The Origin of Stars andPlanetary Systemsedited by

Charles J. LadaHarvard-Smithsonian Center for Astrophysics,Cambridge, MA, U.S.A.

and

Nikolaos D. KylafisDepartment of Physics,University of Crete,Heraklion, Crete, Greece

\<1

Kluwer Academic Publishers

Dordrecht / Boston / London

Published in cooperation with NATO Scientific Affairs Division

Table of Contents

Preface xixParticipants xxiii

I. THE NATURE OF MOLECULAR CLOUDS AND THEIR RELATION TOSTAR FORMATION 1

Molecular CloudsLeo Blitz and Jonathan P. Williams

1. Introduction ; 32. Formation of Molecular Clouds 4

2.1. Galaxy Scale Issues 42.2. The Chaff 72.3. The Association of Atomic and Molecular Gas 10

3. Cloud Structure 123.1. Categorization 123.2. Structure Analysis Techniques 143.3. Clumps 163.4. Fractal Structures 193.5. Departures from Self-Similarity 20

4. The Relation Between Cloud Structure and the IMF 235. Summary 25

The Dynamical Structure and Evolution of Giant Molecular CloudsChristopher F. McKee

1. Introduction: The Observed Characteristics of GMCs 291.1. Chemical and Thermal Properties 301.2. Dynamical Properties 311.3. Star Formation in GMCs 33

2. Dynamical Structure of GMCs 342.1. The Virial Theorem 342.2. Are GMCs Gravitationally Bound? 362.3. Isothermal Clouds 392.4. Magnetic Fields vs. Gravity 40

2.4.1. Magnetic Critical Mass 402.4.2. Toroidal Fields. 422.4.3. Clouds Supported by Both Magnetic and Gas Pressure 432.4.4. Are Clouds Magnetically Supercritical? Theory 452.4.5. Observation of Magnetically Supercritical Clouds 45

2.5. MHD Waves in Molecular Clouds 47

VI

2.5.1. Wave Pressure 472.5.2. Wave Damping ...49

2.6. Polytropic Models for Molecular Clouds 502.6.1. Structure of Polytropes 502.6.2. Stability of Polytropes: Locally

Adiabatic Components 512.6.3. Stability of Polytropes: Globally

Adiabatic Components 532.6.4. Multi-Pressure and Composite Polytropes 54

2.7. Modeling Turbulence in Molecular Clouds 563. Evolution of Molecular Clouds and Star Formation 57

3.1. Formation of GMCs 573.2. Dynamical Evolution of GMCs 583.3. Photoionization-Regulated Star Formation 61

4. Conclusion 63

Physical Conditions in Nearby Molecular CloudsPhilip C. Myers

1. Introduction 672. Nearby Molecular Clouds 67

2.1. Molecular Cloud Constituents 692.2. Cloud Diversity 722.3. Density Probes 73

3. Dense Cores 743.1. Star Formation 763.2. Elongation 763.3. Virial Balance 773.4. Ionization and Field-Neutral Coupling 803.5. Line Width-Size Relations 813.6. Associated Stars 823.7. Core Size 823.8. Temperature 833.9. External Excitation of Core Turbulence 83

4. Cluster-Forming Cores 834.1. Models of Clustered Star Formation 834.2. Observational Studies of Young Clusters 854.3. Kernel Model 86

5. Inward Motions in Dense Cores 875.1. Infall Asymmetry 885.2. Infall Asymmetry in Starless Cores 905.3. Turbulent Cooling Flows 92

6. Summary 93

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Models and Observations of the Chemistry Near YoungStellar ObjectsEwine F. Van Dishoeck and Michiel R. Hogerheijde

1. Introduction .972. Gas-phase Chemistry 101

2.1. Basic Molecular Processes 1012.2. Gas-Phase Networks 1042.3. Gas-Phase Models 105

2.3.1. Depth-dependent vs. Time-dependent models 1052.3.2. The C+ -> C -> CO transition 106

2.4. Fractionation 1072.5. Successes and Problems 108

3. Grain-surface Chemistry 1083.1. Basic Surface Processes 1083.2. Thermal Processing and Evaporation 1103.3. Non-Thermal Desorption I l l3.4. Energetic Processing I l l3.5. Polar and Apolar Ices 1123.6. Gas-Grain Models 1123.7. Successes and Problems 113

