a unified model for bipolar outflows from young stars

A UNIFIED MODEL FOR BIPOLAR OUTFLOWS FROM YOUNG STARS

Hsien Shang,1, 2

Anthony Allen,1, 2

Zhi-Yun Li,2, 3

Chun-Fan Liu,1,2

Mei-Yin Chou,1,3

and Jeffrey Anderson2

Received 2006 March 30; accepted 2006 June 6

ABSTRACT

We develop a unified model for molecular outflows in star formation. The model incorporates essential featuresexpected of the primary wind, which is thought to be driven magnetocentrifugally from close to the central stellar ob-ject, and the ambient core material shaped by anisotropic magnetic support. The primary wind is modeled as a toroi-dally magnetized fast outflow moving radially away from the origin, with an angle-dependent density distribution: adense axial jet surrounded by a more tenuous wide-angle wind, as expected in the X-wind model. If dynamically sig-nificant magnetic fields are present, the star-forming core will settle faster along the field lines than across, forminga toroid-like structure. We approximate the structure with a singular isothermal toroid whose density distribution canbe obtained analytically. The interaction of the laterally stratified wind and the ambient toroid is followed using theZeus2D magnetohydrodynamics (MHD) code. We find that the lobes produced by the interaction resemble manysystematics observed in molecular outflows from very young stars, ranging from Class 0 to I sources. In particular,both the dense axial jet and the wide-angle wind participate in the wind-ambient interaction. In our model, the jet- andwind-driven pictures of molecular outflows are unified. We discuss the observational implications of the unifiedpicture, including the possibility of detecting the primary jet /wind directly.

Subject headinggs: ISM: jets and outflows — stars: pre–main-sequence — stars: winds, outflows

1. INTRODUCTION

Most, perhaps all, stars go through a phase of vigorous out-flow in their infancy. The outflow phenomena, including opticaland radio jets, neutral atomic winds, and bipolar molecular out-flows, are an integral part of the process of star formation (Lada1985; Shu et al. 1987). The molecular outflows are thought to beambient material set into motion by fast-moving jets and winds.The details of the interaction remain unclear. The current debatecenters on whether the molecular outflow is driven primarily bya narrow jet or a more wide-angled wind (Bachiller 1996). Insome sources, the jet scenario appears to fit observations better(e.g., HH 212); in others, a wide-angle wind is required (e.g.,Velusamy & Langer 1998). In this paper we seek to establish aunified picture in which both the jet and wind play a role indriving the outflow. This picture is a direct and unavoidableconsequence of the currently favored theory for jet formation:if jets are launched magnetocentrifugally from either the disksurface (Konigl & Pudritz 2000) or the X-point (Shu et al. 2000),they must first travel away from the rotation axis before be-ing collimated into a narrow beam. Perfect collimation cannotbe achieved for all streamlines, except perhaps at true infinity(Heyvaerts & Norman 1989; Shu et al. 1995). On the astro-physically interesting scales of ambient interaction, there is al-ways a fraction of the magnetocentrifugally driven outflow thatremains hardly collimated—the wide-angle character. Althoughthe exact fraction depends on the distributions of themagnetic fieldand mass loading at the launching surface (e.g., Krasnopolskyet al. 2003), both of which are uncertain, the intrinsic dual-character nature of magnetocentrifugal winds forms the theoret-ical underpinning of our unified model of molecular outflows.

Our model is an extension of the analytic model of Shu et al.(1991, hereafter SRLL91). In SRLL91 the molecular outflow isidentified with a shell created by a wind of angle-dependentmomentum distribution running into an ambient medium ofangle-dependent density distribution, conserving linear momen-tum along each radial direction. Here we use the magnetohydro-dynamics (MHD) code Zeus2D (Stone & Norman 1992) tosimulate the interaction between a magnetized wind with thestructure expected of magnetocentrifugal winds at large distancesand an anisotropic ambient medium shaped by partial magneticsupport. The simulation setup is described in x 2. The numericalresults are presented in x 3 and discussed in x 4. In x 5 we explorethe implications of our proposed unified model on millimeterand submillimeter observations of molecular outflows and theevolutionary status of outflows. In x 6 we summarize our mainconclusions.

2. THEORETICAL BACKGROUNDAND SIMULATION SETUP

Following SRLL91, we specify the angle-dependent ambientdensity distribution as

�(r; � ) ¼ a2

2�Gr 2Q(�) ð1Þ

and the wind momentum per steradian as

Mwvw4�

P(�); ð2Þ

where a is the isothermal sound speed, � ¼ cos � (� is the polarangle), and Mw and vw are the mass loss rate and the velocity ofthe wind, respectively. The dimensionless functions for the an-gular dependences of ambient density Q(�) and wind momen-tum P(�) are determined using theories of the ambient mediumand magnetocentrifugal wind.

1 Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box23-141, Taipei 106, Taiwan; [email protected].

2 Theoretical Institute for Advanced Research in Astrophysics (TIARA),Academia Sinica, and National Tsing Hua University, Taiwan.

3 Department of Astronomy, University of Virginia, P.O. Box 3818,Charlottesville, VA 22903.

845

The Astrophysical Journal, 649:845–855, 2006 October 1

# 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.

2.1. Magnetocentrifugal Winds

The evolution of magnetocentrifugal winds is governed by theset of ideal magnetohydrodynamic equations:

@�

@tþ: = (�v) ¼ 0; ð3Þ

�@v

@tþ:

jvj2

2

!þ (: < v) < v

" #

¼ �:p� �:V þ 1

4�: < Bð Þ < B; ð4Þ

�D

Dt

e

�¼ �p: = vþ �� ; ð5Þ

@B

@t¼ : < (v < B); ð6Þ

with the magnetic field B satisfying the constraint

: = B ¼ 0: ð7Þ

The variables �, v, p, and e represent the density, velocity in theinertia frame, gas pressure, and internal energy density of thefluid, respectively. The notationD/Dt ¼ @ /@t þ v = : is the sub-stantial derivative. The heating and cooling terms inside the fluidelement are � and �, respectively, and V is the gravitational po-tential. The sound speed is given as a2 ¼ �p/�, where � is theratio of specific heats. In the regime of a very cold flow, a � 0,and the gas pressure p and the internal energy e are negligible,as they are for the case of X-wind (Shu et al. 1994a, 1994b). Weassume axial symmetry and adopt an isothermal equation of state.The time evolution of a coldmagnetocentrifugalwind is advancednumerically as a two-dimensional problem in cylindrical coor-dinates ($, z) using Zeus2D (Stone & Norman 1992).

At large distances well beyond the small region of launch, thewind equations can be simplified. Once the flow makes the fastmagnetosonic transition, its magnetic field becomes dominatedby the toroidal component and its velocity field by the poloidalcomponent. In this regime, appropriate for our study of wind-ambient interaction, we have the approximate behaviors v ¼v$ e$þ vz ez and B ¼ Be� for a wind that has reached steadystate. The simplified MHD equations admit a family of exactsteady state solutions (see Li & Shu 1996a and eq. [10] below),which are used to set the initial and boundary conditions for ourZeus2D-based time-dependent simulations.

