807 research outputs found
Evolutionary history of four binary blue stragglers from the globular clusters \omega Cen, M55, 47 Tuc and NGC 6752
Context. Origin and evolution of blue stragglers in globular clusters is
still a matter of debate. Aims. The aim of the present investigation is to
reproduce the evolutionary history of four binary blue stragglers in four
different clusters, for which precise values of global parameters are known.
Methods. Using the model for cool close binary evolution, developed by one of
us (KS), progenitors of all investigated binaries were found and their
parameters evolved into the presently observed values. Results. The results
show that the progenitors of the binary blue stragglers are cool close binaries
with period of a few days, which transform into stragglers by rejuvenation of
the initially less massive component by mass transfer from its more massive
companion overflowing the inner critical Roche surface. The parameters of V209
from \omega Cen indicate that the binary is substantially enriched in helium.
This is an independent and strong evidence for the existence of the helium rich
subpopulation in this cluster.Comment: Accepted by Astronomy and Astrophysic
The radio lighthouse CU Virginis: the spindown of a single main sequence star
The fast rotating star CU Virginis is a magnetic chemically peculiar star
with an oblique dipolar magnetic field. The continuum radio emission has been
interpreted as gyrosyncrotron emission arising from a thin magnetospheric
layer. Previous radio observations at 1.4 GHz showed that a 100% circular
polarized and highly directive emission component overlaps to the continuum
emission two times per rotation, when the magnetic axis lies in the plane of
the sky. This sort of radio lighthouse has been proposed to be due to cyclotron
maser emission generated above the magnetic pole and propagating
perpendicularly to the magnetic axis. Observations carried out with the
Australia Telescope Compact Array at 1.4 and 2.5 GHz one year after this
discovery show that this radio emission is still present, meaning that the
phenomenon responsible for this process is steady on a timescale of years. The
emitted radiation spans at least 1 GHz, being observed from 1.4 to 2.5 GHz. On
the light of recent results on the physics of the magnetosphere of this star,
the possibility of plasma radiation is ruled out. The characteristics of this
radio lighthouse provides us a good marker of the rotation period, since the
peaks are visible at particular rotational phases. After one year, they show a
delay of about 15 minutes. This is interpreted as a new abrupt spinning down of
the star. Among several possibilities, a quick emptying of the equatorial
magnetic belt after reaching the maximum density can account for the magnitude
of the breaking. The study of the coherent emission in stars like CU Vir, as
well as in pre main sequence stars, can give important insight into the angular
momentum evolution in young stars. This is a promising field of investigation
that high sensitivity radio interferometers such as SKA can exploit.Comment: Accepted to MNRAS, 8 pages, 7 figures, updated versio
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