Kavli Affiliate: Gregory J. Herczeg
| First 5 Authors: William J. Fischer, Lynne A. Hillenbrand, Gregory J. Herczeg, Doug Johnstone, Ágnes Kóspál
| Summary:
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 sub-millimeter wavelengths are
sensitive to lower-temperature phenomena, 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.
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