4. Determination of Molecular Abundances 1134.1. Observational Techniques 113

4.1.1. Rotational line emission at(sub-)millimeter wavelengths 113

4.1.2. Vibrational absorption at infrared wavelengths 1154.2. Constraining the Physical Structure 1164.3. From Observations to Abundances 117

5. Chemistry in Pre-stellar Cores 1185.1. Translucent Clouds 1195.2. Dark Cloud Cores 1205.3. Ionization Fraction 122

6. Chemistry in Cold Envelopes Around YSOs 1226.1. Models 1226.2. Observations 123

7. Chemistry in Warm Envelopes Around Newly-formed Stars 1247.1. Warm Envelopes 1247.2. Hot Core Regions 1257.3. Outflow Regions 127

8. Examples 1288.1. An Intermediate-Mass Class 0 YSO in Serpens 1288.2. The W 3 Massive Star-Forming Region 132

9. Chemistry in Circumstellar Disks 13410. Concluding Remarks 134

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II. THE ORIGIN AND EARLY EVOLUTION OF LOW MASS STARS 141

The Formation of Low Mass Stars: An Observational OverviewCharles J. Lada

•1. Introduction '. 1432. Stellar Observations: The Fossil Record 145

2.1. Associations and Clusters 1452.2. Multiplicity 1462.3. The Initial Mass Function 1472.4. The HR Diagram and Early Stellar Evolution 150

3. Giant Molecular Clouds: Sites Star Formation 1523.1. OB Associations and GMCs 1523.2. Physical Properties of GMCs 1563.3. The Structure of GMCS: Dense Cores 158

4. The Young Stellar Objects..-. 1604.1. Clustering 1604.2. Multiplicity 1644.3. The Initial Luminosity and Mass Functions 1654.4. Spectral Classification: The Evolutionary Status and Nature of

Young Stellar Objects 1704.4.1. The Embedded Phase: Protostars 1734.4.2. The Revealed Phase: Pre-Main Sequence Stars 178

5. The YSO Evolutionary Sequence 1835.1. The Pivotal Role of Bipolar Outflows 1835.2. Episodic Accretion? 186

6. Concluding Remarks 187

Low-Mass Star Formation: TheoryFrank H. Shu, Anthony Allen, Hsien Shang, Eve C. Ostriker and Zhi-Yun Li

1. Magnetic Support of Clouds 1932. Fragmentation 1953. Empirical Measurements of the Mass-to-Flux Ration 1964. Turbulent Support of Molecular Clouds 1985. Filamentary Structure in Molecular Clouds 2016. Sustenance of Cloud Turbulence Through YSO Outflows 2027. A Toy Model ofGMC Structure and Evolution 2048. Formation and Evolution of Molecular Cloud Cores 2109. The Pivotal State 21110. Self-Similar Collapse of Singular Isothermal Toroids 212

IX

11. Mass Loss from Stars and Disks 21712. Generalized X-wind Model 21913. Summary 223

Bipolar Molecular OutflowsR. Bachiller & M. Tafalla

1. Introduction 2272. Determining Outflow Properties from CO Observations 230

2.1. CO Spectroscopy 2302.2. Outflow Physical Parameters 2312.3. Outflow dumpiness 232

3. Properties of Classical CO Outflows 2333.1. Outflow Geometry 2343.2. Physical Properties and Kinematics of the Outflow Gas 2363.3. Outflow Energetics 238

4. Properties of Highly Collimated CO Outflows 2394.1. Geometry and Collimation 2404.2. Kinematics and Energetics 240

5. Outflow Evolution 2445.1. Changes in Outflow Collimation 2455.2. Changes in the Kinematics and Energetics 2465.3. Changes in the Physical and Chemical Conditions of the

