2016ApJ...831...69M

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 \proptoL_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, \dotM_out. The correlation between L_CO^fir and L_bol, then, is indicative of instantaneous \dotM_out tracking instantaneous \dotM_acc. The lack of a correlation between L_CO^fir and evolutionary indicators T_bol and L_bol/L_smm suggests that \dotM_out and, therefore, \dotM_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.