[vsnet-chat 4240] DE Dra: Predicted Minima

[Thom Gandet <tgandet@mindspring.com>]

Hi Sebastian,

Sebastian Otero wrote:

> >> DA Dra = 71 Dra = HD193964.
> 
> **Hi, Thom:
>                Actually it is DE Dra, ... or the namimg rules have
> changed!!

   No, the naming rules haven't changed.  My fingers apparently don't
know the diffarance batwaan tha "a" end tha "e" enymore! <G>

>  **Actually it has nothing to do with the epoch published in the GCVS
< (??) It falls in between. using the 5.298111 day period we have:
> JD 2442623.956 **GCVS= JD 2442626.2861** JD 2442629.254

   T0 is the time of periastron passage in the spectrographic orbit.
It is not a predicted time of minimum.  Using the 8th SB catalogue
orbital elements, and P=5.298111, the predicted times of Min. I and
Min. II closest to T0 are:

      Min I: HJD 2441142.9401    Min II: HJD 2441145.8111

The predicted times of eclipse that follow are calculated from these
predicted times.

   The eclipses closest to the GCVS epoch of JD 2442626.2861 are:

      Min I:     2442626.411     Min II:     2442629.254

Apparently, that epoch refers to Min. I.

(Note that it cannot be determined from the spectroscopic orbit
alone which is primary and which is secondary eclipse.)

   Using the period found by Meuninger, P=5.2980361, reduces the
above predicted times of minima by about 0.02 day.

   I haven't been able to locate the Meuninger reference, but there
is a photoelectric series of observations by W. Furtig, reported in
IBVS 1071.  His observations cover only about 1.9h and the depth
of minimum is no less than 0.15 mag; the duration of that eclipse
was, therefore, at least 3.8 hours long.  However, there is no sign
that minimum had been reached, so any time of minimum derived from
those observations is very uncertain.

   Carrying the eclipse predictions from the spectroscopic orbital
elements forward to the times of the Hipparcos observations you
quoted, eclipses would have occurred on the following HJD dates:

            Predicted Minima        Hipparcos Minimum    Probable     
          Min I:        Min II:                          Minimum   
      2448072.771   2448075.742        2448074.50          II         
         8157.540      8160.510           8160.03          II
         8316.582      8319.453           8317.112          I
         8348.371      8351.242           8349.114          I
         8438.439      8441.310           8438.858          I
         8660.853      8663.831           8660.99           I

   If you use the Meuninger period you quoted, the predicted times
should have 0.1 day subtracted from them.

> Probably the period is no less than 5.298111 days... but nothing
> else can be said.

   A great deal more can be said. <G>  The period is, fortunately,
fixed by the spectroscopy: the radial velocity observations span 56
years and they rule out any other greatly different period.  If the
star is an eclipsing binary, then the radial velocity curve fixes
the period,  barring any intrinsic variability.  But any intrinsic
variability greater than 0.15 mag. would almost certainly have
shown up in the radial velocities.  (The Meuninger period is not
significantly different from the spectroscopic period.)

   Kari Tikkanen reported observing minima, of unspecified magnitude,
at JD 2451996.43 and JD 2452013.5.  The spectrographic orbit predicts
eclipses at JD 2451996.189 (Min. II), and at JD 2452014.510 (Min. I).

   Some preliminary sense of the probable duration of eclipse(s) may
be made from the data you've posted:

   We know from Furtig's observations that one of the eclipses is at
least 3.8 hours long and that probably was Min. II.  Hipparcos 
apparently observed Min. II twice; of those observations, the first
one comes 1.24 days before predicted Min. II and the second comes
0.48 days before.  Min II must therefore last more than 1.24 days.
The Hipparcos observations of Min. I occur no more than 0.74 days
after their predicted times, so the duration of Min. I is at least
0.74 days.

   This would be an unusual eclipsing binary since it seems to spend
nearly 40% of its time in eclipse! <G>  Considering the long apparent
duration of eclipse, its spectral type and period, DE Dra is probably
a Beta Lyrae type binary; if so, the light curve will show a continuous
change of luminosity so times of maxima and minima will be difficult
to see.
 
   Without knowing the times of observation more accurately than have
been given in the postings I've seen here (I haven't looked at the
Hipparcos photometry myself), and without knowing the observed
magnitudes, it is impossible to say which is the primary and which is
the secondary.  (Perhaps you posted your observations to vsnet-obs,
but I don't subscribe to it because of the enormous amount of e-mail
it generates.)

   I've calculated predicted times of Minima I and II for the next
thirty days:

                Min. I            Min. II
           JD 2452019.962      2452022.833
                 2025.260         2028.131
                 2030.558         2033.429
                 2035.856         2038.727
                 2041.154         2044.025
                 2046.452         2049.323
                 2051.750         2054.621

   I hope you keen visual observers can observe DE Dra this season
and help to determine which is primary and which is secondary minimum.
This information, combined with the spectroscopy, will provide valuable
insight into the physical processes and characteristics of the system.
Good luck!

> And regarding delta Scorpii, time will tell if there is any periodicity in
> the visual variations. The two maxima shown in our paper are simmetrical.
> But recently the star has remained bright near 1.8. The two brighter sharp
> peaks are some 70 days apart (JD 2451752 - 2451823) and the last maximum
> although difficult to detect seem to have occured near 2451978, more
> consistent with a 77 day variation.

   That's a nice tie-in with the reported radial velocity period of
~80-day.  It would be an interesting project to re-visit the existing
radial velocity data and attempt to correlate them with the
photometry, but one I won't be able to do for some time.

   As you noted, radial velocities and photometry can be severely
effected by the behavior of the circumstellar matter that may
imposes its own periodicities on any otherwise detectable binary
motion.  (There are similarities in this behavior to eclipsing CVs.)

> But interesting..
> 
> Cheers,
> Sebastian.

   Yes, very interesting!  Keep up the good work!


Best regards,
Thom Gandet


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                      Thomas L. Gandet, Director
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