MBCAA Observatory

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 5200K, 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

  • It is not clear why the check star is stronger in the 2nd session (but not WASP-2). May be this comes from the different FWHM, so I try to modify the apertures but I did not get a significant change. However, as this is crowded field, there is not much room to play with this. This may also come from the poor sky of the 1st session and the K star being bluer;
  • The fields of view are offset between the 2 sessions. Having the fields centered the same way would have allowed the use of more comparison and check stars;
  • There is a fairly good fit of my measurements with the BinaryMaker3 synthetic light curve, so this is probably a real detection. However these observervations should be repeated to be absolutly sure.
  • 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.

    Data processing.

    Modelling with BinaryMaker3.



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