2.2. Asymptotic Wind Structure

The salient features of magnetocentrifugal outflows at largedistances can be captured without numerical simulation usingasymptotic analysis. Here we focus on the asymptotic theory ofShu et al. (1995) for X-winds; similar results are expected forwinds driven from a small region (say inside 1 AU) in the innerpart of the accretion disk (Krasnopolsky et al. 2003). The as-ymptotic behaviors of magnetocentrifugal winds form the basesof solutions for the simplified MHD equations.

At large distances from the launching region, Shu et al. (1995)show that the density and toroidal magnetic field distributions inthe X-wind approach

� ! C

�$ 2; B� ! � C

$; ð8Þ

where $ ¼ r sin � is the cylindrical radius from the axis and Cis a quantity that slowly varies logarithmically with distance. Itis proportional to the total current carried in one hemisphere ofthe wind. The quantity � is the (conserved) ratio of magnetic-to-mass flux along a streamline in the wind. The strong, nearlycylindrical, density stratification expressed in equation (3) formsthe basis for interpreting the optical jet as the densest part of awide-angle wind (Shang et al. 1998, 2002). The terminal windvelocity asymptotes to

vw ! (2J � 3� 2C� )1=2VK; ð9Þ

where J is the (conserved) specific angular momentum along astreamline and VK the Keplerian speed at the X-point. The maxi-mum theoretical value of thewind velocity reaches (2J � 3)1

=2VK ,when the contribution of Maxwell torque becomes negligible inthe wind. At large (cylindrical ) distances, where most of the am-bient interaction takes place, the poloidal component of the mag-netic field becomes much smaller than the toroidal component. Itis ignored in our simulation. The toroidally dominated wind maybe stablized by an axial magnetic field coming from the stellarsurface (Shu et al. 1995).For illustrative purposes, we consider the limit at which the

quantities C, �, and vw are constant for all streamlines. The re-sulting asymptotic wind solution is given by

� ($) ¼ D0

$2; B�($) ¼ B0

$; vw ¼ V0; ð10Þ

whereD0,B0, andV0 are constants to be determined. The choicesof their values are guided by theory and observations. The pri-mary jet /wind that drives a molecular outflow is difficult toobserve directly (see, however, x 5), particularly during theembedded phase of star formation, when the molecular outflowis most prominent. The wind speed during this phase is probablyless than that of the optical jet observed during the later revealedphase (typically a few hundred km s�1; Reipurth & Bally 2001),because the central star is less massive and the wind is probablymore heavily loaded, both of which tend to reduce the speed ofa magnetocentrifugal wind. For this reason, we adopt a roundnumber V0 ¼ 50 km s�1 for the protostellar wind.The density constant D0 is related to the wind mass loss rate

Mw through the expression

Mw ¼ 2�D0V0 ln1þ cos �11� cos �1

��; ð11Þ

where �1 is the half-angle subtended by the first grid cell next tothe axis. This limiting angle is used to represent the physicaleffect in which, in the X-wind picture, the magnetocentrifugallydriven wind encloses a light coronal wind, which eliminates thedensity singularity on the axis implied by the idealized asymp-totic density distribution in equation (10). For our standardmodel,we adopt a relatively largemass flux Mw ¼ 10�6 M� yr�1, whichmay be appropriate for the deeply embedded phasewhen themassaccretion rate is high.We have explored other choices of Mw andhave found little qualitative difference.The magnetic field strength in the wind is currently not mea-

surable but can be constrained from theory. In our formulation,themagnetic field constantB0 is related to theAlfvenMach num-ber MA through

B0 ¼ffiffiffiffiffiffiD0

4�

rV0

MA

: ð12Þ

SHANG ET AL.846 Vol. 649

In the X-wind theory, the Mach number asymptotes to MA ¼½(2 J �3�2C� )/C� � 1=2, with J and � being the average specificangular momentum andmagnetic-to-mass flux ratio, respectively.For the streamlines and isodensity contours that were presented inShang et al. (1998, 2002), J � 3:8, � � 1:2, and C � 0:1 for therange 50–500AU in distance. This combination gives an effectiveMach number MA � 6, which serves as our fiducial value. It isconsistent with the values of a few determined from several large-scale numerical simulations of the magnetocentrifugal winds upto 100 AU-scale distances (Krasnopolsky et al. 2003).

2.3. Singular Isothermal Toroids

The jet /wind coming from near a forming star should first in-teract with the protostellar envelope during the embedded phase.The envelope is part of the star-forming core that remains to beaccreted onto the central object (disk plus star). Its structure isdetermined by the processes involved in the formation and col-lapse of the dense core. If magnetic fields are dynamically im-portant in core formation, as envisioned in the ‘‘standard’’ theoryof isolated low-mass star formation (Shu et al. 1987;Mouschovias& Ciolek 1999), the mass distribution in the core should be an-isotropic, because matter can settle more easily along field linesthan across.

The magnetically induced anisotropic mass distribution is il-lustrated clearly in the self-similar model of Li & Shu (1996b),where the radial and angular dependences of the density �(r; � )and magnetic flux�(r; � ) are assumed to take the following sep-arable form:

�(r; � ) ¼ a2

2�Gr 2R(� ); �(r; � ) ¼ 4�a2r

G1=2�(� ): ð13Þ

The dimensionless angular functions of the mass density R(� )and magnetic flux �(� ) are determined by solving two coupledordinary differential equations, subject to appropriate boundaryconditions (see Li & Shu 1996b for details). It turns out thatthe solutions R(� ) and �(� ) describe a linear sequence of singu-lar isothermal toroids characterized by a single parameter H0,which represents the fractional overdensity supported by themagnetic field above that supported by thermal pressure alone.As H0 increases, the toroid becomes increasingly flattened, asshown in Figure 1. As in Li & Shu (1996b), we sometimes usethe number n ¼ 4H0 to label the sequence. In the limit n ! 0, thetoroid becomes a singular isothermal sphere. As n ! 1, thetoroid flattens into a thin sheet. We adopt the function R(� ) forthe angular distribution of the ambient density [i.e., the quantityQ(�) in eq. (1)]. For simplicity, we ignore the magnetic field thatthreads the ambient medium. The ambient field is expected to aidin collimating the molecular outflow. In a future investigation, we

plan to explore this and other effects, including the protostellarcollapse, which is expected to modify the inner portion of theisothermal toroid (Allen et al. 2003, hereafter ALS03; see x 5.4for further discussion).

