Brightness variations of the FUor-type eruptive star V346 Nor

TL;DR

This study tracks the brightness evolution of FUor-type star V346 Nor over decades, revealing a fading event, a halo of scattered light, and the roles of accretion and extinction in its variability.

Contribution

It provides new near-infrared observations, identifies a scattered light halo, and models the interplay of accretion and extinction in the star's brightness changes.

Findings

V346 Nor experienced a 4.6 mag fading around 2010.

A halo of scattered light was discovered around V346 Nor.

Brightness variations are driven by changes in accretion rate and extinction.

Abstract

Decades after the beginning of its FU Orionis-type outburst, V346 Nor unexpectedly underwent a fading event of $\Delta{}K$ = 4.6 mag around 2010. We obtained near-infrared observations and re-analysed data from the VISTA/VVV survey to outline the brightness evolution. In our VLT/NaCO images, we discovered a halo of scattered light around V346~Nor with a size of about 0.04 arcsec (30 au). The VISTA data outlined a well-defined minimum in the light curve at late 2010/early 2011, and tentatively revealed a small-amplitude periodic modulation of 58 days. Our latest data points from 2016 demonstrate that the source is still brightening but has not reached the 2008 level yet. We used a simple accretion disk model with varying accretion rate and line-of-sight extinction to reproduce the observed near-infrared magnitudes and colors. We found that before 2008, the flux changes of V346 Nor were caused by a correlated change of extinction and accretion rate, while the minimum around 2010 was mostly due to decreasing accretion. The source reached a maximal accretion rate of ${\approx}10^{-4} M_{\odot}$ yr$^{-1}$ in 1992. A combination of accretion and extinction changes was already invoked in the literature to interpret the flux variations of certain embedded young eruptive stars.