1,555 research outputs found
Making action-angle disc models for Gaia
A brief review of recent work. I describe dynamical modelling of the Milky
Way using action-angle coordinates. I explain what action-angle coordinates
are, and what progress has been made in the past few years to ensuring they can
be used in reasonably realistic Galactic potentials. I then describe recent
modelling efforts, and progress they have made in constraining the potential of
the Milky Way and the local dark matter density.Comment: 5 pages, published in the proceedings of the GREAT-ITN conference on
"The Milky Way Unravelled by Gaia", Barcelona, December 201
Extending the Hyades
We explore the implications of models of the Hyades moving group in which it
has a resonant origin, for regions of the Galaxy beyond the Solar
neighbourhood. We show that while models associated with different resonances
can produce nearly identical substructure in the local velocity distribution,
the velocity distribution away from the Solar neighbourhood has different
properties for different models. In particular there is a variation between
different models of where in Galactocentric radius the observed Hyades signal
in velocity space is strongest, at a given Galactic azimuth. We note, however,
that the uncertainties in currently available data, primarily due to uncertain
distances to stars, hide these signatures rather effectively, meaning we are
not yet able to determine which resonance is the cause of the Hyades.Comment: 11 pages, 9 figures. MNRAS accepte
Analysing surveys of our Galaxy I: basic astrometric data
We consider what is the best way to extract science from large surveys of the
Milky Way galaxy. The diversity of data gathered in these surveys, together
with our position within the Galaxy, imply that science must be extracted by
fitting dynamical models to the data in the space of the observables. Models
based on orbital tori promise to be superior for this task than traditional
types of models, such as N-body models and Schwarzschild models. A formalism
that allows such models to be fitted to data is developed and tested on
pseudodata of varying richness.Comment: 15 pages, 6 figures, MNRAS accepted, changed to reflect final versio
The uncertainty in Galactic parameters
We reanalyse the measurements of parallax, proper motion, and line-of-sight
velocity for 18 masers in high mass star-forming regions presented by Reid et
al. (2009). We use a likelihood analysis to investigate the distance of the Sun
from the Galactic centre, R_0, the rotational speed of the local standard of
rest, v_0, and the peculiar velocity of the Sun, vsol, for various models of
the rotation curve, and models which allow for a typical peculiar motion of the
high mass star-forming regions.
We find that these data are best fit by models with non-standard values for
vsol or a net peculiar motion of the high mass star-forming regions. We argue
that a correction to vsol is much more likely, and that these data support the
conclusion of Binney (2009) that V_sol should be revised upwards from 5.2 km/s
to 11 km/s. We find that the values of R_0 and v_0 that we determine are
heavily dependent on the model we use for the rotation curve, with
model-dependent estimates of R_0 ranging from 6.7 \pm 0.5kpc to 8.9 \pm 0.9kpc,
and those of v_0 ranging from 200 \pm 20 km/s to 279 \pm 33 km/s. We argue that
these data cannot be thought of as implying any particular values of R_0 or
v_0. However, we find that v_0/R_0 is better constrained, lying in the range
29.9-31.6 km/s/kpc for all models but one.Comment: 8 pages. MNRAS accepted. Revised to reflect final versio
Distances and parallax bias in Gaia DR2
We derive Bayesian distances for all stars in the RV sample of Gaia DR2, and
use the statistical method of Schoenrich, Binney & Asplund(2012) to validate
the distances and test the Gaia parallaxes. In contrast to other methods, which
rely on special sources, our method directly tests the distances to all stars
in our sample. We find clear evidence for a near-linear trend of distance bias
f with distance s, proving a parallax offset delta p. On average, we find delta
p = -0.054 mas (parallaxes in Gaia DR2 need to be increased) when accounting
for the parallax uncertainty under-estimate in the Gaia set (delta p = -0.048
mas on the raw parallax errors) with negligible formal error and a systematic
uncertainty of about 0.006 mas. The value is in concordance with results from
asteroseismic measurements, but differs from the much lower bias found on
quasar samples. We further use our method to compile a comprehensive set of
quality cuts in colour, apparent magnitude, and astrometric parameters. Last,
we find that for this sample delta p appears to strongly depend on the parallax
uncertainty sigma_p (when including the additional 0.043 mas) with a
statistical confidence far in excess of 10\sigma and a proportionality factor
close to 1, though the dependence varies somewhat with sigma_p. Correcting for
the sigma_p dependence also resolves otherwise unexplained correlations of the
offset with the number of observation periods n_{vis} and ecliptic latitude.
