2015ApJ...806..231H

We present an HCO⁺ J = 3 -> 2 survey of Class 0+I and Flat SED young stellar objects (YSOs) found in the Gould Belt clouds by surveys with Spitzer. Our goal is to provide a uniform Stage 0+I source indicator for these embedded protostar candidates. We made single point HCO⁺ J = 3 -> 2 measurements toward the source positions at the CSO and APEX of 546 YSOs (89% of the Class 0+I + Flat SED sample). Using the criteria from van Kempen et al., we classify sources as Stage 0+I or bona fide protostars and find that 84% of detected sources meet the criteria. We recommend a timescale for the evolution of Stage 0+I (embedded protostars) of 0.54 Myr. We find significant correlations of HCO⁺ integrated intensity with α and T_bol but not with L_bol. The detection fraction increases smoothly as a function of α and L_bol, while decreasing smoothly with T_bol. Using the Stage 0+I sources tightens the relation between protostars and high extinction regions of the cloud; 89% of Stage I sources lie in regions with A_V > 8 mag. Class 0+I and Flat SED YSOs that are not detected in HCO⁺ have, on average, a factor of ∼2 higher T_bol and a factor of ∼5 lower L_bolthan YSOs with HCO⁺ detections. We find less YSO contamination, defined as the number of undetected YSOs divided by the total number surveyed, for sources with T_bol≲ 600 K and L_bol≳ 1 L_☉. The contamination percentage is >90% at A_V< 4 mag and decreases as A_Vincreases.