CoRoT-1: observations of the exoplanet transits

Observed: 28 Dec 2008, 18, 21 Mar 2009

Michel Bonnardeau
10 May 2009

Abstract

Amateur observations of this transiting exoplanet are presented and are modelled with BinaryMaker3.

Introduction

CoRoT-1 = GSC4804-02268 (or CoRoT-1b or CoRoT-Exo-1b) is a transiting exoplanet with an orbital period of Porb=1.5089557d and a transit duration of 134.7mn. It was discovered by Barge et al (2008); some data:

mass of the planet=1.03MJ, radius=1.49RJ , temperature=1898°K,
mass of the host star=0.95Mo, radius=1.11Ro, Teff=5950°K,
limb darkening u1+u2=0.71,
inclination=85.1°, eccentricity=0, semi major axis=4.92 star radii,
ephemeris for the transits:
HJD(E)=2,454,159.4532+/-0.0001+(Porb+/-0.0000064)*E

Observations

The observations were carried out with a 203mm f/6.3 SC telescope, a Cousins Rc filter and a SBIG ST7E camera (KAF401E CCD). 124 usable images were obtained in 3 sessions, each with a 200s duration exposure.

For the photometry an ensemble of 5 comparison stars is used:

Designation	Identification GSC4804-	B-V color
C	01354	0.257
C1	02216	0.437
C2	01766	1.150
C3	01170	1.097
C4	02058	0.153

(the B-V colors are obtained from the Tycho2 magnitudes, converted to Johnson-Cousins owing to Mamajek et al (2002) & (2006)).

Two check stars are used:

Designation	Identification GSC4804-	B-V color
K1	01756	2.546
K2	02256	0.367

The color of CoRoT-1 is B-V=0.536. So the comparison stars and K2 are a reasonably good match. K1 is very red but appears to be suitable. (The B-V colors of CoRoT-1 and of the check stars 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 28 Dec 2008

The light curves:


Red: CoRoT-1, Blue: K1 shifted by 1.25 mag, Black: K2 shifted by 1.05 mag. The error bars are +/- the 1-sigma statistical uncertainties.
K1: average mag=12.209, average 1-sigma statistical uncertainty=6mmag, standard deviation=6mmag.
K2: 12.456mag, 7 and 9mmag respectively.

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)):




Blue line: the fit. Green line: below the blue line by 0.1 mag (measurements below this line would be discarded). The measurements (red dots) are all well aligned along the blue line, therefore the sky quality is considered as good.

To assess the telescope stability, I measure the FWHM and check its constancy:

Sessions 18 and 21 March 2009

All the measurements are HERE.

Phase plot

The resulting phase plot:

The measurements may be smoothed by averaging them by bins of 20mn, and for each bin the standard deviation is computed (using the statistical uncertainties as weights):


The average standard deviation is 12mmag.

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); they are HERE.

From a screen copy with BinaryMaker3:

The synthetic light curve of BinaryMaker3 and the smoothed measurements:


The observed transit may be slightly deeper than the synthetic one.

References

Barge P., Baglin A., Auvergne M. et al (2008) A&A 482 L17.

Bilir S., Ak S., Karaali S., Cabrera-Lavers A., Chonis T.S., Gaskell C.M. (2007) MNRAS and arXiv:astro-ph/0711.4356v1.

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.

Astronomical note

Technical notes

Telescope and camera configuration.

Computer and software configuration.

Data processing.

Modelling with BinaryMaker3.