314 research outputs found
Spectroscopy by frequency entangled photon pairs
Quantum spectroscopy was performed using the frequency-entangled broadband
photon pairs generated by spontaneous parametric down-conversion. An absorptive
sample was placed in front of the idler photon detector, and the frequency of
signal photons was resolved by a diffraction grating. The absorption spectrum
of the sample was measured by counting the coincidences, and the result is in
agreement with the one measured by a conventional spectrophotometer with a
classical light source.Comment: 11 pages, 5 figures, to be published in Phys. Lett.
Selfgravitating Gas Spheres in a Box and Relativistic Clusters: Relation between Dynamical and Thermodynamical Stability
We derive a variational principle for the dynamical stability of a cluster as
a gas sphere in a box. Newtonian clusters are always dynamically stable and,
for relativistic clusters, the relation between dynamical and thermodynamical
instabilities is analyzed. The boundaries between dynamically and
thermodynamically stable and unstable models are found numerically for
relativistic stellar systems with different cut off parameters. A criterion
based on binding energy curve is used for determination of the boundary of
dynamical stability.Comment: 10 figure
Multi--Pressure Polytropes as Models for the Structure and Stability of Molecular Clouds. I. Theory
Molecular clouds are supported by thermal pressure, magnetic pressure, and
turbulent pressure. Each of these can be modeled with a polytropic equation of
state, so that overall the total pressure is the sum of the individual
components. We model the turbulent pressure as being due to a superposition of
Alfven waves. The theory of polytropes is generalized to allow for the flow of
entropy in response to a perturbation, as expected for the entropy associated
with wave pressure. The equation of state of molecular clouds is "soft", so
that the properties of the clouds are generally governed by the conditions at
the surface. In general, the polytropes are not isentropic, and this permits
large density and pressure drops to occur between the center and the edge of
the polytropes, as is observed.Comment: Submitted to ApJ with 10 figure
ă»ăă·ă§ăłă21ă»ă€ăăăăăăăă„ăŠăă«ăłă±ă€ăăăăă€ă
æ„äžć°ć
±ćç 究ăçŸä»Łäžćœăšæ±ăąăžăąăźæ°ç°ćąă âĄ21äžçŽăźæ„äžéąäż : éćčŽç 究è
ăźæ玹ăšćŻŸ
Anion-Catalyzed Dissolution of NO_2 on Aqueous Microdroplets
Fifty-seven years after NOx (NO + NO_2) were identified as essential components of photochemical smog, atmospheric chemical models fail to correctly predict âąOH/HO_2âą concentrations under NO_x-rich conditions. This deficiency is due, in part, to the uncertain rates and mechanism for the reactive dissolution of NO_2(g) (2NO_2 + H_2O = NO_3^â + H^+ + HONO) in fog and aerosol droplets. Thus, state-of-the-art models parametrize the uptake of NO_2 by atmospheric aerosol from data obtained on âdeactivated tunnel wall residueâ. Here, we report experiments in which NO_3^â production on the surface of microdroplets exposed to NO_2(g) for 1 ms is monitored by online thermospray mass spectrometry. NO_2 does not dissolve in deionized water (NO_3^â signals below the detection limit) but readily produces NO_3^â on aqueous NaX (X = Cl, Br, I) microdroplets with NO_2 uptake coefficients Îł that vary nonmonotonically with electrolyte concentration and peak at Îł_(max) ~ 10^(â4) for [NaX] ~ 1 mM, which is >10^3 larger than that in neat water. Since I^â is partially oxidized to I_2âą^â in this process, anions seem to capture NO2(g) into XâNO_2âą^â radical anions for further reaction at the air/water interface. By showing that Îł is strongly enhanced by electrolytes, these results resolve outstanding discrepancies between previous measurements in neat water versus NaCl-seeded clouds. They also provide a general mechanism for the heterogeneous conversion of NO_2(g) to (NO_3^â + HONO) on the surface of aqueous media
Simultaneously measuring two ultrashort laser pulses on a single-shot using double-blind frequency-resolved optical gating
We demonstrate a simple self-referenced single-shot method for simultaneously measuring two different arbitrary pulses, which can potentially be complex and also have very different wavelengths. The method is a variation of cross-correlation frequency-resolved optical gating (XFROG) that we call double-blind (DB) FROG. It involves measuring two spectrograms, both of which are obtained simultaneously in a single apparatus. DB FROG retrieves both pulses robustly by using the standard XFROG algorithm, implemented alternately on each of the traces, taking one pulse to be ?known? and solving for the other. We show both numerically and experimentally that DB FROG using a polarization-gating beam geometry works reliably and appears to have no nontrivial ambiguities
Mass Extinctions and The Sun's Encounters with Spiral Arms
The terrestrial fossil record shows that the exponential rise in biodiversity
since the Precambrian period has been punctuated by large extinctions, at
intervals of 40 to 140 Myr. These mass extinctions represent extremes over a
