Red Supergiant Stars as Cosmic Abundance Probes

I am currently working with near-IR spectroscopic data from VLT-KMOS and Keck-MOSFIRE focusing on red supergiant (RSG) stars outside of our Milky Way galaxy.  This project will deliver precise and accurate elemental abundances (including iron and key-alpha group elements) and as such has the potential to revolutionize our understanding of the metal content of galaxies in the Local Universe. Because of the applicability of these results over large distances I am working to deliver this project on the European Extremely Large Telescope with the HARMONI and MOSAIC instruments. 

This project was pioneered by Dr Ben Davies, Prof. Rolf Kudritzki and Dr Chris Evans. 

RSGs are evolved massive stars which have cool outer atmospheres. For context, if we placed a RSG at the centre of our Solar System, the physical extent of the star would extend to beyond the orbit of Saturn! 

As these stars as so bright and cool, their peak luminosities are in the near-IR (~1.1um) which mean that these stars are the most luminous stars in a star-forming galaxy in the near-IR. Coupled with the intrinsic advantages of dust extinction in the near-IR (over that of the optical regime) and the fact that the next generation of telescopes (JWST, E-ELT) will be specialised for study in the near-IR, RSGs will be at the forefront of exploring star-forming environments in extragalactic systems.

Red Supergiants in NGC6822

With Science Verification time on KMOS (data publicly available from the ESO archive) I demonstrated the technique of estimating stellar parameters including metallicity (chemical abundance) from KMOS spectra. Using a multi-object spectrograph to measure metallicities of stars gives us the potential of looking at abundance variations and trends within external galaxies.

One of the interesting (and unexpected) results to come out of our study of RSGs in NGC6822 we showed that the temperature of RSGs is independent of the metallicity of the star: something which is not predicted by evolutionary models. Previous studies show that the spectral appearance of RSGs in the optical changes as the metallicity of the star changes. In lower metallicity systems RSGs appear to have an "earlier spectral type" (Levesque & Massey 2012, ), which is typically attributed to warmer temperatures. However by looking in the near-IR we show that this is not the case and RSGs maintain a constant temperature of ~4000K. (See Tabernero et al. 2018 for more on the temperature of RSGs.)

Large Magellanic Cloud massive star cluster: NGC2100

Using KMOS guaranteed time observations (GTO; data publicly available from the ESO archive) we have measured the Chemical and Kinematical properties of a young star cluster in the LMC (NGC 2100) using RSGs. With these data we were able to place an upper limit on the line-of-sight velocity dispersion of the cluster and calculate, for the first time, its dynamical mass.

Additionally, we estimate stellar parameters, including metallicity, using the a new analysis technique and find good agreement with previous estimates for the metallicity of the cluster. We also show that by using a simulated integrated-light cluster analysis that the same technique can be used an unresolved cluster of stars which are dominated by RSGs. See Patrick, 2016 for details. 

Massive Stars in the Galactic Centre

I am currently working on a project based on studying the most massive stars in a very highly star forming region towards the Galactic Centre. This project aims investigating the origin of some extremely massive stars in the region and finding an answer to the question: Why do some of the most massive stars appear outside of dense cluster environments?