Publications (20)47.75 Total impact
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Article: Magnetospheric accretion on the T Tauri star BP Tauri
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ABSTRACT: From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star BP Tau. Circular polarisation signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarised and circularly polarised spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2% of the stellar surface. The strong mainly-axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully-convective dwarfs. It suggests that magnetic fields of fully-convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their convective zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4 R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau. Comment: MNRAS, in press02/2008; -
Article: Forty eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance
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ABSTRACT: We have conducted a programme to determine the fundamental parameters of a substantial number of eclipsing binaries of spectral types O and B in the Small Magellanic Cloud. New spectroscopic data, obtained with the two-degree-field multi-object spectrograph on the 3.9-m Anglo-Australian Telescope, have been used in conjunction with photometry from the Optical Gravitational Lens Experiment (OGLE-II) database of SMC eclipsing binaries. Previously we reported results for 10 systems; in this second and concluding paper we present spectral types, masses, radii, temperatures, surface gravities and luminosities for the components of a further 40 binaries. The full sample of 50 OB-type eclipsing systems is the largest single set of fundamental parameters determined for high-mass binaries in any galaxy. We find that 21 of the systems studied are in detached configurations, 28 are in semi-detached post-mass-transfer states, and one is a contact binary. Each system provides a primary distance indicator. We find a mean distance modulus to the SMC of 18.91+/-0.03+/-0.1 (internal and external uncertainties; D=60.6+/-1.0 kpc). This value represents one of the most precise available determinations of the distance to the SMC.11/2004; -
Article: A 2dF survey of the Small Magellanic Cloud
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ABSTRACT: We present a catalogue of new spectral types for hot, luminous stars in the Small Magellanic Cloud. The catalogue contains 4161 objects, giving an order of magnitude increase in the number of SMC stars with published spectroscopic classifications. The targets are primarily B- and A-type stars (2862 and 853 objects respectively), with 1 Wolf-Rayet, 139 O-type, and 306 FG stars, sampling the main sequence to ~mid-B. The selection and classification criteria are described, and objects of particular interest are discussed, including UV-selected targets from the Ultraviolet Imaging Telescope (UIT) experiment, Be and B[e] stars, `anomalous A supergiants', and composite-spectrum systems. We examine the incidence of Balmer-line emission, and the relationship between H-gamma equivalent width and absolute magnitude for BA stars. Comment: 27 pages, 17 figures (reduced size). Accepted for publication in MNRAS. Both a postscript version with normal res. figs and the full catalogue is available from http://www.ing.iac.es/~cje/2dF.html06/2004; -
Article: Interacting OB star binaries: LZ Cep, SZ Cam and IU Aur
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ABSTRACT: New spectroscopic observations of three O+O binary stars (LZ Cep, SZ Cam and IU Aur), obtained over a number of years at the Dominion Astrophysical Observatory, are analysed using state-of-the-art cross-correlation techniques to produce radial velocity curves. These results are combined with new (LZ Cep) and published (SZ Cam and IU Aur) photometry to yield masses, temperatures, radii and luminosities. We find that LZ Cep is a semi-detached system, in agreement with the results of Howarth et al., with the secondary star filling its Roche lobe. We find a distance modulus of 9.6 ??? 