[Paper Review] Accretion Variability as a Guide to Stellar Mass Assembly

William J. Fischer, Lynne A. Hillenbrand+4 more|arXiv (Cornell University)|Mar 21, 2022

Astrophysics and Star Formation Studies59 citations

TL;DR

This is a comprehensive review of time-variable accretion in young stellar objects, outlining variability types, diagnostics across wavelengths, and implications for how stars assemble mass, with emphasis on bursts, outbursts, and the need for multiwavelength spectroscopy to distinguish accretion models.

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.

Motivation & Objective

• Motivate the study of time-variable accretion as central to understanding how stars gain mass.
• Summarize observational diagnostics across the electromagnetic spectrum for accretion processes.
• Discuss the diversity of variability and its implications for disk physics and stellar growth.
• Assess how observed luminosities relate to accretion histories and model constraints.
• Highlight unresolved issues and advocate for multiwavelength spectroscopy to distinguish accretion scenarios.

Proposed method

• Synthesize observations of variable accretion from UV/optical/IR to millimeter wavelengths.
• Discuss physical components (magnetospheric accretion, inner disk, envelope) and how each probes accretion.
• Evaluate luminosity distributions of protostars and pre-main-sequence stars with respect to evolutionary state.
• Compare episodic, burst-like, and secular variability models against observed luminosities and lightcurves.
• Propose best practices for follow-up classification and multiwavelength monitoring.

Experimental results

Research questions

• RQ1 What is the role of variable accretion in building stellar mass and determining observed protostellar properties?
• RQ2 Do all stars experience large-amplitude accretion outbursts, or are these events contingent on special circumstances?
• RQ3 What triggers large accretion events and what mechanisms switch them off or modulate them?
• RQ4 How can variability be used to discriminate between disk instabilities and other accretion processes?
• RQ5 How reliably can we measure accretion luminosity from monochromatic observations, and how does variability affect this?
• RQ6 How does variability influence disk chemistry and planet formation?

Key findings

• Protostellar luminosities span 3–4 orders of magnitude at each evolutionary stage, challenging simple steady-accretion models.
• The historical protostellar luminosity problem has faded with revised lifetimes and sensitivities, though a current protostellar luminosity spread persists.
• Multiple time-dependent accretion scenarios (including episodic, secular, and stochastic variability) can reproduce observed luminosity distributions without requiring episodic accretion exclusively.
• Luminosities alone do not distinguish among models; additional constraints from protostellar masses, radii, and accretion rates are essential.
• Mid-infrared and submillimeter variability probes cooler, embedded regions and may reflect distinct physics from shorter wavelengths; multiwavelength diagnostics are crucial.
• A combination of photometric and spectroscopic monitoring, across wavelengths and over time, is needed to robustly constrain accretion physics and stellar mass assembly.