1,685 research outputs found
Effects of magnetic fields on the cosmic-ray ionization of molecular cloud cores
Low-energy cosmic rays are the dominant source of ionization for molecular
cloud cores. The ionization fraction, in turn, controls the coupling of the
magnetic field to the gas and hence the dynamical evolution of the cores. The
purpose of this work is to compute the attenuation of the cosmic-ray flux rate
in a cloud core taking into account magnetic focusing, magnetic mirroring, and
all relevant energy loss processes. We adopt a standard cloud model
characterized by a mass-to-flux ratio supercritical by a factor of about 2 to
describe the density and magnetic field distribution of a low-mass starless
core, and we follow the propagation of cosmic rays through the core along flux
tubes enclosing different amount of mass. We then extend our analysis to cores
with different mass-to-flux ratios. We find that mirroring always dominates
over focusing, implying a reduction of the cosmic-ray ionization rate by a
factor of about 2-3 over most of a solar-mass core with respect to the value in
the intercloud medium outside the core. For flux tubes enclosing larger masses
the reduction factor is smaller, since the field becomes increasingly uniform
at larger radii and lower densities. We also find that the cosmic-ray
ionization rate is further reduced in clouds with stronger magnetic field, e.g.
by a factor of about 4 for a marginally critical cloud. The magnetic field
threading molecular cloud cores affects the penetration of low-energy cosmic
rays and reduces the ionization rate by a factor 3-4 depending on the position
inside the core and the magnetization of the core.Comment: 7 pages, 7 figures, to be published in Astronomy and Astrophysic
Polytropic models of filamentary interstellar clouds - I. Structure and stability
The properties of filamentary interstellar clouds observed at sub-millimetre
wavelengths, especially by the Herschel Space Observatory, are analysed with
polytropic models in cylindrical symmetry. The observed radial density profiles
are well reproduced by negative-index cylindrical polytropes with polytropic
exponent (polytropic index
), indicating either external heating or
non-thermal pressure components. However, the former possibility requires
unrealistically high gas temperatures at the filament's surface and is
therefore very unlikely. Non-thermal support, perhaps resulting from a
superposition of small-amplitude Alfv\'en waves (corresponding to ), is a more realistic possibility, at least for the most massive
filaments. If the velocity dispersion scales as the square root of the density
(or column density) on the filament's axis, as suggested by observations, then
polytropic models are characterised by a uniform width. The mass per unit
length of pressure-bounded cylindrical polytropes depends on the conditions at
the boundary and is not limited as in the isothermal case. However, polytropic
filaments can remain stable to radial collapse for values of the
axis-to-surface density contrast as large as the values observed only if they
are supported by a non-isentropic pressure component.Comment: 9 pages, 4 figures. Accepted for publication in MNRA
Thermodynamics and Chemistry of the Early Universe
The interplay between chemistry and thermodynamics determines the final
outcome of the process of gravitational collapse and sets the conditions for
the formation of the first cosmological objects, including primordial
supermassive black holes. In this chapter, we will review the main chemical
reactions and the most important heating/cooling processes taking place in a
gas of primordial composition, including the effects of local and cosmological
radiation backgrounds.Comment: Preprint of the Chapter "Thermodynamics and Chemistry of the Early
Universe", to be published in the review volume "Formation of the First Black
Holes", Latif M. and Schleicher D.R.G., eds., World Scientific Publishing
Company, 2018, pp. [see
http://www.worldscientific.com/worldscibooks/10.1142/10652
Synchrotron emission in molecular cloud cores: the SKA view
Understanding the role of magnetic fields in star-forming regions is of
fundamental importance. In the near future, the exceptional sensitivity of SKA
will offer a unique opportunity to evaluate the magnetic field strength in
molecular clouds and cloud cores through synchrotron emission observations. The
most recent Voyager 1 data, together with Galactic synchrotron emission and
Alpha Magnetic Spectrometer data, constrain the flux of interstellar cosmic-ray
electrons between MeV and GeV, in particular in the
energy range relevant for synchrotron emission in molecular cloud cores at SKA
frequencies. Synchrotron radiation is entirely due to primary cosmic-ray
electrons, the relativistic flux of secondary leptons being completely
negligible. We explore the capability of SKA in detecting synchrotron emission
in two starless molecular cloud cores in the southern hemisphere, B68 and FeSt
1-457, and we find that it will be possible to reach signal-to-noise ratios of
the order of at the lowest frequencies observable by SKA ( MHz)
with one hour of integration.Comment: 5 pages, 4 figures, accepted by Astronomy & Astrophysic
Active stabilization of a Michelson interferometer at an arbitrary phase with sub-nm resolution
We report on the active stabilization of a Michelson interferometer at an
arbitrary phase angle with a precision better than one degree at nm, which corresponds to an optical path difference between the two arms
of less than 1 nm. The stabilization method is ditherless and the error signal
is computed from the spatial shift of the interference pattern of a reference
laser, measured in real-time with a CCD array detector. We discuss the
usefulness of this method for nanopositioning, optical interferometry and
quantum optical experiments
- …