2.4. Simulation Setup

We carry out axisymmetric simulations using Zeus2D in acylindrical coordinate ($; �; z) system (Stone & Norman 1992).A square computation box is adopted in the $-z plane, with $going from 0 to $max and z from 0 to zmax. We choose $max ¼zmax ¼ 1018 cm, which is well beyond the wind-ambient inter-action region. We initialize the density distribution everywhereinside the box using the toroid solutions (see Fig. 1). Inside asmall box of 1 AU ; 1 AU near the origin, the toroid solution isreplaced with the toroidally magnetized, asymptotic wind so-lution specified by equation (10). The wind solution is reini-tialized at each time step, so that it is kept the same at all times.To cover a wide range in radius, we use a logarithmic grid of500 AU ; 500 AU for the entire computational domain, with theinnermost 20 AU ; 20 AU grid allocated to the small wind in-jection region. The boundary conditions on the axis and equatorare set by symmetry. On the two remaining boundaries, standardoutflow conditions are imposed. For simplicity, we extend theisothermal equation of state of the toroid to the wind and the in-teraction region; this simplification is justified because the in-teraction region is expected to be radiative for typical outflow

Fig. 1.—Examples of singular isothermal toroids used in this paper, corresponding to n ¼ 1, 2, 4, and 6 (left to right). Isodensity contours (thick lines) and magneticfield lines (thin lines) are plotted.

Fig. 2.—Snapshot at 1000 yr of a toroidally magnetized wind running into anear vacuum, after a nearly steady state has been reached, for a mass-loss rate of1 ; 10�6 M� yr�1 and an Alfven Mach number of 6. Note the cylindrical strat-ification of density, as predicted by the free-wind solution.

WIND SIMULATION 847No. 2, 2006

parameters (Hollenbach 1997), and the thermal pressure playslittle dynamical role in the fast-moving, magnetized cold wind.

Before simulating the wind-ambient interaction, we did testruns to see how free winds propagate without external confine-ment. The example is shown in Figure 2, for MA ¼ 6, after thetenuous ambient medium has already been swept outside thesimulation box and a nearly steady state has been reached. Notethat the density is cylindrically stratified, except for a slightpinch near the axis. The fact that the large-scale numerically ob-tained wind solution outside the injection box is close to the ana-lytic solution indicates that the latter satisfies the full MHDwindequations to a good approximation. This is one of the two windsto be injected into the toroids shown in Figure 1 to produce mo-lecular outflows in the next section; the other has a much weakertoroidal field corresponding to MA ¼ 600.

3. NUMERICAL RESULTS

We seek to capture the essential features of molecular outflowswith a model that contains as few free parameters as possiblethrough judicious use of physically motivated idealizations. Ouridealized model is characterized by two dimensionless parame-

ters: the Alfven Mach numberMA, which determines the degreeof wind magnetization, and the parameter n ¼ 4H0, which de-termines the degree of the magnetically induced anisotropy inthe ambient mass distribution. The former completely specifiesthe free wind, and the latter the ambient environment for windinteraction; both parameters characterize the dynamical effectsof the magnetic field, one on the hydrostatic equilibrium of theambient medium, and the other on the density stratification of theprimary wind.For our standard simulations, we fix the Mach number MA to

the fiducial value 6 and vary the ambient parameter n from 1 to 6.The goal here is to illustrate how, for a given driving wind, themorphology and kinematics of molecular outflows change as theambient medium is flattened more and more by a magnetic fieldof increasing strength. For illustrative purposes, we choose fourrepresentative values, n ¼ 1, 2, 4, and 6. The results are shown inFigure 3.In Figure 3 we plot the density distributions of the four cases at

two representative times, when the wind has propagated in theambient toroid for t ¼ 102 and 103 yr. The most striking feature,common to all cases, is the apparent two-component density

Fig. 3.—Snapshots of density structures at 100 (top) and 1000 (bottom) yr for our standard choices of mass-loss rate (1 ; 10�6 M� yr�1) and AlfvenMach number (6)for n ¼ 1, 2, 4, and 6 (left to right). The color shows density variation in a logarithmic scale.


structure: a dense axial jet, which is part of the primary wind, anda dense, closed shell, which encases the jet. Between the jet andshell lies the wide-angle wind. In our model, the jet is barelyslowed down by ambient interaction, because it is the densestpart of the wind and is running into the least dense part of theambient toroid. The jet and part of the wind right next to it areessentially coasting freely. The coasting is particularly clear inthe higher n cases (4 and 6), where the polar ‘‘vacuum’’ region iswider. The fast-moving wind is denser than the ambient toroid inthis region and can propagate along the axis with little imped-iment. The part of the wind closer to the equator has a moredifficult time propagating freely. It is weaker (more tenuous) tobegin with and is running into the denser part of the ambienttoroid. Nevertheless, it is this interaction that is responsible forproducing the bulk of molecular outflow (in mass) in our model.The interaction of the denser part near the axis with the ambientmedium, on the other hand, is largely responsible for the volumeswept out by the primary wind, particularly for the lower n cases.Both parts play a role in shaping the molecular outflow.

As one would expect intuitively, the wind-driven shell iswider for more flattened ambient toroids. It is, however, some-

what surprising that for a moderately flattened toroid of n ¼ 2(orH0 ¼ 0:5, which produces a core elongation that is consistentwith observations; Li & Shu 1996b, Myers et al. 1991), the shellis highly collimated. Even in the n ¼ 4 case where the toroid isstrongly flattened, the shell is well collimated. This apparent‘‘mismatch’’ of degrees of toroid flattening and shell collimationis an indication that the shell collimation is not due to the ex-ternal confinement alone; it is also shaped by the intrinsic densitystratification in the wind, which is conducive to shell collima-tion. The existence of such ‘‘jetlike’’ molecular outflows forreasonable wind and toroid parameters is the most importantquantitative result of our idealized model. It provides a basis forthe unified picture of molecular outflows to be discussed in depthin x 5.

The swept-up shells resemble each other in shape at differenttimes, as evidenced by comparing the top and bottom panels ofFigure 3. The resemblance is to be expected, since both the pri-mary wind and the ambient medium are scale-free. Indeed, theirinteraction should be strictly self-similar in principle. In practice,the finite sizes of the (square) free-wind zone and computationalgrid introduce deviations from strict self-similarity. The similarities

Fig. 4.—Snapshots of density (top) and velocity (bottom) structures for the n ¼ 4 case at times 30, 100, 300, and 1000 yr (left to right). The color shows densityvariation in a logarithmic scale, and the units for velocity are km s�1.


and differences at different times are illustrated more clearly inFigure 4, where the density and velocity distributions for then ¼ 4 case are shown at four representative times. The slowermoving walls of 10–20 km s�1 in the ‘‘classical’’ sense, whichare the signature of a wide-angle wind, are readily seen along theouter lining shells of the lobes. Although the shapes of the outershell remain similar, the density and velocity fields of the windmaterial enclosed inside the shell show noticeable variations: thewind appears more perturbed at later times, particularly in thelower part of the flow close to the origin. Why this is the case isunclear. It may be related to the presence of the toroidal magneticfield, which may drive a (weak) pinch instability that takes arelatively long time to grow to a nonlinear amplitude.

