39 research outputs found
Observational Constraints on the Ages of Molecular Clouds and the Star-Formation Timescale: Ambipolar-Diffusion--Controlled or Turbulence-Induced Star Formation?
We revisit the problem of the star formation timescale and the ages of
molecular clouds. The apparent overabundance of star-forming molecular clouds
over clouds without active star formation has been thought to indicate that
molecular clouds are "short-lived" and that star formation is "rapid". We show
that this statistical argument lacks self-consistency and, even within the
rapid star-formation scenario, implies cloud lifetimes of approximately 10 Myr.
We discuss additional observational evidence from external galaxies that
indicate lifetimes of molecular clouds and a timescale of star formation of
approximately 10 Myr . These long cloud lifetimes in conjunction with the rapid
(approximately 1 Myr) decay of supersonic turbulence present severe
difficulties for the scenario of turbulence-controlled star formation. By
contrast, we show that all 31 existing observations of objects for which the
linewidth, the size, and the magnetic field strength have been reliably
measured are in excellent quantitative agreement with the predictions of the
ambipolar-diffusion theory. Within the ambipolar-diffusion-controlled star
formation theory the linewidths may be attributed to large-scale non-radial
cloud oscillations (essentially standing large-amplitude, long-wavelength
Alfven waves), and the predicted relation between the linewidth, the size, and
the magnetic field is a natural consequence of magnetic support of
self-gravitating clouds.Comment: 7 pages, 2 figures, uses emulateapj; accepted for publication in Ap
Formation of Interstellar Clouds: Parker Instability with Phase Transitions
We follow numerically the nonlinear evolution of the Parker instability in
the presence of phase transitions from a warm to a cold HI interstellar medium
in two spatial dimensions. The nonlinear evolution of the system favors modes
that allow the magnetic field lines to cross the galactic plane. Cold HI clouds
form with typical masses ~= 10^5 M_sun, mean densities ~= 20 cm^-3, mean
magnetic field strengths ~= 4.3 muG (rms field strengths ~= 6.4 muG),
mass-to-flux ratios ~= 0.1 - 0.3 relative to critical, temperatures ~= 50 K,
(two-dimensional) turbulent velocity dispersions ~= 1.6 km s^-1, and
separations ~= 500 pc, in agreement with observations. The maximum density and
magnetic field strength are ~= 10^3 cm^-3 and ~= 20 muG, respectively.
Approximately 60% of all HI mass is in the warm neutral medium. The cold
neutral medium is arranged into sheet-like structures both perpendicular and
parallel to the galactic plane, but it is also found almost everywhere in the
galactic plane, with the density being highest in valleys of the magnetic field
lines. `Cloudlets' also form whose physical properties are in quantitative
agreement with those observed for such objects by Heiles (1967). The nonlinear
phase of the evolution takes ~< 30 Myr, so that, if the instability is
triggered by a nonlinear perturbation such as a spiral density shock wave,
interstellar clouds can form within a time suggested by observations.Comment: 11 pages, 7 figures, accepted for publication in MNRAS;
high-resolution color figures may be found at
https://netfiles.uiuc.edu/mkunz/MKC08
Polarimetric variations of binary stars. IV. Pre-main-sequence spectroscopic binaries located in Taurus, Auriga, and Orion
We present polarimetric observations of 14 pre-main-sequence (PMS) binaries
located in the Taurus, Auriga, and Orion star forming regions. The majority of
the average observed polarizations are below 0.5%, and none are above 0.9%.
After removal of estimates of the interstellar polarization, about half the
binaries have an intrinsic polarization above 0.5%, even though most of them do
not present other evidences for the presence of circumstellar dust. Various
tests reveal that 77% of the PMS binaries have or possibly have a variable
polarization. LkCa3, Par1540, and Par2494 present detectable periodic and
phase-locked variations. The periodic polarimetric variations are noisier and
of a lesser amplitude (~0.1%) than for other types of binaries, such as hot
stars. This could be due to stochastic events that produce deviations in the
average polarization, a non-favorable geometry (circumbinary envelope), or the
nature of the scatterers (dust grains are less efficient polarizers than
electrons). Par1540 is a Weak-line TTauri Star, but nonetheless has enough dust
in its environment to produce detectable levels of polarization and variations.
