Section 4 Flashcards
What happens when star light hits a dust cloud?
Some blue light is reflected
Some blue light is absorbed and reradiated at longer (redder) wavelengths
Red light is transmitted (passes through) more efficiently than blue light
What is reddening?
Red light is transmitted (passes through) more efficiently than blue light e.g Barnard 68
What is flux of radiation and what is it for a black body?
The amount of energy emitted from an object’s surface per unit area per unit time (W m^-2)
For a black body, Stefan-Boltzmann law applies (F = sigma T^4
What type of relationship does flux and luminosity have with distance from a star?
inverse square law
How is brightness of a star at a wavelength measured?
By it’s apparent or absolute magnitude (measured in log scale)
What does a difference of 1 magnitude correspond to?
A difference in brightness by a factor of 2.5
What must be considered if there is dust along the line of sight?
Extinction at a wavelength (A)
To find the observed colour index, what needs to be considered?
The intrinsic colour index and colour excess (added together)
What are extinction and colour excess proportional to?
The column density of dust grains along the line of sight
What is R_v?
How much is in the diffuse ISM?
The ratio of total to selective extinction
R_v = 3.1
When do objects become redder?
When there is more dust along the line of sight
What can happen to radiation when it travels along a distance?
It can be absorbed (by dust grains) or scattered ( a photon with the same energy is reemitted in a different direction)
What happens in local thermodynamic equilibrium?
Sv = Bv(dust)
as it acts like a blackbody
What can make the intensity increase?
Thermal emission from dust
What is emissivity?
The energy per unit volume per unit time emiited into the direction
What does extinction cause?
Radiation at shorter wavelengths (bluer light) to be more strongly scattered and absorbed than that at longer wavelengths (redder light)
What are interstellar dust grain properties?
Extinction efficiency factor
Polarisation of star light
Dichroic extinction
Observations of polarised light and thermal emission
The Zeeman effect
What is the opacity?
The total extinction cross section per mass of interstellar material
What is Qv?
Extinction efficiency factor (Qv_abs + Qv_sca)
What is Mie theory?
describes the scattering of EM radiation by spherical particles (works well for wavelengths between infrared and visible)
Qv = a_d/lamda
How can the empirical form of efficiency be obtained?
From the interstellar extinction curve
What happens to the efficiency at longer wavelengths (far-infrared and mm)?
The ISM is generally transparent (no absorption) so one needs to observe the emission from heated dust clouds to determine extinction and efficiency factor
In the densest clouds and disks what does beta tend towards?
The lower end of the range (at around 1) and it lies closer to 2 in more diffuse environments
What happens once the grain size is larger than wavelength?
The opacity and efficiency no longer depends on wavelength
What processes lead dust to polarise light?
Dichroic extinction
Scattering
Thermal emission
In Milky Way in polarised light image, what does the dark red region represent
More polarised light
Why is there a correlation between polarisation and extinction?
As dust grains are elongated and aligned (so they are also para-magnetic and spinning in a magnetic field)
What happens to grains in the presence of a magnetic field?
Grains polarise radiation through dichroic extinction and rotate around their shorter axis
As they have an electric charge and are paramagnetic they acquire a magnetic moment
What happens to the alignment of the dust grains in a magnetic field?
Their long axis becomes aligned perpendicular to B field due to the torque produced by the field (M x B)
What is dichroic extinction?
Polarisation:grain preferentially absorbs light polarised perpendicular with the B field such that light polarised parallel is transmitted
Where is the electric fieldin dichronic extinction ?
Driving charge down the grain’s long axis (perpendicular to B)
What happens to radiation scattered in directions 90 degrees from n?
The scattered electric field only oscillates along the line that is the projections of the new plane and the radiation is linearly polarised
What happens to radiation which is scattered in other directions?
Partial Polarisation: electric field oscillates along two orthogonal lines but unequal amplitude
What is a property of a scattering event?
Photons are scattered in Sz plane can only scatter in horizontal plane and photons in Sx place can only scatter in vertical component
What happens to light emitted by a star behind a dust cloud?
It is absorbed along the main axis of the dust grains
Which direction is polarisation in the optical (transmitted)?
Orthogonal to magnetic field and to the grain
Which direction is polarisation in the infrared (emitted)?
Perpendicular to magnetic field and orthogonal to the optical polarisation
What is thermal emission perpendicular to?
magnetic field
What is the shape of the magnetic field in the polarised emission?
hour-glass
What is the Zeeman effect?
The splitting of energy levels under the influence of a magnetic field as the magnitude of the splitting is proportional to the magnetic field
(it shows how magnetic field changes)
What is magnetic moment of atoms proportional to and what does this lead to when a magnetic field is present(in terms of zeeman effect) ?
Their total angular momentum
Angular momentum is quantised (and the energy levels) as the magnetic field exerts a force on the atom
What is the splitting between the highest and lowest level for the 21cm line of atomic hydrogen?
2.8 uG Hz
What does self scattering cause?
Polarisation vectors to change orientation due to dust grains interacting with themselves
When does the magnetic field strength increase?
With increased density which indicates that field lines can be compressed along with gas (flux freezing)
What do denser regions have?
A stronger magnetic field (due to flux freezing) leading to an hour-glass magnetic field structure
