1,530 research outputs found
Line and continuum radiative transfer modelling of AA Tau
We present photometric and spectroscopic models of the Classical T Tauri star
AA Tau. Photometric and spectroscopic variability present in observations of AA
Tau is attributed to a magnetically induced warp in the accretion disc,
periodically occulting the photosphere on an 8.2--day timescale. Emission line
profiles show signatures of both infall, attributed to magnetospherically
accreting material, and outflow. Using the radiative transfer code TORUS, we
have investigated the geometry and kinematics of AA Tau's circumstellar disc
and outflow, which is modelled here as a disc wind. Photometric models have
been used to constrain the aspect ratio of the disc, the offset angle of the
magnetosphere dipole with respect to the stellar rotation axis, and the inner
radius of the circumstellar disc. Spectroscopic models have been used to
constrain the wind and magnetosphere temperatures, wind acceleration parameter,
and mass loss rate. We find observations are best fitted by models with a mass
accretion rate of M yr, a dipole offset of
between and , a magnetosphere that truncates the disc from
5.2 to 8.8 R, a mass-loss-rate to accretion-rate ratio of ~ 0.1, a
magnetosphere temperature of 8500 -- 9000 K, and a disc wind temperature of
8000 K.Comment: 22 pages, 32 figures, 4 tables. Accepted by MNRAS. V3: Corrected typ
The Inner Rim of YSO Disks: Effects of dust grain evolution
Dust-grain growth and settling are the first steps towards planet formation.
An understanding of dust physics is therefore integral to a complete theory of
the planet formation process. In this paper, we explore the possibility of
using the dust evaporation front in YSO disks (`the inner rim') as a probe of
the dust physics operating in circumstellar disks. The geometry of the rim
depends sensitively on the composition and spatial distribution of dust. Using
radiative transfer and hydrostatic equilibrium calculations we demonstrate that
dust growth and settling can curve the evaporation front dramatically (from a
cylindrical radius of about 0.5 AU in the disk mid-plane to 1.2 AU in the disk
upper layers for an A0 star). We compute synthetic images and interferometric
visibilities for our representative rim models and show that the current
generation of near-IR long-baseline interferometers (VLTI, CHARA) can strongly
constrain the dust properties of circumstellar disks, shedding light on the
relatively poorly understood processes of grain growth, settling and turbulent
mixing.Comment: 26 pages, 9 figures. Accepted for publication in Ap
Building leadership capacity and future leaders in operational research in low-income countries: why and how?
Very limited operational research (OR) emerges from programme settings in low-income countries where the greatest burden of disease lies. The price paid for this void includes a lack of understanding of how health systems are actually functioning, not knowing what works and what does not, and an inability to propose adapted and innovative solutions to programme problems. We use the National Tuberculosis Control Programme as an example to advocate for strong programme-level leadership to steer OR and build viable relationships between programme managers, researchers and policy makers. We highlight the need to create a stimulating environment for conducting OR and identify some of the main practical challenges and enabling factors at programme level. We focus on the important role of an OR focal point within programmes and practical approaches to training that can deliver timely and quantifiable outputs. Finally, we emphasise the need to measure successful OR leadership development at programme level and we propose parameters by which this can be assessed. This paper 1) provides reasons why programmes should take the lead in coordinating and directing OR, 2) identifies the practical challenges and enabling factors for implementing, managing and sustaining OR and 3) proposes parameters for measuring successful leadership capacity development in OR
Radiative Transfer in Star Formation: Testing FLD and Hybrid Methods
We perform a comparison between two radiative transfer algorithms commonly
employed in hydrodynamical calculations of star formation: grey flux limited
diffusion and the hybrid scheme, in addition we compare these algorithms to
results from the Monte-Carlo radiative transfer code MOCASSIN. In disc like
density structures the hybrid scheme performs significantly better than the FLD
method in the optically thin regions, with comparable results in optically
thick regions. In the case of a forming high mass star we find the FLD method
significantly underestimates the radiation pressure by a factor of ~100.Comment: 4 Pages; to appear in the proceedings of 'The Labyrinth of Star
Formation', Crete, 18-22 June 201
In pursuit of gamma-ray burst progenitors: the identification of a sub-population of rotating Wolf-Rayet stars
Long gamma-ray bursts involve the most powerful cosmic explosions since the
Big Bang. Whilst it has been established that GRBs are related to the death
throes of massive stars, the identification of their progenitors has proved
challenging. Theory suggests that rotating Wolf-Rayet stars are the best
candidates, but their strong stellar winds shroud their surfaces, preventing a
direct measurement of their rotation. Fortunately, linear spectropolarimetry
may be used to probe the flattening of their winds due to stellar spin.
