Binary Red Supergiant Stars
Much work has gone into understanding the binary properties of stars on the main sequence and lower mass red giant stars, however the binary nature of the Red Supergiant Stars (RSGs) remains largely unknown. There are a small number of examples of RSGs in binary systems in the Galaxy, but these seem to be relative odd balls.
I am currently studying the binary nature of the RSG population of the Tarantula region as part of the FLAMES VLT Tarantula Survey (VFTS), a multi-epoch study of a small number of RSGs in one of our nearest neighbor galaxies, the Large Magellanic Cloud. In this study we no examples of RSGs within binary systems similar to those found in the Galaxy, adding strength to the argument that as a would-be RSG in a close binary system evolves towards the red, interaction between the stars prevents the RSG phase of evolution.
Red Supergiant Stars as Cosmic Abundance Probes
I am currently working with near-IR spectroscopic data from KMOS focusing on massive stars in the Galaxy and external environments. My PhD project focused on RSGs in extragalactic environments and more generally, I am interested in the location and distribution of massive stars and how studying these extreme stellar objects can give us insights into stellar evolution and the evolution of galaxies.
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.
In NGC6822, a dwarf irregular galaxy with a turbulent history, we found weak evidence for an abundance gradient within this galaxy.
In the figure on the left, we show metallicities of RSGs shown against their distance from the centre of the NGC6822. Black points show RSG results from Patrick et al. 2015, blue and red points show BSG results from Venn et al. (2001) and Muschielok et al. (1999), respectively. A least-squares fit to the KMOS results reveals a low-significance abundance gradient. For comparison, R25 = 460′′ (= 1.03 kpc; McConnachie 2012).
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.
The Figure on the left shows the radial velocity estimates for this cluster shown against radius from the 'centre' of the cluster. Black circles show RSG results from Patrick et al. 2016. The green dashed line shows the LMC systemic velocity of ∼200 massive stars from (Evans et al. 2015). The solid black line shows the mean cluster velocity. The blue triangles show estimates for two OB-type stars in NGC 2100 (Evans et al. 2015) and the red squares show previous estimates for three of our targets (Jasniewicz & Thevenin 1994).
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?
- Patrick et al. 2017, MNRAS, 468, 492 - Physical properties of the first spectroscopically confirmed red supergiant stars in the Sculptor Group galaxy NGC 55
- Patrick et al. 2016, MNRAS, 458, 3968 - Chemistry and Kinematics of Red Supergiant Stars in the Young Massive Cluster NGC2100
- Patrick et al. 2015, Apj, 803, 14 - Red Supergiant Stars as Cosmic Abundance Probes: KMOS Observations in NGC6822
- IAU Vienna, FM7: Radial Metallicity gradients in Star Forming Galaxies
- Frontiers of the Physics of Massive Stars, Ensenada, Mexico (Invited)
- VFTS11 Tenerife
- EMIR@GTC: aprendiendo a usarlo , Cadiz, España
- JWST@ROE, Edinburgh, UK
- DEX2016, Durham
- Stellar Populations Newton meeting, Sao Paolo, Brazil (Invited)
- SUPA Cormack meeting 2014
- NAM2014, Porstmouth - Massive star evolution and the diversity of supernova parallel
- Durham Edinburgh Extragalactic Workshop
- BRIDGEC workshop 2014, Keele University