Stellar surface information from the Ca II H&K lines – I. Intensity profiles of the solar activity components

The detection of Earth-like planets with the radial-velocity (RV) method is currently limited by the presence of stellar activity signatures. On rotational time-scales, spots and plages (or faculae) are known to introduce different RV signals, but their corrections require better activity proxies. The best-known chromospheric activity proxies in the visible are the Ca II H&K lines, but the physical quantities measured by their profiles need to be clarified. We first investigate resolved images of the Sun in order to better understand the spectrum of plages, spots, and the network using the Meudon spectroheliogram. We show that distinct line profiles are produced by plages, spots, and by the network component and we also derived the centre-to-limb variations of the three profiles. Some care is required to disentangle their contributions due to their similarities. By combining disc-integrated spectra from the ISS high-resolution spectrograph with SDO direct images of the Sun, we managed to extract a high-resolution emission spectrum of the different components, which tend to confirm the spectra extracted from the Meudon spectroheliogram datacubes. Similar results were obtained with the HARPS-N Sun-as-a-star spectra. We concluded using a three-component model that the temporal variation of the popular Sindex contains, on average for the 24th solar cycle: 70 ± 12 per cent of plage, 26 ± 12 per cent of network, and 4 ± 4 per cent of spots. This preliminary investigation suggests that a detailed study of the Ca II H&K profiles may provide rich information about the filling factor and distribution of different types of active regions

Stellar surface information from the Ca II H&K lines I. Intensity profiles of the solar activity components

The detection of Earth-like planets with the radial-velocity method is currently limited by the presence of stellar activity signatures. On rotational timescales, spots and plages (or faculae) are known to introduce different RV signals, but their corrections require better activity proxies. The best-known chromospheric activity proxies in the visible are the Ca II H & K lines, but the physical quantities measured by their profiles need to be clarified. We first investigate resolved images of the Sun in order to better understand the spectrum of plages, spots, and the network using the Meudon spectroheliogram. We show that distinct line profiles are produced by plages, spots, and by the network component and we also derived the center-to-limb variations of the three profiles. Some care is required to disentangle their contributions due to their similarities. By combining disk-integrated spectra from the ISS high-resolution spectrograph with SDO direct images of the Sun, we managed to extract a high-resolution emission spectrum of the different components, which tend to confirm the spectra extracted from the Meudon spectroheliogram datacubes. Similar results were obtained with the HARPS-N Sun-as-a-star spectra. We concluded using a three-component model that the temporal variation of the popular S-index contains, on average for the 24th solar cycle: 70 +/- 12% of plage, 26 +/- 12% of network and 4 +/- 4% of spots. This preliminary investigation suggests that a detailed study of the Ca II H & K profiles may provide rich information about the filling factor and distribution of different types of active regions.Comment: 20 pages, 17 figure

YARARA V2: reaching sub-m s−1 precision over a decade using PCA on line-by-line radial velocities

Context. The detection of Earth-like planets with the radial velocity (RV) method is extremely challenging today due to the presence of non-Doppler signatures such as stellar activity and instrumental signals that mimic and hide the signals of exoplanets. In a previous paper, we presented the YARARA pipeline, which implements corrections for telluric absorption, stellar activity, and instrumental systematics at the spectral level, and then it extracts line-by-line (LBL) RVs with a significantly better precision than standard pipelines. Aims. In this paper, we demonstrate that further gains in RV precision can be achieved by performing principal component analysis (PCA) decomposition on the LBL RVs. Methods. The mean-insensitive nature of PCA means that it is unaffected by true Doppler shifts, and thus can be used to isolate and correct nuisance signals other than planets. Results. We analysed the data of 20 intensively observed HARPS targets by applying our PCA approach on the LBL RVs obtained by YARARA. The first principal components show similarities across most of the stars and correspond to newly identified instrumental systematics for which we can now correct. For several targets, this results in an unprecedented RV root-mean-square of around 90 cm s−1 over the full lifetime of HARPS. We used the corrected RVs to confirm a previously published 120-day signal around 61 Vir, and to detect a super-Earth candidate (K ~ 60 ± 6 cm s−1, m sin i = 6.6 ± 0.7 M⊕) around the G6V star HD 20794, which spends part of its 600-day orbit within the habitable zone of the host star. Conclusions. This study highlights the potential of LBL PCA to identify and correct hitherto unknown, long-term instrumental effects and thereby extend the sensitivity of existing and future instruments towards the Earth analogue regime

