Problem set 2 for ASTR 323: The Local Universe

Due in class Fri Feb 1.

(1) Give one reason why the very first stars to form might be more massive than current-day stars.

(2) Carefully watch the simulation of the growth of a dark halo like the Milky Way's at http://www.ucolick.org/~diemand/vl/movies.html
Note that this simulation is displayed in physical coordinates; the expansion of the universe is shown.
Look at the formation history from z=12 to 0. (The higher-resolution movies, in particular the 218 MB one, are much better for this)

Select 5 of the most massive sub-halos and watch what happens as they are accreted. Are they totally disrupted? If so, how many orbits does this take?

Make an estimate of the where the edge of the halo is at z=5,4,3,2,1.5,1.0,0.5,0.3 and 0. Then make a rough estimate of how many sub-halos are accreted by the big halo in between each of these times, and how massive each one is, compared to the mass of the existing big halo. (You may decide to ignore the smallest halos, but say how you decide this). Plot up the increase of mass of the halo with time. This will involve a lot of stepping through the movie, and some snapshots.

Compare your results with Figure 1 of Diemand et al (ApJ 667, 859, 2007).

(3) Distance scale, group problem:

Find one of the papers in the astronomical literature which derives a distance to an open or globular cluster via main-sequence fitting to nearby field stars with Hipparcos parallaxes. Write a one-page critical summary of the paper, discussing its strengths and any weaknesses that worry you. (Extra credit: find another paper that disagrees with your paper and try to work out why).