Results for "Moira M. Jardine"

total 193522took 0.14s
Coronal stripping in supersaturated starsDec 12 2003A recent unambiguous detection of X-ray rotational modulation of the supersaturated star VXR45 (P = 0.223 days) has shown that its corona has discrete dark and bright X-ray regions. We suggest that due to the rapid rotation, the X-ray emitting corona ... More
Estimating magnetic filling factors from Zeeman-Doppler magnetogramsMar 13 2019Low-mass stars are known to have magnetic fields that are believed to be of dynamo origin. Two complementary techniques are principally used to characterise them. Zeeman-Doppler imaging (ZDI) can determine the geometry of the large-scale magnetic field ... More
Slingshot prominences: nature's wind gaugesOct 22 2018Nov 03 2018Mass loss rates for the tenuous, hot winds of cool stars are extremely difficult to measure, yet they are a crucial ingredient in the stars' rotational evolution. We present a new method for measuring these mass loss rates in young, rapidly-rotating stars. ... More
Self-intersection numbers of curves in the doubly-punctured planeJan 25 2010We address the problem of computing bounds for the self-intersection number (the minimum number of self-intersection points) of members of a free homotopy class of curves in the doubly-punctured plane as a function of their combinatorial length L; this ... More
Self-intersection numbers of curves on the punctured torusJan 20 2009The minimum number of self-intersection points for members of a free homotopy class of curves on the punctured torus is bounded above in terms of the number L of letters required for a minimal description of the class in terms of the generators of the ... More
X-ray Emission From Nearby M-dwarfs: the Super-saturation PhenomenonJul 12 2000A rotation rate and X-ray luminosity analysis is presented for rapidly rotating single and binary M-dwarf systems. X-ray luminosities for the majority of both single & binary M-dwarf systems with periods below $\simeq 5-6$ days (equatorial velocities, ... More
Estimating stellar wind parameters from low-resolution magnetogramsNov 10 2016Stellar winds govern the angular momentum evolution of solar-like stars throughout their main-sequence lifetime. The efficiency of this process depends on the geometry of the star's magnetic field. There has been a rapid increase recently in the number ... More
Modelling stellar coronae from surface magnetograms: the role of missing magnetic fluxJan 14 2010Recent advances in spectropolarimetry have allowed the reconstruction of stellar coronal magnetic fields. This uses Zeeman-Doppler magnetograms of the surface magnetic field as a lower boundary condition. The ZDI maps, however, suffer from the absence ... More
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary DisksJul 18 2008We study the properties of powerful X-ray flares from 161 pre-main sequence (PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula region. Relationships between flare properties, protoplanetary disks and accretion are examined in ... More
Mechanical Equilibrium of Hot, Large-Scale Magnetic Loops on T Tauri StarsDec 22 2011The most extended, closed magnetic loops inferred on T Tauri stars confine hot, X-ray emitting plasma at distances from the stellar surface beyond the the X-ray bright corona and closed large-scale field, distances comparable to the corotation radius. ... More
Modelling stellar coronal magnetic fieldsAug 28 2010Our understanding of the structure and dynamics of stellar coronae has changed dramatically with the availability of surface maps of both star spots and also magnetic field vectors. Magnetic field extrapolations from these surface maps reveal surprising ... More
Prominence formation and ejection in cool starsDec 21 2017Feb 06 2018The observational signatures of prominences have been detected in single and binary G and K type stars for many years now, but recently this has been extended to the M dwarf regime. Prominences carry away both mass and angular momentum when they are ejected ... More
Dust cloud lightning in extraterrestrial atmospheresJul 08 2012Lightning is present in all solar system planets which form clouds in their atmospheres. Cloud formation outside our solar system is possible in objects with much higher temperatures than on Earth or on Jupiter: Brown dwarfs and giant extrasolar gas planets ... More
MOVES II. Tuning in to the radio environment of HD189733bMar 05 2019We present stellar wind modelling of the hot Jupiter host HD189733, and predict radio emission from the stellar wind and the planet, the latter arising from the interaction of the stellar wind with the planetary magnetosphere. Our stellar wind models ... More
