A Disk-based Dynamical Mass Estimate for the Young Binary AK Sco

TL;DR

This paper uses ALMA observations of the AK Sco binary system to measure its mass dynamically from disk rotation, providing a model-independent estimate that challenges some pre-main sequence models and offers a new method for stellar characterization.

Contribution

It introduces a disk-based dynamical mass measurement technique for young binaries using ALMA data, independent of stellar evolutionary models.

Findings

Dynamical mass of AK Sco is 2.49 ± 0.10 M_sun.

Pre-main sequence models overestimate stellar mass by ~10%.

AK Sco's age is estimated at 18 ± 1 Myr, older than expected for its disk presence.

Abstract

We present spatially and spectrally resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations of gas and dust in the disk orbiting the pre-main sequence binary AK Sco. By forward-modeling the disk velocity field traced by CO J=2-1 line emission, we infer the mass of the central binary, $M_\ast = 2.49 \pm 0.10~M_\odot$, a new dynamical measurement that is independent of stellar evolutionary models. Assuming the disk and binary are co-planar within $\sim$2{\deg}, this disk-based binary mass measurement is in excellent agreement with constraints from radial velocity monitoring of the combined stellar spectra. These ALMA results are also compared with the standard approach of estimating masses from the location of the binary in the Hertzsprung-Russell diagram, using several common pre-main sequence model grids. These models predict stellar masses that are marginally consistent with our dynamical measurement (at $\sim 2\,\sigma$), but are systematically high (by $\sim$10%). These same models consistently predict an age of $18\pm1$ Myr for AK Sco, in line with its membership in the Upper Centaurus-Lupus association but surprisingly old for it to still host a gas-rich disk. As ALMA accumulates comparable data for large samples of pre-main sequence stars, the methodology employed here to extract a dynamical mass from the disk rotation curve should prove extraordinarily useful for efforts to characterize the fundamental parameters of early stellar evolution.