Armagh Observatory Consolidated Grant

Lead Research Organisation: Armagh Observatory
Department Name: Astronomy


This research programme combines projects in Solar Physics, Planetary Science and Stellar and Galactic Astrophysics. These fields encompass studies of our Sun and Solar System, and Stars (including the evolution of both single and binary systems, i.e. two stars orbiting around one another) and the role played by stars as tracers for our understanding of the wider Universe.

A key uncertainty in our knowledge is the role of binarity in the evolution of stars, and the interactions of one star with another during their evolution. In this case, at different times, material from one star can flow onto the other (and sometimes vice versa), and more rarely two stars may collide to produce a single more massive object or sometimes a gigantic explosion called a supernova. Such stellar collisions are called mergers, and they lead to objects with unusual chemical composition which contain a 'fossil' record of the two stars' joint history or (in the case of a supernova) to an object with short-lived properties that can be used to probe the most distant parts of the Universe.

A second area of our research on stars concerns the measurement of stellar magnetic fields, and the impact of magnetic fields on a star's evolution. The reason why some stars are magnetic, and others less so, remains a mystery, and our work aims to provide reliable data with which to compare the different ideas. A third is the origin, evolution and fate of the most massive stars in the Universe. Their evolution is dominated by powerful stellar winds. Do such stars explode disruptively at the end of their lives, or do they ultimately collapse to produce black holes; and, in either case, what is the effect of the stellar wind on neighbouring stars and the nearby star-forming regions? This work will significantly advance our understanding of stars.

Our work on the Sun - our nearest Star - has implications not just for understanding stars generally but also for how processes in our Sun's visible atmosphere produce the observed phenomena that ultimately leads to heating of its million-degree Corona and the formation of the Solar Wind. The Sun is a variable star showing a dominant roughly 11-year cycle of magnetic activity between episodes of sunspot maximum and minimum. It is currently observed continuously by a fleet of spacecraft, and our detailed observations from instruments onboard these spacecraft (which cover a very wide range of wavelengths) are designed to improve our understanding of the physics of the Sun's atmosphere and the mechanisms by which it produces occasional massive outbursts of mass and energy. Some of these outbursts have huge power, and can lead not just to the visible appearance of aurorae in the Earth's upper atmosphere but to potentially damaging effects on spacecraft and large-scale power systems on Earth. The variable magnetic activity of the Sun has broad implications for Earth's place in the near-space environment.

Lastly, we seek to understand the origin of our planetary system, and the evolution of the small bodies - comets and asteroids (and their debris) - within it. We will study the newly discovered populations of small satellites orbiting the giant planets, for example Jupiter and Saturn, to test theories of the origin of our Solar System. We will also investigate the detailed processes by which comets decay into meteoroid streams, debris from which may occasionally cross Earth's orbit to produce the well-known phenomenon of a meteor shower - the burning up of small pieces of cometary material in the Earth's atmosphere. Not only are there interesting scientific reasons to study such objects and their interrelationships with each other in the Solar System, but the study of Earth's near-space astronomical environment has important practical benefits, leading to better understanding of the distribution of small bodies on near-Earth orbits and the time-variable risk of collisions with the Earth.

Planned Impact

Our research in Stellar Astrophysics encompasses a wide range of detailed investigations into the formation and evolution of stars, taking into account factors such as mass loss through stellar winds, stellar oscillations, stellar magnetic fields, extreme chemical abundances, and the impact of binarity on our understanding of the evolution of stars and galaxies. The work impacts on many areas of the subject, for example the evolution of ultra-compact binaries and how their study provides crucial input for understanding the first detected gravitational wave events.

One prominent area concerns the question of just how massive can stars be in the near universe. It is thought that a large fraction of a massive star's mass is lost through a stellar wind, and this and related work has a major impact on understanding the evolution of such stars and on various survey projects. Theoretical work on the structure of evolved objects such as sdB stars and R CrB stars is informed by detailed spectroscopic investigations of chemically peculiar stars. Systematic analyses of such stars, including surface chemistry, masses and Galactic distribution, allow us to develop improved models of the frequency and physics of white dwarf mergers in the Galaxy and their role in shaping its evolution.

The last decade has also seen a significant number of photometric variability surveys. One of the few deep, high-cadence surveys is the RApid Temporal Survey (RATS). This survey is unique in that it is sensitive to blue stars as faint as g ~ 22 and to variability on time-scales of a few mins to several hours. With follow-up observations, we expect to double our current number (~120,000) of variable sources. Such data will be required for PLATO (which remains one of the three possible ESA Cosmic Vision projects), as follow-up observations are necessary to determine if the transiting object is a planet or low-mass star. This work will likely increase the number of detected Earth-sized planets.