0.2, which is slightly smaller than the distance modulus to Cep OB2 (= 9.9).The reddening of LZ Cep is also at the lower end of the cluster range, suggesting that the binary is located on the near side of the cluster. SZ Cam is a triple system, and we are able to measure the radial velocities of the three components that are observed in the absorption lines. A light-curve solution employing our new mass ratio shows that the short-period binary is detached. The third body emits about 40 per cent of the radiation from the system, and is probably a binary itself. The distance modulus for SZ Cam (= 9.88) is in good agreement with that of its parent cluster NGC 1502 (= 9.71). IU Aur is another triple system, although we are unable to find spectroscopic evidence for the tertiary. The close binary is semi-detached, with the secondary filling its Roche lobe. The distance modulus of the system is 11.0, which is in approximate agreement with that of the Aur OB1 association (= 10.6 ??? 0.2). We note that the current models for interacting massive binaries all assume case B mass transfer, in which the mass-loser becomes a Wolf–Rayet star. Case A mass-transfer models (in which the mass-loser fills its Roche lobe while still on the main sequence) are required in order to see whether or not they can explain the observed properties of these interacting systems.Monthly Notices of the Royal Astronomical Society 04/2002; 295(2):386 - 396. · 4.90 Impact Factor -
Article: Spectropolarimetric orbits of symbiotic stars: five S-type systems
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ABSTRACT: We present multi-epoch, high signal-to-noise spectropolarimetry of the Raman-scattered emission lines of five S-type symbiotic binaries: CD-43o14304, Hen 1242, M1-21, SY Mus, and V455 Sco. The Raman-line polarizations and magnitudes are used to estimate orbital parameters for these systems. Two of the five systems (CD-43o14304 and SY Mus) have published radial-velocity orbits, which agree with the spectropolarimetric solutions. The physical parameters of the programme stars are consistent with the period-spectral-type relation of Mürset & Schmid (1999).Astronomy and Astrophysics 08/2000; 361:139-152. · 4.59 Impact Factor -
Article: Light-curve analysis and eclipse mapping of the contact binaries KQ GEM and V412 HER
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ABSTRACT: CCD photometry of the short-period binary stars KQ Gem and V412 Her is presented, together with some spectroscopic observations of KQ Gem. Although both systems are classified in the General Catalogue of Variable Stars as having light curves of EB/KW type, our data and analyses, involving light-curve synthesis and stellar surface imaging, show that KQ Gem is an EB system that is in marginal contact and has an enhanced bright region around the substellar point on the secondary component, whilst V412 Her is an EW system, a true contact binary with a mass ratio of 0.46 and both stars having the same surface brightness. The properties of the components of the two systems are compared with other marginal-contact and contact binaries, and a plea is repeated for more theoretical work on the mass/energy interchanges in contact binaries.Monthly Notices of the Royal Astronomical Society 04/1998; 296:100-108. · 4.90 Impact Factor -
Article: Time-series spectropolarimetry of the short-period Wolf-Rayet+O star binary CQ Cephei
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ABSTRACT: We present time-resolved, very precise linear spectropolarimetry of the short-period WR+O star binary CQ Cephei. We find that the N IV 4058 A line is the best spectral diagnostic of the WR orbital motion, and we derive a semiamplitude K(WR) = 290 +/- 1 km/s and a systemic velocity gamma = -72 +/- 1 km/s. We identify an antiphase spectral feature that we attribute to He I 4143 A absorption by the O star. A semiamplitude of 360 +/- 18 km/s is found for the O star, giving a mass ratio (WR:O) of 1.24 +/- 0.06. Spectropolarimetric measurements, combined with published photopolarimetry, enable a determination of the orbital inclination. The emission-line polarization vectors allow us to separate the intrinsic and interstellar polarizations, and we find that the polarization attributable to scattering of the WR radiation in the expanding wind is small but may be increasing towards the UV. We note that the inclinations derived from photometric analyses of eclipsing WR+O binaries are systematically lower than those computed from polarimetric measurements. The light curves from Hiltner are analyzed with LIGHT2 to confirm that the system is in a near-contact state. The resultant masses and astrophysical parameters show a normal main-sequence O star with a WR star that has parameters typical of the WN 6 type. The distance modulus for CQ Cep derived from our results is (V0 - MV) = 12.0.Monthly Notices of the Royal Astronomical Society 10/1997; 291:544. · 4.90 Impact Factor -
