Cosmic Dust and Magnetic Fields

Dust and magnetic fields are ubiquitous in the universe. Interstellar dust is the seeds of star and planet formation. Magnetic fields are expected to play important roles in formation and evolution of stars. The formation of low-mass stars is expected to begin from molecular clouds, evolving to dense cores, protostellar disks, and stars. Magnetic fields are believed to play important roles in these processes. Many instruments with powerful polarimetric capabilities (e.g., SOFIA, BlastPol, Planck, SMA, ALMA, etc.) allow us to map magnetic fields from the large scales of molecular clouds to the small scales of disks. In addition, computational simulations of dust polarimetry is essential to reconstruct realistic structure of magnetic fields. However, previous simulations do not treat grain alignment adequately, due to the lack of a quantitative theory of grain alignment. Such a predictive theory of grain alignment is now available.

Grain Alignment Theory

Observations of starlight polarization indicate that interstellar grains are non-spherical and aligned with the magnetic field. The problem of how dust grains become aligned with the magnetic field is a longstanding problem in astrophysics. Despite significant efforts over the last 60 years, a predictive theory of grain alignment was not available until recent years. Inspired by a popular toy of kids--pinwheel, in Lazarian and Hoang (2007), we realized that a beam of photons would also exert a net torque on a helical grain, the same as a flow of gas exerting a torque on the pinwheel because photons carry intrinsic momentum. With a simple toy model of helical grain, we derived analytical formulas of radiative torques induced by a radiation beam, which reproduce successfully functional forms of radiative torques computed with discrete dipole approximation and induce the similar grain alignment Hoang and Lazarian (2008) . Using the analytical model for experimenting grain alignment in different conditions, my works (Hoang and Lazarian 2009a and Hoang and Lazarian 2009b) identified the basic features of radiative alignment, which are confirmed by numerous observations to date. Finally, a unified theory of grain alignment for grains with iron inclusions is presented in Hoang and Lazarian (2016) .

Physical Modeling of Dust Polarization

To directly test the predictions, extensive ab-initio numerical modeling of dust polarization by radiatively aligned grains have been conducted for a variety of environments, including starless cores, zodiacal cloud, cometary coma (Hoang and Lazarian 2014), and reflection nebula IC 63 (Hoang, Lazarian, and Andersson 2015).

Measuring the Magnetic Field Strength via UV Polarimetry

To date, dust polarization only provides us the direction of magnetic fields projected in the sky, and measuring its strength must rely on other techniques (e.g, Zeeman, Chandrasekhar-Fermi techniques). In Hoang, Lazarian, and Martin (2014) we proposed a novel method to fill the gap based on paramagnetic alignment of small grains. Indeed, while a wealth of observations is in favor of RAT alignment for big interstellar grains, paramagnetic relaxation is inevitable for the alignment of small grains where radiative torque alignment is inefficient. Our numerical calculations show that the degree of alignment of small grains is considerable and increases with the magnetic field strength. Because the alignment of small grains is mostly responsible for UV polarization of starlight, inference of the alignment from UV polarization provides the actual strength of the magnetic field. To infer the degree of alignment from UV polarization, I employed my Monte Carlo simulation-based inversion technique code. Applying to the diffuse ISM, I found a good correspondence with the measurements from other techniques, such as Zeeman effect.

Dust Dynamics: Grain Acceleration

Research on exoplanets and planetary systems recently become an increasingly hot topic among the astrophysics community; and principal mechanisms underlying the planetesimal formation are strongly debated. Dust coagulation is widely believed to be the first step during the process of planetesimal formation. My research has focused on the acceleration of charged grains, which allows us to better understand the rate of dust coagulation and planetesimal formation, eventually.

For grains larger than 0.1 micron, Hoang, Lazarian, and Schlickeiser (2012) identified that the fast modes of MHD turbulence can efficiently accelerate them through transit-time damping (TTD), which arises from resonant interactions of particles with the compressive component of magnetic fluctuations (i.e., the component parallel to the mean magnetic field). For smaller grains, Hoang and Lazarian (2012) find that acceleration of very small dust grains due to random charge fluctuations.

