[vsnet-be 42] RE: [vsnet-chat 4197] Re: re DK CAM hipep.. THANKS!

[Thom Gandet <tgandet@mindspring.com>]

crawl@zoom.co.uk wrote:
> 
> A B type star that's part of a physical binary of 10 to 20 years' period
> that shows photometric variation at periastron...
> 
> ...does that sound at all familiar, Sebastian???? [hint to others, see IBVS
> 5026]
> 
> Given the bit about spectroscopic binaries, maybes del Sco is also a 20 day
> spec bin after all, and then again, maybe not!
> 
> Cheers
> 
> John


John -

   The comparison to delta Sco may be apt, but delta Sco is a Be star
formerly classified as GCAS.  None of the late B-stars I referred to 
have shown emission lines - yet!  My hunch is that Herbig Be stars,
GCAS stars, and "classical" Be stars form an evolutionary sequence;
all of these classes may be really be some form of pre-main sequence
object (the Herbig Be stars certainly are) and are moving onto the
M.S.

   The long-period orbits of the late B-star binaries generally have 
very highly eccentric orbits - like the 10-year orbit of delta Sco.
While multiple star orbits are generally not co-planar, if the
inclination of delta Sco's long-period orbit is near, say 66
the short-period binary - if it exists! - could be an eclipser
with an 80-day period.  (I don't have the Hartkopf interferometry
handy.)

   The inference could be drawn (and I do draw it) that the long-
period companion "stimulates" the emission near periastron passage.
(A periastron distance of 1 AU for such massive stars as these 
involves enormous tidal stresses.)  However, photometric variability
in these late-B stars has only been suspected.  They are, perhaps,
Be-stars in the making.

   Delta Sco, considering its spectral type, its past spectroscopic
behavior and its recent photometric behavior, may be a Herbig Be
star rather than a classical Be star.  The prior lack of convincing
evidence for a spectroscopic period ought to be re-examined, as the
recent photometry by Sebastian and others (IBVS 5026) does show
evidence for a period near 80-days.  However, it is difficult to 
measure radial velocities of Be stars that are meaningful in terms
of Keplerian motion because of effects by gas streams, decretion/
accretion disk activity, etc.

   Delta Sco's behavior is very similar to that of V380 Cep, a
bright Herbig Be star that has shown a clear 3.7-year cycle in the
H-alpha equivalent widths over the last 20 years.  There is also a
long-term trend toward higher EWs with time.  A high-state in the EWs
is predicted sometime before June of this year, and it is expected to
be the highest yet observed.  This star should be monitored closely
for photometric changes during the high-state.  (The radial velocities
are difficult to interpret; they tantalize only!)

   V380 Cep reminds me eerily of the remark made in IBVS 5026 about
Delta Sco: "The current optical outburst is unlike anything previously
seen, in terms of both magnitude and duration. The H{alpha} emission
is also stronger than previously observed, but in detail it does not
show a strong correlation with the brightness variations, which
suggests that the mass loss and luminosity are not strongly coupled."

   Visual observations of V380 Cep are fairly easy to make because the
only good comparison stars are up to 3-4away.  However, there is a
nearby comparison star ideal for PEP (but too far away for most CCDs
- and too bright!).  V380 Cep is imbedded in a large reflection nebula,
and appears in a beautiful photograph in the Gallery section of Sky
and Telescope a couple of months ago.

   I believe the probability is high that many of the Be stars will
turn out to be short-period binaries; at least some of those will
be eclipsers.

  Lizard Hollow Observatory is observing fainter Be-stars, GCAS
stars, the late-B stars I mentioned, and other blue variables that
may be related to the Be-star phenomenon.  Simultaneous photometry
and spectroscopy of these objects would be a fruitful collaboration.
Improved orbits of the long-period interferometric binaries are
also needed.


Cordially,
Thom Gandet


-- 
************************************************************************
                      Lizard Hollow Observatory
                      Thomas L. Gandet, Director
                            PO Box 77021                   
                      Tucson, AZ 85703-7021  USA
************************************************************************
-- 
************************************************************************
                      Lizard Hollow Observatory
                      Thomas L. Gandet, Director
                            PO Box 77021                   
                      Tucson, AZ 85703-7021  USA
************************************************************************

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