Type Ia SupernovaeProgenitors

Type Ia Supernovae

Historical defining characteristics:Generally, lack of lines of hydrogen

Contain a strong Si II absorption feature (6355Å shifted to ~6100Å)

Observational CharacteristicsHomogeneity: nearly 60% homogeneous in terms of spectra, light curves, peak absolute magnitudes MB ~ -18.8+5log(Ho/74)Inhomogeneity: differences in spectra, light curves, do exist. In terms of explosion strength, order:

Very weak, e.g. SN 1991bg, SN 1992k

Weak, e.g. SN 1986G

Normal, ~60%

Very bright, e.g. SN 1991T, ~20%

Energy: ½(~104 km/s)2 ~ ECO->Fe

Degeneracy: Explosive event

Spectrum: Generally no hydrogen

Delay: Explosions can occur with long delay after cessation of star formation.

Basic model: Thermonuclear disruption of mass-accreting white dwarf.

Note: Flame physics is still an unsolved problem, deflagration to detonation?

The following simple facts

Why is identifying the progenitors so important?

The fact that we still don’t know the progenitors of some of the most dramatic explosions – a major embarrassment.For dark energy properties we need to understand the evolution of the luminosity with cosmic time.Feedback – radiative, kinetic, nucleosynthetic input into galaxy evolution.

• Form at Mwd 0.45M≲ ☉

• Can explode at 0.7M∼ ☉ central He ignition

• But composition of ejected matter: He + 56Ni + decay productsInconsistent with observations

• Form from M 8-11.5∼ M☉

• But not numerous enough and expected to collapse to NS rather than explode

Composition of accreting WDO-Ne-MgC-OHe

Yes

No No

At what mass does the WD explode?

Chandrasekhar masscarbon igniters

Carbon ignites at center:

Sub-Chandra mass helium igniters

1. At ~1051 ergs2. 56Ni decay

powers lightcurve

3. X; (Vj) consistent with spectra

4. MCh explains homogeneity

1. Difficult to reach MCh

2. For MWD 1.2M≳ ☉ collapse to NS rather than explosion may ensue

Indirect double detonation or “edge lit” detonation

One detonation propagates outward (through He), inward

pressure wave compresses the C-O, ignites off center, followed

by outward detonation.

+ −

At what mass does the WD explode?Composition of high velocity ejecta

In sub-Chandra mass WDsIntermediate mass elements sandwiched by Ni and He/Ni rich

High velocity Ni, He! No high velocity C!

For progenitors we have two possible scenarios

Double degenerateMerger of two CO WDs

brought together by gravitational radiation

Single degenerateA CO WD accretes material from a main sequence or

red giant companion

Binary WD systems Systems such as:Recurrent novae

Supersoft x-ray sourcesSymbiotic systems

Evolutionary Scenarios

Double degenerate scenarioStrengths

1. Population synthesis predicts the right statistics.

2. Double degenerate systems detected observationally.

3. Mergers with some significant consequences appear inevitable.

4. In ellipticals, consistent with observed x-ray flux (~30x smaller than predicted for SDS).

Weaknesses1. Off-center carbon ignition

may lead to O-Ne-Mg WD and accretion induced collapse rather than SN Ia.

Double degenerate scenario1. The merger of two 0.9 M☉

WDs produced a subluminous SN Ia (SN 1991bg-like).

Double degenerate scenario2. Simulation of 0.6 M☉ +

0.9 M☉ suggested thatSNe Ia could be obtained if

τJ >τ ν

Single degenerate scenarioStrengths

1. If accreted matter can be retained, natural path to increasing mass.

2. Candidate progenitors exist.

Weaknesses1. It is not absolutely clear that a

hydrogen-accreting WD can indeed reach MCh.

2. Limits on the presence of H exclude symbiotics with the highest mass loss rates.

3. In ellipticals, the observed x-ray flux ~30x smaller than predicted.

A few recent observational findings

1. Two populations:“Prompt” – SN Ia rate ~ star formation rate“Delayed” – SN Ia rate ~ stellar mass

⇓a. Rate higher in late type

galaxies.b. For higher z prompt

dominate sample.c. Prompt are brighter.

Recent observational findings2. A decrease with redshift in the strength of intermediate

mass element features (consistent with higher brightness, which implies larger mass of 56Ni).

Recent observational findings3. The luminosity-weighted age of the host galaxy is correlated

with the 56Ni yield ⇒ more massive progenitors give rise to more luminous explosions.

Recent observational findings4. Super-Chandrasekhar

mass progenitors?E.g. SNLS-03D3bbSN 2009dc.

Serendipity and SNe IaA short (10 sec) acquisition image by STIS on board HST of the nucleus of the radio-loud galaxy 3C 78 revealed a point source superposed very near the jet – a SN Ia.

Possibility to Increase SNe Ia Rate?

The shocks produced by jets can either:a. Heat mass-donor star in binaries containing white dwarfs,

thereby increasing the mass transfer rate.or

b. Compress clumps in the interstellar medium thereby increasing the mass accretion rate onto low-velocity white dwarfs.

Prediction: The rate of classical novae should increase too, because novae are obtained when white dwarfs accrete at rates

Serendipity and Classical NovaeAn HST program intended to measure the proper motion of the optical jet in M87 discovered 11 transient sources in the vicinity of the jet.

HST WFPC2HST WFPC2

Conclusions1. Single degenerate scenario could, in principle, explain

everything: prompt are caused by accretion from young main sequence stars (~8M☉), and delayed are caused by accretion from red giant companions (~1M☉).However, models involve maybe too many “moving parts.”

Conclusions2. Double degenerates could perhaps produce SNe Ia,

especially in ellipticals, but many uncertainties remain.3. Is there something to be learned from gamma-ray bursts?

In particular, if indeed both double degenerates and single degenerates contribute to SNe Ia, then it would be hard to believe that mergers and accretion from a red giant, in ellipticals, would appear precisely the same in all two-parameter diagrams (e.g., the separation between long and short GRBs in duration-hardness diagrams).Consequently, there should be some phase space in which the two progenitor families should separate.