Variable Stars

2. Variable Stars • Some Giants and Hypergiants exhibit regular periodic change in luminosity • Mira (Fabricius 1595) changes by factor of 100with period of 332d • LPV like Mira not well modelled

3. Instability Strip • A nearly vertical region traversed by most massive stars on HB • RR Lyrae: PII HB stars with periods of hours. Luminosity varies little (!) • Cepheids(PI) , W Virginis(PII) periods of days.

4. Why They Pulse • Cepheidsoscillate in size (radial oscillation) • Temperature and luminosity peak during rapid expansion • Eddington: Compression increases opacity in layer trapping energy and propelling layer up where it expands, releases energy • Problem: compression reduces opacity due to heating • Solution: compression ionizes Helium so less heating. Expansion reduces ionization – κ-mechanism • Instability strip has partially ionized Helium at suitable depth

5. Why We Care • Leavitt 1908: Period-Luminosity Relation for SMC cepheids • Luminous cepheids have longer periods • With calibration in globular clusters cepheids become standard candles • Later: W Virginis PLR less luminous for same period

6. White Dwarfs • Bessel 1844: Sirius wobbles: a binary • Pup hard to find. Clark 1846 • Orbits: • Spectrum (Adams 1915): • Surface Gravity • Spectrum: Very broad Hydrogen absorption lines • Estimate: • No Hydrogen else fusion

7. Degenerate Matter • White dwarves are the degenerate cores of stars with • Composition is Carbon Oxygen • Masses • Significant mass loss • Chandrasekhar: • Relativity:

8. Mass-Radius

9. Roche Potential • In a binary system matter orbits both stars • Entire system rotates. If dropped from (rotating) rest, where will a stone fall? • Combined gravity and rotation described by Roche potential • Inside each star’s Roche lobe orbits stay close to that star

10. Algol • Eclipsing binary Algolis a puzzle: MS subgiant • Massive A should have evolved earlier? • B started out as the more massive star • In its subgiant phase, atmosphere leaked out of its Roche lobe • Gas lost by B forms accretion disk around A

11. White Dwarf Nova • White dwarves in close binaries can accrete Hydrogen at from partner when it overflows its Roche lobe • Infalling gas compressed to degeneracy and heated by immense surface gravity • Enriched with CNO by turbulent mixing at base • When accumulates, base temperature • CNO fusion explosively heats gas to and luminosity • Radiation pressure ejects accreted material • Total energy released over months • Can recur in • Ejected matter glows at initial • 30/yr in M31

13. Nova Remnants

14. Type-Ia Supernova • Accretion adds to white dwarf mass. What if it exceeds Chandrasekhar limit? • It doesn’t.As increased mass compresses dwarf, pressure and temperature increase • A turbulent convection phase leads to ignition of Carbon fusion • In degenerate dwarf heating does not lead to expansion so violent explosive process fuses substantial fraction of star in a few seconds • Oxygen fusion less complete • Internal temperature exceeds • Fusion releases blowing star apart completely releasing shock wave ejecting matter at high speeds • Luminosity reaches and decays over months • Spectrum has absorption lines of Si but little H He • Decay of readioactive fusion products near iron mass in shell contributes to luminosity at late times

15. What We Know • Nature of Mass donor unclear • Single Degenerate: Donor is MS or giant • Double Degenerate: Donor is White dwarf ripped apart by tidal forces in merger • Likely both occur • Nature of explosion also debated: deflagration or detonation? Degenerate Helium flash trigger or internal CO ignition • Fact: Luminosity (corrected by light curve) almost the same for all Ia Supernovae: Standard Candles!

