Accretion and inter-cycle variations in the PMS interacting binary AK Sco

Abstract

There are only a handful of known short-period pre-main sequence spectroscopic binaries with significant accretion rates (Class II sources). AK Sco stands out in this list because the system is composed of two equal mass F5 stars in a highly eccentric orbit thus both stars get as close as 11 stellar radii at periastron passage. This configuration is optimal for accretion studies because enhance accretion events can be precisely timed at periastron passage. In this work, we present the results from the monitoring of the AK Sco system with Hubble during three consecutive periastron passages. These data provide a unique dataset to spectroscopically characterize accretion and evaluate the inter-cycle variability of the system. Clear evidence of accretion rate enhancement was observed in cycle1 and 3: the blueing of the near UV continuum, the sudden flux increase of important accretion tracers, such as the N V, Si IV and C IV lines, and also of neutral/singly ionized species such as O I and C II. Also, variations in the Si III]/C III] ratio reveals an enhancement of the electron density by an order of magnitude during the periastron passage. Moreover, in cycle 3, the spectral resolution of the observations obtained with the Cosmic Origins Spectrograph enabled discerning that the flow was channelled preferentially into one of the two components. The most remarkable feature in the cycle-to-cycle variations was the detection of a notable increase of the UV flux from cycle 1 to 2 that was not accompanied by enhanced accretion signatures.

Accepted, October 7th, 2020

References in corpus (22)

• Validation of the new Hipparcos reduction
• An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-Through-UV Curve Morphology
• Photoevaporation of Circumstellar Disks by FUV, EUV and X-ray Radiation from the Central Star
• Three Dimensional MHD Simulation of Circumbinary Accretion Disks: Disk Structures and Angular Momentum Transport
• Photoevaporation of Circumstellar Disks around Young Stars
• Probing for Exoplanets Hiding in Dusty Debris Disks: Disk Imaging, Characterization, and Exploration with HST/STIS Multi-Roll Coronagraphy
• The FUV spectrum of TW Hya. I. Observations of H${}_2$ Fluorescence
• Rotation of Jets from Young Stars: New Clues from the Hubble Space Telescope Imaging Spectrograph
• X-ray irradiated protoplanetary disk atmospheres II: Predictions from models in hydrostatic equilibrium
• Astrophysics of Young Star Binaries
• A Far-ultraviolet Atlas of Low-resolution Hubble Space Telescope Spectra of T Tauri Stars
• Pre-main sequence spectroscopic binaries suitable for VLTI observations
• A Hubble Space Telescope Survey of H2 Emission in the Circumstellar Environments of Young Stars
• Gas Accretion from a Circumbinary Disk
• Modelling Circumbinary Gas Flows in Close T Tauri Binaries
• The effects of X-ray photoionization and heating on the structure of circumstellar discs
• XMM-Newton monitoring of the close pre-main-sequence binary AK Sco. Evidence of tide driven filling of the inner gap in the circumbinary disk
• AK Sco, first detection of a highly disturbed atmosphere in a pre-main sequence close binary
• Polarimetric variations of binary stars. VI. Orbit-induced variations in the pre-main-sequence binary AK Sco
• Constraints to the magnetospheric properties of T Tauri stars - II. The Mg II ultraviolet feature
• Protoplanetary disk shadowing by gas infalling onto the young star AK Sco
• Evidence of hot high velocity photoionized plasma falling on actively accreting T-Tauri Stars