Here are some brief descriptions of research I have been working on, together with links to the relevant published articles or preprints.
The term "habitable zone" used to be an esoteric concept on the fringes of astrophysics, describing the region around a star in which a planet could support liquid water. The discovery over the last decade or so that planets are actually very common in the Universe has thrust habitable zones into the common lexicons of both astrophysics and popular science. While the classical definition of a habitable zone remains useful, it is not obvious that all planets in habitable zones will actually be habitable.
In several past works, we have argued that the space weather environment of a planet - the plasma and magnetic field conditions resulting from the host stellar wind - is likely to be crucial to habitability. The link to habitability is through the atmosphere that is continually eroded by the action of the host stellar wind. The key question is whether a planet can hold on to its atmosphere over billion year timescales. The Earth managed it, but evidence suggests that Mars, probably because of its lack of a strong protective magnetic field, has long since lost most of its surface water due to solar wind scouring.
The recently discovered planet in the habitable zone of our closest neighboring star, Proxima Centauri, has been one of the most exciting findings of the last decade. The planet is close enough that next generation telescopes will be able to observe it directly: Proxima b could become the first habitable planet studied in detail. But Proxima is a host star quite different to the Sun - cool and faint, shining with only 1/600th of the Sun's power. The habitable zone of Proxima is then very close to the star, and Proxima b orbits twenty times closer to Proxima than the Earth to the Sun. SAO scientist Cecilia Garraffo has lead a new study of the impact of this proximity to Proxima. Supercomputer model simulations of the wind from Proxima indicate it blows with similar strength to that of the Sun. Being so close though, Proxima b gets blasted by this wind, experiencing a wind pressure several hundred times that experienced by Earth. Twice each 11 day orbit it passes through more dense wind streams that raise the pressure to 2000 times that at Earth. If Proxima b has a magnetic field it might serve as some protection for its atmosphere, but it will be a highly dynamic magnetospheric environment, expanding and compressing like a bellows on timescales of less than a day. The much less severe dynamics of the Earth's magnetosphere are thought to be a factor in the Earth's atmospheric loss, as plasma is released by the forced opening and closing of the magnetic field. It seems doubtful under such conditions that Proxima b will have retained any sort of atmosphere capable of sustaining life.
Detailed atmosphere calculations are needed to assess the true habitability of Proxima b, but the property does not appear quite as appealing as it did in the first glossy brochures. This work was published in the 2016 December 10 edition of the Astrophysical Journal Letters.
Several postings within these pages have discussed new observations and insights into nova explosions. The term "nova" derives from 16th century astronomer Tycho Brahe's nova stella - "new star" - description of supernova SN 1572 in Cassiopeia. Unable to see the progenitor of the explosion, from his perspective it appeared like a bright new star where before there was none. Novae seemed similar - new stars appearing where nothing was visible to the naked eye before. Classical novae are actually like mini supernova
explosions, but rather than the detonation of an entire star the explosion originates on the surface of a white dwarf that has accreted material from a close companion star - see the postings on U Scorpii and V407 Cygni for further details.
The "Fastest Nova in Town", V745 Sco, is a special member of this class. The white dwarf orbits within the dense wind of its red giant companion from which its strong gravity scavenges the hydrogen fuel that powers thermonuclear outbursts at intervals of about 25 years. One persistent problem in understanding observations of nova explosions has been evidence for distinctly aspherical blasts. Why would an explosion over the surface of a very spherical white dwarf be aspherical? We have pursued this problem for several recent nova events using supercomputer hydrodynamical simulations.
The key to the form and evolution of a nova explosion turns out to lie in its immediate environment. The expanding blast wave propagates at different speeds through gas of different density. The denser the gas the slower it moves, but more dense gas also looks brighter when it has been heated by the blast. Salvo Orlando, of the Palermo Astronomical Observatory in Sicily, has lead a study based on simulations of the V745 Sco 2014 event to try to understand Doppler shift observations that revealed a very aspherical explosion. Earlier studies have indicated that a white dwarf orbiting in a dense wind tends to attract higher density gas in the plane of its orbit. The supercomputer simulations confirmed that we cannot reproduce a sufficiently aspherical explosion without this. The result is a blast and ejecta that shoot out poleward, expanding rapidly northward and southward of the white dwarf. The dense equatorial gas instead lights up more brightly in X-rays and UV light, producing a ring-like emission structure. Careful comparison of different simulations with X-ray observations indicate the explosion threw off about 1/10 of an Earth mass, with an energy equivalent to about 1,000,000,000,000,000,000,000,000,000 tonnes of TNT (4x1043 erg). This work was published in the 2017 February 1 edition of Monthly Notices of the Royal Astronomical Society.
