Parallel Session P32: 1045-1230, Friday 16th April 2010

Stellar and Exoplanetary Magnetism

Location: G226


The past few years have seen the detection of magnetic star-planet interaction (SPI), a newly discovered phenomenon that may yield a method of characterising exoplanetary magnetic fields and consequently their internal structure. The existence of magnetic SPI remains controversial, however, if confirmed, it would allow our Solar System to be placed in context with other planetary systems within the Galaxy. As for stellar magnetism, recent discoveries include the possible detection of a remnant fossil field on a hot massive star; the first ever surface maps of pre-main sequence stars in the classical T Tauri phase of their evolution; the discovery of a magnetic cycle on a star possibly induced by a known orbiting close-in giant planet; the rapid increase in field complexity at the transition from completely convective low-mass stars to those with radiative cores; and the discovery of globally structured magnetic fields on the intermediate mass Herbig Ae-Be stars. This session will highlight the latest results in the study of stellar magnetism across the HR diagram, and exoplanetary systems.


  • Scott Gregory (University of Exeter)



16 April, 10:45Stellar magnetism in solar-type stars
Gaitee Hussain (ESO)
16 April, 11:15Magnetic activity in fast rotating M-dwarfs above and below the fully convective boundary
Rob Jeffries (Keele University)
16 April, 11:30Stellar magnetism through the eyes of the FORS1 instrument of the ESO VLT
Stefano Bagnulo (Armagh Observatory)
16 April, 12:00The influence of the stellar wind on close-in giant planets
Aline Vidotto (University of St Andrews)
16 April, 12:15The Doppler Shadow of WASP-3b
Grant Miller (University of St Andrews)


Investigating the X-ray Emission of A-type Stars Through the Magnetic Activity of Unresolved Lower Mass Companions
Robert De Rosa (University of Exeter)

Testing the ability of field extrapolation models to predict the X-ray properties of pre-main sequence stars
Scott Gregory (University of Exeter)

Talk Abstracts

Stellar magnetism in solar-type stars
Hussain, Gaitee
16 April, 10:45

Magnetic fields play a role at almost all evolutionary stages of low mass (F to M-type) stars. On the pre-main sequence, strong kG fields channel accretion from circumstellar disks onto the central T Tauri stars; while in older planet-hosting systems we find evidence of magnetospheric interaction between planets and their host stars.

Zeeman Doppler imaging techniques enable us to map stellar surface magnetic fields in unprecedented detail. Magnetic field measurements made from magnetically-sensitive intensity line profiles further complement and enhance the information gained from these surface magnetic field maps. The latest results suggest that the size and complexity of surface magnetic fields differ in fully convective stars and stars with radiative cores. A further transition occurs as stars spin down over time.

I will review our current understanding and demonstrate how magnetospheric models of low mass stars based on surface magnetic field maps are tested using multi-wavelength observations. Finally, I will give an overview of the rich future prospects of this field in light of new instruments and facilities, both in the short and long-term.

Magnetic activity in fast rotating M-dwarfs above and below the fully convective boundary
Jeffries, Rob, Richard Jackson
Keele University
16 April, 11:15

We present results from surveys of coronal and chromospheric activity among fast rotating M-dwarfs belonging to the young open clusters NGC 2547 and NGC 2516. Our targets are cool cluster members with known rotation periods covering spectral types from K3 to M5. Coronal activity appears to be continuous across the fully convective boundary, where almost all the fully convective stars exhibit saturated X-ray emission. However, the level of saturated chromospheric emission declines by factors of 2-3 for stars of spectral type M3 and cooler, coinciding with both the fully convective boundary and a dramatic increase in average rotation rates. No evidence for "supersaturation" was found in the chromospheres or coronae of the M-dwarfs. We discuss these findings in terms of changing magnetic topologies.

Stellar magnetism through the eyes of the FORS1 instrument of the ESO VLT
Bagnulo, Stefano, L. Fossati, C. Izzo, J.D. Landstreet, C. Folsom, O. Kochukhov
Armagh Observatory
16 April, 11:30

During a full decade of operations, the FORS1 instrument of the ESO Very Large Telescope has collected a large amount of magnetic field measurements of various kinds of stars. Together with the ESPaDOnS instrument of the Canada-France-Hawaii Telescope, with the MuSiCoS and NARVAL instruments of the 2 m Telescope Bernard Lyot of the Pic-du-Midi Observatory, FORS1 has been one of the workhorse instruments for the observational studies of stellar magnetism. Several interesting detections were obtained, some of them were eventually confirmed by follow-up investigations, some were not. We have performed a homogeneus (re-)analysis of the full FORS data archive, and in this talk we will give an updated review of the incidence of the stellar magnetism in various classes of stars.

