The detection of Gl 581 'b' and may be 'c' planet
transit would have established the absolute mass of the
planet and its radius. From these parameters one could
also determine the mean density of the planet 'b' and also
estimate its chemical composition.
Measurements of the light curves were taken between 22.40 hrs on May 4, 2007 and 0200 hrs on 5 May, 2007. The uncertainty on the estimated mid-transit times was 86 minutes.
GJ 581 has a V mag of 10.56 and was strategically
placed on the CCD to include a suitable comparison star
in the frames. As comparison I used the nearby star
BD-06 4172 (V mag of 10.50), located at a dis-
tance nearby. Anything as bright as 8 was avoided in case of descrepancies regarding the scintillation problem.
As check star I used a slighty fainter object (V = 12.2) located nearby.
I collected a total of 230 images with 20s exposure times
was monitored over 3 hours with an average interval
of 12 seconds for each exposure. Practice runs were tried out with other stars at the meridian. Exposure times were worked out with the right balance to avoid over blooming.
Photometric precision is therefore limited by scintillation and therefore I selected a long exposure to get over this problem at 20 seconds. This also avoided any blooming from brighter stars.
I used a programme that could achieve millimag accuracy of 0.001 in post processing for the 230 frames.
To determine whether or not a planet transit was present
in the data, it was important to have the estimated transit duration and
transit depth of GJ 581b. In turn an estimate of the
the radius of the planet, the radius of the star, the or-
bital separation between the two objects, and the orbital
period of the system. If it was an edge-on configuration (i = 90 deg) and a circular orbit, the transit duration could have lasted between 85 to 100 minutes. The data has shown no detectable transit despite obtaining millimag accuracy.
The observation therefore has ruled out transits of GJ581b
for very high inclinations. However, the star appears to
be more photometrically stable at short time scales than
at the time scale of months to years measured by Weis
(1994). Hence the 3.5 hours was sufficient for stability.
This is a process of elimination. Longer monitoring may be required in order to catch the transit as the planet may now be at a lower inclination.
Thanks Alastair for the references, data, etc and also to you Phil for clear sky report. I learnt alot from the whole process. In fact one IFAS member came down to my place to see the early images first results that were captured that morning. My whole weekend has been taken up with the post processing. I also was very lucky regarding the weather.
I hope to make another attempt on a future prediction.
Eamonn A
  