We present key results from the Herschel Orion Protostar Survey (HOPS): spectral energy distributions (SEDs) and model fits of 330 young stellar objects, predominantly protostars, in the Orion molecular clouds. This is the largest sample of protostars studied in a single, nearby star formation complex. With near-infrared photometry from 2MASS, mid-and far-infrared data from Spitzer and Herschel, and submillimeter photometry from APEX, our SEDs cover 1.2 -870 µm and sample the peak of the protostellar envelope emission at ∼ 100 µm. Using mid-IR spectral indices and bolometric temperatures, we classify our sample into 92 Class 0 protostars, 125 Class I protostars, 102 flatspectrum sources, and 11 Class II pre-main-sequence stars. We implement a simple protostellar model (including a disk in an infalling envelope with outflow cavities) to generate a grid of 30,400 model SEDs and use it to determine the best-fit model parameters for each protostar. We argue that far-IR data are essential for accurate constraints on protostellar envelope properties. We find that most protostars, and in particular the flat-spectrum sources, are well fit. The median envelope density and median inclination angle decrease from Class 0 to Class I to flat-spectrum protostars, despite the broad range in best-fit parameters in each of the three categories. We also discuss degeneracies in our model parameters. Our results confirm that the different protostellar classes generally correspond to an evolutionary sequence with a decreasing envelope infall rate, but the inclination angle also plays a role in the appearance, and thus interpretation, of the SEDs.

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 μm and 160 μm that are either too faint (m 24 > 7 mag) to be reliably classified as protostars or undetected in the Spitzer/MIPS 24 μm band. We find that the 11 reddest protostar candidates with log λF λ 70/λF λ 24 > 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 μm 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 λF λ 70/λF λ 24 > 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; T bol < 45 K) and large values of sub-millimeter fluxes (L smm /L bol > 0.6%). Modified blackbody fits to the SEDs provide lower limits to the envelope masses of 0.2-2 M and luminosities of 0.7-10 L . 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.

We have conducted a survey of 328 protostars in the Orion molecular clouds with the Atacama Large Millimeter/ submillimeter Array at 0.87 mm at a resolution of ∼0 1 (40 au), including observations with the Very Large Array at 9mm toward 148 protostars at a resolution of ∼0 08 (32 au). This is the largest multiwavelength survey of protostars at this resolution by an order of magnitude. We use the dust continuum emission at 0.87 and 9mm to measure the dust disk radii and masses toward the Class 0, Class I, and flat-spectrum protostars, characterizing the evolution of these disk properties in the protostellar phase. The mean dust disk radii for the Class 0, Class I, and flat-spectrum protostars are -+ 44.9 3.4 5.8 , -+ 37.0 3.0 4.9 , and -+ 28.5 2.3 3.7 au, respectively, and the mean protostellar dust disk masses are 25.9 -+ 4.0 7.7 , -+ 14.9 2.2 3.8 , -+11.6 1.93.5 Å M , respectively. The decrease in dust disk masses is expected from disk evolution and accretion, but the decrease in disk radii may point to the initial conditions of star formation not leading to the systematic growth of disk radii or that radial drift is keeping the dust disk sizes small. At least 146 protostellar disks (35% of 379 detected 0.87 mm continuum sources plus 42 nondetections) have disk radii greater than 50 au in our sample. These properties are not found to vary significantly between different regions within Orion. The protostellar dust disk mass distributions are systematically larger than those of Class II disks by a factor of >4, providing evidence that the cores of giant planets may need to at least begin their formation during the protostellar phase.

We present far-infrared (57−196 µm) spectra of 21 protostars in the Orion molecular clouds. These were obtained with the Photodetector Array Camera and Spectrometer (PACS) onboard the Herschel Space observatory, as part of the Herschel Orion Protostar Survey (HOPS) program. We analyzed the emission lines from rotational transitions of CO, involving rotational quantum numbers in the range J up = 14−46, using PACS spectra extracted within a projected distance of 2000 AU centered on the protostar. The total luminosity of the CO lines observed with PACS (L CO ) is found to increase with increasing protostellar luminosity (L bol ). However, no significant correlation is found between L CO and evolutionary indicators or envelope properties of the protostars such as bolometric temperature, T bol or envelope density. The CO rotational (excitation) temperature implied by the line ratios increases with increasing rotational quantum number J, and at least 3−4 rotational temperature components are required to fit the observed rotational diagram in the PACS wavelength range. The rotational temperature components are remarkably invariant between protostars and show no dependence on L bol , T bol or envelope density, implying that if the emitting gas is in local thermodynamic equillibrium, the CO emission must arise in multiple temperature components that remain independent of L bol over two orders of magnitudes. The observed CO emission can also be modeled as arising from a single temperature gas component or from a medium with a power-law temperature distribution; both of these require sub-thermally excited molecular gas at low densities (n(H 2 ) 10 6 cm −3 ) and high temperatures (T 2000 K). Our results suggest that the contribution from photodissociation regions (PDRs), produced along the envelope cavity walls from UV-heating, is unlikely to be the dominant component of the CO emission observed with PACS. Instead, the 'universality' of the rotational temperatures and the observed correlation between L CO and L bol can most easily be explained if the observed CO emission originates in shock-heated, hot (T 2000 K), sub-thermally excited (n(H 2 ) 10 6 cm −3 ) molecular gas. Post-shock gas at these densities is more likely to be found within the outflow cavities along the molecular outflow or along the cavity walls at radii several 100−1000 AU.

Synchrotron emission is commonly found in relativistic jets from active galactic nuclei (AGNs) and microquasars, but so far its presence in jets from young stellar objects (YSOs) has not been proved. Here, we present evidence of polarized synchrotron emission arising from the jet of a YSO. The apparent magnetic field, with strength of ~0.2 milligauss, is parallel to the jet axis, and the polarization degree increases toward the jet edges, as expected for a confining helical magnetic field configuration. These characteristics are similar to those found in AGN jets, hinting at a common origin of all astrophysical jets.