Roughly half of all stars orbit around another star making a binary star. Their orbital period is the length of time it takes the stars to make one orbit around their common center of gravity. The longest periods can be hundreds of years and the shortest is 5.4 mins. I study those binaries which have orbital periods less than a handful of hours. These compact binaries contain a white dwarf which is accreting material from a companion star through Roche Lobe overflow and are called Cataclysmic Variables.
I’ve studied this diverse set of binaries over many years and in many wavelengths. Those accreting white dwarfs which have strong magnetic fields (B>10MG) are called polars since they are strongly polarised in the optical. They are also strong X-ray sources with some being intense emitters of soft X-rays – indeed many were discovered through the Rosat all-sky survey.
Those Cataclysmic Variables whose white dwarfs are not strongly magnetic can show regular outbursts – also called dwarf novae – every few weeks, months or years. The Kepler/K2 and TESS space missions have obtained light curves of these binaries which extend over several years in the case of Kepler and a month in the case of TESS. Those binaries which are eclipsing – where the inclination angle of the binary is such that the secondary star can obscure the white dwarf once an orbit – are especially useful systems. Data from eclipsing binaries can be used to measure the mass of each star and show how the accretion disc changes its size as the system brightens during outburst and then returns to quiescence.
The most compact binary systems are those which contain two white dwarfs. Since they are physically small they can get very close indeed. The most compact is HM Cnc which was discovered using Rosat data and then subsequently identified to show optical variability on the same 321 sec period. It is so compact both stars could fit inside the volume of Saturn. These sources are predicted to emit intense amounts of persistent gravitational waves and are verification sources or the Lisa mission due to be launched sometime in the next 15 years.
Gravitational-Wave Optical Transient Observer
The direct detection of gravitional waves using the Ligo gravitational wave detectors in September 2015 was one of humankind’s greatest achievements. It was the equivalent of measuring the distance to the nearest star to our Sun better than the thickness of a human hair. Gravitational waves offer a route straight to the heart of the most extreme systems in nature and environments that are inaccessible to conventional astronomical techniques. This makes them powerful probes of extreme conditions and beacons to the distant universe.
However, gravitational wave detectors are currently not able to accurately pin-point the location in the sky of these waves. It will be rather like the bird watcher hearing an interesting call in the distance; the direction can be determined roughly but then the searcher must scan visually for signs of movement to pinpoint the cause. Although merging black holes are not expected to show an immediate optical signal, merging neutron stars are.
The problem is that the detectors can only locate the merging system to an area thousands of times the area of the moon. If the region can be mapped quickly enough new sources can be identified which were not present before the event took place. This idea was spectacularly demonstrated when in Sept 2017 a merging neutron star binary was detected first in gravitational waves and then a few days later in optical, radio and X-rays. This event became one of the most well studied astronomical events ever made and indicated that gold may well originate in these violent events.
In 2015, the Universities of Warwick and Monash in Australia developed the Gravitational-wave Optical Transient Observer (GOTO). The concept was to have a series of telescopes on two mounts allowing us to cover 100 times the area of the moon in one go. As soon as a gravitational wave was triggered the robotic telescope would start taking images of the part of the sky where the event was expected to be. Since then Armagh Observatory and Planetarium and a number of UK and international groups have joined the GOTO consortium and a prototype has been operating on the mountain top of La Palma in the Canaries.
The prototype telescopes allows us to cover 18 square degrees of sky simultaneously so we can map the whole sky in roughly a week. We have additional telescopes which will be mounted later this year allowing us to cover the sky more quickly. This is essential if we are going to weed out new sources which are not the gravitational wave event but other events such as supernovae, accreting binaries or flare stars. Our design ensures we are able to compete with other world class facilities and can be seen as being a prototype to the much larger LSST telescope being built in Chile.
BlackGem is an international astronomical project, with founder partners the Netherlands Research School for Astronomy (NOVA), Radboud University (Netherlands) and the KU Leuven (Belgium). I am one of the PI-level partners which includes groups in the UK, Israel, USA, Germany, Denmark and Ireland.
The main goal of BlackGem is to detect the optical emission from Gravitational Wave events – one of the most exciting areas in astronomy today. BlackGem is complementary to the GOTO project, in that it can go deeper, but with less sky coverage, and can detect bluer light than GOTO. An initial three telescopes have been built at the European Southern Observatory in Chile and are being commissioned during 2022.
Those of us who have been at high northern (or low southern!) latitudes and been able to see stars in the night sky may been lucky to have seen aurora. These ghostly features are due to flares or other eruptions from the Sun. Mostly the Earth’s magnetic field shields the planet from these bouts of Solar irradiation. However, sometimes they can have a direct impact on the Earth. The Carrington event of 1859 was the most powerful white light flare ever to have been observed on the Sun and caused disruption to the new telegraph network in North America. More recently, in 1989 the electricity supply in Quebec was severely disrupted by a Solar Storm causing wide spread blackouts.
Other stars, in particular stars which have masses less than half of the Sun can show many flares, the most extreme of which can become 9 magnitudes brighter for a short time. For stars with spectral type later than M4 it is thought their interiors are fully convective. However, since these stars have no tachocline (a boundary zone between the radiative and convective zones) the star would not expected to have a significant magnetic field if it was generated in the same manner as the Sun. However, some late M dwarfs show strong flaring activity suggesting these flares may be generated by a different mechanism. In Armagh, Lauren Doyle, Gerry Doyle and myself have been using Kepler, K2 and TESS data to investigate flares from these dwarf stars in more detail.
The light-curves of low mass M dwarfs can show periodic changes in their brightness as the star rotates. This is widely thought to be the result of a dominant, large star-spot which is cooler than its surroundings rotating in and out of view. From solar physics we know that flares typically originate in active regions which host spots, so we would expect flares to originate from the large star-spots in low mass M dwarfs. However, recent studies such as Ramsay et al. (2013), and Doyle et al. (2018) which used Kepler and K2 data, appear to contradict this view. There was no correlation between the flare number and the rotational phase found in any of the M dwarfs suggesting flares do not necessarily originate from the large, dominant star-spot. Now we have used TESS data to study a much larger sample of M dwarf stars – our finding are the same and reported in Doyle et al (2019). This implies we do not understand how flares are generated in these stars.
There are wider implications from this work. Many surveys including the TESS mission are specifically targeting M dwarf stars to search for exoplanets. Since they are physically smaller than Solar type stars it is easier to detect dips in their light curve if a planet is at the right inclination. If the M star is emitting lots of flares how will this effect the atmosphere of any orbiting planet? Will it make these planets unable to host any life due to the high irradiation? It is only by studying the characteristics of the flares — and whether they change their rate over months or years — will this question be able to be answered.
Plato is an ESA mission whose main goal is to discover Earth-like planets which are orbiting Solar type stars in their habitable zone. Although Kepler and TESS have discovered many exo-planets, they have yet to discover such planets. A key part of the Plato mission is to characterise the host stars and the physical characteristics of their exo-planets. Parameters such as age and mass will be determined through studying stellar pulsations. A ground based followup programme is also central to the success of Plato. The reflex motion of the Earth on the Sun’s center of mass is 10 cm/s. I am the ESA Community Scientist and a member of ESA’s Plato Science Working Team. Plato is due to be launched in Dec 2026 on an Ariane 6 launcher.