The CARMENES search for exoplanets around M dwarfs

Abstract

We announce the discovery of two planets orbiting the M dwarfs GJ 251 (0.360 ± 0.015M⊙) and HD 238090 (0.578 ± 0.021M⊙) based on CARMENES radial velocity (RV) data. In addition, we independently confirm with CARMENES data the existence of Lalande 21185 b, a planet that has recently been discovered with the SOPHIE spectrograph. All three planets belong to the class of warm or temperate super-Earths and share similar properties. The orbital periods are 14.24 d, 13.67 d, and 12.95 d and the minimum masses are 4.0 ± 0.4 M⊕, 6.9 ± 0.9 M⊕, and 2.7 ± 0.3 M⊕ for GJ 251 b, HD 238090 b, and Lalande 21185 b, respectively. Based on the orbital and stellar properties, we estimate equilibrium temperatures of 351.0 ± 1.4 K for GJ 251 b, 469.6 ± 2.6 K for HD 238090 b, and 370.1 ± 6.8 K for Lalande 21185 b. For the latter we resolve the daily aliases that were present in the SOPHIE data and that hindered an unambiguous determination of the orbital period. We find no significant signals in any of our spectral activity indicators at the planetary periods. The RV observations were accompanied by contemporaneous photometric observations. We derive stellar rotation periods of 122.1 ± 2.2 d and 96.7 ± 3.7 d for GJ 251 and HD 238090, respectively. The RV data of all three stars exhibit significant signals at the rotational period or its first harmonic. For GJ 251 and Lalande 21185, we also find long-period signals around 600 d, and 2900 d, respectively, which we tentatively attribute to long-term magnetic cycles. We apply a Bayesian approach to carefully model the Keplerian signals simultaneously with the stellar activity using Gaussian process regression models and extensively search for additional significant planetary signals hidden behind the stellar activity. Current planet formation theories suggest that the three systems represent a common architecture, consistent with formation following the core accretion paradigm.

The 10 parsec sample in the Gaia era

“…For instance, Dressing & Charbonneau (2015) predicted 2.5 ± 0.2 small and close-by planets per M star, so we could expect more than 600 new exoplanets to be discovered, outnumbering the number of stellar and sub-stellar objects within 10 pc. Such an optimistic estimation is in line with the recent discovery of small planets around the closest stars, such as Proxima Centauri (two planets: Anglada-Escudé et al 2016;Damasso et al 2020;Kervella et al 2020), Barnard's star (one planet: Ribas et al 2018), or Lalande 21185 (one planet: Díaz et al 2019;Stock et al 2020) 12 . However, even if these new planets are predicted from Kepler's results, we will probably not detect more than a fraction of them in the coming years for several reasons: (i) Planets with periods close to that of stellar rotation will mostly go undetected; (ii) stellar activity will prevent others from being detected; and (iii) close in planets on highly inclined orbits with small sin i values imply small radial-velocity semi-amplitudes.…”

An ultra-short-period transiting super-Earth orbiting the M3 dwarf TOI-1685

“…where σ GP is the amplitude of the GP given in ms −1 , Γ is the amplitude of the GP sine-squared component, α is the square of the inverse length scale of the exponential component of the GP given in d −2 , τ is the time lag in days, and P rot is the period of the GP-QP component given in days. The GP-QP is a kernel that is widely used to model stellar activity signatures (see, e.g., Faria 2017;Nava et al 2020;Stock et al 2020b;Kemmer et al 2020;Bluhm et al 2020, and references therein). The advantage of using a multiplied kernel is due to its exp-sinesquared factor, which enables the modeling of complex periodic signals.…”

TOI-1201 b: A mini-Neptune transiting a bright and moderately young M dwarf

“…where τ = |t i − t j | is the temporal distance between two points, σ GP is the amplitude of the GP modulation, α is the inverse length-scale of the GP exponential component, Γ is the amplitude of the GP sine-squared component, and P rot is the rotational modulation of the GP quasi-periodic component. We followed the approach as discussed by Stock et al (2020b) in order to set up the priors and thus constrained α to refrain from including the samples that exhibit a "plateau-like" behavior (see Fig. 6 in Stock et al 2020b).…”

“…We followed the approach as discussed by Stock et al (2020b) in order to set up the priors and thus constrained α to refrain from including the samples that exhibit a "plateau-like" behavior (see Fig. 6 in Stock et al 2020b). Upon visual inspection of the α-versus-P rot correlation plot when keeping α unconstrained (Fig.…”