297 research outputs found
Macroscopic and Local Magnetic Moments in Si-doped CuGeO with Neutron and SR Studies
The temperature-concentration phase diagram of the Si-doped spin-Peierls
compound CuGeO is investigated by means of neutron scattering and muon
spin rotation spectroscopy in order to determine the microscopic distribution
of the magnetic and lattice dimerised regions as a function of doping. The
analysis of the zero-field muon spectra has confirmed the spatial inhomogeneity
of the staggered magnetisation that characterises the antiferromagnetic
superlattice peaks observed with neutrons. In addition, the variation of the
macroscopic order parameter with doping can be understood by considering the
evolution of the local magnetic moment as well as of the various regions
contributing to the muon signal
Turbulent Control of the Star Formation Efficiency
Supersonic turbulence plays a dual role in molecular clouds: On one hand, it
contributes to the global support of the clouds, while on the other it promotes
the formation of small-scale density fluctuations, identifiable with clumps and
cores. Within these, the local Jeans length \Ljc is reduced, and collapse
ensues if \Ljc becomes smaller than the clump size and the magnetic support
is insufficient (i.e., the core is ``magnetically supercritical''); otherwise,
the clumps do not collapse and are expected to re-expand and disperse on a few
free-fall times. This case may correspond to a fraction of the observed
starless cores. The star formation efficiency (SFE, the fraction of the cloud's
mass that ends up in collapsed objects) is smaller than unity because the mass
contained in collapsing clumps is smaller than the total cloud mass. However,
in non-magnetic numerical simulations with realistic Mach numbers and
turbulence driving scales, the SFE is still larger than observational
estimates. The presence of a magnetic field, even if magnetically
supercritical, appears to further reduce the SFE, but by reducing the
probability of core formation rather than by delaying the collapse of
individual cores, as was formerly thought. Precise quantification of these
effects as a function of global cloud parameters is still needed.Comment: Invited review for the conference "IMF@50: the Initial Mass Function
50 Years Later", to be published by Kluwer Academic Publishers, eds. E.
Corbelli, F. Palla, and H. Zinnecke
Statistics of Core Lifetimes in Numerical Simulations of Turbulent, Magnetically Supercritical Molecular Clouds
We present measurements of the mean dense core lifetimes in numerical
simulations of magnetically supercritical, turbulent, isothermal molecular
clouds, in order to compare with observational determinations. "Prestellar"
lifetimes (given as a function of the mean density within the cores, which in
turn is determined by the density threshold n_thr used to define them) are
consistent with observationally reported values, ranging from a few to several
free-fall times. We also present estimates of the fraction of cores in the
"prestellar", "stellar'', and "failed" (those cores that redisperse back into
the environment) stages as a function of n_thr. The number ratios are measured
indirectly in the simulations due to their resolution limitations. Our approach
contains one free parameter, the lifetime of a protostellar object t_yso (Class
0 + Class I stages), which is outside the realm of the simulations. Assuming a
value t_yso = 0.46 Myr, we obtain number ratios of starless to stellar cores
ranging from 4-5 at n_thr = 1.5 x 10^4 cm^-3 to 1 at n_thr = 1.2 x 10^5 cm^-3,
again in good agreement with observational determinations. We also find that
the mass in the failed cores is comparable to that in stellar cores at n_thr =
1.5 x 10^4 cm^-3, but becomes negligible at n_thr = 1.2 x 10^5 cm^-3, in
agreement with recent observational suggestions that at the latter densities
the cores are in general gravitationally dominated. We conclude by noting that
the timescale for core contraction and collapse is virtually the same in the
subcritical, ambipolar diffusion-mediated model of star formation, in the model
of star formation in turbulent supercritical clouds, and in a model
intermediate between the previous two, for currently accepted values of the
clouds' magnetic criticality.Comment: 25 pages, 8 figures, ApJ accepted. Fig.1 animation is at
http://www.astrosmo.unam.mx/~e.vazquez/turbulence/movies/Galvan_etal07/Galvan_etal07.htm
Interstellar Turbulence and Star Formation
We provide a brief overview of recent advances and outstanding issues in
simulations of interstellar turbulence, including isothermal models for
interior structure of molecular clouds and larger-scale multiphase models
designed to simulate the formation of molecular clouds. We show how
self-organization in highly compressible magnetized turbulence in the
multiphase ISM can be exploited in simple numerical models to generate
realistic initial conditions for star formation.Comment: 8 pages, 5 color figures; submitted to Proceedings of IAU Symposium
270 "Computational Star Formation" held in Barcelona, May 31 - June 4, 201
Interstellar MHD Turbulence and Star Formation
This chapter reviews the nature of turbulence in the Galactic interstellar
medium (ISM) and its connections to the star formation (SF) process. The ISM is
turbulent, magnetized, self-gravitating, and is subject to heating and cooling
processes that control its thermodynamic behavior. The turbulence in the warm
