My favourite star:
Draco 461, an amazing C-star
Evidence of recent star formation
Inma Domínguez
Group of Stellar Evolution and Nucleosynthesis
Universidad de Granada
Carlos Abia, Univ. de Granada
Sergio Cristallo, Oscar Straniero, Osservatorio di Teramo
Ya. V. Pavlenko, Main Astronomical Observatory, Kiev
Synthetic fit to D461 spectrum
4.2 m WHT+ ISIS, Roque de los Muchachos R ~ 6500
IRAF
S/N ~ 60
Model Atmospheres
CaI
log ε(Li)=no Li
1.5
3.0
3.5
LiI
SAM12 (Pavlenko 2003)
Best fit
Teff ~ 3600 K
[Fe/H]=-2.0±0.2
C/O=3-5
log g= 0
ξ=2.5 km/s
Draco Dwarf Spheroidal Galaxy
[Fe/H]~ -2
(Carney & Seitzer; Bell; Aparicio; Bellazzini)
Large Dispersion -3<[Fe/H]<-1.5 (Shetrone et al. 1998)
Stellar population ~ 10 Gyr
No (or slightly) α-enhanced
(Grillmair et al. 1998)
(Shetrone et al. 1998, 2001)
(m-M)o=19.84±0.14
(Bellazzini et al. 2002)
Anomalous Cepheids
(1-2 M)
(Baade & Swope, 1961; Gallart et al. 1999)
Massive and younger stellar population
© SSDS
(Aparicio et al. 2001; Ikuta & Arimoto 2002)
Numerical Method:
Stellar Evolution
FRANEC
1D Hydrostatic Code
¾ Extended Nuclear Network s-process included
couple to Physics: 450 isotopes - 700 reactions
¾ Time dependent mixing
¾ Mass loss: Reimers η=0.4
Thermal Pulses & 3rd Dredge-up
M=1.5 M
†
Z=3 10-4 Y=0.24
•
•
•
•
1st
2nd
3rd
4th
C/O
2.1
10.4
18.1
23.1
ƒ
C/O~5
C/O
12C/13C
C/O …
Low O
2
2
1
Mbol
[Ba/Fe]
1
L3α
2
}
1
Lcno
g-Teff
Teff decreases
D461
Opacity y
C y 3rd D-up
Marigo 2002
Opacity Tables for
different Z
(Solar scaled)
Low Temperature Opacity Tables
Needed !!
D461 is compatible with a
Low Mass AGB + 3rd D-up
C/O
12C/13C
[Ba/Fe]
1.5 M Z=3 10-4
Teff g
Z … α-enhanced „ O … „ better C/O
BUT D461 is Li rich !!!
Model
log ε(Li)
RGB
-0.73
log ε(Li) ~ 3.5
1st
2nd 3rd
4th
-0.54 0.01 0.34 0.55
TBCE < 20 106 K
NO 7Be
Li production in Ì
Cameron-Fowler belt Mechanism
¾ 3He(α,γ)7Be
T> 20-30 106 K
¾ 7Be(e-,ν)7Li
τ1/2 ~ 29 yr (T~ 25 106 K)
7Li(p,α)4He
„ HBB in Intermediate-Massive Ì
T> 2 106 K
τmixing < τ1/2 (7Be + e-)
Low mass Ì „ Extra-mixing or CBP
Wasserburg, Boothroyd, Sackmann 1995
Nollet, Busso, Wasserburg 2003
Extra-Mixing Model
During Interpulse Period
Nollet, Busso, Wasserburg 2003
Convective envelope
_____________________________
∆Mmix~ 5 10v-3
M
CBP 
TCBP
vCBP= vconv
vconv/10
20 - 25 - 30 - 40 106 K
H burning shell
M& ~ 0.05 M/yr
He shell
CO core
vconv /100
Radiative zone
Nuclear burning
log ε(Li)=3.5±0.4
log ε(Li)
40
30
25
TCBP (106 K)
20
1
2
std
log ε(X) = log (X/H) + 12
std
20
25
30
40
log ε(Li)
2nd
0.01
2.24
3.39
3.76
3.76
Extra-mixing in the AGB
¾ Extra-mixing explains observed Li in D461
¾ Does not alter the AGB structure and evolution
¾ 2 parameters: TCBP
vCBP
w Just TCBP critical
¾ Extra-mixing needed to explain
isotope composition of dust grains
Physical model ??
Constraints to D461 Mass & AGE
Luminosity – Core Mass
D461: Mv = -2.74±0.14
(Shetrone et al. 2001)
Occurrence of 3rd D-up
Menv > 0.4-0.5 M
†
M < 2.0 M
> 1 Gyr
(Straniero et al. 2003)
M > 1.3 M
< 3 Gyr
Could be extrinsic and... “old” ??
Close Binary System:
D461 M < 1 M + Companion
¾ Photometric variability (4σ) „ AGB
(Shetrone et al. 2001)
¾ Radial velocity monitoring „ NO companion
(Olszewski et al. 1996)
¾ High Li abundance „ local production
(Abia et al. 1993)
Conclusions
Our favourite hypothesis
D461 is an intrinsic AGB (carbon) star
at the beginning of the TP phase
M > 1.3 M
AGE < 3 Gyr
Evidence of recent star formation in
Draco