Author: Shashwata GangulyPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Morphology, Fragmentation, and Dynamic Balance: an Investigation Into Early Stages of Structure Formation in Molecular Clouds
Author: Shashwata GangulyObservational Studies of Fragmentation in Molecular Clouds
Author: Riwaj PokhrelPublisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
In this dissertation, I explore fragmentation physics in multiple scales in nearby molecular clouds and discuss some implications of fragmentation for cloud structure formation and star formation, primarily by analyzing multi-wavelength observations of dust emission. First, I tested the complete thermal and combined thermal and nonthermal support mechanisms that balance gravitational contraction at multiple scales in the Perseus molecular cloud. I found that the observed multiscale structures in Perseus are consistent with an inefficient thermal Jeans fragmentation, where the Jeans efficiency increases from the largest scale ($\gtrsim$10s of pc) to the smallest scale ($\sim$10s of AU). Next, I studied the effect of the formation of dense self-gravitating structures and star formation on the gas distribution in terms of its column density distribution function (N-PDF). I found that the evolutionary effect of clouds has corresponding changes on the N-PDF functional form, with a lognormal shape in diffuse regions that have negligible star formation, a lognormal and two power-laws in denser regions with moderate star formation, and a lognormal and one power-law in the densest regions with highly efficient clustered star formation. Finally, I explored the variations of star and gas surface densities in twelve molecular clouds using various techniques. I found that the stellar mass surface density of the recently formed stars varies as the square of the gas mass surface density in all twelve clouds. Also, I do not find any evidence of a column density threshold for efficient star formation.
Literature 1991, Part 2
Author: Astronomisches Rechen-InstitutPublisher: Springer Science & Business Media
ISBN: 3662123762
Category : Science
Languages : en
Pages : 1592
Book Description
"Astronomy and Astrophysics Abstracts" appearing twice a year has become oneof the fundamental publications in the fields of astronomy, astrophysics andneighbouring sciences. It is the most important English-language abstracting journal in the mentioned branches. The abstrats are classified under more than a hundred subject categories, thus permitting a quick survey of the whole extended material. The AAA is a valuable and important publication for all students and scientists working in the fields of astronomy and related sciences. As such it represents a necessary ingredient of any astronomical library all over the world.
Scientific and Technical Aerospace Reports
Author: Publisher:
ISBN:
Category : Aeronautics
Languages : en
Pages : 804
Book Description
Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.
Understanding the Structure of Molecular Clouds
Author: Jan OrkiszPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
The new generation of wide-bandwidth high-resolution receivers turns almost any radio observation into a spectral survey. In the case of wide-field imaging of the interstellar medium, such a wealth of data provides new diagnostic tools, but also poses new challenges in terms of data processing and analysis. The ORION-B project aims at observing 5 square degrees of the OB molecular cloud, or about half of the cloud's surface, over the entire 3mm band. The emission of tens of molecular tracers has been mapped, including CO isotopologues, HCO, HCN, HNC, N2H+, methanol, SO, CN...Having access to spatially resolved maps from many molecular species enables us to identify the best tracers of the gas density and illumination. Machine learning techniques have also been applied to these maps, in order to segment the molecular cloud into typical regions based on their molecular emission, and to quantify the most meaningful correlations of different molecular tracers with each other and with physical quantities such as density or dust temperature.The wide-field coverage, together with the spatial and spectral resolution, also allows to characterize statistically the kinematics and dynamics of the gas. The amount of momentum in the compressive and solenoidal (rotational) modes of turbulence are retrieved, showing that the cloud is dominated by solenoidal motions, with the compressive modes being concentrated in two star-forming regions. This result is in line with the overall very low star formation efficiency of the cloud, and highlights the role of compressive forcing in the star formation process.The numerous filaments identified in the molecular cloud also prove to have rather low densities, and are very stable against gravitational collapse. Most filaments are starless, but they show signs of longitudinal and radial fragmentation, which indicates that star formation might occur later on.