Single photon emission and detection at the nanoscale utilizing semiconductor nanowiresSep 02 2010We report recent progress toward on-chip single photon emission and detection in the near infrared utilizing semiconductor nanowires. Our single photon emitter is based on a single InAsP quantum dot embedded in a p-n junction defined along the growth ... More
A general state-sum construction of 2-dimensional topological quantum field theories with defectsFeb 25 2016We derive the general state sum construction for 2D topological quantum field theories (TQFTs) with source defects on oriented curves, extending the state-sum construction from special symmetric Frobenius algebra for 2-D TQFTs without defects (cf. Lauda ... More
Slingshot prominence evolution for a solar-like starFeb 20 2019Although the present-day Sun rotates too slowly to exhibit centrifugally-supported 'slingshot prominences', at some time during its past it may have formed these clouds of cool gas and ejected them into the interplanetary medium. We determine the time ... More
A further 'degree of freedom' in the rotational evolution of starsAug 30 2005Observational and theoretical investigations provide evidence for non-uniform spot and magnetic flux distributions on rapidly rotating stars, which have a significant impact on their angular momentum loss rate through magnetised winds. Supplementing the ... More
Theoretical mass loss rates of cool main-sequence starsNov 14 2006We develop a model for the wind properties of cool main-sequence stars, which comprises their wind ram pressures, mass fluxes, and terminal wind velocities. The wind properties are determined through a polytropic magnetised wind model, assuming power ... More
The contribution of starspots to coronal structureAug 21 2010Significant progress has been made recently in our understanding of the structure of stellar magnetic fields, thanks to advances in detection methods such as Zeeman-Doppler Imaging. The extrapolation of this surface magnetic field into the corona has ... More
Coronal Structure of Low-Mass StarsJul 09 2012We investigate the change in stellar magnetic topology across the fully-convective boundary and its effects on coronal properties. We consider both the magnitude of the open flux that influences angular momentum loss in the stellar wind and X-ray emission ... More
The Search for Super-saturation in Chromospheric EmissionJul 09 2011We investigate if the super-saturation phenomenon observed at X-ray wavelengths for the corona, exists in the chromosphere for rapidly rotating late-type stars. Moderate resolution optical spectra of fast rotating EUV- and X-ray- selected late-type stars ... More
Studying stellar spin-down with Zeeman-Doppler magnetogramsMay 04 2017Magnetic activity and rotation are known to be intimately linked for low-mass stars. Understanding rotation evolution over the stellar lifetime is therefore an important goal within stellar astrophysics. In recent years, there has been increased focus ... More
A spectro-polarimetric study of the planet-hosting G dwarf, HD 147513Oct 07 2015The results from a spectro-polarimetric study of the planet-hosting Sun-like star, HD 147513 (G5V), are presented here. Robust detections of Zeeman signatures at all observed epochs indicate a surface magnetic field, with longitudinal magnetic field strengths ... More
Light Dark Matter Anomalies After LUXNov 08 2013Nov 27 2013We examine the consistency of light dark matter (DM) elastic scattering in CoGeNT, DAMA, and CDMS-Silicon in light of constraints from XENON, CDMS, LUX, PICASSO and COUPP. We consider a variety of operators that have been employed to reconcile anomalies ... More
Magnetic coronae of active main-sequence starsNov 12 2008The coronal structure of main sequence stars continues to puzzle us. While the solar corona is relatively well understood, it has become clear that even stars of the same mass as the Sun can display very non-solar coronal behaviour, particularly if they ... More
Radio emission from exoplanets: the role of the stellar coronal density and magnetic field strengthAug 21 2008The search for radio emission from extra-solar planets has so far been unsuccessful. Much of the effort in modelling the predicted emission has been based on the analogy with the well-known emission from Jupiter. Unlike Jupiter, however, many of the targets ... More
A small survey of the magnetic fields of planet-host starsJul 23 2013Jul 24 2013Using spectropolarimetry, we investigate the large-scale magnetic topologies of stars hosting close-in exoplanets. A small survey of ten stars has been done with the twin instruments TBL/NARVAL and CFHT/ESPaDOnS between 2006 and 2011. Each target consists ... More
The effects of stellar winds on the magnetospheres and potential habitability of exoplanetsSep 03 2014Context: The principle definition of habitability for exoplanets is whether they can sustain liquid water on their surfaces, i.e. that they orbit within the habitable zone. However, the planet's magnetosphere should also be considered, since without it, ... More
