Michael Küffmeier

Synthetic polarization maps around embedded protostars

Co-supervisor: Zhi-Yun Li

Student: Hannah Woodward

Credit: B. Saxton

Stars are born in large assemblies of gas

Star-disk systems form in different environments provided by Giant Molecular Clouds (Size: 10 - 100 pc)

Serpens SMM1 (Le Gouellec et al. 2019)

Zoom-in method

Küffmeier et al. 2017

  • adaptive mesh refinement
  • ideal magnetohydrodynamics
  • turbulence driven by supernovae
  • stars modelled as sink particles

Zoom-in on embedded protostellar multiple

Küffmeier, Calcutt

& Kristensen 2019

Küffmeier, Reißl et al. 2020

bridge structure similar to IRAS 16293--2422 (e.g. Sadavoy+ 2018, van der Wiel+ 2019, Maureira+ 2020)

~1500 AU

Zoom-in simulations

Start from a snapshot of a young Giant Molecular Cloud (GMC)

Model stars as sink particles

Turbulence as a result of supernova feedback

l_{\rm box} = 40\, \rm pc

Adaptive mesh refinement (AMR)

ideal Magnetohydrodynamics (MHD)

Magnetic fields in simulation

Küffmeier, Reißl et al. 2020

...in bridge

Field strength in bridge: 

about 1 to 2 mG

...around primary protostar

Field strength close to foot point:

>100 mG

Dust polarization to measure magnetic fields

Polarization depends on degree of grain alignment and elongation

Credit: B. G. Anderson

Measuring linear polarization of dust grains allows to determine magnetic field orientation ...

... if alignment mechanism is known! 

Stokes vector: description of polarized light

Poincaré sphere; credit: wikipedia

Synthetic observation with POLARIS

Küffmeier, Reißl et al. 2020

Emitted radiation

at 1.3 mm: polarization traces magnetic field structure

Synthetic dust polarization maps at 1.3 mm

Küffmeier, Reißl et al. 2020

Emitted radiation

Polarization fraction in bridge:

a few %

Polarization fraction in bridge:

up to 20 %

IRAS 16293--2422

Sadavoy et al. 2018

alignment efficiency higher than efficiency produced by standard RAT alignment

(also Le Goeullec+20)

Wavelength dependence: 1.3 mm vs 53 micron

Emitted radiation

1.3 mm: good tracer of magnetic field

53 micron: poor tracer of magnetic field

Küffmeier, Reißl et al. 2020

Two reasons for wavelength dependence

Küffmeier, Reißl et al. 2020

Self-scattering

Dichroic extinction

Take-away for scales beyond >100 au

< 200 micron: dichroic extinction and self-scattering; no trace of B

> 200 micron: thermal emission; linear polarization traces B

Conclusion and outlook

Linear polarization of dust reemission at wavelength >200 micron is good tracer of magnetic field structure on scales >100 au

Goal: statistical comparison of synthetic polarization maps with observations (summer project of UVa student Hannah Woodward)

Credit: Pelkonen et al. 2021

Disk size distribution

Alternative/additional effect to external photoevaporation:

Disks are born small in star-forming regions with high ionization

Tobin+ 2019

magnetohydrodynamics

\frac{\partial \mathbf{B}}{\partial t} = \nabla\times (\mathbf{v}\times\mathbf{B})

ideal MHD

\color{red}-\nabla\times[\eta_{\rm O} (\nabla \times \mathbf{B})]
\color{purple}-\nabla\times \{\eta_{\rm H} [(\nabla \times \mathbf{B}) \times \mathbf{B}/B] \}
\color{blue}-\nabla\times \{\eta_{\rm AD} \mathbf{B}/B \times [(\nabla \times \mathbf{B}) \times \mathbf{B}/B] \}

Ohmic dissipation

Hall

ambipolar diffusion

Non-ideal

Resistivity depends on ionization rate

Küffmeier, Zhao & Caselli 2020

Question: What is the effect on disk formation when differing the (cosmic-ray) ionization rate?

In fact, we use a 2D setup (r- and z-coordinate)

Parameter study

Küffmeier, Zhao & Caselli 2020

Effect of ionization on disk size

increasing ionization rate

enhanced magnetic braking

smaller disks

Küffmeier, Zhao & Caselli 2020

Are disks already born small in some regions?

 because of higher (cosmic-ray) ionization rates??