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Abstract
To study transitions of embedded clusters into open clusters, we present visible light spectroscopic observations of 190 young stars in the Cepheus OB3b young cluster. These observations have enabled us to constrain the fate of Cep OB3b, the nearest example of a large young cluster in the late stages of gas dispersal. The Gaia DR2 data has resolved previous ambiguities in the distance to Cep OB3b with a distance of 819~pc. The cluster contains two distinct sub-clusters; the proper motions show the two sub-clusters are moving away from one another. Based on the velocity dispersion measured with the visible spectra, each sub-cluster will form a bound cluster of $\sim$300 stars each. To study the transition point between protostars and pre-main sequence stars, the 66 near-IR high resolution spectroscopy of Orion HOPS protostars is paving the way to observationally constrain the temperatures, luminosities, and radii of a large sample of late stage protostars at a common distance without the need to use theoretical models. We identify temperature sensitive features in the HOPS spectra spectral typing each target with a suite of spectral typing standards, simultaneously, lines without temperature sensitivity will be used to measure veiling. In parallel effort outside of this dissertation, hydrogen lines are currently being fit to measure the accretion luminosity in each HOPS protostar. The hydrogen line fits will be used to correct the intrinsic luminosity from the total luminosity. The luminosities and effective temperatures from spectral types will be used to determine the radii of the protostars and can be combined with masses from model tracks, as well as direct measurements of masses using the Keplerian disk motions resolved in future ALMA data. The combination of radii and masses for dozens of flat spectrum protostars will independently provide the starting point of contraction without need for theoretical models that assume an initial mass and radius. To measure the transition from a collapsing gas fragment to protostar with the high spatial resolution of ALMA and VLA, we present 0.87 and 8 mm continuum images of the theoretically predicted optically thick phase of star formation that has eluded observation for decades. The mm continuum observations resolve the youngest protostars in the Orion Molecular Clouds. Specifically, four irregularly shaped protostars have properties that constrain their age to be younger than 10,000 years. The four detections out of 328 protostars also suggest lifetimes of a few thousand years. The four irregular protostars are optically thick at 0.87~mm and given their masses, radii, and luminosities, appear to be collapsing slowly on Kelvin-Helmholtz timescales, which is also a few thousand years; in comparison free fall times are a few hundred years. The slow contraction is in contradiction with virial arguments, which show that thermal energies at the observed temperatures are much too low to support the inner envelopes against collapse. Other sources of energy, rotation, turbulence and/or magnetic fields, are needed. Future observations are required to constrain the dust law and measure the turbulent and/or rotational motions of the inner envelopes to determine whether they are sufficient to support the envelopes against collapse.