Outflow Gas 2485.4. An Empirical Time Sequence of Low-Mass Outflows 248

6. Shock Effects 2496.1. Shock Heating 2496.2. Shock Chemistry 2506.3. Observations of Shock Chemistry in LI 157 252

7. Properties of the Primary Wind 2557.1. Observations 2567.2. Models 257

7.2.1. Models of Molecular Gas Acceleration 2587.2.2. Wind Launching Models 260

Herbig-Haro FlowsBo Reipurth & A.C. Raga

1. Introduction , 2672. Herbig-Haro Objects at Different Wavelengths 267

2.1. Optical Spectra 2672.2. Infrared Spectra and Images 2712.3. Ultraviolet Spectra 274

3. Morphological and Kinematic Properties of Jets 2744. TheHH 111 Jet Complex 2785. Giant Herbig-Haro Jets 281

5.1. TheGiantHH 111 JetComplex 2835.2. "" The Importance of Giant HH Flows 285

6. Irradiated Jets 2877. Jets From Massive Stars 2888. Theoretical Models of HH Objects 290

8.1. Basic Theoretical Ideas 2908.2. Plane-Parallel and Bow Shocks 2918.3. Working Surfaces 2928.4. Steady Jets 2948.5. Instabilities in Jet Beams 2948.6. Jets From Variable Sources 2958.7. Comparing Models and Observations of HH Jets 297

Magnetic Fields and Star Formation: A Theory Reaching AdulthoodTelemachos Ch. Mouschovias & Glenn E. Ciolek

1. Introduction - A modern Theory of Star Formation 3052. Five-Fluid MHD Description of Protostar Formation 308

2.1. Physical Origin of the Five-Fluid Equations 3082.2. Reduction of the Five-Fluid Equations: Flux-Freezing in

the Plasma 3132.3. Grain Motion and Attachment to the Magnetic Field 3142.4. Modification of the Ion Attachment Parameter by Grains 315

3. Two Fluid Description of Protostar Formation 3153.1. Basic Equations 3153.2. Ambipolar Diffusion: General Considerations,

and Timescales 3163.3. Magnetic Braking 308

4. Results of Two-Fluid Description of Protostar Formation 3194.1. Ambipolar Diffusion in Nonrotating Models 3204.2. Rotating Models: Ambipolar Diffusion and

Magnetic Braking 3245. Protostar Formation in the Four-Fluid Approximation 327

5.1. Main Effects of Grains 3275.2. Detailed Comparison with Observations 3325.3. Effect of Ultraviolet Radiation 332

6. Hydromagnetic Waves, Linewidths, Ambipolar Diffusion, andStar Formation 333

7. Summary 3348. Appendix: Relevant Chemical Reactions in the Presence of

UV Ionization 336

XI

The Nature of Young Solar-Type StarsFrangois Menard & Claude Bertout

1. Introduction 3411.1. Young Solar-Type Stars: The Different Flavors 341

1.1.1. TTauri Stars 3421.1.2. FUOriStars 344

1.2. The Case for Stellar Youth 3451.2.1. Association with OB Associations

and Molecular Clouds 3451.2.2. Location in the HRD and Lithium Abundance 346

2. The Nature of TTS Photometric Variability 3472.1. Historical Background 3472.2. Long-Term Variability 3482.3. Mid-Term Variability 348

2.3.1. Type I: Rotational Modulation Caused by Cold MagneticSpots 349

2.3.2. Types II and Up: Variability Caused byCold and Hot Spots 350

2.3.3. Type III: Variable Obscuration byCircumstellar Dust? 351

2.3.4. Are There Other Mid-TermVariability Components? 352

2.4. Short-Term Variability 3542.5. Conclusions 355

3. The Physics of CTTS Accretion Disks 3553.1. Historical Background 3563.2. Global Disk Observables 3583.3. Direct Observations of Accretion Disks 360

3.3.1. Concluding Remarks 3643.4.. Structure of the Star-Disk Interaction Zone 365

3.4.1. Spectro-Photometric Evidence for Magneto-spheric Accretion 365

3.4.2. Is there an Appropriate Stellar Magnetic Field? 3663.4.3. Stellar Angular Momentum Evolution .' 3663.4.4. Problems and Prospective 367

The Evolution of Pre-Main-Sequence StarsFrancesco Palla

1. The Classical Theory Of Pre:Main-Sequence Evolution 3751.1. Basic Results 375

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1.2. The Influence of Protostellar Evolution 3771.3. The Stellar Birthline 381

2. Pre-Main-Sequence Evolution 3822.1. Protostellar Initial Conditions 3832.2."" Stellar Structure Equations 3852.3. Input Physics 387

