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Lecture 9 Hydrogen Burning Nucleosynthesis, Classical Novae, and X-Ray Bursts

Lecture 9 Hydrogen Burning Nucleosynthesis, Classical Novae, and X-Ray Bursts. Once the relevant nuclear physics is known in terms of the necessary rate factors, l = N A < s v>, the composition can be solved from the coupled set of rate equations:.

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Lecture 9 Hydrogen Burning Nucleosynthesis, Classical Novae, and X-Ray Bursts

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  1. Lecture 9 Hydrogen Burning Nucleosynthesis, Classical Novae, and X-Ray Bursts

  2. Once the relevant nuclear physics is known in terms of the necessary rate factors, l = NA<sv>, 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...

  3. Aside on implicit solution of rate equations note limits:

  4. 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

  5.  = 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.

  6. Steady state after several times these time scales.

  7. Provided steady state has been achieved the abundance ratios are just given by the l’s. 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.

  8. 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 -> 14N; at higher T and over longer time scales16O -> 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.

  9. 18O - made in helium burning in massive stars by14N (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.

  10. 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.

  11. b-Limited or “Hot” CNO cycle 14O (e+n) (p,g) Slowest rates are weak decays of 14O and 15O.

  12. Hot CNO cycle ColdCNO cycle but 13N decays in 10 min

  13. 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 (>1000 year) time scales. • Rise in optical brightness by > 9 magnitudes • Significant brightness change thereafter in < 1000 days • Evidence for mass outflow from 100’s to 5000 km s-1 • Anomalous (non-solar) abundances of elements from carbon to sulfur

  14. 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)

  15. Discovery Aug 29, 1975 Magnitude 3.0 A “fast” nova V1500 Cygni

  16. 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)

  17. Fast nova – rise is very steep and the principal display lasts only a few days. Falls > 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.

  18. Effect of embedded companion star? Recurrent novae – observed to recur on human time scales. Some of these are accretion disk instabilities

  19. Red dwarf stars are very low mass main sequence stars

  20. 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.

  21. 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/

  22. Models

  23. 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

  24. 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.

  25. Truran and Livio (1986) using Iben (1982) – lower limits especially for high masses Mass WD Interval (105 yr) Even though theaverage 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

  26. For typical values the density at ignition is somewhat degenerate: (hydrostatic eq.)

  27. Though partially degenerate and dominated by beta-limited CNO burning at first, the nova instability is basically an example of the thin shell instability.

  28. Basically the limiting condition is that the temperature stays high enough to provide an Eddington luminosity to the layeruntil it is all ejected.

  29. 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

  30. 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.

  31. 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

  32. 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 >10% 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)

  33. 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

  34. Over time, matter is removed from the white dwarf, not added and this poses a problem to making TypeIa supernovae by this route.

  35. 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.

  36. Burst Ignition: Prior to ignition : hot CNO cycle ~0.20 GK Ignition : 3a : Hot CNO cycle II ~ 0.68 GK breakout 1: 15O(a,g) ~0.77 GK breakout 2: 18Ne(a,p) (~50 ms after breakout 1)Leads to rp process and main energy production

  37. 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)

  38. Endpoint: Limiting factor I – SnSbTe Cycle The Sn-Sb-Te cycle Known ground statea emitter (Schatz et al. PRL 86(2001)3471)

  39. 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 > 0.50 may produce many “proton-rich” nuclei above the iron group (part of the “p-process”)

  40. Type I X-Ray Bursts (e.g., Strohmayer & Bildsten 2003) • Burst rise times < 1 s to 10 s • Burst duration 10’s of seconds to minutes • Occur in low mass x-ray binaries • Persistent luminosity from 0.2 Eddington to < 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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