8,241 research outputs found
Observations of interstellar zinc
The International Ultraviolet Explorer observations of interstellar zinc toward 10 stars are examined. It is found that zinc is at most only slightly depleted in the interstellar medium; its abundance may serve as a tracer of the true metallicity in the gas. The local interstellar medium has abundances that apparently are homogeneous to within a factor of two, when integrated over paths of about 500 pc, and this result is important for understanding the history of nucleosynthesis in the solar neighborhood. The intrinsic errors in detecting weak interstellar lines are analyzed and suggestions are made as to how this error limit may be lowered to 5 mA per target observation
DLAs and Galaxy Formation
Damped Lyman-alpha systems (DLAs) are useful probes of star formation and
galaxy formation at high redshift. We study the physical properties of DLAs and
their relationship to Lyman-break galaxies using cosmological hydrodynamic
simulations based on the concordance Lambda cold dark matter model. Fundamental
statistics such as global neutral hydrogen (HI) mass density, HI column density
distribution function, DLA rate-of-incidence and mean halo mass of DLAs are
reproduced reasonably well by the simulations, but with some deviations that
need to be understood better in the future. We discuss the feedback effects by
supernovae and galactic winds on the DLA distribution. We also compute the
[C_II] emission from neutral gas in high-z galaxies, and make predictions for
the future observations by ALMA and SPICA. Agreement and disagreement between
simulations and observations are discussed, as well as the future directions of
our DLA research.Comment: 15 pages, 10 figures. Invited brief review for Modern Physics Letters
A, in pres
A Survey of Metal Lines at High-redshift (I) : SDSS Absorption Line Studies - The Methodology and First Search Results for OVI
We report the results of a systematic search for signatures of metal lines in
quasar spectra of the Sloan Digital Sky Survey (SDSS) Data Release 3(DR3),
focusing on finding intervening absorbers via detection of their OVI doublet.
Here we present the search algorithm, and criteria for distinguishing
candidates from spurious Lyman {} forest lines. In addition, we compare
our findings with simulations of the Lyman {} forest in order to
estimate the detectability of OVI doublets over various redshift intervals. We
have obtained a sample of 1756 OVI doublet candidates with rest-frame
equivalent width > 0.05 \AA{} in 855 AGN spectra (out of 3702 objects with
redshifts in the accessible range for OVI detection). This sample is further
subdivided into 3 groups according to the likelihood of being real and the
potential for follow-up observation of the candidate. The group with the
cleanest and most secure candidates is comprised of 145 candidates. 69 of these
reside at a velocity separation > 5000 km/s from the QSO, and can therefore be
classified tentatively as intervening absorbers. Most of these absorbers have
not been picked up by earlier, automated QSO absorption line detection
algorithms. This sample increases the number of known OVI absorbers at
redshifts beyond z$_{abs} > 2.7 substantially.Comment: 41 pages, 10 figures, 2 tables, accepted by AJ. This is a
substantially altered version, including an appendix with details on the
validity of the search algorithm on one pixel rather than binning. Also note
that M. Pieri was added as autho
Slice Stretching Effects for Maximal Slicing of a Schwarzschild Black Hole
Slice stretching effects such as slice sucking and slice wrapping arise when
foliating the extended Schwarzschild spacetime with maximal slices. For
arbitrary spatial coordinates these effects can be quantified in the context of
boundary conditions where the lapse arises as a linear combination of odd and
even lapse. Favorable boundary conditions are then derived which make the
overall slice stretching occur late in numerical simulations. Allowing the
lapse to become negative, this requirement leads to lapse functions which
approach at late times the odd lapse corresponding to the static Schwarzschild
metric. Demanding in addition that a numerically favorable lapse remains
non-negative, as result the average of odd and even lapse is obtained. At late
times the lapse with zero gradient at the puncture arising for the puncture
evolution is precisely of this form. Finally, analytic arguments are given on
how slice stretching effects can be avoided. Here the excision technique and
the working mechanism of the shift function are studied in detail.Comment: 16 pages, 4 figures, revised version including a study on how slice
stretching can be avoided by using excision and/or shift
Picosecond electrical spectroscopy using monolithic GaAs circuits
This article describes an experimental apparatus for free-space mm-wave transmission measurements (spectroscopy). GaAs nonlinear transmission lines and sampling circuits are used as picosecond pulse generators and detectors, with planar monolithic bowtie antennas with associated substrate lenses used as the radiating and receiving elements. The received pulse is 270 mV amplitude and 2.4 ps rise time. Through Fourier transformation of the received pulse, 30–250 GHz measurements are demonstrated with <=0.3 dB (rms) accuracy
Positivity of Entropy in the Semi-Classical Theory of Black Holes and Radiation
Quantum stress-energy tensors of fields renormalized on a Schwarzschild
background violate the classical energy conditions near the black hole.
Nevertheless, the associated equilibrium thermodynamical entropy by
which such fields augment the usual black hole entropy is found to be positive.
More precisely, the derivative of with respect to radius, at fixed
black hole mass, is found to vanish at the horizon for {\it all} regular
renormalized stress-energy quantum tensors. For the cases of conformal scalar
fields and U(1) gauge fields, the corresponding second derivative is positive,
indicating that has a local minimum there. Explicit calculation
shows that indeed increases monotonically for increasing radius and
is positive. (The same conclusions hold for a massless spin 1/2 field, but the
accuracy of the stress-energy tensor we employ has not been confirmed, in
contrast to the scalar and vector cases). None of these results would hold if
the back-reaction of the radiation on the spacetime geometry were ignored;
consequently, one must regard as arising from both the radiation
fields and their effects on the gravitational field. The back-reaction, no
matter how "small",Comment: 19 pages, RevTe
Analytical and Experimental Evaluation of the Heat Transfer Distribution over the Surfaces of Turbine Vanes
Three airfoil data sets were selected for use in evaluating currently available analytical models for predicting airfoil surface heat transfer distributions in a 2-D flow field. Two additional airfoils, representative of highly loaded, low solidity airfoils currently being designed, were selected for cascade testing at simulated engine conditions. Some 2-D analytical methods were examined and a version of the STAN5 boundary layer code was chosen for modification. The final form of the method utilized a time dependent, transonic inviscid cascade code coupled to a modified version of the STAN5 boundary layer code featuring zero order turbulence modeling. The boundary layer code is structured to accommodate a full spectrum of empirical correlations addressing the coupled influences of pressure gradient, airfoil curvature, and free-stream turbulence on airfoil surface heat transfer distribution and boundary layer transitional behavior. Comparison of pedictions made with the model to the data base indicates a significant improvement in predictive capability
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