Sub-Chandra SN Ia models About the ignition of helium envelop detonation Eli Livne+Ami Glasner Racah Institute of Physics The Hebrew university
The standard scenario I ØFor helium accretion rates around 10-8 M o /yr on top of a CO WD a thick helium layer would accumulate and ignite (Taam 1980 ;Nomoto 1980, 1982 ; Woosley et al. 1980). helium C-O (O-Ne) Ø The possible outcomes: - Helium runaway ignited as a single detonation or a subsonic burning flame, producing a faint SN and leaving behind an intact WD. - The ignited helium detonation leads to a secondary explosion of the CO core as well as disruption of the whole star (Taam 1980; Nomoto 1980, 1982; Woosley et al. 1986; Livne 1990; Livne & Glasner 1990, 1991; Woosley & Weaver 1994). For this scenario there are two possible channels: a) direct ignition of an inward detonation at the interface between the detonated helium and the CO core. b) converging compression wave that steepens as it converges to the inner parts of the WD core and develops to a detonation.
ØTheoretical spectra and light curves are not consistent with those of normal Type Ia supernovae (Hoeflich& Khokhlov 1996a; Nugent et al. 1997; Garcia-Senz et al. 1999) due to over production of iron group elements.
The standard scenario II ØNew observations and theoretical work has shown that much less massive accreted helium envelopes (< 0.1 M ) detonate leading to light curves, spectra and abundances that compare relatively well with observational data (Bildsten et al. 2007; Shen et al. 2010; Kromer et al. 2010; Woosley & Kasen 2011; Poznanski et al. 2010; Kasliwal et al. 2010; Perets et al. 2011; Waldman et al. 2011; Drout et al. 2013, Inserra et al. 2015). ØWith a lower He shell mass, there is no longer a significant over-production of Fe-peak elements at high velocities, which brings model spectra into better agreement with SN Ia observations.
Motivation for the present study Ignition of helium detonation is a key issue since it is the the first step for the whole scenario. Evolutionary 1D models show that the helium envelope is unstable to convection on timescales of days, prior to the runaway. Examination of the relevant timescales shows that the ignition is local (not in a spherical shell) => multidimensional process. Most of the multidimensional numerical models published to date ignited the detonation artificially and ignored the convective flow.
A comment before we start
helium ignition in the published multi-d models
Convection and ignition in 1D models that apply the MLT for the convective flux 1) models show that the helium envelope is unstable to convection on timescales of days, prior to the runaway. 2) mixing-length theory certainly breaks down when the burning time scale becomes shorter than the convective turnover time. (Woosley&Kasen 2011) 3) In most models the MLT becomes unjustified when the base temperature is between 200-300M Kelvin.
Conditions for the ignition of the detonaion, By the Zeldovich mechanism - Shallow temperature gradients are necessary to initiate a detonation. - Convection efficiently keeps the temperature gradient adiabatic. - At each point on that adiabatic temperature profile there is a local burning time taub. - - If there exists regions that are separated by a distance such that the ratio of that distance to the difference in the burning time taub implies a phase velocity that is supersonic, a detonation can form. V phase = 1/( dtaub dr ) taub=time to burn from the existing temperature to about 2*10 9 kelvin
Low temperature:
but.
Higher temperature => Convection?
Temperature V_sound V_phase
sensitivity
Mixing Length Theory (MLT) for convection still legitimate Lower limit for the Zeldovich mechanism
but in 1D
Our study 2D models Initial temperature at the base of the accreted envelope less than 2*10 8 Kelvin burning simple alpha net (13 elements) various numerical recepies for burning in the advective flow
The work space M= dm/dt (helium)= Accreted helium mass= 1 M few 10-8 M /Yr 0.05-0.1 M Initial luminosity of the core= 0.1 L 1D accretion up to maximal temperature of 1.65-2.0 10 8 at the accreted helium envelope. Mapping into the 2D solver to follow the convective flow. C-O (O-Ne) helium The simulated envelope
0.1 M-sol
0.05 M-sol
Before we conclude The reaction: 12 C(p,γ )13 N(α,p )16 O plays a critical role in accelerating the burning during the runaway.
Main results
Insights and issues for further discussion
END