YARARA: Significant improvement of RV precision through post-processing of spectral time-series

Aims: Even the most-precise radial-velocity instruments gather high-resolution spectra that present systematic errors that a data reduction pipeline cannot identify and correct for efficiently. In this paper, we aim at improving the radial-velocity precision of HARPS measurements by cleaning individual extracted spectra using the wealth of information contained in spectra time-series. Methods: We developed YARARA, a post-processing pipeline designed to clean high-resolution spectra from instrumental systematics and atmospheric contamination. Spectra are corrected for: tellurics, interference pattern, detector stitching, ghosts and fiber B contaminations as well as more advanced spectral line-by-line corrections. YARARA uses Principal Component Analysis on spectra time-series with prior information to disentangle contaminations from real Doppler shifts. We applied YARARA on three systems: HD10700, HD215152 and HD10180 and compared our results to the HARPS standard Data Reduction Software and the SERVAL post-processing pipeline. Results: On HD10700, we obtain radial-velocity measurements that present a rms smaller than 1 m/s over the 13 years of the HARPS observations, which is 20 and 10 % better than the HARPS Data Reduction Software and the SERVAL post-processing pipeline, respectively. We also injected simulated planets on the data of HD10700 and demonstrated that YARARA does not alter pure Doppler shifted signals. On HD215152, we demonstrated that the 1-year signal visible in the periodogram becomes marginal after processing with YARARA and that the signals of the known planets become more significant. Finally, on HD10180, the known six exoplanets are well recovered although different orbitals parameters and planetary masses are provided by the new reduced spectra.Comment: 23 pages, 19 figure

Measuring precise radial velocities on individual spectral lines : II. Dependance of stellar activity signal on line depth

Context. Although the new generation of radial-velocity (RV) instruments such as ESPRESSO are expected to reach the long-term precision required to find other earths, the RV measurements are contaminated by some signal from stellar activity. This makes these detections hard. Aims. On real observations, we here demonstrate for the first time the effect of stellar activity on the RV of individual spectral lines. Recent studies have shown that this is probably the key for mitigating this perturbing signal. Results. We estimate that at least 89% of the lines that appear in the spectrum of alpha Cen B for which we measure a reliable RV are correlated with the stellar activity signal. This can be interpreted as those lines being sensitive to the inhibition of the convective blueshift observed in active regions. Because the velocity of the convective blueshift increases with physical depth inside the stellar atmosphere, we find that the effect induced by stellar activity on the RV of individual spectral lines is inversely proportional to the line depth. The stellar activity signal can be mitigated down to 0.8-0.9 m/s, either by selecting lines that are less sensitive to activity or by using the difference between the RV of the spectral lines that are formed at different depths in the stellar atmosphere as an activity proxy. Conclusions. This paper shows for the first time that based on real observations of solar-type stars, it is possible to measure the RV effect of stellar activity on the RV of individual spectral lines. Our results are very promising and demonstrate that analysing the RV of individual spectral lines is probably one of the solutions to mitigate stellar activity signal in RV measurements down to a level enabling the detection of other earths.Comment: 9 pages (main text), 9 pages (appendices), 9 figures (main text

