The fulcrum wavelength of young stellar objects -- the case of LRLL 31

TL;DR

This study investigates the cause and conditions of the fulcrum wavelength in young stellar objects with see-saw IR variability, using radiative transfer models to analyze LRLL 31's disc structure and inclination effects.

Contribution

It identifies the inner rim height change as the likely cause of the fulcrum wavelength and determines its dependence on disc inclination and vertical density profile, especially the exponent β.

Findings

Fulcrum wavelength occurs at high inclinations where the line of sight intersects the disc.

The fulcrum wavelength is most strongly influenced by the vertical density exponent β.

Only flatter discs (β < 1.2) produce a fulcrum wavelength beyond the 10 μm silicate feature.

Abstract

A small subset of young stellar objects (YSOs) exhibit "see-saw" temporal variations in their mid-infrared SED; as the flux short-ward of a fulcrum wavelength ($\lambda_{f}$) increases the flux long-wards of this wavelength decreases (and vice-versa) over timescales of weeks to years. While previous studies have shown that an opaque, axisymmetric occulter of variable height can cause this behaviour in the SED of these objects, the conditions under which a single $\lambda_{f}$ occurs have not previously been determined, nor the factors determining its value. Using radiative transfer modelling, we conduct a parametric study of the exemplar of this class, LRLL 31 to explore this phenomenon, and confirm that the cause of this flux variation is likely due to the change in height of the optically thick inner rim of the accretion disc at the dust sublimation radius, or some other phenomenon which results in a similar appearance. We also determine that a fulcrum wavelength only occurs for high inclinations, where the line of sight intersects the accretion disc. Accepting that the disc of LRLL 31 is highly inclined, the inner rim radius, radial and vertical density profiles are independently varied to gauge what effect this had on $\lambda_{f}$ and its position relative to the silicate feature near $10 \mu$m. While $\lambda_{f}$ is a function of each of these parameters, it is found to be most strongly dependent on the vertical density exponent $\beta$. All other factors being held constant, only for flatter discs ($\beta < 1.2$) did we find a $\lambda_{f}$ beyond the silicate feature.