68 research outputs found
Applying Operations Research techniques to planning of train shunting
In this paper, we discuss a model-based algorithmic approach for supporting planners in the creation of shunt plans for passenger trains. The approach provides an example of a mathematical model and a corresponding solution approach for model based support. We introduce a four-step solution approach and we discuss how the planners are supported by this approach. Finally, we present computational results for these steps and give some suggestions for further research.A* search;railway optimization;real world application;routing
Applying Operations Research techniques to planning of train shunting
In this paper, we discuss a model-based algorithmic approach for supporting planners in the creation of shunt plans for passenger trains. The approach provides an example of a mathematical model and a corresponding solution approach for model based support. We introduce a four-step solution approach and we discuss how the planners are supported by this approach. Finally, we present computational results for these steps and give some suggestions for further research
CSS100603:112253-111037: A helium-rich dwarf nova with a 65 minute orbital period
We present time-resolved optical spectroscopy of the dwarf nova
CSS100603:112253-111037. Its optical spectrum is rich in helium, with broad,
double-peaked emission lines produced in an accretion disc. We measure a line
flux ratio HeI5876/H_alpha = 1.49 +/- 0.04, a much higher ratio than is
typically observed in dwarf novae. The orbital period, as derived from the
radial velocity of the line wings, is 65.233 +/- 0.015 minutes. In combination
with the previously measured superhump period, this implies an extreme mass
ratio of M_2/M_1 = 0.017 +/- 0.004. The H_alpha and HeI6678 emission lines
additionally have a narrow central spike, as is often seen in the spectra of AM
CVn type stars. Comparing its properties with CVs, AM CVn systems and hydrogen
binaries below the CV period minimum, we argue that CSS100603:112253-111037 is
the first compelling example of an AM CVn system forming via the evolved CV
channel.
With the addition of this system, evolved cataclysmic variables (CVs) now
account for seven per cent of all known semi-detached white dwarf binaries with
Porb < 76 min. Two recently discovered binaries may further increase this
figure. Although the selection bias of this sample is not yet well defined,
these systems support the evolved CV model as a possible formation channel for
ultracompact accreting binaries. The orbital periods of the three ultracompact
hydrogen accreting binaries overlap with those of the long period AM CVn stars,
but there are currently no known systems in the period range 67 - 76 minutes.Comment: 10 pages, 6 figures. Accepted for publication in MNRA
XMM-Newton observations of the dwarf nova YZ Cnc in quiescence
We present results from the XMM observations of the dwarf nova YZ Cnc in a
quiescent state. We have performed a detailed time series analysis of the
resulting light curves. Unusually, we do not detect any orbital modulation in
the UV, with only marginal evidence for X-ray modulation on this period.
Although there are peaks in the X-ray periodograms at periods less than 5000 s,
we attribute them to red noise effects and assign significance to them using a
novel approach. The variability in the UV and optical bands can also be
modelled as a result of aperiodic variability (red noise) in the system. There
is evidence that the UV and X-ray fluxes are anti-correlated with a time delay
of about 100 s, with the UV lagging behind the X-ray emission. This
anti-correlation is intriguing, but is only present on two occasions lasting
several 1000 s each. The X-ray spectrum shows similar emission features to
other dwarf novae and is well fitted using a multi-temperature emission model.