The moderately strong (MA ¼ 6) toroidal field is dynami-cally significant in another regard. It keeps the dense ‘‘jet’’ partof the primary wind well collimated despite interaction with theambient medium. Without this field, the jet would spread outgradually and eventually lose its identity inside the shell, asshown in Figure 5, where the Mach number is increased (and thefield strength reduced correspondingly) by a factor of 100. Theupper lobes of the strong magnetized cases also appear moreelongated. The magnetic collimation has implications on high-resolution observations of the jetlike molecular outflow HH 211(see x 5).

To summarize, we have demonstrated that magnetocentrifugalwinds of asymptotic cylindrical density stratification can pro-duce dense shells of varying degrees of collimation, dependingon the degree of magnetically induced flattening in the ambientmedium. We now discuss these results and their observationalimplications in the next two sections.

4. DISCUSSION

4.1. Connection to the SRLL91 Model

In the shell model of SRLL91, the momentum flux of the pri-mary wind is assumed to be exactly balanced by the ram pressureof the swept-up ambient medium along each radial direction. Theresulting angular distribution of the speed, and thus the shape, ofthe shell is determined by a bipolarity function B(�) / (P/Q) 1

=2.Li & Shu (1996b) applied this formalism to the case where theambient density distribution Q corresponds to a singular iso-thermal toroid and the momentum distribution to a cylindricallystratified wind, as in this paper. In particular, they determinedanalytically the shell shape for the n ¼ 2 case. It agrees with ournumerically determined shape for the same case rather well inthe lower part of the shell (from the equator up to the location ofmaximum width), where most of the swept-up material resides.In the upper part, our numerically determined shell closes backonto the axis, whereas the analytic shell given by the bipolarityfunction remains open to infinity. The mathematical reason forthe latter behavior is that, as the axis is approached, the dimen-sionless momentum P goes to infinity, while the densityQ goesto zero, yielding a divergent bipolarity function B. Physically, aswept-up shell of vanishingly small mass must move infinitelyfast in order to absorb the infinite amount of wind momentumper steradian on the axis. However, the shell velocity must bebounded by the speed of the wind pushing the shell from be-hind. As the shell speed approaches the wind speed (from be-low), the approximations used to derive the shell shape in termsof the bipolarity function alone start to break down. Semianalyticsolutions without these approximations are possible and will be

Fig. 5.—Outflows driven by two primary winds of different degrees of magnetization. Note that the axis jet in the more strongly magnetized wind with MA ¼ 6remains collimated, whereas that in the more weakly magnetized case (MA ¼ 600) becomes more decollimated.


presented elsewhere. They show that the shell is closed, withthe equatorial part bounded mostly by the ram pressure of theswept-up ambient material, and the polar part essentially coast-ing with the primary wind, as we found numerically. In addition,our numerical model treats self-consistently the back-reaction ofthe ambient interaction on the primary wind, which is not ac-counted for in the analytic model. Nevertheless, the analyticmodel captures the essential feature of the part of the molecularoutflow driven by the wide-angle character of the wind.

4.2. Structure of Molecular Outflows

In this work, the production of both jetlike and broad outflowlobes is achieved by one single wide-anglewind of stratified den-sity and momentum. There is no need to invoke different driversfor outflows of different degrees of collimation. When properchoices of P and Q are made, the basic shell model of SRLL91is applicable in general and not limited to ones with wide-opencavities as sometimes assumed (Bachiller & Tafalla 1999). Wediscuss how the old divergent pictures of jet-driven and wind-blown mechanisms for molecular outflows can be unified inview of our simulations.

The broad, classical outflows of slow-moving shells form bywind/jet interaction with toroids of wide axial openings (high-nvalues). The primary winds/jets are little confined, except in theequatorial region. They produce wide-opened shells that can eas-ily be recognized as wind-blown cavities near the bases of the out-flows. The range of velocities predicted for the limb-brightenedshells falls within the characteristic velocities associated withCO molecular outflows. In contrast to a pure wide-angle windscenario, the densest part of the primary wind along the axis, ifproperly lit up in optical or infrared, stands out as a jet bisect-ing the parabolically or conically shaped cavities surroundingthe winds. In sources of classical molecular outflows, the jetsmay be mostly atomic and remain undetected in molecular COtransitions.

The narrow, jetlike outflows, on the other hand, form bywind/jet interaction with toroids of narrow axial openings ( low-n val-ues). While the dense, jet part of the primary outflow can prop-agate just as easily as before, the tenuous wide-angle flow has aharder time pushing the ambient toroid material sideways. Theresulting narrower width of the swept-up material gives the mo-lecular outflowan overall jetlike appearance. Suchwell-collimatedoutflows are traditionally interpreted in terms of ambient mate-rial set into motion by a pure jet. In our unified picture, they aredriven by the same underlying (anisotropic, MHD) primarywind as the classical outflows, except that the lateral expansionof the wind is more constrained. The elongated, jetlike outershells that contain the swept-up ambient material enclose ontheir axes ‘‘real,’’ fast-moving jets—the densest part of the pri-mary wind—which, under the right conditions, may be observedby high-density and high-temperature molecular lines within thehigh-speed molecular bullets or a continuous jet. If observed ata high enough resolution, the high-speed, densest jet core can bespatially separated from the lower velocity shells, as appears tobe the case in HH 211 (see x 5.4 below).

Besides the overall outflow morphology, position-velocitydiagrams provide important model constraints, particularly onoutflow kinematics. For example, Lee et al. (2000, 2001, 2002)carried out a systematic study of the features expected of jet-driven bow shocks and broad wind-blown shells using analyticforms (SRLL91; Ostriker et al. 2001) and hydrodynamic simu-lations. They compared the model predictions with observationsin CO (J ¼ 1 0). They concluded that the predicted features foreither model can be found in some sources and sometimes both

in the same source. In other cases, there is no clear evidence forthe features expected of either model. Episodic mass ejectionmay have complicated the identification. Although the situationis not all that clear at present, it is likely that both a jet and a wide-angle wind play a role in producing the observed kinematicfeatures. Models such as ours that involve jet and wind charac-ters simultaneously have the potential to accommodate the diverseobservations in a single framework. We will address this issue ina subsequent communication.

