Activity time series of old stars from late F to early K

Abstract

Context. Stellar activity strongly affects and may prevent the detection of Earth-mass planets in the habitable zone of solar-type stars with radial velocity technics. Astrometry is in principle less sensitive to stellar activity because the situation is more favourable: the stellar astrometric signal is expected to be fainter than the planetary astrometric signal compared to radial velocities. Aims. We quantify the effect of stellar activity on high-precision astrometry when Earth-mass planets are searched for in the habitable zone around old main-sequence solar-type stars. Methods. We used a very large set of magnetic activity synthetic time series to characterise the properties of the stellar astrometric signal. We then studied the detectability of exoplanets based on different approaches: first based on the theoretical level of false positives derived from the synthetic time series, and then with blind tests for old main-sequence F6-K4 stars. Results. The amplitude of the signal can be up to a few times the solar value depending on the assumptions made for activity level, spectral type, and spot contrast. The detection rates for 1 MEarth planets are very good, however, with extremely low false-positive rates in the habitable zone for stars in the F6-K4 range at 10 pc. The standard false-alarm probability using classical bootstrapping on the time series strongly overestimates the false-positive level. This affects the detection rates. Conclusions. We conclude that if technological challenges can be overcome and very high precision is reached, astrometry is much more suitable for detecting Earth-mass planets in the habitable zone around nearby solar-type stars than radial velocity, and detection rates are much higher for this range of planetary masses and periods when astrometric techniques are used than with radial velocity techniques.

Stellar variability in radial velocity

“…Stellar activity also affects the astrometric signal, but for massive planets to be detected with Gaia, the amplitude of the effect is completely negligible for solartype stars. Future high-precision astrometric mission aiming to detect very low mass terrestrial planets will be sensitive to stellar activity however, but to a much lesser extent than radial velocities for similar stars and planets (Makarov et al, 2010;Lagrange et al, 2011;Meunier et al, 2020).…”

“…Dumusque et al (2011) performed fits of the Harvey function for a few stars observed by HARPS, but the sample is too small to derive a trend. Meunier et al (2020) scaled it to granulation amplitude because Roudier et al (2016) showed that supergranulation appears to be related to the behavior of granulation (through the analysis of trees of fragmenting granules).…”

“…All laws are described and discussed in . Other observables than RV were also produced: chromospheric emission log R ′ HK , photometry (Meunier and Lagrange, 2019b), and astrometry (Meunier et al, 2020). An example of time series is shown in Fig.…”

Predictions of Astrometric Jitter for Sun-like Stars. II. Dependence on Inclination, Metallicity, and Active-Region Nesting

“…We note that the plotted standard deviations are affected by the jitter both on the rotational timescale discussed in Section 3.1 and on the activity timescale discussed in Section 3.2. Although the deviations tend to increase with increasing metallicity and degree of nesting, the dependence on inclination is different for σ(∆X) and σ(∆Y ), in accordance with Meunier et al 2020. Before simulating the astrometric jitter time series as it would be obtained by Gaia, we add the solar wobble caused by the Earth to the time series of daily displacements. The motivation behind considering the Sun-Earth system is to determine if the detection of Earth-mass planets in the habitable zones of stars with solar-like magnetic activity is influenced by the jitter due to magnetic activity of the host star.…”

“…Meunier et al (2019) proposed a model to simulate astrometric time series for solar-type stars. Using these simulated time series, Meunier et al (2020) quantified the effect of stellar activity on astrometric measurements, and its impact on the search for Earth-mass planets in the habitable zones of old main-sequence solar-type stars. Shapiro et al (2021, hereafter Paper I) developed a model, that employs the observed distribution of spots and faculae on the visible solar disk, to calculate the astrometric jitter of the Sun.…”