light curves and spectra

5-9 Nov 2012 Tsinghua Transient Workshop

Light curves and Spectra


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

56Ni56Co56Fe


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SN Light Curves


4 main phases1. Shock breakout

- star is hot, L~R*, rapid

2. Recombination phase (H-rich SNe)- envelope recombines, Light emitted: L,

t ~ M(env), R(env)

3. 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


Different types of SNe have different light curves

SN Type

prog Shock Rec Rad. Heat.

Rad. Tail

II RSG (BSG)

IIP (87A)

(small R)

Ib/cCores (WR)

(small R)

(no H)

IaCores (WD)

(small R)

(no H)


Type II SNe79C: 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)


SN 1987A : contributions to the LC

Shock breakout

Radioactive Heating

Radioactive Tail


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!

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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


Adiabatic Cooling• But some luminosity does escape

• If no other heating form,

• Where

• If E(rad) ~ 1/2 E(SN),

• Luminosity in this phase


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


Plateau phase can last for months

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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+


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


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)


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


Radioactive Tail/2• If envelope not massive, eventually even e+ may not

fully deposit, and LC will decline faster


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




SN Light CurvesPeak Lum: 56Ni

Plateau: H-envelope, R*

(SNe IIL: small H-envelope

SN 1987A: small R*)

Tail: 56Ni, M, E


SN Spectra

• Formation, Observables


Early-time spectrum

Homologous expansion

(v ≈ R) Ejecta are dense“Photospheric

Epoch”

τ=1continuum

absorption


• Ejecta are dense

pseudo-photosphere

• Lines have P-Cygni profiles with

But velocities are high:

many lines overlap:

“Line Blanketing”

Early-time spectrum

phabs vv ~


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


Montecarlo spectra• Treatment of ionization/excitation includes

approximate NLTE (nebular approx.)

• Excited states

• Ground/metastable states: LTE

• Ionization: modified Saha


Photon Travel in Montecarlo scheme

Abbott & Lucy 1985


Treatment of Opacities in MC

Mazzali & Lucy 1993


Photon Branching in MC

Mazzali 2000


The effect of Photon Branching

Mazzali 2000


Testing different distances


Testing different risetimes


Late-time spectra

Ejecta are thin:

“Nebular Epoch”

Gas heated by deposition of γ’s and

cooled by forbidden line emission

+e

Spectrum: no continuum.

Emission line profiles depend on velocity, abundance distribution.

Homologous expansion, homogenous density and abundance: parabolic profiles

τ < 1


Late-time spectra

• Solve gamma-ray deposition, NLTE equations for state of gas

• Emission in mostly forbidden lines


Supernova ClassificationMaximum light spectra

H / no H

SNe II SNe I | |

Light Curve shape Si / no Si

SNe IIL SNe IIP SNe Ia He / no He

SNe Ib SNe Ic


Spectral Classification


Supernova ClassificationLate-time spectra (6mo-1yr)

H / no H

SNe II SNe I | | O, H Fe, no O / O

SNe Ia SNe Ib/c


Properties of SNe

early lat e radio gal axy light curve M(Ni) (M)

H He Si H O E S peak tail ~0.6

Ia nar fast ~0.1

Ib nar fast ~0.1

Ic nar fast ~0.1

II brd slow-IIP nar-IIL


SNe II

H lines dominate at all times

H Ca II


Properties of SNe from spectra: SNe II

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SNe II: spectral evolution reflects structure of massive star



Early times: outer layers visible

Late times: inner part exposed


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


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


SNe IIL: small H-envelope

These are rare events, showing a rapid (Linear) decline with no plateau: e.g. SN1980K


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

CSMLate time: core


SNe IIn: extreme case of interactionSimilar 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


SNe IIn spectraDominated by interaction: narrow H lines

indicate massive CSM


SNe IIn spectraDominated by interaction: massive CSM


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

CSM

Late time: core


SNe IIn: ejecta-CSM interaction

Two shock are launched at the contact discontinuity


SNe IIn: ejecta-CSM interactionWind may be clumpy

Narrow (few 100 km/s): clumpy wind

Intermediate (~1000 km/s) : shocked wind

Broad (few 100 km/s): : shocked ejecta


SNe IIb: an intermediate class?Early times:

Both H and He present


SNe IIb: an intermediate class?Late times: O dominates, some weak H also present, with flat-top profile: H in a shell

H-envelope partially stripped



early

late


A binary progenitor?





