TOI-3288 b and TOI-4666 b: two gas giants transiting low-mass stars characterised by NIRPS

TL;DR

This work confirms and characterises two gas giants transiting low-mass stars, TOI-3288 b and TOI-4666 b, using a joint HARPS/NIRPS RV approach augmented by a novel telluric-mitigation post-processing. The NIRPS GTO GATOS program targets TESS-identified giant planets around M dwarfs, employing careful vetting, high-resolution spectroscopy, and multi-band photometry to derive precise masses, radii, and orbital parameters. The results reveal trends of decreasing planetary mass with cooler host spectral type, a metallicity-enrichment effect for more massive planets, and a higher binarity fraction among low-mass-star hosts, consistent with formation theories invoking metal-rich disks and dynamical perturbations. These findings, together with planned expansions of the sample, underscore the value of near-infrared RVs and high-angular-resolution imaging for characterizing and understanding giant planets around the faint end of the main sequence.

Abstract

Gas giant planets orbiting low-mass stars are uncommon outcomes of planet formation. Increasing the sample of well-characterised giants around early M dwarfs will enable population-level studies of their properties, offering valuable insights into their formation and evolutionary histories. We aim to characterise giant exoplanets transiting M dwarfs identified by TESS. High-resolution spectroscopic data are obtained in the optical and nIR, combining HARPS and NIRPS. We derive RVs via the cross-correlation function and implement a novel post-processing procedure to further mitigate telluric contamination in the nIR. The resulting RVs are jointly fit with TESS and ground-based photometry to derive the orbital and physical parameters of the systems. We confirm two gas giants transiting the low-mass stars TOI-3288 A (K9V) and TOI-4666 (M2.5V). TOI-3288 A hosts a Hot Jupiter with a mass of $2.11\pm0.08~M_{\rm Jup}$ and a radius of $1.00 \pm 0.03~R_{\rm Jup}$, with an orbital period of 1.43 days ($T_{\rm eq} = 1059 \pm 20~{\rm K}$). TOI-4666 hosts a $0.70_{-0.06}^{+0.05}~M_{\rm Jup}$ warm Jupiter ($T_{\rm eq} = 713 \pm 14~{\rm K}$) with a radius of $1.11 \pm 0.04~R_{\rm Jup}$, and an orbital period of 2.91 days. We identify a decrease in planetary mass with spectral type, where late M dwarfs host less massive giant planets than early M dwarfs. More massive gas giants that deviate from this trend are preferentially hosted by more metal-rich stars. Furthermore, we find an increased binarity fraction among low-mass stars hosting gas giants, which may play a role in enhancing giant planet formation around low-mass stars. The observed population trends agree with theoretical expectations, where higher metallicity can compensate for lower disk masses, and wide binary systems may influence planet formation and migration through Kozai-Lidov cycles or disk instabilities.

Figures (23)

• Figure 1: HR diagram of all Gaia DR3 nearby stars with a parallax $\pi \geq 5$ mas, using the broad-band $G$ magnitude versus the colour $G_{\rm BP}$ (blue) minus $G_{\rm RP}$ (red). The colours indicate $\log(g)$, stars without a $\log (g)$ measurement are shown in grey. The six targets presented in this paper as part of the NIRPS-GTO giants subprogram are overplotted (black outlined circles), along with five stars identified as giant stars using this method. TOI-3288 and TOI-4666 (black outlined squares), hosting gas giants, are visible on the main sequence. This figure can be generated using Gaia-HR, available at https://github.com/ygcfrensch/Gaia-HR.
• Figure 2: The $5\,\sigma$ magnitude contrast curves as observed by Zorro in both filters as a function of the angular separation out to $1.2\arcsec$. The insets show the reconstructed speckle images of TOI-3288 with a $1\,\arcsec$ scale bar. TOI-3288 was found to have no close companions from the diffraction limit ($0.02\,\arcsec$) out to $1.2\,\arcsec$ to within the magnitude contrast levels achieved.
• Figure 3: Normalized CCF of TOI-3288. The upper panel shows the full CCF, while the lower panel shows the CCF with the BERV window ($\pm \frac{1}{2}{\rm FWHM}$) excluded. The Gaussian fit is indicated in orange (top) and green (bottom). The uncorrected RV is red-shifted by $\sim300$ m/s.
• Figure 4: Phase-folded light curves of TOI-3288 from TESS-SPOC, QLP, and LCO (SSO, SAAO, and CTIO). If a dilution factor was fitted, it is indicated in the legend of each subpanel. Markers with black edges represent data binned per 10 or 15 minutes, as noted in the legend.
• Figure 5: Night-binned RVs of TOI-3288 with the Juliet orbital solution overplotted as a black line. The residuals have an RMS of $\sim 65\,\mathrm{m\,s^{-1}}$. The RVs have been cleaned on telluric contamination. The RVs from before the post-processing steps are shown as gray squares, with dotted lines indicating the difference from the corrected values.