star formation (or not) in magnetised - hs.uni-hamburg.de filestar formation (or not) in magnetised...

Star formation (or not) in magnetised

molecular clouds

Robi BanerjeeUniversity of Hamburg

Based on work by: Bastian Körtgen (Hamburg)co-workers: Daniel Seifried (Köln), Enrique Vazquez-Semadeni (UNAM)

OSSF, Paralia-Keterninis, May 2014, Robi Banerjee

galactic B-fields (e.g. R.Beck 2001)large scale component: B ~ 6µGtotal field strength: > 10 µG

The ISM is highly magnetised

M51

Magnetic Fields in the ISM

µG

Fletcher et al. 2011


M51

• M33: Bpos ~ 100...500 µG in GMCs from polarised CO emission

! sub Alfvenic turbulence

Magnetic Fields in the ISMLi & Henning, Nature 2011


e.g. Pipe nebula: B ! 17µG " 65µG(F.O.Alves, Franco, Girart 2008)



M51

• PLANCK: magnetic field of the Milky Way

ESA PLANCK: Milky Way's magnetic fingerprint, May 2014



M51Crutcher et al. 2010

• Magnetisation of the ISM and cloud cores

B ! n0.65



magnetic flux is frozen into the plasma:!

critical value for collapse:

uniform disc Nakano & Nakamura 1978

spherical structureMouschovias & Spitzer 1976

Impact of Magnetic Fields

= self-gravity / magnetic energy

mass-to-flux ratio:

!


M51

! minimal column density:

! minimal length scale:

! time-scale for colliding flows:

Impact of Magnetic Fields


M51

Crutcher, 2012



M51How do we get from here to here ?

Crutcher, 2012



Dissipation processes

Solutions? • flux loss by:

• Ambipolar Diffusion (Mestel & Spitzer 1956, Shu 1987, Mouschovias 1987)

• Turbulence + AD (e.g. Heitsch et al. 2004)

• Turbulent reconnection (Lazarian & Vishniac 1999)

• Ohmic resistivity (e.g. Dapp & Basu 2010, Krasnopolsky et al. 2010)

• ...


• Ionisation degree of the diffuse ISM:

; K=3#10!3 cm!3, K’=4.6#10!4 cm!3 (e.g. Fiedler & Mouschovias 1993, Hosking & Whitworth 2004)

! xe ~ 10"5 ... 0.1

! AD timescale:

AD in the WNM

1/2


Cloud Cores

M51Williams et al. 1998

• Ionisation degree of cloud cores:

xe ~ 2.5#10"8 ... 2.5#10"7

! ambipolar diffusion time scale:

$AD ~ 10 (xe/10"7) tff

! but cores are already supercritical n > 104 cm"3

! Ncores > Ncrit

! AD is not important for cores


Seifried, et al. 2012, 2013see Poster: 31

collapse of slightlysupercritical cores+ turbulence

! escapes “magnetic braking catastrophe” (Galli et al. 2006, ...)

! overcomes “fragmentation crisis” (Hennebelle & Teyssier 2008,)

200 AU

Collapse of Turbulent Cores


!

!"#

$%&'(

)

$%&'()

$%&'(

)

**$'()

+

,!"#$

-,!"#$

Model parameter:• n = 1 cm"3

• r = 32 ... 64 pc ! Minf = 2.3#104 M⊙

! N ! 7#1020 cm"2

• vinf = 14 km/sec

+ turbulence: vturb = 0.2 ... 12 km/sec+ ambipolar diffusion

• Bx = 1 ... 5 µG ! µ/µcrit = 1.1 (B/3µG)"1

! tcrit ! 15 Myr (B/3µG)

Simulations of colliding flows

see also Vazquez-Semadeni et al. 2007, 2010

MC formation & star formation


influence of magnetic fields

B = 3µG B = 4µG



ideal case with ambipolar diffusionB = 4µG

influence of ambipolar diffusion



1

10

100

1000

19.5 20 20.5 21 21.5 22 22.5 23

|BL

OS|

log(N [cm-2])

B=3µGt=06.51 Myrt=11.51 Myrt=14.00 Myrt=16.51 Myrt=21.51 Myr

Simulations of colliding flows results from head-on colliding flows with different field strengths

SF @ t ~ 20 Myr

no SF !

1

10

19.5 20 20.5 21 21.5

|BL

OS|

log(N [cm-2])

20B=5µG

t=06.51 Myrt=11.51 Myrt=16.51 Myrt=19.76 Myrt=21.51 Myr

B. Körtgen, RB in prep.


Simulations of oblique flows

! !

!"#

$%&'(

)

$%&'()

$%&'(

)

*

+!"#$

,+!"#$

!

Simulations setup of oblique flows! resemble non-ideal MHD

Model parameter:

• % = 0, 30, 60

• n = 1 ... 10 cm"3

• r = 32 ... 64 pc

• vinf = 14 km/sec• vturb = 2..10 km/sec

• Bx = 1 ... 5 µGB. Körtgen, RB in prep.


B = 3µG B = 5µG

Simulations of oblique flows! = 30°


Simulations of oblique flows results from oblique flows with different field strengths at % = 30°

SF @ t ~ 16 Myrno SF !

1

10

100

1000

19.5 20 20.5 21 21.5 22 22.5 23

|BL

OS|

log(N [cm-2])


1

10

19.6 19.8 20 20.2 20.4 20.6 20.8 21

|BL

OS|

log(N [cm-2])

20 t=06.51 Myrt=11.51 Myrt=16.51 Myrt=19.76 Myrt=21.51 Myr

B = 3µG B = 5µG


B = 3µG B = 5µG

Simulations of oblique flows! = 60°


Simulations of oblique flows

SF @ t ⪝ 20 Myrno SF !

1

10

100

1000

19.5 20 20.5 21 21.5 22 22.5 23

|BL

OS|

log(N [cm-2])


1

10

19.6 19.8 20 20.2 20.4 20.6 20.8 21

|BL

OS|

log(N [cm-2])

20 t=06.51 Myrt=11.51 Myrt=16.51 Myrt=19.76 Myrt=21.51 Myr

results from oblique flows with different field strengths at % = 60°


B = 3µG B = 5µG


B = 3µG B = 5µG





0.0001

0.001

0.01

0.1

1

10

100

1000

10000

100000

1e+06

-4 -2 0 2 4

Mass

[M

sol]

log(µ/µcrit)

t=16.54 Myrs

t=19.79 Myrs

t=21.54 Myrs

0.0001

0.001

0.01

0.1

1

10

100

1000

10000

100000

1e+06

-4 -2 0 2 4

Mass

[M

sol]

log(µ/µcrit)

t=16.54 Myrs

t=19.79 Myrs

t=21.54 Myrs

B = 3µG B = 5µG



Conclusion

Crutcher, 2012


Conclusion

Crutcher, 2012

?

star formation (or not) in magnetised - hs.uni-hamburg.de filestar formation (or not) in magnetised...

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