What Are Type Ia Supernovae?

What Are Type Ia Supernovae? Max-Planck-Institut für Astrophysik Based on collaborations with: W. Hillebrandt (MPA Garching) S.E. Woosley (UC Santa Cruz) M. Reinecke (MPA Garching) B. Leibundgut (ESO Garching) and others

SN 1994D

Type Ia Supernovae: Facts... General properties of SNe Ia: Very homogeneous class of events, only small (and correlated) variations. Rise time: ~ 15-20 days Decay time: many months No hydrogen is seen in the spectra! Early spectra: Si, Ca, Mg,...(abs.) Late spectra: Fe, Ni, (emiss.) Very high velocities (~10000 km/s) SN Ia are found in all types of galaxies, including ellipticals Progenitor systems must have long lifetimes Courtesy of the Supernova Cosmology Project

and Theory: Standard model (Hoyle & Fowler 1960): SNe Ia are thermonuclear explosions of C+O white dwarf stars. Evolution to criticality: Accretion from a binary companion leads to growth of the WD to the critical Chandrasekhar mass (~ 1.4 solar masses). After ~1000 years of slow thermonuclear cooking, a violent explosion is triggered at or near the center complete incineration within less than two seconds, no compact remnant!

Deflagrations and Detonations Deflagrations ( Flames ): Subsonic burning fronts, propagating by heat conduction. Laminar flame speed and flame width (Timmes & Woosley 1992): S L ~ 0.001 c, ~ 1 m.1 cm Detonations: Supersonic burning fronts, propagating by shock heating. Detonation width and speed: S D ~ u s ~ 0.1 c, D ~ 100 -In principle, both modes of propagation are allowed in the supernova. Details of the ignition process decide which mode is realized. -Both modes are hydrodynamically unstable in multiple dimensions!

Anatomy of an Explosion I: Prompt Detonation Ignition Prompt Detonation (Arnett 1969; Hansen & Wheeler 1969): Supersonic propagation doesn t allow the star to expand prior to being burned. Almost no production of intermediate mass elements. Ruled out by observations!

Anatomy of an Explosion II: Pure Turbulent Deflagration Ignition Deflagration Phase (many classic references): Burning propagates as a subsonic flame. The Rayleigh-Taylor instability (buoyancy!) produces rising bubbles. Shear flows at the bubble walls produce turbulence (Kelvin- Helmholtz instability). Turbulent combustion : Turbulence increases the flame surface and hence the speed. Under certain conditions, the laminar speed becomes irrelevant (J.N. & Hillebrandt 1995): The turbulent flame speed is ~ equal to the speed of the fastest turbulent eddies! (first observed by Damköhler 1940)

Anatomy of an Explosion III: Delayed Detonation (Khokhlov 1991; Woosley & Weaver 1994) Deflagration Phase Deflagration-Detonation- Transition (DDT) (Zeldovich et al. 1970; Khokhlov 1991; Woosley & Weaver 1994): DDT may be possible as a result of local flame quenching and fast turbulent mixing (Khokhlov 1997, J.N. & Woosley 1997). Advantages for 1D SN Ia models (papers by Nomoto, Höflich, Thielemann, ): Detonation sweeps up unburned C+O, gives additional kick. Transition density is convenient tuning parameter. Problems: Doesn t work if flames can t be quenched (J.N. 1999). May not be needed (new 3D results).

Bottom Lines I: Flames and Detonations Understanding turbulent combustion is crucial for understanding SN Ia explosions: Most important: 1. Effective turbulent burning speed (may be independent of microphysics!) 2. Robustness of flames with regard to turbulent quenching (DDT!) Delayed Detonations: Allow good fits of 1D simulations to observations. Get rid of unburned material. Provide convenient fitting parameter for SN Ia family (transition density). BUT: 1. Physics of DDT indicates very low probability. 2. May not be needed to explain explosion strength.

Zoology: Currently Discussed Explosion Models Type Ia Supernova Explosion Models Merging White Dwarfs Chandrasekhar Mass Models Sub-Chandrasekhar Mass Models Prompt Detonation Initial Deflagration Pure (Fast) Turbulent Deflagration Slow Turbulent Deflagration + DDT Delayed Detonation Pulsational Detonation

Multidimensional Simulations of SN Ia: Where Are We? Warnings: 1. Turbulence is a key element of all Chandrasekhar mass models. 2. No 2D (not to mention 3D) simulation to date reaches the fully turbulent regime! 3. This is what we really simulate:

Large Eddy Simulations of Exploding White Dwarfs 2D simulation of an exploding white dwarf (Reinecke, Hillebrandt & J.N. 1998): Uses a flame capturing/tracking scheme based on a level set method The turbulent flame speed is given by 768x768 simulation: S T = (2 q) 1/2 where q is the turbulent subgrid energy obtained from a subgrid-scale model for the unresolved turbulence (J.N. & Hillebrandt 1995). Large scales solved by higher-order Godunov scheme (PPM) (Colella & Woodward 1984). Movie made by M. Reinecke

Everything is in the details 512x512 1024x1024 256x256 Reinecke 2001: Global energy release is almost independent of resolution by virtue of the subgrid-model.

3D LES of Turbulent Deflagration: A Healthy Explosion? 2D vs. 3D (central ignition): 3D, resolution study: explosion re-collapse explosion re-collapse Reinecke 2001: - Systematically higher energy release in 3D (consistent with Khokhlov 2001). - Weaker dependence on the initial conditions than in 2D.

Problems of the Chandrasekhar Mass Paradigm What are the progenitors? Any single scenario has difficulties explaining SN Ia occurrence in oldest and youngest host populations simultaneously (Howell 2001). Where is the hydrogen? If binary companion is H donor there should be some trace of H in the spectra. None has been found so far (e.g. Cumming et al. 1996). Where is the low-velocity C and O (or Si, Ca)? In multi-d deflagration models some unburned material always remains near the center. Can a delayed detonation get rid of all of it (Khokhlov 2001)? Or maybe fully developed turbulence? Correlation of SN Ia subtype with host population Subluminous events only occur in old pop.s (Howell 2001), overluminous ones in young stellar systems (Hamuy et al. 1996). Why? Nickel masses SN 1991bg-like objects produce only ~0.1 solar masses of Ni. This amount doesn t even unbind a Chandrasekhar mass WD

Zoology: Currently Discussed Explosion Models Type Ia Supernova Explosion Models Merging White Dwarfs Chandrasekhar Mass Models Sub-Chandrasekhar Mass Models Prompt Detonation Initial Deflagration Pure (Fast) Turbulent Deflagration Slow Turbulent Deflagration + DDT Delayed Detonation Pulsational Detonation

Usually Discussed Systematic Effects 1. Supernova evolution 2. Sample evolution 3. Grey dust 4. Lensing effects need to know what they are!

Supernova Evolution low C/O high C/O

Sample Evolution high-mass progenitors low-mass progenitors

Summary: What Can We Expect? If all SNe Ia are Chandrasekhar mass events: Little SN evolution. No sample evolution. If they are either all sub-chandras or all mergers: Possibly substantial SN evolution. No sample evolution. If they are a mix of some or all of the above: Sample and SN evolution.