2.3.1. Equation of State 3872.3.2. Opacity 3882.3.3. Treatment of Convection 3902.3.4. Nuclear Reactions 390

3. A New H-R Diagram 3913.1. Low-Mass Stars: 3923.2. Intermediate-Mass Stars: 3933.3. Massive Stars: 395

4. Tests to PMS Evolutionary Diagrams 3974.1. Comparison of Tracks 3984.2. Lithium Depletion 3994.3. Effects of Mass Accretion 4004.4. Effects of Binaries 4034.5. The Instability Strip for PMS Stars 404

UI. THE ROLE OF CLUTERING IN STAR FORMATION, THE ORIGINOF MASSIVE STARS 409

OB AssociationsA.G.A. Brown, A. Blaauw and R. Hoogerwerf, J.H.J. De Bruijne and P.T. De Zeeuw

1. Introduction 4112. The Importance of Studying OB Associations 4133. „ Defining Characteristics of OB Associations 4154. Recent Research 417

4.1. High-Mass Stars , 4174.2. Low-Mass Stars 421

5. Results from Hipparcos 4225.1. Astrometric Membership Selection 4235.2. Selected Results 4245.3. Mean Distances and Motions 430

6. Gould's Belt and the Origin of the Nearby Associations 4317. Future Perspectives 434

7.1. The Nearby OB Associations 4347.2. Beyond the Galaxy 436

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The Role of Embedded Clusters in Star FormationElizabeth A. Lada

1. Introduction 4412. The Distribution of Star Formation in Giant Molecular Clouds 4423. Properties of Embedded Clusters 451

3.1. Definition and Identification of Embedded Clusters 4513.2. Stellar Densities and Structure 4523.3. The Binary Fraction in Young Clusters 4553.4. Near-Infrared Luminosity Functions 457

4. The Formation of Embedded Clusters 4615. Fate of Embedded Clusters 4646. Circumstellar Disks in Cluster Environments 465

6.1. Near-Infrared Studies of Circumstellar Disks 4676.2. Millimeter Continuum Observations of Circumstellar

Disks and Disk Masses 470

Multiple Stellar Systems: From Binaries to ClustersIan A. Bonnell

1. Introduction 4792. Observations of Binary Systems 480

2.1. Main Sequence Binaries 4802.2. Pre-Main Sequence Binaries 480

3. ' Binary Formation Theories: Fission and Capture 4813.1. Fission 4813.2. Capture 4823.3. Star-Disc Capture , 483

4. Binary Formation through Fragmentation 4844.1. Gravitational Collapse and Fragmentation 4844.2. Fragmentation During Collapse 4884.3. Disc Fragmentation 490

5. Early Evolution of Binary Systems 4935.1. Accretion in Binary Systems 4945.2. Star-Disc Interactions in Binary Systems 494

6. Implications of Binary Star Formation 4967. Stellar Clusters 496

7.1. Formation Mechanism 4977.2. Cluster Dynamics 498

7.2.1. Violent Relaxation 5007.2.2. Mass Segregation 501

7.3. Accretion and StellarMasses 5037.4. Formation of Massive Stars 5057.5. Cluster Dissolution 507

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8. Binary Stars in Clusters 5089. Summary 509

Massive Star FormationEd Churchwell

1. Introduction 5152. Typical Properties of MSF Regions 5163. Morphologies, Ages, and Dynamics of UC HII Regions 517

3.1. Champagne Flow "Blister Models" 5183.2. In-Fall Models 5183.3. Photo-Evaporating Disk Model 5193.4. Pressure Confined UC HII Regions 5203.5. Stellar Wind Supported Bow Shock Model 5213.6. Mass-Loaded Stellar Winds 522

4. The Angular Expansion Rate of G5.89-0.39 5235. Dust Associated with UC HII Regions 5246. Hot Molecular Cloud Cores 5267. Hard X-Ray Emission From MSF Regions 5288. Molecular Outflows Associated with MSF 530

8.1. The Frequency of Outflows from MSF Regions 5308.2. Molecular Bipolar Outflow Properties 5318.3. Issues Raised by Massive Outflows 5398.4. Origin of the Mass in Massive Outflows 540

8.4.1. Accumulated Stellar Winds 5408.4.2. Entrained ISM in Stellar Bipolar Jets 5428.4.3. Swept-UpISM 5448.4.4. Accretion Driven Outflows 546

