Context. Low-mass stars have been recognised as promising targets in the search for rocky, small planets with the potential of supporting life. As a consequence, Doppler search programmes using high-resolution spectrographs like HARPS or HARPS-N are providing huge quantities of optical spectra of M dwarfs. However, determining the stellar parameters of M dwarfs using optical spectra has proven to be challenging. Aims. We aim to calibrate empirical relationships to determine accurate stellar parameters for early-M dwarfs (spectral types M0-M4.5) using the same spectra as those that are used for radial velocity determinations, without the necessity of acquiring IR spectra or relying on atmospheric models and/or photometric calibrations. Methods. Our methodology consists of using ratios of pseudo-equivalent widths of spectral features as a temperature diagnostic, a technique frequently used in solar-type stars. Stars with effective temperatures obtained from interferometric estimates of their radii are used as calibrators. Empirical calibrations for the spectral type are also provided. Combinations of features and ratios of features are used to derive calibrations for the stellar metallicity. Our methods are then applied to a large sample of M dwarfs that are currently being observed in the framework of the HARPS GTO search for extrasolar planets. The derived temperatures and metallicities are used together with photometric estimates of mass, radius, and surface gravity to calibrate empirical relationships for these parameters. Results. A long list of spectral features in the optical spectra of early-M dwarfs was identified. This list shows that the pseudoequivalent width of roughly 43% of the features is strongly anticorrelated with the effective temperature. The correlation with the stellar metallicity is weaker. A total of 112 temperature sensitive ratios were identified and calibrated over the range 3100−3950 K, providing effective temperatures with typical uncertainties of about 70 K. Eighty-two ratios of pseudo-equivalent widths of features were calibrated to derive spectral types within 0.5 subtypes for stars with spectral types between K7V and M4.5V. We calibrated 696 combinations of the pseudo-equivalent widths of individual features and temperature-sensitive ratios for the stellar metallicity over a metallicity range from −0.54 to +0.24 dex, with estimated uncertainties in the range of 0.07−0.10 dex. We provide our own empirical calibrations for stellar mass, radius, and surface gravity. These parameters depend on the stellar metallicity. For a given effective temperature, lower metallicities predict lower masses and radii as well as higher gravities.

We report the discovery of a system of two super-Earths orbiting the moderately active K-dwarf HD 176986. This work is part of the RoPES RV program of G- and K-type stars, which combines radial velocities (RVs) from the HARPS and HARPS-N spectrographs to search for short-period terrestrial planets. HD 176986 b and c are super-Earth planets with masses of 5.74 and 9.18 M⊕, orbital periods of 6.49 and 16.82 days, and distances of 0.063 and 0.119 AU in orbits that are consistent with circular. The host star is a K2.5 dwarf, and despite its modest level of chromospheric activity (log10 (RHK' = –4.90 ± 0.04), it shows a complex activity pattern. Along with the discovery of the planets, we study the magnetic cycle and rotation of the star. HD 176986 proves to be suitable for testing the available RV analysis technique and further our understanding of stellar activity.

Comparisons of the alignment of exoplanets with a common host star and each other can be used to distinguish among concurrent evolution scenarios for the star and the planets. However, multi-planet systems usually host mini-Neptunes and super-Earths, whose sizes make orbital architecture measurements challenging. We introduce the Rossiter-McLaughlin effect Revolutions (RMR) technique, which can access the spin-orbit angle of small exoplanets by exploiting the full extent of information contained in spectral transit time series. We validated the technique through its application to published HARPS-N data of the mini-Neptune HD 3167c (P = 29.8 days), refining its high sky-projected spin-orbit angle (−108.9−5.5+5.4°), and we applied it to new ESPRESSO observations of the super-Earth HD 3167 b (P = 0.96 days), revealing an aligned orbit (−6.6−7.9+6.6°). Surprisingly different variations in the contrast of the stellar lines occulted by the two planets can be reconciled by assuming a latitudinal dependence of the stellar line shape. In this scenario, a joint fit to both datasets constrains the inclination of the star (111.6−3.3+3.1°) and the 3D spin-orbit angles of HD 3167b (29.5−9.4+7.2°) and HD 3167c (107.7−4.9+5.1°). The projected spin-orbit angles do not depend on the model for the line contrast variations, and so, with a mutual inclination of 102.3−8.0+7.4°, we can conclude that the two planets are on perpendicular orbits. This could be explained by HD 3167b being strongly coupled to the star and retaining its primordial alignment, whereas HD 3167c would have been brought to a nearly polar orbit via secular gravitational interactions with an outer companion. Follow-up observations of the system and simulations of its dynamical evolution are required to search for this companion and explore the likelihood of this scenario. HD 3167 b (R = 1.7 REarth) is the smallest exoplanet with a confirmed spectroscopic Rossiter-McLaughlin signal. The RMR technique opens the way to determining the orbital architectures of the super-Earth and Earth-sized planet populations.

