Authors: Federico Sestito, Sara Vitali, Paula Jofre, Kim A. Venn, et. al.
First Author’s Institutions: University of Victoria, Universidad Diego Portales, Millennium Nucleus ERIS
Status: Submitted to A&A [open access]
Many astronomers are extremely interested in Old Things. When we go back to the beginning and look at how galaxies, large-scale structure, or even the Universe as a whole are put together from fundamental building blocks, we gain an invaluable understanding of the context in which we live. Studying the big picture is vitally important.
The problem with studying the big picture, though, is that it tends to be hard to see. Older, fundamental structures are by definition further away, so we’re limited in our ability to characterize them by the capabilities of our telescopes. Many astronomers instead try to understand the formation and evolution of galaxies through a discipline called galactic archeology, in which we use the kinematic and chemical properties of our Milky Way and its nearby galaxies to reconstruct their histories. That’s where today’s paper comes in! The authors analyzed the chemistry of stars in the Sagittarius dwarf galaxy, one of the Milky Way’s closest neighbors, to reconstruct how it might have formed.
The chemistry of stars
Although most of the matter in stars, as well as in the Universe as a whole, is hydrogen, plenty of other elements can be found in a star’s chemical makeup. One of the most well-studied is iron, whose abundance (the percentage of iron in the star’s atmosphere relative to hydrogen, written as [Fe/H]) is used as a shorthand for the amount of metals, or elements heavier than helium, that are present. Iron abundance is also called metallicity for this reason. We measure chemical abundances in units called dex, which refers to their order of magnitude difference from the chemical abundances of the Sun. So the Sun has an iron abundance [Fe/H] = 0 dex, and a star with [Fe/H] = -1.0 dex would have a metallicity ten times lower!
Because iron is produced in supernovae, older stars tend to have less of it than younger stars – there just hadn’t been as many supernovae in the history of the Universe when older stars were born, so the amount of iron available to them at their formation was lower. This means that a star with an iron abundance of less than -2.0, or over 100 times less than that of the Sun, is likely to be extremely old, and studying a population of these very metal-poor stars could yield important insights into the earliest days of a galaxy.
Today’s authors analyzed 12 stars with metallicities lower than -2.0 using the MIKE spectrograph on the Magellan-Clay telescope at the Las Campanas Observatory in Chile. If you’re curious about spectroscopy, we have a whole guide, but the essential idea is that the authors have measured the intensity of light from each star over a range of wavelengths. At certain wavelengths, the intensity dips sharply as light is absorbed by a particular element. The shape of these dips can tell us a lot about the chemical makeup of the star, and therefore the history of the system it was found in!
Stripping back the layers
The mechanisms behind the formation of heavy chemical elements are the so-called r-processes and s-processes, short for rapid and slow neutron-capture processes. These two mechanisms take place in different environments and over vastly different timescales. The r-processes take place in violent environments, like core-collapse (type II) supernovae and neutron star mergers, over very short amounts of time. The s-processes, on the other hand, occur over thousands of years, usually in stars on the asymptotic giant branch (AGB) of the Hertzsprung-Russell diagram. Some elements can be created by both r- and s-processes, others by only one of the two, so by comparing the abundances of different elements the authors can reconstruct the prevalence of each type of environment in the cluster’s history.
The authors measure the relative abundances, shown in Figure 1, of several different elements, all of which are produced through different mechanisms and can tell us different things about the history of the galaxy. The large spread they observe in the ratio of strontium (Sr) to barium (Ba), for example, indicates that both r- and s-processes were present in the galaxy’s early history. Barium is produced mostly via s-processes in AGB stars, whereas strontium can be produced either through s-processes or in supernovae, so that a wide distribution of [Sr/Ba] ratios means that the environment the observed stars formed in likely contained the remnants of both AGB stars and core-collapse supernovae!
We can also look at the α elements, formed via the so-called “α-process” as helium (He) is fused into atoms like carbon (C), silicon (Si), magnesium (Mg), and titanium (Ti). These elements are found in the remnants of type II supernovae, but not in the aftermath of type Ia supernovae, so when their abundances are plotted versus the stars’ overall metallicities, astronomers expect to see a sharp change in slope, called the α-knee, where type 1a supernovae become more common than type II. Today’s authors don’t see the α-knee in their data, which might mean that the history of these early stars included a comparatively small number of stars in binary systems with white dwarfs. Only time, and further study, will tell whether this effect truly indicates a lack of type Ia supernovae and whether this lack persists for younger, more metal-rich stars.
The history of galaxies, even small ones, is complex and difficult to study. Like archeological digs on Earth, we often have to disentangle multiple layers of history as we unravel how they form and evolve over time, both in terms of their kinematics and their chemistry. There’s plenty more to understand when it comes to the earliest building blocks of galaxies, but studies like this give us an important window into ancient times. The more we dig, the more we learn!
Astrobite edited by: Jack Lubin
Featured image credit: Unknown artist, about 1400-1410. Getty Museum, Ms. 33 (88.MP.70), fol. 12
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