The influence of the stellar wind on close-in giant planets
Vidotto, Aline, M. Opher, V. Jatenco-Pereira, T. I. Gombosi
University of St Andrews
16 April, 12:00

Since the first discovery of extrasolar planets, more than 300 planets have been detected, many of them located very near the host star. Because of such extreme proximity, interaction of the planet with the star is expected to give rise to a variety of phenomena. The stellar wind is expected to directly influence the planet and its atmosphere, e.g., by changing the configuration of the planet's magnetosphere, producing nonthermal planetary magnetospheric radio emissions, etc. So far, the few theoretical works investigating the influence of the stellar wind on the magnetosphere of planets were based on simplified treatments of the stellar winds. We developed three-dimensional magnetohydrodynamics models of stellar winds which enabled us to determine self-consistently the characteristics of the impacting wind on the planet. Under realistic stellar wind conditions, we analyzed the influence of the wind on planet migration by estimating the migration time-scale due to drag forces exerted by the stellar wind on a hot-Jupiter. Furthermore, we also estimated the power released from the magnetic interaction (reconnection) between a magnetized wind and the magnetosphere of a close-in giant planet. We show that the characterization of the wind properties of the host star is essential for a deeper understanding of extrasolar planetary systems.

The Doppler Shadow of WASP-3b
Miller, Grant
University of St Andrews
16 April, 12:15

Hot-jupiter planets for at large distances from their host stars then migrate inwards to their observed orbital separations. The mechanisms responsible for this are not fully understood but the spin-orbit alignment of the system gives clues as to how the planets migrated. The spin-orbit alignment can be measured by analysing the Rossiter-McLaughlin effect. This is the radial velocity anomaly caused by a planet as it transits the stellar disc. We describe a new tomographic method for analysing the Rossiter-McLaughlin effect and present the results of its application to the WASP-3 system. We also explain how this method can be used to confirm the existence of planets around early-type stars previously inaccessible to planet hunters due to their rapid rotation and lack of spectral lines.

Poster Abstracts

Investigating the X-ray Emission of A-type Stars Through the Magnetic Activity of Unresolved Lower Mass Companions
De Rosa, Robert, J. Bulger, J. Patience, B. Leland, B. Macintosh
University of Exeter

We present the results of an investigation into the source of X-ray emission from nearby early-type stars. By utilising the ROSAT All-Sky Survey (RASS) bright and faint source catalogues, and the preliminary results from our ongoing multiplicity survey of A-type stars.

Bremsstahlung emission has been detected from numerous main-sequence stars, from O-M. Studies of massive stars show X-ray luminosity scaling as a function of the stellar-wind density, suggesting that these strong radiatively driven winds are the primary generating mechanism. For less massive stars the X-ray generation is thought to be due to a dynamo effect caused by differential rotation. Stars from mid-B to late-A have insufficient luminosity to drive massive winds, nor have sufficiently deep convective envelopes and should theoretically have low X-ray emission. Contrary to these predictions, X-ray emission has been detected from numerous nearby A-type stars. The detection of two low-mass stellar companions to X-ray emitting A-type stars (e.g. Alcor B) would suggest an X-ray generation mechanism for these anomalous stars - unresolved companions.

Within this study we have compiled a distance limited sample of nearby X-ray detected A-type stars, alongside a control sample of non X-ray detected A-type stars. The typical angular resolution achieved in our observations was ~0.1" with the narrowband exposures allowing us to resolve ~10AU binaries, whilst sensitivity to the bottom of the main sequence was achieved at ~1-2" with the longer broadband exposures. We find that a greater proportion of X-ray detected A-type stars have an associated companion compared with those in the control sample, suggesting that unresolved low-mass companions are indeed a source of A-star X-ray emission.

Testing the ability of field extrapolation models to predict the X-ray properties of pre-main sequence stars
Gregory, Scott, E. Flaccomio, C. Argiroffi, J. Bouvier, J.-F. Donati, E. D. Feigelson, K. V. Getman, G. A. J. Hussain, M. Ibrahimov, M. Jardine, F. M. Walter
University of Exeter

By extrapolating from observationally derived magnetic surface maps, obtained through Zeeman-Doppler imaging, models of stellar magnetospheres can be constructed. By assuming that the plasma trapped along the closed field lines is in hydrostatic equilibrium, coronal X-ray emission properties, such as the global X-ray emission measure and the amount of rotational modulation of X-ray emission, can be predicted. For pre-main sequence magnetospheres the analysis can be extended to incorporate accretion flows, and predict the amount of softer X-ray emission from accretion spots that would be observed. I will detail the preliminary results of an ambitious multi-wavelength, multi-observing site, and near contemporaneous campaign, combining spectroscopic optical, nIR, UV, X-ray (200ks, Chandra), spectropolarimetric and photometric monitoring of the accreting pre-main sequence star V2129 Oph. Surprisingly the new magnetic map derived from the 2009 data appears to indicate that the surface magnetic field has undergone little evolution since it was previously observed in 2005. The dataset also allows X-ray emission from the stellar corona and the accretion hotspots to be disentangled and crucially allows the validity, and the predictions, of the 3D field topologies derived via field extrapolation to be critically examined.