16. A Standard Candle

17. Post-MS Massive Star • Massive stars end Main Sequence life • When core Hydrogen fusion ceases core contracts and envelope expands and cools • Shell Hydrogen fusion: Red Supergiant • Core does not become degenerate

18. Massive Star HB • Helium core ignites • Hydrogen fusion in shell • Envelope contracts and heats • Blue Supergiant • Forming CO core

19. Massive Star AGB • CO core collapses until • Carbon fusion produces Mg Ne O • Helium and Hydrogen fusion in shells • Many neutrinos carry energy off • Superwindand mass loss

20. More Onion Shells • At ignite Neon fusion • Produce O Mg… • Neutrinos carry off • Last a few years • Oxygen fusion • Produce Si S P… • Neutrinos carry off • Last about a year • Si fusion • Produce Ni Fe • Neutrinos carry off • Last about a day • Build up inert Fe core • Changes rapid. Envelope never responds • s-process nucleosynthesis produces heavier elements

21. End of the (Si) Day • Inert Fe core • High T photons cause photodisintegration destroying heavy nuclei and absorbing energy • Fe is the end: no more nuclear energy. What next?

22. The Center Cannot Hold • As gravitational crush increases, iron core collapses from size of Earth to a few km in • In core,emits ϒrays leading to photodisintegration of heavy nuclei • Outer layers fall inward at speeds up to • As core collapses electron degeneracy overcome • Electrons forced into • Left with a small, incredibly dense core that is mostly neutrons • Does collapse stop?

23. Boom! • Within 0.25s core is neutrons with radius 20 km and super-nuclear density • Very little light can escape, energy carried off by neutrinos. Power emitted in these exceeds all known stars for 10 s • At this density core collapse stops with bounce • Colliding with infalling layers this triggers shock wave blowing outer star into space (96% of mass for star) • In compressed heated shock wave fusion to Fe and beyondvia r-process • As ejecta thin light can escape. Luminosity reaches • Energy released type-II supernova – gravitationalin origin

24. Seeing Them • Sung dynasty history describes a supernova in 1054 whose remnant – Crab nebula in Taurus – is still visible (M1) • Japanese, Arabic, Native American records concur • Milky Way supernovae also in 1006, 1572, 1604. Estimated every 300 years but obscured by dust • Many visible in other galaxies, currently some 20-30 bright ones

25. SN 2011dh

26. Classification • SN classified by spectrum: • Ia: Strong Si no H He • Ib: Weak H Strong He • Ic: Weak Si no H He • II: Strong H • Iaare nuclear explosion of WD • II IbIcare gravitational core collapse with degrees of envelope loss

27. 168,000 years ago a B3 I supergiant collapsed in LMC • Observed as SN 1987A • Progenitor known – changed theory • Remnants observed in detail

28. The Nebula

29. What we are Seeing

30. Neutrinos • Three hours before the supernova detected, neutrino detectors observed a burst (20) of neutrinos from the right direction. • 20 detected implies 1058emitted carrying 1046 J in agreement with models • Neutrinos get out before shock wave disperses outer layers, so got here before the light • Neutrino Astronomy launched, many new experiments planned

31. What is Left of Core? • Electron degeneracy cannot stop collapse – few electrons • Neutron degeneracy pressure at density • in • Surface gravity • Physics is relativistic • Chandrasekhar Limit depends on rotation • Rapid Rotation expected • High magnetic field frozen in

32. Discovery • Physics Predictions: • Rapid Rotation • Intense magnetic field • High Temperature • Bell 1967: Periodic 1.337s Radio pulses: LGM? • Quickly found other sources: natural • Soon find many pulsars Slow down in

33. LGM Data

34. What are Pulsars? • Rotating star breaks up • Only NS dense enough to survive • Emission aligned to magnetic axis - tilted • Crab pulsar : Neutron star SN remnant

35. How They Work • General Idea: Rapidly changing intense magnetic field creates intense electric field • Lifts charged particles from polar regions into magnetosphere dragged around by rotation • Accelerated to relativistic speeds – emit synchrotron radiation at all wavelengths in direction of magnetic axis • Emitted energy slows rotation • Luminosity of Crab nebula agrees with observed rate of slowing of pulsar • Pulsars observed in all bands