the life cycle of giant molecular clouds charlotte christensen

The Life Cycle of Giant Molecular Clouds

Charlotte Christensen

Observational Constraints onThe Life Cycle of

Giant Molecular Clouds in Milky Way-like Galaxies

Charlotte Christensen

Coming up

• Physical Background

• Lifecycle• Formation•Core Formation• Protostar Formation• Star Formation•Dispersal

• Nagging Questions

Meet the Molecules

Meet the Molecules

HIIHII

Meet the Molecules

HIHI

Meet the Molecules

HH22

Meet the Molecules1212COCO

Meet the Molecules

1313COCO

Meet the Molecules

NHNH33

3 Phase Interstellar Media

• Hot Ionized Medium

• Warm Neutral/Ionized Medium

• Cold Neutral Medium

3 Phase Interstellar Media

• Hot Ionized Medium•HII• T 106 - 107 K• 10-4 - 10-2 cm-3

• Warm Neutral/Ionized Medium• Cold Neutral Medium

Haffner et al, 2003Haffner et al, 2003

3 Phase Interstellar Media

• Hot Ionized Media• Warm Neutral/Ionized Media

•HII & HI• T 6000 -- 12,000K• 0.01 cm-3

• Cold Neutral Media

MW 21cm radiationMW 21cm radiation

Dickey & Lockman, 1990Dickey & Lockman, 1990

3 Phase Interstellar Media

• Hot Ionized Media• Warm Neutral/Ionized Media• Cold Neutral Media

•HI & H2

• T 15 -- 100K• 100 -- 5000 cm-3

Dame et al, 2001Dame et al, 2001

MW CO emissionMW CO emission

Molecular Hydrogen Clouds

• Self-gravitating (rather than diffuse)

• H2, molecules, and dust grains

• 30 - 60% of the gas mass

• Occupy > 1% of the volume

• Site of star formation

Eagle NebulaHST

Size ScalesMass (MO) Size (pc) (cm-3)

Superclouds / GMAs

107 -- --

Giant Molecular Clouds

104 -- 106 50 100

Molecular Clouds 103 -- 104 10 100

Bok Globules 1 -- 1000 1 104

Cores 1 -- 1000 1 104

Size ScalesMass (MO) Size (pc) (cm-3)

Superclouds / GMAs

107 -- --

Giant Molecular Clouds

104 -- 106 50 100

Molecular Clouds 103 -- 104 10 100

Bok Globules 1 -- 1000 1 104

Cores 1 -- 1000 1 104

Some Timescales

• Crossing Time• Time for a sound wave to propagate

through

• c = 10 Myr

• Dynamical Time• Time for a particle to free fall to center

• dyn = G-1/2 2 Myr

• “Dynamic” vs “Quasi-Static” Evolution

Support

• Assume Equilibrium• Virial Theorem

2 T + W = 02 T + W = 0

Kinetic EnergyKinetic Energy

Potential EnergyPotential Energy

Jeans Mass:

Support

• Assume Equilibrium•Outside Pressure

2(T - T2(T - T00) + W = 0) + W = 0

Potential EnergyPotential Energy

KE from External PressureKE from External Pressure

Kinetic EnergyKinetic Energy

Support

• Assume Equilibrium• Turbulence vs Thermal KE

2(T2(T + T + TPP - T - T00) + W = 0) + W = 0

Potential EnergyPotential Energy

KE from External PressureKE from External Pressure

Thermal KEThermal KE

Turbulent KETurbulent KE

Support

• Assume Equilibrium•Magnetic Field

2(T2(T + T + TPP - T - T00) + W + B = 0) + W + B = 0

Potential EnergyPotential Energy

KE from External PressureKE from External Pressure

Thermal KEThermal KE

Turbulent KETurbulent KE

Mag. EnegryMag. Enegry

Support

• Assume Equilibrium•Magnetic Field

2(T2(T + T + TPP - T - T00) + W + B = 0) + W + B = 0

Potential Energy

KE from External Pressure

Thermal KE

Turbulent KE

Mag. EnegryMag. Enegry

Turbulent Support -- Source

• Internal• Stellar Winds• Bipolar Outflows•HII

• External•Density Waves•Differential Rotation• Supernovae•Winds from Massive Stars

Turbulent Support -- Decay

• Close to a Kolmogrov Spectrum

• Cascade down to lower energies• Large eddies form small eddies• Small eddies dissipated through friction

• Timescale: 1 Myr

Magnetic Field Support -- Source

• Galactic Dynamo• Seed Magnetic Field• Differential Rotation• Convection

• Throughout MW• Seen in polarization

and Zeeman splitting

MPIfR Bonn

NGC 6946

Magnetic Field Support -- Decay

• Ambipolar Diffusion -- Decoupling of charged and neutral particles

• Timescale: 10 Myr

• Depends on: •Density•Magnetic Flux• Ionization Fraction

Life Cycle

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Life Cycle

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Theories

• Collisional build up of molecular clouds•Growth time collisional time

• Quiescent growth of ambient H2

• Gravitational/magnetic instability• Shock compression

• Spiral Arms• Supernovae• From HI of H2?

w/ CO

all HIall HI

Correlation with HI

• Filaments of HI around all GMCs

Engargiola et al, 2003Engargiola et al, 2003

M33M33

DensityDensity

Correlation with Spiral Arms

M33M33

• 60% of H2 in spiral arms

• Grand design spirals: • > 90% (Nieten et al. 2006, Garcia-Burillo et al 1993)