4.3. Comparison with Other Works and Future Refinements

Gardiner et al. (2003) simulated the propagation of a wide-angle wind into a collapsing sheet (Hartmann et al. 1996). Forthewide-anglewind, they adopted a smoothed-out density profile,r 2� / �1=2/(� sin2�þ cos2� ), and rB� / sin �½�1=2 /(� sin2� þcos2� )� for the magnetic field (see their eqs. [2] and [3]). Theyinvestigated the � ¼ 1, 9, and 100 cases. The corresponding den-sity contours in the wind appear spherical (� ¼ 1) or egg shaped(� ¼ 100) and are not cylindrically stratified as in our simu-lations. The solution of a collapsing rotating sheet obtained fromHartman et al. (1996) differs substantially from the toroid solu-tions used in this paper. They also obtained a ‘‘jetlike’’ feature; itis formed, however, in the postshock region, perhaps due to ef-fects of shock focusing and amplification.

Cunningham et al. (2005) extended the work of Gardiner et al.(2003) using adaptive mesh refinement (AMR) techniques. Theyalso included molecular, ionic, and atomic species, cooling func-tions, and molecule recombination and dissociation. They wereable to partially resolve the strongly cooling, shocked layersaround the outflow lobes. They found that the shocked wind andambient medium are not completely mixed along the walls of thewind-blown cavity but noted that this result might be affectedby their limited resolution, despite the fact that AMR was used.They designed their simulations to explain the wide outflowsobserved in the BN/KL regions, including maser spots. They didnot explore the formation of the jetlike outflow shells that arecharacteristic of some of our simulations.

We have purposely kept our idealized model as simple as pos-sible to demonstrate the unifying concept that magnetocentrif-ugal winds can produce molecular outflows of varying degreesof collimation, including those with a jetlike appearance. For adetailed comparison with observations, several refinements aredesirable. If the ambient density distribution is indeed shapedby support from an ordered poloidal magnetic field, as assumedhere, we would expect the field to resist lateral expansion andthus aid in shell collimation. Gravitational collapse of the mag-netized toroid, on the other hand, would create a wider evacu-ated polar region near the central object (ALS03), and thus tendto widen the shell at the base. This tendency may be countered,however, by dynamic infall, which tends to strengthen the shellconfinement. In addition, the primary wind may be episodic andcontain a poloidal, as well as a toroidal, magnetic field, particu-larly very close to the axis. It can bemodeledmore self-consistentlyusing results from simulations that follow the wind evolutionfrom the launching surface to large observable distances (e.g.,Krasnopolsky et al. 2003). A potential drawback of our ideal-ized model is the lack of the large bow shocks that are sometimesobserved. They may correspond to either internal or terminatingshocks of the primary wind. For internal shocks, time-dependentwind solutions will be required. The absence of prominent termi-nating bow shocks is due, at least in part, to the zero density onthe axis in the singular isothermal toroid solutions. It is desir-able to replace these idealized solutions with those obtained self-consistently from calculations of magnetized core formation and


evolution. We will explore these and other refinements in futureinvestigations.

5. IMPLICATION FOR OBSERVATIONS

5.1. Extremely High-Velocity Outflows

Some Class 0 outflows, such as L1448-mm, Orion B, HH 211,SVS 13B, and HH 212, show extremely high velocity (EHV)components along the axes. When present, they can carry mo-menta an order of magnitude greater than the broader, ‘‘classi-cal’’ components. Wind-blown shells typically have speeds oftens of km s�1, well below the deprojected speeds inferred for theEHV components, which approach those of optical jets (Bachiller& Tafalla 1999). In our unified model, the EHV component ispart of the integrated picture. These fast-moving components canconceivably be produced by the axial portion of the primary flow,which has not been slowed down much by ambient interaction.The densest innermost jet can escape more or less freely, coatedperhaps with some (partially) swept-up material. They move atspeeds approaching the true driving jet. The highest velocitycomponents are cylindrically stratified in density.

Some support for this interpretation comes from the fact thatsome EHV components consist of discrete ‘‘bullets,’’ some ofwhich are located symmetricallywith respect to the central source.The symmetry implies that the pairs may be ejected in singleevents in opposite directions (Bachiller & Tafalla 1999). A po-tential problem is that the bullets may expand and dissolve due tointernal velocity dispersion as they propagate outward from thecentral object, as pointed out by Richer et al. (1992) for the spe-cific case of bullets in L1448, unless the bullets are confined. Thestrong toroidal field within the inner shells, as shown in our sim-ulation, can naturally collimate the axial portion of the flow as inthe naked jet. They retain their bullet-like appearance along theaxes when the underlying wind is magnetocentrifugally driven.

The discrete nature of CO bullets may be a reflection of thestrong variability of the central engine. The variability timescaleswould be on the order of 102–103 yr, which are not dissimilarto those inferred for discrete HH objects in older, more opticallyrevealed sources. The similarity is not surprising in our picture:both are produced by the (time variable) strongly density-stratifiedmedia confined bymagnetic stresses. In deeply embedded sources,molecules may form with significant abundances in the densest‘‘jet’’ (Glassgold et al. 1989, 1991). High mass-loss rates areexpected of the primary outflows during this phase, because ofactive mass accretion. If the jetlike component is dense enough[n(H2) � 105 106 cm�3], it may be observed as part of the EHVcomponents (or bullets) in CO and even in SiO molecules. De-tections of high-J SiO transitions up to J ¼ 11 10 in L1448-mmandL1157-mmby IRAMand the JamesClerkMaxwell Telescope(JCMT; Nisini et al. 2005) reveal dense and warm conditions sim-ilar to the protostellar outflow HH 211 (see x 5.4). The detectionis also consistent with strong molecule formation behind strongshocks along the axial region (Schilke et al. 1997). As a sourceevolves, the CO and SiO emission weaken, perhaps as a result ofthe expected decline in wind strength. Eventually, the true denseinner jet may be observed in optical or near-IR as a more or lesscontinuous HH jet or discrete HH objects.

5.2. Lighting Up the Remnant Cavities

Bright optical jets or Herbig-Haro knots of emission lines areamong the most spectacular features around T Tauri stars. Bythis time, the early cloud material has been mostly cleared, andthe link to the early outflow activities has weakened. While thehighly collimated morphology argues for jet-driven origins, the

kinematics of the optical jets clearly depicts the dual nature of thedriver. The broad velocity dispersion at the base of a jet clearlysupports the existence of a wide-angle wind. Direct detection ofsuch a component has not been possible, however.The wide-angle wind can be made visible through interac-

