Hydrogen Burning Nucleosynthesis,

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Lecture 9 Hydrogen Burning Nucleosynthesis, Class
ical Novae, and X-Ray Bursts
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Once the relevant nuclear physics is known in
terms of the necessary rate factors, l NAltsvgt,
the composition can be solved from the coupled
set of rate equations
The rather complicated looking restriction on the
second summation simply reflects the necessary
conservation conditions for the generic forward
reaction, I(j,k)L and its reverse, L(k,j)I. k
and j are typically n, p, a, or g. In the
special case of weak interactions one substitutes
for Yjrl or Ykrl, the inverse mean lifetime
against the weak interaction, ???(I or L)
1/??(I or L), where ? can be beta-decay, positron
decay or electron capture. The mean lifetime is
the half-life divided by ln 2 0.693...
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Aside on implicit solution of rate equations
note limits
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How long does that take for a pair of
nuclei? The time to reach steady state (not the
same thing as equilibrium) between two nuclei
connected by a single reaction is approximately
the reciprocal of the destruction rate for the
more fragile nucleus.
The larger term initially
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?? 1 to 10 would be more appropriate for
massive stars where T is this high, so the real
time scale should be about 10 times greater. Also
lengthened by convection.
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Steady state after several times these time
scales.
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Provided steady state has been achieved the
abundance ratios are just given by the ls. After
the operation of the CNO cycle, some nuclei may
achieve super-solar ratios in the stellar
envelope.
More recent measurements of 17O(p,a) suggest that
it is not.
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Hydrogen Burning Nucleosynthesis Summary

• 12C - destroyed, turned into 13C if
  incomplete cycle, 14N
  otherwise
• 13C - produced by incomplete CN cycle.
  Probably made in low mass stars
  and ejected into the ISM by red giant winds
  and plaetary nebulae
• 14N - product of the CNO cycle. At
  comparatively low T, 12C -gt
  14N at higher T and over longer time scales
  16O -gt 14N. Mostly made in low mass
  stars and ejected by red giant
  winds and planetary nebulae. However, some
  part from high mass stars, especially
  at high Z and if the He core
  peenetrates the H-envelope in low Z stars.
• 17O - complicated. Used to be considered a
  massive star product from the CNO
  bicycle. Now new rate measurements suggest that
  it may need to be relegated to
  classical novae
• 15N - certainly not made in the classical
  CNO cycle in stable stars.

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• 18O - made in helium burning in massive
  stars by 14N (???????18F (e
  ??)18O
• 23Na - partly a product of the Ne-Na cycle
  in hydrogen burning, but mostly
  made by carbon burning
• 26Al - gamma-ray line emitter. Partly made in
  hydrogen burning by Mg - Al
  cycle. Mostly made in carbon and neon burning.

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Suppose keep raising the temperature of the CNO
cycle. How fastcan it go?

• As 14N(p,?)15O goes faster and faster there
  comes a point where the decays of 14O and 15O
  cannot keep up with it. ??1/2 (14O) 70.64 s
  against positron emission. ??1/2(15O) 122.
  24 s.
• Material then accumulates in 14O, 15O - more
  than in 14N. The lifetimes of these two
  radioactive nuclei give the energy
  generation that now becomes insensitve to
  temperature and density.
• As the temperature and density continue to rise,
  other reactions become possible.

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b-Limited or Hot CNO cycle
14O
(en)
(p,g)
Slowest rates are weak decays of 14O and 15O.
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Hot CNO cycle
ColdCNO cycle
but 13N decays in 10 min
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Classical Novae

• Distinct from dwarf novae which are probably
  accretion disk instabilities
• Thermonuclear explosions on accreting white
  dwarfs. Unlike supernovae, they recur, though
  generally on long (gt1000 year) time scales.
• Rise in optical brightness by gt 9 magnitudes
• Significant brightness change thereafter in lt
  1000 days
• Evidence for mass outflow from 100s to 5000 km
  s-1
• Anomalous (non-solar) abundances of elements
  from carbon to sulfur

