Activity‐Related Radial Velocity Variation in Cool Stars

Abstract

Planets have been detected orbiting several solar-type stars using high-precision radial velocity (v r) measurements. While changes in v r can be measured with an accuracy of a few m s ?1 , there has been relatively little study of how other astrophysical processes, such as magnetic activity, may e ect the observed velocities. In this paper, we use published data and simple models to explore the contributions to v r from two activity-related sources, starspots and convective inhomogeneities, as these features rotate across the disk and evolve in time. Radial velocity perturbations due to both of these sources increase with rotation and the level of surface activity. Our models indicate that for solar-age G stars, the amplitude of perturbations due to spots is A S < 5 m s ?1 , increasing to A S 30 to 50 m s ?1 for Hyades-age G stars. If f S is the starspot area coverage, we nd A S / f 0:9 S v sin i. The e ects of convective inhomogeneities (as observed in line bisector variations) appear to depend on both rotation and spectral type. Young (active) F and G dwarfs can have convective v r perturbations with amplitudes A C > 50 m s ?1 , while v r amplitudes are reduced for stars with lower v sin i and cooler T e. We show that v r data from the literature display similar trends with v sin i and T e. A S and A C will be strongest at or near timescales related to magnetic activity variations: rotation, active region growth and decay, and activity cycles. Thus, knowledge of these timescales and typical A S and A C values are important in searching for extra-solar planets, especially those around younger, more active stars or those with small v r re ex amplitudes (i.e., < 20 m s ?1). We discuss implications of our results for current planet detections and planet search strategies.

An extreme planetary system around HD 219828

“…It is also known that stellar activity can produce signals that can mimic planetary companions at different timescales (e.g. Saar & Donahue 1997;Queloz et al 2000;Santos et al 2010;Dumusque et al 2012;Kane et al 2016), even for stars with a low activity level (e.g. Santos et al 2014).…”

The HARPS search for southern extra-solar planets

“…Observationally, these inhomogeneities translate into flux variations, as well as into changes of both the shape and the centroid of spectral lines (Saar & Donahue 1997;Queloz et al 2001). Spots also typically impact spectral indices, whether designed to probe the chromosphere (to which photospheric spots have strong magnetic connections) or the photosphere (because spots have cooler spectra).…”

“…As demonstrated by Saar & Donahue (1997), the bisector analysis loses much of its diagnostic power when applied to slow rotators. In simulations of the impact of star spots on radialvelocity and bisector measurements, they found that, for a given spot configuration, the radial velocity varies linearly with v sin i, while the bissector span varies as (v sin i) 3.3 .…”

“…Compared to Sun-like stars, they suffer from some drawbacks: they are faint so photon noise often limits measurements of their radial velocity, and many are at least moderately active and thus prone to so-called "radial-velocity jitter" (Saar & Donahue 1997). On the other hand, the lower masses of M dwarfs result in a higher wobble amplitude for a given planetary mass, and their p-mode oscillations have both smaller amplitudes and shorter periods than those of solar type stars.…”

The planet search program at the ESO Coudé Echelle spectrometer

“…Stellar activity can produce intrinsic RV variability and adds an additional noise source into the RV measurement (e.g. Saar & Donahue 1997). It can even mimic the RV signature of a short-period planet like in the case of HD 166435 (Queloz et al 2001).…”