Article: Spectroscopy and eclipse-mapping of the mass-exchanging binary star V361 LYR
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ABSTRACT: We present the first spectroscopic investigation of the short-period (P = 0.309614 d) mass-exchanging binary system V361 Lyr. The spectral type of the primary component is estimated to be F8-G0. The H-alpha line profile clearly shows two absorption components when the system is at its brightest and bluest stage, around orbital phases 0.35-0.45. The published V and I light curves are analyzed with LIGHT2 to show that the primary component (T= 6200 K) fills its Roche lobe, while the secondary (T = 4500 K) is detached and fills about 57 percent of the available Roche volume. Eclipse maps of both stellar surfaces are derived via DOTS from light curves obtained in 1988, 1989, and 1992. These show that the secondary has a persistent elongated hot structure on its equator with an estimated temperature of 10,000 K which is consistent with the expectations of a theoretical mass transfer stream leaving the primary at its inner Lagrangian point and falling onto the secondary.Monthly Notices of the Royal Astronomical Society 10/1997; 291:749. · 4.90 Impact Factor -
Article: Interacting O-star binaries: V382 Cyg, V448 CYG and XZ CEP
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ABSTRACT: New spectroscopic observations of three interacting O+O-star binaries (V382 Cyg, V448 Cyg and XZ Cep), obtained at the Dominion Astrophysical Observatory, are analysed in conjunction with published photometry, to yield masses, temperatures and radii. We determine masses of 26.0+/-0.7 and 1903+/-0.4Msolar for V382 Cyg, 25.2+/-0.7 and 14.0+/-0.7Msolar for V448 Cyg, and 15.8+/-0.4 and 6.4+/-0.3 Msolar for XZ Cep. We find that V382 Cyg is a contact system, resulting from evolutionary expansion of the components during their main-sequence lifetimes (case A mass transfer). V448 Cyg is an evolved binary that has undergone late case A or early case B mass transfer, with the mass loser now filling its Roche lobe and transferring mass more slowly to the gainer. The distance modulus for V448 Cyg of 11.4 agrees well with that of its parent cluster NGC 6871, while the cluster age (7Myr) places a tight constraint on the evolutionary status of this system. XZ Cep closely resembles V448 Cyg, although its mass is somewhat lower.Monthly Notices of the Royal Astronomical Society 01/1997; 285:277-287. · 4.90 Impact Factor -
Article: New masses for the O-type binary DH Cephei, and the temperatures of O-stars.
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ABSTRACT: Spectroscopic observations, secured in 1973 and 1991, are analysed together with data from Sturm & Simon (1993) and Penny et al. (1996) to yield a new determination of the masses of the O+O-type binary system DH Cep. The masses are 32.7+/-1.7Msun_ for the O5.5V primary and 29.6+/-1.6Msun_ for the O6.5V secondary. Analysis of the light curves from Lines et al. (1986), coupled with temperature and flux ratios from Sturm & Simon, results in revised absolute parameters with final masses increased by 10% relative to those determined by Sturm & Simon. These revised parameters are in excellent accord with the evolutionary models of Schaller et al. (1992), at an age of 1.5Myr. In the context of those evolutionary models, and the work of Schoenberner & Harmanec (1995) on lower-mass binaries, we discuss the properties of 6 O to B0 detached binaries including DH Cep. The masses and radii for the components of these systems are all in remarkable agreement with the Schaller et al. models of solar composition. The O star temperatures need to be reduced by an average ~1000K to achieve agreement in T_eff_ with the models, but this averaged value may hide the need for substantial reductions (~2500K) around O8. We note also some discrepancies between derived visual absolute magnitudes, and the M_v_-spectral type relationship for O stars compiled by Schmidt-Kaler (1982).Astronomy and Astrophysics 09/1996; 314:165-172. · 4.59 Impact Factor -
Article: Raman scattering in symbiotic stars. I. Spectropolarimetric observations.