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• Typically the luminosity rises rapidly to the
  Eddington luminosity for one solar mass (1038
  erg s-1) and stays there for days (fast nova)
  to months (slow nova)
• In Andromeda (and probably the Milky Way) about
  40 per year. In the LMC a few per year.
• Evidence for membership in a close binary
  0.06 days (GQ-Mus 1983)
  2.0 days (GK Per 1901)
  see Warner, Physics of
  Classical Novae,
  IAU Colloq 122, 24 (1990)

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Discovery Aug 29, 1975 Magnitude 3.0
A fast nova
V1500 Cygni
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Nova Cygni 1992
The brightest nova since 1975. Visible to the
unaided eye. Photo at left is from HST in 1994.
Discovered Feb. 19, 1992. Spectrum
showed evidence for ejection of large amounts of
neon, oxygen, and magnesium,
Peak magnitude 4.4 3.2 kpc A neon nova -
ejecta rich in Ne, Mg, O, N Ejecta 2 x
10-4 solar masses H burning ceased after 2
years (uv continuum sudden drop)
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Fast nova rise is very steep and the
principal display lasts only a few days. Falls gt
3 mag within 110 days
Slow nova the decline by 3 magnitudes takes at
least 100 days. There is frequently a decline
and recovery at about 100 days associated with
dust formation.
Very slow nova display lasts for years.
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Effect of embedded companion star?
Recurrent novae observed to recur on human time
scales. Some of these are accretion disk
instabilities
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Red dwarf stars are very low mass main
sequence stars
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An earth mass or so is ejected at speeds of 100s
to 1000s of km/s. Years later the ejected shells
are still visible. The next page shows imgaes
from a ground-based optical survey between 1993
and 1995 at the William Hershel Telescope and the
Anglo-Australian Telescope.
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Nova Persei (1901) GK Per
Nova Hercules (1934) DQ - Her
Nova Pictoris (1927) RR Pic
Nova Cygni (1975) V1500 Cygni
Nova Serpentis (1970) FH Ser
http//www.jb.man.ac.uk/tob/novae/
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Models
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where we have used
This gives a critical mass that decreases rapidly
(as M-7/3) with mass. Since the recurrence
interval is this critical mass divided by the
accretion rate, bursts on high mass white dwarfs
occur more frequently
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Nomoto (1982)
The mass of the accreted hydrogen envelope at the
time the hydrogen ignites is a function of the
white dwarf mass and accretion rate. Bigger
dwarfs and higher accretion rates have smaller
critical masses for surface runaways.
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Truran and Livio (1986) using Iben (1982)
lower limits especially for high masses
Mass WD Interval
(105 yr)
Even though the average mass white dwarf is 0.6
0.7 solar masses the most often observed novae
have masses around 1.14 solar masses. These
would be white dwarfs composed of Ne, O, and Mg.
It is estimated that 1/3 of novae, by number,
occur on NeOMg WDs even though they are quite
rare.
0.60 12.9 0.70 7.3 0.80
4.2 0.90 2.4 1.00
1.2 1.10 0.64 1.20
0.28 1.30 0.09 1.35
0.04
see also Ritter et al, ApJ, 376, 177, (1991)
Politano et al (1990) in Physics of Classical
Novae
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For typical values the density at ignition is
somewhat degenerate
(hydrostatic eq.)
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Though partially degenerate and dominated by
beta-limited CNO burning at first, the nova
instability is basically an example of the thin
shell instability.
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Basically the limiting condition is that the
temperature stays high enough to provide an
Eddington luminosity to the layeruntil it is all
ejected.
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The binding energy per gm of nova material is
This is considerably less than the energy
released by burning a gram of hydrogen to
helium, (6 x 1018 erg gm-1) so most of the
hydrogen is ejected unburned.
However, for a violent outburst, it is not
adequate to use just the CNO in the accreted
matter. Mixing with the substrate must occur and
this enriches the runaway with additional
catalyst for CNO burning
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So the integrated kinetic energies, potential
energy, and light output are all comparable. A
part of this energy may come from a common
envelope effect with the companion star.
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Nucleosynthesis in Novae
Basically 15N and 17O
The mass fraction of both in the ejecta is
0.01, so crudely
Woosley (1986)
approximate Pop I material in the Galaxy within
solar orbit
Novae also make interesting amounts of 22Na and
26Al for gamm-ray astronomy
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Typical temperatures reached in hydrogen burning
in classical novae are in the range 1.5 - 3.0 x
108 K, sufficient that burning is primarily by
the beta-limited CNO cycle. It would take
temperatures of about 3.5 x 108 K to break out
of the CNO cycle and produceheavier elements by
the rp-process.This is not ruled out for the
more massive novae. E.g., 1.35 Msun model reached
356 million K.
Livio and Truran, ApJ, 425, 797, (1994)
Politano et al, ApJ, 448,
807, (1995) Typical heavy element mass
fractions in novae are typically gt10 showing
strong evidence for mixing with the
substrate during or prior to the explosion. E.g.
QU Vul was 76 and 168 times solar in neon at 7.6
and 19.4 yr after explosion
Gehrz et al, ApJ, 672, 1167, (2008)
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Some issues