Our nearest stellar neighbour, the aptly monickered Proxima (actually dubbed "Proxima Centurus" by its discoverer, Robert Innes, of the Union Observatory in Johannesburg), shot to stardom recently after having been found to harbour a habitable zone planet with a mass only 30% larger than that of the Earth. Before this jaunt down the red dwarf carpet, Proxima was merely a conveniently nearby M6 dwarf - a faint, diminutive star with only an eighth of the mass of the Sun and a seventh of its radius. Proxima is cool, in the literal sense - just 3000 K at its surface compared with the Sun's 5800 K. So cool, in fact, that its internal structure is quite different to that of the Sun.
The Sun has an outer convection zone taking up the top 30% or so of its radius. Inside that, its structure is stable and "purely radiative". It has been thought that the interface between the radiative and convective zones on the Sun is key for its dynamo that generates magnetic fields visible at the surface in the form of sunpots and energetic UV to X-ray emission. Proxima does not have a central radiative zone - it is convective all the way to the centre. So, logically, its dynamo should operate quite differently.
An earlier posting presented new X-ray evidence that the dynamos of fully convective stars, like Proxima, are instead surprisingly solar-like. Further evidence has now emerged showing common dynamo action in another way: cyclic behaviour. The solar dynamo has a well-known cycle that results in the surface magnetic field waxing and waning and reversing polarity every 11 years. SAO scientist Brad Wargelin has lead a team analysing long-term optical, UV and X-ray observations of Proxima that have probed its long-term magnetic behaviour. The visible light data reveal a slow modulation in brightness with a period of seven years. This brightening and dimming is the result of changes in the number of starspots on the stellar surface stemming from a magnetic cycle - in Proxima's case a 7 year one instead of 11. X-ray emission measured by a collection of different satellites over the years also shows a sympathetic secular variation in phase with the starspots. The implication of these new observations is that the magnetic fields of all stars with outer convection zones, including the Sun, are generated in the same way within convection zone and do not depend on the presence of a central stable radiative zone. This work was published in the 2017 January 21 edition of Monthly Notices of the Royal Astronomical Society.
A 2013 posting on research examining how much mass stars can lose in coronal mass ejections (CMEs) highlighted a mass and energy "catastrophe" if relations between
flares and CMEs on the Sun are extrapolated to much more magnetically active stars: the implied energies and masses were simply too high to be reasonable. How is the catastrophe to be averted?A failed supercomputer model of a CME on the vey active young star AB Doradus might provide a clue. Magnetically active stars have large-scale magnetic fields up to 100 times that of the Sun. To escape the star, the CME must be ejected with sufficient force to break through the overlying magnetic field canopy. The CME we simulated had the energy of the largest CMEs seen on the Sun - equivalent to about a hundred quadrillion tons of TNT or about 2 billion of the most powerful nuclear warheads ever made - but it failed to break through the magnetic canopy of AB Dor. The rather unspectacular movie of the simulation appears to the right. The crisis could be averted because very active stars hang onto their weaker CMEs and recycle the energy in the corona, reducing the mass and energy budgets to more reasonable values. This work was published in the Proceedings of IAU Symposium 320, held in Hawaii 2015 August.
Stars are giant balls of ionized gas - a plasma of mostly free electrons and protons. Such electrically charged particles are forced to spiral around magnetic fields, and magnetic fields essentially become attached or "frozen in" to the plasma. When the pressure of the plasma is stronger than the magnetic field pressure, plasma motions drag the magnetic field with it. Stellar interiors are very dynamic, with regions of strong convective flows. Stars also do not spin uniformly with the same rotation period - the Sun's equator rotates faster than its pole by 50% or so. There is broad understanding that this differential rotation, combined with convective motions, generates and amplifies the magnetic field by winding and folding it - much like stretching and folding an elastic band - but the details of the process and exactly where in the Sun it happens has remained a subject of intense debate.
When current Earnest Rutherford Fellow Nick Wright of Keele University was a postdoc at Smithsonian, he and I examined the way stellar rotation influenced X-ray output from stars. Faster rotating stars generate more magnetic field and are brighter in X-rays. We were missing some key types of star from the sample though: cool, slowly rotating old M dwarfs - stars with less than half the mass of the Sun. We subsequently observed two of them using NASA's Chandra X-ray Observatory and found X-ray data for two more from older observations. To our surprise, the trend with stellar rotation for these M dwarfs was the same as for Sun-like stars. We were surprised because these stars do not have the central radiative zone that the Sun does, but are convective all the way to the centre. They have no tachocline like that in the Sun, but their magnetic behaviour with rotation is the same. The results indicate that the tachocline is not a significant factor in stellar magnetic dynamos: magnetic field must be generated elsewhere in the convection zone, likely by the differential rotation that also exists there. This study was published in the 2016 July 28 issue of Nature and also featured in a Chandra press release.
