91̽»¨

Abstract:

The variability induced by precipitable water vapour (PWV) can heavily affect the accuracy of time-series photometric measurements gathered from the ground, especially in the near-infrared. We present here a novel method of modelling and mitigating this variability, as well as open-sourcing the developed tool – Umbrella. In this study, we evaluate the extent to which the photometry in three common bandpasses (ŕ, í, ź), and SPECULOOS’ primary bandpass (I + ź ), are photometrically affected by PWV variability. In this selection of bandpasses, the I + ź bandpass was found to be most sensitive to PWV variability, followed by ź, í, and ŕ. The correction was evaluated on global light curves of nearby late M- and L-type stars observed by SPECULOOS’ Southern Observatory (SSO) with the I + ź bandpass, using PWV measurements from the LHATPRO and local temperature/humidity sensors. A median reduction in RMS of 1.1 per cent was observed for variability shorter than the expected transit duration for SSO’s targets. On timescales longer than the expected transit duration, where long-term variability may be induced, a median reduction in RMS of 53.8 per cent was observed for the same method of correction.

Abstract:

Context. In the age of JWST, temperate terrestrial exoplanets transiting nearby late-type M dwarfs provide unique opportunities for characterising their atmospheres, as well as searching for biosignature gases. In this context, the benchmark TRAPPIST-1 planetary system has garnered the interest of a broad scientific community. Aims. We report here the discovery and validation of two temperate super-Earths transiting LP 890-9 (TOI-4306, SPECULOOS-2), a relatively low-activity nearby (32 pc) M6V star. The inner planet, LP 890-9 b, was first detected by TESS (and identified as TOI-4306.01) based on four sectors of data. Intensive photometric monitoring of the system with the SPECULOOS Southern Observatory then led to the discovery of a second outer transiting planet, LP 890-9 c (also identified as SPECULOOS-2 c), previously undetected by TESS. The orbital period of this second planet was later confirmed by MuSCAT3 follow-up observations. Methods. We first inferred the properties of the host star by analyzing its Lick/Kast optical and IRTF/SpeX near-infrared spectra, as well as its broadband spectral energy distribution, and Gaia parallax. We then derived the properties of the two planets by modelling multi-colour transit photometry from TESS, SPECULOOS-South, MuSCAT3, ExTrA, TRAPPIST-South, and SAINT-EX. Archival imaging, Gemini-South/Zorro high-resolution imaging, and Subaru/IRD radial velocities also 91̽»¨ our planetary interpretation. Results. With a mass of 0.118 ± 0.002 Ma, a radius of 0.1556 ± 0.0086 Ra, and an effective temperature of 2850 ± 75 K, LP 890-9 is the second-coolest star found to host planets, after TRAPPIST-1. The inner planet has an orbital period of 2.73 d, a radius of 1.320 a0.027+0.053 Ra, and receives an incident stellar flux of 4.09 ± 0.12 Sa. The outer planet has a similar size of 1.367 a0.039+0.055Ra and an orbital period of 8.46 d. With an incident stellar flux of 0.906 ± 0.026 Sa, it is located within the conservative habitable zone, very close to its inner limit (runaway greenhouse). Although the masses of the two planets remain to be measured, we estimated their potential for atmospheric characterisation via transmission spectroscopy using a mass-radius relationship and found that, after the TRAPPIST-1 planets, LP 890-9 c is the second-most favourable habitable-zone terrestrial planet known so far (assuming for this comparison a similar atmosphere for all planets). Conclusions. The discovery of this remarkable system offers another rare opportunity to study temperate terrestrial planets around our smallest and coolest neighbours.

Abstract:

We present the disco v ery of TOI-2136 b, a sub-Neptune planet transiting a nearby M4.5V-Type star every 7.85 d, identified through photometric measurements from the Transiting Exoplanet Survey Satellite ( TESS ) mission. The host star is located 33 pc away with a radius of R = 0.34 ±0.02 R, a mass of 0 . 34 ±0 . 02 M , and an ef fecti ve temperature of 3342 ±100 K. We estimate its stellar rotation period to be 75 ±5 d based on archi v al long-Term photometry. We confirm and characterize the planet based on a series of ground-based multiwavelength photometry, high-Angular-resolution imaging observations, and precise radial velocities from Canada France Hawaii Telescope (CFHT)/SpectroPolarimetre InfraROUge (SPIRou). Our joint analysis reveals that the planet has a radius of 2.20 ±0.17 R and a mass of 6.4 ±2.4 M. The mass and radius of TOI-2136 b are consistent with a broad range of compositions, from water-ice to gas-dominated worlds. TOI-2136 b falls close to the radius valley for M dwarfs predicted by thermally driven atmospheric mass-loss models, making it an interesting target for future studies of its interior structure and atmospheric properties.

Abstract:

We present a study of photometric flares on 154 low-mass (≤0.2 M⊙) objects observed by the SPECULOOS-South Observatory from 2018 June 1 to 2020 March 23. In this sample, we identify 85 flaring objects, ranging in spectral type from M4 to L0. We detect 234 flares in this sample, with energies between 1029.2 and 1032.7 erg, using both automated and manual methods. With this work, we present the largest photometric sample of flares on late-M and ultra-cool dwarfs to date. By extending previous M dwarf flare studies into the ultra-cool regime, we find M5-M7 stars are more likely to flare than both earlier, and later, M dwarfs. By performing artificial flare injection-recovery tests, we demonstrate that we can detect a significant proportion of flares down to an amplitude of 1 per cent, and we are most sensitive to flares on the coolest stars. Our results reveal an absence of high-energy flares on the reddest dwarfs. To probe the relations between rotation and activity for fully convective stars, we extract rotation periods for fast rotators and lower-bound period estimates of slow rotators. These rotation periods span from 2.2 h to 65 d, and we find that the proportion of flaring stars increases for the most fastest rotators. Finally, we discuss the impact of our flare sample on planets orbiting ultra-cool stars. As stars become cooler, they flare less frequently; therefore, it is unlikely that planets around the most reddest dwarfs would enter the 'abiogenesis' zone or drive visible-light photosynthesis through flares alone.