Radial velocity planet detection biases at the stellar rotational period

Abstract

Future generations of precise radial velocity (RV) surveys aim to achieve sensitivity sufficient to detect Earth mass planets orbiting in their stars' habitable zones. A major obstacle to this goal is astrophysical radial velocity noise caused by active areas moving across the stellar limb as a star rotates. In this paper, we quantify how stellar activity impacts exoplanet detection with radial velocities as a function of orbital and stellar rotational periods. We perform data-driven simulations of how stellar rotation affects planet detectability and compile and present relations for the typical timescale and amplitude of stellar radial velocity noise as a function of stellar mass. We show that the characteristic timescales of quasi-periodic radial velocity jitter from stellar rotational modulations coincides with the orbital period of habitable zone exoplanets around early M-dwarfs. These coincident periods underscore the importance of monitoring the targets of RV habitable zone planet surveys through simultaneous photometric measurements for determining rotation periods and activity signals, and mitigating activity signals using spectroscopic indicators and/or RV measurements at different wavelengths.

Biases in retrieving planetary signals in the presence of quasi-periodic stellar activity

“…The choice of simulating planets with P orb ∼ P rot originates from the intrinsic difficulties encountered in separating the stellar activity from the planetary signal in real RV datasets, when they have very similar periods (e.g. Vanderburg et al 2016).…”

Photometric rotation periods for 107 M dwarfs from the APACHE survey

“…When it comes to M dwarf type planet hosts, the stellar temperate zone, within which the signals of potentially habitable planets can today be readily identified in principle (e.g., Anglada-Escudé et al 2016a;Dittmann et al 2017;Suárez Mascareño et al 2017a;Astudillo-Defru et al 2017b;Bonfils et al 2018;Zechmeister et al 2019), corresponds to orbital periods typically in the range of tens of days (e.g., Kopparapu et al 2013Kopparapu et al , 2014Kopparapu 2018). However, these timescales can coincide with those of stellar rotation periods of low-mass stars in the middle of their main-sequence lifetime (e.g., Barnes 2007;McQuillan et al 2013;Vanderburg et al 2016;Newton et al 2016;Suárez Mascareño et al 2018). Without a detailed understanding of stellar activity-induced effects in RVs to supporting spectroscopic and photometric information, ambiguities in the interpretation of RV signals with periods corresponding to habitable zone distances might be long-lasting (e.g., Robertson et al 2014;Anglada-Escudé & Tuomi 2015;Robertson et al 2015;Anglada-Escudé et al 2016b).…”

“…For the Transiting Exoplanet Survey Satellite (TESS; Ricker et al 2014), launched in 2018 April, small habitable-zone planets around M dwarfs constitute a particularly relevant sample (e.g., Sullivan et al 2015;Barclay et al 2018;Ballard 2019;Kaltenegger et al 2019), as the combination of small telescope aperture and satellite's observing strategy translate in a low mission sensitivity to orbital periods longer than a few tens of days (except near the ecliptic poles) and to planets with radii 2 R ⊕ around solar-type primaries. Measurements of photometric rotation periods for M dwarfs with TESS-detected small-size transit candidates would be of paramount importance to gauge the effects of spots in the analysis of the transit events and to help disentangling planetary RV signals from those related to stellar magnetic activity (e.g., Haywood et al 2014;Vanderburg et al 2016;Dittmann et al 2017;Damasso et al 2018). However, rotation periods of a few tens of days or longer will not be typically accessible to TESS (at least during its nominal mission lifetime), due to its less than a month duration of the observing windows.…”

New Rotation Period Measurements for M Dwarfs in the Southern Hemisphere: An Abundance of Slowly Rotating, Fully Convective Stars

“…Stellar rotation imprints itself on both photometry and radial velocity data, and thus the detection and characterization of planets near the stellar rotation period or its low-order harmonics can be frustrated. For early M dwarfs, the typical rotation periods for older field stars coincides with orbital periods of planets in the habitable zone (Newton et al 2016a;Vanderburg et al 2016). This can inhibit the detection of potentially habitable planets around these stars (e.g.…”