Template:ASTR508/Compact Objects

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Compact Objects

Reading Assignment

Chapter 6 of Collins, George W. (1989), The Fundamentals of Stellar Astrophysics, Freeman, ISBN 0-7167-1993-2, PDF

White Dwarfs

Neutron Stars

Assignment

Part 1: White-Dwarf Cooling

You will use the output from the one-solar-mass star that you did in week 2 to look at white-dwarf cooling. Unfortunately, the program stopped when the luminosity was less than one-tenth of solar, so you will have to restart the calculation where you left off. To do this

  • Copy the directory with your results so far using the following command:
 cp -rp 1M_pre_ms_to_wd 1M_pre_ms_to_wd_continued
  • In the new directory, rename final.mod to restart.mod using
 mv final.mod restart.mod
  • Edit the inlist_1.0 file to use the restart.mod to start. Delete the following line
     create_pre_main_sequence_model = .true.
  • Add the following lines
     load_saved_model = .true.
     saved_model_name = 'restart.mod'
  • Set log_L_lower_limit = -6 in the inlist_1.0 file.
  • Run the new model.

You will find that the history file literally starts where you left off. The model numbers and times are sequential, so you can combine the two history files without too much trouble. Furthermore, the new run will also add new profile files ... it will not overwrite the old ones.

Using the combined history file, I would like for you to construct the cooling curve for the white dwarf in the V-band, so on the x-axis you will have the base-ten logarithm of the age of the white dwarf and on the y-axis you will have the absolute V magnitude. You should define the time of birth of the white dwarf to coincide the local maximum in the luminosity on the AGB.

Also construct the cooling curve using the total luminosity. You should observe several slopes in this cooling curve. Identify the dominant process in each regime, using what you learned in class.

Part 2: Neutron-Star Accretion

You will be looking at the following models in the test_suite: ns_h and ns_he. You will need to change the frequency of the history file output because we will want high time resolution, so change history_interval to 1 in both of the inlists. You can also decrease profile_interval too, if you want to look at the structure of the burning layers for yourself. I would like you to plot luminosity versus time for both of the models and answer the following questions for hydrogen and helium accretion.

  1. What is the rise time of the burst?
  2. How does the luminosity change after the initial rise?
  3. What is the duration of the plateau of the burst, if there is one?
  4. What is the decay time after the burst?

Finally speculate why the behaviour is so different using what you know about hydrogen and helium burning.