Galactic Evolution – Through the Lens of Oxygen?

Title: 18O/17O abundance ratio toward a sample of massive star forming regions with parallax distances

Authors: Chao Ou et al.

First Author’s Institution: Guangxi Key Laboratory for Relativistic Astrophysics, Department of Physics, Guangxi University, Nanning 530004, PR China 

Status: Accepted to MNRAS [closed access]

For a large portion of our history, many scientists (such as Newton) and philosophers (such as Aristotle) believed our universe to be immutable and unchanging. Today, we understand that while this may seem to be the case from our human point of view, in fact the stars and galaxies of our universe are all evolving and changing as time goes on; and this evolution is a process that can be studied and modeled, so as to understand where we came from and where we are going.

To understand how a galaxy evolves over time, one can study the composition of the interstellar medium (ISM) – the gas, dust, and molecules that exist in the vast space in-between stellar systems. This is because, while the ISM is primarily composed of hydrogen gas (the most abundant element in the universe), as stars are born, evolve, and die, they seed the ISM with the heavier elements and molecules they produced during their lives. As a result, the evolution of the galaxy can be traced through the fingerprints of these molecules, left behind in the expanse between the stars.

The authors of today’s paper look to better understand the evolution of our own Milky Way galaxy by studying the ratio of two specific isotopes of oxygen in the ISM – 18O and 17O, two different kinds of oxygen atoms that differ in the number of neutrons in their nucleus. These isotopes are produced by different stellar nucleosynthesis processes, which means that the ratio of the two abundances in different regions of the galaxy can tell us how the stars in that region evolved. Tracing these abundances in the Milky Way can thus reveal larger trends in the evolution of the galaxy itself.

Galactic Fingerprinting

The two main processes involved in creating 18O and 17O inside stars are the triple-alpha process and the CNO cycle, respectively. During the triple-alpha process, three helium nuclei (also known as alpha particles) fuse to create carbon-12 (or 12C); sometimes, if there is enough helium and high enough energy, an additional helium nucleus can fuse with this 12C nucleus to form 18O. Meanwhile, the carbon-nitrogen-oxygen (or CNO) cycle is a cycle in which carbon, nitrogen, and oxygen isotopes form and decay into each other in a loop, absorbing hydrogen and generating helium and energy at different steps in the process. While there are multiple variations on the CNO cycle occurring inside the cores of stars, sometimes the cycle is able to produce the key 17O isotope which the authors are hoping to observe.

The important difference for this study is the type of stars in which these cycles occur. You need a very massive star (13-25 solar masses) to be able to produce 18O via the triple-alpha process, while the CNO cycle is common in stars of low and intermediate mass. Since massive stars burn their fuel faster, 18O is injected into the ISM at a faster rate than 17O, and thus the 18O/17O ratio can tell us how old a particular region of the galaxy is, as this ratio should decrease over time. One hypothesis is that the Milky Way formed inside-out; the innermost regions forming first, and the outer ones later. If so, then the 18O/17O ratio should increase with increasing distance from the center of the galaxy (“galactocentric distance”).

Do the Stars Align?

In order to test this hypothesis (and other models of galaxy evolution), the authors used the Institut de Radioastronomie Millimétrique (IRAM) 30m telescope to observe 51 massive star-forming regions of the galaxy, at different distances from the galactic center. In particular, they looked for the spectroscopic signatures of C17O and C18O, two carbon-oxygen molecules with similar chemical properties – the difference in intensity in the spectral lines of these molecules can be used to derive the abundance ratio of the oxygen isotopes in the region. Additionally, they also used the Yebes 40m telescope to observe five of those regions for the spectral lines of HC18O+ and HC17O+ as well (positively-charged, hydrogen-carbon-oxygen molecules), so as to make sure the 18O/17O abundance ratios derived from different base molecules are consistent.

Figure 1: Abundance of 18O/17O versus galactocentric distance, in kiloparsecs. Blue triangles are results using HCO+ observations, black circles are CO observations with peak brightness temperatures lower than 4 Kelvin, and cyan circles are CO observations with peak brightness temperatures higher than 4K. This latter distinction may be important as C18O might become optically thick at these temperatures, lowering the observed ratio. Figure 2a in the paper.

The results of their observations are shown in Figure 1. In particular, there does appear to be a trend of increasing 18O/17O ratio with galactocentric distance, though this trend is weak. While the error bars for some regions remain large, these results are more precise than those done in previous, similar studies, which lends credibility to the result.

Next, the authors compared their results to the results that would be expected from five different models of galactic chemical evolution. This comparison is shown in Figure 2.

Figure 2: Only the CO-derived abundance ratios, compared to theoretical results from five different galactic chemical abundance models. Solid lines are models published in Romano et al. (2019), dashed lines are models discussed in Colzy et al. (2022). The models are discussed in detail in the linked publications. Figure 3 in the paper.

While some models are clearly closer fits to the observations than others, no model can explain the observed relation between the 18O/17O ratio and galactocentric distance. This is most obvious for distances larger than 11,000 parsecs (11 kpc), where there are few data points; but even at smaller distances where there are many, the scatter in the data points at any particular galactocentric distance is too large when compared to their uncertainties. This seems to suggest that while there is a weak correlation between 18O/17O ratio and galactocentric distance, other factors must contribute to the 18O/17O ratio that are not well understood or have not been taken into account.

In order to resolve these questions, a combination of different advancements will have to be made. Higher-sensitivity instruments will be able to pin down uncertainties even further, as well as observe more star-forming regions in the galaxy, adding more data points to the sample. In particular, observations of sources at distances larger than 11 kpc and at temperatures where C18O might become optically thick (and thus less bright, reducing the intensity of emission) are required to fill out the breadth of results. Spectral lines of additional molecules can be observed to double-check the consistency of  18O/17O abundance results. Finally, improved models of galactic chemical abundances can also help explain the derived 18O/17O ratio and galactocentric distance relationship. Any or all of these factors combined can help shed light into the evolution of our galaxy (and galaxies in general), thus deepening our understanding of the universe in which we live.

Astrobite edited by Megan Masterson

Featured image credit: ESO

About Aldo Panfichi

Hello! I'm currently finishing up my Master's degree in Physics at the Pontificia Universidad Católica del Peru in Lima, Peru, writing a thesis project related to asteroids. I previously got my BSc in Astronomy and Astrophysics at the University of Chicago. In my free time, I like spending time with my friends (and my dogs!), going swimming in the summer, and cozying up inside in the winter, playing games or reading science fiction.

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