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