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ABSTRACT: We present high signal-to-noise, high-dispersion linear spectropolarimetry of the Raman-scattered λλ6825, 7082A emission lines in 27 southern-hemisphere symbiotic systems. We give continuum polarization measurements for all the targets (together with estimates of the foreground interstellar polarization), and new estimates of the red-giant spectral types in 23 systems. We confirm continuum-polarization variability in five out of eight systems with previous high-quality photopolarimetry. The Raman lines are fully resolved; although they display a wide variety of morphologies, both the intensity and polarization profiles appear basically to be triple-peaked. Spectra of five systems observed in both 1992 and 1994 show stable Raman-line intensity profiles, but with significant variations in the polarization structure.Astronomy and Astrophysics Supplement Series 09/1996; 119:61-90. -
Article: Spectropolarimetric orbits of symbiotic stars: SY Muscae.
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ABSTRACT: We present a formalism describing the change in position angle with orbital motion of Raman-scattered lines in symbiotic binaries. Straightforward inversion techniques allow this formalism to be used in the determination of `polarimetric orbits'. The orbital quantities which can be estimated from suitable data are the period P; the eccentricity, e; the inclination, i; the longitude of periastron, ω; and the position angle of the line of nodes, {OMEGA}. We illustrate the method with an analysis of data for SY Mus.Astronomy and Astrophysics 05/1996; 310:235-238. · 4.59 Impact Factor -
Article: Linear spectropolarimetry of the Hα emission line of ζ Puppis.
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ABSTRACT: We present high-S/N spectropolarimetry of the Hα emission line of ζ Puppis. We discuss, and successfully remove, a high-frequency instrumental signature. The corrected data show polarization structure through the emission line, which we attribute to an equatorial density enhancement of the wind. We employ simple (core-halo) numerical models to derive a lower limit of a factor of 1.3 to the implied equator-to-pole density ratio, and to demonstrate that the wind structure predicted by wind-compression theory is capable of reproducing the gross characteristics of the polarization spectrum.Astronomy and Astrophysics 05/1996; 310:533-546. · 4.59 Impact Factor -
Article: On the reflection effect in three sdOB binary stars.
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ABSTRACT: Echelle spectroscopy is presented of three binary systems containing hot subdwarf primary stars and low-mass, low-luminosity secondary stars - HW Vir, AA Dor and KV Vel. The semi-amplitudes of the radial-velocity curves of the primary components are determined to an accuracy of 1-2 per cent. The secondary component of KV Vel displays a rich set of weak emission lines due to C II, C III, C IV, N II, N III and O II, arising from the hemisphere that is irradiated by the hot primary star. For HW Vir and AA Dor, an extensive search, via proven cross-correlation procedures, fails to reveal any spectral signatures attributable to the secondary components. The published multi-colour light curves of these three systems are analysed to confirm some previous results, and to demonstrate that the amplitude and detailed shape of the reflection effect can be matched provided that the irradiated hemispheres of the (otherwise) cool secondary stars have bolometric albedos of unity, and have limb-darkening coefficients much reduced over normal values, or even exhibit limb brightening. Astrophysical parameters are determined, based upon these analyses and assuming, in the cases of HW Vir and AA Dor, a mass of 0.5 Msun for the subdwarf stars. The three hot subdwarfs lie in the region of the HR diagram appropriate for evolution models for binary systems undergoing late case B mass transfer followed by a common-envelope phase. For HW Vir and AA Dor, the cool secondary stars are entirely consistent with ZAMS models at 0.09-0.14 Msun, whilst the secondary in KV Vel has log R/Rsun ≡ 0.2 dex larger than a ZAMS model for its mass of 0.23 Msun.Monthly Notices of the Royal Astronomical Society 03/1996; 279:1380-1392. · 4.90 Impact Factor -
Article: Ten eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance
