# Stellar Atmospheres and Winds

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# General Information

This course consists of 2 main topics. The first half of the semester is used for stellar atmospheres and the second half is dedicated to stellar winds. The examination may constist of a project or a more traditional exam. In 2010 the examination consisted of a project around stellar atmspheres, and an openbook exam of the second part: stellar winds. The project was due to the easter holidays, but it is also discussed during the examination of the second part.

# Exams

## 7 june 2010

### Stellar winds (exam)

Question 1

Give an explanation for de discrepantie between the theoretical mass loss of AGB stars ${\displaystyle 10^{-25}-10^{-9}M_{\odot }/yr}$ and the observed values: ${\displaystyle 10^{-8}-10^{-3}M_{\odot }/yr}$.

Question 2

A figure with 2 spectra of an WN 8 star is given. Both spectra are taken with an interval of 750 days. The star is characterized by the following parameters: ${\displaystyle M=17M_{\odot },R=15R_{\odot },L=10^{5.3}L_{\odot },T_{eff}=30000K,\kappa _{e}=0.31cm^{2}/g}$.

This question was taken from the exercises about line driven winds (ex.5).

• Determine the terminal velocity of the star from the P cygni profiles in the spectra. (The rest wavelength of the lines is given).
• The profile of both lines varies clearly in time. Can you give an explanation for this?
• The mass loss rate of this star can be estimted using the CAK-model. Taking all constants into acount the model is given by:

${\displaystyle M_{CAK}=({\frac {\kappa _{e}}{4\pi c}})^{1/\alpha }{\frac {4\pi }{\kappa _{e}v_{th}}}\alpha 0.32^{1/\alpha }({\frac {1-\alpha }{GM(1-\gamma _{e})}})^{(1-\alpha )/\alpha }L^{1/\alpha }}$

Use this relation to obtain the mass loss rate of the star. Does this mass loss rate follows the relation for the single scattering limit? If yes, is this expected for this type of stars? If no, explain.

Question 3

There is a table given in which the temperature and radiative acceleration of different O based molecules (silicates ect.) is given on different distances from a star, together with these properties of amorphous carbon and Fe. There is also a plot given of the scattering efficiency of the silicates. You may also use other plots from the textbook or slides.

• Discuss the difference in temperature between C, O and Fe based molecules.
• What can you say about the difference in mass loss between Carbon stars and Oxygen stars?

Question 4 (Bonus question)

There is a plot shown of a pretty weird absorption line of water. Give a possible explanation for this form. (It turned out to be a massering line)

### Stellar atmospheres (project)

The project that was made for this part of the course is discussed. They let you explain what you did, and what your results where. Some extra questions are asked.