A Herschel * and APEX census of the reddest sources in Orion: Searching for the youngest protostars

We perform a census of the reddest, and potentially youngest, protostars in the Orion molecular clouds using data obtained with the PACS instrument on board the Herschel Space Observatory and the LABOCA and SABOCA instruments on APEX as part of the Herschel Orion Protostar Survey (HOPS). A total of 55 new protostar candidates are detected at 70 \u3bcm and 160 \u3bcm that are either too faint (m24 > 7 mag) to be reliably classified as protostars or undetected in the Spitzer/MIPS 24 \u3bcm band. We find that the 11 reddest protostar candidates with log \u3bbF\u3bb70/\u3bbF \u3bb24 > 1.65 are free of contamination and can thus be reliably explained as protostars. The remaining 44 sources have less extreme 70/24 colors, fainter 70 \u3bcm fluxes, and higher levels of contamination. Taking the previously known sample of Spitzer protostars and the new sample together, we find 18 sources that have log \u3bbF\u3bb70/ \u3bbF\u3bb24 > 1.65; we name these sources "PACS Bright Red sources," or PBRs. Our analysis reveals that the PBR sample is composed of Class 0 like sources characterized by very red spectral energy distributions (SEDs; Tbol < 45 K) and large values of sub-millimeter fluxes (Lsmm/Lbol > 0.6%). Modified blackbody fits to the SEDs provide lower limits to the envelope masses of 0.2-2 M 99 and luminosities of 0.7-10 L 99. Based on these properties, and a comparison of the SEDs with radiative transfer models of protostars, we conclude that the PBRs are most likely extreme Class 0 objects distinguished by higher than typical envelope densities and hence, high mass infall rates. \ua9 2013. The American Astronomical Society. All rights reserved.Peer reviewed: YesNRC publication: Ye