Transcript Wheeler_Explosion_1x

Explosions of Massive Stars
SUPERNOVAE
Catastrophic explosions that end the lives of stars,
Provide the heavy elements on which planets and
life as we know it depends,
Energize the interstellar gas to form or inhibit
new stars,
Produce exotic compact objects, neutron stars and
black holes,
Provide yardsticks to measure the history and fate
of the Universe.
One type of supernova is powered by the collapse of the core of a
massive star to produce
a neutron star,
or perhaps
a black hole
The mechanism of the explosion is still a mystery.
The other type of supernovae
(Type Ia) is thought to come
from a white dwarf that grows
to an explosive condition in a
binary system.
Chandra X-ray Observatory image
Of Tycho’s supernova of 1572
These explode completely, like a
stick of dynamite, and leave no
compact object (neutron star or
black hole) behind.
Supernovae
Historical Supernovae - in our Milky Way Galaxy observed with naked
eye over 2000 years especially by Chinese (preserved records), but also
Japanese, Koreans, Arabs, Native Americans(?), finally Europeans.
SN 185
earliest record
No NS
SN 386
NS, jet?
SN 1006
brightest
No NS
SN 1054
Crab Nebula
NS, jets
SN 1181
(Radio Source 3C58) NS, jets
SN 1572
Tycho
No NS
SN 1604
Kepler
No NS
~1680
Cas A
NS? Jets
------------------------------------------------------SN 1987A
nearby galaxy
NS? jets
Extragalactic Supernovae
2 in the Whirlpool
Extragalactic Supernovae
All supernovae since 1680, since invention of telescope, modern astronomy,
have been discovered in other galaxies. (G1.9+0.3 was already a supernovae
remnant when discovered)
Galaxies like our Milky Way produce supernovae about once per century.
None since Cas A in about 1680. Our Galaxy is overdue for another!
Recognition (early in the 20th century) that some “novae” (new stars) were
in distant galaxies and hence were 10,000 to 100,000 times brighter than
classical novae in the Milky Way.
Led to the recognition and naming of “super” novae.
Categories of Supernovae
1st category discovered
Type Ia – near peak light, no detectable Hydrogen or Helium in the spectrum,
rather “intermediate mass elements” such as oxygen, magnesium, silicon,
sulfur, calcium. Iron appears later as the light fades.
Type Ia occur in all galaxy types:
In spiral galaxies they tend to avoid the spiral arms, they have had time to drift
away from the birth site  the star that explodes is old
In elliptical galaxies where star formation is thought to have ceased long ago
 the star that explodes is old, billions of years
the progenitor that explodes must be long-lived, not very massive,
suggesting a white dwarf. Sun is long-lived, but won’t explode
Type II Supernovae - “other” type discovered early in the
study of supernovae, show Hydrogen in the spectrum early,
Oxygen, Magnesium, Calcium, later
Most occur in spiral galaxies, in the spiral arms, they have
no time to drift from the birth site
never in elliptical galaxies (no young stars)
Stars with more mass have more fuel, but they burn it at a
prodigous rate, live a shorter time!
The progenitor stars are young, short-lived (millions to
tens of millions of years) massive stars
We expect such stars to evolve to form iron cores and
collapse to a neutron star or black hole.
SN 1999em
“Plateau” light curves of Type II are consistent with explosion in a Red Giant
Betelgeuse is a massive red giant, 15 solar masses: we expect it to become a Type II
supernova. Maybe tonight! Rigel in Orion probably burning He to C/O, explode later.
SN 386, 1181 records are sparse, might have been Type II
Crab was “peculiar” (high helium abundance, slow explosion), but probably a Type II.
Expelled outer hydrogen envelope has been difficult to detect directly, but is inferred.
Cas A was probably something else with a thin layer of Hydrogen (Type IIb).
months
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Crab nebula
Time
Type II are common in other galaxies,
more frequent than Type Ia.
SN1987A was a
“peculiar” Type II
New Types, blurring the old categories, identified in the 1980s, defined by
elements observed in the spectrum.
