Neutron Stars and Black Holes PHYS390: Astrophysics Professor Lee Carkner Lecture 18

Question 1)Should there be a lower limit for the mass of observed white dwarfs? Why or why not?  Yes, size of white dwarf depends on initial mass of star, very low mass stars have not had time to evolve to white dwarf yet

Neutron Stars   None detected until the 1960s   Principally observed as pulsars  rapidly rotating neutron star producing beamed radio emission

Neutron Degeneracy   Density of neutron star ~  Star is like a big ball of ~  Acceleration of gravity at surface ~ trillion meters per second 2   Mass limit of ~3 M sun

Neutron Star Formation   Start with iron core  At high densities electrons become relativistic and combine with protons to produce neutrons (and neutrinos)   As density increases neutrons “drip” outside of nuclei  Can form superconducting superfluid

Neutron Star Structure   inner crust of heavy nuclei and free neutrons   interior mostly neutrons  maybe a core of sub- nuclear particles?

Rotation   Ratio of initial and final periods: P f /P i = (R f /R i ) 2   End up with neutron star rotation periods ~ 1 second

Flux Freezing   Magnetic fields get “frozen” into core material and concentrated as core shrinks  B f /B i = (R i /R f ) 2  Again, hard to know initial core magnetic field  Typical neutron star B ~10 8 T

Pulsars   P ~ 1 sec  Only something very small and compact could change that fast  Many pulsars have large space motions   Can be found in the center of SNR

Pulsar Model   Changing magnetic field produces magnetic dipole radiation   If the cone intersects the Earth, we see the radio pulse   Energy is drawn from rotation and the pulsar slows down over time

Black Hole   Gravity is so strong that the escape velocity exceeds the speed of light  Point occurs at the Schwarzschild radius R S = 2GM/c 2  Marks the event horizon   At the center is the singularity   Even outside of the event horizon, tidal forces are very strong  Material nearing a black hole is violently ripped apart  Can heat up material causing it to emit

Rotation   Maximum angular momentum is: L max = GM 2 /c   May cause frame dragging of local spacetime

Types of Black Holes  3-15 M sun, stellar remnant black holes   M sun : intermediate mass black holes   possible explanation for superbright X-ray sources  M sun, supermassive black holes   Create Active Galactic Nuclei (AGN) when active, hard to find if not active

NS and BH Binaries   Called an X-ray binary  If the mass of the compact object is greater than ~3 Msun, it is a black hole   More than anything else but annihilation

Types of Binaries  X-ray pulsar  Matter falls onto pulsar, heating it up to X-ray temperatures (10 7 K)   X-ray hot spot may be eclipsed  Mass transfer may spin-up the pulsar, decreasing the period  X-ray burster  If the magnetic field is too weak the material will build up in a layer on the surface 

Next Time  Test 3  Same format as 1 and 2  For Friday:  Read  Homework: 24.15, 24.32