Smithsonian astrophysicist Vallia Antoniou and University of Crete astronomer Andreas Zezas are leading an international team studying the Small Magellanic Cloud using an extensive series of observations made by the Chandra X-ray Observatory. Lying "just" 200,000 light years away, the SMC is our closest galaxy. Much smaller than the Milky Way and termed a "dwarf irregular galaxy", the SMC harbours an interesting population of stars. Having undergone extensive episodes of star formation over the last few million to tens of millions of years - very recently in astronomical terms when the age of the Universe is measured in billions of years - it is a good place to go hunting for interesting younger and more massive stars. One such object retrieved from the net of Chandra observations is the X-ray pulsar SXP214.
X-ray pulsars are magnetized neutron stars in a close binary system with a more normal stellar companion. The neutron star scavenges
SXP214 was discovered by Southampton University astronomer Malcolm Coe from X-ray observations made back in 2009. It is a neutron star spinning with a period of about 200s in an eccentric orbit around a "Be star". Be-type stars are young stars several times more massive than the Sun and rotate so rapidly that they fling material off themselves to form a circumstellar disk. This disk proved key to understanding the Chandra observations of SXP214. In a study lead by Smithsonian astrophysicist JaeSub Hong, the X-ray emission from the neutron star during the 14 hours of observations was found to both brighten and become "softer" - become relatively more bright at lower X-ray energies. The neutron star was caught in the act of plunging through the disk. Material in the disk fed the accretion onto the star, making its X-ray emission brighter. As it emerged, the lower energy X-rays that were absorbed by the murky disk began to shine through. The study was published in the 2016 July 20 edition of the Astrophysical Journal, and also features in an American Astronomical Society NOVA research highlight.
The fastest nova in town featured in an earlier posting detailed the fascinatingly rapid evolution of the V745 Scorpii event followed in the UV and X-rays by NASA's Swift satellite. Nova explosions are mini versions of supernova explosions that occur on the surface of a white dwarf. The fuel for the explosion is hydrogen-rich gas accreted from a close companion star. V745 Sco is special because it is a "recurrent" nova - a nova observed to have had more than a single outburst. All novae likely go through many explosion cycles, but for the great majority the time between events is many thousands of years. The explosion cycle time is controlled by the speed with which new fuel is accreted, and by the mass of the white dwarf star.
V745 Sco is also special because it is a symbiotic nova - a white dwarf accreting from the extended atmosphere and wind of an evolved giant star rather than from a K or M dwarf star like most novae. The explosion sets off a blast wave through the gas. The shock wave heats the gas to millions of degrees and we can observe it in the X-rays with sophisticated instruments such as the Chandra X-ray Observatory High Energy Transmission Grating Spectrometer. Careful analysis of data obtained by Chandra two weeks after the nova discovery enabled us to study the blast wave in detail. We found that the blast was quite aspherical, and could deduce the energy of the explosion and verify estimates of the density of the red giant wind in the immediate vicinity. We could also estimate the mass thrown off in the explosion and compare it with the mass needed to trigger the blast for various white dwarf masses. The mass ejected - a few millionths of a solar mass - appears to be less than that required to trigger an explosion even for the most massive white dwarf. The white dwarf must be close to the Chandrasekhar Limit to have undergone a nova at all. But also some mass must have been retained on the white dwarf. This means the white dwarf is growing in mass, and creeping inexorably toward a supernova demise. This work was published in the 2016 July 10 edition of the Astrophysical Journal.
The classical definition of a star's "habitable zone" is the orbital distance at which a planet can be warmed sufficiently to sustain liquid water on its surface. In earlier postings on studies of the radiation environments of M dwarf stars, we highlighted an additional criterion: the magnetic activity of the planet-hosting star. Magnetism is responsible for what are arguably the most interesting aspects of the behaviour of stars: UV and X-ray emission, flares and prominences, coronal mass ejections, and rarified but hot and energetic winds. These phenomena are more than of academic interest: they drive planetary atmospheric loss, influence the chemistry, and over time can
Universitäts-Sternwarte München and European Southern Observatory PhD student Julian Alvarado-Gomez has been leading a study to understand the magnetic activity of planet-hosting stars. The first step is to measure the stellar surface magnetic field - a non-trivial task and an activity representing an entire subfield of stellar astronomy in itself. A technique called "Zeeman-Doppler Imaging" is employed - see the posting on a particular application of this to the young planet-hosting Sun-like star, HD 1237. The next step involves using the surface magnetic field map to drive a sophisticated numerical supercomputer model of the outer atmosphere and magnetosphere of the star.
