110,182 research outputs found
Mass segregation in very young open clusters -- A case study of NGC 2244 and NGC 6530
We derive the proper motions, membership probabilities, and velocity
dispersions of stars in the regions of the young (about 2-4 Myr-old) open
clusters NGC 2244 (the central cluster in the Monoceros R2 association) and NGC
6530 (the dominant cluster in the Sgr OB1 association) from photographic plate
material obtained at Shanghai Astronomical Observatory, with time baselines of
34 and 87 years, respectively. Both clusters show clear evidence of mass
segregation, but they do not exhibit any significant velocity-mass (or,
equivalently, a velocity-luminosity) dependence. This provides strong support
for the suggestion that the observed mass segregation is -- at least partially
-- due to the way in which star formation has proceeded in these complex
star-forming regions (``primordial'' mass segregation). Based on arguments
related to the clusters' published initial mass functions, in conjunction with
our new measurements of their internal velocity dispersions (35 and 8 km/s for
NGC 2244 and NGC 6530, respectively), we provide strong arguments in favor of
the dissolution of NGC 2244 on very short time-scales, while we speculate that
NGC 6530 may be more stable against the effects of internal two-body
relaxation. However, this latter object may well be destroyed by the strong
tidal field prevalent at its location in the Galactic plane in the direction of
the Galactic Center.Comment: 36 pages, 10 figures, accepted to A
Identification of a spatio-temporal model of crystal growth based on boundary curvature
A new method of identifying the spatio-temporal transition rule of crystal growth is introduced based on the connection between growth kinetics and dentritic
morphology. Using a modified three-point-method, curvatures of the considered crystal branch are calculated and curvature direction is used to measure growth
velocity. A polynomial model is then produced based on a curvature-velocity relationship to represent the spatio-temporal growth process. A very simple simulation
example is used initially to clearly explain the methodology. The results of identifying a model from a real crystal growth experiment show that the proposed
method can produce a good representation of crystal growth
Intrinsic electron-doping in nominal "non-doped" superconducting (La,Y)CuO thin films grown by dc magnetron sputtering
The superconducting nominal "non-doped" (LYCO) thin
films are successfully prepared by dc magnetron-sputtering and in situ
post-annealing in vacuum. The best more than 13K is achieved in the
optimal LYCO films with highly pure c-axis oriented T'-type structure. In the
normal state, the quasi-quadratic temperature dependence of resistivity, the
negative Hall coefficient and effect of oxygen content in the films are quite
similar to the typical Ce-doped T'-214 cuprates, suggesting that T'-LYCO shows
the electron-doping nature like known n-type cuprates, and is not a band
superconductor as proposed previously. The charge carriers are considered to be
induced by oxygen deficiency.Comment: 5 pages, 7 figure
Dynamic Willingness to Pay: An Empirical Specification and Test
In a static setting, willingness to pay for an environmental improvement is equal to compensating variation. However, in a dynamic setting characterized by uncertainty, irreversibility, and the potential for learning, willingness to pay may also contain an option value. In this paper, we incorporate the dynamic nature of the value formulation process into a study using a contingent valuation method, designed to measure the value local residents assign to a north-central Iowa lake. Our results show that willingness to pay is highly sensitive to the potential for future learning. Respondents offered the opportunity to delay their purchasing decisions until more information became available were willing to pay significantly less for improved water quality than those who faced a now-or-never decision. The results suggest that welfare analysts should take care to accurately represent the potential for future learning.Clear Lake, contingent valuation, water quality, willingness to pay.
Spatial oscillations in the spontaneous emission rate of an atom inside a metallic wedge
A method of images is applied to study the spontaneous emission of an atom
inside a metallic wedge with an opening angle of , where N is an
arbitrary positive integer. We show the method of images gives a rate formula
consistent with that from Quantum Electrodynamics. Using the method of images,
we show the correspondence between the oscillations in the spontaneous emission
rate and the closed-orbits of emitted photon going away and returning to the
atom inside the wedge. The closed-orbits can be readily constructed using the
method of images and they are also extracted from the spontaneous emission
rate.Comment: 8 figure
Non-LTE analysis of copper abundances for the two distinct halo populations in the solar neighborhood
Two distinct halo populations were found in the solar neighborhood by a
series of works. They can be clearly separated by [alpha\Fe] and several other
elemental abundance ratios including [Cu/Fe]. Very recently, a non-local
thermodynamic equilibrium (non-LTE) study revealed that relatively large
departures exist between LTE and non-LTE results in copper abundance analysis.
We aim to derive the copper abundances for the stars from the sample of Nissen
et al (2010) with both LTE and non-LTE calculations. Based on our results, we
study the non-LTE effects of copper and investigate whether the high-alpha
population can still be distinguished from the low-alpha population in the
non-LTE [Cu/Fe] results. Our differential abundance ratios are derived from the
high-resolution spectra collected from VLT/UVES and NOT/FIES spectrographs.
Applying the MAFAGS opacity sampling atmospheric models and spectrum synthesis
method, we derive the non-LTE copper abundances based on the new atomic model
with current atomic data obtained from both laboratory and theoretical
calculations. The copper abundances determined from non-LTE calculations are
increased by 0.01 to 0.2 dex depending on the stellar parameters compared with
the LTE results. The non-LTE [Cu/Fe] trend is much flatter than the LTE one in
the metallicity range -1.6<[Fe/H]<-0.8. Taking non-LTE effects into
consideration, the high- and low-alpha stars still show distinguishable copper
abundances, which appear even more clear in a diagram of non-LTE [Cu/Fe] versus
[Fe/H]. The non-LTE effects are strong for copper, especially in metal-poor
stars. Our results confirmed that there are two distinct halo populations in
the solar neighborhood. The dichotomy in copper abundance is a peculiar feature
of each population, suggesting that they formed in different environments and
evolved obeying diverse scenarios.Comment: 9 pages, 7 figures, 2 table
Open clusters: their kinematics and metellicities
We review our work on Galactic open clusters in recent years, and introduce
our proposed large program for the LOCS (LAMOST Open Cluster Survey). First,
based on the most complete open clusters sample with metallicity, age and
distance data as well as kinematic information, some preliminary statistical
analysis regarding the spatial and metallicity distributions is presented. In
particular, a radial abundance gradient of - 0.058 0.006 dex kpc
was derived, and by dividing clusters into age groups we show that the disk
abundance gradient was steeper in the past. Secondly, proper motions,
membership probabilities, and velocity dispersions of stars in the regions of
two very young open clusters are derived. Both clusters show clear evidence of
mass segregation, which provides support for the ``primordial'' mass
segregation scenarios. Based on the great advantages of the forthcoming LAMOST
facility, we have proposed a detailed open cluster survey with LAMOST (the
LOCS). The aim, feasibility, and the present development of the LOCS are
briefly summarized.Comment: 7 pages, 4 figures, submitted to Proceeding of IAU Symposium No.248:
"A Giant Step:from Milli- to Micro-arcsecond Astrometry
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