New atmosphere models for massive stars: line-blanketing effects and wind properties of O stars

Fabrice Martins

Observatoire Midi-Pyr\'en\'ees, Toulouse, France

Massive stars are at a cornerstone of modern astrophysics. Their
nucleosynthesis produces the elements heavier than Oxygen which are
spread in the interstellar medium when they end their life as
supernovae. They also emit high energy photons which ionise the
surrounding medium, creating HII regions. They continuously emit
powerful winds which interact with the interstellar medium, giving
birth to bubbles, cavities and triggering the collapse of molecular
clouds. The very first massive stars (the so-called population III
stars) are also thought to be the precursors of gamma-ray bursts and
may be responsible for the reionisation of the Universe at high
redshift. Understanding quantitatively these objects is then crucial
for a number of astrophysical studies. This requires the development
of complex evolutionary models to explain the structure of their
interior and their nucleosynthesis. And atmosphere models are
necessary to make the link between the interior of the star and the
observable quantities, and to constrain the stellar and wind
properties of massive stars.

This thesis focuses on the second kind of models. The main reason is
that significant progress has been made in the modelling of massive
stars atmospheres in the last few years. In particular, it is now
possible to include reliably metals in such models. This allows
the production of realistic models and synthetic spectra which can be
used to improve our knowledge of the stellar and wind properties. In
this thesis, we have built such new atmosphere models computed mainly
with the code CMFGEN (Hillier \& Miller 1998).

The first part of this work has been devoted to the analysis of the
effects of the inclusion of metals in atmosphere models (the
line-blanketing effects). We have confirmed the expected fact that
both the atmospheric structure and the emergent spectrum are modified
by the presence of metals. The
temperature is increased in the interior of the atmosphere
(backwarming effect) and reduced in the outer layers (line-cooling
effect). The ionisation is also higher in the interior and
lower in the upper atmosphere. This change of ionisation modifies the
strength of He lines used for the spectral classification compared to
models without metals, lowering the effective temperature scale of O
dwarfs by 1500 (4000) K for late (early) type O dwarfs
with solar abundances.
For a lower metallicity typical of the Small
Magellanic Cloud, the reduction of the T$_{\rm eff}$ - scale is
roughly half that of the solar case. We also investigate the effect
of line-blanketing on the spectral energy distribution of O stars.
In particular, a study of compact Galactic HII regions observed in
the mid-IR by ISO reveals that the new generation of atmosphere models
allows a better, although not perfect, reproduction of the excitation
sequences defined by ratios of nebular lines of the same element
emitted in the HII region.

The second part of this thesis is devoted to the study of wind
properties of dwarf O stars thanks to new atmosphere models. We first
focus on the stellar components of the High Excitation Blob N81 in the
Small Magellanic Cloud. The quantitative spectroscopic analysis of UV
STIS spectra reveals that these stars are young O dwarfs with lower
luminosities than typical O stars of the same spectral type and
showing very weak winds. These characteristics may indicate that they
belong to the class of Vz stars, a class of O stars thought to lie
close to the ZAMS. With mass loss rates of the order of $10^{-8..-9}$
M$_{\odot}$ yr$^{-1}$, the winds are weaker than ever observed for
such stars, and are weaker by 1 to 2 orders of magnitude compared to
the predictions of hydrodynamical simulations. The modified wind
momenta show the same trend, indicating possibly a break-down of the
modified wind momentum - luminosity relation (WLR) or a steeper slope
at lower metallicity. Different hypothesis are investigated to explain
this strange behaviour (low metallicity, decoupling, line strength
parameterisation in hydrodynamical simulations) without success. A
possible link between the youth of the star and the weakness of
developing winds may possibly explain such a behaviour.
To better understand the conditions under which weak winds appear, we
then analyse a sample of Galactic stars known to display qualitatively
weak winds. Mass loss
rates as low as $10^{-9.5}$ M$_{\odot}$ yr$^{-1}$ are found in the
faintest stars, showing that metallicity is not the main reason for
the reduction of the wind strength. The
break-down of the WLR around $\log
\frac{L}{L_{\odot}} = 5.2$ seems to be confirmed. Bright stars also
show winds slightly reduced compared to previous analysis due to the
inclusion of clumping in the models. However, the reason for the
reduced wind strength in low luminosity stars remains unknown.

Reference: Martins, F., 2004, PhD Thesis, Universit\'e Paul Sabatier, Toulouse, France
Status: Other


Comments: PhD thesis. Supervisor: Daniel Schaerer. Co-supervisor: Mohammad Heydari-Malayeri