Post on 21-Dec-2015
Supernova
PHYS390 Astrophysics
Professor Lee Carkner
Lecture 16
High Mass Stars
Variability and mass loss
Produce heavy elements in cores
Which produces even heavier elements
Luminous Blue Variables
T ~ 15000-30000 K, L ~ 106 Lsun
Occupy an instability zone on the HR diagram
What accounts for mass loss and variability? Pulsation Rapid rotation
Wolf-Rayet Stars Hot, bright stars with rapid
rotation and mass loss
Have strong emissions lines of different elements
WN: He and N
WC: He and C from triple alpha
WO: O from C + He burning
High Mass Evolution
High mass stars go through more post-MS stages compared to low mass stars
Can end up in a Wolfe-Rayet stage where outer layers are stripped away
End in supernova instead of PN stage
Types of Supernovae
A point source that get brighter
A supernova is a very bright nova
Accretion onto white dwarf causing core collapse (Type Ia)
Core collapse of high mass star (Type Ib, Ic, and II)
Classification
Type I have no H lines Must be from stars that have
lost their outer layers
Type Ib have strong He Type II have strong H
Type II-P have a plateau in the light curve
Type II-L have a more rapid drop off
Core Collapse
Excluding the Type Ia, we can refer to the rest as core collapse supernova
Generates about 1046 J of energy
mostly in the form of neutrinos
The Core
Lighter on outside, heavier towards middle
As the fusion products move towards iron, the energy released per nuclei decreases
Iron can’t be burned, so star can’t stay in HSE
Core Processes
Produces protons T and P are also high enough that the protons
can fuse with electrons, producing neutrons
Core’s ability to support outer layers drops rapidly
Removing electrons decreases electron degeneracy pressure
Explosion
hard to do
This causes the collapse to rebound and send a shock wave out
A neutrinosphere forms behind the shock The shock front is so dense it can absorb neutrinos to
power the shock back out
about billion suns
Supernova 1987A
Located in a Milky Way satellite called the Large Magellanic Cloud
Progenitor was 12th magnitude blue supergiant mass ~ 20 Msun
End Products If initial star was smaller
than about 35 Msun, core will form a neutron star
Else it will form a black hole
Can produce strong synchrotron emission from high velocity electrons spiraling around magnetic field lines
Remnant can also collide with ISM causing emission and triggering star formation
Radioactive Decay
The decay of these isotopes add energy to the supernova and effect the shape of the light curve
Most important reaction is 56Ni decaying to 56Co and then 56Fe With half life of 6.1 days and 77.7 days
Half-Life
How much energy is released by decay? Decay goes as:
N(t) = N0e-t
Where N0 is the initial number of atoms and N is the number at some time t
= (ln 2)/1/2
where 1/2 is the half-life (time for ½ of atoms to decay)
Light Curve Slope
The magnitude of the supernova is proportional to
dMbol/dt = 1.086
For example supernova 1987A has a light curve after 200 days well matched by 56Co and 57Co
Elemental Abundances H and He are the most
plentiful
Li, Be, B underabundant
Some elements (C, O, Ne, …) are abundant because the are created in post-main sequence stars
Fe created in supernovae cores
Next Time
Read 16.1-16.5 Homework: 16.1b,c,d, 16.4