Tim J. Harries

Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph

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.

Benchmark problems for continuum radiative transfer. High optical depths, anisotropic scattering, and polarisation

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. 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. 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.

On the formation of Hα line emission around classical T Tauri stars

We present radiative transfer models of the circumstellar environment of classical T Tauri stars, concentrating on the formation of the Hα emission. The wide variety of line profiles seen in observations are indicative of both inflow and outflow , and we therefore employ a circumstellar structure that includes both magnetospheri c accretion and a disc wind. We perform systematic investigations of the model parameters for the wind and the magnetosphere to search for possible geometrical and physical conditions which lead to the types of profiles seen in observations. We find that the hybrid models can repro duce the wide range of profile types seen in observations, and that the most common profile t ypes observed occupy a large volume of parameter space. Conversely, the most infrequently observed profile morphologies require a very specific set of models parameters. We find our mo del profiles are consistent with the canonical value of the mass-loss rate to mass-accretion rate ratio (� = 0.1) found in earlier magneto-hydrodynamic calculations and observations, but the models with 0.05 < � < 0.2 are still in accord with observed Hα profiles. We investigate the wind contribution to the line profile as a function of model parameters, and examine the rel iability of Hα as a mass accretion diagnostic. Finally, we examine the Hα spectroscopic classification used by Reipurth et. al, and discuss the basic physical conditions that are re quired to reproduce the profiles in each classified type.

Probing the circumstellar structure of Herbig Ae/Be stars

We present Hspectropolarimetry observations of a sample of 23 Herbig Ae/Be stars. A change in the linear polarisation across His detected in a large fraction of the objects, which indicates that the regions around Herbig stars are flattened (disc- like) on small scales. A second outcome of our study is that the spectropolarimetric signatures for the Ae stars differ from those of the Herbig Be stars, with characteristics changing from depolarisation across Hin the Herbig Be stars, to line polarisations in the Ae group. The frequency of depolarisations detected in the Herbig Be stars (7/12) is particularly interesting as, by analogy to classical Be stars, it may be the best evidence to date that the higher mass Herbig stars are surrounded by flattened structures. For the Herbig Ae stars, 9 out of 11 show a line polarisation effect that can be understood in terms of a compact Hemission that is itself polarised by a rotating disc-like circumstellar medium. The spectropolarimetric difference between the Herbig Be and Ae stars may be the first indication that there is a transition in the Hertzsprung-Russell Diagram from magnetic accretion at spectral type A to disc accretion at spectral type B. Alternatively, the interior polarised line emission apparent in the Ae stars may be masked in the Herbig Be stars due to their higher levels of Hemission.

A 2dF survey of the Small Magellanic Cloud

We present a catalogue of new spectral types for hot, luminous stars in the Small Magellanic Cloud (SMC). 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 one 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 )e xperiment, Be and B[e] stars, ‘anomalous A supergiants’ and composite-spectrum systems. We examine the incidence of Balmer-line emission, and the relationship between Hγ equivalent width and absolute magnitude for BA stars. Ke yw ords: stars: early-type ‐ stars: emission-line, Be ‐ stars: fundamental parameters ‐ Hertzsprung‐Russell (HR) diagram ‐ Magellanic Clouds.

Three-dimensional dust radiative-transfer models: the Pinwheel Nebula of WR 104

We present radiative-transfer modelling of the dusty spiral Pinwheel Nebula observed around the Wolf‐Rayet/OB-star binary WR 104. The models are based on the three-dimensional radiative-transfer code TORUS, modified to include an adaptive mesh that allows us to adequately resolve both the inner spiral turns (subau scales) and the outer regions of the nebula (distances of 10 4 au from the central source). The spiral model provides a good fit to both the spectral energy distribution and Keck aperture masking interferometry, reproducing both the maximum entropy recovered images and the visibility curves. We deduce a dust creation rate of 8 ± 1 ×

Surface magnetic fields on two accreting T Tauri stars: CV Cha and CR Cha

We have produced brightness and magnetic field maps of the surfaces of CV Cha and CR Cha: two actively accreting G- and K-type T Tauri stars in the Chamaeleon I star-forming cloud with ages of 3–5 Myr. Our magnetic field maps show evidence for strong, complex multipolar fields similar to those obtained for young rapidly rotating main-sequence stars. Brightness maps indicate the presence of dark polar caps and low-latitude spots – these brightness maps are very similar to those obtained for other pre-main-sequence and rapidly rotating main-sequence stars. Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star. We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence.

Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars

We present Hα spectropolarimetry observations of a sample of 10 bright T Tauri stars, supplemented with new Herbig Ae/Be star data. A change in the linear polarization across Hα is detected in most of the T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret in terms of a compact source of line photons that is scattered off a rotating accretion disc. We find consistency between the position angle (PA) of the polarization and those of imaged disc PAs from infrared and millimetre imaging and interferometry studies, probing much larger scales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find the polarization PA to be perpendicular to the imaged disc, which is expected for single scattering. On the other hand, the polarization PA aligns with the outer disc PA for the T Tauri stars DR Tau and SU Aur and FU Ori, conforming to the case of multiple scattering. This difference can be explained if the inner discs of Herbig Ae stars are optically thin, whilst those around our T Tauri stars and FU Ori are optically thick. Furthermore, we develop a novel technique that combines known inclination angles and our recent Monte Carlo models to constrain the inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, we consider the connection of the inner disc structure with the orientation of the magnetic field in the foreground interstellar medium: for FU Ori and DR Tau, we infer an alignment of the stellar axis and the larger magnetic field direction.

Radiation hydrodynamics of triggered star formation: the effect of the diffuse radiation field

We investigate the effect of including diffuse field radiation when modelling the radiatively driven implosion of a Bonnor–Ebert sphere (BES). Radiation–hydrodynamical calculations are performed by using operator splitting to combine Monte Carlo photoionization with grid-based Eulerian hydrodynamics that includes self-gravity. It is found that the diffuse field has a significant effect on the nature of radiatively driven collapse which is strongly coupled to the strength of the driving shock that is established before impacting the BES. This can result in either slower or more rapid star formation than expected using the on-the-spot approximation depending on the distance of the BES from the source object. As well as directly compressing the BES, stronger shocks increase the thickness and density in the shell of accumulated material, which leads to short, strong, photoevaporative ejections that reinforce the compression whenever it slows. This happens particularly effectively when the diffuse field is included as rocket motion is induced over a larger area of the shell surface. The formation and evolution of ‘elephant trunks’ via instability is also found to vary significantly when the diffuse field is included. Since the perturbations that seed instabilities are smeared out elephant trunks form less readily and, once formed, are exposed to enhanced thermal compression.

Multidimensional models of hydrogen and helium emission line profiles for classical T Tauri stars: method, tests and examples

We present multidimensional non-LTE radiative transfer models of hydrogen and helium line profiles formed in the accretion flows and the outflows nea r the star-disk interaction regions of classical T Tauri stars (CTTSs). The statistical eq uilibrium calculations, performed under the assumption of the Sobolev approximation using the radiative transfer code TORUS, has been improved to include He I and He II energy levels. This allows us to probe the physical conditions of the inner wind of CTTSs by simultaneously modelling the robust wind diagnostic line He I �10830 and the accretion diagnostic lines such as Pa� , Br and He I �5876. The code has been tested in one and two dimensional problems, and demonstrated its performance. We apply the model to the complex flow geometries of CTTSs. Example model profiles are computed using the combinations of (1) magnetos pheric accretion and disc wind, and (2) magnetospheric accretion and the stellar wind. In both cases, the model produces line profiles which are very similar to those found in observation s. Our models are consistent with the scenario in which the narrow blueshifted absorption component of He I �10830 seen in observations is caused by a disc wind, and the wider blueshifted absorption component (the P-Cygni profile) is caused by a dipolar stellar wind. Our prel iminary calculations suggest that the temperature of the disc wind and stellar winds cannot be much higher than �10,000 K, on the basis of the strengths of hydrogen lines. Similarly the t emperature of the magnetospheric accretion cannot be much higher than �10,000 K. With these low temperatures, we find that the photoionzation by high energy photons (e.g. X-rays) is necessary to produce He I �10830 in emission and to produce the blueshifted absorption components.