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