1,550 research outputs found
Chemical modeling of the L1498 and L1517B prestellar cores: CO and HCO+ depletion
Prestellar cores exhibit a strong chemical differentiation, which is mainly
caused by the freeze-out of molecules onto the grain surfaces. Understanding
this chemical structure is important, because molecular lines are often used as
probes to constrain the core physical properties. Here we present new
observations and analysis of the C18O (1-0) and H13CO+ (1-0) line emission in
the L1498 and L1517B prestellar cores, located in the Taurus-Auriga molecular
complex. We model these observations with a detailed chemistry network coupled
to a radiative transfer code. Our model successfully reproduces the observed
C18O (1-0) emission for a chemical age of a few 10^5 years. On the other hand,
the observed H13CO+ (1-0) is reproduced only if cosmic-ray desorption by
secondary photons is included, and if the grains have grown to a bigger size
than average ISM grains in the core interior. This grain growth is consistent
with the infrared scattered light ("coreshine") detected in these two objects,
and is found to increase the CO abundance in the core interior by about a
factor four. According to our model, CO is depleted by about 2-3 orders of
magnitude in the core center.Comment: Accepted for publication in A&
Specific heat in two-dimensional melting
We report the specific heat around the melting transition(s) of
micrometer-sized superparamagnetic particles confined in two dimensions,
calculated from fluctuations of positions and internal energy, and
corresponding Monte Carlo simulations. Since colloidal systems provide single
particle resolution, they offer the unique possibility to compare the
experimental temperatures of peak position of and symmetry breaking,
respectively. While order parameter correlation functions confirm the
Kosterlitz-Thouless-Halperin-Nelson-Young melting scenario where translational
and orientational order symmetries are broken at different temperatures with an
intermediate so called hexatic phase, we observe a single peak of the specific
heat within the hexatic phase, with excellent agreement between experiment and
simulation. Thus, the peak is not associated with broken symmetries but can be
explained with the total defect density, which correlates with the maximum
increase of isolated dislocations. The absence of a latent heat strongly
supports the continuous character of both transitions
Experimental investigation of the freely cooling granular gas
Using diamagnetically levitated particles we investigate the dynamics of the
freely cooling granular gas. At early times we find good agreement with Haff's
law, where the time scale for particle collisions can be determined from
independent measurements. At late times, clustering of particles occurs. This
can be included in a Haff-like description taking into account the decreasing
number of free particles. With this a good description of the data is possible
over the whole time range.Comment: 4 pages, 5 figure
Education and Freedom of Choice: Evidence from Arranged Marriages in Vietnam
Using household data from Vietnam, we provide evidence on the causal effects of education on freedom of spouse choice. We use war disruptions and spatial indicators of schooling supply as instruments. The point estimates indicate that a year of additional schooling reduces the probability of an arranged marriage by about 14 percentage points for an individual with 8 years of schooling. We also estimate bounds that do not rely on the exact exclusion restrictions (lower bound is 6-7 percentage points). The impact of education is strong for women, but much weaker for men.Arranged Marriage, Education, Schooling, Freedom of choice, Development, Vietnam, Social Interactions
Using Chemistry to Unveil the Kinematics of Starless Cores: Complex Radial Motions in Barnard 68
We present observations of 13CO, C18O, HCO+, H13CO+, DCO+ and N2H+ line
emission towards the Barnard 68 starless core. The line profiles are
interpreted using a chemical network coupled with a radiative transfer code in
order to reconstruct the radial velocity profile of the core. Our observations
and modeling indicate the presence of complex radial motions, with the inward
motions in the outer layers of the core but outward motions in the inner part,
suggesting radial oscillations. The presence of such oscillation would imply
that B68 is relatively old, typically one order of magnitude older than the age
inferred from its chemical evolution and statistical core lifetimes. Our study
demonstrates that chemistry can be used as a tool to constrain the radial
velocity profiles of starless cores.Comment: 12 pages, 3 figures, to appear in the Astrophysical Journal Letter
Non-collinear magnetism in Al-Mn topologically disordered systems
We have performed the first ab-initio calculations of a possible complex
non-collinear magnetic structure in aluminium-rich Al-Mn liquids within the
real-space tight-binding LMTO method. In our previous work we predicted the
existence of large magnetic moments in Al-Mn liquids [A.M. Bratkovsky, A.V.
Smirnov, D. N. Manh, and A. Pasturel, \prb {\bf 52}, 3056 (1995)] which has
been very recently confirmed experimentally. Our present calculations show that
there is a strong tendency for the moments on Mn to have a non-collinear
(random) order retaining their large value of about 3~. The d-electrons
on Mn demonstrate a pronounced non-rigid band behaviour which cannot be
reproduced within a simple Stoner picture. The origin of the magnetism in these
systems is a topological disorder which drives the moments formation and
frustrates their directions in the liquid phase.Comment: 10 pages, RevTex 3.0, 24kb. 3 PS figures available on request from
[email protected] The work has been presented at ERC
``Electronic Structire of Solids'' (Lunteren, The Netherlands, 9-14 September
1995
Frank's constant in the hexatic phase
Using video-microscopy data of a two-dimensional colloidal system the
bond-order correlation function G6 is calculated and used to determine the
temperature-dependence of both the orientational correlation length xi6 in the
isotropic liquid phase and the Frank constant F_A in the hexatic phase. F_A
takes the value 72/pi at the hexatic to isotropic liquid phase transition and
diverges at the hexatic to crystal transition as predicted by the KTHNY-theory.
This is a quantitative test of the mechanism of breaking the orientational
symmetry by disclination unbinding
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