An international team of astronomers, led by Vilppu Piirola
of Tuorla
Observatory, have made extensive observations of the polarised
light coming from W Serpentis, a stellar pair in the constellation
of Serpens, the Snake.
W Serpentis is an unusual kind of binary star in which matter is
being transferred quite rapidly from the lighter to the heavier
star (i.e. the 'primary' star of the system). The matter doesn't
necessarily stream directly onto the primary, but can collect in a
so-called accretion disk in which the primary star is
embedded. Embedded is the key word here, because it is usually
assumed that the primary star is nevertheless well and truly
visible in such systems -- however, in W Ser-type stars, the
accretion disk may be so thick and influential on the overall light
from the system, that disentangling the contributions from due to
the primary star and the accretion disk is difficult. In the case
of W Ser, the secondary star, from which matter is being
transferred, is so faint that it can only be seen from the eclipse
effects when passing in front of the primary.
A way to break through the ambiguities is to observe the star
not in ordinary light, but in polarised light. By monitoring how
the brightness and polarisation of the star changes during several
full rotations of the system (which takes around 14 days) with the
Nordic Optical Telescope, and the
remotely controlled KVA-60 telescope, Vilppu Piirola, Andrei
Berdyugin and Seppo Mikkola (of Tuorla Observatory) and
George Coyne, S.J. (of the Vatican Observatory) have been able to
put together a detailed picture of the system.
The astronomers found that the primary star in the system very
likely has a bright spot or jet high on its surface, close to one
of its poles. The figures show a sketch of the system, with the
spot clearly visible on the primary star, and appearing with
different aspect angles, and sometimes almost dissapearing, as the
system rotates through a full cycle.
Somewhat contrary to expectation, the team found no evidence for
an accretion disk from the polarisation observations, although the
disk could certainly be present all the same. If the disk were
particularly thick and dense, then its emitted light might have
very little polarisation, making its detection difficult. The team
found other things though: firstly, evidence for a shell of hot
plasma --- a natural source of polarisation -- around the primary;
and secondly, evidence of polarised emission from the stream of
matter connecting the two stars.
What exactly is that hot spot/jet doing up at the pole? The team
speculates that the hot spot is associated with an accretion disk
so thick that it envelopes the primary star entirely --- the
obscured central object can only be seen through the scattered
radiation in the polar directions of the disk, where the
circumstellar envelope is thinner and light can escape more easily
from the hot central star.
The study of W Ser has been published in the
Astrophysical Journal.
Posted November 19th 2005.
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Schematic of the W Ser system, as reconstructed by polarised
light observations made at the Nordic Optical Telescope. The
figures show the binary star system as it rotates through a full
cycle, from top to bottom. The primary is the smaller one of the
system -- nevertheless it is quite massive and hot as stars go; the
lighter (but larger) star is the much less massive secondary. The
primary is shown surrounded by an atmosphere of ionised gas, and
with a bright spot (or jet) at high latitude. The thin line
represents the stream of matter flowing from the lighter to the
heavier star. All of these components contribute to the
polarisation of the light emitted by the system.
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