Crossfield, Ian J. M. and Knutson, Heather and Fortney, Jonathan and Showman, Adam P. and Cowan, Nicholas B. and Deming, Drake (2012) Spitzer/MIPS 24 μm Observations of HD 209458b: Three Eclipses, Two and a Half Transits, and a Phase Curve Corrupted by Instrumental Sensitivity Variations. Astrophysical Journal, 752 (2). Art. No. 81. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120720-132144478
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We report the results of an analysis of all Spitzer/MIPS 24 μm observations of HD 209458b, one of the touchstone objects in the study of irradiated giant planet atmospheres. Altogether, we analyze two and a half transits, three eclipses, and a 58 hr near-continuous observation designed to detect the planet's thermal phase curve. The results of our analysis are: (1) a mean transit depth of 1.484% ± 0.033%, consistent with previous measurements and showing no evidence of variability in transit depth at the 3% level. (2) A mean eclipse depth of 0.338% ± 0.026%, somewhat higher than that previously reported for this system; this new value brings observations into better agreement with models. From this eclipse depth we estimate an average dayside brightness temperature of 1320 ± 80 K; the dayside flux shows no evidence of variability at the 12% level. (3) Eclipses in the system occur 32 ± 129 s earlier than would be expected from a circular orbit, which constrains the orbital quantity ecos ω to be 0.00004 ± 0.00033. This result is fully consistent with a circular orbit and sets an upper limit of 140 m s^(–1) (3σ) on any eccentricity-induced velocity offset during transit. The phase curve observations (including one of the transits) exhibit an anomalous trend similar to the detector ramp seen in previous Spitzer/IRAC observations; by modeling this ramp we recover the system parameters for this transit. The long-duration photometry which follows the ramp and transit exhibits a gradual ~0.2% decrease in flux over ~30 hr. This effect is similar to that seen in pre-launch calibration data taken with the 24 μm array and is better fit by an instrumental model than a model invoking planetary emission. The large uncertainties associated with this poorly understood, likely instrumental effect prevent us from usefully constraining the planet's thermal phase curve. Our observations highlight the need for a thorough understanding of detector-related instrumental effects on long timescales when making the high-precision mid-infrared measurements planned for future missions such as EChO, SPICA, and the James Webb Space Telescope.
|Additional Information:||© 2012 American Astronomical Society. Received 2012 February 7; accepted 2012 April 11; published 2012 May 29. We thank Brad Hansen for many informative discussions, and Alberto Noriega-Crespo and James Colbert of the Spitzer Science Center for discussions about calibration of, and systematics in, MIPS 24μm photometry. We thank the referee for useful comments and the suggestion to expand our discussion of the residual noise properties. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, the California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. We received free software and services from SciPy, Matplotlib, and the Python Programming Language. This research made use of Tiny Tim/Spitzer, developed by John Krist for the Spitzer Science Center; the Center is managed by the California Institute of Technology under a contract with NASA.|
|Subject Keywords:||eclipses; infrared: planetary systems; planetary systems; planets and satellites: individual (HD 209458b); stars: individual (HD 209458); Sun: fundamental parameters; techniques: photometric|
|Official Citation:||Spitzer/MIPS 24 μm Observations of HD 209458b: Three Eclipses, Two and a Half Transits, and a Phase Curve Corrupted by Instrumental Sensitivity Variations Ian J. M. Crossfield et al. 2012 ApJ 752 81|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||20 Jul 2012 21:29|
|Last Modified:||18 Jan 2013 23:18|
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