Magnetic fields in the early phase of massive star formation

Transcription

1 Magnetic fields in the early phase of massive star formation FLASH workshop in Hamburg Daniel Seifried Hamburger Sternwarte, University of Hamburg (Robi Banerjee, Ralf Klessen, Ralph Pudritz, Dennis Duffin)

2 Introduction Massive star formation not yet fully understood Difficulties in observing massive star formation: Hard to observe (located in star clusters, hidden by surrounding gas) Short evolution time of central object Greater distances than low-mass SF-regions Recent advances reveal evidence for similar formation mechanism of low- and high-mass stars disks and outflows around massive protostars Observed magnetic fields reveal significant strengths

3 Introduction Massive star formation not yet fully understood Difficulties in observing massive star formation: Hard to observe (located in star clusters, hidden by surrounding gas) Short evolution time of central object Greater distances than low-mass SF-regions Recent advances reveal evidence for similar formation mechanism of low- and high-mass stars disks and outflows around massive protostars Observed magnetic fields reveal significant strengths Star formation reveals high complexity and spans several orders of magnitude in density, lengthscales, etc. Recent advances in massive star formations simulations (3D) including magnetic fields

4 Observations: Massive cores High-mass star forming cores: Masses: Msun Sizes: pc Slightly rotating Magnetic fields: μg Hour-glass shaped field structure Dragged in by gravitational collapse Rotation velocity and magnetic field line structure (G ) Girart et al. 2009

5 Observations: Outflows Outflows are omnipresent in star formation Detected for the first time in 1890 (T-Tauri) Magnetically driven Observed for a wide range of stellar masses and protostellar evolutionary phases Wide spread in outflow properties Outflow velocities range from ~ km s-1 up to ~ 100 km s-1 Typical sizes: 0.1 pc 1 pc (ejecting objects ~ 10 AU) Mass-loss rates: 10-7 Msun yr Msun yr-1 Luminosities between 10-3 Lsun and 100 Lsun Collimation factors up to > 20 Herbig Haro HH47 J. Morse/STScI, & NASA

6 Outflow driving mechanism Constraint to magnetically driven outflows, no radiation driven outflows Two different regimes in outflows driven by magnetic fields Lorentz force Magneto-centrifugal acceleration Blandford & Payne 1982 Magnetic-pressure driven outflow Lynden-Bell 1996 Both regimes can be described by one generalised set of equations and are typically both contained within a single outflow

7 Numerics Solving magnetohydrodynamical equations using Tree gravity solver (see Richard Wunsch's talk) Speed up by a factor ~ 3 compared to multigrid solver 3-wave MHD-solver (Bouchut, Klingenberg, Waagan 2007, 2010) preserving positivity of density and internal energy high stability: important for violent star formation process Molecular line cooling + dust cooling Line cooling of individual molecules (tabulated) Thermal radiation of dust Subcycling if energy change > 20 % Sink particle to follow long term evolution (see Robi Banerjee's talk) Only interacting gravitationally with gas Magnetic fields are left unaltered

8 Initial condition Initial conditions: molecular cloud core of 100 Msun (~ 55 Jeans masses) Density profile: Diameter of 0.25 pc Slightly rotating Highly magnetised: B ~ μg Magnetic field parallel to rotation axis In total 12 simulations ρ(r) ~ r AMR levels with a max. resolution of 4.5 AU Required computation time ~ 1 Mio. CPU-h

9 Disk formation I What do we know about disks in massive star forming regions? Two types of massive disk are typically observed: disks with rotational support: Keplerian disk disks without rotational support: sub-keplerian disks Star formation simulations with realistic magnetic field strengths find sub- Keplerian disks Wang et al, 2010, C18O transition in S255IR, line-of-sight velocity

10 Disk formation II Can we reproduce this findings? weak magnetic field: Keplerian disk Edge-on view of the disk: Seifried et al. 2011, 2012 Strongly pinched magnetic field in the midplane (white lines)

11 Disk formation II Can we reproduce this findings? weak magnetic field: Keplerian disk strong magnetic field: sub-keplerian disk with almost radial infall Edge-on view of the disk: Seifried et al. 2011, 2012 Strongly pinched magnetic field in the midplane (white lines) Magnetic braking catastrophe: Angular momentum gets removed by magnetic field

12 Outflows results Very strong shock fronts, Mach numbers up to ~ 50 Weak field case: Keplerian disk Well collimated High velocities Strong field case: Sub-Keplerian disk Poorly collimated Low velocities Very different morphologies for weak field and strong field cases Caused by underlying disk structure

13 Outlook So far turbulence was neglected completely What happens if supersonic motions are included in initial conditions? Turbulence run no turbulence run Keplerian disk are reobtained even for strong magnetic fields Turbulence avoids magnetic braking catastrophe

14 Summary 12 collapse simulations of magnetised massive cloud cores Tree gravity solver 3-wave MHD solver Tabulated cooling rates 13 levels of refinement, 1 Mio. CPU-h Keplerian disk and sub-keplerian disk are found Suppression of Keplerian disk formation for strong fields due to magnetic braking Outflows are common feature in star formation Large variability of outflow properties due to different disk structures Inclusion of turbulence solves the magnetic braking problem

15 Thank you for your attention!