In Solar Physics, the community is poised to answer a number of important scientific questions, for example owing to synergy between Hinode and missions such as SDO, STEREO and RHESSI and the future IRIS. The present rise to solar maximum will allow us to obtain a much larger sample of focused observations than during the last few years, enabling us to identify how solar activity is driven and how it impacts on Earth and its environment. The exceptionally low solar activity of the first years of Hinode operations (during which only two X-flares were observed) are behind us, and Hinode's "Active Sun" programme has as its unifying theme the effects produced by magnetic flux emergence from the solar interior - a key strength of the Observatory's Solar Physics group.

Lastly, the question how planetary systems form remains unresolved. Despite the fact that many hundred of exo-planets are now known, our Solar System still provides the best detailed tests of planetary system formation models. Although we cannot observe this happening directly, we can test models by comparing their predictions with the characteristics of the satellite systems of the major planets. Our work also impacts on the origin and evolution of objects in the inner and outer Solar System; comparative planetology; and meteor physics. The study of interplanetary matter is important for developing an understanding of how planetary systems work: it illustrates a process of decay and replenishment that has necessarily occurred throughout the life of the Solar System. Secondly, given the importance of space missions to society, the economic benefits are substantial. The space industry will benefit from improved detailed knowledge and understanding of the near-Earth and interplanetary environment. Finally, public understanding of science benefits greatly from news stories describing research on disintegrating comets and meteor displays.


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Description The evolution and fate of very massive stars (VMS) are tightly connected to their mass-loss properties. Their initial and final masses differ significantly as a result of mass loss. VMS have strong stellar winds and extremely high ionising fluxes, which are thought to be critical sources of both mechanical and radiative feedback in giant ionized hydrogen regions. However, how VMS mass-loss properties change during stellar evolution is poorly understood.
In the framework of the VLT-Flames Tarantula Survey (VFTS), Armagh astronomers and a team of European astronomers explored the mass-loss transition region from optically thin O to denser WNh star winds, thereby testing theoretical predictions. Joachim selected 62 VMS, an unprecedented sample of stars with the highest masses and luminosities known, and he performed a spectral analysis of optical VFTS as well as near-infrared data using a non-LTE radiative transfer code to obtain stellar and wind parameters. For the firsttime, we observationally resolved the transition between optically thin O and optically thick WNh VMS winds. Our results suggest the existence of a kink between both mass-loss regimes, in agreement with recent Monte Carlo simulations. For the optically thick regime, we confirm the steep dependence on the Eddington factor from previous theoretical and observational studies. The transition occurs on the main-sequence near a million solar luminosities or a mass of 80-90M?. Above this limit we find that, even when accounting for moderate wind clumping, wind mass-loss rates are enhanced with respect to standard prescriptions currently adopted in stellar evolution calculations. We also show that this results in substantial helium surface enrichment. Based on our spectroscopic analyses, we are able to provide the most accurate ionising fluxes for VMS known to date, confirming the pivotal role of VMS in ionising and shaping their environments.

On the solar work, one of the main completed work started with the idea of providing spectroscopic insight into the coronal-wave phenomenon by analysing SUMER observations during one such event for the first time. During the course of the study, we extracted a wealth of information and provided for the first time a spectroscopic description of an asymmetric eruptive filament. We found that the coronal wave registered by EIT, TRACE, SUMER, and CDS was caused by a combined flare with two impulsive peaks.
A CME was generated during the flare and a coronal wave was
observed as the CME propagated across the solar disk in a
non-radial, south-north direction. We clearly identified all the
features that describe the bi-modal nature of the phenomenon
EUV/EIT/coronal wave, i.e. a fast-mode wave and CME-driven
compression. A "breaking" of the CME leading edge is followed
by an asymmetric filament moving "through" the partly disin-
tegrated dimming/CME and the CME-driven compression. We
suggested that either CME-CME interaction or impulsive CME
bubble over-expansion in the low solar atmosphere as a possible cause for the CME disintegration. The filament rises to a certain height and it appears to fall back to its source region. No signature of the wave was found in the spectroscopic SUMER and CDS data. An earlier detected hot-plasma emission in the Mg x 609.79 A° by HS was found to be caused by a blend from the O iv 609.83 A° line. The eruptive filament is recorded only in a transition-region temperature range 4.9-5.8 K (log Tmax) and no coronal emission was detected. Thanks to the time-space combined imaging and spectroscopic data, a sharp increase of the filament Doppler shift observed in the spectroscopic data was found be caused by the movement of the fast moving leading edge of the asymmetric filament under the SUMER slit rather than being produced by a filament acceleration. We found that the twisted body of the filament rises with a speed of up to 150 km/s while moving laterally. The leading edge of the filament lifts up almost twice as quick with speeds of around 300 km/s.
Exploitation Route See the research papers for more details.
Sectors Education,Other