Late Stages of Stellar Evolution

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Hot, high-mass stars have a big impact on the galaxy through their energetic outflows, mainly ionised gas. These are not only detected in thermal emission but can produce non-thermal radiation from shocks and particle acceleration, such as when the winds from two binary companions collide.

High-mass stars evolve so rapidly that detection of strong activity is a sign that star formation is ongoing or very recent. Thus, understanding their lifetimes and behaviour helps us use them as tools to trace the evolution of distant galaxies as well as individual stars in our own. Their mass loss rates are hard to measure due to clumping and rapid variability, so that although they are an important contributor to chemical enrichment this is not yet well quantified, nor are the precise mechanisms by which massive star cluster outflows may impact on nearby molecular clouds to trigger star formation - or even cause material to be lost from galaxies altogether.

The COBRaS (Cyg OB2 Radio Survey) e-MERLIN Legacy project, led by Raman Prinja (UCL) is tackling this by a deep survey at 1.6 and 5 cm wavelengths, to distinguish thermal and non-thermal radiation and resolve binary interactions.

See also: The e-MERLIN Cyg OB2 Radio Survey (COBRaS): Massive and young stars in the Galaxy (PDF).

Several types of highly evolved star develop vigorous stellar winds that eject a substantial fraction of the original stellar mass into the interstellar medium. These stars are often classified as either oxygen-rich (most carbon is bound is CO and there is abundant oxygen to combine with other elements) or carbon rich. The oxygen-rich stars typically generate silicate dust in their stellar winds.

Oxygen-rich evolved stars may host up to three types of maser from the molecules SiO, H2O and OH. The SiO masers from closest to the star, in a zone that is repeatedly heated by shock-waves resulting from stellar pulsations. The H2O masers form further out, in a zone where the pulsations transform into a steady outward wind. This zone is shared with masers from the 1665 and 1667-MHz transitions of OH. Much further out again, we find masers from the 1612-MHz OH transition.

Recent work has replaced a radiation transfer model based on the large-velocity-gradient approximation with more exact spherical accelerated lambda iteration. The model is able to calculate, for the SiO masers, the projected radii of maser zones for many lines as a function of stellar phase, and these can be compared with observations.

Coming Soon.

Coming Soon