Astrochemical evolution along star formation: Overview of the IRAM Large
  Program ASAI
  Evidence is mounting that the small bodies of our Solar System, such as
comets and asteroids, have at least partially inherited their chemical
composition from the first phases of the Solar System formation. It then
appears that the molecular complexity of these small bodies is most likely
related to the earliest stages of star formation. It is therefore important to
characterize and to understand how the chemical evolution changes with
solar-type protostellar evolution. We present here the Large Program
"Astrochemical Surveys At IRAM" (ASAI). Its goal is to carry out unbiased
millimeter line surveys between 80 and 272 GHz of a sample of ten template
sources, which fully cover the first stages of the formation process of
solar-type stars, from prestellar cores to the late protostellar phase. In this
article, we present an overview of the surveys and results obtained from the
analysis of the 3 mm band observations. The number of detected main isotopic
species barely varies with the evolutionary stage and is found to be very
similar to that of massive star-forming regions. The molecular content in O-
and C- bearing species allows us to define two chemical classes of envelopes,
whose composition is dominated by either a) a rich content in O-rich complex
organic molecules, associated with hot corino sources, or b) a rich content in
hydrocarbons, typical of Warm Carbon Chain Chemistry sources. Overall, a high
chemical richness is found to be present already in the initial phases of
solar-type star formation.
