The red dwarf pair GJ65 AB: inflated, spinning twins of Proxima

Abstract

The nearby red dwarf binary GJ65 AB (UV+BL Ceti, M5.5Ve+M6Ve) is a cornerstone system to probe the physics of very low-mass stars. The radii of the two stars are currently known only from indirect photometric estimates, however, and this prevents us from using GJ65 AB as calibrators for the mass-radius (M-R) relation. We present new interferometric measurements of the angular diameters of the two components of GJ65 with the VLTI/PIONIER instrument in the near-infrared H band: θ UD (A) = 0.558 ± 0.008 ± 0.020 mas and θ UD (B) = 0.539 ± 0.009 ± 0.020 mas. They translate into limb-darkened angular diameters of θ LD (A) = 0.573 ± 0.021 mas and θ LD (B) = 0.554 ± 0.022 mas. Based on the known parallax, the linear radii are R(A) = 0.165 ± 0.006 R and R(B) = 0.159 ± 0.006 R (σ(R)/R = 4%). We searched for the signature of flares and faint companions in the interferometric visibilities and closure phases, but we did not identify any significant signal. We also observed GJ65 with the VLT/NACO adaptive optics and refined the orbital parameters and infrared magnitudes of the system. We derived masses for the two components of m(A) = 0.1225 ± 0.0043 M and m(B) = 0.1195 ± 0.0043 M (σ(m)/m = 4%). To derive the radial and rotational velocities of the two stars as well as their relative metallicity with respect to Proxima, we also present new individual UVES high-resolution spectra of the two components. Placing GJ65 A and B in the mass-radius diagram shows that their radii exceed expectations from recent models by 14 ± 4% and 12 ± 4% , respectively. Following previous theories, we propose that this discrepancy is caused by the inhibition of convective energy transport by a strong internal magnetic field generated by dynamo effect in these two fast-rotating stars. A comparison with the almost identical twin Proxima, which is rotating slowly, strengthens this hypothesis because the radius of Proxima does not appear to be inflated compared to models.

The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

“…This value agrees within error bars with the inclination angles which follow from the above v sin i, the rotation periods determined by Barnes et al (2016), and the radii measured by Kervella et al (2016).…”

“…The binary system was observed on the night of Sep 21, 2013, when 4 spectra of GJ65 A and 4 spectra of GJ65 B were recorded, and on the night of Sep 24, 2013, when another 4 spectra of GJ65 B were obtained. According to the orbital parameters derived by Kervella et al (2016), the projected separation of the components was 2.2 ′′ at the time of ESPaDOnS observations, implying that the stars were well isolated by the 1.6…”

“…Here we take advantage of the recent refinement of the fundamental parameters of GJ65 by interferometry and highcontrast imaging (Kervella et al 2016) and an accurate spectroscopic determination of the rotational properties of the components (Barnes et al 2016) to characterise magnetic field strength and topology in both stars. These direct constraints on the surface stellar magnetic field are essential for understanding dynamo action in fully convective stars and interpreting activity patterns of the GJ65 components.…”

Proxima’s orbit around α Centauri

“…Through an analogy with the Sun, they estimate an upper limit of +33 m s −1 , and variability with the magnetic field strength and location of stellar spots. The presence of magnetic field is likely to inhibit the convective flow (Kervella et al 2016a), hence affecting the convective pattern at the surface. The velocity of Proxima, however, is stable at a level of a few m s −1 over long periods (Barnes et al 2014;Anglada-Escudé et al 2016).…”

First light for GRAVITY: Phase referencing optical interferometry for the Very Large Telescope Interferometer

“…The well studied M-dwarf binary GJ 65 AB (BL Cet+ UV Cet, WDS J01388-1758AB) consists of two very low mass M5.5Ve and M6Ve main-sequence dwarfs with masses of M(A) = 0.123 M and M(B) = 0.120 M (Kervella et al 2016). The two stars orbit each other with a period of P = 26.28 yr and a semi-major axis a = 2.05 .…”