We introduce hydrodynamic simulations in which a protostar captures a cloudlet with a relatively small angular momentum. The cloudlet accretes onto the protostar and perturbs the gas disk rotating around the protostar. This cloudlet capture can reproduce some features observed in the molecular emission lines from TMC-1A. First, the cloudlet can reproduce the blue asymmetry observed in the CS emission. Second, the cloudlet can explain the slow infall observed in the C18O emission. Third, the impact of the cloudlet can explain the offset of the SO emission from the disk center. We also argue that a warm gas should confine the cloudlet through pressure. A protostar may obtain substantial mass by capturing cloudlets.

Far-infrared spectroscopy reveals gas cooling and its underlying heating due to physical processes taking place in the surroundings of protostars. These processes are reflected in both the chemistry and excitation of abundant molecular species. Here, we present the Herschel-PACS far-IR spectroscopy of 90 embedded low-mass protostars from the WISH (van Dishoeck et al. 2011), DIGIT (Green et al. 2013), and WILL surveys (Mottram et al. 2017). The 5 × 5 spectra covering the ∼50″ × 50″ field-of-view include rotational transitions of CO, H2O, and OH lines, as well as fine-structure [O I] and [C II] in the ∼50-200 μm range. The CO rotational temperatures are typically ∼300 K, with some sources showing additional components with temperatures as high as ∼1000 K. The H2O / CO and H2O / OH flux ratios are low compared to stationary shock models, suggesting that UV photons may dissociate some H2O and decrease its abundance. Comparison to C shock models illuminated by UV photons show a good agreement between the line emission and the models for pre-shock densities of 105 cm−3 and UV fields 0.1-10 times the interstellar value. The far-infrared molecular and atomic lines are the unique diagnostic of shocks and UV fields in deeply-embedded sources.

High-energy radiation and particles profoundly affect circumstellar disk gas and solids. We discuss stellar high-energy sources and summarize their effects on circumstellar disks.