Article by Aaron Golden, Visiting Astronomer at the Armagh Observatory and Planetarium
Stephen Bourke works at the Department of Space, Earth and Environment, Onsala Space Observatory in Sweden, and Aaron Golden at the School of Maths in NUI Galway, and is a visiting astronomer at the Armagh Observatory and Planetarium. The I-LOFAR observations were taken as part of LOFAR proposal LC9_040 “A search for aurora on nearby flare stars using LOFAR”.
I-LOFAR reached another milestone on the night of the 6th of March, when the entire LOFAR telescope network across the European continent including the Birr outstation was for the first time used by Irish astronomers Stephen Bourke and Aaron Golden to observe the nearby flare star CN Leonis. The team hope to ‘catch’ a stellar flare exploding in the star’s corona, and to use the radio observations taken at Birr and across the LOFAR network to understand how such flares evolve over time and how similar they are to the solar flares we experience here on Earth. CN Leo is a small, red dwarf star about 8 light years away in the constellation Leo, and is likely to possess a planetary system. In fact, we now know that the vast majority of stars in the galaxy that have planetary systems that could harbour habitable planets orbit red dwarf stars like CN Leo, so a really important question to answer is whether or not such planets could survive the really very powerful stellar flares we see from many of these red dwarfs. Studying the way in which such stellar flares occur and how they interact with their local environments using I-LOFAR offers a new window on this important area of astronomy.
Variable stars in the night sky have been known since antiquity – some of you may have heard of the naked-eye star Algol, at the end of the slightly skewed ‘T’ that forms the constellation Perseus. It is an eclipsing binary, whereby the passage of the cooler, and dimmer, companion star passes in front of the larger, brighter primary star – in effect, making Algol looking perceptively dimmer, about 1.3 magnitudes every 3 days. Stars that dim through eclipses are extremely useful to us, as observations can be used to study stellar atmospheres, and most famously, if the dimming is caused by a planet crossing the stellar disk, we can measure that too – in fact this is how all of the exoplanets that are regularly announced by NASA using their Kepler/K2 orbiting observatory are made. Thanks to this technique, we now know that there are thousands of exoplanets orbiting nearby stars, which in many respects is almost as revolutionary a concept as Copernicus’ proposal that the planets go around the sun.
The most interesting thing about planets is the possibility that life could exist on them, and astronomers have already embarked on studies to try and determine if the ingredients for life as we know it are present on these exoplanets. There are many ways, both direct and indirect, to try and see if an exoplanet might fit the bill.
How close is an orbiting planet to a star? Too close, and the perpetual roasting heat will bake away any atmosphere, such as what we see with Mercury. Too far away, and the planet will exist in an endlessly freezing state, the star being too far away to allow a rocky world like ours to sustain a gaseous atmosphere, critically in the temperature regime that allows water to stably exist in liquid form. The distance bounded by this inner & outer limit is known as the habitable zone, and where it can be determined is almost entirely based on how hot the central star is, which can be determined from the colours of the star itself. But this is only half of the story – for our solar system, the planets Venus, Earth and Mars lie in the Sun’s habitable zone, yet we all know only one is actually habitable.
Direct observations can resolve whether an exoplanet is ‘habitable’ by studying the minuscule difference in the observed spectrum of a star as a planet passes in front of it. These changes come from light scattering through the planet’s thin atmosphere and using the largest of ground based telescopes, along with the Hubble Space Telescope, signatures of water have been found on other worlds. How can you determine directly what a ‘living planet’ looks like compared to an inert one? Remarkably enough, this is where the Moon is very useful. You might have noticed when the sky is sufficiently dark that you can still sort of see the other part of a bright crescent moon – the part of the moon supposed to be in shadow from the sun. Through binoculars or a telescope, this ‘ashen light’ is gloriously apparent. This the light of our planet reflecting off the dark side of the moon. Beautiful that it is, it’s also a signature of what reflected light is like from a ‘living’ planet, and astronomers can take a spectrum of the Sun’s normal light, and a spectrum of this ‘earthshine’, subtract one from the other, and hey-presto, have a spectrum of what Earth actually looks like. Its then easy to detect the interesting signatures associated with water, with various oxygen species, and the broad humps and bumps corresponding to the oceans or the forested landmasses. One day in the very near future, astronomers will be able to make these same types of observations for the nearest exoplanets.
So what has all this got to do with CN Leo, and the observations recently performed by I-LOFAR?
CN Leo is a cool M dwarf – its a lot cooler than our Sun, so its habitable zone is closer in. The galaxy has a lot more of these types of stars, than stars like our Sun, and perhaps more pertinently, the vast majority of exoplanets discovered to date orbit stars like CN Leo. The other thing about stars like CN Leo is that they are pretty old, so given what we know about our own ‘family history’ i.e. the billion or so years it took life to evolve, this would tend to ‘shorten the odds’ of habitability. That’s the good news.
The bad news is that stars like CN Leo undergo, for reasons we still don’t fully understand, frequent and at times very violent flare events – like the giant solar flares we occasionally hear about originating from our own Sun. When a stellar flare occurs, enormous amounts of energy are released as electromagnetic radiation and high energy particles, and this event can have devastating implications for anything nearby, such as a planet, as the impact of this energy can in effect strip away and evaporate any atmosphere, and bathe its surface in lethal ionizing radiation. On Earth we are incredibly fortunate that our planet possesses a magnetic field. Like iron filings sprinkled on top of a bar magnet, the torus like magnetic field lines create a cocoon, known as the magnetosphere, that shields us from the searing solar wind 24/7, and which buckles yet remains resilient when a solar flare’s particle bomb – a coronal mass ejection – hits, providing us with the beautiful light show that are the aurorae.
For M dwarfs though, the flares are much more common, and much more violent – the repeat offenders make up a well studied group known as ‘flare stars’, and CN Leo is one of these. Just like our sun, the best way to try and understand the origin and evolution of a flare is to observe it happening, using many different types of observations – in X-rays, in the optical, using radio waves – as these all probe different physical components of the process, and so allow us to fit together the jigsaw of the underlying physics involved. Its only very recently that we have been able to study the Universe in the radio waves more normally associated with transistor radios, and using the LOFAR telescope we will be able for the first time have a critically important missing piece in that jigsaw puzzle. This is why our recent observations of CN Leo with LOFAR also involved Jodrell Bank’s e-MERLIN radiotelescope array, John Moore’s University’s robotic Liverpool Telescope at the Roque de los Muchachos Observatory in the Canary Islands, and NASA’s Neil Gehrels Swift Observatory.
And there is the potential of a ‘bonus’. The beautiful colors we see in optical light from the aurorae also have a distinct signature in radio light, and frequency on the dial we need to set our receivers to so we can listen to the Earth’s aurorae are set by the Earth’s magnetic field. Fortuitously, radiotelescopes like LOFAR can be tuned to that set of ‘planetary’ frequencies, and so detect the distant dance of an aurora from an as yet undetected exoplanet orbiting CN Leo, an exoplanet that had a sufficiently strong magnetic field to protect it from CN Leo’s stellar flares. And if so for the CN Leo system, why not for many of the other exoplanetary systems orbiting other M dwarfs in our little corner of the galaxy?