FAUST. XXXI. Grain properties and variability of three sources in GSS 30

To advance our understanding of dust properties in class 0/I young stellar objects, it is essential to resolve and characterize their structures at multiple wavelengths and investigate how both grain growth and environmental factors shape their spectral properties. We present 0 angular resolution ALMA observations of the GSS 30 complex at 1.2–3.0,mm, collected as part of the FAUST large programme, with a linear resolution of 69 au. We analysed the dust continuum emission and performed model fitting to constrain the dust mass and the disk structure. For IRS3, the spectral index increases radially from 2.0 at the centre to 2.5 at the disk edge. In contrast, it decreases from 2.0 to as low as 1.6–1.8 along the outflow direction. An asymmetric distribution shows α as low as 1.6 to the northeastern blueshifted lobe, possibly due to cold outer envelope dust obscuring warmer inner regions. Spectral energy distribution fitting indicates maximum dust grain sizes of tens of microns and a dust mass of 650-1510 M_⊕. IRS1 is connected with an extended structure to the north-east. It may be an outflow–disk complex or a trailing structure associated with a circumbinary disk, although we cannot rule out the possibility that it could also be a gas streamer accreting onto IRS1. At the centre of IRS1, the spectral index is $<$0.8, which is consistent with marginally optically thick free-free emission. IRS2 exhibits brightness variations over 420 seconds of observation, with a multi-epoch comparison suggesting a flare lasting tens of minutes, indicating magnetic activity in the central star. This study presents a detailed millimetre multi-wavelength analysis of the GSS 30 IRS3 and IRS1 systems, highlighting the crucial role of environmental factors -- specifically dust obscuration and the connecting streamer structures -- in shaping the observed morphology of these early-stage disks. We also find that the IRS2 exhibits millimetre variability, which may originate from magnetic flares at the surface of the protostar.