A CHEOPS-enhanced view of the HD 3167 system

Much remains to be understood about the nature of exoplanets smaller than Neptune, most of which have been discovered in compact multi-planet systems. With its inner ultra-short period planet b aligned with the star and two larger outer planets d-c on polar orbits, the multi-planet system HD 3167 features a peculiar architecture and offers the possibility to investigate both dynamical and atmospheric evolution processes. To this purpose we combined multiple datasets of transit photometry and radial velocimetry (RV) to revise the properties of the system and inform models of its planets. This effort was spearheaded by CHEOPS observations of HD 3167b, which appear inconsistent with a purely rocky composition despite its extreme irradiation. Overall the precision on the planetary orbital periods are improved by an order of magnitude, and the uncertainties on the densities of the transiting planets b and c are decreased by a factor of 3. Internal structure and atmospheric simulations draw a contrasting picture between HD 3167d, likely a rocky super-Earth that lost its atmosphere through photo-evaporation, and HD 3167c, a mini-Neptune that kept a substantial primordial gaseous envelope. We detect a fourth, more massive planet on a larger orbit, likely coplanar with HD 3167d-c. Dynamical simulations indeed show that the outer planetary system d-c-e was tilted, as a whole, early in the system history, when HD 3167b was still dominated by the star influence and maintained its aligned orbit. RV data and direct imaging rule out that the companion that could be responsible for the present-day architecture is still bound to the HD 3167 system. Similar global studies of multi-planet systems will tell how many share the peculiar properties of the HD 3167 system, which remains a target of choice for follow-up observations and simulations

The mean longitudinal magnetic field and its uses in radial-velocity surveys

Funding: This work has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation under grants 51NF40_182901 and 51NF40_205606. XD acknowledges the support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement SCORE No 851555) and from the Swiss National Science Foundation under the grant SPECTRE (No 200021_215200). RDH is funded by the UK Science and Technology Facilities Council (STFC)’s Ernest Rutherford Fellowship (grant number ST/V004735/1). SD acknowledges support from the STFC consolidated grant number ST/V000721/1. BSL is funded by a UK Science and Technology Facilities Council (STFC) studentship (ST/V506679/1). XD acknowledges funding by the French National Research Agency in the framework of the Investissements d’Avenir program (ANR-15-IDEX-02), through the funding of the ‘Origin of Life’ project of the Grenoble-Alpes University. ACC acknowledges support from STFC consolidated grant numbers ST/R000824/1 and ST/V000861/1. BK acknowledges funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 865624, GPRV) SHS gratefully acknowledges support from NASA XRP grant 80NSSC21K0607 and NASA EPRV grant 80NSSC21K1037. NP acknowledges the Scholarship program funded by the Knut and Alice Wallenberg Foundation.This work focuses on the analysis of the mean longitudinal magnetic field as a stellar activity tracer in the context of small exoplanet detection and characterization in radial-velocity (RV) surveys. We use Solar Dynamics Observatory/Helioseismic and Magnetic Imager filtergrams to derive Sun-as-a-star magnetic field measurements, and show that the mean longitudinal magnetic field is an excellent rotational period detector and a useful tracer of the solar magnetic cycle. To put these results into context, we compare the mean longitudinal magnetic field to three common activity proxies derived from HARPS-N Sun-as-a-star data: the full width at half-maximum, the bisector span, and the S-index. The mean longitudinal magnetic field does not correlate with the RVs and therefore cannot be used as a one-to-one proxy. However, with high cadence and a long baseline, the mean longitudinal magnetic field outperforms all other considered proxies as a solar rotational period detector, and can be used to inform our understanding of the physical processes happening on the surface of the Sun. We also test the mean longitudinal magnetic field as a ‘stellar proxy’ on a reduced solar data set to simulate stellar-like observational sampling. With a Gaussian Process regression analysis, we confirm that the solar mean longitudinal magnetic field is the most effective of the considered indicators, and is the most efficient rotational period indicator over different levels of stellar activity. This work highlights the need for polarimetric time series observations of stars.Peer reviewe

The CORALIE survey for southern extrasolar planets XIX. Brown dwarfs and stellar companions unveiled by radial velocity and astrometry

