4,345 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
Variable-focus liquid lens for miniature cameras
The meniscus between two immiscible liquids can be used as an optical lens. A change in curvature of this meniscus by electrowetting leads to a change in focal distance. It is demonstrated that two liquids in a tube form a self-centered lens with a high optical quality. The motion of the lens during a focusing action was studied by observation through the transparent tube wall. Finally, a miniature achromatic camera module was designed and constructed based on this adjustable lens, showing that it is excellently suited for use in portable application
Variable-focus liquid lens for miniature cameras
The meniscus between two immiscible liquids can be used as an optical lens. A change in curvature of this meniscus by electrowetting leads to a change in focal distance. It is demonstrated that two liquids in a tube form a self-centered lens with a high optical quality. The motion of the lens during a focusing action was studied by observation through the transparent tube wall. Finally, a miniature achromatic camera module was designed and constructed based on this adjustable lens, showing that it is excellently suited for use in portable application
The 32-GHz performance of the DSS-14 70-meter antenna: 1989 configuration
The results of preliminary 32 GHz calibrations of the 70 meter antenna at Goldstone are presented. Measurements were done between March and July 1989 using Virgo A and Venus as the primary efficiency calibrators. The flux densites of theses radio sources at 32 GHz are not known with high accuracy, but were extrapolated from calibrated data at lower frequencies. The measured value of efficiency (0.35) agreed closely with the predicted value (0.32), and the results are very repeatable. Flux densities of secondary sources used in the observations were subsequently derived. These measurements were performed using a beamswitching radiometer that employed an uncooled high-electron mobility transistor (HEMT) low-noise amplifier. This system was installed primarily to determine the performance of the antenna in its 1989 configuration, but the experience will also aid in successful future calibration of the Deep Space Network (DSN) at this frequency
Out-breeding behaviour and xenophobia in the Damaraland mole-rat, Cryptomys damarensis
Out-breeding behaviour and xenophobia were investigated in laboratory colonies of the Damaraland mole-rat, Cryptomys damarensis. Foreign males and/or females were introduced into reproductively quiescent colonies and colonies which were actively breeding. Although males attempted to mate with familiar or related females, females only mated with foreign males. This suggests that female avoidance of incest is responsible for outbreeding in this species. Resident males in reproductively quiescent colonies did not attack foreign males or females. However, resident males in actively breeding colonies attacked and kilted foreign males. Resident females in reproductively quiescent colonies attacked foreign females but attempted to mate with foreign males. However, in colonies in which the breeding female was approaching parturition, resident non-breeding females also attacked foreign males. Once foreign females attained reproductive status in the colony into which they were introduced they killed all the resident females. These results suggest that xenophobia in the Damaraland mole-rat is influenced by whether or not the colony is actively breeding, and by the reproductive state of the breeding female
Hierarchical fragmentation and collapse signatures in a high-mass starless region
Aims: Understanding the fragmentation and collapse properties of the dense
gas during the onset of high-mass star formation. Methods: We observed the
massive (~800M_sun) starless gas clump IRDC18310-4 with the Plateau de Bure
Interferometer (PdBI) at sub-arcsecond resolution in the 1.07mm continuum
andN2H+(3-2) line emission. Results: Zooming from a single-dish low-resolution
map to previous 3mm PdBI data, and now the new 1.07mm continuum observations,
the sub-structures hierarchically fragment on the increasingly smaller spatial
scales. While the fragment separations may still be roughly consistent with
pure thermal Jeans fragmentation, the derived core masses are almost two orders
of magnitude larger than the typical Jeans mass at the given densities and
temperatures. However, the data can be reconciled with models using
non-homogeneous initial density structures, turbulence and/or magnetic fields.
While most sub-cores remain (far-)infrared dark even at 70mum, we identify weak
70mum emission toward one core with a comparably low luminosity of ~16L_sun,
re-enforcing the general youth of the region. The spectral line data always
exhibit multiple spectral components toward each core with comparably small
line widths for the individual components (in the 0.3 to 1.0km/s regime). Based
on single-dish C18O(2-1) data we estimate a low virial-to-gas-mass ratio
<=0.25. We discuss that the likely origin of these spectral properties may be
the global collapse of the original gas clump that results in multiple spectral
components along each line of sight. Even within this dynamic picture the
individual collapsing gas cores appear to have very low levels of internal
turbulence.Comment: 8 pages, 4 figures, A&A in pres
High-Resolution Observations in B1-IRS: ammonia, CCS and water masers
We present a study of the structure and dynamics of the star forming region
B1-IRS (IRAS 03301+3057) using the properties of different molecules at high
angular resolution (~4''). We have used VLA observations of NH3, CCS, and H2O
masers at 1 cm. CCS emission shows three clumps around the central source, with
a velocity gradient from red to blueshifted velocities towards the protostar,
probably due to the interaction with outflowing material. Water maser emission
is elongated in the same direction as a reflection nebula detected at 2micron
by 2MASS, with the maser spots located in a structure of some hundreds of AU
from the central source, possibly tracing a jet. We propose a new outflow model
to explain all our observations, consisting of a molecular outflow near the
plane of the sky. Ammonia emission is extended and anticorrelated with CCS. We
have detected for the first time this anticorrelation at small scales (1400 AU)
in a star forming region.Comment: 6 pages, 3 figures. To appear in the Proceedings of the 2004 European
Workshop: "Dense Molecular Gas around Protostars and in Galactic Nuclei",
Eds. Y.Hagiwara, W.A.Baan, H.J.van Langevelde, 2004, a special issue of ApSS,
Kluwe
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