Variability of Disk Emission in Pre-Main Sequence and Related Stars IV. Investigating the Structural Changes in the Inner Disk Region of MWC 480

TL;DR

This study analyzes multi-epoch near-infrared observations of the protoplanetary disk around MWC 480, revealing significant variability in accretion rates and disk emission, and models the variability as driven by inner disk winds rather than structural rim changes.

Contribution

It demonstrates that inner disk winds better explain near-infrared variability in MWC 480 than rim height changes, advancing understanding of disk dynamics.

Findings

Accretion rates vary by over 50% between epochs.

Near-IR flux variability is about 30%.

Inner disk winds explain IR variability better than rim height changes.

Abstract

We present five epochs of near IR observations of the protoplanetary disk around MWC 480 (HD31648) obtained with the SpeX spectrograph on NASA's Infrared Telescope Facility (IRTF) between 2007 and 2013, inclusive. Using the measured line fluxes in the Pa beta and Br gamma lines, we found the mass accretion rates to be (1.43 - 2.61)x10^-8 Msun y^-1 and (1.81 - 2.41)x10^-8 Msun y^-1 respectively, but which varied by more than 50% from epoch to epoch. The spectral energy distribution (SED)reveals a variability of about 30% between 1.5 and 10 microns during this same period of time. We investigated the variability using of the continuum emission of the disk in using the Monte-Carlo Radiative Transfer Code (MCRT) HOCHUNK3D. We find that varying the height of the inner rim successfully produces a change in the NIR flux, but lowers the far IR emission to levels below all measured fluxes. Because the star exhibits bipolar flows, we utilized a structure that simulates an inner disk wind to model the variability in the near IR, without producing flux levels in the far IR that are inconsistent with existing data. For this object, variable near IR emission due to such an outflow is more consistent with the data than changing the scale height of the inner rim of the disk.