Role of Magnetic Field in Star Formation & Galactic Structure (Dec 20–22, 2017)

The Role of Magnetic Field in Star Formationin the Disk of Milky Way Galaxy
Shu-ichiro Inutsuka (Nagoya University)
Main Collaborators:

Tsuyoshi Inoue, Doris Arzoumanian,
Masato Kobayashi (Nagoya Univ)
Masanobu Kunitomo (Tokyo Univ)
Kazunari Iwasaki, Kengo Tomida (Osaka Univ)
Takashi Hosokawa (Kyoto Univ)
Philippe André (CEA, Saclay)
Dec 20–22, 2017

Kagoshima University• Observational Introduction• Formation of Molecular Clouds• Dynamics of Filaments

Outline– Mass Function of Dense Cores  IMF• Cloud/Star Formation in the Galactic Disk
– Accelerated Star Formation– SF Efficiency & Schmidt-Kennicutt Law– Mass Function of Molecular Clouds• Implication for Observation– Cloud-to-Cloud Velocity Dispersion, Ridge,– Intermediate Mass SF• Conclusion & Remaining Questions

Star Formation is Inefficient!
• Mass of Molecular Clouds (~10K): MMC ~ 109 M
• Typical Density observed by 12CO: nCO ~ 102 /cm3
• Free-Fall Time for 102 cm−3 ~ 106 yr• Star Formation Rate (if at Free-Fall Rate) too large!
RSF = 109M /106yr = 103 M /yr
Observed SF Rate, RSFR, obs ~ 100 M /yr
 Either Slow or Very inefficient (~10-3)!
tGas = Μgas / RSFR, obs ~ 100 Gyr

Schmidt-Kennicutt Law of SF
ΣSFR [M /kpc2 yr]

Star Formation Rate

High Density Tracer

Lada+2010,2012,2013

Kennicutt 1998

Σgas [M/pc2]
Timescale: Σgas / ΣSFR ~ Gyr• Column Density: Σgas [M/pc2]• SF Rate: ΣSFR [M /kpc2 yr]• Timescale: Μ /(SFR) ~ 20Myr
See also Gao & Solomon 2004; Wu+2005; Bigiel etal. 2008,2010,2011, Shimajiri+2017

Highlight of Herschel Result (André+2010)

2Cs2/G

Self-Gravity Essential in Filaments

Formation ofMolecular Clouds

Radiative Equilibrium for a given density
Solid: NH=1019cm-2,

Dashed: 1020cm-2

ρ ×102Warm
MediumCold Neutral
Mediume.g., Wolfire et al. 1995, Koyama & SI 2000

Compression of Magnetized WNM
Can direct compression of magnetized WNM
create molecular clouds?
 No, not at once!
Inoue & SI (2008) ApJ 687, 303
Inoue & SI (2009) ApJ 704, 161

Essentially same result byHeitsch+2009; Körtgen & Banerjee 2015; Valdivia+2016

We need multiple episodes of compression.

Further Compress. of Mole. Clouds
MultipleCompressions ofMolecular Cloud
Magnetized
Massive Filaments& Striations
Agree with Many
Observations!

Black Lines: Magnetic Field Lines
Self-Gravity Included, SI, Inoue, Iwasaki, & Hosokawa 2015

Formation of Molecular Clouds
Can direct compression of magnetized WNM
create molecular clouds?  Not at once.
We need multiple episodes of compression.
Inoue & SI (2008) ApJ 687, 303; Inoue & SI (2009) ApJ 704, 161
Inoue & SI (2012) ApJ 759, 35 Transformation of HI to H2tform = a few107yr
Further Compression of Molecular Clouds
Magnetized Massive Filaments & Striations= “Herschel Filaments”

Dynamical Timescales of Star Formation
Observational Demography of YSOs (e.g., Fuller&Myers1985)
 NTTauri / Nprotostar ~101.5-2  Tprotostar ~105 yr
 # of Dense Cores: NnoIR / N+IR ~ 101  Tcore ~106 yrc.f. Tambipolar ~107 yr & Tfreefall ~105 yr for n=104/cc

Gravitational collapse of a core is not quasi-steady!
Dynamical Gravitational Collapse in Dense Cores!
Dynamical Evolution in Self-Gravitating Filament with
Mline ~ 2Cs2/G

Evolutionary Timescales
Cloud Formation

107yr

SupercriticalFilaments
MolecularClouds (12CO)

Mass Function ofMolecular Cloud Coresand IMF

Massive Stars through Filaments(Peretto+2013)• Uniform but Different Velocity in Each Filament• Infall through Filament ~ 10-3 M/yr
Nicely Understood in Filament Paradigm

Applicability of Filament Paradigm for Massive Stars
Aquila CMF from HerschelAndré+2010; Könyves+2010
Massive stars can beformed in filaments!