Rosolowsky et al, 2007

Age Limits

• = 10-20 Myr• Collisional build

up of molecular clouds• = 2000 Myr

• Quiescent growth of ambient H2

• H2 = 0.3 MO pc2

• = 100 MyrEngargiola et al, 2003Engargiola et al, 2003

M33M33

Shocks

• Observation of a shocked GMA

Tosaki, 2007Tosaki, 2007

1212CC 1313CC

M31

GMC Formation -- Conclusions

• Formed primarily from either HI or H2

• Compressed to self-gravitating clouds in spiral arms

Life Cycle

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Cloud Core Formation

• GMC is supported by:• Magnetic flux• Turbulence

• Support is removed either• Slowly by Ambipolar diffusion• Fast by decay of turbulence and

turbulence amplified diffusion

• Cores (regions 2-4 times ambient density) form at 10% efficiency

Lagoon Nebula

Initial Conditions

• Cloud envelope is• In non-equilibrium•Magnetically subcritical (Cortes et al, 2005)

• Very inhomogenous

Carina, HST

Observations of Cores

Myers & Fuller, 1991

Observations of Cores

• Cores are:•Non-isotropic•More prolate than oblate•Not necessarily aligned

with the magnetic field (Glenn 1999)

Prolate

Oblate

Ratio of Clouds without Stars

• One last test of timescale:•NNS/NT = NS/ T

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Ratio of Clouds without Stars

• Very few MW GMCs without SF

• 25% of GMCs in other galaxies have no associate HII regions (Blitz, 2006)

Engargiola, et al 2003Engargiola, et al 2003

M33 -- Distance between GMC and HII

Ratio of Clouds without Stars

• NNS/NT = NS/ T 1/4

• Dynamic Collapse

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Life Cycle

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Core Collapse to Protostar

• Overdensties collapse

• Collapse regulated by• Turbulence•Magnetic Field

• Fragmentation

• Protostar formation when core becomes opaque

Core Sizes &Densities

Radius (pc)

Lee et al, 1999

Enoch et al, 2008

Log Density

Protostar Formation

Size

Magnetic Support

• Cores are (probably) supercritical, i.e. not supported by the magnetic field

• M/B = c G-1/2

• c 0.12

Crutcher, 1999

Critical

Turbulence

• Cores are turbulent

• Motions are Supersonic

• Turbulence from shocks or MHD waves

Myers & Khersonsky, 1994

MHD Turbulence

• Dependent on Ionization

• Decays by ***

• Decay rate is still comparable to non-magnetic turbulence

• Speeds close to Alfven speed

Time Scales

• We have flow of material onto magnetically-unsupported cores

• Larger, more massive cores collapse to protostars

• How fast does this happen?

Time Scales -- Spiral Arm Offset

Time Scales -- Spiral Arm Offset

Tosaki, 2002

M51 13CO12CO H

Time Scales -- Spiral Arm Offset

• Difference between peaks 10 Myr

• Long delay of SF OR staggered SF

Tosaki, 2002

Time Scales --Statistcs

• Ratio of clouds without protostars:•NNSC/NC = NSC/ C

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Time Scales --Statistics

• Optically Selected MW Cores:•NNSC/NC = 306/400

(Lee & Myers, 1999)

• Perseus, Serpens, & Ophiuchus:•NNSC/NC = 108/200

(Enoch et al, 2008)

• 25% - 50% of core life before SF (Enoch et al, 2008)

Time Scales --Statistics

• Lifetime of a protostar 2 - 5 x 105 Myr

• Lifetime of a core 0.3 - 1 x 106 Myr

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

0.5 Myr

Life Cycle

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Stars Form

• Powered by gravitational energy

• Envelopes of accreting material

• T Tauri Stars

Trifid, HST

Size

Hatchel & Fullerl, 2008

Younger Protostar

Older Protostar

Starless

Perseus Cores

Time Scale

• T Tauri Problem•Most stars

form within 3 Myr

Palla & Stahler, 2000

Location

Huff & Stahler, 2006

Time Scale

• Star formation lasts 2 - 4 Myr

• Clouds gone after 5 - 10 Myr

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

2 - 4 Myr

Lifecycle

Cloud Formation

Cloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Clouds Dispersing

Leisawitz, 1989

Proximity to New Stars

• Star clusters older than 10 Myr have no associated clouds

Leisawitz, 1989

Cascading SF

• Dispersing clouds may spark SF elsewhere

Hartmann

M51, HST

Putting it all TogetherCloud Core Formation

Protostar Collapse

Stars Form

Cloud Dispersal

Cloud Formation

Cascading SF

0 1 4

10 - 20 Myr

Nagging Questions

• Do clouds form from HI of H2?

• How long before cores form?

• What effect does the magnetic field have on turbulence?

Thanks

• Tom Quinn, Fabio Governato, Julianne Dalcanton, Andrew Connely, Bruce Hevly

• Adrienne and David for making me dinner

• Everybody who came to my practice talk

Gas In-fall Onto Cores

Lee, 2001

Alignment

MHD Turbulence

Padoan, 2004

Core Densities

Enoch, 2008

Location

Huff & Stahler, 2006

More Dispersal

Jorgensen, 2007