L1551 IRS5 Class I binary system. (a, b) FAUST maps at 1.3 and 3.0 mm, at the resolution of $1\stackrel{″}{.}0$. White contours show the [3, 6, 10, 100]σ, with σ_1.3 mm = 0.13 mJy beam⁻¹ and σ_3.0 mm = 0.08 mJy beam⁻¹. The red- and blue-shifted CO(2−1) emission show the [3, 12, 33]σ_CO emission (σ_CO = 0.5 mJy beam⁻¹ km s⁻¹). (c) Dust spectral index map. White contours follow those in (a) while the purple one shows the 3σ level in (b). All maps are masked at 3σ and the beams are shown in the upper corners.