tion with a flared disk or a flattened envelope. An oblique shockforms at the interface, which may excite line emissions, depend-ing on the strengths of the shocks. Li & Shu (1996a) investigatedsuch interaction with a flared disk and concluded that the shockedstreamlines skimming through the surface of a flared disk maybe too weak for observable signatures on the large scale (k10–100 AU). Large-scale oval-shaped optical and near-infrared emis-sions surrounding some optical jets, whose jet signature is obvi-ous, may in fact be the products of direct interaction of wide-anglewinds with the remnant envelopes. The phenomena in opticallines [S ii] and H have only been seen in L1551 IRS 5 (Daviset al. 1995) and in FS Tau B (Eisloffel & Mundt 1998) on thescales of several hundred to 1000 AU. Smaller but similar struc-tures are seen around T Tau (Robberto et al. 1995; Herbst et al.1996, 1997) as thick nebulosity. In particular, shocked fluores-cent H2 has been detected that traces cavity-like structures nearthe base of a poorly collimated outflow down to the 100AU levelin T Tau (Saucedo et al. 2003). Since a relatively high tempera-ture of�103 K is required for the emission, the emitting region isexpected to be strongly shocked, most likely by a low-density,invisible, wide-angle wind (Saucedo et al. 2003). Takami et al.(2004) detected a slow, warmmolecular H2 outflow fromDGTauon the�40–80AU scale,with a peak velocity of�15–20 km s�1.The number density inferred for H2 is high, with n(H2)k4 ;104 cm�3. They interpret it as a separate disk wind from someregion farther out. Since DG Tau is known to have a remnantenvelope, it is equally possible that the H2 emission comesfrom the interaction region between the envelope and a wide-anglemagnetocentrifugal wind. The famousmicrojet of DG Taumay come from the denser axial part of the same wind. Detailedexcitation mechanisms are required to quantify the optical andIR emission expected from the wind-envelope interaction.

5.3. Protostellar Outflow HH 211

HH 211 is the prototype of the class of jetlike molecularoutflows (Gueth & Guilloteau 1999). It has a dynamical ageof �750 yr, which is among the youngest for Class 0 sources(Bachiller 1996). The outflow appears highly collimated in bothlow- and high- velocity CO (J ¼ 1 0 and 2 1) emission. Thelow-velocity component seems to coincide with the H2 emission,outlining a cavity surrounding the high-velocity component. Themorphology of CO emission is suggestive of a molecular out-flow driven by a jet. However, the position-velocity diagrams ofthe source show parabolic features (Hirano et al. 2006) that arecharacteristic of thin shells swept up by wide-angle winds (Leeet al. 2001). We suggest that HH 211 is an example of the earliestphase of a molecular outflow driven by a stratified magneto-centrifugal wind.SiO emissions from high rotational transitions J ¼ 5 4 (Hirano

et al. 2006) and J ¼ 8 7 (Palau et al. 2006) are detected us-ing the Submillimeter Array (SMA). They reveal structures notdetected by earlier observations of lower transition J ¼ 1 0(Chandler & Richer 2001) and CO J ¼ 2 1 and 1 0 (Gueth &Guilloteau 1999). Near the base of the outflow, the SiO exhibits abroad velocity dispersion. The dispersion narrows down as onemoves outwards. Such a pattern is sometimes observed in nakedoptical jets and is explained as a collimating jet in X-wind theory(Shang et al. 1998). This unique kinematic signature providesa glimpse into the primary wind that drives the outflow.


The knotty SiO emission shows a dual character. Beyond thefirst bright knot, the velocity increases linearly with distance, asobserved in CO transitions J ¼ 3 2, 2 1, and 1 0. There is asystematic shift in velocity between the SiO and CO EHVcomponents, with SiOmoving faster byk5 km s�1 (Hirano et al.2006; Palau et al. 2006). The velocity shift seems larger in J ¼8 7 than in J ¼ 5 4. The high density (k106 cm�3) and warmgas temperature (k140 K) inferred from the line ratios of SiOknots point to an origin for the SiO-emitting region that is dif-ferent from that of the cold, low-velocity CO shell, which weinterpret as ambient swept-up material. The density-stratifiedstructures within the outflow lobe are identified with the highestvelocities. Together with the images in CO and SiO, such fastestmoving and highest density emission seems to come from the innerpart of the jetlike lobes, which is unresolved at current resolution.

This jet-shell nature of the dense gas distribution is an es-sential feature of our unified model of molecular outflows. TheSiO and CO emissions combined clearly depict a fast-moving jetand an elongated, jetlike outer shell that consists of swept-upambient material. With the cylindrically stratified momentuminput, the velocity stratification finds a natural explanation in theshells of structures within the lobes. The spectacular HH 211appears to have the most detailed correspondence between themodel and observations. In at least one other source, HH 212, theflattened NH3 core, elongated CO shell, and highly collimatedH2 jet (Lee et al. 2006) also strongly resemble the prominent fea-tures shown in Figure 3.

It is very promising that SiO molecules may exist in situ in theprimary fast MHD inner-disk wind. Glassgold et al. (1989,1991)showed SiO molecules can form, together with CO, in signifi-cant abundances in a fast wind from a protostar. When the mass-loss rates are sufficiently high, the conversion into molecules canreach completion for CO and SiO. Existing clues seem to supportsuch potential for deeply embedded protostars. Sources like HH 211may have extremely dense primary winds of k10�6 M� yr�1. Itcould serve as the dense environment for the complete conversionof SiOmolecules near the base of the outflow. The only puzzlingaspect is why the CO low-J emission is missing and undetectedfrom the primary jet while multiple J levels of SiO are emittingstrongly. One possibility might be that population inversion oc-curs in the warm-temperature regime appropriate for SiO, andthe lowest J levels of CO are underpopulated. It would be inter-esting to search for high-J transitions for CO in HH 211. Moredetailed modeling of the kinematics seen in HH 211 with simplechemistry is required to make definite predictions.

5.4. A Heuristic Sequence in Time

The protostellar envelope is the immediate environment that aprimary protostellar jet /wind runs into on the way out. The en-velope mass distribution plays a role in determining the shape ofthe interaction region—the molecular outflow—particularly nearthe base. The distribution evolves on the timescale of envelopecollapse. Specific examples of envelope evolution can be foundin ALS03, who numerically followed the collapse of magnetizedsingular isothermal toroids from the inside out. The collapsingmaterial settles preferentially along field lines into a dense equa-torial pseudodisk, which grows in size as the collapse proceeds.Asmore and morematerial is channeled into the growing pseudo-disk, an increasingly wider evacuated region is produced aroundthe magnetic pole. The expected widening of the polar region intime may be represented, in a heuristic way, by a sequence of(static) toroids with an increasing degree of flattening (i.e., in-creasing value of n; see Fig. 1). From this angle, the snapshotsshown in Figure 3 for the structures produced by a single primary

wind in ambient toroids of increasing flatness may be viewed asa time sequence of the primary wind expanding into a single, col-lapsing envelope (F. Shu 2004, private communication). Theyreveal a clear trend: the wind-blown outflow lobe widens withtime, which is in agreement with observations (e.g., Bachiller &Tafalla 1999; Arce & Sargent 2005, 2006).