The computer model solves all the equations involving heating and cooling, forces and motions of the tenuous outer atmosphere of the star and its magnetic field. The gas is mostly a fully-ionized hydrogen plasma comprising electrons and protons. Unlike neutral gas, plasma interacts with magnetic fields. The outer atmosphere is a battle between the heated plasma and the magnetic field that tries to contains it. If the plasma wins, it breaks open the magnetic field to freedom in the form of an outflowing stellar wind; if the magnetic field wins, it contains the plasma as a multi-million degree X-ray emitting corona. This type of modelling was applied to three planet-hosting stars for which surface magnetic field maps were derived, HD 1237, HD 147513, and HD 22049 (better known as Epsilon Eridani). The results, published in the 2016 March 14 edition of Astronomy & Astrophysics, were used to visualize the stars' coronae and magnetic field structures, and to verify that the methods could successfully match observations of their energetic emissions. The next step is to push the modeling further out, to the orbits of their planets and probe their plasma and magnetic field space environments.
In 1947, legendary Mount Wilson Observatory stellar spectroscopist Paul Merrill noticed some peculiar features in photographic spectrograms of an otherwise fairly normal looking yellow giant, HD 117555. Spectral lines appeared very broad compared with other giants, and there was a strong emission line of hydrogen - Halpha - that is not usually present in giant star spectra. Merrill deduced that the lines were broad because of the Doppler effect across the surface of the spinning star - and that the giant must be an extremely rapid rotator. The equatorial rotation speed is in fact about 185 km/s, and the rotation period was subsequently discovered from the light modulation caused by large star spots rotating in and out of view. It is 2.4 days - more than ten times shorter than the Sun's. The hydrogen emission line is caused by surface magnetic activity driven to a frenzy by the fast rotation.
Later given the variable star name FK Comae, HD 117555 is thought to be rotating so fast because it is the result of a "binary merger" - a rare object formed from two stars orbiting so close to each other that they eventually merge into a single rapidly spinning star. Latter day legendary University of Colorado astrophysicist, Tom Ayres, is leading a project to use FK Com as a way of studying the processes of stellar magnetic activity at their extremes. The "COordinated Campaign of Observations and Analysis, Photosphere to Upper Atmosphere, of a Fast-rotating Star" - COCOA-PUFS for short, of course - is using the Hubble Space Telescope (HST) and Chandra X-ray Observatory to study UV and X-ray emission, together with supporting photometry and spectropolarimetry from the ground to monitor the visible light signatures of star spots and other fingerprints of magnetic activity. The figure shows a series of HST spectra obtained at different rotational phases of two lines of ionized magnesium that are formed in the stellar chromosphere. The lines are extremely broad, but sliced open by narrow absorption features at the line centres. The initial data analysis, published in the 2016 March edition of the Astrophysical Journal Supplement Series, has revealed a highly extended, dynamic, 10 million degree K coronal magnetosphere around the star, threaded by cooler structures that might be similar to solar prominences.
On November 22, 2014 the All-Sky Automated Survey for Supernovae (ASAS-SN) discovered a bright object, dubbed ASASSN-14li, that appeared to be coincident with the center of the galaxy PGC 043234 lying 290 million light years away. The way the light intensity from the event evolved matched that expected from a stellar "tidal disruption event". A group of us lead by astrophysicist Jon Miller at the University of Michigan used three X-ray telescopes - NASA's Chandra X-ray Observatory, Swift Gamma Ray Burst Explorer, and ESA's XMM-Newton - to observe the high energy emission expected from the nascent disk of shredded star material.
The Chandra and XMM-Newton spectra showed evidence of material moving outward from the object at speeds of a few hundred km per second - too slow to escape the gravitational field of the black hole. This material is probably levitated by the pressure of the intense light from the energised gas, similar to the radiatively-driven outflows of massive stars discussed in a recent posting. The gas flow is consistent with a rotating wind from the inner region of the disk, or with a filament of disrupted stellar gas at the further reaches of its elliptical orbit. The results will help constrain theories of tidal disruption and accretion disk formation. This study was published in the October 22nd issue of Nature and also featured in a press release.
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