HR 363 was classified M2S by Keenan (1954), who noted its strong
BaII nm line, and M3IIS by Yamashita (1967).
Keenan &
Boeshaar (1980) reclassified it later on as
in their revised
classification scheme (where the first digit is a temperature
index, and the second one is an index of ZrO strength, 2
corresponding to ZrO < TiO). More recently, Sato & Kuji (1990)
have classified HR 363 as M2III. These authors stress that,
although HR 363 is often considered as an S star, its salient
spectral features are those typical of M giants, except for a
strong BaII
nm line.
As for HD 191226, a near-infrared spectrum has been obtained for
HR 363 with the Aurélie spectrograph at OHP, and yields a M3III
spectral type.
Finally, regarding its Tc-poor
nature, we refer to Little et al. (1987) and
Smith & Lambert
(1988).
The orbital solution for HR 363 listed in Table 2
is based on 49 CORAVEL radial-velocity
measurements covering 1.1 orbital cycle (from 1983 to 1997), to
which one older measurement obtained in 1976 has been added
(Table 3). This early measurement (obtained at the
1.52-m telescope of Haute-Provence Observatory on the photographic
spectrum GA 2881) substantially
improves the period determination, since it brings the orbital
coverage to 1.65 cycle. Its accuracy is only 0.8 km s, compared to
an average of 0.30 km s
for the CORAVEL measurements. It
has therefore been attributed a weight of 0.25 in the orbital solution
(compared to 1 for the CORAVEL measurements).
The radial-velocity curve, folded with the orbital period, is
presented in Fig. 2. This figure presents as well
the other measurements
(namely the three radial velocities from the Fick Observatory,
the 15 measurements from Brown et al., and the 6 radial velocities we
obtained from photographic spectra at OHP)
that were not used in the orbital solution,
since they would degrade its accuracy. These observations are nevertheless
compatible with the computed solution.
Note that the residuals
are significantly larger than the accuracy of the measurements.
This jitter is likely due to envelope pulsations or to atmospheric
motions, as discussed by Jorissen et al. (1998; see their Fig. 1).
As for HD 191226, the mass function is compatible with a WD
companion,
since
for the red giant implies
for the unseen companion, given
. If the companion is to be a WD with a mass typical of field WD's
(0.58
; Reid 1996), the orbital inclination has to be close to
90
, and the system may be an eclipsing binary.
This star has been observed with the International Ultraviolet
Explorer (IUE) and ROSAT satellites. Although there is only
marginal
evidence for an UV continuum from a hot companion
(Ake et al.
1988),
HR 363 is a strong source of hard X-rays (Jorissen et al. 1996).
These X-rays are not expected to come from a hot corona, because
with B-V = 1.7, HR 363 lies far to
the right of the region of the Hertzsprung-Russell diagram
populated
by class III giants with a hot corona (Hünsch et al. 1996).
The hard X-rays in HR 363 are therefore likely powered by
mass transfer in the binary system. The same holds true for the
weak
HeI nm emission line observed in HR 363
(Brown et al.
1990), since that line is generally absent in cool M giants, but is
frequent in interacting binary systems like symbiotic stars (Brown
et al. 1990 and references therein).
It is somewhat surprising, though, that HR 363 behaves as an
interacting binary system while having the longest known period
(4593 d = 12.6 y) among S stars (Jorissen et al. 1998).
That question is discussed in more details in Sect. 4.
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