9. Summary 548

Masers in Star-Forming RegionsN.D. Kylafis and K.G. Pavlakis

1. Introduction 5532. Basic Concepts 554

2.1. Amplification 5552.2. Saturation 5552.3. Thermalization 5562.4. Beaming 5562.5. Geometry and Apparent Size 557

2.5.1. Elongated Structures 5582.5.2. Nearly Spherical Structures 558

2.6. Variability 558

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2.7. Spectra and Line Widths 5592.8. Polarization 560

3. Laboratory versus Astronomical Masers 5604. Usefulness of Masers 5615. Maser Models 562

5.1. Requirements 5625.2. Basic Equations..' 5635.3. Collisional and Radiative Pumping 563

6. Masers in Star-Forming Regions 5646.1. OH Masers 564

6.1.1. Observations 5656.1.2. Theoretical Calculations 566

6.2. H2O Masers 5676.2.1. Observations 5686.2.2. Theoretical Calculations 568

7. Location of Masers 5698. Conclusions 571

IV. THE PHYSICS OF CIRCUMSTELLAR DISKS AND PLANETFORMATION 577

Circumstellar DisksSteven V.W. Beckwith

1. Introduction 5792. The Early Solar System 5803. How Do We Know That Disks Exist? 5814. Spectral Energy Distributions 585

4.1. Flat, Black Disks 5864.2. . Modifying the Disk Shape, Flaring 5934.3. Exceptions: Flat Spectrum Sources 5964.4. Very Long Wavelength Emission: Measuring

Disk Mass 6004.5. Summary 602

5. Properties of Disks 6035.1. How Many Young Stars Have Disks? 6035.2. Disk Lifetimes 6045.3. Disk Masses 6045.4. Particle Sizes 606

6. Are Disks Important? 608

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Accretion Disks and Eruptive PhenomenaScott J. Kenyon

1. Introduction 6132. Steady Disks.. 616

2.1. Accretion Luminosities and Temperatures 6162.2. Turbulent Viscosity and Disk Timescales 6212.3. Disk Energy Distributions 623

3. Unstable Accretion Disks 6244. Disk Eruptions in Pre-Main Sequence Stars 628

4.1. Basic Properties of FU Orionis Objects 6304.2. FU Orionis Objects as Accretion Disks 6324.3. Observational Tests of Disk Models 6334.4. The Importance of FU Ori Eruptions 636

The Formation of PlanetsSteven P. Ruden

1. Introduction 6431.1. Overview: From Dust to Planets 6441.2. Future Research Questions 646

2. Structure and Evolution of Disks 6472.1. The Minimum-Mass Solar Nebula (MMSN) 6492.2. Nonlinear Nature of Disk Physics 652

3. From Grains to Planetesimals 6523.1. Particle-Gas Dynamics 6523.2. Midplane Settling and Growth 655

3.2.1. Radial Distribution of Solids inProtoplanetary Disks 657

,,3.3. Planetesimal Formation 6583.4. Prospects for Observing the Early Planet Formation Epoch 6603.5. Future Research Questions 661

4. From Planetesimal to Planet 6624.1. Orderly Growth 6634.2. Runaway Growth 6644.3. Final Accumulation Stages 6674.4. Future Research Questions 667

5. The Formation of Gas Giant Planets 6685.1. The Core-Instability Scenario 6685.2. Tidal Interaction 6715.3. Tidal Migration... 6735.4. The Survival of Giant Planets 6755.5. Future Research Questions 677

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Extrasolar Planets: Techniques, Results, and the FutureG. W. Marcy and R.P. Butler

1. Introduction.....: 6812. The Doppler Detection Technique 6833. Doppler Results: Properties of the Candidate Planets 685

3.1. Giant Planets Orbiting Within 0.1 AU 6873.2. Mass Distribution of Planetary Companions 6893.3. SemiMajor Axes of Jupiter-Mass Companions 6913.4. Orbital Eccentricity of Jupiter-Mass Companions 6933.5. Metallicity 695

4. Formation of Giant Planets 6985. Future Planet-Detection Techniques 699

5.1. Transits and Microlensing 6995.2. Astrometry 7005.3. Microlensing 7015.4. Space-Borne Nulling Interferometry 702

6. Summary 702