269 research outputs found
Kinetic Theory of Transient Condensation and Evaporation at a Plane Surface
The phenomenon of transient condensation onto, or evaporation from, a liquid sheet in contact
with its pure vapor is treated from a kinetic theory viewpoint. The Maxwell moment method is used
to formulate the detailed transient problem. A steady surface mass flux rate exists for times large in
comparison with the collision time, that is, in the continuum regime, and explicit formulas are given
for this limit. The complete gasdynamic field, however, is nonsteady for all times. The calculations are
carried out utilizing four moments, and the effects of incorporating additional moments are negligible.
Finally, the analysis is extended to incorporate imperfect mass and temperature accommodation.
Examination of the transient solution and a matched asymptotic "quasisteady" solution shows that
the gasdynamic field consists of a diffusion process near the liquid surface coupled through an expansion
or compression wave to the constant far field state
The Flux Auto- and Cross-Correlation of the Lyman-alpha Forest. II. Modelling Anisotropies with Cosmological Hydrodynamic Simulations
The isotropy of the Lyman-alpha forest in real-space uniquely provides a
measurement of cosmic geometry at z > 2. The angular diameter distance for
which the correlation function along the line of sight and in the transverse
direction agree corresponds to the correct cosmological model. However, the
Lyman-alpha forest is observed in redshift-space where distortions due to
Hubble expansion, bulk flows, and thermal broadening introduce anisotropy.
Similarly, a spectrograph's line spread function affects the autocorrelation
and cross-correlation differently. In this the second paper of a series on
using the Lyman-alpha forest observed in pairs of QSOs for a new application of
the Alcock-Paczynski (AP) test, these anisotropies and related sources of
potential systematic error are investigated with cosmological hydrodynamic
simulations. Three prescriptions for galactic outflow were compared and found
to have only a marginal effect on the Lyman-alpha flux correlation (which
changed by at most 7% with use of the currently favored variable-momentum wind
model vs. no winds at all). An approximate solution for obtaining the zero-lag
cross-correlation corresponding to arbitrary spectral resolution directly from
the zero-lag cross-correlation computed at full-resolution (good to within 2%
at the scales of interest) is presented. Uncertainty in the observationally
determined mean flux decrement of the Lyman-alpha forest was found to be the
dominant source of systematic error; however, this is reduced significantly
when considering correlation ratios. We describe a simple scheme for
implementing our results, while mitigating systematic errors, in the context of
a future application of the AP test.Comment: 20 page
Irradiation of amorphous Ta42Si13N45 film with a femtosecond laser pulse
Films of 260nm thickness, with atomic composition Ta42Si13N45, on 4″ silicon wafers, have been irradiated in air with single laser pulses of 200 femtoseconds duration and 800nm wave length. As sputter-deposited, the films are structurally amorphous. A laterally truncated Gaussian beam with a near-uniform fluence of ∼0.6J/cm2 incident normally on such a film ablates 23nm of the film. Cross-sectional transmission electron micrographs show that the surface of the remaining film is smooth and flat on a long-range scale, but contains densely distributed sharp nanoprotrusions that sometimes surpass the height of the original surface. Dark field micrographs of the remaining material show no nanograins. Neither does glancing angle X-ray diffraction with a beam illuminating many diffraction spots. By all evidence, the remaining film remains amorphous after the pulsed femtosecond irradiation. The same single pulse, but with an enhanced and slightly peaked fluence profile, creates a spot with flat peripheral terraces whose lateral extents shrink with depth, as scanning electron and atomic force micrographs revealed. Comparison of the various figures suggests that the sharp nanoprotrusions result from an ejection of material by brittle fraction and spallation, not from ablation by direct beam-solid interaction. Conditions under which spallation should dominate over ablation are discusse
The Interaction of Entropy Fluctuations with Turbine Blade Rows; A Mechanism of Turbojet Engine Noise
The theory relating to the interaction of entropy fluctuations ('hot spots'), as well as vorticity and pressure, with blade rows is described. A basic feature of the model is that the blade rows have blades of sufficiently short chord that this is negligible in comparison with the wavelength of the disturbances. For the interaction of entropy with a blade row to be important, it is essential that the steady pressure change across the blade row should be large, although all unsteady perturbations are assumed small. A number of idealized examples have been calculated, beginning with isolated blade rows, progressing to single and then to several turbine stages. Finally, the model has been used to predict the low-frequency rearward-radiated acoustic power from a commercial turbojet engine. Following several assumptions, together with considerable empirical data, the correct trend and level are predicted, suggesting the mechanism to be important at low jet velocities
21-cm synthesis observations of VIRGOHI 21 - a possible dark galaxy in the Virgo Cluster
Many observations indicate that dark matter dominates the extra-galactic
Universe, yet no totally dark structure of galactic proportions has ever been
convincingly identified. Previously we have suggested that VIRGOHI 21, a 21-cm
source we found in the Virgo Cluster using Jodrell Bank, was a possible dark
galaxy because of its broad line-width (~200 km/s) unaccompanied by any visible
gravitational source to account for it. We have now imaged VIRGOHI 21 in the
neutral-hydrogen line and find what could be a dark, edge-on, spinning disk
with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has
unquestionably been involved in an interaction with NGC 4254, a luminous spiral
with an odd one-armed morphology, but lacking the massive interactor normally
linked with such a feature. Numerical models of NGC 4254 call for a close
interaction ~10^8 years ago with a perturber of ~10^11 solar masses. This we
take as additional evidence for the massive nature of VIRGOHI 21 as there does
not appear to be any other viable candidate. We have also used the Hubble Space
Telescope to search for stars associated with the HI and find none down to an I
band surface brightness limit of 31.1 +/- 0.2 mag/sq. arcsec.Comment: 8 pages, accepted to ApJ, uses emulateapj.cls. Mpeg animation (Fig.
