Measurement of the Spin-Orbit Angle of Exoplanet HAT-P-1b

TL;DR

This paper measures the spin-orbit angle of exoplanet HAT-P-1b using spectroscopic observations, finding it nearly aligned, and refines its orbital parameters to better understand its migration history.

Contribution

It provides the first precise measurement of the spin-orbit angle for HAT-P-1b and refines its orbital period and eccentricity constraints.

Findings

Spin-orbit angle   3.7 b1 2.1 degrees.

Orbital eccentricity upper limit of 0.067 at 99% confidence.

Refined transit ephemeris with significantly reduced uncertainty.

Abstract

We present new spectroscopic and photometric observations of the HAT-P-1 planetary system. Spectra obtained during three transits exhibit the Rossiter-McLaughlin effect, allowing us to measure the angle between the sky projections of the stellar spin axis and orbit normal, \lambda = 3.7 +/- 2.1 degrees. The small value of \lambda for this and other systems suggests that the dominant planet migration mechanism preserves spin-orbit alignment. Using two new transit light curves, we refine the transit ephemeris and reduce the uncertainty in the orbital period by an order of magnitude. We find a upper limit on the orbital eccentricity of 0.067, with 99% confidence, by combining our new radial-velocity measurements with those obtained previously.