## $\alpha$ Eri: rotational distortion, stellar and circumstellar activity

M.M.F. Vinicius$^1$
J. Zorec$^2$
N.V. Leister$^1$
R.S. Levenhagen$^1$

1 - Instituto de Astronomia, Geof\'{\i}sica e Ci\^encias Atmosf\'ericas
da Universidade de S\~ao Paulo, CUASO, 05508-900 S\~ao Paulo SP,
Brazil
2 - Institut d'Astrophysique de Paris, UMR 7095 CNRS-Universit\'e
Pierre \& Marie Curie, 98bis Boulevard Arago, 75014 Paris, France

We explore the geometrical distortion and the stellar and
circumstellar activity of $\alpha$~Eri (HD 10144), the brightest Be star in
the sky. We present a thorough discussion of the fundamental parameters of the
object for an independent determination of its rotational distortion. We used
stellar atmosphere models and evolutionary tracks calculated for fast rotating
early-type stars. If the star is a rigid rotator, its angular velocity rate is
$\Omega/\Omega_c \simeq$ 0.8, so that its rotational distortion is smaller
than the one inferred from recent interferometric measurements. We then
discuss the stellar surface activity using high resolution and high S/N
spectroscopic observations of He\,{\sc i} and Mg\,{\sc ii} lines, which
concern a period of H$\alpha$ line emission decline. The variations in the
He\,{\sc i} lines are interpreted as due to non-radial pulsations. Time series
analysis of variations was performed with the {\sc cleanest} algorithm, which
enabled us to detect the following frequencies: 0.49, 0.76, 1.27 and 1.72 c/d
and pulsation degrees $\ell \sim (3-4)$ for $\nu$ = 0.76 c/d; $\ell \sim (2-3)$ for $\nu$ = 1.27 c/d and $\ell \sim (3-4)$ for $\nu$ = 1.72 c/d. The
study of the absolute deviation of the He\,{\sc i} $\lambda$6678 \AA\
spectral line revealed mass ejection between 1997 and 1998. We conclude that
the lowest frequency found, $\nu =$ 0.49 c/d, is due to the circumstellar
environment, which is present even at epochs of low emission in the wings of
He\,{\sc i} $\lambda$6678 \AA\ and Mg\,{\sc ii} $\lambda$4481 \AA\ line
profiles, as well as during nearly normal aspects of the H$\alpha$ line.
This suggests that there may be matter around the star affecting some spectral
regions, even though the object displays a B-normal like phase. The long-term
changes of the H$\alpha$ line emission in $\alpha$~Eri are studied. We pay
much attention to the H$\alpha$ line emission at the epoch of interferometric
observations. The H$\alpha$ line emission is modeled and interpreted in terms
of varying structures of the circumstellar disc. We conclude that during the
epoch of interferometric measurements there was enough circumstellar matter
near the star to produce $\lambda 2.2\mu$m flux excess, which could account
for the overestimated stellar equatorial angular diameter. From the study of
the latest \bbe\ phase transition of $\alpha$~Eri we concluded that the H$\alpha$ line emission formation regions underwent changes so that: a) the
low H$\alpha$ emission phases are characterized by extended emission zones in
the circumstellar disc and a steep outward matter density decline; b) during
the strong H$\alpha$ emission phases the emitting regions are less extended
and have a constant density distribution. The long-term variations of the
H$\alpha$ line in $\alpha$~Eri seem to have a 14-15 year cyclic \bbe phase
transition. The disc formation time scales, interpreted as the periods during
which the H$\alpha$ line emission increases from zero to its maximum, agree
with the viscous decretion model. On the other hand, the time required for the
disc dissipation ranges from 6 to 12 years which questions the viscous disc
model.

Reference: Astronomy and Astrophysics
Status: Manuscript has been accepted