The Accretion History of EX Lup: A Century of Bursts, Outbursts, and Quiescence

TL;DR

This study reconstructs a century-long accretion history of EX Lup, revealing two types of bursts with distinct characteristics and suggesting different underlying mechanisms, while analyzing their impact on stellar mass and surface features.

Contribution

It introduces a novel method to estimate historical accretion rates from optical brightness correlations and characterizes two distinct burst classes in EX Lup's accretion history.

Findings

Major outbursts reach accretion rates of ~10^{-7} M_sun/yr

Characteristic bursts reach accretion rates of ~10^{-8} M_sun/yr

Total accreted mass during bursts is about twice that during quiescence

Abstract

EX Lup is the archetype for the class of young stars that undergoes repeated accretion outbursts of $\sim 5$ mag at optical wavelengths and that last for months. Despite extensive monitoring that dates back 130 years, the accretion history of EX Lup remains mostly qualitative and has large uncertainties. We assess historical accretion rates of EX Lup by applying correlations between optical brightness and accretion, developed on multi-band magnitude photometry of the $\sim 2$ mag optical burst in 2022. Two distinct classes of bursts occur: major outbursts ($\Delta V\sim5$ mag) have year-long durations, are rare, reach accretion rates of $\dot{M}_{\rm acc}\sim10^{-7}~M_\odot~{\rm yr^{-1}}$ at peak, and have a total accreted mass of around 0.1 Earth masses. The characteristic bursts ($\Delta V\sim2$ mag) have durations of $\sim 2-3$ months, are more common, reach accretion rates of $\dot{M}_{\rm acc}\sim10^{-8}~M_\odot~{\rm yr^{-1}}$ at peak, and have a total accreted mass of around $10^{-3}$ Earth masses. The distribution of total accreted mass in the full set of bursts is poorly described by a power law, which suggests different driving causes behind the major outburst and characteristic bursts. The total mass accreted during two classes of bursts is around two times the masses accreted during quiescence. Our analysis of the light curves reveals a color-dependent time lag in the 2022 post-burst light curve, attributed to the presence of both hot and cool spots on the stellar surface.