Lecture 19

1. Lecture 19 The fate of massive stars: supernovae

2. Massive stars • Helium burning continues to add ash to the C-O core, which continues to contract and heat up. • Carbon is ignited, forming

3. Shell structure • If each reaction has time to reach equilibrium, the stellar interior will consist of shells of different composition and reactions • Oxygen is ignited next producing a Silicon core.

4. Silicon burning • Silicon burning produces numerous elements near the iron peak of stability • The most abundant: • Further reactions are endothermic and thus do not provide stellar luminosity.

5. Timescales • As the iron peak is approached, the energy released per unit mass of reactant decreases. Thus the timescale becomes shorter and shorter

6. Photodisintegration • During Silicon burning the core has reached extremely high temperatures and densities: • The photons produced are so energetic they can destroy heavy nuclei, reversing the process of fusion. In particular:

7. Core collapse • The inner core collapses, leaving the surrounding material suspended above it, and in supersonic free-fall at velocities of ~100,000 km/s. • The core density increases to 3x the density of an atomic nucleus and becomes supported by neutron degeneracy pressure. • The core rebounds somewhat, sending pressure waves into the infalling material

8. Stalled shocks • As the shock wave propagates outward and encounters the infalling core, the high temperatures result in further photodisintegration. • This removes a lot of energy from the shock: it loses 1.7x1044 J of energy for every 0.1MSun of iron it breaks down. • If the iron core is too large, the shock becomes a stationary accretion shock, with matter accreting onto it.

9. Instability growth • The rapid growth of long-wavelength mode instabilities may play a role

10. Explosion • As the shock moves toward the surface, it drives the hydrogen-rich envelope in front of it. • When the expanding shell becomes optically thin, the radiation can escape, in a burst of luminosity that peaks at about 1036 W

11. Break

12. Light curves • After the initial burst of luminosity, the supernova slowly fades away over a period of several hundred days. • As the shock wave propagates through the star, it creates a large amount of heavy, radioactive elements. • Each species decays exponentially with a unique timescale

13. Radioactive decay • For example, the following beta-decay reaction occurs: • This decay is a statistical process: the rate of decay must be proportional to the number of atoms in the gas: • where l is the decay constant, and is characteristic of each radioactive element.

14. Example: radioactive decay • The energy released by the decay of one cobalt-56 atom is 3.72 MeV. Given 0.075 MSun of this isotope (this is how much was estimated to have been produced in SN1987A) how much energy does the decay release? • (for t measured in years) • The initial luminosity is 2.5x108 LSun. After one year it has decreased to 9.9x106 LSun.

15. Remnants • If the star is relatively low mass, roughly M<25MSun, it can be supported by neutron degeneracy and becomes a neutron star. • For more massive stars, the gravitational attraction overcomes neutron degeneracy, and the core collapses to form a black hole.

16. Supernova remnants • Crab nebula: believed to be the remnant of the supernova that went off in 1054 A.D. • Nebula is still expanding, at ~1450 km/s • The source of the luminosity and electrons is a pulsar in the centre of the nebula. • The Crab nebula is ~2 kpc away, with an angular size of 4x2 arcminutes. The expansion velocity is measured from the Doppler shift to be 1450 km/s. Estimate the age of the nebula. How bright would the supernova that gave rise to the Crab nebula have been?

17. Supernova remnants • Cygnus loop: this is a ~15,000 year old remnant. • The filaments are caused by shocks encountering the interstellar medium. These shocks excite the gas which then emits emission lines. A small part of the remnant, expanding left to right

18. SN1987A • Occurred in the Large Magellanic cloud, a small galaxy near the Milky Way.

19. SN1987A progenitor • Progenitor was a much smaller star than usually responsible for Type II explosions. • Smaller stars are denser, so more energy was required to lift the atmosphere, and this resulted in a slower brightening and fainter peak luminosity.

20. SN1987A light curve • The initial decay mostly tracks Co-56, followed by Co-57 • This reaction produces high energy gamma rays which were detected for the first time, confirming the presence of this isotope.

21. Neutrinos • Neutrinos produced in part by this decay were also detected: this was the first time neutrinos were detected from an astronomical source other than the Sun.

22. SN1987A: the rings • The central ring is due to ejection by a stellar wind prior to the explosion. • Lies in the plane that contains the centre of explosion • Glows due to [OIII] emission, excited by radiation from the explosion

23. SN1987A: the rings • The central ring is due to ejection by a stellar wind prior to the explosion. • When the shock wave from the explosion reached this ring, in 2004, it excited the gas causing it to glow brightly.

24. SN1987A: the rings • The two other rings are not in the plane of the explosion, but in front of and behind the star • The explanation of these rings is still unknown