Early Class 0 outflows, such as HH 211 and L1448-mm, havejetlike morphologies. Their lobes can be modeled by interactionwith toroids of n between 1 and 2.More evolved Class 0 sources,such as L1157-mm, start to appear more wind-blown, and toroidswith n between 2 and 4 may be needed to describe their shapes.Wide-open cavities seen around Class I sources can be accom-modated bymore flattened toroids of perhaps n � 6. The remnantoutflows from transition objects to Class II sources require highlyflattened envelopes, corresponding to n36. In the context ofour heuristic time sequence linked by toroids of different degreesof flattening, we suggest the following evolutionary sequence:HH 211 and L1448-mm ! L1157-mm and BHR 71 ! BB35,L483, and L1527 ! L1551 IRS 5 ! HL Tau and HH 30. Thisnaturally follows the evolutionary sequence of young stars fromClass 0 to I to II.

The heuristic time sequence is consistent with the empiri-cal time sequence suggested by Bachiller & Tafalla (1999). Or-dered by their apparent dynamical ages, HH 211 (�750 yr) andL1448-mm (�3500 yr; Bachiller et al. 1990) are the youngestClass 0 sources known. L1157-mm (�15000 yr; Bachiller et al.2001) is older than both HH 211 and L1448-mm, which seems tobe consistent with the analysis by their respective opening of thecavities. The older (105–106 yr) outflows are driven by Class Isources, such as L43, L1551, and B5 (Richer et al. 2000) andMonR2 and NGC 2071 (Bachiller & Tafalla 1999). They appearcharacteristically with low-velocity emissions from wider COcavities (with opening angles close to 90�) and are visible inoptical or 2 �m wavelengths. These older sources have beenextensively interpreted within the original SRLL91 framework.Large episodic ejections could reset the clock with new genera-tions of shock structures for the identification of the dynamicages, and the age estimation based on noticeable shock tips maybe the lower limit of the system. The excavated surroundings,however, are irreversible once opened by the natural flatteningof gravitational collapse or dispersed by winds.

In addition, SiO molecules have been found to trace the evo-lutionary stages of outflows within the youngest Class 0 sources(Bachiller & Tafalla 1999). The same sequence identified by theapparent collimation factors of the lobes also follows the mor-phological associations with the SiO outflows. The SiO may bedetected preferentially in the youngest Class 0 sources with thehighest SiO abundances. HH 211 and L1448-mm are known tocontain SiO jets and an unusually high SiO abundance�10�6 to10�5 (Martın-Pintado et al. 1992; Bachiller et al. 1991; Hiranoet al. 2006), a factor �105 above the value in the ambient qui-escent clouds. The abundances, however, quickly drop to about�10�9 to 10�8 (e.g., Bachiller 1996) and are detected only nearthe tips of the outflows in sources such as L1157-mm (Mikamiet al. 1992;Zhang et al. 1995, 2000;Gueth et al. 1998;Bachiller et al.2001), IRAS 03282 (Bachiller et al. 1994), BHR 71 (Garay et al.1998), and NGC 1333 IRAS 2A (Jørgensen et al. 2004). Similarabundances (10�7 to 10�8 ) are also inferred from some CO bul-lets in L1441-mm and L1157-mm with high-J transitions fromSiO molecules (Nisini et al. 2005). No SiO emission has beendetected in L483 (Tafalla et al. 2000) or L1527, which are con-sidered to be near the end of their Class 0 phase, or in any out-flows that are driven by Class I or II sources. The detection ofSiO in those youngest Class 0 sources may not be coincidental;


the physical conditions in their primary winds may be partic-ularly conducive to the in situ formation and excitation of SiO.

6. CONCLUSION: A UNIFIED PICTUREOF MOLECULAR OUTFLOWS

We have carried out numerical simulations of magnetocentrif-ugal winds of cylindrical density stratification interacting withambient toroids of different degrees of flattening. The resultingstructure shows two prominent dense features: a shell of mostlyswept-up ambient material and a jet along the axis that is thedensest part of the primary wind. The shell can be either well col-limated, as observed for the class of jetlike molecular outflows,or wide open, as in the classical molecular outflows. These twoclasses of outflows were traditionally interpreted using two sep-arate pictures: jet-driven for the former and wind-blown for thelatter. They are now unified in a single picture that involves asingle physically motivated, stratified MHD wind running intoambient media of different density distributions. Indeed, theprototype of jetlike outflows, HH 211, which was previouslythought to be incompatible with the wind model, may providethe strongest support yet for the unified wind model.

Our result that the morphology of molecular outflow is shapedto a large extent by the ambient mass distribution has implica-tions for outflow evolution. We envision the mass distribution tobe relatively isotropic during the earliest Class 0 phase, with anarrow low-density funnel near the axis produced by anisotropicenvelope support from magnetic fields and/or rotation. In thisstage, shells of molecular outflow are well collimated, and the jetnature of the primary wind may be observable in high-density

and high-temperature lines, as either a more or less continuousjet or discrete bullets. As the source evolves, the collapsing am-bient envelope is expected to become more flattened through,e.g., the growth of amagnetic pseudodisk. The flattening enablesthe wide-angle component of the primary wind to open up thebase of the wind cavity, creating a broad, classical molecular out-flow. The jetlike and classical molecular outflows are thus uni-fied in an evolutionary sequence.Our framework has implications beyond molecular outflows.

It can be used to explore the evolution of the primary wind itself,the densest part of which may be observable in molecular linesat early times and in optical / IR as the source becomes more re-vealed. The more tenuous wide-angle wind may be made visiblein optical / IR at large scales through ambient interaction, prob-ably with an ambient envelope. Perhaps most importantly, quan-titative calculations based on this framework that include bothinfall and outflow may help address the fundamental question instar formation: what determines the mass of a star?

The authors would like to thank Al Glassgold, Naomi Hirano,Paul Ho, Susana Lizano, Mario Tafalla, and Frank Shu for theircritical comments and discussions. This work is supported bythe Theoretical Institute for Advanced Research in Astrophys-ics (TIARA) operated under Academia Sinica and the NationalScience Council Excellence Projects program in Taiwan admin-istered through grants NSC 94-2752-M-007-001 and NSC 94-2752-M-001-00; Z.-Y. L. and J. A. acknowledge support fromNASA grant NAG5-12102 and NSF grant AST 03-07368.

