Binary S stars share many properties with symbiotic stars, since
both families consist of a cool red giant and a WD companion in
systems
with orbital periods of a few hundred to a few thousand days
(Jorissen
1997). Some kind of symbiotic activity should thus be expected
among
binary S stars as well.
Table 4 lists those systems where the usual
signatures of
binary interaction have been probed.
In that table, refers to the luminosity in the
ROSAT hard band (0.5 - 2.4 keV) taken from Jorissen et al. (1996),
and adapted to the new HIPPARCOS distances (Van Eck et al.
1998).
Hard X-rays have been observed in HR 363 and HD 35155, and appear
strongly variable.
In the UV domain, several stars exhibit
strong emission lines of highly ionized species typical of
interacting binary systems
(like CIV
nm). In the column labelled "UV em. lines'',
"IB'' stands for
"interacting binary'', "no'' stands for no emission lines seen, and
"WD''
indicates that the UV spectrum fits that of a clean WD.
The continuum UV luminosity in the 125.0 - 195.0 nm band is
often larger than would
be expected from an isolated WD [see column "
'']. In
Table 4,
, where the average flux
density
in the 125.0 - 195.0 nm domain is
taken from Johnson et al. (1993) and the distance d from
Van Eck
et al. (1998) or
Eggen (1972). The UV luminosity is often strongly variable, in which
case the different observed values are listed.
The HeI
1083.0 nm triplet generally confirms the UV
diagnostics.
The three binary S
stars flagged as "interacting binaries" from their UV features are
also
those showing strong and variable HeI
nm lines,
whereas
the two stars (HR 363 and HD 191226) with no UV emission lines
exhibit
only a weak HeI triplet in emission.
Two important conclusions may be drawn from the observations
summarized in Table 4:
(i) the level of binary interaction does not appear to be
correlated with the orbital period. HD 49368 (=V613 Mon) for
instance exhibits
a much higher level of activity than the shorter-period system HD
191226, whereas the shortest-period system HD 121447 does not show
any
sign of interaction at all. Moreover, the maximum X-ray luminosities of HR
363
and HD 35155 are comparable despite very different orbital
periods;
(ii) the activity appears to be strongly variable.
Orbital modulation is likely a major cause of the activity
variations,
as shown by Shcherbakov & Tuominen (1992) and Ake
et al. (1994) in the well-documented case of HR 1105.
However, at a given phase, important cycle-to-cycle variations
remain
(Ake et al. observed a variation by a factor of 3 in the
nm flux of HR 1105 at phase 0.3 in two different orbital
cycles),
suggesting the existence of yet another cause of (secular)
variability.
Various physical processes are able to produce hard photons
modulated
by the orbital motion in a binary system:
1. Heating of the red-giant hemisphere facing a hot WD;
2. Accretion-powered hot spot;
3. Stream of gas from the red-giant wind, heated when funneled
through the
inner Lagrangian point.
A strong sensitivity of the activity level to the binary separation
is expected in the first and second cases. In the first case, the
orbital separation directly controls the dilution suffered by the hot
radiation when it reaches the giant atmosphere. In the second case,
the mass accretion rate by the secondary roughly scales as
(where
is the wind mass-loss
rate of the giant, and k is the ratio between the orbital and the
wind
velocities) in the case
of supersonic Bondi-Hoyle accretion in a detached system (see
Theuns et
al. 1996). The previous relation reduces to
(where A is the orbital separation) when k << 1 (i.e.
).
From a detailed analysis of the variations with orbital phase of
the UV flux level and the HeI nm line shape,
Shcherbakov & Tuominen (1992) and Ake et al. (1994)
favour the third process as the origin of the hard photons,
i.e. funneling of the red-giant wind through the inner Lagrangian
point.
The existence of such funneled streams is moreover predicted by
smooth particle hydrodynamics simulations of mass transfer in
detached binary systems (Theuns & Jorissen 1993;
Theuns et al.
1996).
Different viewing angles of the stream during the orbital cycle
account for
the orbital modulation, whereas long-term fluctuations of the
mass-loss rate account for the secular variations (like those observed
in Mira variables, and associated with a clumpy and non-spherically
symmetric wind; e.g. Whitelock et al. 1997; Lopez et al. 1997;
Olofsson 1997).
The wind mass-loss rate of the red giant, rather than the orbital
separation, is expected to be the dominant factor controlling the
activity level in this case.
The absence of any correlation between the
orbital periods and the activity levels in the sample
of S stars listed in Table 4 therefore suggests
that
streams like the one observed in the system HR 1105 might in fact
be responsible for the activity observed in other S stars as well.
The absence of any activity observed in the system HD 121447,
despite
the fact that it is the closest system in the sample, may then be
attributed to its low luminosity (
=-1.4), and therefore
low mass-loss rate.
Among the more luminous S stars (
), differences in
their
mass-loss rates may account for their different activity levels
(compare e.g. HD 35155 and HR 1105 having different activity levels
despite similar periods and spectral types,
or HD 35155 and HR 363 having the same X-ray flux at
very different orbital periods).
Future detailed studies of this class of mass-losing, binary red giants may thus be expected to shed light on the mass-loss process, as well as on the physics of interacting binaries.
AcknowledgementsWe wish to express our thanks to Tom Ake for communicating us results in advance of publication. Data and bibliographic references made available by the Centre de Données Stellaires (Strasbourg) were of great help in the present study. This work was supported in part by the Fonds National de la Recherche Scientifique (Belgium, Switzerland).
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