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Light curves and Spectra

Light curves and Spectra

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Light curves and Spectra

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  1. Light curves and Spectra Tsinghua Transient Workshop

  2. SN Light Curves • A SN shines for different reasons, and different types of SN may only show some of the various mechanisms • Some SN classification is done on the basis of the Light curve properties • The only phase common to all SNe is the radioactive phase, with 56Ni56Co56Fe Tsinghua Transient Workshop

  3. Tsinghua Transient Workshop

  4. SN Light Curves Tsinghua Transient Workshop

  5. 4 main phases • Shock breakout - star is hot, L~R*, rapid • Recombination phase (H-rich SNe) • envelope recombines, Light emitted: L, t ~ M(env), R(env) • Radioactive heating (long diffusion times) • 56Ni, 56Co decay: ’s, deposition, optical photons L ~ M(56Ni), M*, ; t ~ M*, , KE • Radioactive tail (short diffusion times) • 56Co decay: prompt optical photons L ~ M(56Ni); t ~ M*, , KE Tsinghua Transient Workshop

  6. Different types of SNe have different light curves Tsinghua Transient Workshop

  7. Type II SNe 79C: IIL (small H env. - no Rec. Phase) 93J: Ib (very small H env.: He lines) 87A: IIP-pec (BSG prog - small R) 97D: IIP (faint) (large envelope, small KE - long plateau) Tsinghua Transient Workshop

  8. SN 1987A : contributions to the LC Radioactive Heating Shock breakout Radioactive Tail Tsinghua Transient Workshop

  9. Shock breakout • Explosion KE of SN ~ , >> binding En of star  Expansion velocity is supersonic: Shock Wave • When this reaches the surface, the star gets hot and bright • Thermal En.:  • If (1 ‘foe’), RSG progenitor T~106K • But • Very bright! Tsinghua Transient Workshop

  10. Shock breakout /2 • But this phase is very short-lived (~1day): • Adiabatic cooling: • Radiation dominates: •  • Gas cools before it can contribute radiation to the LC Tsinghua Transient Workshop

  11. Adiabatic Cooling • But some luminosity does escape • If no other heating form, • Where • If E(rad) ~ 1/2 E(SN), • Luminosity in this phase Tsinghua Transient Workshop

  12. Recombination Phase • H envelope recombines when T~ 6000 K (T~12000 K for He envelope) • Most opacity in H-rich SNe is Thomson scattering on free electrons • When H recombines, opacity drops • Recombined envelope ~ transparent to photons • Photosphere follows ionization front • Recombination wave moves inwards in vel space • During Recombination phase, both Rph and L ~ constant: PLATEAU • This is only true if H-envelope is massive Tsinghua Transient Workshop

  13. Plateau phase can last for months Tsinghua Transient Workshop

  14. Radioactive heating • Adiabatic and recombination Luminosity only high if R large, E/M large (M small), H-envelope present. • Otherwise, need other source of energy • In SNe, 56Ni is produced: this is radioactive =8.8d =111d 56Ni 56Co  56Fe  e+  e+ Tsinghua Transient Workshop

  15. Radioactive decay/2 • Energy produced: • 56Ni: 3.9 1010 erg/s/g • 56Co: 6.8 109 erg/s/g • ~96% of energy carried by ’s, rest by e+ • ’s are efficiently trapped: k ~ 0.3 cm2/g • Thermalisation to optical photons • Optical photons must random-walk their way out in a large optical depth environment: kopt~0.1cm2/g Tsinghua Transient Workshop

  16. Radioactive part of LC • When photons escape SN becomes bright • But the SN ejecta expand: density decreases and so does opacity • Basic property: Maximum light occurs when heating = cooling (Arnett’s Rule) • L(Max)  M(56Ni) • Radioactive heating dominates LC if R* small, no H-envelope: Type I SNe (also SN1987A after shock breakout) Tsinghua Transient Workshop

  17. Radioactive Tail • At late times, opt<1, <1 • Only e+ deposit: ke+ ~ 7cm2/g, e+>>1 • LC follows 56Co decay rate (optical photons immediately emitted) • m = 0.98 mag/100d Tsinghua Transient Workshop

  18. Radioactive Tail/2 • If envelope not massive, eventually even e+ may not fully deposit, and LC will decline faster Tsinghua Transient Workshop

  19. Radioactive Tail/3 • For massive envelope (eg SN1987A) 56Co decay effective for a long time (2-3 yr), then other radioactive species with long decay times (eg 44Ti, 57Co) take over Tsinghua Transient Workshop