and hot ionized components of the ISM appears to be trans- or subsonic, and
thus to behave nearly incompressibly. However, the neutral warm and cold
components are highly compressible, as a consequence of both thermal
instability in the atomic gas and of moderately-to-strongly supersonic motions
in the roughly isothermal cold atomic and molecular components. Within this
context, we discuss: i) the production and statistical distribution of
turbulent density fluctuations in both isothermal and polytropic media; ii) the
nature of the clumps produced by thermal instability, noting that, contrary to
classical ideas, they in general accrete mass from their environment; iii) the
density-magnetic field correlation (or lack thereof) in turbulent density
fluctuations, as a consequence of the superposition of the different wave modes
in the turbulent flow; iv) the evolution of the mass-to-magnetic flux ratio
(MFR) in density fluctuations as they are built up by dynamic compressions; v)
the formation of cold, dense clouds aided by thermal instability; vi) the
expectation that star-forming molecular clouds are likely to be undergoing
global gravitational contraction, rather than being near equilibrium, and vii)
the regulation of the star formation rate (SFR) in such gravitationally
contracting clouds by stellar feedback which, rather than keeping the clouds
from collapsing, evaporates and diperses them while they collapse.Comment: 43 pages. Invited chapter for the book "Magnetic Fields in Diffuse
Media", edited by Elisabete de Gouveia dal Pino and Alex Lazarian. Revised as
per referee's recommendation
Theory of helimagnons in itinerant quantum systems
The nature and effects of the Goldstone mode in the ordered phase of helical
or chiral itinerant magnets such as MnSi are investigated theoretically. It is
shown that the Goldstone mode, or helimagnon, is a propagating mode with a
highly anisotropic dispersion relation, in analogy to the Goldstone mode in
chiral liquid crystals. Starting from a microscopic theory, a comprehensive
effective theory is developed that allows for an explicit description of the
helically ordered phase, including the helimagnons, for both classical and
quantum helimagnets. The directly observable dynamical spin susceptibility,
which reflects the properties of the helimagnon, is calculated.Comment: 20 pp., 1 eps fig; corrects various typos and incorrect prefactors in
Phys Rev B versio
A Simple Perspective on the Mass-Area Relationship in Molecular Clouds
Despite over 30 years of study, the mass-area relationship within and among
clouds is still poorly understood both observationally and theoretically.
Modern extinction datasets should have sufficient resolution and dynamic range
to characterize this relationship for nearby molecular clouds, although recent
papers using extinction data seem to yield different interpretations regarding
the nature and universality of this aspect of cloud structure. In this paper we
try to unify these various results and interpretations by accounting for the
different ways cloud properties are measured and analyzed. We interpret the
mass-area relationship in terms of the column density distribution function and
its possible variation within and among clouds. We quantitatively characterize
regional variations in the column density PDF. We show that structures both
within and among clouds possess the same degree of "universality", in that
their PDF means do not systematically scale with structure size. Because of
this, mass scales linearly with area.Comment: 10 pages, 8 figures, MNRAS in pres
The Local Leo Cold Cloud and New Limits on a Local Hot Bubble
We present a multi-wavelength study of the local Leo cold cloud (LLCC), a
very nearby, very cold cloud in the interstellar medium. Through stellar
absorption studies we find that the LLCC is between 11.3 pc and 24.3 pc away,
making it the closest known cold neutral medium cloud and well within the
boundaries of the local cavity. Observations of the cloud in the 21-cm HI line
reveal that the LLCC is very cold, with temperatures ranging from 15 K to 30 K,
and is best fit with a model composed of two colliding components. The cloud
has associated 100 micron thermal dust emission, pointing to a somewhat low
dust-to-gas ratio of 48 x 10^-22 MJy sr^-1 cm^2. We find that the LLCC is too
far away to be generated by the collision among the nearby complex of local
interstellar clouds, but that the small relative velocities indicate that the
LLCC is somehow related to these clouds. We use the LLCC to conduct a shadowing
experiment in 1/4 keV X-rays, allowing us to differentiate between different
possible origins for the observed soft X-ray background. We find that a local
hot bubble model alone cannot account for the low-latitude soft X-ray
background, but that isotropic emission from solar wind charge exchange does
reproduce our data. In a combined local hot bubble and solar wind charge
exchange scenario, we rule out emission from a local hot bubble with an 1/4 keV
emissivity greater than 1.1 Snowdens / pc at 3 sigma, 4 times lower than
previous estimates. This result dramatically changes our perspective on our
local interstellar medium.Comment: 13 pages, 12 figures. Accepted for publication in the Astrophysical
Journal. Vector figure version available at
http://www.astro.columbia.edu/~jpeek
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