Detect hot star (B1 Ia) spectrum in spectum of SN1993J


SN1993J had lost most of its H-envelope to a companion

Now companion is more massive



New evolutionary track of companion after mass accretion


SNe Ib

Early: optical: He , Ca, Si, some O IR: characteristic He lines

He lines require non-thermal excitation by fast particles


SNe Ib

Late: O, Ca, Mg



Early times: see He H-envelope lost

Late times: see CO core, as in SNe II

Stripped star




SNe Ic

Early: optical: Fe , Ca, Si, O IR: evidence of He lines unclear


SNe Ib/c

Early: He (Ib), Ca, Si, some O Late: O dominates, Ca

He lines require non-thermal excitation by fast particles


SNe Ic

Late: O, Ca, Mg

Early times: see CO core H- and H envelopes lost

Late times: see CO core, as in SNe II, Ib

Star more stripped




A Wolf-Rayet progenitor ?- SN2002ap

Progenitor detection difficult. Probably a Wolf-Rayet star (stripped massive star)


An evolutionary sequence among core-collapse SNe

early late


Btw, SNe Ib v. Ic: Helium

Ib

Ic

Ic

2.058µm1.083µm

(Taubenberger et al. 2006)

Strongest HeI lines in IR. 1 can cause confusion, 2 line unique


Most nearby long-soft GRBs come with type Ic SNe

stripped stars are more fun


Significance of spectrum

Broad lines

Large Kinetic Energy

“Hypernovae”

(only SN1998bw was associated with a GRB)

Narrow lines

“normal” KE (1 foe)

Normal SN Ic

Mazzali et al. 2002


SNe Ib/c cover a range of Lum

• SN1998bw was as bright as a SN Ia

• It produced much more 56Ni than `normal’ core-collapse SNe (~ 0.5 M)


SNe span a range of KE

• GRB/SNe have very high expansion velocities (optical velocities up to 0.1c track relativistic properties)

• XRF/SNe have lower velocities

after Pian et al. 2006, Nature

5-9 Nov 2012 Tsinghua Transient WorkshopIwamoto et al. 1998

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SN 1998bw: modelling

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GRB/SNe are highly aspherical

• Evidence in nebular spectrum (Oxygen line broader than Fe lines, Mazzali et al. 2001) but also

in light curve


56Fe

16O

Spherical

Aspherical

FeII] 5200A

[OI] 6300A

Observed

Aspherical explosion:

aspect-dep line shape

Orientation 15 deg

Maeda et al. 2002


Was SN 2003jd = 98bw off-axis?• It was almost as

bright at peak as SN1998bw (Mv = -18.7)

• Early-time spectra had broad lines, but closer to SN2002ap


Prediction from asphericity: off-axis GRB/SNe

• Double-peaked [O I] line indicates edge-on SN

• SN Ic 2003jd had broad lines was luminous, and showed a double-peaked [O I] line

• but the presence of an off-axis GRB seems ruled out by radio limits (Soderberg et al. 2006)

• So, something’s missing

Mazzali et al. (2005)


Which stars become which SNe?

SN types of nonrotating massive single stars as a function of initial metallicity and initial mass. Green horizontal hatching indicates the domain where SNe IIp occur. At the high-mass end of the regime they may be weak and observationally faint because of fallback of 56Ni. These weak SNe IIp should preferentially occur at low metallicity. At the upper right-hand edge of the SN II regime, close to the green line of loss of the hydrogen envelope, SNe IIL/b that have a H-envelope of ~2 Mo; are made (purple cross-hatching). In the upper right-hand quarter of the figure, above both the lines of H-envelope loss and direct black hole formation, SNe Ib/c occur; in the lower part of their regime (middle of the right half of the figure) they may be weak and observationally faint because of fallback of 56Ni, similar to the weak SNe IIp. In the direct black hole regime no "normal" (non–jet-powered) SNe occur since no SN shock is launched. An exception are pulsational pair-instability SNe (lower right-hand corner; brown diagonal hatching) that launch their ejection before the core collapses. Below and to the right of this we find the (nonpulsational) pair-instability SNe (red cross-hatching), making no remnant, and finally another domain where black hole are formed promptly at the lowest metallicities and highest masses (white) where no SNe are made. Single WDs also do not make SNe (white strip at the very left).


Spectra of SNe: SNe Ia

Early phase: absorption lines Late phase: nebular lines


The case of SNe Ia ASTRO: Calibration of SN luminosity

– Brighter SNe have broader LCs (Phillips 93)

PHYS: What causes Lum-light curve relation?

Observed normalised


The case of SNe Ia• Observations near maximum

– Composition of outer layers, energetics

• Importance of late, nebular phase– Properties of inner layers, dominated by 56Ni

Mazzali et al. 1998Early Late


The case of SNe Ia• Abundance tomography

– Model time series of spectra– Montecarlo and NLTE techniques– Complete description of SN

(Mazzali et al. 2008)


The case of SNe Ia

Mazzali et al. (2007)

56Ni

Si

stable Fe

• The key to understanding Zorro diagram SN Ia behaviour– Study many SNe– Mass (Ni + Si) ~ const

• Mass burned ~ const• KE ~ const• Lum M(56Ni)• LC width opacity• M(Fe group)

– Links to progenitor/ explosion

light curves and spectra

Documents

henvelope present

iil small h env

h iacores wd small r

small r ibccores wr

r large

radioactive phase

maximum light

free electronswhen h