Open windows for a light axigluon explanation of the top forward-backward asymmetryDec 07 2012Feb 09 2013The top forward-backward asymmetry (tAFB) measured at the Tevatron remains one of the most puzzling outstanding collider anomalies. After two years of LHC running, however, few models for tAFB remain consistent with LHC data. In this paper we take a detailed ... More
On the Effect of Nuclear Response Functions in Dark Matter Direct DetectionJan 15 2014We examine the effect of nuclear response functions, as laid out in [Fitzpatrick et al, arXiv:1203.3542], on dark matter (DM) direct detection in the context of well-motivated UV completions, including electric and magnetic dipoles, anapole, spin-orbit, ... More
Orbiting Clouds of Material at the Keplerian Co-Rotation Radius of Rapidly Rotating Low Mass WTTs in Upper ScoFeb 06 2017Using K2 data, we have identified 23 very low mass members of the $\rho$ Oph and Upper Scorpius star-forming region as having periodic photometric variability not easily explained by well-established physical mechanisms such as star spots, eclipsing binaries, ... More
The impact of meridional circulation on stellar butterfly diagrams and polar capsApr 05 2006Observations of rapidly rotating solar-like stars show a significant mixture of opposite-polarity magnetic fields within their polar regions. To explain these observations, models describing the surface transport of magnetic flux demand the presence of ... More
Why are accreting T Tauri stars observed to be less luminous in X-rays than non-accretors?Apr 23 2007Accreting T Tauri stars are observed to be less luminous in X-rays than non-accretors, an effect that has been detected in various star forming regions. To explain this we have combined, for the first time, a radiative transfer code with an accretion ... More
Ionisation in atmospheres of Brown Dwarfs and extrasolar planets II Dust-induced collisional ionizationMay 23 2011Observations have shown that continuous radio emission and also sporadic H-alpha and X-ray emission are prominent in singular, low-mass objects later than spectral class M. These activity signatures are interpreted as being caused by coupling of an ionised ... More
Why are accreting T Tauri stars less luminous in X-rays than non-accretors?Jul 03 2007Accreting T Tauri stars are observed to be less luminous in X-rays than non-accretors, an effect that has been detected in various star forming regions. To explain this we have combined, for the first time, a radiative transfer code with an accretion ... More
Transit Variability in Bow Shock-Hosting PlanetsFeb 08 2011We investigate the formation of bow shocks around exoplanets as a result of the interaction of the planet with the coronal material of the host star, focusing on physical causes that can lead to temporal variations in the shock characteristics. We recently ... More
Prospects for Detection of Exoplanet Magnetic Fields Through Bow-Shock Observations During TransitsNov 15 2010An asymmetry between the ingress and egress times was observed in the near-UV light curve of the transit planet WASP-12b. Such asymmetry led us to suggest that the early ingress in the UV light curve of WASP-12b, compared to the optical observations, ... More
Using Chandra/LETG spectra to probe stellar coronaeMar 09 2004We probe the relationship between surface magnetic fields and the X-ray emitting corona in the rapidly rotating star AB Dor. Circularly polarised spectra have been inverted to produce a surface (photospheric) magnetic field map. This surface map has been ... More
Early UV Ingress in WASP-12b: Measuring Planetary Magnetic FieldsSep 29 2010Recently, Fossati et al. observed that the UV transit of WASP-12b showed an early ingress compared to the optical transit. We suggest that the resulting early ingress is caused by a bow shock ahead of the planetary orbital motion. In this Letter we investigate ... More
Slingshot prominences above stellar X-ray coronaeJun 10 2005We present a new model for the coronal structure of rapidly rotating solar-type stars. The presence of prominences trapped in co-rotation 2 to 5 stellar radii above the stellar surface has been taken as evidence that the coronae of these stars must be ... More
Magnetic activity and hot Jupiters of young Suns: the weak-line T Tauri stars V819 Tau and V830 TauSep 07 2015We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri stars (wTTSs) V819 Tau and V830 Tau within the MaTYSSE programme, involving the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope. At ~3 Myr, ... More
More Rapidly Rotating PMS M Dwarfs with Light Curves Suggestive of Orbiting Clouds of MaterialDec 12 2017In a previous paper, using data from K2 Campaign 2, we identified 11 very low mass members of the $\rho$ Oph and Upper Scorpius star-forming region as having periodic photometric variability and phased light curves showing multiple scallops or undulations. ... More
