Accretion and Intercycle 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 enhanced 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 data set to spectroscopically characterize accretion and evaluate the intercycle variability of the system. Clear evidence of accretion rate enhancement was observed in cycles 1 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 reveal 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 us to discern that the flow was channeled 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 cycle 2 that was not accompanied by enhanced accretion signatures.