star formation melvin hoare university of...

Star Form

ation

Melvin Hoare

University of Leeds

Overview

•Sites of star form

ation

•Gravitational collapse

•• •Disks and Outflows

•Evolutionary Stages

•Massive star form

ation

2

Sites of Star Form

ation

•Stars for in spiral arm

s

•Compression of H I gas via

spiral density wave

•Form

s Giant Molecular

•Form

s Giant Molecular

Clouds GMCs

3

Molecular Clouds

The Milky Way as seen in Integrated 13CO Maps

4

Molecular Clouds

5

Courtesy of the Galactic Ring Survey (http://www.bu.edu/galacticring)

Molecular Clouds

•Filamentary, clumpy, hierarchical on wide range of scales:

•massive clumps, several pc and masses ~1000 Mo

•small dense cores , 0.1 pc and masses of order 1 Mo,

6

Initial Conditions

7Submm Dust emission

C18O emission

Cloud Support

•Clouds are

dominated by non-

8

dominated by non-

therm

al motions

-Turbulence

-Magnetic support

Physical Conditions

•Cold (20 K), dense 104cm

-3, ice form

s on grains

Deep absorption feature at 9.7µm

99

Water ice -found in dense clouds

Gravitational Collapse

The critical mass is known as the Jeans criterion i.e.

The balance between therm

al support and gravity leads to

The critical mass is known as the Jeans criterion i.e.

Free-fall tim

e

10

8 Mo cloud collapse in ~105yr

Fragmentation

The Initial Mass Function

-Initial collapse is

isothermal

-

Orion Nebula Cluster (ONC)

Pleiades

Average Galactic Field IMF

-Jeans m

ass decreases

-Smaller fragments

become unstable

-Stops when clumps

become optically thick

and no longer

isothermal

M35

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isothermal

Kroupa 2002, Science, 295, 82

Progress of collapse

12

Effects of Rotation

•We can determ

ine the

radius where F

C= F

G

-this is referred to as the

centrifugal radius

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centrifugal radius

Effects Magnetic Fields

B Field

Ambipolar Diffusion

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-Neutrals can drift relative to the magnetic field opposed

by only collisions with ions

-Timescale for this process is typically longer than tff

Effects of Magnetic Fields

Magnetic Fields

B Field

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-Collapse of magnetically supported cloud should lead

to hourglass shape

-Need to loose large amount of magnetic flux during

collapse

•SMA observations of

polarized sub-m

m

emission (Girat et al

2006)

2006)

•Shows hourglass field

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Accretion Discs

Rotation leads to accretion via a disc

17

Mannings Nature 388, 555-557 (7 August 1997)

Accretion Disc Spectrum

Viscous dissipation in disc in optically thick disc gives

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Accretion Discs Observed

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Accretion Discs Observed

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Collapse of a 100 solar mass protostellar

core to a massive star (Krumholz, Klein, &

McKee 2007)

Jets and Outflows

•Highly collim

ated

jets are invariably a

part of star

form

ation

form

ation

•Extend over pc

distances and end

in a bow shock -

Herbig-Haro

emission

•Some episodic and

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•Some episodic and

precessing

Jets and Outflows

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NICMOS

WFPC2

VLA 8 GHz

•Ionized gas

observed but

thought to be

24

thought to be

mostly 90%

neutral

Jet Proper Motions

Material moving at around 500 km s-1

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Bipolar Molecular Outflows

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MHD Driving and Collimation Mechanisms

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•Disc wind

model (Ouyed et

al. 2003)

X XXX- ---wind model

wind model

wind model

wind model (Shu et al. 1997)

(Shu et al. 1997)

(Shu et al. 1997)

(Shu et al. 1997)

•X-w

ind interaction between rotating stellar field and disc

•Infall channelled along m

agnetic fields

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X XXX- ---ray Activity in Orion Cluster

ray Activity in Orion Cluster

ray Activity in Orion Cluster

ray Activity in Orion Cluster

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www.astro.psu.edu/co

up

Most (all?) stars form

in clusters

Most (all?) stars form

in clusters

Most (all?) stars form

in clusters

Most (all?) stars form

in clusters

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Evolution

•Form

ing stars

evolve from

deeply

embedded

embedded

phases to

optically visible

•Adams et al

(1987), Andre et

al. (1993)

classification

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scheme Class 0,

I, II, III

Disc Dispersal

•Disc disperses

over time as

fraction of

objects with IR

objects with IR

excess due to

discs decreases

with age of

cluster (Haisch

et al. 2001)

•Likely due to

planet form

ation

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planet form

ation

Massive Star Form

ation

Kelvin-Helmholtz Timescale

•Massive star starts core hydrogen burning whilst still

deeply embedded and accreting

•Radiation pressure on infalling dust is very high

•Accretion at high rates via a disc overcomes this

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Krumholz et al 2005

Evolution in Massive Star Form

ation

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Infrared dark clouds –

opaque at 8 m

icrons

(Rathborne et al.

2005)

IRDCs

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Spitzer IRDC

24 µm

450 µm

Rathborne et al. 2005

Massive Young Stellar Objects

Massive Young Stellar Objects

Massive Young Stellar Objects

Massive Young Stellar Objects

•Mid-IR bright point

sources

•Luminous but not

yet ionizing

yet ionizing

surroundings

•Bipolar outflows

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www.ast.leeds.ac.uk/R

MS

UCHII regions

UCHII regions

UCHII regions

UCHII regions

•Cometary

UCHIIs imply

stars born in

stars born in

density gradient

ie off-centre

37

Arthur & Hoare 2006

www.ast.leeds.ac.uk/Co

rnish

Triggered Star Form

ation

Triggered Star Form

ation

Triggered Star Form

ation

Triggered Star Form

ation

•Expanding H II regions

compress surrounding cloud

triggering further

gravitational collapse

gravitational collapse

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Danger: Massive stars

about

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