The M4 Transition: Toward a comprehensive understanding of the transition into the fully convective regimeDec 12 2010Dec 15 2010The difference in stellar structure above and below spectral type ~M4 is expected to be a very important one, connected directly or indirectly to a variety of observational phenomena in cool stars---such as rotation, activity, magnetic field generation ... More
Minimum periods of homeomorphisms of orientable surfacesMar 30 2012One of the main problems of the theory of dynamical systems is the determination of the existence of periodic orbits of a self-map and more generally, the structure of the set of periods. Define the minimum period of a class os self-maps of a fixed set ... More
Tevatron Top $A_{FB}$ Versus LHC Top PhysicsJul 21 2011Aug 12 2011We carry out a comprehensive analysis of models for top A_{FB} at CDF in light of new top data arriving from the LHC. We begin with a careful Tevatron analysis, considering in general which sets of effective vertices give rise to a large forward-backward ... More
On Models of New Physics for the Tevatron Top A_FBMar 17 2011CDF has observed a top forward-backward asymmetry discrepant with the Standard Model prediction at 3.4 \sigma. We analyze models that could generate the asymmetry, including flavor-violating W's, horizontal Z'_Hs, triplet and sextet diquarks, and axigluons. ... More
Searching for Top Flavor Violating ResonancesJan 31 2011We study new top flavor violating resonances that are singly produced in association with a top at the LHC. Such top flavor violating states could be responsible for the Tevatron top forward-backward asymmetry. Since top flavor violating states can directly ... More
Non-Abelian number theory and the structure of curves on surfacesAug 09 2016In this note we study numerically the combinatorics of curves and geodesics on the torus with one boundary component. A potential computational difficulty is avoided by counting inside specific orbits of the mapping class group up to a certain length, ... More
Coronal structure of the cTTS V2129 OphFeb 15 2008The nature of the magnetic coupling between T Tauri stars and their disks determines not only the mass accretion process but possibly the spin evolution of the central star. We have taken a recently-published surface magnetogram of one moderately-accreting ... More
The global magnetic topology of AB DoradusMay 09 2002We have used Zeeman-Doppler maps of the surface field of the young, rapid rotator AB Dor (P = 0.514 days) to extrapolate the coronal field, assuming it to be potential. We find that the topology of the large-scale field is very similar in all three years ... More
Using Kepler transit observations to measure stellar spot belt migration ratesFeb 16 2012Planetary transits provide a unique opportunity to investigate the surface distributions of star spots. Our aim is to determine if, with continuous observation (such as the data that will be provided by the Kepler mission), we can in addition measure ... More
Stellar Differential Rotation and Coronal TimescalesJul 12 2014We investigate the timescales of evolution of stellar coronae in response to surface differential rotation and diffusion. To quantify this we study both the formation time and lifetime of a magnetic flux rope in a decaying bipolar active region. We apply ... More
Minimal intersection of curves on surfacesJun 16 2007May 06 2008In the eighties Goldman discovered a Lie algebra structure on the vector space generated by the free homotopy classes of oriented curves on an oriented surface. The Lie bracket [a,b] is defined as the signed sum over the intersection points of a and b ... More
Ionisation in atmospheres of Brown Dwarfs and extrasolar planets I The role of electron avalancheOct 21 2010Brown Dwarf and extrasolar planet atmospheres form clouds which strongly influence the local chemistry and physics. These clouds are globally neutral obeying dust-gas charge equilibrium which is, on short time scales, inconsistent with the observation ... More
Modeling X-ray emission from stellar coronaeSep 24 2008By extrapolating from observationally derived surface magnetograms of low-mass stars we construct models of their coronal magnetic fields and compare the 3D field geometry with axial multipoles. AB Dor, which has a radiative core, has a very complex field, ... More
Mapping potential fields on the surfaces of rapidly rotating starsNov 13 2000We present a technique that combines Zeeman Doppler imaging (ZDI) principles with a potential field mapping prescription in order to gain more information about the surface field topology of rapid rotators. This technique is an improvement on standard ... More