Every study using Gaia DR2 parallaxes/distances should investigate the
sensitivity of their results on the parallax biases described here and - for
fainter samples - in the DR2 astrometry paper.Comment: 14 pages, 13 figures, accepted in MNRAS. The derived distances, as
well as stellar positions and kinematics are found at
https://zenodo.org/record/255780
Dynamical models of the Galaxy
I discuss the importance of dynamical models for exploiting survey data,
focusing on the advantages of "torus" models. I summarize a number of
applications of these models to the study of the Milky Way, including the
determination of the peculiar Solar velocity and investigation of the Hyades
moving group.Comment: 4 pages, 3 figures, to appear in proceedings of "Assembling the
Puzzle of the Milky Way
Palliative stenting for oesophagogastric cancer: tumour and host factors and prognosis
Objectives: Palliative self-expandable metallic stent (SEMS) insertion is common in patients not suitable for resection of oesophagogastric (OG) cancer. Factors which may determine survival, however, are not clear. The present study examined the relationship between tumour and host factors, including the systemic inflammatory response and survival of patients undergoing palliative SEMS insertion.
Methods: Patients with a diagnosis of OG cancer who were considered suitable for palliative SEMS only without systemic therapy were identified. Patient characteristics including Eastern Cooperative Oncology Group performance status, radiological stage and modified Glasgow Prognostic Score (mGPS: 0—C-reactive protein (CRP) ≤10 mg/L; 1—CRP >10 mg/L; 2—CRP >10 mg/L; albumin <35 g/L) were recorded prospectively. The relationship between such characteristics and 3-month survival was examined.
Results: 203 patients were included in the final analysis. All patients died during follow-up, with median survival from diagnosis 75 days (IQR 47–157). 78% of patients were systemically inflamed (mGPS >1). On multivariate analysis, only poor performance status (HR 1.23, p=0.025), metastatic disease (HR 2.27, p<0.001) and mGPS (HR 1.25, p=0.021) were associated with shorter survival. The combination of performance status and mGPS stratified 3-month survival of patients without metastatic disease from 88% to 20% (p<0.001) and patients with metastases from 43% to 6% (p=0.059). Similar results were observed when analysis was restricted to patients with oesophageal and junctional cancer (M0: 83%–20%, p=0.008; M1: 33%–8%, p=0.082).
Conclusion: Performance status, metastatic disease and mGPS are independent predictors of survival in patients with OG cancer undergoing palliative SEMS insertion. These routinely available markers provide a rational system on which to base decisions regarding prognosis and treatment
Nitrogen-rich transition metal nitrides
The solid state chemistry leading to the synthesis and characterization of metal nitrides with N:M ratios >1 is summarized. Studies of these compounds represent an emerging area of research. Most transition metal nitrides have much lower nitrogen contents, and they often form with non- or sub-stoichiometric compositions. These materials are typically metallic with often superconducting properties, and they provide highly refractory, high hardness materials with many technological applications. The higher metal nitrides should achieve formal oxidation states (OS) attaining those found among corresponding oxides, and they are expected to have useful semiconducting properties. Only a very few examples of such high OS nitrogen-rich compounds are known at present. The main group elements typically form covalently bonded nitride ceramics such as Si3N4, Ge3N4 and Sn3N4, and the early transition metals Zr and Hf produce Zr3N4 and Hf3N4. However, the only main example of a highly nitrided transition metal compound known to date is Ta3N5 that has a formal oxidation state +5 and is a semiconductor with visible light absorption leading to applications as a pigment and in photocatalysis. New synthesis routes are being explored to study the possible formation of other N-rich materials that are predicted to exist by ab initio calculations. There is a useful interplay between theoretical predictions and experimental synthesis studies at ambient and high pressure conditions, as we explore and establish the existence and structure–property relations of these new nitride compounds and polymorphs. Here we review the state of current investigations and indicate possible new directions for further work
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