Observational characteristics of early star formation
Date: 4 November 2019
University of Exeter
PhD in Physics
The formation of low mass stars has been modelled in detail for several decades and in the last 30 years infrared and submillimetre telescopes have observed protostellar sources and discovered faint, embedded protostars, outflows, jets and discs. The first hydrostatic core (FHSC), the first stable pressure–supported object to form ...
The formation of low mass stars has been modelled in detail for several decades and in the last 30 years infrared and submillimetre telescopes have observed protostellar sources and discovered faint, embedded protostars, outflows, jets and discs. The first hydrostatic core (FHSC), the first stable pressure–supported object to form during the star formation process, was first predicted in 1969 but has not been definitively observed. Specific criteria are required to distinguish it from other faint pre– and protostellar objects observationally. In this thesis, we develop synthetic observations of the early stages of star formation to help determine how to identify the FHSC in nature. We present synthetic spectral energy distributions (SEDs) from 3-D radiation hydrodynamical simulations of collapsing pre–stellar cores for a variety of initial conditions. Variations in the initial rotation rate, radius and mass lead to differences in the location of the SED peak and far–infrared flux. We then attempt to fit the SEDs of 15 candidate FHSCs with model SEDs. This showed that the SED can provide an insight into the nature of these sources and enable sources that are probably more evolved to be ruled out. Next, we produced spectral line observations for CO, HCO+, CS and SO, calculated from radiation (magneto)hydrodynamical models, chemical modelling and Monte Carlo radiative transfer. Many common molecules are strongly depleted except for in the inner few 10s of AU , after the formation of the FHSC. HCO+ (1−0) and SO (87−76) spectra show variations which may allow a candidate FHSC to be distinguished from a more evolved object. It may also be possible to detect the rotation of the outflow in CO (4 − 3) and (3 − 2) lines for nearby (∼ 150 pc) sources. Lastly, we applied the chemical model to a large–scale simulation of cluster formation. This showed, again, that the abundances of most species are depleted with respect to abundances in the interstellar medium and are only released into the gas phase in small scale regions where protostars are forming. This work also highlighted the importance of considering the initial conditions carefully because the physical history has a substantial effect on the calculated abundances in the outer, low density regions.
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