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The problematic modelling of RCrB atmospheres
Bengt Gustafsson Department of Astronomy and Space Physics
Uppsala University
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Hydrogen-Deficient StarsTübingen, September 2007
Standard MARCS models
• 1D (plane-parallel or spherically symmetric)• Detailed blanketing• LTE • Mixing-Length Convection
See
Asplund, Gustafsson, Kiselman & Eriksson (1997): A&A 318, 521
Asplund, Gustafsson, Lambert & Rao (2000): A&A 353, 287
as well as
Eriksson, Edvardsson, Gustafsson & Plez (2007), in preparation
• No HI and H - opacity =>
Heavy blanketing =>
Steepened
grad T
Increasing Teff => increasing =>
decreasing and Pg, unaltered Pe.
Increasing Teff => increasing =>
decreasing and Pg, unaltered Pe.
Model-structure variations with fundamental parameters
/,
Density inversion -- Super Eddington?
/,
Density inversion -- Super Eddington?
< 0 in ioniz. zone
>1< 0
/,
Density inversion -- Super Eddington?
< 0 in ioniz. zone
>1< 0
=> < 0
Density inversion occurs (first) due to ionization -- not radiative force
• Yet, >1 does not automatically lead to mass flows -- a positive pressure grandient may balance
• Additional effects due to Pdyn
• Instabilities deserve further studies!
Border case at = 1?
Super-Eddington luminosities cause RCB declines?
From Asplund & Gustafsson (1996), ASP Conf. 96, 39
RCB:s evolve from right to left:Expansion => Cooling => StabilityLBV:s from left to right:Expansion => Cooling => Instability
Yet very uncertain whether effect works, See Asplund (1998), A&A 330, 641Effects of spheriicity and convection!
Low H increases line blanketing from CI and other atoms => flux pushed redwards. So does also CI continuum
Dominating opacity sources
Total
He I
Mg
e-
C I
He-N I
See also Pavlenkos talk!
A reasonable fit to observed
fluxes
CI lines at 5000-7000Å
~ 8.5 eV
gf values from TOP data base
W ~ l/
mainly from
CI bf, ~ 9.2 eV
Data also from TOP
Incidently, also other opacities (He-, C- , e- ) reflect C abundance since most electrons come from C
C = [C]pred - [C]obs
C = [C]pred - [C]obs
A real problem!
””The Carbon Problem”The Carbon Problem”
What could be the reason?
• Errors in FP:s? No!• Errors in codes etc? No!• W? No!• Extra-photospheric flux? (> 3x photosp., No!) • Basic atomic data for CI in error? (~10-30%, much too little!)• CI opacity not dominant? (C/He = 1%, must be lowered by more than x
20, inconsistent with hot RCrB stars and EHe stars)• NLTE? (~ 2%, Asplund & Ryde 1996, No (?))• Model atmospheres? - incomplete opacities? - sphericity? - dep. from hydrostatic equilibrium? Hardly! - temperature inhomogeneities? Not per se - errors in structures due, e.g. due to dynamical fluxes
, • New better opacities
More heavy blanketing
Steeper grad T
Sphericity
=> Steeper grad T
Effects on
abundances :
~ ± 0.1 dex
New MARCS
New Marcs models (Eriksson et al. 2007, in prep)
Goes the wrong way for C I!
Decrease grad T in CI-line forming layers!
This works reasonably well but requires
Fheat ~ 4P (4T3T)s
~ 10% Ftot
Compare to
Fmech ~ vturb3
vturb ~ 40 km/s
C problem also for [CI]Pandey et al. (2004), MNRAS 353, 143
… however not for CII (?)
To do:
• C2?
• CI and C2 in IR (He- takes over in )
• Explore accurately normal supergiants
• 3D HD simulations3D HD simulations
No real progress in 8 years. Errors in abundances at least x2 - x4 in absolute numbers. Time to resolve this now?
No real progress in 8 years. Errors in abundances at least x2 - x4 in absolute numbers. Time to resolve this now?
”Truth is the daughter of time, and I feel no shame of being her midwife”
Johannes Kepler