A New 24 micron Phase Curve for upsilon Andromedae b

TL;DR

This study reports the detection of 24 micron phase variations from upsilon Andromedae b, revealing a large phase offset and temperature contrast that challenge existing atmospheric circulation models.

Contribution

It provides the first detailed 24 micron phase curve of upsilon Andromedae b with improved calibration and analysis, including the largest observed phase offset for an exoplanet.

Findings

Detected 24 micron flux variations consistent with planetary orbit

Observed a phase offset of ~80 degrees, the largest for an exoplanet

Derived a planetary temperature contrast of >900 K

Abstract

We report the detection of 24 micron variations from the planet-hosting upsilon Andromedae system consistent with the orbital periodicity of the system's innermost planet, upsilon And b. We find a peak-to-valley phase curve amplitude of 0.00130 times the mean system flux. Using a simple model with two hemispheres of constant surface brightness and assuming a planetary radius of 1.3 Jupiter radii gives a planetary temperature contrast of >900 K and an orbital inclination of >28 degrees. We further report the largest phase offset yet observed for an extrasolar planet: the flux maximum occurs ~80 degrees before phase 0.5. Such a large phase offset is difficult to reconcile with most current atmospheric circulation models. We improve on earlier observations of this system in several important ways: (1) observations of a flux calibrator star demonstrate the MIPS detector is stable to 10^-4 on long timescales, (2) we note that the background light varies systematically due to spacecraft operations, precluding use of this background as a flux calibrator (stellar flux measured above the background is not similarly affected), and (3) we calibrate for flux variability correlated with motion of the star on the MIPS detector. A reanalysis of our earlier observations of this system is consistent with our new result.