Type Ib: no (or very little) Hydrogen, but Helium early, near maximum
brightness; Oxygen, Magnesium, Calcium later on
Type Ic: no (or very little) Hydrogen, no (or very little) Helium early, near
maximum brightness; Oxygen, Magnesium, Calcium later on
Explode in the spiral arms of spiral galaxies
Never in elliptical galaxies
 massive stars,
expect neutron star
or black hole
Like Type II, but have somehow lost their outer layers of Hydrogen or
even Helium  wind or binary mass transfer.
weeks
Type Ib, Type Ic Light Curve
Similar to a Type Ia, usually, but not always,
dimmer, consistent with a star that has lost
its outer, Hydrogen envelope (or even
Helium for a Type Ic)
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Type Ia
Type II
Type Ib, Ic
Time
Cas A seems to have been dim at
explosion, some evidence for a little
Hydrogen in the remnant now. Recent
spectrum of light from peak reflected
from dust, arriving “now” shows it was
closely related to a Type Ib, a Type IIb
like SN2011dh.
Single star: Type II
Same evolution
inside star, thermal
pressure, regulated
burning, shells of
heavier elements,
whether hydrogen
envelope is there or
not.
Same star in binary: Type Ib/c
Single star: Type II
Same star in binary: Type Ib/c
Both types
leave
behind a
neutron star
Rotating,
magnetic
radio
pulsar.
Neutron
star in
binary
system,
X-ray
source
Some basics of core collapse
Iron core
Collapse
Electron capture
Neutrinos
Neutron star (or sometimes a black hole)
1053 ergs released in binding energy of neutron star (G MNS2/RNS).
Need 1051 to blow up (NEVER “go supernova”), overcome binding
energy of core (G MFe2/RFe).
Asymmetries, maybe also jets or other irregularities (Lecture 2).
Nuclear physics:
Protons and neutrons attract each other.
The strong nuclear force binds protons and neutrons together in atomic
nuclei.
Short range force, acts only when protons and neutrons are nearly touching.
Protons have positive electrical charge. They repel one another at large
distances.
The strong nuclear force can, and does overwhelm the charge repulsion if the
protons and neutrons are close enough together.
Evolution of Stars - gravity vs. charge repulsion
Why do you have to heat a fuel to burn it?
H  He  C  O
more protons, more charge repulsion,
must get ever hotter to burn ever “heavier”
fuel
Just what massive stars do!
Support by gas plus radiation pressure.
When fuel runs out, core loses energy, but
gravity squeezes, core contracts and HEATS UP
Negative specific heat.
Overcomes higher charge repulsion, burns new,
heavier fuel, until get to iron
Massive stars make a succession of heavier elements
Sun, National
Ignition Facility
Measure of
binding
energy of
protons and
neutrons in
the atomic
nucleus
Special role of Iron - 26p, 30n, most tightly bound arrangement of protons and
neutrons.
Endothermic - must put energy in to break iron apart into lighter elements or to
forge heavier elements. Irons absorbs energy, lowers pressure, core contracts,
iron absorbs more energy, more contraction…
=> The iron core quickly collapses! Catastrophic death of the star.
Iron core of massive star absorbs energy.
When iron core forms - star is doomed to collapse.
Iron core collapses in about 1 second to form a neutron star (or maybe a
black hole), composed essentially of all neutrons.
Neutrons are formed when protons and electrons combine.
p + e  n + v neutrino,
Action of Weak Nuclear Force
One neutrino is generated for every proton that is converted, a star’s worth
of protons
lots of neutrinos (how many?)
During iron core collapse, essentially all protons and electrons are
converted to neutrons with the emission of a neutrino.
Neutrinos have a tiny mass, no electrical charge, interact little with normal
matter, only through weak nuclear force.
Normal stellar matter is essentially invisible to neutrinos.
100x more energy is created in iron core collapse to a neutron star than is
needed to explode the star
But
99% of the energy of collapse is carried off by neutrinos
Collapse of iron core to form neutron star
is halted by the repulsive strong nuclear
force at very close distances, high
compaction of neutrons (somewhat
uncertain)
+ degeneracy pressure of neutrons
Charge repulsion
among protons,
zero for neutrons
Nuclear
attraction among
protons and
neutrons
Nuclear repulsion at
very short distances
Maximum mass of a neutron star is 1.5 to 2 solar masses
Supernova 1987A!
Core collapse, confirmation of neutrino production, radioactive decay of 56Ni .
