What does a more massive neutron star constitute?

tomjennings

a. it rotates faster
b. it is smaller
c. it is more luminous
d. it has stronger pulses

I'm stumped on this practice question for my final. I'm thinking that a. and d. contradict each other since doesn't an older, slower rotating neutron strong emit longer, stronger pulses? C. seems to make a little sense since the more massive a star, the greater its pressure, temperature and thus its luminosity, but that's only for main sequence stars and a neutron star is degenerate so its temperature has no effect on its pressure and its luminosity, right? B. seems to be a trick, but this idea holds for very large stars that have flimsy, expanded outer layers and small, compressed cores with a high mass and low volume, right?

I'm trying to think through this question, but I clearly have no idea what the answer is. Does anybody

SHISHKABOB

Using conservation of angular momentum, you should be able to consider a properly.

For b remember that neutron stars are neutron stars because the pressure inside of them is dominated by neutron degeneracy pressure, make sure to consider what that means.

For c it looks like you've figured it out.

I don't know much about neutron star pulses though >.>

Chronos

a. Not necessarily. Depends on initial conditions.
b. False. More mass = more size. Further compression is not possible.
c. Not necessarily, an accreting neutron star can be more luminous.
d. Not necessarily. Related to a.

If you disregard accretion, c is the best choice among these options. A more massive star is larger, hence has more surface area.

Drakkith

Are you sure Chronos? I've read that adding mass actually decreases the radius of a white dwarf or neutron star. The wiki article on white dwarfs says the same thing too. See here: http://en.wikipedia.org/wiki/File:Wh...***-radius.jpg

goldsax

i to thought that increased mass in the case of neutron stars lead to smaller radius due to increased gravity until Tolman–Oppenheimer–Volkoff limit is reached

mfb

Maybe it is not monotonic?

"Depends on initial conditions." is the important point for the situation.
- The angular velocity depends on the rotation of the initial star (and maybe influences from other stars nearby, if in a multiple star system)
- The size should depend on the angular velocity
- The luminosity depends on its age, and it is larger if mass is flowing towards the neutron star
- pulse strength depends on the situation, too

Chronos

Agreed. The density does increase sufficiently to reduce neutron star radius as mass increases. See figure 8 of http://diposit.ub.edu/dspace/bitstre...2/1/165558.pdf. So [b] is the correct answer.