26,293 research outputs found
Spokes cluster: The search for the quiescent gas
Context. Understanding the role of fragmentation is one of the most important
current questions of star formation. To better understand the process of star
and cluster formation, we need to study in detail the physical structure and
properties of the parental molecular cloud. The Spokes cluster, or NGC 2264 D,
is a rich protostellar cluster where previous N2H+(1-0) observations of its
dense cores presented linewidths consistent with supersonic turbulence.
However, the fragmentation of the most massive of these cores appears to have a
scale length consistent with that of the thermal Jeans length, suggesting that
turbulence was not dominant. Aims. These two results probe different density
regimes. Our aim is to determine if there is subsonic or less-turbulent gas
(than previously reported) in the Spokes cluster at higher densities. Methods.
We present APEX N2H+(3-2) and N2D+(3-2) observations of the NGC2264-D region to
measure the linewidths and the deuteration fraction of the higher density gas.
The critical densities of the selected transitions are more than an order of
magnitude higher than that of N2H+(1-0). Results. We find that the N2H+(3-2)
and N2D+(3-2) emission present significantly narrower linewidths than the
emission from N2H+(1-0) for most cores. In two of the spectra, the nonthermal
component is close (within 1-sigma) to the sound speed. In addition, we find
that the three spatially segregated cores, for which no protostar had been
confirmed show the highest levels of deuteration. Conclusions. These results
show that the higher density gas, probed with N2H+ and N2D+(3-2), reveals more
quiescent gas in the Spokes cluster than previously reported. More high-angular
resolution interferometric observations using high-density tracers are needed
to truly assess the kinematics and substructure within NGC2264-D. (Abridged)Comment: 8 pages, 4 figures. Accepted in A&
Fold-Saddle Bifurcation in Non-Smooth Vector Fields on the Plane
This paper presents results concerning bifurcations of 2D piecewise-smooth
dynamical systems governed by vector fields. Generic three parameter families
of a class of Non-Smooth Vector Fields are studied and its bifurcation diagrams
are exhibited. Our main result describes the unfolding of the so called
Fold-Saddle singularity
Mapping prior information onto LMI eigenvalue-regions for discrete-time subspace identification
In subspace identification, prior information can be used to constrain the
eigenvalues of the estimated state-space model by defining corresponding LMI
regions. In this paper, first we argue on what kind of practical information
can be extracted from historical data or step-response experiments to possibly
improve the dynamical properties of the corresponding model and, also, on how
to mitigate the effect of the uncertainty on such information. For instance,
prior knowledge regarding the overshoot, the period between damped oscillations
and settling time may be useful to constraint the possible locations of the
eigenvalues of the discrete-time model. Then, we show how to map the prior
information onto LMI regions and, when the obtaining regions are non-convex, to
obtain convex approximations.Comment: Under revie
Growth patterns and scaling laws governing AIDS epidemic in Brazilian cities
Brazil holds approximately 1/3 of population living infected with AIDS
(acquired immunodeficiency syndrome) in Central and South Americas, and it was
also the first developing country to implement a large-scale control and
intervention program against AIDS epidemic. In this scenario, we investigate
the temporal evolution and current status of the AIDS epidemic in Brazil.
Specifically, we analyze records of annual absolute frequency of cases for more
than 5000 cities for the first 33 years of the infection in Brazil. We found
that (i) the annual absolute frequencies exhibit a logistic-type growth with an
exponential regime in the first few years of the AIDS spreading; (ii) the
actual reproduction number decaying as a power law; (iii) the distribution of
the annual absolute frequencies among cities decays with a power law behavior;
(iv) the annual absolute frequencies and the number of inhabitants have an
allometric relationship; (v) the temporal evolution of the annual absolute
frequencies have different profile depending on the average annual absolute
frequencies in the cities. These findings yield a general quantitative
description of the AIDS infection dynamics in Brazil since the beginning. They
also provide clues about the effectiveness of treatment and control programs
against the infection, that has had a different impact depending on the number
of inhabitants of cities. In this framework, our results give insights into the
overall dynamics of AIDS epidemic, which may contribute to select empirically
accurate models.Comment: 12 pages, 6 figure
Upper limit on mh in the MSSM and M-SUGRA vs. prospective reach of LEP
The upper limit on the lightest CP-even Higgs boson mass, mh, is analyzed
within the MSSM as a function of tan(beta) for fixed mtop and Msusy. The impact
of recent diagrammatic two-loop results on this limit is investigated. We
compare the MSSM theoretical upper bound on mh with the lower bound obtained
from experimental searches at LEP. We estimate that with the LEP data taken
until the end of 1999, the region mh < 108.2 GeV can be excluded at the 95%
confidence level. This corresponds to an excluded region 0.6 <= tan(beta) <=
1.9 within the MSSM for mtop = 174.3 GeV and Msusy <= 1 TeV. The final
exclusion sensitivity after the end of LEP, in the year 2000, is also briefly
discussed. Finally, we determine the upper limit on mh within the Minimal
Supergravity (M-SUGRA) scenario up to the two-loop level, consistent with
radiative electroweak symmetry breaking. We find an upper bound of mh \approx
127 GeV for mtop = 174.3 GeV in this scenario, which is slightly below the
bound in the unconstrained MSSM.Comment: 10 pages, 3 figure
A dense micro-cluster of Class 0 protostars in NGC 2264 D-MM1
We present sensitive and high angular resolution (~1") 1.3 mm continuum
observations of the dusty core D-MM1 in the Spokes cluster in NGC 2264 using
the Submillimeter Array. A dense micro-cluster of seven Class 0 sources was
detected in a 20" x 20" region with masses between 0.4 to 1.2 solar masses and
deconvolved sizes of about 600 AU. We interpret the 1.3 mm emission as arising
from the envelopes of the Class 0 protostellar sources. The mean separation of
the 11 known sources (SMA Class 0 and previously known infrared sources) within
D-MM1 is considerably smaller than the characteristic spacing between sources
in the larger Spokes cluster and is consistent with hierarchical thermal
fragmentation of the dense molecular gas in this region.Comment: Accepted for publication in the Astrophysical Journal Letter
- …