REFERENCES

Allen, A., Li, Z.-Y., & Shu, F. H. 2003, ApJ, 599, 363 (ALS03)Arce, H. G., & Sargent, A. I. 2005, ApJ, 624, 232———. 2006, ApJ, 646, 1070Bachiller, R. 1996, ARA&A, 34, 111Bachiller, R., Martın-Pintado, J., & Fuente, A. 1991, A&A, 243, L21Bachiller, R., Martın-Pintado, J., Tafalla, M., Cernicharo, J., & Lazareff, B.1990, A&A, 231, 174

Bachiller, R., Perez Gutierrez, M., Kumar, M. S. N., & Tafalla, M. 2001, A&A,372, 899

Bachiller, R., & Tafalla, M. 1999, in The Origin of Stars and Planetary Systems,ed. C. J. Lada & N. D. Kylafis (NATO ASI Ser. C, 540; Dordrecht: Kluwer),227

Bachiller, R., Terebey, S., Jarrett, T., Martın-Pintado, J., Beichman, C. A., &van Buren, D. 1994, ApJ, 437, 296

Chandler, C. J., & Richer, J. S. 2001, ApJ, 555, 139Cunningham, A., Frank, A., & Hartmann, L. 2005, ApJ, 631, 1010Davis, C. J., Mundt, R., Eisloffel, J., & Ray, T. P. 1995, AJ, 110, 766Eisloffel, J., & Mundt, R. 1998, AJ, 115, 1554Garay, G., Kohnenkamp, I., Bourke, T. L., Rodrıguez, L. F., & Lehtinen, K. K.1998, ApJ, 509, 768

Gardiner, T. A., Frank, A., & Hartmann, L. 2003, ApJ, 582, 269Glassgold, A. E., Mamon, G. A., & Huggins, P. J. 1989, ApJ, 336, L29———. 1991, ApJ, 373, 254Gueth, F., & Guilloteau, S. 1999, A&A, 343, 571Gueth, F., Guilloteau, S., & Bachiller, R. 1998, A&A, 333, 287Hartmann, L., Calvet, N., & Boss, A. 1996, ApJ, 464, 387Herbst, T. M., Beckwith, S. V. W., Glindemann, A., Tacconi-Garman, L. E.,Kroker, H., & Krabbe, A. 1996, AJ, 111, 2403

Herbst, T. M., Robberto, M., & Beckwith, S. V. W. 1997, AJ, 114, 744Heyvaerts, J., & Norman, C. 1989, ApJ, 347, 1055Hirano, N., Liu, S.-Y., Shang, H., Ho, P. T. P., Huang, H.-C., Kuan, Y.-J.,McCaughrean, M. J., & Zhang, Q. 2006, ApJ, 636, L141

Hollenbach, D. 1997, in IAU Symp. 182, Herbig-Haro Flows and the Birth ofStars, ed. B. Reipurth & C. Bertout (Dordrecht: Kluwer), 181

Jørgensen, J. K., Hogerheijde, M. R., Blake, G. A., van Dishoeck, E. F., Mundy,L. G., & Schoier, F. L. 2004, A&A, 415, 1021

Konigl, A., & Pudritz, R. E. 2000, in Protostars and Planets IV, ed. V. Mannings,A. P. Boss, & S. S. Russell (Tucson: Univ. Arizona Press), 759

Krasnopolsky, R., Li, Z., & Blandford, R. D. 2003, ApJ, 595, 631

Lada, C. J. 1985, ARA&A, 23, 267Lee, C.-F., Ho, P. T. P., Beuther, H., Bourke, T. L., Zhang, Q., Hirano, N., &Shang, H. 2006, ApJ, 639, 292

Lee, C.-F., Mundy, L. G., Reipurth, B., Ostriker, E. C., & Stone, J. M. 2000,ApJ, 542, 925

Lee, C.-F., Mundy, L. G., Stone, J. M., & Ostriker, E. C. 2002, ApJ, 576, 294Lee, C.-F., Stone, J. M., Ostriker, E. C., & Mundy, L. G. 2001, ApJ, 557, 429Li, Z.-Y., & Shu, F. H. 1996a, ApJ, 468, 261———. 1996b, ApJ, 472, 211Martın-Pintado, J., Bachiller, R., & Fuente, A. 1992, A&A, 254, 315Mikami, H., Umemoto, T., Yamamoto, S., & Saito, S. 1992, ApJ, 392, L87Mouschovias, T. Ch., & Ciolek, G. E. 1999, in The Origin of Stars andPlanetary Systems, ed. C. J. Lada & N. D. Kylafis (NATO ASI Ser. C, 540;Dordrecht: Kluwer), 305

Myers, P. C., Fuller, G. A., Goodman, A. A., & Benson, P. J. 1991, ApJ, 376, 561Nisini, B., Codella, C., Giannini, T., Bachiller, R., Santiago Garcia, J., &Richer, J. S. 2005, in The Dusty and Molecular Universe: A Prelude toHerschel and ALMA (ESA SP-577; Noordwijk: ESA), 397

Ostriker, E. C., Lee, C.-F., Stone, J. M., & Mundy, L. G. 2001, ApJ, 557, 443Palau, A., et al. 2006, ApJ, 636, L137Reipurth, B., & Bally, J. 2001, ARA&A, 39, 403Richer, J. S., Hills, R. E., & Padman, R. 1992, MNRAS, 254, 525Richer, J. S., Shepherd, D. S., Cabrit, S., Bachiller, R., & Churchwell, E. 2000,in Protostars and Planets IV, ed. V. Mannings, A. P. Boss, & S. S. Russell(Tucson: Univ. Arizona Press), 867

Robberto, M., Clampin, M., Ligori, S., Paresce, F., Sacca, V., & Staude, H. J.1995, A&A, 296, 431

Saucedo, J., Calvet, N., Hartmann, L., & Raymond, J. 2003, ApJ, 591, 275Schilke, P., Walmsley, C. M., Pineau des Forets, G., & Flower, D. R. 1997,A&A, 321, 293

Shang, H., Glassgold, A., Shu, F., & Lizano, S. 2002, ApJ, 564, 853Shang, H., Shu, F. H., & Glassgold, A. E. 1998, ApJ, 493, L91Shu, F. H., Adams, F. C., & Lizano, S. 1987, ARA&A, 25, 23Shu, F., Najita, J., Ostriker, E., & Shang, H. 1995, ApJ, 455, L155Shu, F., Najita, J., Ostriker, E., Wilkin, F., Ruden, S., & Lizano, S. 1994a, ApJ,429, 781

Shu, F. H., Najita, J., Ruden, S. P., & Lizano, S. 1994b, ApJ, 429, 797Shu, F., Najita, J., Shang, H., & Li, Z.-Y. 2000, in Protostars and Planets IV, ed.V. Mannings, A. P. Boss, & S. S. Russell (Tucson: Univ. Arizona Press), 789


Shu, F., Ruden, S., Lada, C. J., & Lizano, S. 1991, ApJ, 370, L31 (SRLL91)Stone, J. M., & Norman, M. L. 1992, ApJS, 80, 791Tafalla, M., Myers, P. C., Mardones, D., & Bachiller, R. 2000, A&A, 359, 967Takami, M., Chrysostomou, A., Ray, T. P., Davis, C., Dent, W. R. F., Bailey, J.,Tamura, M., & Terada, H. 2004, A&A, 416, 213

Velusamy, T., & Langer, W. D. 1998, Nature, 392, 685Zhang, Q., Ho, P. T. P., & Wright, M. C. H. 2000, AJ, 119, 1345Zhang, Q., Ho, P. T. P., Wright, M. C. H., & Wilner, D. J. 1995, ApJ, 451, L71


a unified model for bipolar outflows from young stars

Documents