• Burning is not violent enough to give fast
  novae unless the accreted layer is significantly
  enriched with CNO prior to or early during the
  runaway. Also nucleosynthesis strongly
  suggests mixing.
• Relation to Type Ia supernovae. How to grow
  MWD?
• How hot do they get?

Shear mixing during accretion Convective
undershoot during burst
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Over time, matter is removed from the white
dwarf, not added and this poses a problem to
making TypeIa supernovae by this route.
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The rp-Process
Wallace and Woosley, ApJS, 45, 389 (1981)
Aside - T to produce heavy elements is reduced if
there is a lot of Ne and Mg already present as in
novae on NeOMg white dwarfs.
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Burst Ignition
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The rp-Process

• 15O(???)19Ne comparable to Hburning lifetime
• 19Ne(p,?)20Na appoximatelyequal to 19Ne positron
  decay
• rp-process limited byweak interactions,
  not15O(???)19Ne.
• e) (?,p) reactions startto bridge waiting points

Wallace and Woosley, ApJS, 45, 389, (1981)
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Endpoint Limiting factor I SnSbTe Cycle
(Schatz et al. PRL 86(2001)3471)
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Principal Application(s)

• Type I X-ray bursts on accreting neutron stars
• Unusually violent novae using Mg or Ne as
  starting point
• Neutrino-driven wind. Early on in a supernova
  explosion proton-capture in a region with Ye gt
  0.50 may produce many proton-rich nuclei
  above the iron group (part of the
  p-process)

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Type I X-Ray Bursts (e.g., Strohmayer Bildsten
2003)

• Burst rise times lt 1 s to 10 s
• Burst duration 10s of seconds to minutes
• Occur in low mass x-ray binaries
• Persistent luminosity from 0.2 Eddington to lt
  0.01 Eddington
• Spectrum softens as burst proceeds. Spectrum
  thermal. A cooling blackbody
• Lpeak 3.8 x 1038 erg s-1. Evidence for radius
  expansion above that. T initially 3 keV,
  decreases to 0.5 keV, then gets hotter again.

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Burst energythermonuclear
Persistent fluxgravitational energy (much more
energy)
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• Of 13 known luminous globular cluster x-ray
  sources, 12 show x-ray bursts. Over 50 total
  X-ray bursters are known.
• Distances 4 12 kpc. Until 2005, none outside
  our galaxy. Now two discovered in M31 (Pietsch
  and Haberl, AA, 430, L45 (2005).
• Some superbursts observed lasting for several
  hours
• Low B-field lt 108-9 gauss
• Rapid rotation (at break up?)
• Very little mass lost (based upon models)

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(Woosley Taam 1976)
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Woosley et al, ApJS, 151,
175 (2004)
Reaction network
red triangles have experimentally determined
masses. The rest are theoretical more or less
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from He burning
Total nuclear energy generation at this stage is
3.4 x 1035 erg s-1. The time is one minute before
the burst.
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Just before the burst starts, most of the layer
is convective. The total power is 8 x 1037 erg
s-1, but only 1.3 x 1035 erg s-1 is escaping from
the surface - small compared with the accretion
luminosity.
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Woosley et al (2004)
on a longer time scale
Times offset by 41,700 s of accretion at 1.75 x
10-9 solar masses/yr
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begining of second burst
maximum T developed in the burst about 1.5 x 109
K
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Fourteen consecutive flashes. The first is a
start up transient.
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Embarrassingly good agreement!
Model A3
GS 1826-24
Heger, Cumming, Gallaoway and Woosley (2007, ApJ,
671, 141)