480 research outputs found
On the Fractal Distribution of HII Regions in Disk Galaxies
In this work we quantify the degree to which star-forming events are clumped.
We apply a precise and accurate technique to calculate the correlation
dimension Dc of the distribution of HII regions in a sample of disk galaxies.
Our reliable results are distributed in the range 1.5<Dc<2.0. We get
significant variations in the fractal dimension among galaxies, contrary to a
universal picture sometimes claimed in literature. The faintest galaxies tend
to distribute their HII regions in more clustered (less uniform) patterns.
Moreover, the fractal dimension for the brightest HII regions within the same
galaxy seems to be smaller than for the faintest ones suggesting some kind of
evolutionary effect.Comment: 6 pages including 4 figures. To appear in Highlights of Spanish
Astrophysics V: Proceedings of the VIII Scientific Meeting of the Spanish
Astronomical Society held in Santander, July 7-11, 200
A catalogue of open cluster radii determined from Gaia proper motions
In this work we improve a previously published method to calculate in a
reliable way the radius of an open cluster. The method is based on the
behaviour of stars in the proper motion space as the sampling changes in the
position space. Here we describe the new version of the method and show its
performance and robustness. Additionally, we apply it to a large number of open
clusters using data from Gaia DR2 to generate a catalogue of 401 clusters with
reliable radius estimations. The range of obtained apparent radii goes from
Rc=1.4+-0.1 arcmin (for the cluster FSR 1651) to Rc=25.5+-3.5 arcmin (for
NGC~2437). Cluster linear sizes follow very closely a lognormal distribution
with a mean characteristic radius of Rc=3.7 pc, and its high radius tail can be
fitted by a power law as N \propto Rc^(-3.11+-0.35). Additionally, we find that
number of members, cluster radius and age follow the relationship Nc \propto
Rc^(1.2+-0.1) Tc^(-1.9+-0.4) where the younger and more extensive the cluster,
the more members it presents. The proposed method is not sensitive to low
density or irregular spatial distributions of stars and, therefore, is a good
alternative or complementary procedure to calculate open cluster radii not
having previous information on star memberships.Comment: 13 pages including 9 figures and 2 tables (main table will be
available online). Accepted for publication in MNRA
NGC 2548: clumpy spatial and kinematic structure in an intermediate-age galactic cluster
NGC 2548 is a 400-500 Myr old open cluster with evidence of spatial
substructures likely caused by its interaction with the Galactic disk. In this
work we use precise astrometric data from the Carte du Ciel - San Fernando
(CdC-SF) catalogue to study the clumpy structure in this cluster. We confirm
the fragmented structure of NGC 2548 but, additionally, the relatively high
precision of our kinematic data lead us to the first detection of substructures
in the proper motion space of a stellar cluster. There are three spatially
separated cores each of which has its own counterpart in the proper motion
distribution. The two main cores lie nearly parallel to the Galactic plane
whereas the third one is significantly fainter than the others and it moves
toward the Galactic plane separating from the rest of the cluster. We derive
core positions and proper motions, as well as the stars belonging to each core.Comment: 9 pages, 8 figures, 4 tables. Accepted for publication in MNRA
A method for determining the radius of an open cluster from stellar proper motions
We propose a method for calculating the radius of an open cluster in an
objective way from an astrometric catalogue containing, at least, positions and
proper motions. It uses the minimum spanning tree (hereinafter MST) in the
proper motion space to discriminate cluster stars from field stars and it
quantifies the strength of the cluster-field separation by means of a
statistical parameter defined for the first time in this paper. This is done
for a range of different sampling radii from where the cluster radius is
obtained as the size at which the best cluster-field separation is achieved.
The novelty of this strategy is that the cluster radius is obtained
independently of how its stars are spatially distributed. We test the
reliability and robustness of the method with both simulated and real data from
a well-studied open cluster (NGC 188), and apply it to UCAC4 data for five
other open clusters with different catalogued radius values. NGC 188, NGC 1647,
NGC 6603 and Ruprecht 155 yielded unambiguous radius values of 15.2+/-1.8,
29.4+/-3.4, 4.2+/-1.7 and 7.0+/-0.3 arcmin, respectively. ASCC 19 and Collinder
471 showed more than one possible solution but it is not possible to know
whether this is due to the involved uncertainties or to the presence of complex
patterns in their proper motion distributions, something that could be inherent
to the physical object or due to the way in which the catalogue was sampled.Comment: 12 pages including 14 figures and 1 table. Accepted for publication
in MNRA
Nozzle for a needle-free injection system
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, June 2005.Includes bibliographical references (leaf 33).The purpose of this thesis was to develop an optimized nozzle for the needle-free injection device currently under construction in MIT's Bio-Instrumentation Laboratory. Initial predictions from ANSYS, a finite element modeling program, indicated that the injection performance could be noticeably improved with a new nozzle design. After running several flow simulations, a final nozzle design was selected, and a strategy was developed to manufacture the new nozzle. The new nozzle was placed in the injection device and measurements of the jet velocity were recorded via a high speed camera. A 2mm long nozzle with a contoured profile consisting of a linear segment tangent to an arc segment at the nozzle exit produced an exit velocity of 45.5m/s at the end of the injection stroke. This showed almost a 19 percent increase in velocity compared to the older nozzle which produced 38. lm/s upon termination of the injection cycle. However, the results of the new nozzle vary from injection to injection. Thus there is a need for continued testing in the future, and possibly more refined measuring techniques such as depth of penetration into the gel or developing improvements with the current video setup.by Gabriel Nestor Sanchez.S.B
The Method of Multiple Scales: Asymptotic Solutions and Normal Forms for Nonlinear Oscillatory Problems
AbstractThe method of multiple scales is implemented in Maple V Release 2 to generate a uniform asymptotic solutionO(ϵr) for a weakly nonlinear oscillator.In recent work, it has been shown that the method of multiple scales also transforms the differential equations into normal form, so the given algorithm can be used to simplify the equations describing the dynamics of a system near a fixed point.These results are equivalent to those obtained with the traditional method of normal forms which uses a near-identity coordinate transformation to get the system into the “simplest” form.A few Duffing type oscillators are analysed to illustrate the power of the procedure. The algorithm can be modified to take care of systems of ODEs, PDEs and other nonlinear cases
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