arXiv subjects

Ryuichi Kurosawa

Publications and source records attributed to Ryuichi Kurosawa.

27 records · Page 2Linked to original sources

arXiv2007

Three-Dimensional Simulations of Inflows Irradiated by a Precessing Accretion Disk in Active Galactic Nuclei: Formation of Outflows

We present three-dimensional (3-D) hydrodynamical simulations of gas flows in the vicinity of an active galactic nucleus (AGN) powered by a precessing accretion disk. We consider the effects of the radiation force from such a disk on its environment on a relatively large scale (up to ~10 pc. We implicitly include the precessing disk by forcing the disk radiation field to precess around a symmetry axis with a given period ($P$) and a tilt angle ($Θ$). We study time evolution of the flows irradiated by the disk, and investigate basic dependencies of the flow morphology, mass flux, angular momentum on different combinations of $Θ$ and $P$. We find the gas flow settles into a configuration with two components, (1) an equatorial inflow and (2) a bipolar inflow/outflow with the outflow leaving the system along the poles (the directions of disk normals). However, the flow does not always reach a steady state. We find that the maximum outflow velocity and the kinetic outflow power at the outer boundary can be reduced significantly with increasing $Θ$. We also find that of the mass inflow rate across the inner boundary does not change significantly with increasing $Θ$. (Abbreviated)

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arXiv2006

Radial and rotational velocities of young brown dwarfs and very low-mass stars in the Upper Scorpius OB association and the rho Ophiuchi cloud core

We present the results of a radial velocity (RV) survey of 14 brown dwarfs (BDs) and very low-mass (VLM) stars in the Upper Scorpius OB association (UScoOB) and 3 BD candidates in the rho Ophiuchi dark cloud core. We obtained high-resolution echelle spectra at the Very Large Telescope using Ultraviolet and Visual Echelle Spectrograph (UVES) at two different epochs for each object, and measured the shifts in their RVs to identify candidates for binary/multiple systems in the sample. The average time separation of the RV measurements is 21.6d, and our survey is sensitive to the binaries with separation < 0.1 au. We found that 4 out of 17 objects (or 24^{+16}_{-13} per cent by fraction) show a significant RV change in 4-33d time scale, and are considered as binary/multiple `candidates.' We found no double-lined spectroscopic binaries in our sample, based on the shape of cross-correlation curves. The RV dispersion of the objects in UScoOB is found to be very similar to that of the BD and VLM stars in Chamaeleon I (Cha I). We also found the distribution of the mean rotational velocities (v sin i) of the UScoOB objects is similar to that of the Cha I, but the dispersion of v sin i is much larger than that of the Cha I objects.

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arXiv2006

On the formation of H-alpha 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 H-alpha 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 magnetospheric 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 reproduce the wide range of profile types seen in observations, and that the most common profile types 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 model profiles are consistent with the canonical value of the mass-loss rate to mass-accretion rate ratio (mu=0.1) found in earlier magneto-hydrodynamic calculations and observations, but the models with 0.05<mu<0.2 are still in accord with observed H-alpha profiles. We investigate the wind contribution to the line profile as a function of model parameters, and examine the reliability of H-alpha as a mass accretion diagnostic. Finally, we examine the H-alpha spectroscopic classification used by Reipurth et. al, and discuss the basic physical conditions that are required to reproduce the profiles in each classified type.

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arXiv2005

T Tauri stellar magnetic fields: He I measurements

We present measurements of the longitudinal magnetic field in the circumstellar environment of seven classical T Tauri stars. The measurements are based on high-resolution circular spectropolarimetry of the He I 5876 emission line, which is thought to form in accretion streams controlled by a stellar magnetosphere. We detect magnetic fields in BP Tau, DF Tau and DN Tau, and detect statistically significant fields in GM Aur and RW Aur A at one epoch but not at others. We detect no field for DG Tau and GG Tau, with the caveat that these objects were observed at one epoch only. Our measurements for BP Tau and DF Tau are consistent, both in terms of sign and magnitude, with previous studies, suggesting that the characteristics of T Tauri magnetospheres are persistent over several years. We observed the magnetic field of BP Tau to decline monotonically over three nights, and have detected a peak field of 4kG in this object, the highest magnetic field yet observed in a T Tauri star. We combine our observations with results from the literature in order to perform a statistical analysis of the magnetospheric fields in BP Tau and DF Tau. Assuming a dipolar field, we determine a polar field of ~3kG and a dipole offset of 40deg for BP Tau, while DF Tau's field is consistent with a polar field of ~-4.5kG and a dipole offset of 10deg. We conclude that many classical T Tauri stars have circumstellar magnetic fields that are both strong enough and sufficiently globally-ordered to sustain large-scale magnetospheric accretion flows.

