Revealing the dust grain size in the inner envelope of the Class I protostar Per-emb-50

Context. A good constraint of when the growth of dust grains from sub-micrometer to millimeter sizes occurs, is crucial for planet formation models. This provides the first step towards the production of pebbles and planetesimals in protoplanetary disks. Currently, it is well established that Class II objects have large dust grains. However, it is not clear when in the star formation process this grain growth occurs.Aims. We use multi-wavelength millimeter observations of a Class I protostar to obtain the spectral index of the observed flux densities alpha(mm) of the unresolved disk and the surrounding envelope. Our goal is to compare our observational results with visibility modeling at both, 1.3 and 2.7 mm simultaneously.Methods. We present data from NOEMA at 2.7 mm and SMA at 1.3 mm of the Class I protostar, Per-emb-50. We model the dust emission with a variety of parametric and radiative-transfer models to deduce the grain size from the observed emission spectral index.Results. We find a spectral index in the envelope of Per-emb-50 of alpha(env) = 3.3 +/- 0.3, similar to the typical ISM values. The radiative-transfer modeling of the source confirms this value of alpha(env) with the presence of dust with a a(max) <= 100 mu m. Additionally, we explore the backwarming effect, where we find that the envelope structure affects the millimeter emission of the disk.Conclusions. Our results reveal grains with a maximum size no larger than 100 mu m in the inner envelope of the Class I protostar Per-emb-50, providing an interesting case to test the universality of millimeter grain growth expected in these sources.