Transcript Stellar Evolution

Supernovae and
Gamma-Ray Bursts
Summary of Post-Main-Sequence
Evolution of Stars
Supernova
Fusion
proceeds;
formation
of Fe core.
M > 8 Msun
Subsequent
ignition of
nuclear
reactions
involving
heavier
elements
Fusion stops
at formation
of C,O core.
M < 4 Msun
Fusion of Heavier Elements
4 He → 16 O + g
C
+
6
2
8
16 O + 4 He → 20 Ne + g
8
2
10
12
16
16 O → 28 Si + 4 He
O
+
8
8
14
2
Onset of Si burning at T ~ 3x109 K
→ formation of S, Ar, …;
→ formation of 5426Fe and 5626Fe
Final stages of fusion happen extremely rapidly:
Si burning lasts only for ~ 2 days.
→ iron core
The Life “Clock” of a
Massive Star (> 8 Msun)
Let’s compress a massive star’s life into one day…
H → He
11 12 1
Life on the Main Sequence
+ Expansion to Red Giant:
22 h, 24 min.
2
10
9
3
4
8
H burning
7
6
5
H → He
He → C, O
11 12 1
2
10
He burning:
(Horizontal Branch)
1 h, 35 min, 53 s
9
3
4
8
7
6
5
H → He
He → C, O
C → Ne, Na, Mg, O 10
11 12 1
2
3
9
C burning:
6.99 s
4
8
7
6
5
C → Ne, Na, Mg, O
H → He
He → C, O
Ne → O, Mg
Ne burning:
6 ms
23:59:59.996
H → He
He → C, O
C → Ne, Na, Mg, O
Ne → O, Mg
O → Si, S, P
O burning:
3.97 ms
H → He
He → C, O
23:59:59.99997
C → Ne, Na, Mg, O
Ne → O, Mg
O → Si, S, P
Si → Fe, Co, Ni
Si burning:
0.03 ms
The final
0.03 msec!!
Observations of Supernovae
Total energy output:
DEne ~ 3x1053 erg
(~ 100 L0 tlife,0)
DEkin ~ 1051 erg
DEph ~ 1049 erg
Lpk ~ 1043 erg/s
~ 109 L0
Supernovae can easily be
seen in distant galaxies.
~ Lgalaxy!
SN 2006X in M 100
Observed with the MDM 1.3 m telescope
Type I and II Supernovae
Core collapse of a massive star:
Type II Supernova
Light curve
shapes
dominated by
delayed energy
Type II P
input due to
radioactive
decay of 5628Ni
Collapse of an accreting
White Dwarf exceeding the
Chandrasekhar mass limit
Type II L
→ Type Ia Supernova.
Type I: No hydrogen lines in the spectrum
Type Ib: He-rich
Type II: Hydrogen lines in the spectrum
Type Ic: He-poor
The Famous Supernova of 1987:
SN 1987A
Before
At maximum
Unusual type II
Supernova in the
Large Magellanic
Cloud in Feb. 1987
Progenitor: Blue
supergiant (denser
than normal SN II
progenitor)
20 M0;
lost ~ 1.4 – 1.6 M0
prior to SN
Evolved from red to
blue ~ 40,000 yr
prior to SN
The Remnant of SN 1987A
Ring due to SN ejecta
catching up with pre-SN
stellar wind; also
observable in X-rays.
vej ~ 0.1 c
Neutrinos from SN1987
have been observed by
Kamiokande (Japan)
Escape before shock
becomes opaque to
neutrinos → before peak
of light curve
provided firm upper limit on ne mass: mne < 16 eV
Remnant of SN1978A in X-rays
Color
contours:
Chandra
X-ray
image
White
contours:
HST optical
image
Supernova Remnants
X-rays
The Crab Nebula:
Remnant of a
supernova observed
in a.d. 1054
Optical
The Cygnus Loop
The VeilANebula
Cassiopeia
Synchrotron Emission and
Cosmic-Ray Acceleration
The shocks of supernova
remnants accelerate
protons and electrons to
extremely high,
relativistic energies.
→“Cosmic Rays”
In magnetic fields,
these relativistic
electrons emit
Synchrotron Radiation.
Synchrotron Radiation
Power-law distribution of relativistic electrons:
Ne(g) ~ g-p
In
Opt.
thick
n5/2
jn ~ n-a
a = (p-1)/2
kn ~ n-b
b = (p+4)/2
Opt. thin
n-(p-1)/2
n
Synchrotron Spectra of SNR shocks (I)
Electrons are accelerated at the shock
front of the supernova remnant:
Ne = Ne(g, t)
. ) + Q(g,t)
∂Ne/∂t = -(∂/∂g)(gN
e
Ne (g,t)
Q(g,t) = Q0 g-q
g-q
Uncooled
Cooled
g-(q+1)
gc
g
Synchrotron Spectra of SNR shocks (II)
Resulting synchrotron spectrum:
Opt. thin, uncooled
In
Opt.
thick
n-(q-1)/2
n5/2
Opt. thin, cooled
n-q/2
nsy,c = nsy (gc)
Find the age of the remnant from
.
t = (gc/g[gc])
n
Gamma-Ray Bursts
(GRBs)
Short (sub-second to minutes)
flashes of gamma-rays
GRB Light Curves
Long GRBs (duration > 2 s)
Short GRBs (duration < 1 s)
Possibly two different types of GRBs: Long and short bursts
General Properties
• Random distribution in the sky
• Approx. 1 GRB per day observed
• No repeating GRB sources
Afterglows of GRBs
On the day of the GRB
3 days after the GRB
X-ray afterglow of GRB 970228
(GRBs are named by their date: Feb. 28, 1997)
Most GRBs have gradually decaying afterglows in X-rays,
some also in optical and radio.
1 day after GRB
2 days after GRB
Optical afterglow of GRB 990510 (May 10, 1999)
Optical afterglows of GRBs are
extremely difficult to localize:
Very faint (~ 18 – 20 mag.);
decaying within a few days.
Optical Afterglows of GRBs
Host Galaxy
Optical Afterglow
Optical afterglow of GRB 990123,
observed with Hubble Space
Telescope (HST/STIS)
Long GRBs are
often found in the
spiral arms (star
forming regions!)
of very faint host
galaxies
Energy Output of GRBs
Observed brightness
combined with large
distance implies huge
energy output of
GRBs, if they are
emitting isotropically:
E ~ 1054 erg
L ~ 1051 erg/s
… another one, observed Energy equivalent to the entire mass
by us with the MDM 1.3 m of the sun (E = mc2), converted into
gamma-rays in just a few seconds!
telescope on Kitt Peak!
Beaming
Evidence that GRBs are not
emitting isotropically (i.e. with
the same intensity in all
directions), but they are beamed:
E.g., achromatic breaks
in afterglow light curves.
GRB 990510
Models of GRBs (I)
There’s no consensus about what causes GRBs.
Several models have been suggested, e.g.:
Hypernova:
Supernova explosion of a
very massive (> 25 Msun) star
Iron core collapse
forming a black hole;
Material from the outer
shells accreting onto
the black hole
Accretion disk =>
Jets => GRB!
Models of GRBs (II)
Black-hole – neutron-star merger:
Black hole and neutron star (or
2 neutron stars) orbiting each
other in a binary system
Neutron star will be
destroyed by tidal effects;
neutron star matter accretes
onto black hole
=> Accretion disk
=> Jets => GRB!
Model works probably
only for short GRBs.