David Rees

Bio: David Rees is an academic researcher from Commonwealth Scientific and Industrial Research Organisation. The author has contributed to research in topics: Zeeman–Doppler imaging & Stars. The author has an hindex of 9, co-authored 14 publications receiving 2020 citations.

Spectropolarimetric observations of active stars

Abstract: This paper reports the results of five years (five runs, 23 nights) of spectropolarimetric observations of active stars with the UCL Echelle Spectrograph of the Anglo-Australian Telescope. 225 circularly (and four linearly) polarized spectra were recorded on 28 objects (21 active stars and seven calibration standards) using the new technique of Zeeman-Doppler imaging. To extract polarization echelle spectra from raw frames, we developed a new dedicated automatic software package (called ESpRIT, which utilizes optimal extraction techniques) whose detailed description is given in the paper. For each recorded spectrum, we extract 'mean' polarized and unpolarized profiles using 'least-squares deconvolution', a technique similar to cross-correlation, which can enhance enormously the sensitivity of Zeeman-Doppler imaging, by up to 7.5 mag in flux with respect to a single average line analysis or by 4.5 mag compared with the older technique of Donati et al. in the particular case of a K1 star. Magnetic field is detected unambiguously on 14 objects, namely the weak-line T Tauri star V410 Tau, the pre-main-sequence binary HD 155555, the ZAMS stars AB Dor and LQ Hya, the dwarf flare star CC Eri, the RS CVn systems HR 1099, EI Eri, TY Pyx, CF Tue, SZ Psc, II Peg, IM Peg and IL Hya, and the FK Com star YY Men. Marginal field detections are also obtained for the weak-line T Tauri star HD 283572 and the Herbig Ae star HD 104237. Except on HR 1099 and II Peg, our results represent the first direct field detections ever reported on these objects, and in particular the first direct field detection on as young a star as V410 Tau. Most of the magnetic signatures we detect on cool stars show several sign reversals throughout the line profile, indicating that the parent field structure is rather complex and must feature (as expected) many small-scale magnetic regions of different polarities. For all stars on which Zeeman detections are recorded with sufficient accuracy (namely LQ Hya, CC Eri, HR 1099, El Eri, II Peg, IL Hya and YY Men), differential least-squares deconvolution from both the blue and the red parts of the spectral domain indicates that the magnetic regions we detect are mostly 500 to 1000 K cooler than, and sometimes at the same temperature as, but never warmer than the surrounding photosphere. Serendipitous results include the first detection (i) of small-amplitude radial velocity variations (1.3 kms -1 peak to peak) of the Herbig Ae star HD 104237 with small enough a period (37.5 ± 1 min) that they must be due to stellar pulsations and (ii) of the solar-like secondary component of the RS CVn system IL Hya.

Dynamo processes and activity cycles of the active stars AB Doradus, LQ Hydrae and HR 1099

Abstract: In this paper, we present new brightness and magnetic surface images of the young K0 dwarfs AB Doradus and LQ Hydrae, and of the K1 subgiant of the RS CVn system HR 1099 (=V711 Tauri), reconstructed from Zeeman–Doppler imaging spectropolarimetric observations collected at the Anglo-Australian Telescope during five observing campaigns (totalling 50 nights), from 1998 January to 2002 January. Along with the older images of the same stars (published in previous papers), our complete data set represents the first long-term series on temporal fluctuations of magnetic topologies of very active stars. All of the magnetic images presented here indicate that large regions with predominantly azimuthal magnetic fields are continuously present at the surfaces of these stars. We take this as further evidence that the underlying dynamo processes that produce them are probably distributed throughout the entire convective zone (and not confined at its base, as in the Sun). We speculate that the radial and azimuthal field maps that we recover correspond, respectively, to the poloidal and toroidal components of the large-scale dynamo field. We find, in particular, that some signatures, for instance the relative fraction of magnetic energy stored in the large-scale poloidal and toroidal field components, and the polarity of the axisymmetric component of the field, are variable with time, and provide potentially fruitful diagnostics for investigating magnetic cycles in active stars other than the Sun. We report here the detection of partial polarity switches in some of the axisymmetric field components of two of our programme stars (AB Dor and LQ Hya), suggesting that the dynamo operating in these stars may be cyclic.

