7,221 research outputs found
The origin of the Gaia phase-plane spiral
A simple model is presented of the formation of the spiral the (z,v_z) phase
plane of solar-neighbourhood stars that was recently discovered in Gaia data.
The key is that the frequency Omega_z at which stars oscillate vertically
depends on angular momentum about the z axis in addition to the amplitude of
the star's vertical oscillations. Spirals should form in both and
whenever a massive substructure, such as the Sgr dwarf galaxy, passes through
the Galactic plane. The model yields similar spirals to those observed in both
and . The primary driver is the component of the tidal force that
lies in the plane. We investigate the longevity of the spirals and the mass of
the substructure, but the approximations inherent in the model make
quantitative results unreliable. The work relies heavily on a self-consistent,
multi-component model of our Galaxy produced by the AGAMA package for f(J)
modelling.Comment: 6 pages submitted to MNRA
Migration and kinematics in growing disc galaxies with thin and thick discs
We analyse disc heating and radial migration in N-body models of growing disc
galaxies with thick and thin discs. Similar to thin-disc-only models, galaxies
with appropriate non-axisymmetric structures reproduce observational
constraints on radial disc heating in and migration to the Solar Neighbourhood
(Snhd). The presence of thick discs can suppress non-axisymmetries and thus
higher baryonic-to-dark matter fractions are required than in models that only
have a thin disc. Models that are baryon-dominated to roughly the Solar radius
R_0 are favoured, in agreement with data for the Milky Way. For inside-out
growing discs, today's thick-disc stars at R_0 are dominated by outwards
migrators. Whether outwards migrators are vertically hotter than non-migrators
depends on the radial gradient of the thick disc vertical velocity dispersion.
There is an effective upper boundary in angular momentum that thick disc stars
born in the centre of a galaxy can reach by migration, which explains the
fading of the high-alpha sequence outside R_0. Our models compare well to Snhd
kinematics from RAVE-TGAS. For such comparisons it is important to take into
account the azimuthal variation of kinematics at R ~ R_0 and biases from survey
selection functions. The vertical heating of thin disc stars by giant molecular
clouds is only mildly affected by the presence of thick discs. Our models
predict higher vertical velocity dispersions for the oldest stars than found in
the Snhd age-velocity dispersion relation, possibly because of measurement
uncertainties or an underestimation of the number of old cold stars in our
models.Comment: accepted for publication in MNRAS, 22 pages, 11 figures, 1 Table,
Appendi
The detection and treatment of distance errors in kinematic analyses of stars
We present a new method for detecting and correcting systematic errors in the
distances to stars when both proper motions and line-of-sight velocities are
available. The method, which is applicable for samples of 200 or more stars
that have a significant extension on the sky, exploits correlations between the
measured U, V and W velocity components that are introduced by distance errors.
We deliver a formalism to describe and interpret the specific imprints of
distance errors including spurious velocity correlations and shifts of mean
motion in a sample. We take into account correlations introduced by measurement
errors, Galactic rotation and changes in the orientation of the velocity
ellipsoid with position in the Galaxy. Tests on pseudodata show that the method
is more robust and sensitive than traditional approaches to this problem. We
investigate approaches to characterising the probability distribution of
distance errors, in addition to the mean distance error, which is the main
theme of the paper. Stars with the most overestimated distances bias our
estimate of the overall distance scale, leading to the corrected distances
being slightly too small. We give a formula that can be used to correct for
this effect. We apply the method to samples of stars from the SEGUE survey,
exploring optimal gravity cuts, sample contamination, and correcting the used
distance relations.Comment: published in MNRAS 14 pages, 8 figures, 2 tables, corrected eq.(35),
minor editin
Age velocity dispersion relations and heating histories in disc galaxies
We analyse the heating of stellar discs by non axisymmetric structures and
giant molecular clouds (GMCs) in N-body simulations of growing disc galaxies.
The analysis resolves long-standing discrepancies between models and data by
demonstrating the importance of distinguishing between measured age-velocity
dispersion relations (AVRs) and the heating histories of the stars that make up
the AVR. We fit both AVRs and heating histories with formulae proportional to
t^beta and determine the exponents beta_R and beta_z derived from in-plane and
vertical AVRs and ~beta_R and ~beta_z from heating histories. Values of beta_z
are in almost all simulations larger than values of ~beta_z, whereas values of
beta_R are similar to or mildly larger than values of ~beta_R. Moreover, values
of beta_z (~beta_z) are generally larger than values of beta_R (~beta_R). The
dominant cause of these relations is the decline over the life of the disc in
importance of GMCs as heating agents relative to spiral structure and the bar.
