) data cubes, we identify five outflows, whose driving sources are VLA 1623, EL 32, LFAM 26, EL 29, and IRS 44. Among the identified outflows, the most luminous outflow is the one from the prototypical Class 0 source, VLA 1623. We also discover that the EL 32 outflow located in the Oph B2 region has very extended blueshifted and redshifted lobes with wide opening angles. This outflow is most massive and has the largest momentum among the identified outflows in the CO (J = 1 – 0) map. We estimate the total energy injection rate due to the molecular outflows identified by the present and previous studies to be about 0.2 L ☉, larger than or at least comparable to the turbulence dissipation rate [(0.03 – 0.1)L ☉]. Therefore, we conclude that the protostellar outflows are likely to play a significant role in replenishing the supersonic turbulence in this clump.
The Astrophysical Journal 12/2010; 726(1):46. · 6.02 Impact Factor
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Fumitaka Nakamura,
Yuhei Kamada,
Takeshi Kamazaki,
Ryohei Kawabe,
Yoshimi Kitamura,
Yoshito Shimajiri,
Takashi Tsukagoshi,
Kengo Tachihara,
Toshiya Akashi,
Kenta Azegami,
Norio Ikeda,
Yasutaka Kurono, Zhi-Yun Li,
Tomoya Miura,
Ryoichi Nishi,
Tomofumi Umemoto
[show abstract]
[hide abstract]
ABSTRACT: We present the results of CO (J=3-2) and CO (J=1-0) mapping observations toward the active cluster forming clump, L1688, in the rho Ophiuchi molecular cloud. From the CO (J=3-2) and CO (J=1-0) data cubes, we identify five outflows, whose driving sources are VLA 1623, EL 32, LFAM 26, EL 29, and IRS 44. Among the identified outflows, the most luminous outflow is the one from the prototypical Class 0 source, VLA 1623. We also discover that the EL 32 outflow located in the Oph B2 region has very extended blueshifted and redshifted lobes with wide opening angles. This outflow is most massive and have the largest momentum among the identified outflows in the CO (J=1-0) map. We estimate the total energy injection rate due to the molecular outflows identified by the present and previous studies to be about 0.2 L_solar, larger than or at least comparable to the turbulence dissipation rate [~(0.03 - 0.1) L_solar]. Therefore, we conclude that the protostellar outflows are likely to play a significant role in replenishing the supersonic turbulence in this clump. Comment: 37 pages, 9 figures, accepted for publication in The Astrophysical Journal
10/2010;
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[hide abstract]
ABSTRACT: Stars form predominantly in clusters inside dense clumps of molecular clouds that are both turbulent and magnetized. The typical size and mass of the cluster-forming clumps are $\sim 1$ pc and $\sim 10^2 - $ 10$^3$ M$_\odot$, respectively. Here, we discuss some recent progress on numerical simulations of clustered star formation in such parsec-scale dense clumps with emphasis on the role of magnetic fields. The simulations have shown that magnetic fields tend to slow down global gravitational collapse and thus star formation, especially in the presence of protostellar outflow feedback. Even a relatively weak can retard star formation significantly, because the field is amplified by supersonic turbulence to an equipartition strength. However, in such a case, the distorted field component dominates the uniform one. In contrast, if the field is moderately strong, the uniform component remains dominant. Such a difference in the magnetic structure is observed in simulated polarization maps of dust thermal emission. Recent polarization measurements show that the field lines in nearby cluster-forming clumps are spatially well-ordered, indicative of a rather strong field. In such strongly-magnetized clumps, star formation should proceed relatively slowly; it continues for at least several global free-fall times of the parent dense clump ($t_{\rm ff}\sim $ a few $\times 10^5$ yr). Comment: 8 pages, proceedings of Computational Star Formation (IAU 270)
08/2010;
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ABSTRACT: Magnetic fields are generally expected to increase the characteristic mass of stars formed in stellar clusters, because they tend to increase the effective Jeans mass. We test this expectation using adaptive mesh refinement (AMR) magnetohydrodynamic simulations of cluster formation in turbulent magnetized clumps of molecular clouds, treating stars as accreting sink particles. We find that, contrary to the common expectation, a magnetic field of strength in the observed range decreases, rather than increases, the characteristic stellar mass. It (1) reduces the number of intermediate-mass stars that are formed through direct turbulent compression, because sub-regions of the clump with masses comparable to those of stars are typically magnetically subcritical and cannot be compressed directly into collapse, and (2) increases the number of low-mass stars that are produced from the fragmentation of dense filaments. The filaments result from mass accumulation along the field lines. In order to become magnetically supercritical and fragment, the filament must accumulate a large enough column density (proportional to the field strength), which yields a high volume density (and thus a small thermal Jeans mass) that is conducive to forming low-mass stars. We find, in addition, that the characteristic stellar mass is reduced further by outflow feedback. The conclusion is that both magnetic fields and outflow feedback are important in shaping the stellar initial mass function (IMF). Comment: Accepted to ApJL
08/2010;
