The galaxy’s red giant bones

Location in the galactic plane of stars in the sample from Miglio et al. (Fig 1b)

Title: Galactic archaeology: mapping and dating stellar populations with asteroseismology of red-giant stars
Author: A. Miglio, C. Chiappini, T. Morel, M. Barbieri et al.
Institution:  University of Birmingham, UK

I’ll admit, I was disappointed to learn that archaeology is limited to the study of human history – if you want dinosaur bones, you want a paleontologist. This means that astronomy might be stuck with yet another inappropriate piece of terminology: “galactic archaeology” is the term that has come to refer to using the motions and chemical compositions of stars of different ages to learn about the history of the Milky Way. It seems to me that “galactic paleontology” might be a bit more accurate (full disclosure: I’m a dinosaur fan). But that’s coming from someone who took a class devoted to dinosaurs in college; what do you all think? I hope to see galactic archaeology v. galactic paleontology fought out in the comments.

The paper for today’s astrobite digs into the topic of galactic archaeology using asteroseismology, which is the study of stellar pulsations (see these astrobites). Asteroseismology is an important tool because it provides the only way to directly measure the mass and radii of distant stars, although it relies on using colors to determine a star’s temperature. The authors apply asteroseismology methods to measure the stellar masses and radii of 2000 red giant stars. Red giants are evolved stars and have a range of masses, ages, and chemical compositions. The red giants in their sample include stars in two distinct parts of the galaxy, as shown to the right. I’ll call them the “blue” and “red” samples (this is just to distinguish between their samples – all the stars are red giant stars). Importantly, the stars in the “blue” sample are located closer to the galactic plane than the stars in the “red” sample.

Vertical location of stars in their sample. The y axis shows the distance from (i.e. height above or below) the galactic plane. Two samples, the "red" and "blue" samples are shown. (Fig 1a).

Miglio et al. look at the stellar masses and radii of the “blue” and “red” samples. The radii of the two samples are indistinguishable within the uncertainties, but the mass distributions are different: the “red” sample is skewed towards lower masses. When they simulate the population of stars they would expect to see in the two samples, they find that the “red” sample is expected to be older. Previous work shows that the age difference would result in a difference in mass distribution, as is seen.

The authors interpret the age/mass differences between the “red” and “blue” sample as being due to their different locations in the galaxy. Stars in the “red” sample are farther from the galactic plane, and are on average older (and therefore more low mass) stars. Studying stars at different heights above the galactic plane therefore lets us do galactic archaeology, because we are studying stars of different ages. (This idea has been used previously, for example by West et al., who call the method “galactic stratigraphy.”)

Pulsating red giants are particularly promising targets for galactic archaeology. The ages of these stars span most of the the Milky Way’s history and, with a good estimate of the chemical composition, the ages of the stars can even be measured. Because they are bright, they can be seen out to large distances in the Milky Way, allowing us to probe stars very far from the galactic plane. Because they are pulsating, asteroseismology can be used to measure accurate masses, radii and distances.

About Elisabeth Newton

Elisabeth was a Harvard graduate student and an astrobites and ComSciCon co-founder and is now a professor at Dartmouth College.

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