4,636 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
The role of accretion disks in the formation of massive stars
We present radiation hydrodynamics simulations of the collapse of massive
pre-stellar cores. We treat frequency dependent radiative feedback from stellar
evolution and accretion luminosity at a numerical resolution down to 1.27 AU.
In the 2D approximation of axially symmetric simulations, it is possible for
the first time to simulate the whole accretion phase of several 10^5 yr for the
forming massive star and to perform a comprehensive scan of the parameter
space. Our simulation series show evidently the necessity to incorporate the
dust sublimation front to preserve the high shielding property of massive
accretion disks. Our disk accretion models show a persistent high anisotropy of
the corresponding thermal radiation field, yielding to the growth of the
highest-mass stars ever formed in multi-dimensional radiation hydrodynamics
simulations. Non-axially symmetric effects are not necessary to sustain
accretion. The radiation pressure launches a stable bipolar outflow, which
grows in angle with time as presumed from observations. For an initial mass of
the pre-stellar host core of 60, 120, 240, and 480 Msol the masses of the final
stars formed in our simulations add up to 28.2, 56.5, 92.6, and at least 137.2
Msol respectively.Comment: 4 pages, 2 figures, Computational Star Formation Proceedings IAU
Symposium No. 270, 2010, Ed.: J. Alves, B. Elmegreen, J. Girart & V. Trimbl
A search for 183-GHz emission from water in late-type stars
A search was made for 183 GHz line emission from water vapor in the direction of twelve Mira and two semiregular variables. Upper limits to the emission are in the range of 2000 to 5000 Jy. It is estimated that thermal emission from the inner regions of late type stellar envelopes will be on the order of ten Jy. Maser emission, according to one model, would be an order of magnitude stronger. From the limited set sampled, the possibility of very strong maser emission at 183 GHz cannot yet be ruled out
Abatement of Type 1 Diabetes as a Result of Polychlorinated Biphenyl (PCB)-153 Exposure in the Non-Obese Diabetic (NOD) Mouse Model
Type 1 diabetes {Tl D) is an autoimmune disorder characterized by the Tcell- mediated destruction of insulin-producing P-cells in the pancreatic islets of Langerhans. The steady increase in prevalence and incidence of Tl D across the globe suggests disease onset may be contributed by various environmental factors besides genetics, such as persistent organic pollutants (POP). Given that polychlorinated biphenyl (PCB)-153 is a highly abundant POP in both the environment and mammalian tissues, there is reason to believe the compound may be an environmental factor influencing disease susceptibility and onset. Non-obese diabetic (NOD) mice, the best experimental model for studying TID, were exposed to intraperitoneal injections of PCB-153 in a 10-day acute (50mg/Kg or 0.5mg/Kg) or 16-week chronic (12.5mg/Kg or 0.125mg/Kg) fashion. Analysis of various immune parameters, including T-cell types and subtypes, T-cell proliferative responses, as well as their cytokine secretions, revealed that both acute and chronic exposure to PCB-153 caused significant immunosuppression in all PCB-153- exposed mice. Based on the significant decreases in c04+ T-helper cells and reduced secretion of interleukin {IL)-2 it is plausible to believe that T-helper 1 {TH1) cells are the most susceptible cell population to PCB-153 exposure. This is further supported by the decrease in TlD incidence observed in mice chronically exposed to either dose of PCB-153. Overall, this study not only reveals for the first time the protective effects of PCB-153 exposure on TlD, but brings the awareness about PCB-153\u27s antidiabetogenic immunosuppressive effects in the context of influencing the action of other co-pollutants, as PCB-153 might mask other pollutants\u27 effects on TIO development
Thirteen-color narrow-band photometry of one thousand bright stars
Thirteen-color narrow-band photometry of one thousand bright star
Star Formation with Adaptive Mesh Refinement Radiation Hydrodynamics
I provide a pedagogic review of adaptive mesh refinement (AMR) radiation
hydrodynamics (RHD) methods and codes used in simulations of star formation, at
a level suitable for researchers who are not computational experts. I begin
with a brief overview of the types of RHD processes that are most important to
star formation, and then I formally introduce the equations of RHD and the
approximations one uses to render them computationally tractable. I discuss
strategies for solving these approximate equations on adaptive grids, with
particular emphasis on identifying the main advantages and disadvantages of
various approximations and numerical approaches. Finally, I conclude by
discussing areas ripe for improvement.Comment: 8 pages, to appear in the Proceedings of IAU Symposium 270:
Computational Star Formatio
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