Ordinary Meeting, 2005 May 25
Outbursts, Orbits and Oscillations
Dr Norton subtitled the talk to come 'Time Domain Astrophysics: Compact Interacting Binary Systems and Extrasolar Planets', explaining that this concisely summarised it. 'Time domain astrophysics', he added, was the study of how the brightnesses of objects varied over time. Whereas some objects – nebulae and galaxies – could be resolved and imaged, many others, such as stars, could not, appearing only as points of light. While much could often be learnt from the spectra of their light, if time variations could be detected in their brightnesses, these could then provide valuable new insights into their nature.
Systems whose brightnesses varied dramatically invariably involved extreme physical conditions. To undergo variability on short timescales, they had to be physically compact: a well-tested physical law stated that no information could propagate faster than the velocity of light. For an object to flare significantly within a period of hours, the region of space powering this could not exceed a few light-hours across, otherwise its parts could not 'know' that each other were flaring, barring baroque explanations involving the intelligent prearrangement of such synchrony.
The speaker explained that it was thus understood that these systems were orbiting pairs of stars, one of which was gravitationally sucking gas from its companion's surface. If the former was particularly dense – one of the degenerate compact objects (COs) formed at the ends of the lives of stars, a white dwarf, neutron star, or black hole – then this gas could achieve tremendous temperatures as it collapsed down into the tiny space surrounding its attractor, thus generating a vast amount of light from a very compact volume of space. These were the 'Compact Interacting Binary Systems' of his subtitle.
With reference to the final part of his title, 'Extrasolar Planets', he explained that many of the techniques developed for studying variable stars had now been found to be of great value in searching for Earth-like planets around other stars; he would explain why at the end of the talk.
He explained that the transfer of mass in Compact Interacting Binary Systems could take place by one of two mechanisms. In the first scenario, the CO actively drew material from its companion: either the donor star was of low mass and its gas only weakly bound, or it orbited very close to its compact companion; in either case its outer layers might feel greater attraction to the CO than to the star of which it was part. The technical name for this was Roche-lobe overflow, the 'Roche-lobe' being the point in space between the two stars where their gravitational attractions exactly counterbalanced each other, and 'overflow' referring to the fact that in these systems, the donor star's gas extended beyond this point, such that its outer layers attracted towards the CO.
In the second scenario, the CO played a more passive rôle, and the donor star was at the opposite extreme of mass – a hot massive star, whose intense heat produced a substantial stellar wind. The orbiting CO would capture some of this wind without needing to actively strip material.
The speaker went on to describe the light-curve signatures characteristic of various types of system: objects containing each of the three types of CO had distinct behaviour, he explained. In most cases, the variability was most apparent at either optical and infrared wavelengths, or in X-rays. An animation of the X-ray sky over the four-year period 1996-2000, compiled by a team at MIT from the results of the Rossi X-ray Timing Explorer (RXTE) satellite, illustrated this. The brightest objects were almost exclusively accreting COs, and so the view appeared much more dynamic than its familiar visible counterpart.
He turned first to classical novae, objects which flared over a period of days, later decaying back to normality over tens of days, before repeating the process on a timescale theorised to be ~10,000 years, though tricky to confirm observationally. This behaviour resulted from the accretion of hydrogen gas onto a white dwarf – an object of a little less than 1.4 times the mass of the Sun, but comparable size to the Earth. It was thought that as the accreted gas accumulated on the white dwarf's surface, its pressure built up, until reaching a critical point where nuclear fusion became possible, whereupon it burnt explosively to form helium, producing the observed flare.
Another type of variable star, X-ray bursters, produced quite different light-curves, but from a very similar process. They produced rapid bursts of X-rays, lasting only a few seconds, but repeating every few hours; a famous example was Cygnus X-2. These flares resulted from the same thermonuclear explosions as those of classical novae, but here the hydrogen was not accreting onto a white dwarf, rather an even denser neutron star – an object of between 1.4 and 2.5 times the mass of the Sun, yet compressed to so great a density that it was contained within a mere 10 km radius. The process was entirely analogous, but the timescale more rapid because of the smaller central attractor.
