3,640 research outputs found
Are there MACHOs in the Milky Way halo?
Microlensing searches aim to detect compact halo dark matter via its
gravitational lensing effect on stars within the Large Magellanic Cloud. The
most recent results have led to the claim that roughly one fifth of the
galactic halo dark matter may be in the form of compact, solar-mass objects. We
analyze this hypothesis by considering the goodness-of-fit of the best-fit halo
dark matter solutions to the observational data. We show that the distribution
of the durations of the observed microlensing events is narrower than that
expected to result from a standard halo lens population at 90 to 95%
confidence. Only when the fraction of expected background (non-halo) events is
significantly increased does the discrepancy between the observed and
theoretical event duration distributions disappear. This indicates that it is
possible that most of the lenses responsible for the observed microlensing
events are not located within the Milky Way halo.Comment: 5 pages, 2 figures, minor changes to discussion and additional
references, version to appear in Astronomy and Astrophysic
A process chain for integrating microfluidic interconnection elements by micro- overmoulding of thermoplastic elastomers
This paper presents a process chain for in-line integration of microfluidic
interconnection elements by a variant of micro-injection moulding (mu IM). A
SEBS-based thermoplastic elastomer (TPE) was moulded over polymethylmethacrylate
(PMMA) to produce a hybrid microfluidic structure with an aspect ratio of 2. The
process chain implemented micro-milling for fabricating micro-structured tool
inserts, and mu IM and micro-overmoulding was used for replication. A two-plate
mould was used for moulding the substrate, whilst a three-plate mould with a
replaceable insert was used for TPE overmoulding. The presented application was
an interconnect system for a microfluidic device, which enabled direct fitting
of standard tubes into microfluidic substrates. A leakage test showed that the
interconnection was leak-proof within a range of flow rates between 0.32 and
0.62 ml min(-1)
High-energy Astrophysics and the Virtual Observatory
The Virtual Observatory (VO) will revolutionise the way we do Astronomy by
allowing easy access to all astronomical data and by making the handling and
analysis of datasets at various locations across the globe much simpler and
faster. I report here on the need for the VO and its status in Europe,
concentrating on the recently started EURO-VO project, and then give two
specific applications of VO tools to high-energy astrophysics.Comment: 12 pages, 3 figures, invited talk at the Workshop ``Multifrequency
Behaviour of High Energy Cosmic Sources'', Vulcano, Italy, May 2005, F.
Giovannelli et al., in pres
Creating movable interfaces by micro-powder injection moulding
This paper presents a novel in situ technique to produce articulated components with high-precision, micro-scale movable interfaces by micro-powder injection moulding (μPIM). The presented process route is based on the use of micro-scale sacrificial layer between the movable subcomponents which is eliminated during the debinding step, creating a dimensionally-controlled, micro-scale mobile interface. The fabrication technique combines the advantages of micro-powder overmoulding, catalytic debinding and sintering. The demonstrated example was a finger bone prosthesis joint consisting of two sub-components with an interface between components of 200 μm in size. The geometries of the sub-components were designed such that they are inseparable throughout the process whilst allowing them to move relative to each other after the debinding stage. The components produced showed the feasibility of the process route to produce readily-assembled meso-, and potentially micro-, scale articulated system
AX J0049.4-7323 - a close look at a neutron star interacting with a circumstellar disk
Detailed evidence on the system AX J0049.4-7323 is presented here to show how
the passage of the neutron star in the binary system disrupts the circumstellar
disk of the mass donor Be star. A similar effect is noted in three other
Be/X-ray binary systems. Together the observational data should provide
valuable tools for modelling these complex interactions.Comment: 4 pages, accepted for publication in MNRA
Properties of RR Lyrae stars in the inner regions of the Large Magellanic Cloud. II. The extended sample
All galaxies that have been adequately examined so far have shown an extended
stellar halo. To search for such a halo in the LMC we have obtained
low-resolution spectra for 100 LMC RR Lyrae stars, of which 87 are in the field
and 13 in the clusters NGC1835 and NGC2019. We measured radial velocities for
87 LMC RR Lyrae stars, and metallicities for 78 RR Lyrae stars, nearly tripling
the previous sample. These targets are located in 10 fields covering a wide
range of distances, out to 2.5 degrees from the center of the LMC. Our main
result is that the mean velocity dispersion for the LMC RR Lyrae stars is
50+-2km/s. This quantity does not appear to vary with distance from the LMC
center. The metallicity shows a Gaussian distribution, with mean
[Fe/H]=-1.53+-0.02dex, and dispersion 0.20 dex in the Harris metallicity scale,
confirming that they represent a very homogeneous metal-poor population. There
is no dependence between the kinematics and metallicity of the field RR Lyrae
star population. Using good quality low-resolution spectra from FORS1, FORS2
and GEMINI-GMOS we have found that field RR Lyrae stars in the LMC show a large
velocity dispersion and that this indicate the presence of old and metal-poor
stellar halo. All the evidence so far for the halo, however, is from the