  20. SN Light Curves Peak Lum: 56Ni Plateau: H-envelope, R* (SNe IIL: small H-envelope SN 1987A: small R*) Tail: 56Ni, M, E Tsinghua Transient Workshop

  21. SN Spectra • Formation, Observables Tsinghua Transient Workshop

  22. Homologous expansion (v ≈ R) Ejecta are dense “Photospheric Epoch” Early-time spectrum absorption continuum τ=1 Tsinghua Transient Workshop

  23. Early-time spectrum • Ejecta are dense  pseudo-photosphere • Lines have P-Cygni profiles with But velocities are high: many lines overlap: “Line Blanketing” Tsinghua Transient Workshop

  24. Montecarlo approach • SN envelope expands like Hubble flow: • Photons continously redshifted • They can only interact with the next red line • Easy to treat in MC Tsinghua Transient Workshop

  25. Montecarlo spectra • Treatment of ionization/excitation includes approximate NLTE (nebular approx.) • Excited states • Ground/metastable states: LTE • Ionization: modified Saha Tsinghua Transient Workshop

  26. Photon Travel in Montecarlo scheme Abbott & Lucy 1985 Tsinghua Transient Workshop

  27. Treatment of Opacities in MC Mazzali & Lucy 1993 Tsinghua Transient Workshop

  28. Photon Branching in MC Mazzali 2000 Tsinghua Transient Workshop

  29. The effect of Photon Branching Mazzali 2000 Tsinghua Transient Workshop

  30. Testing different distances Tsinghua Transient Workshop

  31. Testing different risetimes Tsinghua Transient Workshop

  32. Late-time spectra Spectrum: no continuum. Emission line profiles depend on velocity, abundance distribution. Homologous expansion, homogenous density and abundance: parabolic profiles Ejecta are thin: “Nebular Epoch” Gas heated by deposition of γ’s and cooled by forbidden line emission τ < 1 Tsinghua Transient Workshop

  33. Late-time spectra • Solve gamma-ray deposition, NLTE equations for state of gas • Emission in mostly forbidden lines Tsinghua Transient Workshop

  34. Supernova Classification Maximum light spectra H / no H SNe II SNe I | | Light Curve shape Si / no Si SNe IIL SNe IIP SNe IaHe / no He SNe Ib SNe Ic Tsinghua Transient Workshop

  35. Spectral Classification Tsinghua Transient Workshop

  36. Supernova Classification Late-time spectra (6mo-1yr) H / no H SNe II SNe I | | O, H Fe, no O / O SNe IaSNe Ib/c Tsinghua Transient Workshop

  37. Properties of SNe Tsinghua Transient Workshop

  38. SNe II H Ca II H lines dominate at all times Tsinghua Transient Workshop

  39. Properties of SNe from spectra: SNe II Tsinghua Transient Workshop

  40. Properties of SNe from spectra: SNe II Tsinghua Transient Workshop

  41. SNe II: spectral evolution reflects structure of massive star Early times: outer layers visible Late times: inner part exposed Tsinghua Transient Workshop

  42. SN1987A - confirmation of core collapse • Core-collapse of massive star • Catalogued star SK-69 202 • M=17M • Teff=17000 • Log L/ L = 5.0 • Star has disappeared • Neutrinos confirm neutron star formation • No pulsar or neutron star yet seen Tsinghua Transient Workshop

  43. Red supergiant progenitor - SN2003gd SN1987A progenitor was a blue supergiant. Progenitor detection difficult. Only one example of a red supergiant of a normal Type II supernova Tsinghua Transient Workshop

  44. SNe IIL: small H-envelope These are rare events, showing a rapid (Linear) decline with no plateau: e.g. SN1980K Tsinghua Transient Workshop

  45. SNe IIL: small H-envelope Spectra show weak absorptions, often emission lines, indicative of interaction with surrounding CSM gas Early time: small H-envelope + CSM Late time: core CSM Tsinghua Transient Workshop

  46. SNe IIn: extreme case of interaction Similar to IIL: early signs of interaction, but interaction luminosity sustains LC for a long time: e.g. SN1995G These can be among the brightest SNe Tsinghua Transient Workshop

  47. SNe IIn spectra Dominated by interaction: narrow H lines indicate massive CSM Tsinghua Transient Workshop

  48. SNe IIn spectra Dominated by interaction: massive CSM Tsinghua Transient Workshop

  49. SNe IIn: massive H-envelope Star collapsed while H-envelope was being shedded, SN strongly interacts with surrounding CSM gas Early time: small H-envelope + CSM Late time: core CSM Tsinghua Transient Workshop

  50. SNe IIn: ejecta-CSM interaction Two shock are launched at the contact discontinuity Tsinghua Transient Workshop