NGTS-EB-7, an eccentric, long-period, low-mass eclipsing binary

TL;DR

This paper reports the discovery of NGTS-EB-7, a long-period, eccentric eclipsing binary with a low-mass M dwarf secondary, providing valuable data to test stellar evolution models for fully convective stars.

Contribution

It presents the first detailed characterization of a long-period, eccentric M dwarf eclipsing binary with a well-constrained secondary star, expanding the sample for testing stellar models.

Findings

Secondary star has a radius of 0.125 R_sun and a mass of 0.096 M_sun.

The system has a highly eccentric orbit with e=0.71436.

The binary's properties are constrained to better than 5%.

Abstract

Despite being the most common types of stars in the Galaxy, the physical properties of late M dwarfs are often poorly constrained. A trend of radius inflation compared to evolutionary models has been observed for earlier type M dwarfs in eclipsing binaries, possibly caused by magnetic activity. It is currently unclear whether this trend also extends to later type M dwarfs below the convective boundary. This makes the discovery of lower-mass, fully convective, M dwarfs in eclipsing binaries valuable for testing evolutionary models especially in longer-period binaries where tidal interaction between the primary and secondary is negligible. With this context, we present the discovery of the NGTS-EB-7 AB system, an eclipsing binary containing a late M dwarf secondary and an evolved G-type primary star. The secondary star has a radius of $0.125 \pm 0.006 R_\odot$ , a mass of $0.096 \pm 0.004 M_\odot$ and follows a highly eccentric $(e=0.71436 \pm 0.00085)$ orbit every $193.35875 \pm 0.00034$ days. This makes NGTS-EB-7 AB the third longest-period eclipsing binary system with a secondary smaller than $200 M_J$ with the mass and radius constrained to better than $5 \%$. In addition, NGTS-EB-7 is situated near the centre of the proposed LOPS2 southern field of the upcoming PLATO mission, allowing for detection of the secondary eclipse and measurement of the companion`s temperature. With its long-period and well-constrained physical properties - NGTS-EB-7 B will make a valuable addition to the sample of M dwarfs in eclipsing binaries and help in determining accurate empirical mass/radius relations for later M dwarf stars.