WASP-2: possible detection of the exoplanet transit
Observed: 5 Aug, 11 Oct 2008
An example of an image:
The comparison stars are inside the yellow squares.
The ensemble aperture photometry is performed with the AstroMB program (version 2.8*), which fits the comparison stars with the Pogson's law, using their statistical uncertainties as weights. The magnitudes of the comparison stars are taken from the UCAC2 catalog.
Session 5 Aug 2008
The light curves:
Red: WASP-2, Blue: the check star shifted by -1mag. The error bars are +/- the 1-sigma statistical uncertainties.
The average 1-sigma statistical uncertainties of the check star is 8mmag and the standard deviation is equal.
The sky quality during the session is assessed by computing the extra losses.
This is done by fitting the photometry zero point as a function of the
air mass by a line and by taking the difference (Gary (2007)):
The airmass during the session.
Blue line: the fit of the photometry zero points versus the airmass.
The extra losses during the session.
The extra losses are very bad (they should be around 0). Actually I had to stop the observing session because of clouds. Normally I would discard such a session. However the check star is constant and the WASP-2 ingress seems to be there. As I do not have many observations, I keep it.
To assess the telescope stability, I measure the FWHM and check its constancy:
The average FWHM is 5.456".
Session 11 Oct 2008
The light curves:
The average 1-sigma statistical uncertainties of the check star is 9mmag and the standard deviation is 8mmag. (The SD smaller than the average SU probably comes from the varying FWHM, see below).
The average magnitude of the check star is 12.456 while it was 12.477 during the previous session.
The extra losses look good:
The FWHM increases during the session. Actually I started to observe
at dawn, while the telescope was not fully thermalized:
The average is 5.729".
I discarded the following sessions:
8 July 2008: there was an obstruction in the line of sight;
3 Aug 2008: there was a focusing problem with the FWHM very large and very variable and that shows up in the light curves of WASP-2 and of the K;
6 and 8 Nov 2007: use of a Clear filter, the data are noisy.
The resulting phase plot:
The depth of the transit is about 20mmag, in agreement with what it should be (see Transitsearch).
A synthetic light curve may be generated with BinaryMaker3, a software program to study binary stars. I use then the published parameters (see the Introduction).
For the limb-darkening, Charbonneau et al (2007) use a quadratic law (with u1=0.2835 and u2=0.2887). BinaryMaker3 allows only a linear law so I use X=u1+u2. All the parameters are HERE.
Some BinaryMaker3 screen copies:
Red crosses: my measurements, Blue circles: the synthetic light curve.
In the field there is also the variable ASAS203114+0627.3 which is listed in the AAVSO VSX as of the MISC type with a period of 45days and an amplitude variation of 0.16mag.
I measure the magnitudes:
5 Aug 2008: 11.142+/-0.005
11 Oct 2008: 11.066+/-0.005
where the uncertainties are the standard deviations. During each session, the light curve looks constant.
Bilir S., Ak S., Karaali S., Cabrera-Lavers A., Chonis T.S., Gaskell C.M. (2007) MNRAS and arXiv:astro-ph/0711.4356v1.
Charbonneau D. et al (2007) ApJ 658 1322.
Collier Cameron A. et al (2006) MNRAS 375 951 and arXiv:astro-ph/0609688v3.
Daemgen S. et al (2009) A&A and arXiv:astro-ph/0902.2179v1
Gary B. (2007) Exoplanet observing for amateurs Reductionist Publications. (May be downloaded from http://brucegary.net/book_EOA/x.htm).
Mamajek E.E., Meyer M.R., Liebert J. (2002) AJ 124 1670 Appendix C; Erratum (2006) AJ 131 2360.
Modelling with BinaryMaker3.
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