A DISK-BASED DYNAMICAL MASS ESTIMATE FOR THE YOUNG BINARY AK SCO

I Czekala,D J Wilner,K G Stassun,S M Andrews,E L N Jensen,G Torres

doi:10.1088/0004-637x/806/2/154

I Czekala, D J Wilner + Show 4 more

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https://doi.org/10.1088/0004-637x/806/2/154

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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 (pre-MS) 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, , a new dynamical measurement that is independent of stellar evolutionary models. Assuming the disk and binary are co-planar within ?2?, 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-MS model grids. These models predict stellar masses that are marginally consistent with our dynamical measurement (at ?2?), but are systematically high (by ?10%). These same models consistently predict an age of 18 ? 1 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-MS 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.

Highlights

Precise measurements of the physical properties of pre-main sequence stars are fundamental to testing the theoretical predictions of stellar evolution models
We employed a forward-modeling approach that uses a parametric prescription for the disk structure, and simulates the observed visibilities by assuming the molecular level populations are in local thermodynamic equilibrium (LTE) and propagating synthetic photons through the model structure with the RADMC-3D radiative transfer code
We have focused on a detailed modeling analysis of the spatially and spectrally resolved CO J = 2–1 line emission, to map out the disk velocity field and make a dynamical estimate of the total mass of the host binary

Summary

Introduction

Precise measurements of the physical properties of pre-main sequence (pre-MS) stars are fundamental to testing the theoretical predictions of stellar evolution models. The recent review of pre-MS benchmarks by Stassun et al (2014) identified only 21 lowmass EBs that have sufficiently precise measurements of their physical parameters to be suitable for testing evolutionary models. The same review presented new evidence that many of the benchmark pre-MS EBs may have their temperatures and/or radii altered by the influence of tertiary companions which, while representing interesting physics in their own right, render them less suitable to direct tests of the evolutionary models that do not include such effects (see Gómez Maqueo Chew et al 2012)

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