Zeeman-Doppler imaging of active stars. III: Instrumental and technical considerations

Abstract: An account is given of the instrumental setup and observational procedure that led to the qualifying of Zeeman-Doppler imaging and to new detections of magnetic fields on four stars HR 1099, σ 2 CrB, II Peg and UX Ari

Multi-Object Tracking in Video

Abstract: This paper reports on tracking of multiple objects using color histogram backprojection and motion cues. Four tasks which facilitate this are discussed. The first is an adaptive color histogram backprojection (which builds upon the works of Swain and Ballard) and its application to tracking of multiple objects in video sequences. The second task is designing efficient fast blob detectors for selecting regions of interest in video sequences. The third is motion detection based on color histogram backprojection. Achieving these tasks led to multi-objects tracking. Various video sequences were used to demonstrate effective tracking of multiple objects. Notably, we created an interactive multiple objects tracker (CLICK-IT) which in its present form is set at three objects but can be extended easily. CLICK-IT (CSIRO Laboratory for Imaging by Content and Knowledge?Interactive Television) is a PC-based system which provides the user with an intelligent highlighter pen for sports action replay. It is intended as a truly interactive improvement on the drawing pad technology currently used for video annotation in sports broadcasting. The system uses computer vision techniques to focus attention and track particular objects (player(s), ball, horse(s), ?) and semi-automatically annotate the dynamic scene. This paper describes the system including the user interface, the tracking technology based on color and motion information, and system performance evaluation in applications to surveillance-like sequences, running, rugby league football, basketball and soccer. Finally, video scene detection based on color histogram is discussed.

High-Energy Processes in Young Stellar Objects

Abstract: ▪ Abstract Observational studies of low-mass stars during their early stages of evolution, from protostars through the zero-age main sequence, show highly elevated levels of magnetic activity. This activity includes strong fields covering much of the stellar surface and powerful magnetic reconnection flares seen in the X-ray and radio bands. The flaring may occur in the stellar magnetosphere, at the star-disk interface, or above the circumstellar disk. Ionization from the resulting high-energy radiation may have important effects on the astrophysics of the disk, such as promotion of accretion and coupling to outflows, and on the surrounding interstellar medium. The bombardment of solids in the solar nebula by flare shocks and energetic particles may account for various properties of meteorites, such as chondrule melting and spallogenic isotopes. X-ray surveys also improve our samples of young stars, particularly in the weak-lined T Tauri phase after disks have dissipated, with implications for our underst...

Magnetic Fields of Nondegenerate Stars

Abstract: Magnetic fields are present in a wide variety of stars throughout the HR diagram and play a role at basically all evolutionary stages, from very-low-mass dwarfs to very massive stars, and from young star-forming molecular clouds and protostellar accretion discs to evolved giants/supergiants and magnetic white dwarfs/neutron stars. These fields range from a few μG (e.g., in molecular clouds) to TG and more (e.g., in magnetic neutron stars); in nondegenerate stars in particular, they feature large-scale topologies varying from simple nearly axisymmetric dipoles to complex nonaxsymmetric structures, and from mainly poloidal to mainly toroidal topologies. After recalling the main techniques of detecting and modeling stellar magnetic fields, we review the existing properties of magnetic fields reported in cool, hot, and young nondegenerate stars and protostars, and discuss our understanding of the origin of these fields and their impact on the birth and life of stars.

Magnetic fields of non-degenerate stars

Abstract: Magnetic fields are present in a wide variety of stars through out the HR diagram and play a role at basically all evolutionary stages, from very-low- mass dwarfs to very massive stars, and from young star-forming molecular clouds and protostellar accretion discs to evolved gi- ants/supergiants and magnetic white dwarfs/neutron stars. These fields range from a fewG (e.g., in molecular clouds) to TG and more (e.g., in magnetic neutron stars); in non-degenerate stars in particular, they feature large-scale topologies varying f rom simple nearly-axisymmetric dipoles to complex non-axsymmetric structures, and from mainly poloidal to mainly toroidal topology. After recalling the main techniques of detecting and modelling stellar magnetic fields, we review the existing properties of magnetic fields reported in cool, hot and young non-degenerate stars and protostars, and discuss our understanding of the origin of these fields and their impact on the birth and life of stars.

Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence

Abstract: We present a series of models for the plasma properties along open magnetic flux tubes rooted in solar coronal holes, streamers, and active regions. These models represent the first self-consistent solutions that combine (1) chromospheric heating driven by an empirically guided acoustic wave spectrum; (2) coronal heating from Alfven waves that have been partially reflected, then damped by anisotropic turbulent cascade; and (3) solar wind acceleration from gradients of gas pressure, acoustic wave pressure, and Alfven wave pressure. The only input parameters are the photospheric lower boundary conditions for the waves and the radial dependence of the background magnetic field along the flux tube. We have not included multifluid or collisionless effects (e.g., preferential ion heating), which are not yet fully understood. For a single choice for the photospheric wave properties, our models produce a realistic range of slow and fast solar wind conditions by varying only the coronal magnetic field. Specifically, a two-dimensional model of coronal holes and streamers at solar minimum reproduces the latitudinal bifurcation of slow and fast streams seen by Ulysses. The radial gradient of the Alfven speed affects where the waves are reflected and damped, and thus whether energy is deposited below or above the Parker critical point. As predicted by earlier studies, a larger coronal "expansion factor" gives rise to a slower and denser wind, higher temperature at the coronal base, less intense Alfven waves at 1 AU, and correlative trends for commonly measured ratios of ion charge states and FIP-sensitive abundances that are in general agreement with observations. These models offer supporting evidence for the idea that coronal heating and solar wind acceleration (in open magnetic flux tubes) can occur as a result of wave dissipation and turbulent cascade.

Mass loss from hot massive stars

Abstract: Mass loss is a key process in the evolution of massive stars, and must be understood quantitatively if it is to be successfully included in broader astrophysical applications such as galactic and cosmic evolution and ionization. In this review, we discuss various aspects of radiation driven mass loss, both from the theoretical and the observational side. We focus on developments in the past decade, concentrating on the winds from OB-stars, with some excursions to the winds from Luminous Blue Variables (including super-Eddington, continuum-driven winds), winds from Wolf–Rayet stars, A-supergiants and Central Stars of Planetary Nebulae. After recapitulating the 1-D, stationary standard model of line-driven winds, extensions accounting for rotation and magnetic fields are discussed. Stationary wind models are presented that provide theoretical predictions for the mass-loss rates as a function of spectral type, metallicity, and the proximity to the Eddington limit. The relevance of the so-called bi-stability jump is outlined. We summarize diagnostical methods to infer wind properties from observations, and compare the results from corresponding campaigns (including the VLT-flames survey of massive stars) with theoretical predictions, featuring the mass loss-metallicity dependence. Subsequently, we concentrate on two urgent problems, weak winds and wind-clumping, that have been identified from various diagnostics and that challenge our present understanding of radiation driven winds. We discuss the problems of “measuring” mass-loss rates from weak winds and the potential of the NIR Br α -line as a tool to enable a more precise quantification, and comment on physical explanations for mass-loss rates that are much lower than predicted by the standard model. Wind-clumping, conventionally interpreted as the consequence of a strong instability inherent to radiative line-driving, has severe implications for the interpretation of observational diagnostics, since derived mass-loss rates are usually overestimated when clumping is present but ignored in the analyses. Depending on the specific diagnostics, such overestimates can amount to factors of 2 to 10, and we describe ongoing attempts to allow for more uniform results. We point out that independent arguments from stellar evolution favor a moderate reduction of present-day mass-loss rates. We also consider larger scale wind structure, interpreted in terms of co-rotating interacting regions, and complete this review with a discussion of recent progress on the X-ray line emission from massive stars. Such emission is thought to originate both from magnetically confined winds and from non-magnetic winds, in the latter case related to the line-driven instability and/or clump-clump collisions. We highlight as to how far the analysis of such X-ray line emission can give further clues regarding an adequate description of wind clumping.