The evolution of far-infrared CO emission from protostars

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, we computed $L_{\mathrm{bol}}$, $T_{\mathrm{bol}}$ and $L_{\mathrm{bol}}/L_{\mathrm{smm}}$ 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 ($L_{\mathrm{CO}}^{\mathrm{fir}}$) and the bolometric luminosity ($L_{\mathrm{bol}}$) of the protostars with $L_{\mathrm{CO}}^{\mathrm{fir}}$ $\propto$ $L_{\mathrm{bol}}^{0.7}$. We, however, do not find a strong correlation between $L_{\mathrm{CO}}^{\mathrm{fir}}$ and protostellar evolutionary indicators, $T_{\mathrm{bol}}$ and $L_{\mathrm{bol}}/L_{\mathrm{smm}}$. FIR CO emission from protostars traces the currently shocked gas by jets/outflows, and $L_{\mathrm{CO}}^{\mathrm{fir}}$ is proportional to the instantaneous mass loss rate, ${\dot{M}}_{\mathrm{out}}$. The correlation between $L_{\mathrm{CO}}^{\mathrm{fir}}$ and $L_{\mathrm{bol}}$ is indicative of instantaneous ${\dot{M}}_{\mathrm{out}}$ tracking instantaneous ${\dot{M}}_{\mathrm{acc}}$. The lack of correlation between $L_{\mathrm{CO}}^{\mathrm{fir}}$ and evolutionary indicators $T_{\mathrm{bol}}$ and $L_{\mathrm{bol}}/L_{\mathrm{smm}}$ suggests that ${\dot{M}}_{\mathrm{out}}$ and, therefore, ${\dot{M}}_{\mathrm{acc}}$ 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 (e.g., Bontemps et al. 1996), 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.

Accepted for publication in ApJ