WASP-2: possible detection of the exoplanet transit
Observed: 5 Aug, 11 Oct 2008
Michel Bonnardeau
13 April 2009
Updated 28 April 2009 (BinaryMaker3 modelling)
Revised and updated 9 May 2009
Abstract
Amateur observations of this transiting exoplanet are presented and are modelled with BinaryMaker3.
Introduction
WASP-2 = GSC522-01199 is a transiting exoplanet with an orbital period
of 2.152226d and a transit duration of 107.9mn. It was discovered by Collier
Cameron et al (2006); some data from exoplanet.eu
(as of Apr 2009):
mass of the planet 0.914MJ, radius 1.017RJ
mass of the star 0.84Mo, radius 0.834Ro, Teff 5200°K, distance 144pc
inclination 84.8°, eccentricity 0, semi-major axis 0.03138au
ephemeris for the transits:
HJD(E)=2,453,991.5146+/-0.0010+(2.152226+/-0.000004)*E
The host star may be in a binary system with an orbital period of 1400yr (Daemgen et al (2009)).
Observations
The observations were carried out with a 203mm f/6.3 SC telescope, a Cousins Rc filter and a SBIG ST7E camera (CCD KAF401E). 73 usable images were obtained in 2 sessions. Each image has an exposure duration of 200s.
For the photometry an ensemble of 4 comparison stars is used:
| Designation | Identification GSC522- |
B-V color |
| C1 | 00878 | 0.958 |
| C2 | 01676 | 0.714 |
| C3 | 01100 | 0.452 |
| C4 | 00780 | 0.912 |
(the B-V colors are obtained from the Tycho2 magnitudes, converted to Johnson-Cousins owing to Mamajek et al (2002) & (2006)).
WASP-2 has a color B-V=0.936, the check star is GSC522-01840 with B-V=0.526. So the comparison stars are a reasonably good color match. (The B-V colors of WASP-2 and of the K star are computed from their CMC14 r' and 2MASS J, H, Ks magnitudes owing to Bilir et al (2007)).
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".
Other sessions
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.
Phase plot
The resulting phase plot:
The depth of the transit is about 20mmag, in agreement with what it
should be (see Transitsearch).
BinaryMaker3 modelling
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.
Discussion
ASAS2031+06
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.
References
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.
Technical notes
Telescope and camera configuration.
Computer and software configuration.
Modelling with BinaryMaker3.
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