New Model Atmospheres for Hydrogen-Deficient Stars

May 3

rd

, 2006 Natalie Behara Armagh Observatory

Hydrogen -Deficient stars

Wolf-Rayet central stars of planetary nebulae

Evolved, massive and extremely hot ( up to ~ 50 000 K) Surface composition is dominated by helium, and typically showing broad wind emission lines of elements like carbon, nitrogen, or oxygen.

R Coronae Borealis stars

F or G variable supergiants, composition dominated by helium and carbon Changes in brightness are irregular and unpredictable.

Extreme helium stars

Early-type supergiants practically void of hydrogen in their atmosphere.

21 extreme helium stars detected in the galaxy Composition dominated by helium, with significant amounts of carbon, nitrogen and oxygen, traces of other metals

Evolutionary Models: Extreme Helium Stars

Final helium-shell flash in a post-AGB star

During contraction from the AGB to the WD track, stars may experience a helium shell flash. This causes large-scale mixing and a brief expansion of the envelope to giant dimensions.

Evidence: FG Sge, V4334 Sge and V652 Aql observed to evolve from faint blue star to red supergiant on timescales of 3 ~ 50 years.

Merger of CO and He white dwarf

Accretion from the disk onto the surviving WD creates a star with a degenerate CO core and a helium envelope. Chemical abundances will help determine a star’s evolutionary history

Stellar Atmospheres

Apart from astroseismology, stellar interiors are effectively invisible to the external observer, so all the information we receive from stars originate from their atmosphere.

Energy transport mechanism of the atmosphere is radiation, and understanding how radiation interacts with matter affecting the emergent line and continuous spectrum is key to modelling atmospheres.

Low H abundance

A direct consequence of low hydrogen abundance is that the continuum opacity, normally dominated by hydrogen, is reduced. Although the abundances of species other than helium and carbon are not significantly different from solar, the metal line spectrum is correspondingly magnified several fold.

Sterne - Model Atmosphere Code

LTE code originally developed to study hydrogen-deficient stars (Wolf & Schonberner, 1974).

Optimizied for stars with T

eff

between 10 000 K and 35 000 K, and extreme compositions.

Model Assumptions

Plane-parallel geometry Steady-state & LTE Hydrostatic equilibrium Radiative equilibrium

Recent revisions

• Continuous opacities updated • Method for treating the line opacities updated

Stellar Opacity

Scattering processes:

Continuous Opacity

Free-Free

Photon scattered by an electron, atom or molecule.

Bound-Free

Line Opacity

Bound-Bound

Opacity Sources: Bound-Free

STERNE 2

Photoionisation cross-sections calculated by Kurucz (1970) & Peach(1970)

STERNE 3

Opacity Project cross-sections (1995,1997)

HI HeI, HeII CI, CII, CIII NI, NII, NIII OI MgI, MgII AlI SiI, SiII CaII HI HeI, HeII LiI, LiII, LiIII BeI, BeII, BeIII, Be IV BI, BII, BIII, BIV CI, CII, CIII, CIV, CV, CVI NI, NII, NIII, NIV OI, OII, OIII, OIV FI, FII, FIII, FIV NeI, NeII, NeIII, NeIV NaI, NaII, NaIII, NaIV MgI, MgII, MgIII, MgIV AlI, AlII, AlIII, AlIV SiI, SiII, SiIII, SiIV SI, SII, SIII, SIV ArI, ArII, ArIII, ArIV CaI, CaII, CaIII, CaIV

Iron Project cross-sections (1997) H-, He- and C- are common to both

FeI, FeII, FeIII

Revising the Bound-Free Opacity

Opacity Project C I cross-section compared to the Peach(1970) approximation (red curve).

Iron Project Fe I cross section compared to the hydrogenic approximation.

Comparison: Opacity

 

bf bf STERNE

3

STERNE

2

Results: Continuous Opacity

An increase in the CII opacity when using the OP cross-sections leads to an increase in the continuum opacity at l > 1000 angstroms.

Ratio of CII opacity computed with the OP cross-sections to the opacity computed using the Peach data.

Effect of the CII opacity on the emergent flux. The dotted line represents the model with the new opacities. T eff = 20 000 K

Revising the Bound-Bound Opacity

Opacity Distribution Functions

 Describes the line opacity  fixed chemical composition:

bb

for a T and gas pressure assuming a l

bb

  l

bb



T

,

P

,

compositio n



Advantages

- once ODFs are calculated, models can be quickly computed

Disadvantages

- ODFs only available for a few selected mixtures - all layers of the atmosphere must have the same composition

Opacity Sampling

Direct calculation of the line opacity at each wavelength point for all layers in a model atmosphere.

Advantages

- allows individualized abundances - allows for a stratified atmospheres

Disadvantages

- line selection and line profile calculations much more costly than ODF table interpolations

Results: Hydrogen-rich atmosphere

Results: Helium-rich atmosphere

Results

Hydrogen-rich atmospheres Helium-rich atmospheres

Future Work

Applications

Extreme Helium stars & He sdB stars

Measure effective temperatures, gravities and compositions by fitting LTE model atmospheres.

Further code development

Modelling stratified atmospheres of chemically-peculiar stars

Observational evidence seems to show that element stratification is present in the atmospheres of several types of stars. The accumulation or depreciation of the elements as a function of depth will modify the atmospheric structure of such stars.

A version of STERNE which self-consistently solves for the stratification profiles of the elements and the atmospheric structure is currently being developed.