Topic 6

Supernovae

Introduction •  We talk of supernovae (SN) in this course on

Nuclear Astrophysics because SN are responsible for dispersing pre-processed elements into the interstellar medium for further processing during (new) star formation

•  SN are the death throes of massive stars –  Typical kinetic energy release: 1044 J –  Typical total energy release: 1046 J (100x that of the Sun

for 10 billion years) –  Temporarily outshine an entire galaxy (magnitude -19 to

-20) –  Optical light is only ~ 1% of total output (majority as

neutrinos)

General SN Properties •  In general there are 2 basic processes that can occur at the

end of a star’s life cycle which can result in a supernova –  Runaway thermonuclear explosion –  Core collapse

•  However, they are classified according to the appearance (or not) of spectral lines, in particular H, at maximum light –  Type I - no H lines in spectra, sub-divisions according to light curve,

spectra, progenitor mass, site, etc. •  Type Ia - have no H lines but He and strong Si II are seen •  Type Ib - lacks H and Si II lines •  Type Ic - no H, Si II and He lines

–  Type II - evidence of H emission lines, sub-classification on light curves

•  Type II P - flat plateau part of light curve •  Type II L - linear light curve

Type Ia Supernovae •  Observations

–  H deficient –  Seen in all types of galaxies (in particular, associated with

populations of older stars between the arms) - implies stars have long lives before exploding

–  Presence of Si II seen at maximum light (2nd strongest distiguishing feature)

–  Accounts for ~ 75% of all type I SN –  Light curve is identical and reproducible

•  Initial sharp rise to peak at absolute magnitude -19.5 •  Long, almost linear (exponential?) fall-off after ~ 50 days •  Use as standard candles by cosmologists

–  At peak brightness BB spectrum at ~ 15000K plus Si II, Ca II, S II, Mg II and O I absorption lines - later on Fe II

–  Expansion velocities 10,000 - 13,000 km/s at maximum –  No remnant is left, just an expanding cloud of gas

Type Ia Light Curve

Type Ia Supernovae •  Model (1)

–  White Dwarf (WD) in a quiescent binary system –  Roughly equal masses of C and O supported by electron

pressure –  This is the evolution endpoint of 1 to 8 solar mass stars –  Outer H-rich envelope lost, CO dwarf is the star’s core –  Accretes matter from binary companion –  Chandrasekhar mass limit is exceeded, density and

temperature increase, thermonuclear burning of Carbon is triggered

–  Electron pressure doesn’t react to additional heat - star doesn’t expand. Heat accelerates burning of C then O to cause “runaway thermonuclear reaction”

Type Ia Supernovae •  Model (2)

– Accretion of new material releases gravitational energy at the surface of the star

– Released energy is radiated as UV – More accretion compressed previously accreted

matter, releases grav. energy in the interior – This energy is transported to the surface

(radiative cooling) and also heats interior (compressional heating)

–  Interior temperature is defined by the relative rates of these 2 heating/cooling processes

Type Ia supernovae •  Model (3)

–  One of 2 possibilities for the final explosion •  Supersonic detonation triggers shock - burning - shock •  Subsonic deflagration wave

Carbon burning propagates outwards but not explosively. Behind this wave material undergoes explosive burning of Si, Ne, C and O.

–  Material incinerates to form 56Ni. –  In both cases process is rapid, whole star is disrupted,

no remnant remains –  All of this depends on rate of accretion - if it is low the

WD can “flash” causing nova –  Late-time light curve is thought to be dominated by

56Ni ! 56Co ! 56Fe

Type Ia SN

Type Ib Supernovae •  Observations

– Like Type Ia no H at maximum light – Also no Si II – Strong He lines (weak/no He: SN Ic) – Optical maximum ~ 1.5 mag dimmer than SN Ia – Only observed in spiral galaxies (esp. near

active star-forming regions) suggests a link to moderate mass stars

– Lower expansion velocities than type Ia – Late-time spectrum dominated by O I, Ca II, Na,

Mg I emission lines

Type Ib/Ic Supernovae

•  (Possible) models – Hydrogen (and possibly He) “denuded” cores of

massive stars – Hydrogen lost either via

•  Stellar winds (radiation pressure linked to high luminosity)

