RUI: The Evolution of Sun-like Stars Through Their Mass Accreting Phase

Bucknell University, Lewisburg PA

Investigators

Abstract

AST-0071063 LADD It is abundantly clear that stars must form from the aggregation of diffuse interstellar material. However, how this process occurs, and specifically which mechanisms play a dominant role in the evolution, are still poorly understood. Stars in the process of formation do not accrete all of the material energetically available to them, but instead reject some fraction of their surrounding dense cores during the formation process. While the rejection process is not well understood, it is suspected that jets and bipolar outflows from the forming star play a major role. The goal of this research program is to understand the time evolution of stars and their placental cores during the critical phase when the mass of a forming star is determined. This phase is dominated by the gravitational infall of material from the core onto the star, and the ballistic ejection of material from the immediate vicinity of the star in the form of bipolar jets and outflows. Both processes have substantial impacts on the core - the former removes mass, while the latter injects energy. The complicated interplay between these two processes strongly affects the evolution of the core, and, since the core is the mass reservoir out of which the star will form, may even set the star's final mass. While the mass of a forming star is impossible to measure directly, the mass of the circumstellar core can be inferred from observations of the rotational transitions of its molecular constituents. Furthermore, the energetics of the core material can be measured from the velocity structure of this spectral line emission. This research activity seeks to track the evolution of circumstellar core material through the main mass accreting phase of star formation, via studies of populations of forming stars. An initial study of sources in the Taurus molecular cloud indicates that the circumstel-lar core is emptied on about a hundred million year time scale, with the most rapid decrease taking place at the earliest times. The rate of mass loss from the circumstellar core is consistent with that expected from standard models of accreting protostars; except during these earliest phases when the mass loss rate is larger than that expected from these models. The analysis of the velocity structure of these cores suggests that bipolar outflows from the forming star couple to core material more efficiently during the earliest times, during which outflows may inject substantial energy into the core. In addition, maps of the spatial distribution of the "stirred-up" core material indicate that this interaction takes place in the inner core, near the forming star. This research program will apply the methodology proved for modeling core forming stars in the Taurus region to all very young stars in large complexes nearer than 350 parsec. The survey observations will be conducted such that the linear spatial resolution will be constant, producing a data set free of distance-dependent effects, and facilitating comparisons between star forming complexes. The new observations will build on the results from the Taurus survey by increasing the sample size by at least a factor of five, and will constrain models of the time-evolution of forming stars during the critical phase in which they accrete most of their mass. This project is funded by the Division of Astronomical Sciences. ***