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ABSTRACT: We present the first results of an observational programme to measure the fundamental parameters of over 100 eclipsing binaries in the Small Magellanic Cloud (SMC). The spectroscopic data have been obtained by using the two degree field (2dF) multi-object spectrograph on the 3.9-m Anglo-Australian Telescope, and have been used in conjunction with photometry from the Optical Gravitational Lensing Experiment (OGLE) data base of SMC eclipsing binaries. In this first paper, we discuss 10 systems: three are detached early-B binaries, six are in a semi-detached configuration, and one is in a marginal contact state. We conclude that the semi-detached systems are undergoing the slow mass-transfer phase of case-A binary evolution, in which the mass donor has reached its Roche lobe while still on the main sequence. Each system provides a primary distance indicator. By constructing a new calibration between spectral type and temperature for O and early B stars, we find a mean distance modulus to the SMC of 18.89 ′ 0.04 (statistical) ′0.10 (systematic). This value represents one of the most precise determinations to date of the distance to the SMC.Harries, T.J. and Hilditch, R.W. and Howarth, I.D. (2003) Ten eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance. Monthly Notices of the Royal Astronomical Society, 339 (1). pp. 157-172. ISSN 00358711. -
Article: The magnetic field and wind confinement of theta1 Orionis C
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ABSTRACT: We report the detection, through spectropolarimetric observations, of a strong dipolar magnetic field of presumably fossil origin at the surface of the very young O star θ1 Ori C. The Stokes V signatures we detect are variable with time, the variations being consistent with rotational modulation. A detailed modelling of our observations indicates that this dipole field has an intensity of 1.1±0.1 kG and is inclined at 42°±6° with respect to the rotation axis (assumed to be inclined at 45° to the line of sight). We find, in particular, that the positive magnetic pole comes closest to the observer when the variable Hα emission component observed on this star reaches maximum strength. This discovery represents the first definite detection of a magnetic field in an O star, as well as the first detection of a fossil field in a very young star. We also investigate in this paper the magnetic confinement of the radiatively driven wind of θ1 Ori C in the context of the magnetically confined wind-shock model of Babel & Montmerle. In the case of θ1 Ori C, this model predicts the formation of a large magnetosphere (extending as far as 2–3R∗) , consisting of a very hot post-shock region (with temperatures in excess of 10 MK and densities of about 1010–1011 cm-3) generated by the strong collision of the wind streams from both stellar magnetic hemispheres, as well as a dense cooling disc forming in the magnetospheric equator. We find that this model includes most of the physics required to obtain a satisfactory level of agreement with the extensive data sets available for θ1 Ori C in the literature (and, in particular, with the recent X-ray data and the phase-resolved spectroscopic observations of ultraviolet and optical wind lines) provided that the mass-loss rate of θ1 Ori C is at least 5 times smaller than that predicted by radiatively driven wind models. We finally show how new observations with the XMM or Chandra spacecraft could help us constrain this model much more tightly and thus obtain a clear picture of how magnetic fields can influence the winds of hot stars.Donati, J.-F. and Babel, J. and Harries, T.J. and Howarth, I.D. and Petit, P. and Semel, M. (2002) The magnetic field and wind confinement of theta1 Orionis C. Monthly Notices of the Royal Astronomical Society, 333 (1). pp. 55-70. ISSN 00358711. -
Article: Raman scattering in symbiotic stars. II. Numerical models
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ABSTRACT: We present numerical models of Raman-line formation in symbiotic systems, based on Monte-Carlo simulations of scattering of O$\thinspace$ vi resonance-line photons from a hot source in the extended, expanding atmosphere of a cool companion, and we explore the sensitivity of the model results to the input parameters. The model successfully reproduces the observed line ratios, velocities, and polarizations, and the viewing-angle (orbital) dependence of the line strengths and polarized intensities. In principle, the position-angle dependence of the polarization in resolved Raman lines provides a diagnostic of the extent of the ionized region in symbiotic systems; however, axisymmetric geometries with `up-down' symmetry, such as we use, fail to reproduce the most commonly observed dependence of PA on velocity. Raman-line polarization morphology is shown to be a powerful probe of the conditions in the red-giant wind; this is particularly true if observations are available at different orbital phases, so that geometrical and physical variables may more easily be isolated.http://dx.doi.org/10.1051/aas:1997110. -
Article: Benchmark problems for continuum radiative transfer
