The DIGIT Program
Dust, ice, and gas evolve as they move from envelopes of forming stars into circumstellar disks where they can become the main building blocks of planets. Herschel-PACS is uniquely suited to trace this evolution through broadband emission from dust, through specific features of solids which reveal elemental composition and water content, and through spectral bands of the main icy components. Both atomic and molecular lines, in particular the [O I], H2O and OH lines, will be used to follow the gas, study the interchange between gas and ice, and trace the oxygen budget. The gas and dust spectral features are at the same time excellent probes of macroscopic parameters, such as temperature, UV and X-ray fields, density and thermal structures of envelopes and disks, and dynamical mixing processes. Our sample covers sources with a range in evolutionary state from embedded objects with massive envelopes to weak-line T Tauri stars with dissipating disks, and with a range in luminosity, spectral type, and dust characteristics. Both our high S/N PACS full spectral range scans (complemented by Spitzer mid-IR spectral scans) and our targeted, deep gas-phase line measurements will have lasting archival value.