  Observations of interstellar material infalling onto star- and planet-forming
systems have become increasingly common thanks to recent advancements in radio
interferometry. These structures replenish disks with fresh material, have the
potential to significantly alter their dynamics, trigger the formation of
substructures, induce shocks, and modify their physical and chemical
properties. In this study, we combine new ALMA band 3 and archival band 6
observations to characterize the dust content and mass infall rate of a 4,000
au arc-like structure infalling onto M512, a class I young stellar object
located in the Lynds 1641 region of the Orion A molecular cloud. We measure for
the first time spectral index maps and derive a dust opacity index profile
along a streamer, constraining grain properties and its dust mass. We measure a
spectral index $\alpha \sim$ 3.2 across the entire structure, and a dust
opacity index $\beta \sim$ 1.6. Given grain properties consistent with the
measured $\beta$, the structure can host up to 245 M$_{\oplus}$ of dust, being
comparable or even exceeding the mass of the inner, unresolved 600 au, which
contains the protoplanetary disk of M512. Such a massive streamer can strongly
affect the evolution of the star- and planet-forming inner system. Assuming
typical ISM dust-to-gas ratio of 1%, free-fall timescales (50 kyr) imply total
mass infall rates up to 1.5 $\cdot$ 10$^{-6}$ M$_{\odot}$/yr. M512 has been
classified as an outbursting source with multi-epoch photometry, thus
representing an interesting case study to explore the possible connection
between infalling streamers and accretion outbursts.
