Topic: Formation and Evolution of Cool Stars and Brown Dwarfs
In the era of the Spitzer and Herschel space observatories, the discovery and characterization of young low-mass stars and brown dwarfs is extending to star-forming regions that contain nearly the full range of stellar masses, allowing a detailed study of how star formation depends on environment. We are pursuing this goal with the Herschel Orion Protostar Survey (HOPS), which targets 330 protostellar candidates in the Orion molecular clouds, the closest star-forming region that contains massive stars. HOPS is a multiwavelength program that combines Herschel 70 and 160 micron photometry and 50-200 micron spectroscopy, Spitzer 3.6-40 micron photometry and spectroscopy, 0.8-2.4 micron IRTF spectroscopy, 1.6 micron HST imaging, and sub-mm APEX imaging. The sample encompasses all embedded phases of protostellar evolution, from the beginning of Class 0 through the end of Class I, and a wide range of formation environments, from dense clusters to relative isolation. With a grid of radiative transfer models, we fit the 1-870 micron spectral energy distributions of the 330 protostars to determine their envelope densities, cavity opening angles, inclinations, and total luminosities. We also measure the equivalent widths of their near-infrared hydrogen lines and use standard relationships to determine what fraction of the total luminosity is due to disk accretion. We find that: 1) Protostellar luminosities are distributed over three orders of magnitude at each evolutionary stage, consistent with previous studies of star-forming regions closer than Orion and broader than predicted by constant-infall models. 2) There is no evidence for evolution in the median luminosity with SED class. 3) Seventeen of the 330 protostars have reddening-corrected bolometric luminosities less than 0.1 L_sun; these may be proto-brown-dwarfs. 4) Our Herschel images contain a sample of previously unknown faint Class 0 protostars. 5) After accounting for the newly detected protostars, the ratio of Class 0 to Class I sources is 1 to 2, larger than previously estimated from Spitzer studies of nearby star-forming regions and implying a longer Class 0 phase. 6) Protostars are on average more luminous in more crowded regions and regions of greater gas column density. 7) The accretion luminosities of Class I protostars measured from hydrogen lines are statistically similar to those of more evolved T Tauri stars in Taurus-Auriga. 8) Evolution of the near-IR spectrum of a recently announced outbursting protostar in the sample indicates that it has transitioned to the FU Orionis regime.