Production of the Elements:
In massive stars (more than about 12 - 15 times the Sun) the core is composed
of Helium or heavier elements: Carbon, Oxygen, Magnesium, Silicon,
Calcium, finally Iron.
The intermediate-mass elements are produced in the star before the
explosion and then expelled into space.
In exploding white dwarfs (arising in stars with mass less than about 8 times
the Sun), the core is composed of Carbon and Oxygen, and the explosion
creates the intermediate-mass elements, Magnesium, Silicon, Calcium,
and also Iron.
(between about 8 and about 12 solar masses, different story, maybe collapsing
white dwarfs)
Common Elements in Supernovae: Alpha Chain Nuclei
H -> He (2 protons, 2 neutrons)
2 Helium -> unstable, no such element
3 Helium -> Carbon (6 protons, 6 neutrons)
4 Helium -> Oxygen (8 protons, 8 neutrons)
5 Helium -> Neon (10 protons, 10 neutrons)
6 Helium -> Magnesium (12 protons, 12 neutrons)
7 Helium -> Silicon (14 protons, 14 neutrons)
Then Sulfur, Calcium, Titanium.
Common “intermediate mass” elements
(heavier than hydrogen and helium, lighter
than iron) that are forged in stars, and in
their explosions, are built on building blocks
of helium nuclei.
Light Curves
weeks
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Type Ia
Type II
Ejected matter must expand and dilute before photons
can stream out and supernova becomes bright: must
expand to radius ~ 100  Earth orbit
Maximum light output ~ 2 weeks after explosion
Type Ib, Ic
Time
Type II in red giants have head start, radius already about the size of Earth’s
orbit; light on plateau comes from heat of original explosion
Ejected matter cools as it expands: for white dwarf (Type Ia) or bare core (Type
Iib, Type Ib, Ic) tiny radius about the size of Earth, must expand huge factor >
1,000,000 before sufficiently transparent to radiate.
All heat of explosion is dissipated in the expansion
By time they are transparent enough to radiate, there is no original heat left to
radiate
Need another source of energy for Type I a, b, c to shine at all!
Type Ia start with C, O: number of protons equal to number of neutrons
(built from helium building blocks)
Iron has 26p 30n not equal
C, O burn too fast (~1 sec) for weak nuclear force to convert p to n.
Similar argument for Type Ib, Ic, core collapse. Shock wave hits silicon
layer that surrounds the iron core. Silicon has #p = #n, burns too quickly for
weak nuclear force to convert p to n.
Fast explosion of C/O in Type Ia and shock hitting layer of Si in Type Ib, Ic
make element closest to iron (with same total p + n), but with #p = #n, Nickel56.
Nickel-56: 28p, 28n total 56 -- Iron-56: 26p, 30n total 56
Ni-56 is unstable to radioactive decay
Nature wants to produce iron at bottom of nuclear “valley”
Decay caused by (slow) weak force p  n
Nickel -56 -rays
heat
Cobalt-56
-rays
heat
Iron-56
28p
“half-life” 27p
“half-life” 26p
28n
6.1 days
77 d
29n
30n
Secondary heat from radioactive decay -rays makes Type I a, b, c shine
Type Ia are brighter than Type Ib and Ic
because they produce more nickel-56 in the
original explosion.
The thermonuclear burning of C and O in a
white dwarf makes about 0.5 - 0.7 solar
masses of nickel-56.
weeks
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Type Ia
Type II
Type Ib, Ic
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A core collapse explosion that blasts the silicon layer makes about 0.1 solar
masses of Nickel-56.
Type II also produce about 0.1 solar mass of nickel-56, but the explosion
energy radiated from the red giant envelope in the plateau tends to be
brighter. After the envelope has expanded and dissipated, the remaining
radioactive decay of Cobalt-56 is seen.
The real thing: post-explosion nucleosynthesis with MESA
Explosion Minilab
Explodes a massive, non-rotating core-collapse model.
Models are assigned from the set of masses 15, 20, 25, 27, 30, 35
M and either Z or 0.1 Z.
We provide the initial model, invoking a thermal bomb and the
appropriate inlists.
Run for 10 minutes, however far that gets.
Crowd source the post-explosion composition plots.
Monitor the location and velocity of the shock wave(s).
Determine the amount of 56Ni produced.