A fourth interesting case is W134, which displays rapid changes in polarization
that could be due to eclipses. We compare the observations with some of our
numerical simulations, and also show that an analysis of the periodic
polarimetric variations with the Brown, McLean, & Emslie (BME) formalism to
find the orbital inclination is for the moment premature: non-periodic events
introduce stochastic noise that partially masks the periodic low-amplitude
variations and prevents the BME formalism from finding a reasonable estimate of
the orbital inclination.Comment: 70 pages, 20 figures, to be published in the Astronomical Journa
The Hall-Petch effect as a manifestation of the general size effect
The experimental evidence for the Hall-Petch dependence of strength on the
inverse square-root of grain size is reviewed critically. Both the classic data
and more recent results are considered. While the data can be fitted to the
inverse square-root dependence excellently (but using two free fitting
parameters for each dataset), it is also consistent with a dependence on the
simple inverse of grain size (with one free fitting parameter for each
dataset). There have been difficulties, recognised for half-a-century, in
explaining the inverse square-root expression. A Bayesian analysis shows that
the data strongly supports the simple inverse expression proposed. Since this
expression derives from underlying theory, it is also more readily explicable.
It is concluded that the Hall-Petch effect is not to be explained by the
variety of theories found in the literature, but is a manifestation of, or
underlain by, the general size effect observed throughout micromechanics, due
to the inverse relationship between the stress required and the space available
for dislocation sources to operate.Comment: Paper presented at Plasticity 2014, The Bahama
Radio emission from Supernova Remnants
The explosion of a supernova releases almost instantaneously about 10^51 ergs
of mechanic energy, changing irreversibly the physical and chemical properties
of large regions in the galaxies. The stellar ejecta, the nebula resulting from
the powerful shock waves, and sometimes a compact stellar remnant, constitute a
supernova remnant (SNR). They can radiate their energy across the whole
electromagnetic spectrum, but the great majority are radio sources. Almost 70
years after the first detection of radio emission coming from a SNR, great
progress has been achieved in the comprehension of their physical
characteristics and evolution. We review the present knowledge of different
aspects of radio remnants, focusing on sources of the Milky Way and the
Magellanic Clouds, where the SNRs can be spatially resolved. We present a brief
overview of theoretical background, analyze morphology and polarization
properties, and review and critical discuss different methods applied to
determine the radio spectrum and distances. The consequences of the interaction
between the SNR shocks and the surrounding medium are examined, including the
question of whether SNRs can trigger the formation of new stars. Cases of
multispectral comparison are presented. A section is devoted to reviewing
recent results of radio SNRs in the Magellanic Clouds, with particular emphasis
on the radio properties of SN 1987A, an ideal laboratory to investigate
dynamical evolution of an SNR in near real time. The review concludes with a
summary of issues on radio SNRs that deserve further study, and analyzing the
prospects for future research with the latest generation radio telescopes.Comment: Revised version. 48 pages, 15 figure
Origin of Galactic and Extragalactic Magnetic Fields
A variety of observations suggest that magnetic fields are present in all
galaxies and galaxy clusters. These fields are characterized by a modest
strength (10^{-7}-10^{-5} G) and huge spatial scale (~Mpc). It is generally
assumed that magnetic fields in spiral galaxies arise from the combined action
of differential rotation and helical turbulence, a process known as the
alpha-omega dynamo. However fundamental questions concerning the nature of the
dynamo as well as the origin of the seed fields necessary to prime it remain
unclear. Moreover, the standard alpha-omega dynamo does not explain the
existence of magnetic fields in elliptical galaxies and clusters. The author
summarizes what is known observationally about magnetic fields in galaxies,
clusters, superclusters, and beyond. He then reviews the standard dynamo
paradigm, the challenges that have been leveled against it, and several
alternative scenarios. He concludes with a discussion of astrophysical and
early Universe candidates for seed fields.Comment: 67 pages, 17 figures, accepted for publication in Reviews of Modern
Physic
Role of the lesion scar in the response to damage and repair of the central nervous system
Traumatic damage to the central nervous system (CNS) destroys the blood-brain barrier (BBB) and provokes the invasion of hematogenous cells into the neural tissue. Invading leukocytes, macrophages and lymphocytes secrete various cytokines that induce an inflammatory reaction in the injured CNS and result in local neural degeneration, formation of a cystic cavity and activation of glial cells around the lesion site. As a consequence of these processes, two types of scarring tissue are formed in the lesion site. One is a glial scar that consists in reactive astrocytes, reactive microglia and glial precursor cells. The other is a fibrotic scar formed by fibroblasts, which have invaded the lesion site from adjacent meningeal and perivascular cells. At the interface, the reactive astrocytes and the fibroblasts interact to form an organized tissue, the glia limitans. The astrocytic reaction has a protective role by reconstituting the BBB, preventing neuronal degeneration and limiting the spread of damage. While much attention has been paid to the inhibitory effects of the astrocytic component of the scars on axon regeneration, this review will cover a number of recent studies in which manipulations of the fibroblastic component of the scar by reagents, such as blockers of collagen synthesis have been found to be beneficial for axon regeneration. To what extent these changes in the fibroblasts act via subsequent downstream actions on the astrocytes remains for future investigation