Other objects seemed not to fall into either of these classes, however. Among them were dwarf novae, observed to flare periodically on a timescale of days or weeks. The variability of these was believed to arise from a quite distinct mechanism. Whenever material was drawn onto a CO, it was thought to form a disc-like structure, called an accretion disc, as it spiralled inward towards it captor. In dwarf novae, it was thought that the flaring was caused not on the attractor's surface, but by an instability in the disc itself.
At the onset of accretion, material would begin building up in the disc: the rate of gas entering it would exceed the rate of its delivery onto the white dwarf's surface. As the density of the disc rose, so too would its temperature, eventually exceeding the ionisation temperature of hydrogen, around 4,000°C, whereupon it would suddenly glow brightly and become opaque. The viscosity of this plasma would be much greater than that of the neutral gas from which it had formed, and consequently the disc would rapidly collapse onto its attractor. As its density now became much lower, it would cool and return to a neutral state, ready for the cycle to recommence.
As with classical novae, directly comparable but observationally quite distinct, neutron star systems were also found. These, called soft X-ray transients, the term 'soft' referring here to the low-energy X-rays which they emitted, were among the most dramatically variable systems in the Universe, producing vast flares of X-rays lasting for several months, recurring every few years. As with X-ray bursters, the mechanism behind their variability was essentially identical to that of their white-dwarf-containing counterparts, but taking place on a different scale.
Dr Norton then went on to describe some of the other objects found in the zoo of variable stars. He turned first to intermediate polar systems, a class of cataclysmic variable stars – this being a collective term for all variable stars containing white dwarfs. These, he explained, could be recognised by their characteristic variability on both short – perhaps 15-minute – time-scales, and over much longer periods. Using the light-curve of AO Piscium as an example, he showed how it pulsed periodically at quite precise 15-minute intervals, and how the brightnesses of these spikes was modulated on a timescale of hours, once again, with quite precise period.
He used these systems to introduce one of the most powerful tools of time domain astrophysics: Fourier transform time series analysis. Any plot of how some measured quantity varied over time could be decomposed into a sum of sine waves of different frequencies; by plotting the amplitudes of these as a function of their frequency, one could obtain a frequency spectrum – a map of the timescales on which that quantity varied. Applying this to AO Piscium, he showed that it allowed the intermediate polar's modulation to be characterised much more precisely: the frequency spectrum showed two sharp peaks, one representing 805-second periodicity, and the other 3.59-hour periodicity. Other peaks, at exact multiples, or harmonics, of these frequencies could be ignored: they resulted from the object's light-curve not exactly matching the sine waves used to construct the spectrum, instead having some more complicated waveform of matching period. He showed how, now that the system's period was so precisely determined, its light-curve could be folded onto it, and data from many periods combined to give an accurate profile of the waveform.
The speaker explained that this dual-period behaviour was believed to arise from the accretion of material onto a white dwarf with a strong magnetic field. Whilst magnetism would not affect neutral infalling matter, it would exert a force on ionised material, allowing it to move freely only along the field's direction. This was relevant because, as the accreted material spiralled inwards, the originally neutral gas would heat up and ionise. Suddenly it would become funnelled by the magnetic field, allowing it only to travel in one direction. Now, the star's magnetic field would resemble that of the Earth, emanating vertically from its surface at two magnetic poles, before turning parallel to its surface and connecting around to the opposite pole. This configuration would prevent material from accreting onto its magnetic equator, where the field lay parallel to the star's surface, rendering downward motion impossible. Accretion would, however, be possible at the magnetic poles, where the field was vertical. Thus, its magnetic poles would glow much more brilliantly than the rest of its surface.
The short timescale pulsing of intermediate polars could be explained by a misalignment of the star's magnetic field with its rotation axis; generally these lay in different directions. Thus, as the star rotated, its bright magnetic poles would periodically rotate past our line of sight, giving rise to the observed pulses. The longer timescale modulation, for example, the 3.59-hour period of AO Piscium, could be attributed to the orbital period of the white dwarf around its donor companion, the clarity of our line of sight changing, depending upon the orientation of the two stars. In some cases, AO Piscium being one, brief eclipses were seen during this cycle, suggesting that the white dwarf passed behind its companion.
