New Model Atmospheres for Hydrogen-Deficient Stars

May 3rd, 2006

Natalie Behara

Armagh Observatory

Hydrogen -Deficient stars

Extreme helium stars

R Coronae Borealis stars

Wolf-Rayet central stars of planetary nebulae

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

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.

F or G variable supergiants, composition dominated by helium and carbon

Changes in brightness are irregular and unpredictable.

Evolutionary Models: Extreme Helium Stars

Final helium-shell flash in a post-AGB star

Merger of CO and He white dwarf

Chemical abundances will help determine a star’s evolutionary history

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.

Accretion from the disk onto the surviving WD creates a star with a degenerate CO core and a helium envelope.

Stellar Atmospheres

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.

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

Sterne - Model Atmosphere Code

Model Assumptions

Plane-parallel geometry

Steady-state & LTE

Hydrostatic equilibrium Radiative equilibrium

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

Optimizied for stars with Teff between 10 000 K and 35 000 K, and extreme compositions.

Recent revisions

• Continuous opacities updated

• Method for treating the line opacities updated

Stellar Opacity

Free-FreeBound-Free

Bound-Bound

Scattering processes: Photon scattered by an electron, atom or molecule.

Continuous Opacity

Line Opacity

Opacity Sources: Bound-FreeSTERNE 2 STERNE 3

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

HI

HeI, HeII

CI, CII, CIII

NI, NII, NIII

OI

MgI, MgII

AlI

SiI, SiII

CaII

Opacity Project cross-sections (1995,1997)

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

H-, He- and C- are common to both

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)

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

2

3

STERNEbf

STERNEbf

Results: Continuous Opacity

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

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

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

Revising the Bound-Bound Opacity

Opacity Distribution Functions

Opacity Sampling

Describes the line opacity bb

for a T and gas pressure assuming a fixed chemical composition:

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

ncompositioPTbbbb ,, Advantages

- allows individualized abundances

- allows for a stratified atmospheres Disadvantages

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

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

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.