Accretion Variability as a Guide to Stellar Mass Assembly

TL;DR

This paper reviews how variable accretion in young stars, observed across multiple wavelengths, informs our understanding of stellar mass assembly, highlighting diverse phenomena and unresolved issues in the field.

Contribution

It provides a comprehensive review of observational variability phenomena in young stellar objects and discusses their implications for stellar mass growth.

Findings

Diverse lightcurves and spectra challenge simple classification.

Mid-infrared and submillimeter observations probe different physical processes.

Understanding variability is key to unraveling stellar mass assembly.

Abstract

Variable accretion in young stellar objects reveals itself photometrically and spectroscopically over a continuum of timescales and amplitudes. Most dramatic are the large outbursts (e.g., FU Ori, V1647 Ori, and EX Lup type events), but more frequent are the less coherent, smaller burst-like variations in accretion rate. Improving our understanding of time-variable accretion directly addresses the fundamental question of how stars gain their masses. We review variability phenomena, as characterized from observations across the wavelength spectrum, and how those observations probe underlying physical conditions. The diversity of observed lightcurves and spectra at optical and infrared wavelengths defies a simple classification of outbursts and bursts into well-defined categories. Mid-infrared and submillimeter wavelengths are sensitive to lower-temperature phenomena and more embedded, younger sources, and it is currently unclear if observed flux variations probe similar or distinct physics relative to the shorter wavelengths. We highlight unresolved issues and emphasize the value of spectroscopy, multiwavelength studies, and ultimately patience in using variable accretion to understand stellar mass assembly.