PhD colloquium, location: Lecture Hall 0.012, AIfA
Ben Hastings
The formation mechanisms of classical emission line stars
Classical emission line stars are fast rotators hosting a decretion disc in which the emission lines form. Since their discovery in 1866, the formation mechanism of these stars remains poorly understood. To explain their rapid rotation, two channels are proposed; envelope spin-up due to the transport of angular momentum from their contracting core during hydrogen burning, and accretion-induced spin-up during mass transfer in interacting binaries.
The open cluster NGC 330, which contains a rich population of classical emission line stars is taken as a test case. To determine the dominant formation channel, we produce synthetic populations of exclusively single and binary stars, respectively. The synthetic population of single stars is generated from a grid of detailed numerical models of rotating stars. While the synthetic binary population is produced from a simple analytic model so that uncertainties in binary evolution may be probed.
One one hand, there is little evidence that the high initial rotation rates required to form emission line stars through the single star channel are realised in nature. Therefore, the large number of relatively unevolved emission line stars in NGC 330 are unlikely to be explained by single star evolution.
On the other hand, it is found that given specific, albeit extreme assumptions, binary evolution can explain the observed population of emission line stars. One requirement is for mass-transfer to be non-conservative. However, evidence for this is distinctly lacking, with several post-mass transfer systems suggesting rather conservative evolution.
We conclude that likely, both channels contribute to the production of emission line stars, and that the relative contributions are sensitive to the stellar birth spin distribution and to the mass transfer efficiency in interacting binary systems.