30,003 research outputs found
Turbulent molecular clouds
Stars form within molecular clouds but our understanding of this fundamental
process remains hampered by the complexity of the physics that drives their
evolution. We review our observational and theoretical knowledge of molecular
clouds trying to confront the two approaches wherever possible. After a broad
presentation of the cold interstellar medium and molecular clouds, we emphasize
the dynamical processes with special focus to turbulence and its impact on
cloud evolution. We then review our knowledge of the velocity, density and
magnetic fields. We end by openings towards new chemistry models and the links
between molecular cloud structure and star--formation rates.Comment: To be published in AARv, 58 pages, 13 figures (higher resolution
figures will be available on line
Primordial molecular clouds
It is now well known that a primordial chemistry, involving light elements
produced during the nucleosynthesis period, might develop during the hydrogen
post-recombination era. In particular, molecular ions and primordial molecules
such as H2, HD and LiH will be produced. We summarize this primordial chemistry
after the recombination epoch, and then present a simple gravitational collapse
model of a cloud. The potentiality of fragmentation of this collapsing
protoclouds through the thermal instability is also discussed. We suggest that
this study could also be extended to the CO molecule, because the carbon
reservoir molecule CO has already been observed in high redshifts objects.Comment: 12 pages, 1 figures, Invited Talk at 3K Cosmology Conference-Roma
October 9
Molecular Clouds as Cosmic Ray Laboratories
We will here discuss how the gamma-ray emission from molecular clouds can be
used to probe the cosmic ray flux in distant regions of the Galaxy and to
constrain the highly unknown cosmic ray diffusion coefficient. In particular we
will discuss the GeV to TeV emission from runaway cosmic rays penetrating
molecular clouds close to young and old supernova remnants and in molecular
clouds illuminated by the background cosmic ray flux.Comment: to appear on Proceedings of 25th Texas Symposium on Relativistic
Astrophysic
The Formation and Destruction of Molecular Clouds and Galactic Star Formation
We describe an overall picture of galactic-scale star formation. Recent
high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with
cooling/heating and thermal conduction have shown that the formation of
molecular clouds requires multiple episodes of supersonic compression. This
finding enables us to create a scenario in which molecular clouds form in
interacting shells or bubbles on a galactic scale. First we estimate the
ensemble-averaged growth rate of molecular clouds over a timescale larger than
a million years. Next we perform radiation hydrodynamics simulations to
evaluate the destruction rate of magnetized molecular clouds by the stellar FUV
radiation. We also investigate the resultant star formation efficiency within a
cloud which amounts to a low value (a few percent) if we adopt the power-law
exponent -2.5 for the mass distribution of stars in the cloud. We finally
describe the time evolution of the mass function of molecular clouds over a
long timescale (>1Myr) and discuss the steady state exponent of the power-law
slope in various environments.Comment: 7 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysic
Supernovae in Molecular Clouds
Supernovae are expected to occur near the molecular material in which the
massive progenitor star was born, except in cases where the photoionizing
radiation and winds from the progenitor star and its neighbors have cleared out
a region. The clumpy structure in molecular clouds is crucial for the remnant
evolution; the supernova shock front can become radiative in the interclump
medium and the radiative shell then collides with molecular clumps. The
interaction is relevant to a number of phenomena: the hydrodynamics of a
magnetically supported dense shell interacting with molecular clumps; the
molecular emission from shock waves, including the production of the OH 1720
MHz maser line; the relativistic particle emission, including radio synchrotron
and gamma-ray emission, from the dense radiative shell; and the possible
gravitational instability of a compressed clump.Comment: 10 pages, 2 figures, review for proceedings of the Maryland
conference on Young Supernova Remnant
Physical Properties and Galactic Distribution of Molecular Clouds identified in the Galactic Ring Survey
We derive the physical properties of 580 molecular clouds based on their 12CO
and 13CO line emission detected in the University of Massachusetts-Stony Brook
(UMSB) and Galactic Ring surveys. We provide a range of values of the physical
properties of molecular clouds, and find a power-law correlation between their
radii and masses, suggesting that the fractal dimension of the ISM is around
2.36. This relation, M = (228 +/- 18) R^{2.36+/-0.04}, allows us to derive
masses for an additional 170 GRS molecular clouds not covered by the UMSB
survey. We derive the Galactic surface mass density of molecular gas and
examine its spatial variations throughout the Galaxy. We find that the
azimuthally averaged Galactic surface density of molecular gas peaks between
Galactocentric radii of 4 and 5 kpc. Although the Perseus arm is not detected
in molecular gas, the Galactic surface density of molecular gas is enhanced
along the positions of the Scutum-Crux and Sagittarius arms. This may indicate
that molecular clouds form in spiral arms and are disrupted in the inter-arm
space. Last, we find that the CO excitation temperature of molecular clouds
decreases away from the Galactic center, suggesting a possible decline in the
star formation rate with Galactocentric radius. There is a marginally
significant enhancement in the CO excitation temperature of molecular clouds at
a Galactocentric radius of about 6 kpc, which in the longitude range of the GRS
corresponds to the Sagittarius arm. This temperature increase could be
associated with massive star formation in the Sagittarius spiral arm
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