AK Sco, first detection of a highly disturbed atmosphere in a pre-main sequence close binary

TL;DR

This paper reports the first detection of a highly disturbed and dense atmosphere in a pre-main sequence close binary star system, AK Sco, revealing effects of gravitational tides and internal energy release.

Contribution

It provides the first evidence of a highly disturbed atmosphere in a PMS close binary, highlighting the impact of gravitational tides and internal heating.

Findings

Detected a highly disturbed atmosphere with sigma ~ 100 km/s

Measured a dense atmosphere with ne = 1.6e10 cm^-3

Atmospheric heating likely due to internal energy release

Abstract

AK Sco is a unique source: a ~10 Myrs old pre-main sequence spectroscopic binary composed of two nearly equal F5 stars that at periastron are separated by barely eleven stellar radii so, the stellar magnetospheres fill the Roche lobe at periastron. The orbit is not yet circularized (e=0.47) and very strong tides are expected. This makes of AK Sco, the ideal laboratory to study the effect of gravitational tides in the stellar magnetic field building up during pre-main sequence (PMS) evolution. In this letter, the detection of a highly disturbed (sigma ~ 100 km/s) and very dense atmosphere (ne = 1.6e10cm-3) is reported. Significant line broadening blurs any signs of ion belts or bow shocks in the spectrum of the atmospheric plasma. The radiative loses cannot be accounted solely by the dissipation of energy from the tidal wave propagating in the stellar atmosphere; neither by the accreting material. The release of internal energy from the star seems to be the most likely source of the plasma heating. This is the first clear indication of a highly disturbed atmosphere surrounding a pre-main sequence close binary.