3,389 research outputs found
A Spitzer View of the Young Open Cluster NGC 2264
We have performed mid-IR photometry of the young open cluster NGC 2264 using
the images obtained with the Spitzer Space Telescope IRAC and MIPS instruments
and present a normalized classification scheme of young stellar objects in
various color-color diagrams to make full use of the information from
multicolor photometry. These results are compared with the classification
scheme based on the slope of the spectral energy distribution (SED).
From the spatial distributions of Class I and II stars, we have identified
two subclusterings of Class I objects in the CONE region of Sung et al. The
disked stars in the other star forming region S MON are mostly Class II
objects. These three regions show a distinct difference in the fractional
distribution of SED slopes as well as the mean value of SED slopes. The
fraction of stars with primordial disks is nearly flat between log m = 0.2 --
-0.5, and that of transition disks is very high for solar mass stars. In
addition, we have derived a somewhat higher value of the primordial disk
fraction for NGC 2264 members located below the main pre-main sequence locus
(so-called BMS stars). This result supports the idea that BMS stars are young
stars with nearly edge-on disks. We have also found that the fraction of
primordial disks is very low near the most massive star S Mon and increases
with distance from S Mon.Comment: 38 pages, 21 figures, 5 tables (AJ accepted
Drift and trapping in biased diffusion on disordered lattices
We reexamine the theory of transition from drift to no-drift in biased
diffusion on percolation networks. We argue that for the bias field B equal to
the critical value B_c, the average velocity at large times t decreases to zero
as 1/log(t). For B < B_c, the time required to reach the steady-state velocity
diverges as exp(const/|B_c-B|). We propose an extrapolation form that describes
the behavior of average velocity as a function of time at intermediate time
scales. This form is found to have a very good agreement with the results of
extensive Monte Carlo simulations on a 3-dimensional site-percolation network
and moderate bias.Comment: 4 pages, RevTex, 3 figures, To appear in International Journal of
Modern Physics C, vol.
Frequency dependent effective conductivity of two-dimensional metal-dielectric composites
We analyze a random resistor-inductor-capacitor lattice model of
2-dimensional metal-insulator composites. The results are compared with
Bruggeman's and Landauer's Effective Medium Approximations where a discrepancy
was observed for some frequency zones. Such a discrepancy is mainly caused by
the strong conductivity fluctuations. Indeed, a two-branches distribution is
observed for low frequencies. We show also by increasing the system size that
at the so-called Drude peak vanishes; it increases for vanishing losses.Comment: 7 pages including all figures, accepted in Int. J. Mod. Phys.
A dual modelling of evolving political opinion networks
We present the result of a dual modeling of opinion network. The model
complements the agent-based opinion models by attaching to the social agent
(voters) network a political opinion (party) network having its own intrinsic
mechanisms of evolution. These two sub-networks form a global network which can
be either isolated from or dependent on the external influence. Basically, the
evolution of the agent network includes link adding and deleting, the opinion
changes influenced by social validation, the political climate, the
attractivity of the parties and the interaction between them. The opinion
network is initially composed of numerous nodes representing opinions or
parties which are located on a one dimensional axis according to their
political positions. The mechanism of evolution includes union, splitting,
change of position and of attractivity, taken into account the pairwise node
interaction decaying with node distance in power law. The global evolution ends
in a stable distribution of the social agents over a quasi-stable and
fluctuating stationary number of remaining parties. Empirical study on the
lifetime distribution of numerous parties and vote results is carried out to
verify numerical results
Resonances in low-energy positron-alkali scattering
Close-coupling calculations were performed with up to five target states at energies in the excitation threshold region for positron scattering from Li, Na and K. Resonances were discovered in the L = 0, 1 and 2 channels in the vicinity of the atomic excitation thresholds. The widths of these resonances vary between 0.2 and 130 MeV. Evidence was found for the existence of positron-alkali bound states in all cases
Circumstellar Disks in the Outer Galaxy: the Star-Forming Region NGC 1893
It is still debated whether star formation process depends on environment. In
particular it is yet unclear whether star formation in the outer Galaxy, where
the environmental conditions are, theoretically, less conducive, occurs in the
same way as in the inner Galaxy. We investigate the population of NGC1893, a
young cluster ~3-4 Myr in the outer part of the Galaxy (galactic radius >11
Kpc), to explore the effects of environmental conditions on star forming
regions. We present infrared observations acquired using the IRAC camera
onboard the Spitzer Space Telescope and analyze the color-color diagrams to
establish the membership of stars with excesses. We also merge this information
with that obtained from Chandra ACIS-I observations, to identify the Class III
population. We find that the cluster is very rich, with 242 PMS Classical
T-Tauri stars and 7 Class 0/I stars. We identify 110 Class III candidate
cluster members in the ACIS-I field of view. We estimate a disk fraction for
NGC1893 of about 67%, similar to fractions calculated for nearby star forming
regions of the same age. Although environmental conditions are unfavorable,
star formation can clearly be very successful in the outer Galaxy, allowing
creation of a very rich cluster like NGC1893.Comment: 10 pages,7 figures,4 table
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