The Role of Magne c Fields in Structuring Clouds and Forming Stars
Zhi-Yun Li (University of Virginia)
Collaborators: Che-Yu Chen (University of Virginia) Laura Fissel (NRAO) Patrick King (University of Virginia) Outline • Why should one care about magne c fields? • Are there measurable magne c effects on cloud structures? • What is the origin of magne cally aligned stria ons? (preliminary work) • Summary & outlook I. Why should one care? Why should one care?
M = cr 2⇡G1/2 1 M for atomic ISM
! If strong enough, may make star forma on difficult Why should one care? Why should one care? Magne c braking catastrophe
Equatorial density Magne c field strength
Krasnopolsky, Li, Shang, Zhao (2012)
! If strong enough, may make disk forma on difficult (e.g., Allen et al. 2003, Machida et al. 2005, Banerjee & Pudritz 2006, Price & Bate 2007, Mellon & Li 2008, Hennebelle & Fromang 2008, Duffin & Pudritz 2009, Seifried et al. 2012, Santos-Lima et al. 2012, SEE WURSTER AND COMMERCON) Evidence for magne c fields? Evidence for magne c fields?
Cepheus Ophiuchus Aquila Lupus
Taurus
Perseus
! Both atomic ISM & molecular clouds are magne zed 2. Measurable magne c effects on cloud structures? Yes, at least in some cases A. Examples B. Sta s cs
Cepheus Ophiuchus Aquila Lupus
Taurus
Perseus
Chamaeleon-Musca Orion Example #1: Field-aligned HI fibers (S. Clark, Peek & Putman 2014) Example #2: Field-aligned stria ons in HI absorp on (CNM)
Riegel-Crutcher cloud
(McClure-Griffiths et al. 2006) Example #3: Field aligned CO stria ons 12CO map of Taurus clouds – CO Stria ons (P. Goldsmith+08) Example #4: Field-aligned Herschel (dust) stria ons
(Palmeirim+2013) Dense filament perpendicular to B field… Measurable magne c effects on cloud structures?
Planck XXXII: diffuse gas >5o from Galac c plane
HRO = Histogram of More polarized ridges Rela ve Orienta on (J. Soler+2013)
Less polarized ridges
Diffuse structures preferen ally aligned wi/ B field… Measurable magne c effects on cloud structures?
Planck XXXV: nearby molecular clouds
High column density Low column density
Diffuse, aligned; dense, perpendicular (not always clear, e.g., Lupus) Measurable magne c effects on cloud structures?
4 BLASTpol: Vela C GMC (~700 pc, 5x10 M", massive star forming)
Herschel map (Hill+2011)
High column density Low column density Measurable magne c effects on cloud structures?
BLASTpol: Vela C GMC (a part shown)
BLASTpol (PI: Mark Devlin)
(Courtesy: Laura Fissel, >3,000 vectors, 3 bands) Measurable magne c effects on cloud structures?
BLASTpol:
Vela C GMC (Full map)
(J. Soler+2017)
(Courtesy: Laura Fissel, >3,000 vectors, 3 bands) Measurable magne c effects on cloud structures?
(Soler+2017) ! Diffuse, aligned Dense, perpendicular South-Ridge
!Unclear Measurable magne c effects on cloud structures?
• Yes, in some cases (diffuse, parallel; dense, perpendicular) e.g., Taurus, South-ridge of Vela C, Chamaeleon-Musca
• But…not always true e.g., South-nest of Vela C 3. Rela ve orienta on of magne c fields & structures: why transi on from parallel to perpendicular? • One possible explana on proposed in Chen, King & Li (2016)
• Based on simula ons of Low column density Chen & Ostriker (2015)
Rela ve orienta on of magne c fields & structures: why transi on from parallel to perpendicular?
Model parameters: a=0.2 km/s, v=2 km/s (M=10), δv=0.14 km/s
-3 0 n0=1000 cm , B0=10 μG, θ=20 Magne zed colliding flow simula ons of Chen & Ostriker (2015)
x
Z
Edge-on view along y-axis Magne zed colliding flow simula ons of Chen & Ostriker (2015)
x
y
Face-on view along z-axis Fields strength-density correla on in the simula on
Post-shock layer
Pre-shock Inclined field lines
# X (Chen, King, Li 2016)
Z# Density# Fields strength-density correla on & field-structure alignment in 3D • Alignment angle between density gradient and B field n B cos = r · n B |r || | (Soler +2013) Inclined field lines • B field along filaments if # X = 90 (cos = 0) • B field perpendicular to filaments if =0 (cos = 1) Z# Fields strength-density correla on & field-structure alignment in 3D
(Soler +2013) Inclined field lines
• B field along filaments if # X = 90 (cos = 0) • B field perpendicular to filaments if =0 (cos = 1) Z# Fields strength-density correla on & field-structure alignment in 3D
(Soler +2013) Inclined field lines
# X Why?
Z# Fields strength-density correla on & field-structure alignment in 3D
Sub-Alfvenic
Super-Alfvenic magne c
Density # Inclined field lines
# X Why?
! Sub- to Z# Super-Alfvenic Rela ve orienta on of fields & structures in 2D: Correla on between B field (from polariza on) & column density structures?
Synthe c polariza on
Low column density
Face-on view High column density along z-axis 4. Origin of field-aligned stria ons?
X # Inclined field lines
# X
Z# Dense sub-layer: unique to oblique MHD shocks
Shocked layer B field
(e.g., Shu’s textbook)
Inclined field lines
#
X
B field
Dense sub-layer! sheet w/ parallel B Z# Magne zed dense sub-layer as “bamboo sheet”
Shocked layer B field
(e.g., Shu’s textbook) B field # X
#
Z
Y #
Inclined field lines
#
X
B field
Dense sub-layer! sheet w/ parallel B Z#
Vz(y)=V0 +0.14 sin(8⇡y)
Column density
B field
#
X
# Z
Y # Inclined field lines
#
X Y#
Z#
Turbulent converging flow
Column density
B field
#
X
# Z
Y # Inclined field lines
#
X Y#
Z# Summary & Outlook
• Magne c fields important theore cally (Mestel & Spitzer 1956) • Pervasive in ISM and molecular clouds • Magne cally aligned stria ons & perpendicular dense filaments • One interpreta on: rela ve orienta on changes from sub- to super-Alfvenic transi on • Stria ons: perturbed dense sub-layer w/ B field • Exci ng me: BLAST-TNG (Dec 2017) 2.5’#25’’, 6 x increase in resolu on, faster mapping Vela C cloud: ~1,000#100,000 vectors • Longer term: by-product of B mode cosmology?