Spectropolarimetry surveys show that an 80% majority of WR stars have
spherically symmetric winds and are thus rotating slowly, yet a small 20%
minority display a spectropolarimetric signature indicative of rotation. Here
we find a highly significant correlation between WR objects that carry the
signature of stellar rotation and the subset of WR stars with ejecta nebulae
that have only recently transitioned from a red sugergiant or luminous blue
variable phase. As these youthful WR stars have yet to spin-down due to mass
loss, they are the best candidate GRB progenitors identified to date. When we
take recently published WR ejecta nebula numbers we find that five out of the
six line-effect WR stars are surrounded by ejecta nebulae. The statistics imply
that the null hypothesis of no correlation between line-effect WR stars and
ejecta nebulae can be rejected at the 0.0004% level. Given that four
line-effect and WR ejecta nebula have spectroscopically been confirmed to
contain nucleosynthetic products, we argue that the correlation is both
statistically significant and physically convincing. The implication is that we
have identified a WR sub-population that fulfills the necessary criteria for
making GRBs. Finally, we discuss the potential of identifying GRB progenitors
via spectropolarimetry with extremely large telescopes.Comment: 5 pages, accepted for publication in Astronomy & Astrophysics Letters
(small textual changes
Forty eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance
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.Comment: paper accepted on 22 November 2004 for publication by MNRAS; 26
pages, 6 tables, 12 figure
Spectropolarimetric observations of Herbig Ae/Be Stars I: HiVIS spectropolarimetric calibration and reduction techniques
Using the HiVIS spectropolarimeter built for the Haleakala 3.7m AEOS
telescope in Hawaii, we are collecting a large number of high precision
spectropolarimetrc observations of stars. In order to precisely measure very
small polarization changes, we have performed a number of polarization
calibration techniques on the AEOS telescope and HiVIS spectrograph. We have
extended our dedicated IDL reduction package and have performed some hardware
upgrades to the instrument. We have also used the ESPaDOnS spectropolarimeter
on CFHT to verify the HiVIS results with back-to-back observations of MWC 361
and HD163296. Comparision of this and other HiVIS data with stellar
observations from the ISIS and WW spectropolarimeters in the literature further
shows the usefulness of this instrument.Comment: 35 pages, 44 figures, Accepted by PAS
Discovery of a 500 au protobinary in the massive prestellar core G11.92-0.61 MM2
Funding: C.J.C. acknowledges support from the University of St Andrews Restarting Research Funding Scheme (SARRF), which is funded through the SFC grant reference SFC/AN/08/020. J.D.I. acknowledges support from the UK’s STFC under ST/T000287/1. S.Z. is funded by the China Scholarship Council–University of St Andrews Scholarship (PhD programmes, No. 201806190010). T.J.H. is funded by a Royal Society Dorothy Hodgkin Fellowship.We present high-resolution ( 24.7 L⊙ for MM2E and L* > 12.6 L⊙ for MM2W. The compact sources are connected by a "bridge" of lower-surface-brightness dust emission and lie within more extended emission that may correspond to a circumbinary disk. The circumprotostellar gas mass, estimated from ~0.2" resolution VLA 0.9 cm observations assuming optically thin emission, is 6.8 ± 0.9 M⊙. No line emission is detected towards MM2E and MM2W in our high-resolution 1.3 mm ALMA observations. The only line detected is 13CO J=2-1, in absorption against the 1.3 mm continuum, which likely traces a layer of cooler molecular material surrounding the protostars. We also report the discovery of a highly asymmetric bipolar molecular outflow that appears to be driven by MM2E and/or MM2W in new deep, ~0.5" resolution (1680 au) ALMA 0.82 mm observations. This outflow, traced by low-excitation CH3OH emission, indicates ongoing accretion onto the protobinary system. Overall, the super-Alfvenic models of Mignon-Risse et al. (2021) agree well with the observed properties of the MM2E/MM2W protobinary, suggesting that this system may be forming in an environment with a weak magnetic field.Publisher PDFPeer reviewe
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