Stephanie Monty
Astronomer and postdoctoral researcher in Galactic Archaeology

High Precision Chemical Abundances of Stars

I use high resolution spectroscopic data (R>45,000) to measure chemical abundances (everything heavier than He!) in stars. The chemical content of stars can reflect both, i) the elements created within the stars via nucleosynthesis and/or ii) the primordial chemical content of the gas in which the star formed. I generally explore the second scenario, using the abundances I measure to examine what the chemical content of the Universe was like when the star formed. Which elements I measure and how much of each is contained in a given star can inform what types of objects enriched the Universe early on. For example, the abundance of the heavy element Eu relative to other elements can tell me how common exotic phenomena like neutron star mergers were at the time the star formed.

Recently, in Monty et al. 2022, I measured the chemical abundances of stars in the globular clusters, NGC 288 and NGC 362. Two results from this study were, i) that both clusters showed significant star-to-star differences in many elements (see the figure) and ii) that NGC 362 showed an enhancement in Eu which we suggest is primoridal. The first conclusion implies the primoridal gas in which the stars formed was inhomogeneous - informing cluster formation theories. The second conclusion implies that exotic processes like neutron star mergers were important in the environment where the cluster formed.

Coupling Chemistry and Dynamics to Explore Object Histories

To augment my chemical abundance measurements, I explore the dynamical properties of stars and globular clusters. To do this, I combine the positions and velocities of objects from high resolution spectra and astrometrically by the Gaia space telescope (sci.esa.int/gaia/), with dynamical modeling software like galpy and AGAMA to the orbits of objects. Orbital characteristics of objects reveal what component of the Milky Way they belong to (e.g. the Milky Way halo vs the disc). They can also be used to explore if the object formed within the Milky Way or externally in another galaxy that was accreted earlier on in the Milky Way's history (via galactic cannibalisim!).

In Monty et al. 2020 I re-examined a set of 55 Milky Way halo stars from the study of Stephens & Boesgaard, 2002, re-deriving the chemical abundances and orbital characteristics of the sample. Through calculating the actions of the stars in the sample (orbital characteristics that remain the same for all time, under certain assumptions), we identified 11 stars that were dynamically coincident with the Gaia-Encelaudus-Sausage (GSE, Belokurov et al. 2018, Helmi et al. 2018) dwarf galaxy accretion event and another 17 associated with the Sequoia (Myeong et al. 2019) accretion event (see figure). When examining the trend of [α/Fe] vs. metallicity we found unique locations of the "low-alpha knee" in GSE vs. MW and Sequoia vs. MW stars - suggesting unique star formation histories.

Dynamical Evolution of Globular Clusters

Globular clusters are fantastic objects to study both chemically and dynamically. Chemically, they host a plethora of bright, ancient stars making them great targets for chemical abundance analysis. Dynamically, they serve as test particles to probe the gravitational potential of the Milky Way (acting like point particles) and as hosts of hundreds of thousands to millions of test particles probing gravitational interactions within the cluster. Thankfully, decades of research into the theoretical models of the dynamical evolution of clusters has laid the groundwork for follow-up today with high fidelity N-body simulations. With the aid of graphical processing units (GPUs), N-body simulations can now track the evolution of millions of stars within a cluster over the course of a billions of years and multiple orbits around the Milky Way.

Using a GPU-accelerated version of NBODY6 NBODY6+P3T written by Anthony Arnold (UQ), I'm investigating the dynamical history of some of the most extreme Milky Way globular clusters. Stay tuned!

I'm a passionate believer that science should be available to everyone and that everyone has a responsibility to ensure this is the case. I have led and co-led different cultural initiatives throughout my undergrad and graduate career with a focus on improving the culture within academia, specifically for underrepresented groups. An example of this was my participation in the creation of a Cultural Action Plan at the Research School of Astronomy & Astrophysics, ANU and the creation of the first Behaviours Guideline and Value Statement for the School. I have also mentored students ranging from highschoolers (through Science Mentors ACT) to undergraduates throughout my undergraduate and graduate degrees and have enjoyed every minute of it. In the future, I hope to engage with people who may have less direct exposure and interaction with science. If you have any suggestions of organisations you know that do this, please contact me!

Past Outreach & Upcoming Public Talks

• Newspaper article in the Canberra Times Sunday Space: "How are humans similar in make-up to stars and planets?""
• Institute of Astronomy, Cambridge Public Talk on Galactic Archaeology on February 1st, 2023.
  The talk is available to watch: here
• Online, Raincoast Education Society 2023 Speaker Series on Februrary 23rd, 2023. This talk will be advertised soon and will be available to watch over Zoom with an RSVP.