We measure a relatively high X-ray luminosity of ~1.4x10^{32} ergs/s, although
this is consistent with a low binary inclination. Finally, we find evidence for
a possible -1200km/s blue shift in the fitted Fe K line energies, possibly
indicating the presence of an outflow in this low inclination system.Comment: 9 pages, accepted for publication in A&
Active Disk Building in a local HI-Massive LIRG: The Synergy between Gas, Dust, and Star Formation
HIZOA J0836-43 is the most HI-massive (M_HI = 7.5x10^10 Msun) galaxy detected
in the HIPASS volume and lies optically hidden behind the Milky Way. Markedly
different from other extreme HI disks in the local universe, it is a luminous
infrared galaxy (LIRG) with an actively star forming disk (>50 kpc), central to
its ~ 130 kpc gas disk, with a total star formation rate (SFR) of ~20.5 Msun
yr^{-1}. Spitzer spectroscopy reveals an unusual combination of powerful
polycyclic aromatic hydrocarbon (PAH) emission coupled to a relatively weak
warm dust continuum, suggesting photodissociation region (PDR)-dominated
emission. Compared to a typical LIRG with similar total infrared luminosity
(L_TIR=10^11 Lsun), the PAHs in HIZOA J0836-43 are more than twice as strong,
whereas the warm dust continuum (lambda > 20micron) is best fit by a star
forming galaxy with L_TIR=10^10 Lsun. Mopra CO observations suggest an extended
molecular gas component (H_2 + He > 3.7x10^9 Msun) and a lower limit of ~ 64%
for the gas mass fraction; this is above average compared to local disk
systems, but similar to that of z~1.5 BzK galaxies (~57%). However, the star
formation efficiency (SFE = L_IR/L'_CO) for HIZOA J0836-43 of 140 Lsun (K km
s^{-1} pc^2)^{-1} is similar to that of local spirals and other disk galaxies
at high redshift, in strong contrast to the increased SFE seen in merging and
strongly interacting systems. HIZOA J0836-43 is actively forming stars and
building a massive stellar disk. Its evolutionary phase of star formation
(M_stellar, SFR, gas fraction) compared to more distant systems suggests that
it would be considered typical at redshift z~1. This galaxy provides a rare
opportunity in the nearby universe for studying (at z~0.036) how disks were
building and galaxies evolving at z~1, when similarly large gas fractions were
likely more common.Comment: Accepted for publication in The Astrophysical Journal. 16 pages, 8
figure
The variability plane of accreting compact objects
Recently, it has been shown that soft-state black hole X-ray binaries and
active galactic nuclei populate a plane in the space defined by the black hole
mass, accretion rate and characteristic frequency. We show that this plane can
be extended to hard-state objects if one allows a constant offset for the
frequencies in the soft and the hard state. During a state transition the
frequencies rapidly move from one scaling to the other depending on an
additional parameter, possibly the disk-fraction. The relationship between
frequency, mass and accretion rate can be further extended by including weakly
accreting neutron stars. We explore if the lower kHz QPOs of neutron stars and
the dwarf nova oscillations of white dwarfs can be included as well and discuss
the physical implications of the found correlation.Comment: Accepted for publication in MNRA
an overview of the MHONGOOSE survey: Observing nearby galaxies with MeerKAT
© Copyright owned by the author(s). MHONGOOSE is a deep survey of the neutral hydrogen distribution in a representative sample of 30 nearby disk and dwarf galaxies with H I masses from ⌠106 to ⌠1011 M, and luminosities from MR ⌠12 to MR ⌠â22. The sample is selected to uniformly cover the available range in log(MHI). Our extremely deep observations, down to H I column density limits of well below 1018 cmâ2 â or a few hundred times fainter than the typical H I disks in galaxies â will directly detect the effects of cold accretion from the intergalactic medium and the links with the cosmic web. These observations will be the first ever to probe the very low-column density neutral gas in galaxies at these high resolutions. Combination with data at other wavelengths, most of it already available, will enable accurate modeling of the properties and evolution of the mass components in these galaxies and link these with the effects of environment, dark matter distribution, and other fundamental properties such as halo mass and angular momentum. MHONGOOSE can already start addressing some of the SKA-1 science goals and will provide a comprehensive inventory of the processes driving the transformation and evolution of galaxies in the nearby universe at high resolution and over 5 orders of magnitude in column density. It will be a Nearby Galaxies Legacy Survey that will be unsurpassed until the advent of the SKA, and can serve as a highly visible, lasting statement of MeerKATâs capabilities
The Expanding Fireball of Nova Delphini 2013
A classical nova occurs when material accreting onto the surface of a white
dwarf in a close binary system ignites in a thermonuclear runaway. Complex
structures observed in the ejecta at late stages could result from interactions
with the companion during the common envelope phase. Alternatively, the
explosion could be intrinsically bipolar, resulting from a localized ignition
on the surface of the white dwarf or as a consequence of rotational distortion.
Studying the structure of novae during the earliest phases is challenging
because of the high spatial resolution needed to measure their small sizes.
Here we report near-infrared interferometric measurements of the angular size
of Nova Delphini 2013, starting from one day after the explosion and continuing
with extensive time coverage during the first 43 days. Changes in the apparent
expansion rate can be explained by an explosion model consisting of an
optically thick core surrounded by a diffuse envelope. The optical depth of the
ejected material changes as it expands. We detect an ellipticity in the light
distribution, suggesting a prolate or bipolar structure that develops as early
as the second day. Combining the angular expansion rate with radial velocity
measurements, we derive a geometric distance to the nova of 4.54 +/- 0.59 kpc
from the Sun.Comment: Published in Nature. 32 pages. Final version available at
http://www.nature.com/nature/journal/v515/n7526/full/nature13834.htm
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