Ionisation in atmospheres of Brown Dwarfs and extrasolar planets III. Breakdown conditions for mineral cloudsJan 31 2013Electric discharges were detected directly in the cloudy atmospheres of Earth, Jupiter and Saturn, are debatable for Venus, and indirectly inferred for Neptune and Uranus in our solar system. Sprites (and other types of transient luminous events) have ... More
Combinatorial Lie bialgebras of curves on surfacesMay 22 2001Nov 15 2006Goldman and Turaev found a Lie bialgebra structure on the vector space generated by non-trivial free homotopy classes of curves on a surface. When the surface has non-empty boundary, this vector space has a basis of cyclic reduced words in the generators ... More
Self-intersections are empirically GaussianNov 28 2010In an orientable surface with boundary, free homotopy classes of curves on surfaces are in one to one correspondence with cyclic reduced words in a set of standard generators of the fundamental group. The combinatorial length of a class is the number ... More
Self-intersection numbers of length-equivalent curves on surfacesNov 03 2013Two free homotopy classes of closed curves in an orientable surface with negative Euler characteristic are said to be length equivalent if for any hyperbolic structure on the surface, the length of the geodesic in one class is equal to the length of the ... More
Mass Accretion onto T Tauri StarsJun 28 2006It is now accepted that accretion onto classical T Tauri stars is controlled by the stellar magnetosphere, yet to date most accretion models have assumed that their magnetic fields are dipolar. By considering a simple steady state accretion model with ... More
Stellar mass ejectionsNov 12 2008It has been known for some time now that rapidly-rotating solar-like stars possess the stellar equivalent of solar prominences. These may be three orders of magnitude more massive than their solar counterparts, and their ejection from the star may form ... More
Instabilities in the AetherDec 05 2008Mar 23 2009We investigate the stability of theories in which Lorentz invariance is spontaneously broken by fixed-norm vector "aether" fields. Models with generic kinetic terms are plagued either by ghosts or by tachyons, and are therefore physically unacceptable. ... More
Meter- to Millimeter Emission from Cool Stellar Systems: Latest Results, Synergies Across the Spectrum, and Outlook for the Next DecadeNov 08 2018Radio observations of cool stellar systems provide unique information on their magnetic fields, high-energy processes, and chemistry. Buoyed by powerful new instruments (e.g. ALMA, JVLA, LOFAR), advances in related fields (e.g., the Gaia astrometric revolution), ... More
Sigma-Model AetherDec 05 2008Jan 16 2009Theories of low-energy Lorentz violation by a fixed-norm "aether" vector field with two-derivative kinetic terms have a globally bounded Hamiltonian and are perturbatively stable only if the vector is timelike and the kinetic term in the action takes ... More
Identifying the Theory of Dark Matter with Direct DetectionJun 15 2015Nov 19 2015Identifying the true theory of dark matter depends crucially on accurately characterizing interactions of dark matter (DM) with other species. In the context of DM direct detection, we present a study of the prospects for correctly identifying the low-energy ... More
Confronting Top AFB with Parity Violation ConstraintsMar 06 2012We consider the implications of low-energy precision tests of parity violation on t-channel mediator models explaining the top AFB excess measured by CDF and D0. Flavor-violating u-t or d-t couplings of new scalar or vector mediators generate at one-loop ... More
Large-scale magnetic topologies of late M dwarfsMay 30 2010Jun 25 2010We present here the final results of the first spectropolarimetric survey of a small sample of active M dwarfs, aimed at providing observational constraints on dynamo action on both sides of the full-convection threshold (spectral type M4). Our two previous ... More
Test ideals in diagonal hypersurface rings IIJul 12 2002Dec 30 2002Let $R=k[x_1, ..., x_n]/(x_1^d + ... + x_n^d)$, where $k$ is a field of characteristic $p$, $p$ does not divide $d$ and $n \geq 3$. We describe a method for computing the test ideal for these diagonal hypersurface rings. This method involves using a characterization ... More
Closed string operators in topology leading to Lie bialgebras and higher string algebraDec 27 2002This paper explains the conjectured algebraic duality between genus zero Gromov-Witten theory and genus zero "Closed String topology". This duality in another perspective is discussed on page 87 of the book "Frobenius manifold, quantum cohomology, and ... More
The Goldman bracket determines intersection numbers for surfaces and orbifoldsSep 04 2012Mar 15 2014In the mid eighties Goldman proved an embedded curve could be isotoped to not intersect a closed geodesic if and only if their Lie bracket (as defined in that work) vanished. Goldman asked for a topological proof and about extensions of the conclusion ... More