background of smaller events and the natural process of species extinction. We
point out that the non-terrestrial phenomena proposed to explain these events,
such as boloidal impacts (a candidate for the end-Cretaceous extinction), and
nearby supernovae, are collectively far more effective during the solar
system's traversal of spiral arms. Using the best available data on the
location and kinematics of the Galactic spiral structure (including distance
scale and kinematic uncertainties), we present evidence that arm crossings
provide a viable explanation for the timing of the large extinctions.Comment: Latex, aaspp4.sty, 8 pages, 2 Postscript figures, to appear in New
Astronomy 3 (1998) 51-5
Habitable Zones in the Universe
Habitability varies dramatically with location and time in the universe. This
was recognized centuries ago, but it was only in the last few decades that
astronomers began to systematize the study of habitability. The introduction of
the concept of the habitable zone was key to progress in this area. The
habitable zone concept was first applied to the space around a star, now called
the Circumstellar Habitable Zone. Recently, other, vastly broader, habitable
zones have been proposed. We review the historical development of the concept
of habitable zones and the present state of the research. We also suggest ways
to make progress on each of the habitable zones and to unify them into a single
concept encompassing the entire universe.Comment: 71 pages, 3 figures, 1 table; to be published in Origins of Life and
Evolution of Biospheres; table slightly revise
Dynamical stability of infinite homogeneous self-gravitating systems: application of the Nyquist method
We complete classical investigations concerning the dynamical stability of an
infinite homogeneous gaseous medium described by the Euler-Poisson system or an
infinite homogeneous stellar system described by the Vlasov-Poisson system
(Jeans problem). To determine the stability of an infinite homogeneous stellar
system with respect to a perturbation of wavenumber k, we apply the Nyquist
method. We first consider the case of single-humped distributions and show
that, for infinite homogeneous systems, the onset of instability is the same in
a stellar system and in the corresponding barotropic gas, contrary to the case
of inhomogeneous systems. We show that this result is true for any symmetric
single-humped velocity distribution, not only for the Maxwellian. If we
specialize on isothermal and polytropic distributions, analytical expressions
for the growth rate, damping rate and pulsation period of the perturbation can
be given. Then, we consider the Vlasov stability of symmetric and asymmetric
double-humped distributions (two-stream stellar systems) and determine the
stability diagrams depending on the degree of asymmetry. We compare these
results with the Euler stability of two self-gravitating gaseous streams.
Finally, we determine the corresponding stability diagrams in the case of
plasmas and compare the results with self-gravitating systems
The Lifetimes and Evolution of Molecular Cloud Cores
We discuss the lifetimes and evolution of clumps and cores formed as
turbulent density fluctuations in nearly isothermal molecular clouds. In the
non-magnetic case, clumps are unlikely to reach a hydrostatic state, and
instead are expected to either proceed directly to collapse, or else
``rebound'' towards the mean pressure and density of the parent cloud.
Rebounding clumps are delayed in their re-expansion by their self-gravity. From
a simple virial calculation, we find re-expansion times of a few free-fall
times. In the magnetic case, we present a series of driven-turbulence,
ideal-MHD isothermal numerical simulations in which we follow the evolution of
clumps and cores in relation to the magnetic criticality of their ``parent
clouds'' (the numerical boxes). In subcritical boxes, magnetostatic clumps do
not form. A few moderately-gravitationally bound clumps form which however are
dispersed by the turbulence in < 1.3 Myr. An estimate of the ambipolar
diffusion (AD) time scale t_AD in these cores gives t_AD > 1.3 Myr, only
slightly longer than the dynamical times. In supercritical boxes, some cores
become locally supercritical and collapse in typical times ~ 1 Myr. We also
observe longer-lived supercritical cores that however do not collapse because
they are smaller than the local Jeans length. Fewer clumps and cores form in
these simulations than in their non-magnetic counterpart. Our results suggest
that a) A fraction of the cores may not form stars, and may correspond to some
of the observed starless cores. b) Cores may be out-of-equilibrium structures,
rather than quasi-magnetostatic ones. c) The magnetic field may help reduce the
star formation efficiency by reducing the probability of core formation, rather
than by significantly delaying the collapse of individual cores.Comment: Accepted in ApJ. Originally submitted as astro-ph/0208245. Completely
rewritten, now including numerical simulations. Animations available at
http://www.astrosmo.unam.mx/~e.vazquez/turbulence_HP/movies/VKSB04.htm
- âŠ