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ABSTRACT: Aims. Solving the continuum radiative transfer equation in high opacity media requires sophisticated numerical tools. In order to test the reliability of such tools, we present a benchmark of radiative transfer codes in a 2D disc configuration.Methods. We test the accuracy of seven independently developed radiative transfer codes by comparing the temperature structures, spectral energy distributions, scattered light images, and linear polarisation maps that each model predicts for a variety of disc opacities and viewing angles. The test cases have been chosen to be numerically challenging, with midplane optical depths up $10^6$, a sharp density transition at the inner edge and complex scattering matrices. We also review recent progress in the implementation of the Monte Carlo method that allow an efficient solution to these kinds of problems and discuss the advantages and limitations of Monte Carlo codes compared to those of discrete ordinate codes.Results. For each of the test cases, the predicted results from the radiative transfer codes are within good agreement. The results indicate that these codes can be confidently used to interpret present and future observations of protoplanetary discs.http://dx.doi.org/10.1051/0004-6361/200811555. -
Article: Benchmark problems for continuum radiative transfer. High optical depths, anisotropic scattering, and polarisation
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ABSTRACT: Aims. Solving the continuum radiative transfer equation in high opacity media requires sophisticated numerical tools. In order to test the reliability of such tools, we present a benchmark of radiative transfer codes in a 2D disc configuration. Methods: We test the accuracy of seven independently developed radiative transfer codes by comparing the temperature structures, spectral energy distributions, scattered light images, and linear polarisation maps that each model predicts for a variety of disc opacities and viewing angles. The test cases have been chosen to be numerically challenging, with midplane optical depths up 10^6, a sharp density transition at the inner edge and complex scattering matrices. We also review recent progress in the implementation of the Monte Carlo method that allow an efficient solution to these kinds of problems and discuss the advantages and limitations of Monte Carlo codes compared to those of discrete ordinate codes. Results: For each of the test cases, the predicted results from the radiative transfer codes are within good agreement. The results indicate that these codes can be confidently used to interpret present and future observations of protoplanetary discs.Astronomy and Astrophysics, v.498, 967-980 (2009). -
Article: Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph
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ABSTRACT: From observations collected with the ESPaDOnS spectropolarimeter, we report the discovery of magnetic fields at the surface of the mildly accreting classical T Tauri star V2129 Oph. Zeeman signatures are detected, both in photospheric lines and in the emission lines formed at the base of the accretion funnels linking the disc to the protostar, and monitored over the whole rotation cycle of V2129 Oph. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized line profiles. We reconstruct the large-scale magnetic topology at the surface of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to be rather complex, with a dominant octupolar component and a weak dipole of strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar radial field spots located at high latitudes and coinciding with cool dark polar spots at photospheric level. This large-scale field geometry is unusually complex compared to those of non-accreting cool active subgiants with moderate rotation rates. As an illustration, we provide a first attempt at modelling the magnetospheric topology and accretion funnels of V2129 Oph using field extrapolation. We find that the magnetosphere of V2129 Oph must extend to about 7R* to ensure that the footpoints of accretion funnels coincide with the high-latitude accretion spots on the stellar surface. It suggests that the stellar magnetic field succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of V2129 Oph. The magnetospheric geometry we derive produces X-ray coronal fluxes typical of those observed in cTTSs.
Top co-authors
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Institutions
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2008
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University of Exeter
Exeter, ENG, United Kingdom
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1997–2002
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University of St Andrews
- School of Physics and Astronomy
Saint Andrews, SCT, United Kingdom
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2000
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University College London
- Department of Physics and Astronomy
London, ENG, United Kingdom
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1997–1998
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Andrews University
Berrien Springs, MI, USA
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