Context. Since 1998, a planet-search around main sequence stars within 50 pc in the southern hemisphere has been underway with the CORALIE spectrograph at La Silla Observatory. Aims. With an observing time span of more than 20 yr, the CORALIE survey is able to detect long-term trends in data with masses and separations large enough to select ideal targets for direct imaging. Detecting these giant companion candidates will allow us to start bridging the gap between radial-velocity-detected exoplanets and directly imaged planets and brown dwarfs. Methods. Long-term precise Doppler measurements with the CORALIE spectrograph reveal radial-velocity signatures of massive planetary companions and brown dwarfs on long-period orbits. Results. In this paper, we report the discovery of new companions orbiting HD 181234, HD 13724, HD 25015, HD 92987 and HD 50499. We also report updated orbital parameters for HD 50499b, HD 92788b and HD 98649b. In addition, we confirm the recent detection of HD 92788c. The newly reported companions span a period range of 15.6–40.4 yr and a mass domain of 2.93–26.77 MJup, the latter of which straddles the nominal boundary between planets and brown dwarfs. Conclusions. We report the detection of five new companions and updated parameters of four known extrasolar planets. We identify at least some of these companions to be promising candidates for imaging and further characterisation.

Aims. The HArps-n red Dwarf Exoplanet Survey (HADES) is providing a major contribution to the widening of the current statistics of low-mass planets, through the in-depth analysis of precise radial-velocity (RV) measurements in a narrow range of spectral sub-types. Using the HARPS-N spectrograph we reach the precision needed to detect small planets with a few earth masses. Our survey is mainly focused on the M-dwarf population of the northern hemisphere. Methods. As part of that program, we obtained RV measurements of Gl 686, an M1 dwarf at d = 8.2 pc. These measurements show a dispersion much in excess of their internal errors. The analysis of data obtained within an intensive observing campaign demonstrates that the excess dispersion is due to a coherent signal with a period of 15.53 d. Almost simultaneous photometric observations were carried out within the APACHE and EXORAP programs to characterize the stellar activity and to distinguish periodic variations related to activity from signals due to the presence of planetary companions, complemented also with ASAS photometric data. We used a Bayesian framework to estimate the orbital parameters and the planet minimum mass, and to properly treat the activity noise. We took advantage of the available RV measurements for this target from other observing campaigns. The analysis of the RV composite time series from the HIRES, HARPS, and HARPS-N spectrographs, consisting of 198 measurements taken over 20 yr, enabled us to address the nature of periodic signals and also to characterize stellar physical parameters (mass, temperature, and rotation). Results. We report the discovery of a super-Earth orbiting at a distance of 0.092 AU from the host star Gl 686. The planet has a minimum mass of 7.1 ± 0.9 M⊕ and an orbital period of 15.532 ± 0.002 d. The analysis of the activity indexes, of the correlated noise through a Gaussian process framework, and of the photometry provides an estimate of the stellar rotation period at 37 d, and highlights the variability of the spot configuration during the long timespan covering 20 yr. The observed periodicities around 2000 d likely point to the existence of an activity cycle.

