The HADES RV programme with HARPS-N at TNG

Abstract

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.

A Collage of Small Planets from the Lick–Carnegie Exoplanet Survey: Exploring the Super-Earth and Sub-Neptune Mass Regime*

“…Having discovered these planets via the radial velocity method, we investigate whether they might make good candidates for additional detection/characterization methods such as transit, astrometric, and direct imaging observations. We calculate transit probabilities using Winn (2010), and astrometric semi-amplitudes and maximum projected separations using Perryman (2011). We find that due to the combination of low mass and relatively short orbital periods, none of the five planets detailed in this work are expected to produce an astrometric semi-amplitude larger than 5 micro-arcseconds placing them all beyond the reach of Gaia's detection threshold.…”

HADES RV programme with HARPS-N at TNG

“…Previous works also suggest that the gas-giant planetmetallicity correlation is also apparent in the M dwarf sample but the correlation is not present when Neptunian and smaller planets are considered (Bonfils et al 2007;Johnson & Apps 2009;Schlaufman & Laughlin 2010;Rojas-Ayala et al 2012;Terrien et al 2012;Neves et al 2013;Courcol et al 2016). More recently, Pinamonti et al (2019) found a moderate-to-weak dependence of the planetary minimum mass on stellar metallicity for M dwarfs.…”

Unbound Close Stellar Encounters in the Solar Neighborhood

“…There is one other pair in Figure 8 that doesn't match well with the bound pairs -that containing the planet hosting star GJ 685 (Pinamonti et al 2019). This falls within the intermediate range we identified in § 2 as likely contaminated by hierarchical triples.…”