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ABSTRACT: Disk formation in magnetized cloud cores is hindered by magnetic braking. Previous work has shown that for realistic levels of core magnetization, the magnetic field suppresses the formation of rotationally supported disks during the protostellar mass accretion phase of low-mass star formation both in the ideal MHD limit and in the presence of ambipolar diffusion for typical rates of cosmic ray ionization. Additional effects, such as ohmic dissipation, the Hall effect, and protostellar outflow, are needed to weaken the magnetic braking and enable the formation of persistent, rotationally supported, protostellar disks. In this paper, we first demonstrate that the classic microscopic resistivity is not large enough to enable disk formation by itself. We then experiment with a set of enhanced values for the resistivity in the range $\eta=10^{17}$--$10^{22}$ cm^2/s. We find that a value of order $10^{19}$ cm^2/s is needed to enable the formation of a 100 AU-scale Keplerian disk; the value depends somewhat on the degree of core magnetization. The required resistivity is a few orders of magnitude larger than the classic microscopic values. Whether it can be achieved naturally during protostellar collapse remains to be determined. Comment: 10 pages, 11 figures
06/2010;
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ABSTRACT: (Abridged) We investigate massive star formation in turbulent, magnetized, parsec-scale clumps of molecular clouds including protostellar outflow feedback using Enzo-based MHD simulations with accreting sink particles and effective resolution $2048^3$. We find that, in the absence of regulation by magnetic fields and outflow feedback, massive stars form readily in a turbulent, moderately condensed clump of $\sim 1,600$ solar masses, along with a cluster of hundreds of lower mass stars. The massive stars are fed at high rates by (1) transient dense filaments produced by large-scale turbulent compression at early times, and (2) by the clump-wide global collapse resulting from turbulence decay at late times. In both cases, the bulk of the massive star's mass is supplied from outside a 0.1 pc-sized "core" that surrounds the star. In our simulation, the massive star is clump-fed rather than core-fed. The need for large-scale feeding makes the massive star formation prone to regulation by outflow feedback, which directly opposes the feeding processes. The outflows reduce the mass accretion rates onto the massive stars by breaking up the dense filaments that feed the massive star formation at early times, and by collectively slowing down the global collapse that fuel the massive star formation at late times. The latter is aided by a moderate magnetic field of strength in the observed range. We conclude that the massive star formation in our simulated turbulent, magnetized, parsec-scale clump is outflow-regulated and clump-fed (ORCF for short). An important implication is that the formation of low-mass stars in a dense clump can affect the formation of massive stars in the same clump, through their outflow feedback on the clump dynamics. Comment: 36 pages, 9 figures, submitted to ApJ, contact the authors for movies of the simulations
08/2009;
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ABSTRACT: We present time-dependent, numerical simulations of the magnetocentrifugal model for jet formation, in an axisymmetric geometry, using a modification of the ZEUS3D code adapted to parallel computers. The gas is supposed cold with negligible thermal pressure throughout. The number of boundary conditions imposed on the disk surface is that necessary and sufficient to take into account information propagating upstream from the fast and Alfvén critical surfaces, avoiding overdetermination of the flow and unphysical effects, such as numerical "boundary layers" that otherwise isolate the disk from the flow and produce impulsive accelerations. It is known that open magnetic field lines can either trap or propel the gas, depending upon the inclination angle, θ, of the poloidal field to the disk normal. This inclination is free to adjust, changing from trapping to propelling when θ is larger than θc ~ 30°; however, the ejected mass flux is imposed in these simulations as a function of the radius alone. As there is a region, near the origin, where the inclination of field lines to the axis is too small to drive a centrifugal wind, we inject a thin, axial jet, expected to form electromagnetically near black holes in active galactic nuclei and Galactic superluminal sources. Rapid acceleration and collimation of the flow is generally observed when the disk field configuration is propelling. We parameterize our runs using a magnetic flux Ψ R and mass flux j = ρvz R. We show in detail the steady state of a reference run with parameters eΨ = -1/2, ej = 3/2, finding that the wind leaves the computational volume in the axial direction with an Alfvén number MA ~ 4, poloidal speed vp ~ 1.6vK0, collimated inside an angle θ ~ 11°. We show also the thrust T, energy L, torque G, and mass discharge of the outgoing wind, and we illustrate the dependence of these quantities with the exponents eΨ and ej.
The Astrophysical Journal 01/2009; 526(2):631. · 6.02 Impact Factor
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[hide abstract]
ABSTRACT: 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 long-standing theoretical problem. As a step toward addressing this problem, we perform the first large-scale 3D MHD simulations that extend to a distance ~102 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 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.
The Astrophysical Journal 12/2008; 653(1):L33. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We numerically follow the time evolution of axisymmetric outflows magnetocentrifugally driven 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 that outlined by the contour of constant density at 104 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 outward or pinch inward, depending on the conditions at the launching surface, particularly the mass flux distribution. We find that even though a dense, jetlike 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 the Hubble Space Telescope 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.
The Astrophysical Journal 12/2008; 595(2):631. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: 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. Comment: submitted to ApJ
04/2008;
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[hide abstract]
ABSTRACT: 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.
03/2007;
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[show abstract]
[hide abstract]
ABSTRACT: 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. Comment: 10 pages, 2 figures, accepted for publication in ApJL
10/2006;
