On the evolutionary status of Be stars. I. Field Be stars near the Sun

J. Zorec$^1$, Y. Fr\'emat$^2$, L. Cidale$^3$

$^1$ Institut d'Astrophysique de Paris, UMR 7095 CNRS-Universit\'e
Pierre \& Marie-Curie, 98bis Bd. Arago, 75014 Paris, France
$^2$ Royal Observatory of Belgium, 3 Av. Circulaire, B-1180 Bruxelles
$^3$ Facultad de Ciencias Astron\'omicas y Geof\'\i sicas, Universidad
de La Plata, Paseo del Bosque S/N, 1900 La Plata, Argentina

A sample of 97 galactic field Be stars were studied by taking into
account the effects induced by the fast rotation on their fundamental
parameters. All program stars were observed in the BCD spectrophotometric
system in order to minimize the perturbations produced by the circumstellar
environment on the spectral photospheric signatures. This is one of the first
attempts at determining stellar masses and ages by simultaneously using model
atmospheres and evolutionary tracks, both calculated for rotating objects. The
stellar ages ($\tau$) normalized to the respective inferred time that each
rotating star can spend in the main sequence phase ($\tau_{\rm MS}$) reveal a
mass-dependent trend. This trend shows that: a) there are Be stars spread
over the whole interval $0 \la \tau/\tau_{\rm MS} \la 1$ of the main sequence
evolutionary phase; b) the distribution of points in the ($\tau/\tau_{\rm
MS},M/M_{\odot}$) diagram indicates that in massive stars ($M \ga 12M_{\odot}$)
the Be phenomenon is present at smaller $\tau/\tau_{\rm MS}$ age ratios than
for less massive stars ($M \la 12M_{\odot}$). This distribution can be due
to: $i$) higher mass-loss rates in massive objets, which can act to reduce the
surface fast rotation; $ii$) circulation time scales to transport angular
momentum from the core to the surface, which are longer the lower the stellar

Reference: Astronomy and Astrophysics
Status: Manuscript has been accepted



Email: zorec@iap.fr