Abstract
We present observations of seven transits and seven eclipses of the transiting planet system HD 189733 taken with Spitzer IRAC at 8 microns. We use a new correction for the detector ramp variation with a double-exponential function. Our main findings are: (1) an upper limit on the variability of the day-side planet flux of 2.7% (68% confidence); (2) the most precise set of transit times measured for a transiting planet, with an average accuracy of 3 seconds; (3) a lack of transit-timing variations, excluding the presence of second planets in this system above 20% of the mass of Mars in low-order mean-motion resonance at 95% confidence; (4) a confirmation of the planet's phase variation, finding the night side is 64% as bright as the day side, as well as an upper limit on the night-side variability of 17% (68% confidence); (5) a better correction for stellar variability at 8 micron causing the phase function to peak 3.5 hrs before secondary eclipse, confirming that the advection and radiation timescales are comparable at the 8 micron photosphere; (6) variation in the depth of transit, which possibly implies variations in the surface brightness of the portion of the star occulted by the planet, posing a fundamental limit on non-simultaneous multi-wavelength transit absorption measurements of planet atmospheres; (7) a measurement of the infrared limb-darkening of the star, in agreement with stellar atmosphere models; (8) an offset in the times of secondary eclipse of 69 sec, which is mostly accounted for by a 31 sec light travel time delay and 33 sec delay due to the shift of ingress and egress by the planet hot spot; this confirms that the phase variation is due to an offset hot spot on the planet; (9) a retraction of the claimed eccentricity of this system due to the offset of secondary eclipse; and (10) high precision measurements of the parameters of this system.
Highlights
The planet system HD 189733 (Bouchy et al 2005) is one of the best-studied transiting planet systems due to two factors: its close proximity to our solar system, making its star one of the brightest transit host stars, and the large size of the planet relative to the star, making the transits deep
Note that this aperture size contains the bulk of the target flux, and is near the minimum in flux just inside the first Airy ring; this makes our photometry less sensitive to variations in position. We have fit both stars with the measured point response function (PRF) for Infrared Array Camera (IRAC) Channel 4, and we find that the contribution of the M-dwarf within this aperture is less than 0.06% of the target star flux for all of our observations
The analysis of 14 transits and eclipses in this paper has made several improvements to the data reduction and modeling; in particular, we have found a better function for fitting the detector ramp of IRAC Channel 4, a double exponential
Summary
The planet system HD 189733 (Bouchy et al 2005) is one of the best-studied transiting planet systems due to two factors: its close proximity to our solar system, making its star one of the brightest transit host stars, and the large size of the planet relative to the star, making the transits deep. In addition to yielding a longitudinal map of the planet (Cowan & Agol 2008) which indicated an offset peak in brightness, attributed to advection of energy by a superrotating equatorial jet (Showman & Guillot 2002; Cooper & Showman 2005), this observation yielded the most precise measurement of the depth of secondary eclipse, as well the most precise times of transit and secondary eclipse, for any extrasolar planet. We show that the transit depth appears to vary, which we hypothesize is due to variations in the stellar surface brightness within the path of the planet (Section 6.6) and we show that the dayside planet flux measured from the secondary eclipses appears not to vary within the uncertainties (Section 6.7) We discuss these results and compare to models in the conclusions (Section 7). These data were used by Carter & Winn (2010) to place a constraint on the oblateness of HD 189733b, while for the purposes of this paper we assume the planet to be spherical
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