2) available at ftp://ftp.naic.edu/pub/publications/minchin/video2.mp
Quasars and their host galaxies
This review attempts to describe developments in the fields of quasar and
quasar host galaxies in the past five. In this time period, the Sloan and 2dF
quasar surveys have added several tens of thousands of quasars, with Sloan
quasars being found to z>6. Obscured, or partially obscured quasars have begun
to be found in significant numbers. Black hole mass estimates for quasars, and
our confidence in them, have improved significantly, allowing a start on
relating quasar properties such as radio jet power to fundamental parameters of
the quasar such as black hole mass and accretion rate. Quasar host galaxy
studies have allowed us to find and characterize the host galaxies of quasars
to z>2. Despite these developments, many questions remain unresolved, in
particular the origin of the close relationship between black hole mass and
galaxy bulge mass/velocity dispersion seen in local galaxies.Comment: Review article, to appear in Astrophysics Update
The Calibration of Monochromatic Far-Infrared Star Formation Rate Indicators
(Abridged) Spitzer data at 24, 70, and 160 micron and ground-based H-alpha
images are analyzed for a sample of 189 nearby star-forming and starburst
galaxies to investigate whether reliable star formation rate (SFR) indicators
can be defined using the monochromatic infrared dust emission centered at 70
and 160 micron. We compare recently published recipes for SFR measures using
combinations of the 24 micron and observed H-alpha luminosities with those
using 24 micron luminosity alone. From these comparisons, we derive a reference
SFR indicator for use in our analysis. Linear correlations between SFR and the
70 and 160 micron luminosity are found for L(70)>=1.4x10^{42} erg/s and
L(160)>=2x10^{42} erg/s, corresponding to SFR>=0.1-0.3 M_sun/yr. Below those
two luminosity limits, the relation between SFR and 70 micron (160 micron)
luminosity is non-linear and SFR calibrations become problematic. The
dispersion of the data around the mean trend increases for increasing
wavelength, becoming about 25% (factor ~2) larger at 70 (160) micron than at 24
micron. The increasing dispersion is likely an effect of the increasing
contribution to the infrared emission of dust heated by stellar populations not
associated with the current star formation. The non-linear relation between SFR
and the 70 and 160 micron emission at faint galaxy luminosities suggests that
the increasing transparency of the interstellar medium, decreasing effective
dust temperature, and decreasing filling factor of star forming regions across
the galaxy become important factors for decreasing luminosity. The SFR
calibrations are provided for galaxies with oxygen abundance 12+Log(O/H)>8.1.
At lower metallicity the infrared luminosity no longer reliably traces the SFR
because galaxies are less dusty and more transparent.Comment: 69 pages, 19 figures, 2 tables; accepted for publication on Ap
The Flux Auto- and Cross-Correlation of the Lyman-alpha Forest. I. Spectroscopy of QSO Pairs with Arcminute Separations and Similar Redshifts
The Lyman-alpha forest has opened a new redshift regime for cosmological
investigation. At z > 2 it provides a unique probe of cosmic geometry and an
independent constraint on dark energy that is not subject to standard candle or
ruler assumptions. In Paper I of this series on using the Lyman-alpha forest
observed in pairs of QSOs for a new application of the Alcock-Paczynski test,
we present and discuss the results of a campaign to obtain moderate-resolution
spectroscopy (FWHM ~ 2.5 Angstroms) of the Lyman-alpha forest in pairs of QSOs
with small redshift differences (Delta z 2.2) and arcminute
separations (< 5'). This data set, composed of seven individual QSOs, 35 pairs,
and one triplet, is also well-suited for future investigations of the coherence
of Lyman-alpha absorbers on ~ 1 Mpc transverse scales and the transverse
proximity effect. We note seven revisions for previously published QSO
identifications and/or redshifts.Comment: 20 page
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