The recent detection of shock precursors toward the very young L1448-mm outflow offers us the possibility to study the grain chemistry during the first stages of the shock evolution, constraining the molecules ejected from grains and the species formed in gas phase. Observations of key molecules in the grain chemistry such as SiO, CH_3OH, SO, CS, H_2S, OCS, and SO_2 toward this outflow are presented. The line profiles and the derived abundances show three distinct velocity regimes that trace the shock evolution: the preshock, the shock precursor, and the postshock gas. The SiO, CH_3OH, SO, and CS abundances are enhanced with respect to the quiescent gas by 1 order of magnitude in the shock-precursor component, and by 3 orders of magnitude in the postshock gas. The derived SiO and CH_3OH abundances are consistent with the recent ejection of these molecules from grains. Since H_2S is only enhanced in the shock-precursor component and OCS and SO_2 are undetected, SO and CS are the most abundant sulfur-bearing species in the grain mantles of L1448-mm. The ejection of mainly SO and CS rather than H_2S or OCS from grains suggests that the sulfur chemistry will depend on the chemical "history" of the grain mantles in outflows and hot cores.
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