Rotational Brownian motion (astronomy)

Rotational Brownian motion, in the context of astronomy, refers to the random, fluctuating rotational movement exhibited by small particles or bodies suspended in a gaseous or fluid medium due to thermal fluctuations. This motion arises from the constant bombardment of the particle by surrounding molecules, imparting random torques that cause it to rotate.

While translational Brownian motion, the random translational movement, is more widely known, rotational Brownian motion plays a significant role in the dynamics of dust grains in interstellar space, protoplanetary disks, and planetary atmospheres. The magnitude of the rotational Brownian motion depends on the temperature of the surrounding medium, the size and shape of the particle, and the viscosity of the medium.

The rotational Brownian motion can have several important effects on astronomical phenomena:

• Alignment of dust grains: In the presence of an external magnetic field, dust grains tend to align themselves. Rotational Brownian motion works against this alignment, causing grains to misalign and reducing the polarization of starlight. The degree of alignment depends on the competition between the aligning torque and the randomizing effect of Brownian motion.

• Radiative transfer: The orientation of dust grains affects the scattering and absorption of light. Rotational Brownian motion influences the average orientation of grains, thus impacting the radiative transfer properties of the medium.

• Chemical reactions: The rotational motion of small particles can influence the rate of chemical reactions on their surfaces. The rotation can enhance mixing of reactants on the surface and affect the sticking probability of molecules.

• Aerodynamic torques: In environments with strong flows or velocity gradients, aerodynamic torques can also influence the rotation of particles. Rotational Brownian motion can either compete with or enhance the effects of these aerodynamic torques.

The study of rotational Brownian motion in astronomy is important for understanding the physical and chemical processes occurring in various astrophysical environments. Detailed modeling and simulations are used to accurately predict the effects of rotational Brownian motion on the observed properties of these environments.