•  Transfer to a binary companion

– May be exploding Wolf-Rayet and/or Helium (Ic) stars

Type II Supernovae •  Observations

–  Presence of H (stars have retained their H envelopes) –  Found only in spiral galaxies, mainly in arms –  At maximum light peak magnitude can differ by 2.0 –  Wide range of expansion velocities (2000 - 20000 km/s) –  At maximum light almost continuous spectrum, some

(weak) H, He absorption –  Late-time features include Ca II, Fe II, Ti II, Sc II

absoprtion lines –  Lot of variation - probably due to different progenitor

mass

SN I vs SN II light curves

SN II-L vs SN II-P light curves

Type II Supernovae •  Model (1)

–  Core collapse of a massive star at the end of thermonuclear fusion cycle

–  How massive? Typically 8 to 50 solar masses (text books differ on these figures)

–  We saw in Topic 4 that such a massive star evolves to a layered body where different fuels are burned (“onion” analogy)

–  Recall also from Topic 4 that as higher mass fuels are burned the relative time burning is less and less

–  Fusion continues until an iron core is established –  At this point no further burning can take place

Type II Supernovae •  Model (2)

–  As usual, as the fuel in exhausted gravitationaal 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

56Fe ! 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

Type II Supernovae •  Model (3)

– As the core further collapses the transition to a neutron star commences

– Density reaches 1017 kg.m-3 and electrons are forced into protons causing neutrons to form

– The inner core, about 1.4 solar masses then rapidly cools via neutrino radiation

–  In fact neutrinos carry off ~99% of the energy of the collapse (few x 1046J)

– 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”

Type II Supernovae •  Model (4)

–  Following this “bounce” the subsequent shock heats up and disrupts the remainder of the star causing lots of processed material to be dispersed in the interstellar medium

–  Neutron star is left behind –  Rebounding outer core shock loses energy dissipatively

(neutrinos, photodissociation) –  Models show the shock should quickly lose its kinetic

energy –  So why do we see the SN explosion?? –  It may be that neutrinos themselves play a part in the

creation of the SN explosion

Type II Supernovae •  Model (5)

– What is true is that the inner heavy element burning layers of the star must be expelled at high velocity with some shock mechanism or else material would continue to fall back into the neutron star and a black hole would form

– As the shock passes through new fusion takes place and much 56Ni is produced

– Again, as in the discussion for SN type Ia the subsequent radioactive decays to 56Co (half-life 6.1 days) and then 56Fe (half-life 77.2 days) drives the late-light curves of these SN

Type II Supernovae •  Model (6)

– Shock continues through rest of star, arriving at the surface after 2h to 1d (depending on progenitor mass)

– SN is now “visible” – Early light is in UV, as surface expands and

cools light looks like a BB with T=6000K – As the shock envelope continues to travel out

and cool Hydrogen recombination kicks in •  Removes opacity •  Luminosity starts to decline dramatically

–  Late light from radioactive decay of 56Co

SN 1987A •  SN 1987A was a supernova in

the Large Magellanic Cloud and was visible from Earth with the naked eye despite being 51kpc away

•  2-3 hours before visible light 3 different neutrino observatories detected a total of 24 neutrinos - significantly above observed background levels"

•  This was the first time neutrinos emitted from a supernova had been observed directly

•  The observations were consistent with theoretical supernova models in which 99% of the energy of the collapse is radiated away in neutrinos. The observations are also consistent with the models' estimates of a total neutrino count of 1058 with a total energy of 1046 Joules.

SN 1987A - a mystery •  SN 1987A appears to be a core-collapse supernova, which should

result in a neutron star. Since the supernova first became visible, astronomers have been searching for the collapsed core but have not detected it. HST images show no evidence of a neutron star. Two possibilities for the 'missing' neutron star are being considered:"–  the neutron star is enshrouded in dense dust clouds so that it cannot be

seen"–  large amounts of material fell back on the neutron star, so that it further

collapsed into a black hole."

Solved Supernova Problem •  A 10 solar mass star explodes leaving

behind a neutron star of radius 10 km and density 6.7 x 1017 kg.m-3. A theorist proposes that 10% of the mass difference is released as high energy (10 TeV) photons. Assuming the photons are released isotropically and that the supernovae takes place on the other side of the galaxy at 50 kpc from Earth estimate the flux of high energy photons incident on the Earth after the explosion. (1 TeV = 1012eV)"

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

Solved Problem solution