Context. The Transiting Exoplanet Survey Satellite (TESS) is revolutionising the search for planets orbiting bright and nearby stars. In sectors 3 and 4, TESS observed TOI-402 (TIC-120896927), a bright V=9.1 K1 dwarf also known as HD 15337, and found two transiting signals with periods of 4.76 and 17.18 days and radii of 1.90 and 2.21 R ⊕ , respectively. This star was observed prior to the TESS detection as part of the radial-velocity (RV) search for planets using the HARPS spectrometer, and 85 precise RV measurements were obtained before the launch of TESS over a period of 14 years. Aims. In this paper, we analyse the HARPS RV measurements in hand to confirm the planetary nature of these two signals.Methods. HD 15337 happens to present a stellar activity level similar to the Sun, with a magnetic cycle of similar amplitude and RV measurements that are affected by stellar activity. By modelling this stellar activity in the HARPS radial velocities using a linear dependence with the calcium activity index log(R HK ), we are able, with a periodogram approach, to confirm the periods and the planetary nature of TOI-402.01 and TOI-402.02. We then derive robust estimates from the HARPS RVs for the orbital parameters of these two planets by modelling stellar activity with a Gaussian process and using the marginalised posterior probability density functions obtained from our analysis of TESS photometry for the orbital period and time of transit. Results. By modelling TESS photometry and the stellar host characteristics, we find that TOI-402.01 and TOI-402.02 have periods of 4.75642±0.00021 and 17.1784±0.0016 days and radii of 1.70±0.06 and 2.52±0.11 R ⊕ (precision 3.6 and 4.2%), respectively. By analysing the HARPS RV measurements, we find that those planets are both super-Earths with masses of 7.20±0.81 and 8.79±1.68 M ⊕ (precision 11.3 and 19.1%), and small eccentricities compatible with zero at 2σ. Conclusions. Although having rather similar masses, the radii of these two planets are very different, putting them on different sides of the radius gap. By studying the temporal evolution under X-ray and UV (XUV) driven atmospheric escape of the TOI-402 planetary system, we confirm, under the given assumptions, that photo-evaporation is a plausible explanation for this radius difference. Those two planets, being in the same system and therefore being in the same irradiation environment are therefore extremely useful for comparative exoplanetology across the evaporation valley and thus bring constraints on the mechanisms responsible for the radius gap.

We aim to investigate the presence of signatures of magnetic cycles and rotation on a sample of 71 early M-dwarfs from the HADES RV programme using high-resolution time-series spectroscopy of the Ca II H&K and Hα chromospheric activity indicators, the radial velocity series, the parameters of the cross correlation function and the V -band photometry. We used mainly HARPS-N spectra, acquired over 4 yr, and add HARPS spectra from the public ESO database and ASAS photometry light-curves as support data, extending the baseline of the observations of some stars up to 12 yr. We provide log10(R′HK) measurements for all the stars in the sample, cycle length measurements for 13 stars, rotation periods for 33 stars and we are able to measure the semi-amplitude of the radial velocity signal induced by rotation in 16 stars. We complement our work with previous results and confirm and refine the previously reported relationships between the mean level of chromospheric emission, measured by the log10(R′HK), with the rotation period, and with the measured semi-amplitude of the activity induced radial velocity signal for early M-dwarfs. We searched for a possible relation between the measured rotation periods and the lengths of the magnetic cycle, finding a weak correlation between both quantities. Using previous v sin i measurements we estimated the inclinations of the star’s poles to the line of sight for all the stars in the sample, and estimate the range of masses of the planets GJ 3998 b and c (2.5–4.9 and 6.3–12.5 M⊕), GJ 625 b (2.82 M⊕), GJ 3942 b (7.1–10.0 M⊕) and GJ 15A b (3.1–3.3 M⊕), assuming their orbits are coplanar with the stellar rotation.

Context. Small rocky planets seem to be very abundant around low-mass M-type stars. Their actual planetary population is however not yet precisely understood. Currently, several surveys aim to expand the statistics with intensive detection campaigns, both photometric and spectroscopic. Aims. The HADES program aims to improve the current statistics through the in-depth analysis of accurate radial-velocity (RV) monitoring in a narrow range of spectral sub-types, with the precision needed to detect small planets with a few Earth masses. Methods. We analyse 106 spectroscopic HARPS-N observations of the active M0-type star GJ 685 taken over the past five years. We combine these data with photometric measurements from different observatories to accurately model the stellar rotation and disentangle its signals from genuine Doppler planetary signals in the RV data. We run an MCMC analysis on the RV and activity index time series to model the planetary and stellar signals present in the data, applying Gaussian Process regression technique to deal with the stellar activity signals. Results. We identify three periodic signals in the RV time series, with periods of 9, 24, and 18 d. Combining the analyses of the photometry of the star with the activity indexes derived from the HARPS-N spectra, we identify the 18 d and 9 d signals as activity-related, corresponding to the stellar rotation period and its first harmonic, respectively. The 24 d signal shows no relation to any activity proxy, and therefore we identify it as a genuine planetary signal. We find the best-fit model describing the Doppler signal of the newly found planet, GJ 685 b, corresponding to an orbital period Pb = 24.160−0.047+0.061 d and a minimum mass MP sin i = 9.0−1.8+1.7 M⊕. We also study a sample of 70 RV-detected M-dwarf planets, and present new statistical evidence of a difference in mass distribution between the populations of single- and multi-planet systems, which can shed new light on the formation mechanisms of low-mass planets around late-type stars.