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Stellar Astrophysics

One major facet of astronomy research at the OU is the study of stellar astrophysics. This is accomplished via a variety of means, including theoretical investigations employing sophisticated population synthesis codes and observations made across the electromagnetic spectrum with world-leading facilities including The European Southern Observatory's Very Large Telescopes (VLT), the Hubble Space Telescope (HST), the Southern African Large Telescope (SALT), and the Atacama Large Millimetre Array (ALMA). More detailed descriptions of some of our research projects can be found below. We also deploy our fully autonomous robotic telescopes in Tenerife (COAST and PIRATE) for monitoring and surveying of bright targets.

Massive stars – from formation to death.

Despite compromising only 0.00003% of the total number of main sequence stars, stars with masses twenty or more times greater than the Sun play a disproportionate role in the evolution of the Universe and their host galaxies. They are the progenitors of some of the most luminous transients in the Universe -  gamma-ray bursts and  core-collapse supernovae. Indeed the coalescing black hole and neutron star binaries that have been detected by the gravitational waves they emit are descendants of such massive stars. But how do such stars evolve to form such energetic phenomena?  And how massive does Nature permit stars be born - a crucial parameter if one is to understand the formation of the 30 solar mass black holes that merged to form the first gravitational wave source to be detected – GW150914. Our research aims to address such questions, focusing on constraining the lifecycle of massive stars - from birth to death and beyond - and the stellar  hierarchies in which they reside.

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The view of the central regions of our Galaxy obtained at near-IR wavelengths by the HST.

Hierarchical star formation and the central nuclear starburst

Observations of star forming galaxies reveals that the dominant  mode of star formation is hierarchical, with stars forming in either diffuse associations or complexes of individual star clusters. In the past decade IR surveys have allowed us to   recognise such structures throughout the Milky Way, causing us to completely reassess the canonical  picture of the Milky Way as being largely devoid of the vigorous star formation observed in many external galaxies. One particular region of interest is the central molecular zone at the heart of our galaxy. Home to a number of very massive stellar clusters and with apparently isolated massive stars strewn throughout its volume, vigorous  star formation has been underway for at least the last 6Myr. Clark and Lohr are undertaking an extensive VLT+HST observational study of this  circumnuclear  starburst at the heart of our Galaxy. These data will allow the construction  of a full census of massive stars - and their 3-d motions - within this region and hence constrain the recent star formation  history, mass function and total energetic output of the resultant stellar population. This is the  only  nuclear starburst for which such  scientific investigation  is possible -  since its proximity means we can resolve  individual stars -  and hence our only opportunity  to produce a template for other (circumnuclear) starburst  clusters in external galaxies.

The lifecycle of massive stars

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The Arches massive star cluster.

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The Quintuplet massive star cluster.

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The Westerlund 1 massive star cluster.

Another reason to study young starburst clusters is that they function as the perfect laboratories for massive stellar astrophysics - a field that has received renewed impetus with LIGO's discovery of a number of merging black hole binaries. Specifically how do the progenitors of such events form, how many should we expect and what mass ranges shouldthey span? To answer these questions requires detailed knowledge of both the lifecycle of massive stars and their binary properties, both of which are very uncertain; indeed we currently cannot predict how massive a star may be born, nor what mass ranges yield neutron stars and which black holes. In order to address these questions Clark is leading  a comprehensive survey of Galactic clusters, supported by significant amounts of HST, VLT and ALMA  time.  With ages of between 1-20Myr such clusters  host stars with masses ranging from ~10 to >>100 times the mass of the Sun. Indeed our observations of the Arches cluster has identifed  the most massive star identified within the Milky Way, with a current mass ~83 times that of the Sun implying a mass at birth of 120-150 solar masses!

Could this star evolve to form a high mass black hole, similar to those implicated in the gravitational wave source GW150914? This would depend on how much mass it sheds prior to core collapse. Unlike stars like the Sun, which lose a comapratively small proportion of their mass, very massive stars lose the majority of their masses via powerful stellar winds – for example a star sixty times the mass of the Sun may exect to lose 75% of its mass by the time it explodes as a supernova. By considering an ensemble of clusters of differing ages we can probe how  mass-loss rates   vary as a function  of initial stellar mass and in collaboration with researchers at UCL we have pioneered the use of millimetre observations with ALMA to determine the mass-loss rates of over fifty very massive stars at all stages of their evolution within the Westerlund 1 massive cluster.

