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[vsnet-ecl 101] Resolving the mystery of theta 1 Ori A
Dear Colleagues,
I would like to draw your attention to this seasons minima of the
long-period eclipsing binary theta 1 Ori A (05 35 15.8 -05 23 14 J2000)
and an observational programme on this system. From a new derivation of
the ephemeris
JD MinI = 2442752.022(+/-0.004$) + 65.43280(+/-0.00008) x E
and orbital solution, the eclipses should occur at
JD Date UT
2451520.017 1999 12 7.517 MinI ** tomorrow **
2451528.6+/-1.0 1999 12 16.1 MinII
2451585.450 2000 2 10.950 MinI
2451594.0+/-1.0 2000 2 19.5 MinII
2451650.883 2000 4 16.383 MinI
2451659.4+/-1.0 2000 4 24.9 MinII
with secondary minimum at phase 0.130.
So far no observations of the secondary eclipse have been made so these
are particularly important. The published light curves of the primary
minimum are now relatively old and there is only one recent time of
minimum. Primary minimum is about 1.0 mag deep and last for about 10
hours (FWHM) so only part is visible at one site. Nothing is known about
the secondary minimum, even its timing has an uncertainty of one day, but
it is also not likely to be very deep, possibly 0.05 mag or even 0.02 mag.
The secondary component is probably a pre-main-sequence star on its final
approach to the main sequence and its temperature is directly related to
the depth of the secondary minimum. It we know one we can get the other.
Further details are given in IBVS 4809.
Secondary minimum lasts about as long as the primary but its timing has an
uncertainty is about one day. What is required are observations during the
whole period of secondary eclipse, obviously not all from the same site,
and observations out of eclipse for reference using the same observational
set up. Even if the individual observations are not reliable to 0.01 or
0.02 mag this does not matter if there are enough measurements to produce
a reliable mean magnitude over a period of a few hours. If no eclipse is
seen these measurements can provide a very valuable upper limit on the
eclipse and constraint on the temperature of the secondary.
Observationally the system is not easy. It is bright, V=6.7, and is the
westernmost member of the Trapezium, so there is potential confusion from
other stars.
If you use a CCD (most people) you will need to use filters or reduce the
telescope aperture to ensure the detector is not saturated. This has to be
a simple and easily reproduced arrangement. The image scale needs to be
large enough to ensure good separation of A and B, probably about 1-2
arcsec/pixel. If in doubt try it and see. Be careful of very short
exposures. Aperture photometry of the sky around A will need to avoid any
contamination from B. Include component E in the aperture with A. Use the
same parameters for the in and out of eclipse observations.
If you do photoelectric photometry you will need an aperture small enough
to exclude the other bright stars and a telescope drive reliable enough
not to wander about. All measurements of the star will include the faint
companion theta 1 Ori E at 4 arcsec. The next closest star is theta 1 Ori
B = BM Ori at 9 arcsec. The sky position should be chosen carefully to
avoid obvious stars but be as evenly illuminated by the nebula as
possible, not a dark position away from the cluster. It should represent
the local sky background. This is probably a challenge beyond most amateur
PEP equipment.
Comparison stars used in the past have been theta 1 Ori C and D. If you
use a CCD measure both C (+faint companion) and D
Sort of to scale theta 1 Ori
----------- 22 arcsec
. . N
B | B = BM Ori
E --W
A
D .
.C . dots are other faint stars
.
If you have any questions then please e-mail me, and any observations you
contribute to the collaboration will be most welcome.
Thank you.
Chris Lloyd
I will eventually set up a web page at
http://astro1.bnsc.rl.ac.uk/cl/www/RandomVariable.html
vsnet-adm@kusastro.kyoto-u.ac.jp
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