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 ~ K, L ~ 10 6 L sun   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 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 12 th magnitude blue supergiant  mass ~ 20 M sun

End Products  If initial star was smaller than about 35 M sun, 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 56 Ni decaying to 56 Co and then 56 Fe  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) = N 0 e - t  Where N 0 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 dM bol /dt =   For example supernova 1987A has a light curve after 200 days well matched by 56 Co and 57 Co

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  Homework: 16.1b,c,d, 16.4