Massive Binaries

Another important factor in massive stellar evolution is the way in which  stars in binaries interact with one another, essentially stripping mass away from one member which is then accreted onto another. In this way the evolution of both stars may be profoundly altered. Utilising the VLT we have been  able to utilise multi-epoch observations of cluster members in order to identify binaries and subsequent determine their properties such as orbital period, inclinations and stellar masses. This has led to us being able to demonstrate that in some situations binary-driven mass loss is so extreme that neutron stars form from stars that are initially so massive (>40 solar masses) that one would expect black holes to form instead.

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Artists impression of the young and highly magentic neutron star within Westerlund 1.

Stellar transients

Stellar winds and binary interactions are not the only mechanism by which stars may lose mass - as stars approach the Eddington limit, dynamical instabilities can  initiate transient eruptions during which tens of solar masses may be lost over years to decades. These are the  so-called luminous blue variables (LBVs), of which Eta  Carinae is the most famous example. LBVs have received much interest recently given  their identification as the direct progenitors of some of the most luminous type-IIn core-collapse    SNe ever detected - a finding that directly contradicts current stellar evolution theory. Clark has a  long standing interest in studying LBVs and their transient eruptive episodes, both in the Milky Way  and external galaxies, which  more closely resemble the host galaxies  of highly luminous type IIn SNe. In our own galaxy the proximity of sources enables us to resolve their ejection nebulae, providing a `fossil' record of the long-term mass loss history of their host stars, while multi-object spectrographs and  robotic  telescopes allow us to mount monitoring campaigns covering whole galaxies and multiple LBVs in   order to characterise the duty cycles and  properties of the eruptive events, which both lead to mass ejection and appear to pressage the death of some LBVs as core-collapse SNe.

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Spectacular emission nebulae associated with the LBV eta Carina and the H-depleted Wolf-Rayet WR124.

Monitoring the variable Sky: searching for electromagnetic counterparts of gravitational wave sources and other transients

The Nobel Prize winning discovery of gravitational waves by LIGO in 2015, and the discovery of gravitational waves from a binary neutron star merger in 2017 by LIGO and Virgo, was accompanied by an extensive observing campaign searching for electromagnetic counterparts of these gravitational wave sources. We have configured our Tenerife-based robotic telescope PIRATE to autonomously respond to gravitational wave alerts by imaging a mosaic of the on-sky gravitational wave source error ellipse. We were a signed up partner of the LIGO/Virgo EM consortium during the O1 and O2 LIGO observing runs, and will continue to search for counterparts using public alerts in O3. This wide-angle survey serendipitously reveals numerous other transients and variable sources for further study. We also follow up Gaia Science Alerts, with a particular focus on cataclysmic variables and microlensing sources. The frequency and characteristics of binary microlensing events provide diagnostics for the ubiquity and parameter distribution of multiple stellar systems, and holds potential for the study of circumbinary planets.

Stellar and binary population synthesis studies of low-mass systems

We conduct Galactic stellar and binary population synthesis studies with a focus on low-mass systems, and on modelling observable population characteristics of eclipsing binary stars such as those in the Kepler Eclipsing Binary Catalogue. Comparison with observed samples allows us to constrain ill-understood physical mechanisms affecting the formation and evolution of single and multiple stellar systems. Gaia Date Release 2 represents an important calibrator for population models and signals a step change in the diagnostic power of the population synthesis approach.

Mining archival time-domain datasets – pulsators and eclipsing systems.

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Artists impression of a doubly eclipsing hierarchical qunitple star system. Credit: M. Lohr

We lead efforts to mine large photometric data archives, such as that of SuperWASP, to investigate aspects of stellar variability and multiplicity. Recent highlights have included the identification of over 140 eclipsing binaries near to the observed contact binary short-period limit (i.e. with 4.5h < P < 5.5h), increasing the known population of such stars by an order of magnitude. A more general survey of SuperWASP eclipsing binaries allowed us to characterize orbital period changes in almost 14,000 systems and so determine their higher-order multiplicity fraction as around 24%. We also carried out the systematic investigation of 10-year lightcurves of a dozen eclipsing post-common-envelope binaries which has revealed coherent eclipse timing variations in 4 systems, providing likely evidence for circumbinary objects (possibly planets) in each case. Another survey led to a catalogue of almost 5000 RR Lyrae stars, of which nearly 3000 were newly recognised, and the identification of almost 1000 Blazhko effect systems, of which around 900 were newly recognised as displaying this phenomenon, including 20 or so displaying extreme amplitude modulation. The SuperWASP archive has also allowed us to uncover intriguing, unique objects, such as the only known doubly-eclipsing hierarchical quintuple system and the highest amplitude, and brightest, semi-detached eclipsing binary containing a delta Scuti star. These objects inform our understanding of how stellar systems may be characterised, as well as how they form and evolve.

A Citizen Science project at the Zooniverse website allows volunteers to classify the lightcruves of 1.6 million SuperWASP variable stars.