My recent work (Hoang, Lazarian, and Schlickeiser 2015) shows that dust grains can be accelerated by powerful radiation pressure to speeds close to that of light, which may act as primary particles of ultrahigh energy cosmic rays if relativistic dust could survive their journey to Earth. Our quantitative calculations show that relativistic dust has little chance surviving in the interstellar medium due to Coulomb explosions.

Publications

  1. Soam, A., et al., including Thiem Hoang, Magnetic fields towards Ophiuchus-B derived from SCUBA-2 polarization measurements, 2018, ApJ, in press
  2. Kwon, J., et al., including Thiem Hoang, A First Look at BISTRO Observations of the rho-Oph-A core , 2018, ApJ, 859,4
  3. Hoang Thiem, and Lazarian,A., A unified model of grain alignment: radiative alignment of interstellar grains with magnetic inclusions," 2016, ApJ, 831, 159, arXiv:1605.02828
  4. Hoang Thiem, Lazarian A., Andersson, B-G "Modelling grain alignment by radiative torques and hydrogen formation torques in reflection nebula," 2015, MNRAS, 448, 1178
  5. Hoang Thiem, Lazarian A., and Martin P.G., "A novel method for measuring interstellar magnetic fields using UV polarimetry," 2014, Astrophysical Journal, 790, 6
  6. Hoang Thiem, Lazarian A., and Schlickeiser, R., On Origin and Destruction of Relativistic Dust and its Implication for Ultrahigh Energy Cosmic Rays," 2015, Astrophysical Journal, 806, 255
  7. Hoang Thiem, Lazarian A., Numerical Studies for Acceleration of small dust grains due to charge fluctuations," 2012, Astrophysical Journal, 761, 96
  8. Hoang Thiem, Lazarian A., Schlickeiser R., Revisiting acceleration of charged grains in MHD turbulence," 2012, Astrophysical Journal, 747, 54
  9. Ivlev, A., Lazarian, A., Tsytovich, V.N., de Angelis, U., Hoang Thiem, Morfill, G.E., "Acceleration of Small Astrophysical Grains due to Charge Fluctuations," 2010, Astrophysical Journal, 723, 612
  10. Hoang Thiem, Lazarian A., "Grain Alignment by Radiative Torques in Special Astrophysical Environments," 2014, MNRAS, 438, 680-703
  11. Andersson B.-G., Piirola V., J. De Buizer, J., et al. (Hoang Thiem), "Evidence for H2 Formation Driven Dust Grain Alignment in IC 63," 2013, Astrophysical Journal, 775, 84
  12. Hoang Thiem, Lazarian A., "Alignment of dust grains: effects of internal relaxations of energy and complex radiation fields," 2009, Astrophysical Journal, 697, 1316-1333
  13. Hoang Thiem, Lazarian A., "Radiative torques alignment in the presence of pinwheel torques," 2009, Astrophysical Journal, 695, 1457-1476
  14. Hoang Thiem, Lazarian A., "Radiative torques alignment: Essential physical processes," 2008, MNRAS, 388, 117-143
  15. Lazarian A., Hoang Thiem, "Alignment of Dust with Magnetic Inclusions: Radiative Torques and Superparamagnetic Barnett and Nuclear Relaxation," 2008, Astrophysical Journal Letter, 676, L25
  16. Whittet D., Hough J. H., Lazarian A., Hoang Thiem, "The Efficiency of Grain Alignment in Dense Interstellar Clouds: A Reassessment of Constraints from Near Infrared Polarization," 2008, Astrophysical Journal, 674, 304
  17. Lazarian A., Hoang Thiem, "Subsonic Mechanical Alignment of irregular grains," 2007, Astrophysical Journal Letter, 669, L77
  18. Lazarian A., Hoang Thiem, "Radiative torques: analytical model and basic properties," 2007, MNRAS, 378, 910-946