Young, active radio stars in the AB Doradus moving group

Abstract

Context. Precise determination of stellar masses is necessary to test the validity of pre-main-sequence (PMS) stellar evolutionary models, whose predictions are in disagreement with measurements for masses below 1.2 M ⊙ . To improve such a test, and based on our previous studies, we selected the AB Doradus moving group (AB Dor-MG) as the best-suited association on which to apply radio-based high-precision astrometric techniques to study binary systems. Aims. We seek to determine precise estimates of the masses of a set of stars belonging to the AB Dor-MG using radio and infrared observations. Methods. We observed in phase-reference mode with the Very Large Array (VLA) at 5 GHz and with the European VLBI Network (EVN) at 8.4 GHz the stars HD 160934, EK Dra, PW And, and LO Peg. We also observed some of these stars with the near-infrared CCD AstraLux camera at the Calar Alto observatory to complement the radio observations. Results. We determine model-independent dynamical masses of both components of the star HD 160934, A and c, which are 0.70±0.07 M ⊙ and 0.45±0.04 M ⊙ , respectively. We revised the orbital parameters of EK Dra and we determine a sum of the masses of the system of 1.38±0.08 M ⊙ . We also explored the binarity of the stars LO Peg and PW And. Conclusions. We found observational evidence that PMS evolutionary models underpredict the mass of PMS stars by 10%−40%, as previously reported by other authors. We also inferred that the origin of the radio emission must be similar in all observed stars, that is, extreme magnetic activity of the stellar corona that triggers gyrosynchrotron emission from non-thermal, accelerated electrons.

The SPOTS Models: A Grid of Theoretical Stellar Evolution Tracks and Isochrones for Testing the Effects of Starspots on Structure and Colors

“…Magnetic fields are known to inhibit convection in sunspots, and they could also clearly impact energy transport, and by extension stellar structure, in heavily spotted stars. A number of papers have incorporated models of magnetic effects into stellar interiors models to establish their predicted influences (e.g Spruit 1982;Spruit & Weiss 1986;Mullan & MacDonald 2001;Chabrier et al 2007;Feiden & Chaboyer 2013;Jackson & Jeffries 2014;Somers & Pinsonneault 2015a;Azulay et al 2017, and references therein). These papers have provided explanations for a number of outstanding problems with pre-main sequence stars.…”

Dynamical masses of M-dwarf binaries in young moving groups

“…In addition, many M-dwarf binaries have now been monitored for well over a decade with high-resolution imaging (Janson et al 2014b), which means that orbital constraints will soon become available even for moderately wide binaries of ∼40 yr orbital periods. The fact that many of these young binaries are also significantly radio emitting (Azulay et al 2017a) is another factor that brings additional potential since it allows for closer pairs to be resolved with VLBI than is possible at visible/infrared wavelengths and can provide absolute astrometry allowing for mass ratio determinations and independent parallax estimations. Furthermore, VLBI imaging is relatively insensitive to day-night cycles, and could thus potentially reach orbital epochs that are unattainable to visible/infrared imaging during the times of the year when the target is on the dayside of Earth.…”

“…Another hypothesis, as argued in e.g. Azulay et al (2017a), is that the high stellar activity for low-mass stars alters the radii and effective temperatures of the components from the theoretical expectations. In this scenario, however, it is somewhat surprising that the deviation from the isochrones is largest among the oldest stars in our sample, even though such stars are statistically the least active.…”

“…One of the scientific aims of this study is to compare the results for AB Dor Ba/Bb with other low-mass YMG binaries in the literature; therefore, we have collected the relevant isochronal quantities for all low-mass components in YMG binaries with good individual dynamical mass constraints that we are aware of. Masses, and in some cases luminosities, come from Close et al (2007) Hormuth et al (2007) is based on the distance from the original Hipparcos reduction, which is inconsistent with newer values such as the VLBI parallax in Azulay et al (2017a); for these cases we calculated an L bol in the same way as described for AB Dor Ba/Bb above, with small variations depending on which unresolved and resolved photometric data points were available for each target. We find log L bol /L of −1.56 ± 0.07 for V 343 Nor B, −0.97 ± 0.02 for HD 160934 A, −1.40 ± 0.03 for HD 160934 C, and −2.49 ± 0.02 for both J1036 B and C.…”

“…HD 160934 has been identified as an ABMG member (López-Santiago et al 2006); this classification has varied substantially in the literature (see e.g. Azulay et al 2017a) and across different versions of the BANYAN tool (e.g. Gagné et al 2014Gagné et al , 2018, and we thus consider it an unclear case.…”