We examine how age errors and biases in solar neighbourhood surveys influence
the measured AVR: they tend to decrease beta values by smearing out ages and
thus measured dispersions. We compare AVRs and velocity ellipsoid shapes
sigma_z/sigma_R from simulations to Solar neighbourhood data. We conclude that
for the expected disc mass and dark halo structure, combined GMC and spiral/bar
heating can explain the AVR of the Galactic thin disc. Strong departures of the
disc mass or the dark halo structure from expectation spoil fits to the data.Comment: Accepted for publication in MNRAS, 19 pages, 8 figures, 1 tabl
Overwrite fabrication and tuning of long period gratings
The central wavelengths of the resonance bands are critical aspect of the performance of long period gratings (LPGs) as sensors, particularly for devices designed to operate near the phase matching turning point (PMTP), where the sensitivity to measurements can vary rapidly. Generally, LPGs are characterized by their period, but the amplitude of the amplitude of the index modulation is also an important factor in determining the wavelengths of the resonance bands. Variations in fabrication between LPG sensors can increase or decrease the sensitivity of the LPG to strain, temperature or surrounding refractive index. Here, the technique of overwritten UV laser fabrication is demonstrated. It is shown that, on repeated overwriting, the resonance bands of an LPG exhibit significant wavelength shift, which can be monitored and which can be used to tune the resonance bands to the desired wavelengths. This technique is applied to periods in the range 100 to 200 ”m, showing the cycle-to-cycle evolution of the resonance bands near the PMTPs of a number of cladding modes. The use of online monitoring is shown to reduce the resonance band sensor-to-sensor central wavelength variation from 10 nm to 3 nm
Star Formation in Disk Galaxies. III. Does stellar feedback result in cloud death?
Stellar feedback, star formation and gravitational interactions are major
controlling forces in the evolution of Giant Molecular Clouds (GMCs). To
explore their relative roles, we examine the properties and evolution of GMCs
forming in an isolated galactic disk simulation that includes both localised
thermal feedback and photoelectric heating. The results are compared with the
three previous simulations in this series which consists of a model with no
star formation, star formation but no form of feedback and star formation with
photoelectric heating in a set with steadily increasing physical effects. We
find that the addition of localised thermal feedback greatly suppresses star
formation but does not destroy the surrounding GMC, giving cloud properties
closely resembling the run in which no stellar physics is included. The
outflows from the feedback reduce the mass of the cloud but do not destroy it,
allowing the cloud to survive its stellar children. This suggests that weak
thermal feedback such as the lower bound expected for supernova may play a
relatively minor role in the galactic structure of quiescent Milky Way-type
galaxies, compared to gravitational interactions and disk shear.Comment: 15 pages, 15 figures, accepted for publication in Ap
Forecasting Intraday Time Series with Multiple Seasonal Cycles Using Parsimonious Seasonal Exponential Smoothing
This paper concerns the forecasting of seasonal intraday time series. An extension of Holt-Winters exponential smoothing has been proposed that smoothes an intraday cycle and an intraweek cycle. A recently proposed exponential smoothing method involves smoothing a different intraday cycle for each distinct type of day of the week. Similar days are allocated identical intraday cycles. A limitation is that the method allows only whole days to be treated as identical. We introduce an exponential smoothing formulation that allows parts of different days of the week to be treated as identical. The result is a method that involves the smoothing and initialisation of fewer terms than the other two exponential smoothing methods. We evaluate forecasting up to a day ahead using two empirical studies. For electricity load data, the new method compares well with a range of alternatives. The second study involves a series of arrivals at a call centre that is open for a shorter duration at the weekends than on weekdays. By contrast with the previously proposed exponential smoothing methods, our new method can model in a straightforward way this situation, where the number of periods on each day of the week is not the same.Exponential smoothing; Intraday data; Electricity load; Call centre arrivals.
A solution to the slow stabilisation of surface pressure sensors based on the Wilhelmy method
Dynamic measurement of surface pressure is of particular interest in the field of Langmuir
monolayers, where the change in surface pressure throughout an experiment can provide information
on the properties of the monolayer forming material, or on the reaction kinetics of
the monolayerâs interaction with other materials. One of the most common methods for the
measurement of dynamic surface pressure is the Wilhelmy plate method. This method measures
changes in the forces acting upon a thin plate of material at the air-water interface; this
measurement is then converted to surface pressure. One version of this method, which uses filter
paper plates at the air-water interface, is particularly popular due to their relatively low cost.
However, it has been seen that the use of filter paper plates attached to a Wilhelmy balance requires
an initial stabilisation period lasting several hours, during which the readings drift from
the original baseline. Here the cause of this drift is explored, considering how changes in the
weight of the plate over time influence the assumptions on which the surface pressure is derived
from the measurements made by the Wilhelmy balance. A simple method for preventing
this drift through pre-soaking of the filter paper plates is presented
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