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, $P_{ext}$. The velocity dispersion in assembled clouds appears to follow the power-law, $\sigma_{gas}\propto P_{ext}^{0.23}$. Also, the power-law tail at higher densities becomes shallower with increasing magnitude of external pressure, for magnitudes, $P_{ext}/k_{B}\lesssim 10^{7}$ K cm$^{-3}$, at higher magnitudes such as those typically found in the Galactic CMZ ($P_{ext}/k_{B} > 10^{7}$ K cm$^{-3}$), 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$^{4}$ K cm$^{-3}$ - 10$^{8}$ K cm$^{-3}$.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 $K$-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