Distance of optically obscured evolved stars
March 24, 2022
May 24, 2022
As intermediate-mass stars, precursors of planetary nebulae, head towards their final fate, they pass through the red-giant stage where they experience an increase of mass loss. This induces the creation of a circumstellar envelope of dust and gas. By the very end of this evolutionary stage, for those objects exhibiting the highest mass-loss rate, the amount of dust in the circumstellar
envelope is such that it blocks optical radiation, turning them into so-called OH/IR stars. These stars are commonly observed throughout the Galaxy and are also observed in the Magellanic
Clouds. Since optically obscured, the measurement of their distances using optical parallaxes as e.g. delivered by Gaia, is not possible. This issue can be circumvented thanks to maser
emission. As their name gives it away, the physical conditions turn out to be ideal for a strong (1612-MHz) OH maser emission to be produced in the outer layers of the radially-expanding spherical circumstellar envelope. Combining single-dish monitoring and interferometric mapping of this OH maser emission, the “phase-lag” method provides a way to measure their distance. We have been revisiting this method through a project called “NRT phase-lag distance”. Here we present the method itself and the modus operandi of our project. We also present an analysis of the faint emission in the outer OH maser shell of OH 83.4-0.9 and use this analysis to discuss the limitations of the “phase-lag” method. Finally we compare the distances obtained from this method with those obtained from optical and radio astrometry.
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