Abstract
Several studies have shown that the occultation of stellar active regions by the transiting planet can generate anomalies in the high-precision transit light curves, and these anomalies may lead to an inaccurate estimate of the planetary parameters (e.g., the planet radius). Since the physics and geometry behind the transit light curve and the Rossiter- McLaughlin effect (spectroscopic transit) are the same, the Rossiter-McLaughlin observations are expected to be affected by the occultation of stellar active regions in a similar way. In this paper we perform a fundamental test on the spin-orbit angles as derived by Rossiter-McLaughlin measurements, and we examine the impact of the occultation of stellar active regions by the transiting planet on the spin-orbit angle estimations. Our results show that the inaccurate estimation on the spin-orbit angle due to stellar activity can be quite significant (up to 30 degrees), particularly for the edge-on, aligned, and small transiting planets. Therefore, our results suggest that the aligned transiting planets are the ones that can be easily misinterpreted as misaligned owing to the stellar activity. In other words, the biases introduced by ignoring stellar activity are unlikely to be the culprit for the highly misaligned systems.
Highlights
As a star rotates, the part of its surface that rotates toward the observer will be blue-shifted and the part that rotates away will be red-shifted
During the transit of a planet, the corresponding rotational velocity of the portion of the stellar disk that is blocked by the planet is removed from the integration of the velocity over the entire star, creating the radial velocity (RV) signal which is known as the Rossiter-McLaughlin (RM) effect (Rossiter 1924; McLaughlin 1924)
In this paper we performed a fundamental test on the spin-orbit angles as derived by RM measurements, and we examined the impact of stellar active regions occultation by the transiting planet on the spin-orbit angle estimations
Summary
The part of its surface that rotates toward the observer will be blue-shifted and the part that rotates away will be red-shifted. In the era of high-precision RV measurements, like those provided by HARPS and HARPS-N and the upcoming spectrograph ESPRESSO, determination of the spin-orbit angle and v sin i can be influenced by second-order effects such as the convective blueshift (Shporer & Brown 2011; Cegla et al 2016), the differential stellar rotation (Albrecht et al 2012), and the microlensing effect due to the transiting planet’s mass (Oshagh et al 2013b) It has been shown in several studies that the occultation of stellar active regions (i.e., stellar spots and plages) by the transiting planet can generate anomalies in the high-precision transit light curves and may lead to an incorrect estimate of the planetary parameters (e.g., Sanchis-Ojeda et al 2011, 2013; Oshagh et al 2013c, 2015a,b; Barros et al 2013). The detection of these anomalies in the transit light curves becomes the norm in the exoplanet community after the high-precision photometric observations achieved by space-based telescopes (such as CoRoT and Kepler)
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