Abstract

We investigate the evolution of far-IR CO emission from protostars observed with Herschel/PACS for 50 sources from the combined sample of HOPS and DIGIT Herschel key programs. From the uniformly sampled spectral energy distributions, whose peaks are well sampled, we computed the L-bol, T-bol, and L-bol/L-smm for these sources to search for correlations between far-IR CO emission and protostellar properties. We find a strong and tight correlation between far-IR CO luminosity (L-CO(fir)) and the bolometric luminosity (L-bol) of the protostars with L-CO(fir) proportional to L-bol(0.7). We, however, do not find a strong correlation between L-CO(fir) and protostellar evolutionary indicators, T-bol and L-bol/L-smm. FIR. CO emission from protostars traces the currently shocked gas by jets/outflows, and far-IR CO luminosity, L-CO(fir), is proportional to the instantaneous mass-loss rate, (M) over dot(out). The correlation between L-CO(fir) and L-bol, then, is indicative of instantaneous (M) over dot(out) tracking instantaneous (M) over dot(acc). The lack of a correlation between L-CO(fir) and evolutionary indicators T-bol and L-bol L-smm suggests that (M) over dot(out) and, therefore, (M) over dot(acc) do not show any clear evolutionary trend. These results are consistent with mass accretion/ejection in protostars being episodic. Taken together with the previous finding that the time-averaged mass-ejection/accretion rate declines during the protostellar phase, our results suggest that the instantaneous accretion/ejection rate of protostars is highly time variable and episodic, but the amplitude and/or frequency of this variability decreases with time such that the time-averaged accretion/ejection rate declines with system age.