The extraordinary far-infrared variation of a protostar: Herschel/PACS observations of LRLL54361

TL;DR

This study presents Herschel/PACS observations revealing significant far-infrared flux variations in the protostar LRLL54361, correlated with shorter wavelength changes, indicating periodic heating of the disk and envelope due to binary accretion pulses.

Contribution

First detection of large far-infrared flux variability in a protostar correlated with shorter wavelengths, linking accretion pulses to long-wavelength emission changes.

Findings

Flux increased by a factor of six at 70 μm

Flux increased by about a factor of two at 160 μm

Far-infrared variability is consistent with a light echo from outflow cavities

Abstract

We report Herschel/PACS photometric observations at 70 {\mu}m and 160 {\mu}m of LRLL54361 - a suspected binary protostar that exhibits periodic (P=25.34 days) flux variations at shorter wavelengths (3.6 {\mu}m and 4.5 {\mu}m) thought to be due to pulsed accretion caused by binary motion. The PACS observations show unprecedented flux variation at these far-infrared wavelengths that are well cor- related with the variations at shorter wavelengths. At 70 {\mu}m the object increases its flux by a factor of six while at 160{\mu}m the change is about a factor of two, consistent with the wavelength dependence seen in the far-infrared spectra. The source is marginally resolved at 70 {\mu}m with varying FWHM. Deconvolved images of the sources show elongations exactly matching the outflow cavities traced by the scattered light observations. The spatial variations are anti-correlated with the flux variation indicating that a light echo is responsible for the changes in FWHM. The observed far-infrared flux variability indicates that the disk and en- velope of this source is periodically heated by the accretion pulses of the central source, and suggests that such long wavelength variability in general may provide a reasonable proxy for accretion variations in protostars.