Fragmentation: Toward Accurate Calculations on Complex Molecular Systems
Author: Mark S. GordonPublisher: John Wiley & Sons
ISBN: 1119129249
Category : Science
Languages : en
Pages : 376
Book Description
Fragmentation: Toward Accurate Calculations on Complex Molecular Systems introduces the reader to the broad array of fragmentation and embedding methods that are currently available or under development to facilitate accurate calculations on large, complex systems such as proteins, polymers, liquids and nanoparticles. These methods work by subdividing a system into subunits, called fragments or subsystems or domains. Calculations are performed on each fragment and then the results are combined to predict properties for the whole system. Topics covered include: Fragmentation methods Embedding methods Explicitly correlated local electron correlation methods Fragment molecular orbital method Methods for treating large molecules This book is aimed at academic researchers who are interested in computational chemistry, computational biology, computational materials science and related fields, as well as graduate students in these fields.
The Micro- and Macro- Physics of Giant Molecular Cloud Formation
Author: Kendall P. HallPublisher:
ISBN:
Category :
Languages : en
Pages : 136
Book Description
Giant molecular clouds are the birthplaces of stars, stars which gift back enriched gas, more endowed with metals than the gas they were formed from.Giant molecular clouds, as the formation sites of stars, are integral to the understanding of galaxy structure formation, star-formation, and ultimately the creation of life-bearing planets. They are complex structures, forming through difference mechanisms, all of which are important to study. For stars to be able to form, cool, molecular gas must exist. This work adds to the growing understanding of how molecules form on small scales,specifically how to trace molecular hydrogen gas that does not have corresponding CO emission: ``CO-dark" molecular hydrogen gas. Many tracers are suggested in the literature, but this thesis focuses on the use of the 158 micron line of C+, the 18 cm lines of OH, and the 9 cm lines of CH as diffuse gas tracers. They are studied in the context of two photodissociation regions/cloud boundaries in the Perseus giant molecular cloud. [CII] emission is found to better correlate with neutral atomic gas, whereas OH correlates well with visual extinction and molecular hydrogen depending on the gas density, and CH correlates best with molecular hydrogen. The conversion between CH integrated intensity and molecular hydrogen column density has less scatter than any other conversion factor, including those for 12CO, OH, and [CII]. This work also encompasses the formation of giant molecular clouds on larger scales. A currently ongoing project aims to study thermal pressure across an individual molecular cloud, for the first time showing observationally how the pressure changes within the different environments of a single cloud.
International Aerospace Abstracts
Author: Recent Advances in the Collapse and Fragmentation of Turbulent Molecular Cloud Cores
Author: Publisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
The formation of Giant Molecular Clouds (GMCs) sets the stage for the formation of protostellar systems by the gravitational collapse of dense regions within the GMC that fragment into smaller core components that in turn condense into stars. Developing a comprehensive theory of star formation remains one of the most elusive, and most important, goals of theoretical astrophysics. Inherent in the difficulty in attaining this goal is that the gravitational collapse depends critically upon initial conditions within the cores which only recently have been known with sufficient accuracy to permit a realistic theoretical attack on the problem. Observations of stars in the vicinity of the Sun show that binary systems are prevalent and appear to be a general outcome of the collapse and fragmentation process. Despite years of progress, theoretical studies have still not determined why binary stars occur with such frequency, or indeed, even what processes determine the transition from single stars to binaries and thence to multiple stellar systems. One of the major goals of this research is to understand the nature of the formation of binary and multiple stellar systems with typical low mass stars 0.2 to 3 M{sub {circle_dot}} and the physical properties of these systems. Basic questions concerning this process remain unanswered. What determines the fraction of an unstable cloud that will fragment into protostellar objects? What determines the pattern of stellar clustering into binaries and multiple systems? Even after fragmentation occurs, we have little understanding of the subsequent collapse. Consequently, it is unclear how the mass distribution of fragments maps onto eventual stellar masses, something we must understand to explain the stellar initial mass function (IMF). We will first discuss the development of the numerical methodology that will contribute to answering these questions. This technology consists of a 3D parallel, adaptive mesh refinement (AMR) self-gravitational, radiation-hydrodynamics code that we have developed. We will present new results for the gravitational collapse and fragmentation of marginally stable turbulent molecular cloud cores and follow the collapse of high mass fragments as they interact with the radiation of the protostars forming at their centers. We will discuss the theoretical difficulties in forming binary stars and the role of turbulence in their formation.
Author: P. Padoan