SI & Miyama 1997

Larger Wavelength
 Massive Core

Mass Function of Cores in a Filament
Inutsuka 2001, ApJ 559, L149

Line-Mass Fluctuation of Filaments
Initial Power SpectrumP(k) ∝ k –1.5
Mass Function

dN/dM∝M –2.5
Observation of Both Perturbation
Spectrum and Mass Function(cf. Hennebelle & Chabrier 2008;
Shadmehri & Elmegreen 2011)

SI & Miyama 1997

P (k) ∝ k -1.5
SI & Miyama 1997

direct test!

t/tff = 0 (dotted) , 2, 4, 6, 8, 10 (solid)

“A possible link between the power spectrum ofinterstellar filaments and the origin of theprestellar core mass function”
Roy, André, Arzoumanian et al. (2015) A&A 584, A111

δ ...
Gaussian Press-Schecter

P (k) ∝ k n
n= −1.6±0.3≈ 5/3: Kolmogorov!

Supporting Inutsuka 2001

How About Binary Statistics?
André+2010; Könyves+2010“Mapping the core mass function on to the stellar initial mass function:
multiplicity matters”, Holman, Walch, Goodwin & Whitworth 2013, MNRAS

Need for Non-Self-Similar Mapping???

CMF to Stellar IMF with Binary SF
Aquila CMF from Herschel∼2 Stars Created in a Core& Binary Frequency ∝ M*

André+2010; Könyves+2010

Self-Similar Mapping from Log-Normal + Power Law
Applicable for any SF Efficiency (M*=ηMcore)
and any Power Law Slope (Misugi & SI 2018, in prep)
See also Whitworth & Lomax 2015, MNRASCore MF  Stellar System MF
Mass Function of Dense Cores

Aquila CMF from HerschelAndré+2010; Könyves+2010

SI & Miyama 1997

Slope of System MF = Slope of IMF

Possible Mission of JCMT-BISTRO
Aquila CMF from Herschel

BISTRO Obs

filament ⊥ B?
filament // filament?

Intermediate Mass SF
in Filament Paradigm

Könyves+2010

Peretto+2013

Massive Star Formation

Filament Paradigm
Completely Successful???
?

Other Modes of StarFormation?
Cloud Collision (Fukui, Tan, Tasker, Dobbs,...)
Collect & Collapse (by Expanding HII Regions)(Elmegreen-Lada, Whitworth, Palouš, Deharveng, Zavagno,…)Toward Global Pictureof Star Formation
Multiple Compressions Neededfor Molecular Cloud Formation

tform = a few 107yr(1 Compression in 1Myr)

Network of Expanding Shells
Multiple Episodes of Compression Formation of Magnetized Molecular Clouds
GMCCollisionDenseHI Shell

MolecularCloudLong (>10Myr) Exposure Picture!
Each bubble clearly visible only for short time (<Myr).
SI+2015

Cloud-to-Cloud Velocity Dispersion
Multiple Episodes ofCompression 
Formation of MagnetizedMolecular Clouds

Shell Expansion
1 km/sVelocities <10~Observation
Stark & Brand 1989

Cloud-to-Cloud
Velocity Dispersion

Network of Expanding Shells
Multiple Episodes of Compression 
Formation of Magnetized Molecular Clouds
GMCCollision

Peretto+2013
Inoue & Fukui 2013
Fukui+2012

DenseHI Shell

MolecularCloudSF starts in filaments once ML~ML,crit=2Cs2/G
and may intensify with increasing Mcloud !

Accelerated Star Formation
Palla & Stahler 2000

NumberTaurus-Auriga

Molecular Cloud Growth
Gradual Activation of SF
Multiple Episode of SF inOB-Association

Age t (Myr)

Also in Lupus,Chamaeleon, ρ ophiuchi,
Upper Scorpius, IC 348,and NGC 2264

Star Formation Efficiency in Dense Gas
Herschel Observation (e.g., Andre+2014, Könyves+2015)

Mcore / Mfilament <~ 15%Star Formation Efficiency in Dense Core: εcore

εcore ~ 33%

Star Formation Efficiency in Dense Gas: εdense gas εdense gas= Mcore / Mfilament × εcore ~ 5%
Consumption Timescale of Dense Gas: tdense gastdense gas−1 = (106 yr)−1 × εdense gas = (20Myr)−1
 tdense gas ~ 20Myr (eg. Lada+2010, Andre+2014)

How Many Generations of Filaments?
Star Formation Efficiency in Dense Gas: εdense gas εdense gas= Mcore / Mfilament × εcore ~ 5%

Typical Mass of Star Forming Filaments: L ~ 3pc, MLine ~2Cs2/G

M = MLine × L ~ 60Msun
Total Mass of Stars Created in a Filament: 60Msun × εdense gas ~ 3Msun

 Total Mass of YSOs: M*total# of Filaments to Form Stars = M*total/3Msun Multiple Generations of Filaments Needed!

Applicability of PresentScenario
Implication to Observation

M51 Synchrotron

λ=6 cm radio emission at 15 arcsec resolution fromVLA and Effelsberg

Polarized Intensity (Fletcher+ 2011)

Various Energy Densities
Polarization B-vectors of IC 342
6cm VLA and Effelsberg telescopes

Beck 2015
Eturb
equipartition assumed

Etot.mag=ECR
Eordered.mag
EWIM

Magnetic energy dominates thermal energy!

