# Radiative processes

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# General

This course is given by prof. H. Van Winckel. It consists of 13 two-hour lectures in the first semester. There is a 160 pages course and slides available on Toledo. The exercises at the end of the chapters are old exam questions. The exam is open book, with a written preparation and an oral examination. Each question counts for around the same points. Do not forget to bring your calculator as you will need it! You will need probably all the given time to finish the exam, so keep this in mind and do not spend to much time on parts you don't get/know.

# Exams

## Examen 21 January 2011 Afternoon

Vraag 1: Short Questions

• A non-illuminated optical thin source has a line. Is it emission or absorption? If the line is optically thin but the continuum optically thick, what can you say? What if the opposite (line thick and continuum thin) is true?
• What is the energy density of the CMB? If 1-z = T_emitted/T_observed, what is the energy density of the CMB for redshift 4? And what was it when the decoupling happened (z = 1089)?
• A pointlike object in the sky, measured at radio wavelength. Assume it is thermal and has a certain temperature, determine $$F_{\nu }$ $ if you know it is a circular projection at an angle of 10 arcseconds.

Vraag 2: There is a plasma stream falling on to a White Dwarf (WD) of 0.5 M_solar and R = 8000 Km. The energy of the infalling matter is absorbed onto an area of 1% of the total WD surface. The mass transfer is $10^{-9}M_{solar}/year$ The energy of the mass is absorbed in the region of 1m deep into the surface. This region is heated by the plasma and is optically thin. The density of this area is: $\rho =10^{-2}$ • What is the number density in the shock absorbing region. Assume the plasma consists of H only.
• Calculate the potential energy per second (J/s) from accreting material by a fall from infinity.
• The potential energy is converted fully to thermal energy. What is the power dropped in $1m^{3}$ ?
• The radiative power of the shock absorbing region is all from the accretion power. Use the integrated volume emissivity of the thermal bremsstralung to find the temperature and the piek in the radiation. (Gauntfactor is 1).

Vraag 3 Two figures are given, both containing pieces of the spectra of two stars. What can you deduce from these figures?

## Exam 15 January 2010 Afternoon

Vraag 1: Short questions

• A non-illuminated optical thin source has a line. Is it emission or absorption? If the line is optically thin but the continuum optically thick, what can you say? What if the opposite (line thick and continuum thin) is true?
• In the derivation of the electron emission, we used the assumption that the collisions happen at non-relativistic speed. For what temperature does this hold? Using the Maxwell distribution, motivate the assumption. If we get a X-ray of 10 keV, does the classical approach hold?
• What is the energy density of the CMB? If 1-z = T_emitted/T_observed, what is the energy density of the CMB for redshift 4? And what was it when the decoupling happened (z = 1089)?
• Assume TE. How many photons are there in a cube with a side the same length as the maximal wavelength? Is this temperature dependant?

Vraag 2 Assume the Earth has no atmosphere, and is perfectly emitting and absorbing. Calculate the temperature at the equator and at the poles. Make a drawing to clearify your answers. a) The Earth has infinite conductivity. b) The Earth has zero conductivity and rotates synchrone with its orbit (cfr. the moon around the Earth, so you have a side of the Earth that's always away from the sun). Express your answer with help of the polar angle. c) The Earth has zero conductivity and rotates very fast, but it's polar axis is now perpendicular on the line planet-sun. Bijvraag: welke benadering is het meest toepasselijk voor de echte aarde?

Vraag 3 Two figures are given, both containing pieces of the spectra of two stars. What can you deduce from these figures?

## Exam 15 January 2010 Morning

Vraag 1: Short questions

• Show that if stimulated emission is neglected, so that there are only two Einstein coefficients left in the transfer equation. An appropriate relation between the Einstein coefficients will be consistent with the Thermal equilibrium between the atom and the radiation field of a Wien spectrum, not a Planck spectrum.
• The strength of CO lines in the IR increases in metal poor objects. This seems contradictory. Can you explain this.

Vraag 2:

Describe the continuum processes in the stellar atmospheres which are relevant for the Sun. Compare the effects for a star with the same effective temperature but with a metalicity of 1/1000 solar.

Vraag 3:

There is a plasma stream falling on to a White Dwarf (WD) of 0.5 M_solar and R = 8000 Km. The energy of the infalling matter is absorbed onto an area of 1% of the total WD surface. The mass transfer is $10^{-9}M_{solar}/year$ The energy of the mass is absorbed in the region of 1m deep into the surface. This region is heated by the plasma and is optically thin. The density of this area is: $\rho =10^{-2}$ • What is the number density in the shock absorbing region. Assume the plasma consists of H only.
• Calculate the potential energy per second (J/s) from accreting material by a fall from infinity.
• The potential energy is converted fully to thermal energy. What is the power dropped in $1m^{3}$ ?
• The radiative power of the shock absorbing region is all from the accretion power. Use the integrated volume emissivity of the thermal bremsstralung to find the temperature and the piek in the radiation. (Gauntfactor is 1).

Vraag 4: Explain a hermes spectrum. (From a nova)