NGTS-EB-8: A double-lined eclipsing M+M binary discovered by citizen scientists

TL;DR

This work presents NGTS-EB-8, a double-lined eclipsing binary of two nearly equal-mass M-dwarfs, initially flagged as a giant-planet candidate by citizen scientists. Using high-resolution spectra from GHOST and NIRPS, the authors confirm the SB2 nature and perform a joint photometric-RV analysis with four radius-ratio priors, favoring a High $k$-range solution. The derived parameters place both components in the fully convective regime and highlight radius inflation trends in M-dwarfs, while also illustrating how deep all-sky surveys can inadvertently pick up eclipsing binaries masquerading as planets. The study underscores the crucial role of reconnaissance spectroscopy in vetting planet candidates and provides a valuable benchmark for testing stellar evolution models at the low-mass end. Future spectroscopic upgrades and monitoring will further constrain masses, radii, and activity, clarifying the system’s role in radius inflation and planet-occurrence studies around low-mass stars.

Abstract

We report the identification and characterization of a new binary system composed of two near-equal mass M-dwarfs. The binary NGTS-EB-8 was identified as a planet candidate in data from the Next Generation Transit Survey (NGTS) by citizen scientists participating in the Planet Hunters NGTS project. High-resolution spectroscopic observations reveal the system to be a double-lined binary. By modeling the photometric and radial velocity observations, we determine an orbital period of 4.2 days and the masses and radii of both stars to be $M_A=0.250^{+0.005}_{-0.004}$ M$_{\odot}$, $M_B=0.208^{+0.005}_{-0.004}$ M$_{\odot}$, $R_A=0.255^{+0.004}_{-0.005}$ R$_{\odot}$, $R_B=0.233^{+0.006}_{-0.005}$ R$_{\odot}$. We detect Balmer line emission from at least one of the stars but no significant flare activity. We note that both components lie in the fully convective regime of low-mass stars ($<0.35$ M$_{\odot}$), therefore can be a valuable test for stellar evolutionary models. We demonstrate that the photometric observations, speckle imaging and initial radial velocity measurements were unable to identify the true nature of this system and highlight that high-resolution spectroscopic observations are crucial in determining whether systems such as this are in fact binaries.

Figures (15)

• Figure 1: ATLAS photometric data for NGTS-EB-8. $o$-band and $c$-band measurements are shown in orange and cyan, respectively.
• Figure 2: CCF for the first GHOST spectrum of NGTS-EB-8 cross-correlated with the HARPS M5 mask. The solid blue line shows the median CCF. The orange and green dashed lines show the Gaussian fits for the primary and secondary component, respectively
• Figure 3: The CCF for the NIRPS spectrum of NGTS-EB-8 from the night of 2023 April 27 cross-correlated with the NIRPS M2 mask. The solid blue line shows the CCF. The vertical dash-dot pink line shows the barycentric Earth radial velocity (BERV) value that coincides with the sky emission peak. The orange and green dashed lines show the Gaussian fits for the primary and secondary component, respectively.
• Figure 4: Telluric line identification for the Na I region, zoomed on the Na I feature around 819 nm. (Top panel) GHOST spectrum of NGTS-EB-8 shown in black. Regions identified as telluric absorption features are highlighted in red and marked with $\oplus$. (Bottom panel) GHOST spectrum of the telluric standard star (HIP 60985) in blue and the synthetic PHOENIX spectrum that represents an A0V star in orange.
• Figure 5: Relative flux calibration process for the VO feature around 746 nm. (Top panel) GHOST spectrum of telluric standard star in black with the continuum fit shown in blue. (Middle panel) PHOENIX synthetic spectrum with $T_{\rm eff}$$=10000$ K, $\log g$$=3.50$ and [Fe/H]$=-0.5$ shown in black with the continuum fit shown in blue. (Bottom panel) GHOST spectrum of NGTS-EB-8 shown in black prior to the relative flux calibration and shown in blue following the relative flux calibration.