S4 - Interstellar dust Flashcards
What effect does interstellar dust have on light (below FIR)?
When light enters a dust cloud, some blue (shorter wavelength) light is reflected whilst the rest is absorbed and re-radiated at longer (redder) wavelengths (reddening). Red light is transmitted through dust clouds more efficiently than blue light.
Objects become redder when there is more dust along the line of sight.
What is extinction?
Extinction is the dimming of light from distant objects due to interactions with dust in the ISM along the line of sight.
What is a problem with Mie theory?
Dust grains are assumed to be compact spheres, however, they look like elongated, fluffy and porous aggregates.
Do dust grains respond to magnetic fields?
Yes, dust grains are para-magnetic and are aligned perpendicular to the magnetic field.
What mechanisms can lead dust grains to polarise light?
Dichroic extinction, scattering, and thermal emission.
Explain what dichroic extinction is.
Dichroic extinction is extinction by dust grains aligned by a magnetic field, which causes polarization of starlight.
Dust grains have a small electric charge and are paramagnetic so they have a magnetic moment. Interaction with a magnetic field then creates a torque which gradually forces the grain’s short axis to align with the field. Thus the dust grains tend to line up such that their long axis is aligned perpendicular to B. This direction becomes the one of maximum absorption of incoming radiation. Light parallel to the magnetic field is transmitted completely.
i.e. Dust grains are aligned perpendicular to the magnetic field so they absorb radiation aligned with their long axis and let radiation parallel to the magentic field through (polarising the light).
Explain polarised thermal emission.
Light emitted by a star behind a dust cloud is absorbed along the main axis of the dust grains. Optical light is polarised perpendicular to the dust grain (i.e. the component of light perpendicular to the long-axis of the dust grain is transmitted and parallel to the long-axis is absorbed). Absorbed light is re-emitted by the dust grain as infrared radiation along the long-axis of the grain. The result is light from a dust cloud having an optical component perpendicular to the dust grains and an infrared component parallel to the dust grains.
Explain the Zeeman effect and what it can be used for?
Atoms have magnetic moments which are proportional to their total angular momentum, J. When a magnetic field is present, it exerts a force on the atom which splits apart the angular momentum energy levels (Zeeman effect). This can be seen by the splitting of a spectral line into several components. The magnitude of line-splitting is proportional to the magnetic field strength so it can be used to measure the strength of magnetic fields.
What is the magnetic field distribution within a molecular cloud?
Ranging from 10μG in the outer parts to about 50μG in compact central regions. There is a gradient of magnetic field strength within a molecular cloud.
What is the relation, if any, between magnetic field strength and density of a molecular cloud?
Typically, the magnetic field strength increases with density: this is an indication that field lines can be compressed along with the gas in a molecular cloud.
What is the polarising effect of scattering on light?
Scattering partially polarises light in directions other than the original direction of propagation.
