47,683 research outputs found
Early phases of star formation: testing chemical tools
The star forming processes strongly influence the ISM chemistry. Nowadays,
there are available many high-quality databases at millimeter wavelengths.
Using them, it is possible to carry out studies that review and deepen previous
results. If these studies involve large samples of sources, it is preferred to
use direct tools to study the molecular gas. With the aim of testing these
tools such as the use of the HCN/HNC ratio as a thermometer, and the use of
HCO, HCN, NH, and CH as "chemical clocks",
we present a molecular line study towards 55 sources representing massive young
stellar objects (MYSOs) at different evolutive stages: infrared dark clouds
(IRDCs), high-mass protostellar objects (HMPOs), hot molecular cores (HMCs) and
ultracompact HII regions (UCHII). We found that the use of HCN/HNC ratio as an
universal thermometer in the ISM should be taken with care because the HCN
optical depth is a big issue that can affect the method. Hence, this tool
should be used only after a careful analysis of the HCN spectrum, checking that
no line, neither the main nor the hyperfine ones, present absorption features.
We point out that the analysis of the emission of HCO, HCN,
NH, and CH could be useful to trace and distinguish regions
among IRDCs, HMPOs and HMCs. The molecular line widths of these four species
increase from the IRDC to the HMC stage, which can be a consequence of the gas
dynamics related to the star-forming processes taking place in the molecular
clumps. Our results do not only contribute with more statistics regarding to
probe such chemical tools, useful to obtain information in large samples of
sources, but also complement previous works through the analysis on other types
of sources.Comment: accepted to be published in Research in Astronomy & Astrophysics,
October 13, 202
Degenerate Fermi Gas of Sr
We report quantum degeneracy in a gas of ultra-cold fermionic Sr
atoms. By evaporatively cooling a mixture of spin states in an optical dipole
trap for 10.5\,s, we obtain samples well into the degenerate regime with
. The main signature of degeneracy is a change in the
momentum distribution as measured by time-of-flight imaging, and we also
observe a decrease in evaporation efficiency below .Comment: 4 pages, 3 figure
Bose-Einstein Condensation of Sr Through Sympathetic Cooling with Sr
We report Bose-Einstein condensation of Sr, which has a small,
negative s-wave scattering length (\,). We overcome the poor
evaporative cooling characteristics of this isotope by sympathetic cooling with
Sr atoms. Sr is effective in this role in spite of the fact that
it is a fermion because of the large ground state degeneracy arising from a
nuclear spin of , which reduces the impact of Pauli blocking of
collisions. We observe a limited number of atoms in the condensate
() that is consistent with the value of and the
optical dipole trap parameters.Comment: 4 pages, 4 figure
Geometric Hamilton-Jacobi Theory for Nonholonomic Dynamical Systems
The geometric formulation of Hamilton--Jacobi theory for systems with
nonholonomic constraints is developed, following the ideas of the authors in
previous papers. The relation between the solutions of the Hamilton--Jacobi
problem with the symplectic structure defined from the Lagrangian function and
the constraints is studied. The concept of complete solutions and their
relationship with constants of motion, are also studied in detail. Local
expressions using quasivelocities are provided. As an example, the nonholonomic
free particle is considered.Comment: 22 p
Experimental study of optimal measurements for quantum state tomography
Quantum tomography is a critically important tool to evaluate quantum
hardware, making it essential to develop optimized measurement strategies that
are both accurate and efficient. We compare a variety of strategies using
nearly pure test states. Those that are informationally complete for all states
are found to be accurate and reliable even in the presence of errors in the
measurements themselves, while those designed to be complete only for pure
states are far more efficient but highly sensitive to such errors. Our results
highlight the unavoidable tradeoffs inherent to quantum tomography.Comment: 5 pages, 3 figure
Electronic phase separation in the rare earth manganates, (La1-xLnx)0.7Ca0.3MnO3 (Ln = Nd, Gd and Y)
All the three series of manganates showsaturation magnetization
characteristic of ferromagnetism, with the ferromagnetic Tc decreasing with
increasing in x up to a critical value of x, xc (xc = 0.6, 0.3, 0.2
respectively for Nd, Gd, Y). For x > xc, the magnetic moments are considerably
smaller showing a small increase around TM, the value of TM decreasing slightly
with increase in x or decrease in . The ferromagnetic compositions (x xc)
show insulator-metal (IM) transitions, while the compositions with x > xc are
insulating. The magnetic and electrical resistivity behavior of these
manganates is consistent with the occurrence of phase separation in the
compositions around xc, corresponding to a critical average radius of the
A-site cation, , of 1.18 A. Both Tc and TIM increase linearly when < rA
> > or x xc as expected of a homogenous ferromagnetic phase. Both Tc
and TM decrease linearly with the A-site cation size disorder at the A-site as
measured by the variance s2. Thus, an increase in s2 favors the insulating AFM
state. Percolative conduction is observed in the compositions with > <
rAc >. Electron transport properties in the insulating regime for x > xc
conforms to the variable range hopping mechanism. More interestingly, when x >
xc, the real part of dielectric constant (e') reaches a high value (104-106) at
ordinary temperatures dropping to a very small (~500) value below a certain
temperature, the value of which decreases with decreasing frequency.Comment: 27 pages; 11 figures, Submitted to J.Phys:Condens Matte
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