Two Upper Limits on the Rossiter-McLaughlin Effect, with Differing Implications: WASP-1 has a High Obliquity and WASP-2 is Indeterminate

TL;DR

This study measures the Rossiter-McLaughlin effect in two exoplanet systems, revealing high stellar obliquity in WASP-1 and inconclusive results for WASP-2, challenging previous assumptions about star-planet alignment.

Contribution

The paper provides the first upper limits on the RM effect for WASP-1 and WASP-2, offering new insights into stellar obliquities and challenging prior claims of retrograde orbit in WASP-2b.

Findings

WASP-1 likely has a high obliquity due to near pole-on viewing angle.

No significant RM signal detected for WASP-2, contradicting earlier retrograde orbit claims.

Results suggest cool stars may not always have low obliquities.

Abstract

We present precise radial-velocity measurements of WASP-1 and WASP-2 throughout transits of their giant planets. Our goal was to detect the Rossiter-McLaughlin (RM) effect, the anomalous radial velocity observed during eclipses of rotating stars, which can be used to study the obliquities of planet-hosting stars. For WASP-1 a weak signal of a prograde orbit was detected with ~2sigma confidence, and for WASP-2 no signal was detected. The resulting upper bounds on the RM amplitude have different implications for these two systems, because of the contrasting transit geometries and the stellar types. Because WASP-1 is an F7V star, and such stars are typically rapid rotators, the most probable reason for the suppression of the RM effect is that the star is viewed nearly pole-on. This implies the WASP-1 star has a high obliquity with respect to the edge-on planetary orbit. Because WASP-2 is a K1V star, and is expected to be a slow rotator, no firm conclusion can be drawn about the stellar obliquity. Our data and our analysis contradict an earlier claim that WASP-2b has a retrograde orbit, thereby revoking this system's status as an exception to the pattern that cool stars have low obliquities.