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Stars and Planet Formation > Section 4 > Flashcards

Section 4 Flashcards

1
Q

What happens when star light hits a dust cloud?

A

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

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2
Q

What is reddening?

A

Red light is transmitted (passes through) more efficiently than blue light e.g Barnard 68

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3
Q

What is flux of radiation and what is it for a black body?

A

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

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4
Q

What type of relationship does flux and luminosity have with distance from a star?

A

inverse square law

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5
Q

How is brightness of a star at a wavelength measured?

A

By it’s apparent or absolute magnitude (measured in log scale)

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6
Q

What does a difference of 1 magnitude correspond to?

A

A difference in brightness by a factor of 2.5

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7
Q

What must be considered if there is dust along the line of sight?

A

Extinction at a wavelength (A)

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8
Q

To find the observed colour index, what needs to be considered?

A

The intrinsic colour index and colour excess (added together)

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9
Q

What are extinction and colour excess proportional to?

A

The column density of dust grains along the line of sight

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10
Q

What is R_v?
How much is in the diffuse ISM?

A

The ratio of total to selective extinction
R_v = 3.1

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11
Q

When do objects become redder?

A

When there is more dust along the line of sight

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12
Q

What can happen to radiation when it travels along a distance?

A

It can be absorbed (by dust grains) or scattered ( a photon with the same energy is reemitted in a different direction)

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13
Q

What happens in local thermodynamic equilibrium?

A

Sv = Bv(dust)
as it acts like a blackbody

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14
Q

What can make the intensity increase?

A

Thermal emission from dust

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15
Q

What is emissivity?

A

The energy per unit volume per unit time emiited into the direction

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16
Q

What does extinction cause?

A

Radiation at shorter wavelengths (bluer light) to be more strongly scattered and absorbed than that at longer wavelengths (redder light)

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17
Q

What are interstellar dust grain properties?

A

Extinction efficiency factor

Polarisation of star light

Dichroic extinction

Observations of polarised light and thermal emission

The Zeeman effect

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18
Q

What is the opacity?

A

The total extinction cross section per mass of interstellar material

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19
Q

What is Qv?

A

Extinction efficiency factor (Qv_abs + Qv_sca)

20
Q

What is Mie theory?

A

describes the scattering of EM radiation by spherical particles (works well for wavelengths between infrared and visible)

Qv = a_d/lamda

21
Q

How can the empirical form of efficiency be obtained?

A

From the interstellar extinction curve

22
Q

What happens to the efficiency at longer wavelengths (far-infrared and mm)?

A

The ISM is generally transparent (no absorption) so one needs to observe the emission from heated dust clouds to determine extinction and efficiency factor

23
Q

In the densest clouds and disks what does beta tend towards?

A

The lower end of the range (at around 1) and it lies closer to 2 in more diffuse environments

24
Q

What happens once the grain size is larger than wavelength?

A

The opacity and efficiency no longer depends on wavelength

25
Q

What processes lead dust to polarise light?

A

Dichroic extinction

Scattering

Thermal emission

26
Q

In Milky Way in polarised light image, what does the dark red region represent

A

More polarised light

27
Q

Why is there a correlation between polarisation and extinction?

A

As dust grains are elongated and aligned (so they are also para-magnetic and spinning in a magnetic field)

28
Q

What happens to grains in the presence of a magnetic field?

A

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

29
Q

What happens to the alignment of the dust grains in a magnetic field?

A

Their long axis becomes aligned perpendicular to B field due to the torque produced by the field (M x B)

30
Q

What is dichroic extinction?

A

Polarisation:grain preferentially absorbs light polarised perpendicular with the B field such that light polarised parallel is transmitted

31
Q

Where is the electric fieldin dichronic extinction ?

A

Driving charge down the grain’s long axis (perpendicular to B)

32
Q

What happens to radiation scattered in directions 90 degrees from n?

A

The scattered electric field only oscillates along the line that is the projections of the new plane and the radiation is linearly polarised

33
Q

What happens to radiation which is scattered in other directions?

A

Partial Polarisation: electric field oscillates along two orthogonal lines but unequal amplitude

34
Q

What is a property of a scattering event?

A

Photons are scattered in Sz plane can only scatter in horizontal plane and photons in Sx place can only scatter in vertical component

35
Q

What happens to light emitted by a star behind a dust cloud?

A

It is absorbed along the main axis of the dust grains

36
Q

Which direction is polarisation in the optical (transmitted)?

A

Orthogonal to magnetic field and to the grain

37
Q

Which direction is polarisation in the infrared (emitted)?

A

Perpendicular to magnetic field and orthogonal to the optical polarisation

38
Q

What is thermal emission perpendicular to?

A

magnetic field

39
Q

What is the shape of the magnetic field in the polarised emission?

A

hour-glass

40
Q

What is the Zeeman effect?

A

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)

41
Q

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) ?

A

Their total angular momentum

Angular momentum is quantised (and the energy levels) as the magnetic field exerts a force on the atom

42
Q

What is the splitting between the highest and lowest level for the 21cm line of atomic hydrogen?

A

2.8 uG Hz

43
Q

What does self scattering cause?

A

Polarisation vectors to change orientation due to dust grains interacting with themselves

44
Q

When does the magnetic field strength increase?

A

With increased density which indicates that field lines can be compressed along with gas (flux freezing)

45
Q

What do denser regions have?

A

A stronger magnetic field (due to flux freezing) leading to an hour-glass magnetic field structure

Stars and Planet Formation (14 decks)

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