NOVAE AND PRE-SUPERNOVA STARS

WHITE DWARFS IN BINARY SYSTEMS

  • The first WD detected, Sirius B, is the companion of the very bright star, Sirius (A)
  • Many WDs form in binaries and if the two stars are close
    NOVAE and CATACYLSMIC VARIABLES can be produced.

    EVOLUTION OF CLOSE BINARIES w/ WDs

  • More massive star leaves MS first, becomes RG, then WD
  • Less massive star swells as it leaves MS, fills its Roche lobe
  • Mass then flows from companion star onto WD through inner Lagrangean point
  • This mass forms an ACCRETION DISK around the WD
  • (The mass stream hits the outer part of the accretion disk and makes a HOT SPOT)

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    ACCRETION DISKS

  • Viscosity in the ACCRETION DISK (AD) causes its gas to:
    lose its angular momentum and SPIRAL INTO THE WD;
  • as it does so, it gets very hot and EMITS ULTRAVIOLET RADIATION
    often, more radiation comes from the disk than from the WD
  • INSTABILITIES in the AD can cause dramatic variations in rate of inflow
  • therefore, AD Luminosity varies a lot --
    CATACLYSMIC VARIABLES produced this way.

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    NOVAE

  • As H gas from companion builds up on WD surface it gets hotter and denser
  • Eventually (typically after 10^3 or 10^4 yrs) it IGNITES
  • This THERMONUCLEAR DETONATION (of pp chains to He, mainly)
    produces a huge burst of POWER -- this is a NOVA.
  • Most of the gas is expelled in a rapidly expanding shell.
  • Luminosity rises between 5 and 12 magnitudes (100--63,000 times)
  • in just a few days; rapid decline over a couple of weeks
    is followed by slow decline to original low L in a few years.
  • While most of the accreted H is blasted off in nova explosions
    some (plus some created He) does remain on the WD surface and the WD's mass increases.
  • As long as mass continues to flow from the companion,
    many such explosions can occur -- RECURRENT NOVAE

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    EVOLUTIONS OF MASSIVE STARS

    ( > 7 or 8 M_sun)

    These stars won't leave behind WDs since their degenerate
    cores will exceed the CHANDRASEKHAR LIMIT of about 1.4 M_sun.

    EVOLUTIONARY HISTORY

  • MS
  • H is exhausted in core
  • H shell burning starts, with modest increase in L and
  • fast decrease in T_surface (fast move to right on H-R diagram)
  • He fusion starts (non-degenerately, so no flash)
  • Modest increase in L and core He burning -- a SUPERGIANT
  • He is exhausted in core
  • So far, pretty similar to lower mass stars studied already, but

    Now we feel the big difference: higher M means gravity can crush
    the C core until it reaches T > 7 x 10^8 K so

  • Carbon CAN ALSO FUSE
  • 12^C + 4^He -- > 16^O + gamma-ray
  • Some: 16^O + 4^He -- > 20^Ne + gamma
  • Also some: 12^C + 12^C -- > 24^Mg + gamma
  • This fuel produces less energy per mass so C is burnt quickly.

    Most such stars will have Oxygen cores that can also fuse,

  • typically needs T > 1 x 10^9 K!
  • 16^O + 4^He -- > 20^Ne + gamma
  • 20^Ne+ 4^He -- > 24^Mg + gamma

    At late stages, such a star resembles an ONION:

  • inert H envelope is very extended
  • inside that is an H fusing shell (T ~ 10^7 K)
  • inside: He layer
  • inside: He fusing shell (T ~ 10^8 K)
  • inside: C fusing
  • inside: O fusing (T ~ 10^9 K)
  • inside: Ne, Mg fusing
  • inside: Si, S, Ca fusing (takes only a second or so!)
  • inside: Iron (Fe) CORE (T ~ 10^10 K)

    Note, the more common heavy elements have an even number of protons since
    they arise by these reactions which are built via 4^He nuclei.
    H and He alone were made in the BIG BANG.
    All other elements (up to iron) are made in PRE-SUPERNOVAE stars.
    Those heavier than Fe are mainly made in the SUPERNOVA EXPLOSION (see later discussion).

    Iron is the endpoint of this thermonuclear fusion chain since
    it has the minimum binding energy per nucleon -- it costs
    energy to build elements heavier than Fe, while energy was
    given off while building those elements ligher than Fe.

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    APPROACH TO NEUTRON STARS AND SUPERNOVAE

  • the Iron core collapses after excess neutrons build up
  • e.g. 16^O + 16^O -- > 31^S + n
  • Si fusion up to Fe takes < 1 day to complete!
  • Mass of Fe core grows to exceed Chandrasekhar limit
  • -- > CORE COLLAPSE

    Key details:

  • the Pressure drops as photons are absorbed:
  • 56^Fe + gamma -- > 13 (4^He) + 4 n
  • 4^He + gamma -- > 2 p + 2n
  • This occurs when density exceeds 10^9 g/cm^3 (500 big cars
    crushed to a teaspoon!) and T > 5 billion degrees!
  • This PHOTODISINTEGRATION occurs in < 0.1 second!
  • Further NEUTRONIZATION occurs when electrons are crushed into protons:
  • p + e -- > n + nu (weak nuclear reaction)
  • Atoms disappear and become nuclear matter, with
  • density about 4 x 10^14 g/cm^3 !

  • Once NEUTRONIZATION is nearly complete, the core collapse
  • is halted by a combination of NEUTRON DEGENERACY PRESSURE
  • and the REPULSIVE PART OF THE STRONG NUCLEAR FORCE.
  • The core, with, R about 10 km, becomes a
  • NEUTRON STAR (NS -- to which we'll return shortly)

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    FORMATION of a TYPE II SUPERNOVA

  • Ca, Si, S, Mg, Ne, O, C layers continue to burn and
    collapse onto the NS core.
  • BUT huge NEUTRINO PRESSURES build up, and, in addition, the
    NS is so "stiff" that matter hitting it BOUNCES from a SHOCK.
  • EXPLOSIVE NUCLEOSYNTHESIS produces ELEMENTS HEAVIER THAN IRON
    and also helps BLAST OFF MOST OF THE STAR'S ENVELOPE.
  • This rapidly expanding star gets very luminous, very fast:
  • A SUPERNOVA