Massive Star Formation in Ridge

Battersby+2014

Extensive Herschel Studies on Massive Star
Formation in “Ridges”

Ridge orEdge-On
Shell?

Battersby+2014

Edge-On View ofCompressed Shell
 Ridge or Bar!

Bubbles (cyan dashed circles)
HII regions (cyan solid circles)
SNR 3C 391 (yellow oval)
Wolf–Rayet star WR 121b (red oval)

Advent of Large Surveys such as FUGIN

Numerous Straight
Ridges or Bars! Why?

Edge-On View of Compressed Shells =Ridges or Bars!  Bar // B Obs Proof of Cloud Formation Theory!!!

Destruction of
Molecular Clouds
How to Stop
Star Formation?
Radiative Feedback
Photodissociation Critical!
c.f. Dale, Walch,…

Expanding HII Region in Magnetized
Molecular CloudSh104Deharveng et al. 2003

Radiation Magnetohydrodynamics Calculation
UV/FUV + H2 + CO Chemistry (Hosokawa & SI 2005, 2006ab, 2007)

Disruption ofMagnetized
MolecularClouds
Feedback due to UV/FUVin a Magnetized Cloudby MHD version of
Hosokawa & SI (2005,2006ab)30M star destroys
105M H2 gas
in 4Myrs!

Mg(M*)
Non-Star Forming Gas−β+1=1.3−βCentral+1=1.5Stellar Mass, M* / M
−β+1=1.7
Exponent of IMF

M*−β+1 Mg(M*)
Central Stellar Mass, M* / M(SI, Inoue, Iwasaki, & Hosokawa 2015 A&A 580, A49)

Disruption ofMagnetized
MolecularClouds
Feedback due to UV/FUVin a Magnetized Cloudby MHD version ofHosokawa & SI (2005,2006ab)30M star destroys
105M H2 gas
in 4Myrs!

Mg(M*)
Non-Star Forming Gas−β+1=1.3−β+1=1.5−β+1=1.7
Exponent of IMF

M*−β+1 Mg(M*)
Central Stellar Mass, M* / M(SI, Inoue, Iwasaki, & Hosokawa 2015 A&A 580, A49)

Star Formation Efficiency &Schmidt-Kennicutt-Law (SI+2015)
105M molecular cloud destroyed by M* > 30M in 4Myrs!

Suppose Mtotal ∼103M stars formed in 105M ∼1 massive (>30M) star for std IMF
Zuckerman & Evans 1974

103MεSF = 5= 0.01
10 MStar Formation Time

Cloud Disruption Time: Τd=4Myr+T*
Gas Dissipation time: τdis =TdεSF∼ 1.4GyrNo Dependence
on Mass SchmidtKennicutt Law

Star Formation Efficiency, KS-Law
Mg molecular gas (H2) dispersed by Md*
β: exponent of IMF
M*m : Effective Minimum Stellar MassIf Mg =105, Md*=30M, M*m=0.1M, β =2.5,103M
εSF = 5= 0.01 observation β of IMF
10 M

(SI, Inoue, Iwasaki, & Hosokawa 2015 A&A 580, A49)

How far can we trace back?
10gas fraction

10Solar System

5Present0.2

SFR

14 13 12 11 10 900

SFR (Msun/yr)

Redshift

1

0.4 0.6 0.8
gas fraction

21538

765Age [Gyr]

432

10

Haywood+2016

Present SF mode responsible for z < 2in Our Galaxy!

Summary• Fragmentation of FilamentsCore Mass FunctionIMF• Bubble-Dominated Formation of Molecular CloudsUnified Picture of Star Formationδvcloud-cloud ~ 101km/s
Accelerated Star Formation
Star Formation Efficiency: εSF~10−2
Schmidt-Kennicutt Law: tdisp(total gas)~1Gyrtdisp(dense gass) ~ 20Myr
SI, Inoue, Iwasaki, & Hosokawa 2015, A&A 580, A49
Kobayashi+2017, ApJ 836, 175; Kobayashi+2017, PASJ submitted

Open Question: Characteristic Widths of Filaments

Which is determinant, NH or Filament-Width?

AquilaPolarisHerschel filaments have almost the same radii!
Aquila: 2R=0.1pc & ML = 2Cs2/G  NH ≈ 1022cm-2 (Av= several)
Polaris: 2R=0.1pc & ML < 2Cs2/G  NH < 1022cm-2 (Av< several)“Column Density Threshold” is a consequence?

Open Questions (Personal)• Why 0.1pc Width?
• Herschel Observation (e.g., Andre+2014, Könyves+2015)

Mcore / Mfilament < 15%• Gas Dissip. time: τdis =TdWhy?

∼ 1.4Gyr  KS LawεSF
if SF Efficiency εSF ~ 10−2 & Td~10Myr

Timescale ofMassive SF