Algebraic characterization of simple closed curves via Turaev's cobracketSep 14 2010The vector space $\V$ generated by the conjugacy classes in the fundamental group of an orientable surface has a natural Lie cobracket $\map{\delta}{\V}{\V\times \V}$. For negatively curved surfaces, $\delta$ can be computed from a geodesic representative ... More
Accretion discs, low-mass protostars and planets: probing the impact of magnetic fields on stellar formationApr 04 2008Whereas the understanding of most phases of stellar evolution made considerable progress throughout the whole of the twentieth century, stellar formation remained rather enigmatic and poorly constrained by observations until about three decades ago, when ... More
M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planetsNov 20 2013We perform three-dimensional numerical simulations of stellar winds of early-M dwarf stars. Our simulations incorporate observationally reconstructed large-scale surface magnetic maps, suggesting that the complexity of the magnetic field can play an important ... More
Secondary radio eclipse of the transiting planet HD 189733 b: an upper limit at 307-347 MHzJan 15 2009We report the first attempt to observe the secondary eclipse of a transiting extra-solar planet at radio wavelengths. We observed HD 189733 b with the Robert C. Byrd Green Bank Telescope of the NRAO over about 5.5 hours before, during and after secondary ... More
Understanding the Angular Momentum Loss of Low-Mass Stars: The Case of V374 PegJan 06 2011Recently, surface magnetic field maps had been acquired for a small sample of active M dwarfs, showing that fully convective stars (spectral types ~M4 and later) host intense (~kG), mainly axi-symmetrical poloidal fields. In particular, the rapidly rotating ... More
Large-scale magnetic topologies of M dwarfsSep 26 2008We present here the first results of a spectropolarimetric analysis of a small sample (about 20) of active stars ranging from spectral type M0 to M8, which are either fully-convective or possess a very small radiative core. This study aims at providing ... More
Powerful Winds from Low-Mass Stars: V374 PegOct 22 2010Jan 06 2011The rapid rotation (P=0.44 d) of the M dwarf V374Peg (M4) along with its intense magnetic field point toward magneto-centrifugal acceleration of a coronal wind. In this work, we investigate the structure of the wind of V374Peg by means of 3D magnetohydrodynamical ... More
Time-scales of close-in exoplanet radio emission variabilityApr 22 2015We investigate the variability of exoplanetary radio emission using stellar magnetic maps and 3D field extrapolation techniques. We use a sample of hot Jupiter hosting stars, focusing on the HD 179949, HD 189733 and tau Boo systems. Our results indicate ... More
The magnetic fields of accreting T Tauri starsSep 24 2008Models of magnetospheric accretion on to classical T Tauri stars often assume that the stellar magnetic field is a simple dipole. Recent Zeeman-Doppler imaging studies of V2129 Oph and BP Tau have shown however that their magnetic fields are more complex. ... More
Shock formation around planets orbiting M-dwarf starsDec 07 2011Bow shocks can be formed around planets due to their interaction with the coronal medium of the host stars. The net velocity of the particles impacting on the planet determines the orientation of the shock. At the Earth's orbit, the (mainly radial) solar ... More
Polar Fields for AB DorJul 10 2003Polar spots are often observed on rapidly-rotating cool stars, but the nature of the magnetic field in these spots is as yet unknown. While Zeeman-Doppler imaging can provide surface magnetic field maps over much of the observed stellar surface, the Zeeman ... More
Modelling surface magnetic field evolution on AB Doradus due to diffusion and surface differential rotationOct 29 2001From Zeeman Doppler images of the young, rapidly-rotating K0 dwarf AB Doradus, we have created a potential approximation to the observed radial magnetic field and have evolved it over 30 days due to the observed surface differential rotation, meridional ... More
Modeling the corona of AB DoradusSep 24 2001Oct 01 2001We present a technique that combines Zeeman Doppler imaging (ZDI) principles with a potential field mapping prescription in order to gain more information about the surface field topology of rapid rotators. This technique is an improvement on standard ... More
Stellar Coronal Response to Differential Rotation and Flux EmergenceMar 10 2016We perform a numerical parameter study to determine what effect varying differential rotation and flux emergence has on a star's non-potential coronal magnetic field. In particular we consider the effects on the star's surface magnetic flux, open magnetic ... More