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arXiv2005

Time-series Paschen-beta spectroscopy of SU Aurigae

We present time-series echelle spectra of the Pa-beta line of the T Tauri star SU Aur, observed over three consecutive nights. The line shows strong variability (~10 per cent) over the velocity range (100 km/s, 420 km/s) in the red broad absorption component, and weaker variability (~2 per cent) over the velocity range (-200 km/s, 0 km/s) in the blue wing. The variability in the velocity range (-200 km/s, 0 km/s) is correlated with that in (200 km/s, 400 km/s), and the variability in these velocity ranges anti-correlates with that in (0 km/s, 100 km/s). The mean spectrum from the second night shows a suggestion of a blue-shifted absorption component at about -150 km/s, similar to that found in the H-alpha and H-beta lines. We find the position of the subpeak in the red absorption component changes steadily with time, and its motion modulates on half the rotational period. We also find that the modulation of the line equivalent width is associated with a half and a third of the rotational period, which is consistent with the surface Doppler images of SU Aur. Radiative transfer models of a rotationally modulated Pa-beta line, produced in the shock-heated magnetospheric accretion flow, are also presented. Models with a magnetic dipole offset reproduce the overall characteristics of the observed line variability, including the line equivalent width and the motion of the subpeak in the red absorption trough.

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arXiv2005

Emission-line profile modelling of structured T Tauri magnetospheres

We present hydrogen emission line profile models of magnetospheric accretion onto Classical T Tauri stars. The models are computed under the Sobolev approximation using the three-dimensional Monte Carlo radiative-transfer code TORUS. We have calculated four illustrative models in which the accretion flows are confined to azimuthal curtains - a geometry predicted by magneto-hydrodynamical simulations. Properties of the line profile variability of our models are discussed, with reference to dynamic spectra and cross-correlation images. We find that some gross characteristics of observed line profile variability are reproduced by our models, although in general the level of variability predicted is larger than that observed. We conclude that this excessive variability probably excludes dynamical simulations that predict accretion flows with low degrees of axisymmetry.

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arXiv2004

Linear spectropolarimetry of young and other emission line stars

The aim of this article is to demonstrate the useful role that can be played by spectropolarimetric observations of young and evolved emission line stars that analyse the linearly polarized component in their spectra. At the time of writing, this demonstration has to be made on the basis of optical data since there is no common-user infrared facility, in operation, that offers the desired combination of spectral resolution and sensitivity. Here we focus on what can be learned from linear spectropolarimetry alone at reasonably high spectral resolution and at $10^3 < $S/N$ < 10^4$. And we remind that the near infrared (1--2 micron) has the potential to out-perform the optical as a domain to work in because of the greatly reduced interstellar obscuration at these wavelengths. This point has been reached at a time when theory, exploiting flexible Monte Carlo methods, is fast becoming a powerful tool. In short we have the complex phenomena, and the rise of the modelling capability to match -- good data are the missing link.

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arXiv2004

Synthetic infrared images and spectral energy distributions of a young low-mass stellar cluster

We present three-dimensional Monte Carlo radiative transfer models of a very young (<10^5 years old) low mass (50 Msun) stellar cluster containing 23 stars and 27 brown dwarfs. The models use the density and the stellar mass distributions from the large-scale smoothed particle hydrodynamics (SPH) simulation of the formation of a low-mass stellar cluster by Bate, Bonnell and Bromm. We compute the spectral energy distributions of the whole cluster and the individual objects, simulated far-infrared Spitzer Space Telescope (SST) images and construct colour-colour diagrams (near-infrared HKL and SST mid-infrared bands). The presence of accretion discs around the light sources influences the morphology of the dust temperature structure on a large scale (up to a several 10^4 au). A considerable fraction of the interstellar dust is underheated compared to a model without the accretion discs because the radiation from the light sources is blocked/shadowed by the discs. The spectral energy distribution (SED) of the model cluster with accretion discs shows excess emission at lambda = 3-30 micron and lambda > 500 micron compared to that without accretion discs. Our model with accretion discs around each object shows a similar distribution of spectral index (2.2-20 micron) values (i.e. Class 0--III sources) as seen in the Rho Ophiuchus cloud. We confirm that the best diagnostics for identifying objects with accretion discs are mid-infrared (lambda = 3-10 micron) colours (e.g. SST IRAC bands) rather than HKL colours.

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arXiv2004

Three-dimensional dust radiative-transfer models: The Pinwheel Nebula of WR104

We present radiative-transfer modelling of the dusty spiral Pinwheel Nebula observed around the Wolf-Rayet/OB-star binary WR104. 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 (sub-AU 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 x 10^{-7} solar masses per year, corresponding to approximately 2% by mass of the carbon produced by the Wolf-Rayet star. Simultaneous modelling of the imaging and spectral data enables us to constrain both the opening-angle of the wind-wind collision interface and the dust grain size. We conclude that the dust grains in the inner part of the Pinwheel nebula are small (~100A), in agreement with theoretical predictions, although we cannot rule out the presence of larger grains (~1 micron) further from the central binary. The opening angle of the wind-wind collision interface appears to be about 40 degrees, in broad agreement with the wind parameters estimated for the central binary. We discuss the success and deficiencies of the model, and the likely benefits of applying similar techniques to the more the more complex nebulae observed around other WR/O star binaries.

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