spectroscopy of the inner LMC regions, similar to the inner flattened halo in
our Galaxy. Further study is necessary to confirm this important result.Comment: 12 pages, 4 figure
Flatness optimization of micro-injection moulded parts: The case of a PMMA microfluidic component
Micro-injection moulding (µ-IM) has attracted a lot of interest because of its potential for the production of low-cost, miniaturized parts in high-volume. Applications of this technology are, amongst others, microfluidic components for lab-on-a-chip devices and micro-optical components. In both cases, the control of the part flatness is a key aspect to maintaining the component's functionality. The objective of this work is to determine the factors affecting the flatness of a polymer part manufactured by µ-IM and to control the manufacturing process with the aim of minimizing the in-process part deformation. As a case study, a PMMA microfluidic substrate with overall dimensions of 10 mm diameter and 1 mm thickness was investigated by designing a µ-IM experiment having flatness as the experimental response. The part flatness was measured using a micro-coordinate measuring machine. Finite elements analysis was also carried out to study the optimal ejection pin configuration. The results of this work show that the control of the µ-IM process conditions can improve the flatness of the polymer part up to about 15 µm. Part flatness as low as 4 µm can be achieved by modifying the design of the ejection system according to suggested guideline
The OGLE View of Microlensing towards the Magellanic Clouds. II. OGLE-II SMC data
The primary goal of this paper is to provide the evidence that can either
prove or falsify the hypothesis that dark matter in the Galactic halo can clump
into stellar-mass compact objects. If such objects existed, they would act as
lenses to external sources in the Magellanic Clouds, giving rise to an
observable effect of microlensing. We present the results of our search for
such events, based on the data from the second phase of the OGLE survey
(1996-2000) towards the SMC. The data set we used is comprised of 2.1 million
monitored sources distributed over an area of 2.4 square degrees. We found only
one microlensing event candidate, however its poor quality light curve limited
our discussion on the exact distance to the lensing object.
Given a single event, taking the blending (crowding of stars) into account
for the detection efficiency simulations, and deriving the HST-corrected number
of monitored stars, the microlensing optical depth is tau=(1.55+-1.55)10e-7.
This result is consistent with the expected SMC self-lensing signal, with no
need of introducing dark matter microlenses. Rejecting the unconvincing event
leads to the upper limit on the fraction of dark matter in the form of MACHOs
to f<20 per cent for deflectors' masses around 0.4 Msun and f<11 per cent for
masses between 0.003 and 0.2 Msun (95 per cent confidence limit). Our result
indicates that the Milky Way's dark matter is unlikely to be clumpy and form
compact objects in the sub-solar-mass range.Comment: Accepted for publication in MNRAS. Data in electronic form are
available on the OGLE's website: http://ogle.astrouw.edu.pl
Theory and practice of microlensing lightcurves around fold singularities
Among all galactic microlensing events, those involving a passage of the
observed source star over the caustic created by a binary lens are particularly
useful in providing information about stellar atmospheres, the dynamics of
stellar populations in our own and neighbouring galaxies, and the statistical
properties of stellar and sub-stellar binaries. This paper presents a
comprehensive guide for modelling and interpreting the lightcurves obtained in
events involving fold-caustic crossings. A new general, consistent, and optimal
choice of parameters provides a deep understanding of the involved features,
avoids numerical difficulties and minimizes correlations between model
parameters. While the photometric data of a microlensing event around a caustic
crossing itself do not provide constraints on the characteristics of the
underlying binary lens and does not allow predictions of the behaviour of other
regions of the lightcurve, vital constraints can be obtained in an efficient
way if these are combined with a few simple characteristics of data outside the
caustic crossings. A corresponding algorithm containing some improvements over
an earlier approach which takes into account multi-site observations is
presented and discussed in detail together with the arising parameter
constraints paying special attention to the role of source and background
fluxes.Comment: 19 pages with 7 EPS figures embedded, LaTeX2e using mn2e.cls. Final
version, tables clarifying meaning and constraints on parameters added. This
is a preprint of an Article accepted for publication in Monthly Notices of
the Royal Astronomical Society, (C) 2004 The Royal Astronomical Societ
Finding outlier light-curves in catalogs of periodic variable stars
We present a methodology to discover outliers in catalogs of periodic
light-curves. We use cross-correlation as measure of ``similarity'' between two
individual light-curves and then classify light-curves with lowest average
``similarity'' as outliers. We performed the analysis on catalogs of variable
stars of known type from the MACHO and OGLE projects and established that our
method correctly identifies light-curves that do not belong to those catalogs
as outliers. We show how our method can scale to large datasets that will be
available in the near future such as those anticipated from Pan-STARRS and
LSST.Comment: 16 pages, 24 figure
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