2,318 research outputs found
On the impact of the magnitude of Interstellar pressure on physical properties of Molecular Cloud
Recently reported variations in the typical physical properties of Galactic
and extra-Galactic molecular clouds (MCs), and in their ability to form stars
have been attributed to local variations in the magnitude of interstellar
pressure. Inferences from these surveys have called into question two
long-standing beliefs that the MCs : 1 are Virialised entities and (2) have
approximately constant surface density i.e., the validity of the Larson's third
law. In this work we invoke the framework of cloud-formation via collisions
between warm gas flows. Post-collision clouds forming in these realisations
cool rapidly and evolve primarily via the interplay between the Non-linear Thin
Shell Instability (NTSI), and the self-gravity. Over the course of these
simulations we traced the temporal evolution of the surface density of the
assembled clouds, the fraction of dense gas, the distribution of gas column
density (NPDF), and the Virial nature of the assembled clouds. We conclude,
these physical properties of MCs not only exhibit temporal variation, but their
respective peak-magnitude also increases in proportion with the magnitude of
external pressure, . The velocity dispersion in assembled clouds
appears to follow the power-law, . Also,
the power-law tail at higher densities becomes shallower with increasing
magnitude of external pressure, for magnitudes,
K cm, at higher magnitudes such as those typically found in the Galactic
CMZ ( K cm), the power-law shows significant
steepening. Thus while our results are broadly consistent with inferences from
various recent observational surveys, it appears, MCs hardly exhibit a unique
set of properties, but rather a wide variety, that can be reconciled with a
range of magnitudes of pressure between 10 K cm - 10 K
cm.Comment: 20 pages, 11 Figures, 1 Table, To appear in Monthly Notice of the RA
On the star-forming ability of Molecular Clouds
The star-forming ability of a molecular cloud depends on the fraction of gas
it can cycle into the dense-phase. Consequently, one of the crucial questions
in reconciling star-formation in clouds is to understand the factors that
control this process. While it is widely accepted that the variation in ambient
conditions can alter significantly the ability of a cloud to spawn stars, the
observed variation in the star-formation rate in nearby clouds that experience
similar ambient conditions, presents an interesting question. In this work we
attempted to reconcile this variation within the paradigm of colliding flows.
To this end we develop self-gravitating, hydrodynamic realisations of identical
flows, but allowed to collide off-centre. Typical observational diagnostics
such as the gas-velocity dispersion, the fraction of dense-gas, the column
density distribution ({\small N-PDF}), the distribution of gas mass as a
function of -band extinction and the strength of compressional/solenoidal
modes in the post-collision cloud were deduced for different choices of the
impact parameter of collision. We find that a strongly sheared cloud is
terribly inefficient in cycling gas into the dense phase and that such a cloud
can possibly reconcile the sluggish nature of star-formation reported for some
clouds. Within the paradigm of cloud-formation via colliding flows this is
possible in case of flows colliding with a relatively large impact parameter.
We conclude that compressional modes - though probably essential - are
insufficient to ensure a relatively higher star-formation efficiency in a
cloud.Comment: 12 pages, 8 figures; To appear in MNRA
How galaxies lose their angular momentum
The processes are investigated by which gas loses its angular momentum during
the protogalactic collapse phase, leading to disk galaxies that are too compact
with respect to the observations. High-resolution N-body/SPH simulations in a
cosmological context are presented including cold gas and dark matter. A halo
with quiet merging activity since z~3.8 and with a high spin parameter is
analysed that should be an ideal candidate for the formation of an extended
galactic disk. We show that the gas and the dark matter have similar specific
angular momenta until a merger event occurs at z~2 with a mass ratio of 5:1.
All the gas involved in the merger loses a substantial fraction of its specific
angular momentum due to tidal torques and falls quickly into the center.
Dynamical friction plays a minor role,in contrast to previous claims. In fact,
after this event a new extended disk begins to form from gas that was not
involved in the 5:1 merger event and that falls in subsequently. We argue that
the angular momentum problem of disk galaxy formation is a merger problem: in
cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1
are expected to occur in almost all galaxies. We suggest that energetic
feedback processes could in principle solve this problem, however only if the
heating occurs at the time or shortly before the last substantial merger event.
Good candidates for such a coordinated feedback would be a merger-triggered
star burst or central black hole heating. If a large fraction of the low
angular momentum gas would be ejected as a result of these processes, late-type
galaxies could form with a dominant extended disk component, resulting from
late infall, a small bulge-to-disk ratio and a low baryon fraction, in
agreement with observations.Comment: 7 pages, 5 figures, submitted to MNRAS. Request for high resolution
figures to the author
Have Pentaquark States Been seen?
The status of the search for pentaquark baryons is reviewed in light of new
results from the first two dedicated experiments from CLAS at Jefferson Lab and
of new analyses from several laboratories on the . Evidence for
and against two heavier pentaquark states is also discussed.Comment: Added some references, corrected typo
Monte Carlo simulations of the halo white dwarf population
The interpretation of microlensing results towards the Large Magellanic Cloud
(LMC) still remains controversial. Whereas white dwarfs have been proposed to
explain these results and, hence, to contribute significantly to the mass
budget of our Galaxy, there are as well several constraints on the role played
by white dwarfs. In this paper we analyze self-consistently and simultaneously
four different results, namely, the local halo white dwarf luminosity function,
the microlensing results reported by the MACHO team towards the LMC, the
results of Hubble Deep Field (HDF) and the results of the EROS experiment, for
several initial mass functions and halo ages. We find that the proposed
log-normal initial mass functions do not contribute to solve the problem posed
by the observed microlensing events and, moreover, they overproduce white
dwarfs when compared to the results of the HDF and of the EROS survey. We also
find that the contribution of hydrogen-rich white dwarfs to the dynamical mass
of the halo of the Galaxy cannot be more than .Comment: 17 pages, 10 figures; accepted for publication in Astronomy and
Astrophysic
QCD radiative and power corrections and Generalized GDH sum rules
We extend the earlier suggested QCD-motivated model for the -dependence
of the generalized Gerasimov-Drell-Hearn (GDH) sum rule which assumes the
smooth dependence of the structure function , while the sharp dependence
is due to the contribution and is described by the elastic part of the
Burkhardt-Cottingham sum rule. The model successfully predicts the low crossing
point for the proton GDH integral, but is at variance with the recent very
accurate JLAB data. We show that, at this level of accuracy, one should include
the previously neglected radiative and power QCD corrections, as boundary
values for the model. We stress that the GDH integral, when measured with such
a high accuracy achieved by the recent JLAB data, is very sensitive to QCD
power corrections. We estimate the value of these power corrections from the
JLAB data at . The inclusion of all QCD corrections leads
to a good description of proton, neutron and deuteron data at all .Comment: 10 pages, 4 figures (to be published in Physical Review D
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