DASCH on KU Cyg: a ~5 year dust accretion event in ~1900

TL;DR

This paper reports the discovery of a five-year dust accretion event around 1900 in the binary system KU Cyg, observed through a century-long light curve, revealing unique dust dynamics different from typical R Coronae Borealis stars.

Contribution

It presents the first detailed analysis of a long-duration dust accretion event in KU Cyg using DASCH data, highlighting its distinct light curve shape and potential dust origin mechanisms.

Findings

A ~5-year fading and brightening event around 1900 was identified.

The event's light curve differs from RCB star dust processes, indicating different dust dynamics.

Dust extinction likely caused the fading, with quick brightening due to dust evaporation.

Abstract

KU Cyg is an eclipsing binary consisting of a F-type star accreting through a large accretion disk from a K5III red giant. Here we present the discovery of a 5-yr dip around 1900 found from its 100 yr DASCH (Digital Access to a Sky Century at Harvard) light curve. It showed a ~0.5 mag slow fading from 1899 to 1903, and brightened back around 1904 on a relatively shorter timescale. The light curve shape of the 1899-1904 fading-brightening event differs from the dust production and dispersion process observed in R Coronae Borealis (RCB) stars, which usually has a faster fading and slower recovery, and for KU Cyg is probably related to the accretion disk surrounding the F star. The slow fading in KU Cyg is probably caused by increases in dust extinction in the disk, and the subsequent quick brightening may be due to the evaporation of dust transported inwards through the disk. The extinction excess which caused the fading may arise from increased mass transfer rate in the system, or from dust clump ejections from the K giant.