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Protostellar objects are the links between dense molecular cloud cores and embedded pre-main sequence stars. The study of protostars is expected to provide crucial insight about the star formation process and, indirectly, about the formation of our own solar system.
Both theoretical considerations and our intuition suggest that stars form as the result of the collapse of a portion of a molecular cloud. Whether the resulting star will have a high or low mass is a function of the physical conditions of the parent cloud. Warm ($>20$~K) cores tend to produce high mass stars, while cool cores produce lower mass stars. Cool clouds (with their low mass offspring) are distributed uniformly throughout the galactic disk. Conversely, warm clouds (which also tend to be the most massive) are found principally along the spiral arms.
A molecular cloud core may be supported initially by a combination of turbulent pressure, rotation, and magnetic fields. Magnetic pressure is expected to diminish over time as a result of ambipolar diffusion. If turbulent motions are to remain sub-Alfv\'enic, they must decrease in magnitude as well. Ambipolar diffusion may play an important role in regulating the collapse of cloud cores. The role of magnetic fields is particularly important in low mass cloud cores which do not have sufficient gravitational energy to overwhelm the energy contained in the magnetic field. Such cores must wait until the magnetic field leaks out through ambipolar diffusion before collapse can proceed. The ambipolar diffusion time scale could span $\sim 10^6$~years. Before collapse begins, cloud cores are found to rotate slowly. This slow rotation may result from magnetic braking and is typically insufficient to support a cloud core against gravitational collapse. Once a protostellar core becomes sufficiently centrally condensed, the collapse proceeds from the inside-out. In cases where external shocks are present (e.g. from an AGB star wind, a nearby molecular outflow, or a distant supernova), collapse may occur from the outside-in. This may have been the situation for our own Sun. As the collapse proceeds over many orders of magnitude in size and density, a point will be reached where angular momentum will slow or stop the infall process and a protostellar accretion disk will form. The embedded protostar may then use magnetic braking and/or mass loss, in the form of a molecular outflow, to shed angular momentum in the newly formed accretion disk and allow the mass accretion to proceed. A protostar is expected to spend $\sim 10^5$~years in the mass accretion/outflow phase.
An object can be identified as a candidate protostar from continuum observations at far-infrared and submillimeter wavelengths. Their large accretion luminosities and locations in dense dust cores should place the peak in the energy distribution of protostars at these wavelengths. To probe the dynamics and chemistry of protostars, spectroscopic observations are required. Because of the relatively low temperatures and high densities expected in star formation regions, molecular line observations at millimeter and submillimeter wavelengths offer the best opportunities for studying the conditions in young protostellar objects.
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