Turning to one final type of variable star, Dr Norton moved onto X-ray pulsars; as with all the preceeding cataclysmic variables, intermediate polars were not without neutron-star-based counterparts. He reminded members that ordinary pulsars were stars which emitted radio pulses at precise time intervals, attributed to beams of radio radiation emanating from a neutron star's magnetic poles, turning periodically past our line of sight as the star rotated. X-ray pulsars were similar, only the pulses were seen in X-rays, and were attributed to the accretion of material down onto these poles.
The speaker's final topic was how the study of variable stars had led to the development of a new technique for detecting planets around other stars. Previously, searches had focussed upon looking for small motions of stars induced by the gravitational pull of orbiting planets; he reminded members that planets did not strictly circle their Suns, but rather both planet and Sun circled their common centre of mass, it was just that, as the Sun was so much heavier, its motion was comparatively tiny. Thus, planet-bearing stars could be identified by the characteristic swaying back and forth of their Doppler shifts. To date, over 120 planets had been discovered by this route, but most were so-called 'Hot Jupiters', large planets in close orbits (within 1 AU) around their parent stars, which produced the largest, most-easily detectable, stellar motions.
Another route was to look for the small diminutions in stars' intensities which might result from planets transiting across their faces. This method would be considerably more sensitive to Earth-like planets, and had already borne fruits, making its maiden discovery in 2001, in the form of a planet around HD209458, found by the Hubble Space Telescope; five other discoveries had since been made. The light-curves of these transits needed careful scrutiny to distinguish them from variable stars, but they had a characteristic U-shape: flat throughout the transit, with limb-darkening at either end.
Dr Norton then went on to describe the SuperWASP project, which hoped to use images from dedicated wide-field cameras to detect an estimated 1,000 new planets over the coming five years, working on the basis that one in every thousand main-sequence stars was expected to show a transit every ~4 days. The methodology was simple: as much of the sky as possible would be imaged each night, and photometry performed on every star; a light-curve would be constructed for each, looking for the characteristic signature of a transit. The Liverpool Telescope would perform spectral follow-up observations on any potential discoveries, seeking to confirm them by the traditional Doppler shift method.
The project would consist of two observatories: one in the northern hemisphere, amongst the Isaac Newton Group (ING) of telescopes in La Palma, the other in the southern hemisphere, in Sutherland, South Africa. Each would have eight cameras on a robotic mount, with slightly offset 7.8° fields. The lenses were off-the-shelf Canon stock, with 200-mm focal length and 11.1-cm aperture (f/1.8), connected to 2048 × 2048 CCD arrays. He recalled with amusement the setback that the project had faced when, partway through construction, Canon had discontinued their chosen model of lens; after the initial frustration, it had proved to be a blessing in disguise when cheap stock became available on eBay, though claiming expenses for this purchase had initially raised a few eyebrows.
It was reported that SuperWASP's northern observatory in La Palma had operated manned for 80 nights between April 2004 and the following autumn, surveying 7% of the sky each night using five cameras. It would eventually be fully robotic, and image the entirety of the visible sky at least once per night. The first 80-night run had produced 4 Tb of data; this eventual set-up would generate 10 Tb/yr, and so a well-automated pipeline would be required to process this volume of data. The flat-field calibration, aperture photometry, and construction and analysis of light-curves, would all be fully-automated.
To close, Dr Norton added that a lot of valuable data would come out of SuperWASP quite apart from extrasolar planet detections: it would detect anything in the range mag 6-16 which varied, providing an extensive survey of variable stars, as well as detecting novae, supernovae and near-Earth asteroids.
Following the applause for Dr Norton's talk, the President congratulated him on providing such a clear explanation of the complex taxonomy of variable stars. Mr Nick James asked how well SuperWASP's pipeline was working, remarking that historically such automation had tended to sound the death knell for amateur work. The speaker replied that the construction of light-curves was working well; none of the light-curves in his talk had been touched up. By contrast, the classification of objects was not yet automated, and was done by eye at present.
Mr Mark Armstrong asked what discoveries had been made in SuperWASP's first run. The speaker reported that several hundred new variable stars had been identified, but no extrasolar planets discovered. Mr Maurice Gavin asked what colour pass-band it used. The speaker replied that it sought to capture as much light as possible, and so imaged in white light.
The President then proceeded to welcome the evening's next speaker, Mr Martin Mobberley, to present his Sky Notes.