04/02/2013 Topic 6 Supernovae Introduction • Supernovae (SNe) are the explosive deaths of stars • Important for Nuclear Astrophysics because – extreme conditions in the SN explosion allow production of elements not created, or much less frequently created, in normal stellar interiors – explosion also disseminates this material into the interstellar medium where it can be incorporated into new stars – explosion may also trigger new star formation by creating shock waves in nearby gas 1 04/02/2013 Supernova Classification • Based mainly on spectral features, with some input from light curve shape (photometry) Typical Ia Si II (615 nm) Fainter, lower ejecta 27% No nd Iax I velocity, no 2 max in IR Hydrogen Strong He features Ib 7% No Si II No/weak He Ic 14% Hydrogen vanishes at late times, II Ib IIb 4% Plateau in light curve II-P 43% Hydrogen II Hydrogen always Linear decline II-L 2% present Narrow lines IIn 3% Relative frequencies are quite uncertain—error in these numbers up to ±50% for smaller numbers. SN I vs SN II light curves Slope at late times powered by 56 Ni 56 Co + e + + ν (t 1/2 = 6.08 days) 56 Co 56 Fe + e + + ν (t 1/2 = 77.27 days) 2 04/02/2013 Supernova rates e.g. arXiv:1006.4613v2 (Lick survey): 2.84 ± 0.60 total number SN/100 years 2.30 ± 0.48 core collapse SN/100 years … in Milky Way Type Ia Supernovae • Diagnostic features: absence of H lines, presence of strong ionised Si feature near 615 nm – “P Cygni profile”, showing outflow – initial ejecta velocities ≥10 000 km s −1 • Occur in all types of galaxies, including elliptical, though more common in late types – therefore not due to massive stars • Spectra show intermediate-mass elements (Si, Ca, Mg, S, O) near maximum light; Fe II, Fe III, Co III at late times – this looks like C/O burning – relative intensities of Co III/Fe III indicate that late-time light is powered by radioactive decay of 56 Co to 56 Fe – consistent with production of few tenths of solar mass of 56 Ni • Light curves and other properties all very similar Everything points to explosive destruction of a white dwarf 3 04/02/2013 Type Ia Light Curves SN2003du Correlation of luminosity with decline rate Type Ia Supernovae • Typical white dwarf has mass 0.6 M – need to increase this to Chandrasekhar limit, 1.4 M, to explode – obvious route to this is mass transfer in a binary system • Two possible mechanisms • Single Degenerate scenario – more massive star in binary system evolves to white dwarf, which then accretes mass from companion (main sequence star or red giant—several possible scenarios) – tricky to “tune” accretion so as to avoid classical nova (which blows off more mass than has been accreted) • Double Degenerate scenario – two white dwarfs in binary will lose energy by gravitational radiation, eventually spiralling inward to coalesce • SD is preferred, but neither model is totally unproblematic, and both may occur 4 04/02/2013 Type Ia Supernovae Type Ia Supernovae Many possible SD evolutionary paths! (from Wang and Han 2012) 5 04/02/2013 Type Ia Supernovae • Once WD reaches 1.4 M it will collapse—gravity will overcome electron degeneracy pressure • Three possible mechanisms for resulting explosion – prompt detonation : burning proceeds as supersonic shock wave which does not allow unburnt material time to expand before being consumed • this tends to convert entire star to 56 Ni, which is not observed (intermediate-mass elements seen in spectrum)—therefore disfavoured – pure deflagration : burning front moves outward subsonically, producing complex turbulent boundary layer propagating outward • it is not absolutely certain that this can generate an explosion – delayed detonation : initial deflagration converts to detonation as it propagates into lower-density regions • this definitely does explode, and is preferred scenario Type Iax Supernovae • New class (proposed by Foley et al., December 2012) • Minority of SNe Ia – have low ejecta velocity (<8000 km/s) – lack secondary maximum in R and I bands – are fainter than standard SNe Ia (absolute magnitudes from −14.2 to −18.9) and do not obey standard peak magnitude–decline rate correlation – appear to be found principally in late-type galaxies (one in an S0) • These may be pure-deflagration events which fail to disrupt the WD completely, leaving a surviving WD remnant – for every 100 SNe Ia, about 30( ±15) SNe Iax (large minority) 6 04/02/2013 Nucleosynthesis in SNe Ia • Nuclear statistical equilibrium mostly iron peak elements • Some intermediate-mass elements from partially burned C/O SN II 1.E+00 SN Ia 1.E-02 1.E-04 1.E-06 1.E-08 1.E-10 10 20 30 40 50 60A 70 Core Collapse Supernovae • All other supernova classes (Ibc, II) are caused by the core collapse of a massive star • We saw in Topic 4 that stars >~10 M will