A historical planet-search on a sample of 1647 nearby southern main sequence stars has been ongoing since 1998 with the CORALIE spectrograph at La Silla Observatory, with a backup subprogram dedicated to the monitoring of binary stars. We review 25 years of CORALIE measurements and search for Doppler signals consistent with stellar or brown dwarf companions to produce an updated catalog of both known and previously unpublished binary stars in the planet-search sample, assessing the binarity fraction of the stellar population and providing perspective for more precise planet-search in the binary sample. We perform new analysis on the CORALIE planet-search sample radial velocity measurements, searching for stellar companions and obtaining orbital solutions for both known and new binary systems. We perform simultaneous radial velocity and proper motion anomaly fits on the subset of these systems for which Hipparcos and Gaia astrometry measurements are available, obtaining accurate estimates of true mass for the companions. We find 218 stars in the CORALIE sample to have at least one stellar companion, 130 of which are not yet published in the literature and for which we present orbital solutions. The use of proper motion anomaly allow us to derive true masses for the stellar companions in 132 systems, which we additionally use to estimate stability regions for possible planetary companions on circumprimary or circumbinary orbits. Finally, we produce detection limit maps for each star in the sample and obtain occurrence rates of $0.43^{+0.23}_{-0.11}\%$ and $12.69^{+0.87}_{-0.77}\%$ for brown dwarf and stellar companions respectively in the CORALIE sample.Comment: 34 pages, 15 figures, accepted for publication in A&

HD152843 b & c: the masses and orbital periods of a sub-Neptune and a super-puff Neptune

Funding: BN would like to acknowledge support from STFC Consolidated Grant ST/S000488/1 (PI Balbus), and the University of Southern Queensland SAGE program. BN would also like to thank Dr Su Wang for her insights on planetary system dynamics. ACC acknowledges support from STFC consolidated grant number ST/V000861/1, and UKSA grant number ST/X002217/1. The HARPS-N project has been funded by the Prodex Program of the Swiss Space Office (SSO), the Harvard University Origins of Life Initiative (HUOLI), the Scottish Universities Physics Alliance (SUPA), the University of Geneva, the Smithsonian Astrophysical Observatory (SAO), and the Italian National Astrophysical Institute (INAF), the University of St Andrews, Queen’s University Belfast, and the University of Edinburgh.We present the characterisation of the two transiting planets around HD 152843 (TOI 2319, TIC 349488688) using an intensive campaign of HARPS-N radial velocities, and two sectors of TESS data. These data reveal a unique and fascinating system: HD 152843 b and c have near equal masses of around 9 M⊕ but differing radii of 3.05 ± 0.11 R⊕ and 5.94 -0.16 +0.18 R⊕, respectively, and orbital periods of 11.62071-0.000106+9.6e-05 days and 19.502104 -8.5e-05 +7.4e-05 days. This indicates that HD 152843 c is in the lowest fifth-percentile in density of the known exoplanet population, and has the longest orbital period among these low density planets. Further, HD 152843 c’s radius places it in the 'Saturn valley', the observed lack of planets larger than Neptune, but smaller than Saturn. The orbital periods of these planets indicate they are near a 5 : 3 mean motion resonance, indicating the possibility of transit timing variations, and hints at the possibility of interaction with a third planet at some point in the evolution of this system. Further, the brightness of the host star and the low density of HD 152843 c make it a key target for atmospheric characterisation.Peer reviewe

HD152843 b & c: the masses and orbital periods of a sub-Neptune and a super-puff Neptune

We present the characterisation of the two transiting planets around HD 152843 (TOI 2319, TIC 349488688) using an intensive campaign of HARPS-N radial velocities, and two sectors of TESS data. These data reveal a unique and fascinating system: HD 152843 b and c have near equal masses of around 9 M$_\oplus$ but differing radii of $3.05 \pm 0.11$ R$_\oplus$ and $5.94^{+0.18}_{-0.16}$ R$_\oplus$ , respectively, and orbital periods of $11.62071^{+9.6e-05}_{-0.000106}$ days and $19.502104^{+7.4e-05}_{-8.5e-05}$ days. This indicates that HD 152843 c is in the lowest fifth percentile in density of the known exoplanet population, and has the longest orbital period among these low density planets. Further, HD 152843 c's radius places it in the Saturn valley, the observed lack of planets larger than Neptune, but smaller than Saturn. The orbital periods of these planets indicate they are near a 5:3 mean motion resonance, indicating the possibility of transit timing variations, and hints at the possibility of interaction with a third planet at some point in the evolution of this system. Further, the brightness of the host star and the low density of HD 152843 c make it a key target for atmospheric characterisation.Comment: Submitted for review to the Monthly Notices of the Royal Astronomical Society. 13 pages, 16 figure