Star formation (or not) in magnetised - hs.uni-hamburg.de fileStar formation (or not) in magnetised...
Transcript of 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
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
M51
• M33: Bpos ~ 100...500 µG in GMCs from polarised CO emission
! sub Alfvenic turbulence
Magnetic Fields in the ISMLi & Henning, Nature 2011
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
e.g. Pipe nebula: B ! 17µG " 65µG(F.O.Alves, Franco, Girart 2008)
Magnetic Fields in the ISM
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
M51
• PLANCK: magnetic field of the Milky Way
ESA PLANCK: Milky Way's magnetic fingerprint, May 2014
Magnetic Fields in the ISM
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
M51Crutcher et al. 2010
• Magnetisation of the ISM and cloud cores
B ! n0.65
Magnetic Fields in the ISM
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
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:
!
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
M51
! minimal column density:
! minimal length scale:
! time-scale for colliding flows:
Impact of Magnetic Fields
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
M51
Crutcher, 2012
Magnetic Fields in the ISM
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
M51How do we get from here to here ?
Crutcher, 2012
Magnetic Fields in the ISM
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
M51How do we get from here to here ?
Crutcher, 2012
Magnetic Fields in the ISM
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
M51How do we get from here to here ?
Crutcher, 2012
Magnetic Fields in the ISM
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
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)
• ...
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
• 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
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
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
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
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
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
!
!"#
$%&'(
)
$%&'()
$%&'(
)
**$'()
+
,!"#$
-,!"#$
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
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
influence of magnetic fields
B = 3µG B = 4µG
Simulations of colliding flows
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
ideal case with ambipolar diffusionB = 4µG
influence of ambipolar diffusion
Simulations of colliding flows
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
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.
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
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.
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
B = 3µG B = 5µG
Simulations of oblique flows! = 30°
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
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])
t=06.51 Myrt=11.51 Myrt=15.59 Myrt=16.51 Myrt=18.34 Myr
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
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
B = 3µG B = 5µG
Simulations of oblique flows! = 60°
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
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])
t=06.51 Myrt=11.51 Myrt=16.51 Myrt=19.76 Myrt=21.51 Myr
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. Körtgen, RB in prep.
B = 3µG B = 5µG
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
B = 3µG B = 5µG
Simulations of oblique flows results from oblique flows with different field strengths at % = 60°
B. Körtgen, RB in prep.
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
Simulations of oblique flows results from oblique flows with different field strengths at % = 60°
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
B. Körtgen, RB in prep.
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
Conclusion
Crutcher, 2012
OSSF, Paralia-Keterninis, May 2014, Robi Banerjee
Conclusion
Crutcher, 2012
?