The Soft X-ray Lightcurves of Partially Eclipsed Stellar FlaresAug 19 2011Most stellar flares' soft X-ray lightcurves possess a `typical' morphology, which consists of a rapid rise followed by a slow exponential decay. However, a study of 216 of the brightest flares on 161 pre-main sequence stars, observed during the Chandra ... More
Exoplanet Transit Variability: Bow Shocks and Winds Around HD 189733bSep 11 2013By analogy with the solar system, it is believed that stellar winds will form bow shocks around exoplanets. For hot Jupiters the bow shock will not form directly between the planet and the star, causing an asymmetric distribution of mass around the exoplanet ... More
The coronal topology of the rapidly rotating K0 dwarf, AB Doradus I. Using surface magnetic field maps to model the structure of the stellar coronaJul 22 2002We re-analyse spectropolarimetric data of AB Dor taken in 1996 December using a surface imaging code that can model the magnetic field of the star as a non-potential current-carrying magnetic field. We find that a non-potential field needs to be introduced ... More
Classical T Tauri stars: magnetic fields, coronae, and star-disc interactionsOct 30 2013The magnetic fields of young stars set their coronal properties and control their spin evolution via the star-disc interaction and outflows. Using 14 magnetic maps of 10 classical T Tauri stars (CTTSs) we investigate their closed X-ray emitting coronae, ... More
The non-dipolar magnetic fields of accreting T Tauri starsJul 04 2008Models of magnetospheric accretion on to classical T Tauri stars often assume that stellar magnetic fields are simple dipoles. Recently published surface magnetograms of BP Tau and V2129 Oph have shown, however, that their fields are more complex. The ... More
Planetary protection in the extreme environments of low-mass starsOct 02 2013Recent results showed that the magnetic field of M-dwarf (dM) stars, currently the main targets in searches for terrestrial planets, is very different from the solar one, both in topology as well as in intensity. In particular, the magnetised environment ... More
X-ray emission from T Tauri starsJan 10 2006We have modelled the X-ray emission of T Tauri stars assuming that they have isothermal, magnetically-confined coronae. These coronae extend outwards until either the pressure of the hot coronal gas overcomes the magnetic field, or, if the corona interacts ... More
Rotationally Modulated X-ray Emission from T Tauri StarsSep 25 2006We have modelled the rotational modulation of X-ray emission from T Tauri stars assuming that they have isothermal, magnetically confined coronae. By extrapolating surface magnetograms we find that T Tauri coronae are compact and clumpy, such that rotational ... More
Modelling the Hidden Magnetic Field of Low-Mass StarsJan 18 2014Zeeman-Doppler imaging is a spectropolarimetric technique that is used to map the large-scale surface magnetic fields of stars. These maps in turn are used to study the structure of the stars' coronae and winds. This method, however, misses any small-scale ... More
An algebraic characterization of simple closed curves on surfaces with boundaryJan 25 2008Sep 29 2015We characterize in terms of the Goldman Lie algebra which conjugacy classes in the fundamental group of a surface with non empty boundary are represented by simple closed curves. We prove the following: A non power conjugacy class X contains an embedded ... More
String TopologyNov 21 1999Consider two families of closed oriented curves in a d-manifold. At each point of intersecction of a curve of one family with a curve of the other family, form a new closed curve by going around the first curve and then going around the second. Typically, ... More
The shocking transit of WASP-12b: Modelling the observed early ingress in the near ultravioletJun 15 2011Near ultraviolet observations of WASP-12b have revealed an early ingress compared to the optical transit lightcurve. This has been interpreted as due to the presence of a magnetospheric bow shock which forms when the relative velocity of the planetary ... More
The coronal structure of Speedy Mic - II: Prominence masses and off-disc emissionOct 04 2006Observations of stellar prominences on young rapidly rotating stars provide unique probes of their magnetic fields out to many stellar radii. We compare two independently obtained datasets of the K3 dwarf Speedy Mic using the Anglo-Australian Telescope ... More
Inferring X-ray coronal structures from Zeeman-Doppler imagesJul 24 2002We have modelled the X-ray emission from the young rapid rotator AB Doradus (P = 0.514 days) using as a basis Zeeman-Doppler maps of the surface magnetic field. This allows us to reconcile the apparently conflicting observations of a high X-ray emission ... More