Download Classical Novae on a Helium White Dwarf

Classical Novae on a Helium
White Dwarf
Irit Idan (Technion)
Lars Bildsten ((KITP, UCSB)
Ken Shen (UCSB)
Introduction
• The evolution of a low mass star on the RG branch can be
halted due to the filling of the RL - low mass (M<0.48 M)
He core
• Howell et al 2001 - 20% of CVs with Porb<2 hr - He WDs
Tight orbits -> contact leading
to accretion of cosmic-mix
material onto a pure He WD at
a very low accretion rate 10-11
M yr-1
•
Shara, Prialnik, Kovetz (1993) –
accretion onto M=0.4M,Tc=107K He WD at
accretion rate of 10-9 Myr-1 for 10 cycles of nova
outburst.
•
•
•
•
•
•
extremely slow nova.
mild outbursts.
time between outburst 106yr, Macc<10-3M
decreasing core temperature.
high luminosity - over 1000yr for L>L .
mass of the WD increase slowly.
Goals
• Study Mign and the Mej, evolution and the time scales
on He WDs - accretion rate scenario
• Abundances – No source of C/O from the WD. The
Tmax in a hydrostatic flash on a low mass He WD is 
2-3 x 108 K (Sugimoto & Fujimoto 1978).
• Can the high temperatures (>2-3x108K) at the base of
the burning H layer can ignite the underlying Helium
WD and make it a low-mass Helium-burning star ?
Method
Study the accretion onto a small, cold (Tc=6E6K)
He WD (98% He and 2% N) both analytically
and numerically.
Using the Prialnik and Kovetz code hydrodynamic, Lagrangian stellar evolution
code.
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•
•
•
OPAL opacities
extended nuclear reactions network
mass loss algorithm
diffusion
Timescale for thermal diffusion into the core
- Analytic estimate
For low mass and cold WD –
the time between outbursts 108 yr
significant thermal coupling between the accreting envelope
and the core. Timescale for heat transport between r0 and r in
non-convective regions (Henyey, L.,&L'Ecuyer, J. 1969)
3  r  cP 1/ 2


( 3 )  dr 


16ac  r0 T

2
Numerical Estimate - Thermal
Diffusion Time
The timescale
for the coupling
 time between
outburst for low
accretion rates.
Ethermal(Env) << Ethemal(Core)
Multicycle Evolution Code constant accretion rate
Chemical diffusion during accumulation
3
3


2.6

10
gcm
For M env  10 M , T  10 K and
The diffusion timescale of H into He core is  9  107 yr
3
7
Maximum Temperatures
For fixed core mass,
envelope mass, and
composition, there is a
unique maximum base
temperature for the fully
convective envelope.
Maximum Temperatures - Multicycle
15
x 10
7
14
x 10
7
12
10
Tmax [K]
Tmax [K]
10
8
6
5
4
2
0
0
0
1
2
3
Time [yr]
4
Md=0.2M
5
6
x 10
9
0
1
2
3
4
5
Time [yr]
Md=0.05M
6
x 10
9
The average outburst parameters
Mv = -4.5
LBol (max)  1.25 10 L
5
SS phase1200 year
Abundances
Element
Solar
Ejecta He WD
Ejecta CO WD
H
He
Z
C12
C13
N14
N15
O16
O17
0.7
0.28
0.02
3.90(-3)
4.30(-5)
1.00(-3)
3.60(-6)
9.40(-3)
3.50(-6)
0.65
0.33
0.02
2.80(-4)
9.09(-5)
9.00(-3)
3.00(-7)
1.05(-2)
1.36(-3)
0.622
0.258
0.12
depletion
enhancement
enhancement
Conclusions
• Study Mign and the time scales on He WDs good agreement between analytical results and
multicycle calculations.
• Extremely slow nova
• Large ejected mass and low metalicity.
• Time between outbursts - 108yr
• Core temperature depend on the accretion rate.
• High luminosity - over 1000yr for L>L -SS
• The Tmax - 108K.
But- Ethermal(Envelope) << Ethemal(Core)
3 mkT
0.6
m
45
Ethermal ( ENV ) 
 4.1 10 erg T7
2  mp
 103 M
Ethermal (Core)  3
mkT
4 M core
 4.9  10 47 erg T7
i m p
i 0.4 M
Ideal gas
liquid ions
The ratio
Tenv M env 0.4 M 0.6 i,core
8.3 10
3
Tcore 10 M M core env 4
3
Chemical diffusion during accumulation
Time between outbursts for accretion rate of 10-11
Myr-1 is 108 yr
Diffusion is important.
2
H
~
,
D
D
H 
3(2 kT )
1
2
1
i
 1.03
4
i
 ( kT ) 
 ln 1 

4 n Z

3
1
2
Z e
i
i
6
cm
s
exp( ) 
1/ 2
1

2
1
3

g
5/2
16 n ( m ) Z Z e 
1/ 2
P
2
T
5/2
7
1/ 2
A
1
Z Z 
2
3
1
2
i
i
T  exp( ) 

  ln 1  14.3  Z




2.4 106 yr 37/3
3
1/ 2
7
2
3
i
2
Z
5 2 i i
( ) 5/2
g7 T7