From astrobites: “The Mass Controversy of Retired A-Stars”

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Title: Asteroseismic masses of retired planet-hosting A-stars using SONG
Authors: Dennis Stello, Daniel Huber, Frank Grundahl, et al.
First Author’s Institution: School of Physics, University of New South Wales, Australia

Status: Accepted to MNRAS, open access

There’s an old saying, sometimes called Segal’s Law: “A person with one watch knows what time it is; a person with two is never sure.” Today’s paper concerns a similar problem in astronomy. The authors were measuring the masses of a certain group of red giants called “Retired A-Stars”. They find that these stars are less massive on average than previous measurements which used different techniques, enough to suggest that there’s a problem– either with the technique used in today’s paper, or with the previous technique.

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Figure 1: Models of retired A-star evolution for a range of stellar masses, from 1 solar mass (at the bottom) to 2.8 solar masses (at the top). For each track, the filled points represent 50 million years of evolution. As can be seen, the higher mass stars evolve much faster. This is Figure 1 from today’s paper.

Retired A-Stars

When stars come towards the end of their life, or “retire”, they go through a phase in which they grow into giants. The nature of a giant star depends on the mass it had before it grew into a giant. The type of stars today’s authors studied are “retired A-stars”: giants which evolved from A-class stars.

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http://www.guildcompanion.com/scrolls/2011/oct/ssg03.html

A-class stars begin life with approximately 1.4–2.1 times the mass of the Sun—in other words they are high mass stars, but not the highest mass stars around. Retired A-stars are particularly interesting because several exoplanets have been found around them.

The mass of a star leaves an imprint on the star’s spectrum, which means we can measure it by comparing a spectrum of the star to model spectra. Of course, this method relies on the models used being reliable; if the models are off, the mass we measure will be off as well. Indeed, this is what happened. The masses measured in the original study were called into question by a later study, which argued that the derived masses were higher than you would expect—higher-mass stars pass through and leave the giant phase faster, so you would expect the population of retired A-stars to be dominated by stars at the lower end of the spectrum (see Figure 1). Several follow-up papers have been published, but the issue is still not resolved.

See the full article here .

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