121 research outputs found
The local density of matter mapped by Hipparcos
We determine the velocity distribution and space density of a volume complete
sample of A and F stars, using parallaxes and proper motions from the Hipparcos
satellite. We use these data to solve for the gravitational potential
vertically in the local Galactic disc, by comparing the Hipparcos measured
space density with predictions from various disc models. We derive an estimate
of the local dynamical mass density of 0.102 +/- 0.010 solar masses per cubic
parsec which may be compared to an estimate of 0.095 solar masses per cubic
parsec in visible disc matter. Our estimate is found to be in reasonable
agreement with other estimates by Creze et al. and Pham, also based on
Hipparcos data. We conclude that there is no compelling evidence for
significant amounts of dark matter in the disc.Comment: 9 pages, 7 figures, accepted by MNRA
White dwarfs and Galactic dark matter
We discuss the recent discovery by Oppenheimer et al (2001) of old, cool
white dwarf stars, which may be the first direct detection of Galactic halo
dark matter. We argue here that the contribution of more mundane white dwarfs
of the stellar halo and thick disk would contribute sufficiently to explain the
new high velocity white dwarfs without invoking putative white dwarfs of the
dark halo. This by no means rules out that the dark matter has been found, but
it does constrain the overall contribution by white dwarfs brighter than M_V ~
16 to significantly less than 1% of the Galactic dark matter. This work
confirms a similar study by Reyle et al (2001).Comment: 8 pages, 8 figures. MNRAS style fil
The colours of the Sun
We compile a sample of Sun-like stars with accurate effective temperatures,
metallicities and colours (from the UV to the near-IR). A crucial improvement
is that the effective temperature scale of the stars has recently been
established as both accurate and precise through direct measurement of angular
diameters obtained with stellar interferometers. We fit the colours as a
function of effective temperature and metallicity, and derive colour estimates
for the Sun in the Johnson/Cousins, Tycho, Stromgren, 2MASS and SDSS
photometric systems. For (B-V)_Sun, we favour the ``red'' colour 0.64 versus
the ``blue'' colour 0.62 of other recent papers, but both values are consistent
within the errors; we ascribe the difference to the selection of Sun-like stars
versus interpolation of wider colour-Teff-metallicity relations.Comment: 5 pages, 2 figures, accepted by MNRA
Is it Raining Outside? Detection of Rainfall using General-Purpose Surveillance Cameras
In integrated surveillance systems based on visual cameras, the mitigation of
adverse weather conditions is an active research topic. Within this field, rain
removal algorithms have been developed that artificially remove rain streaks
from images or video. In order to deploy such rain removal algorithms in a
surveillance setting, one must detect if rain is present in the scene. In this
paper, we design a system for the detection of rainfall by the use of
surveillance cameras. We reimplement the former state-of-the-art method for
rain detection and compare it against a modern CNN-based method by utilizing 3D
convolutions. The two methods are evaluated on our new AAU Visual Rain Dataset
(VIRADA) that consists of 215 hours of general-purpose surveillance video from
two traffic crossings. The results show that the proposed 3D CNN outperforms
the previous state-of-the-art method by a large margin on all metrics, for both
of the traffic crossings. Finally, it is shown that the choice of
region-of-interest has a large influence on performance when trying to
generalize the investigated methods. The AAU VIRADA dataset and our
implementation of the two rain detection algorithms are publicly available at
https://bitbucket.org/aauvap/aau-virada.Comment: 10 pages, 7 figures, CVPR2019 V4AS worksho
Probing the Galaxy's bars via the Hercules stream
It has been suggested that a resonance between a rotating bar and stars in
the solar neighbourhood can produce the so called 'Hercules stream'. Recently,
a second bar may have been identified in the Galactic centre, the so called
'long bar', which is longer and much flatter than the traditional Galactic bar,
and has a similar mass. We looked at the dynamical effects of both bars,
separately and together, on orbits of stars integrated backwards from local
position and velocities, and a model of the Galactic potential which includes
the bars directly. Both bars can produce Hercules like features, and allow us
to measure the rotation rate of the bar(s). We measure a pattern speed, for
both bars, of 1.87 +/- 0.02 times the local circular frequency. This is on par
with previous measurements for the Galactic bar, although we do adopt a
slightly different Solar motion. Finally, we identify a new kinematic feature
in local velocity space, caused by the long bar, which is tempting to identify
with the high velocity 'Arcturus' stream.Comment: 9 pages, 10 figures, Accepted to MNRAS, Corrected for errat
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