fuse successively heavier elements – each successive stage requires higher temperature, is less efficient in generating energy, and hence lasts for less time – star develops “onion” structure with layers of increasingly heavy elements from hydrogen envelope to heavy core – this continues until Si fusion creates (in a few days) an iron core exceeding 1.4 M • Stars of ~8-10 M form an O-Ne-Mg core – this will also collapse and generate a supernova 7 04/02/2013 Core Collapse Supernovae • Collapse of the iron core – As usual, as the fuel is exhausted gravitational collapse occurs causing heating. Usually this ignites the next fuel. – However in this case the mean photon energy becomes such that photodissociation of Iron can take place. This is the so-called Iron-Helium phase transition γ + 56 Fe 13 4He + 4n −124.4 MeV – Note that this process is endothermic – This energy is provided at the expense of the gravitational field – This accelerates the core collapse Core Collapse Supernovae • Neutron star formation – As the core further collapses the transition to a neutron star commences – Density reaches 10 17 kg.m -3 and electrons are forced into protons causing neutrons to form − – This generates a burst of neutrinos from p + e n + νe – In fact neutrinos carry off ~99% of the energy of the collapse (few x 10 46 J)—but at this early stage they are trapped, as collapse is faster than neutrino diffusion time – Collapse is rapid, neutron star forms, outer core of star is not collapsing (infalling) as fast and so hits the neutron star and “bounces off” 8 04/02/2013 • Prompt shock – Bounce creates a shock wave which propagates outward – Shock dissociates nuclei into free protons and neutrons • using up 8-9 MeV/nucleon – Neutrino burst emerges at shock breakout – Energy loss due to dissociation of nuclei and neutrino cooling will cause shock to stall Initial shock will not cause explosion Markus Weiland, LMU • Delayed neutrino heating mechanism – Neutrinos carry 99% of the supernova energy – Neutrino cooling − • inverse β decay: e + p n + νe + e + n p + ν̅e • pair production: e − + e + ν + ν̅ • Bremsstrahlung: N + N N + N + ν + ν̅ – Neutrino heating − • n + νe e + p + • p + ν̅e e + n – If a few % of neutrino energy goes into heating, shock will revive Explosion!! Markus Weiland, LMU 9 04/02/2013 Nucleosynthesis in CCSNe • Explosive fusion – Pre-existing iron-peak elements are mostly dissociated during core collapse – However, propagating shock wave will initiate explosive burning of Si, O, Ne and C • Si burning produces iron-peak elements including 56 Ni whose decay powers late-time exponential decay of light curve • O burning produces many α-particles, which will enhance production of “ α-process isotopes” such as 16 O, 20 Ne, 24 Mg (effectively bound states of α-particles) • this occurs at temperatures of several ×10 9 K and high densities—conditions for nuclear statistical equilibrium Nucleosynthesis in CCSNe • p and r processes – Early ejecta are proton rich • this may be where “bypassed nuclei” (p-process nuclei) are formed by proton capture (rp-process) – Later ejecta may be neutron rich • high neutron number densities possible site for r-process • however unclear if this really happens • for low-metallicity SNe r-process may occur further out, in He-shell of original star (cold r-process) Markus Weiland, LMU 10 04/02/2013 Electron Capture Supernovae • Stars of 8-10 M form an O-Ne-Mg core from carbon burning – this is supported by electron degeneracy pressure (i.e. all accessible electron quantum states are full) × 12 −3 24 – at 1.4 M and 4.5 10 kg m electron capture on Mg − 24 24 becomes energetically favoured: e + Mg Na + νe – this removes electron pressure support core collapses – simulations indicate fairly weak explosion forming sub- Chandrasekhar mass neutron star, with much less heavy 56 element ejection than standard CCSN (<0.001 M Ni, compared to 0.01−0.1 M for typical CCSN) Observations of CCSNe • Progenitors are massive stars – therefore should expect SNe to occur in association with star formation: not in elliptical galaxies, S0 galaxies, spiral galaxies bulges • Most evolved massive stars should have an outer envelope of hydrogen – therefore expect strong hydrogen lines in spectrum • These features match observations of Type II supernovae • Type Ib/c supernovae are also associated with star formation, but don’t have strong hydrogen – progenitors’ hydrogen envelopes lost through stellar winds or binary mass transfer – Distinction is not sharp: Type IIb appears to be genuine intermediate (very low-mass H envelope, quickly dispersed) 11 04/02/2013 Type II-P Supernovae • Strong hydrogen features at maximum light hydrogen envelope in progenitor star • Plateau is caused as visible photosphere (at ~5000K)