Issue |
A&A
Volume 638, June 2020
|
|
---|---|---|
Article Number | A54 | |
Number of page(s) | 18 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361/201937354 | |
Published online | 10 June 2020 |
Radial-velocity variations due to meridional flows in the Sun and solar-type stars: impact on exoplanet detectability
Univ. Grenoble Alpes, CNRS, IPAG,
38000
Grenoble,
France
e-mail: nadege.meunier@univ-grenoble-alpes.fr
Received:
19
December
2019
Accepted:
7
April
2020
Context. Stellar variability due to magnetic activity and flows at different spatial scales strongly impacts radial velocities. This variability is seen as oscillations, granulation, supergranulation, and meridional flows. The effect of this latter process is currently poorly known but could affect exoplanet detectability.
Aims. We aim to quantify the amplitude of the meridional flow integrated over the disc and its temporal variability, first for the Sun, as seen with different inclinations, and then for other solar-type stars. We then want to compare these amplitudes with low-mass exoplanetary amplitudes in radial velocity.
Methods. We used long time series (covering two 11-yr cycles) of solar latitudinal meridional circulation to reconstruct its integrated contribution and study its properties. We then used scaling laws from hydrodynamical simulations relating the amplitude of the meridional flow variability with stellar mass and rotation rate to estimate the typical amplitude expected for other solar-type stars.
Results. We find typical rms of the order of 0.5–0.7 m s−1 (edge-on) and 1.2–1.7 m s−1 (pole-on) for the Sun (peak-to-peak amplitudes are typically 1–1.4 m s−1 and 2.3–3.3 m s−1 resp.), with a minimal jitter for an inclination of 45–55°. This signal is significant compared to other stellar activity contributions and is much larger than the radial-velocity signal of the Earth. The variability is strongly related to the activity cycle, with maximum flows during the descending phase of the cycle, and possible variability on timescales lower than the cycle period. Extension to other solar-type stars shows that the variability due to meridional flows is dominated by the amplitude of the cycle of those stars (compared with mass and rotation rate), and that the peak-to-peak amplitudes can reach 4 m s−1 for the most variable stars when seen pole-on. The meridional flow contribution sometimes represents a high fraction of the convective blueshift inhibition signal, especially for quiet, low-mass stars. For fast-rotating stars, the presence of multi-cellular patterns should significantly decrease the meridional flow contribution to the radial-velocity signal.
Conclusions. Our study shows that these meridional flows could be critical for exoplanet detection. Low inclinations are more impacted than edge-on configurations, but these latter still exhibit significant variability. Meridional flows also degrade the correlation between radial velocities due to convective blueshift inhibition and chromospheric activity indicators. This will make the correction from this signal challenging for stars with no multi-cellular patterns, such as the Sun for example, although there may be some configurations for which the line shape variations may be used if the precision is sufficient.
Key words: Sun: activity / Sun: photosphere / techniques: radial velocities / stars: kinematics and dynamics / stars: activity / planetary systems
© N. Meunier and A.-M. Lagrange 2020
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.