Gravitational collapse of magnetized clouds II. The role of Ohmic dissipation

We formulate the problem of magnetic field dissipation during the accretion phase of low-mass star formation, and we carry out the first step of an iterative solution procedure by assuming that the gas is in free-fall along radial field lines. This so-called ``kinematic approximation'' ignores the back reaction of the Lorentz force on the accretion flow. In quasi steady-state, and assuming the resistivity coefficient to be spatially uniform, the problem is analytically soluble in terms of Legendre's polynomials and confluent hypergeometric functions. The dissipation of the magnetic field occurs inside a region of radius inversely proportional to the mass of the central star (the ``Ohm radius''), where the magnetic field becomes asymptotically straight and uniform. In our solution, the magnetic flux problem of star formation is avoided because the magnetic flux dragged in the accreting protostar is always zero. Our results imply that the effective resistivity of the infalling gas must be higher by several orders of magnitude than the microscopic electric resistivity, to avoid conflict with measurements of paleomagnetism in meteorites and with the observed luminosity of regions of low-mass star formation.Comment: 20 pages, 4 figures, The Astrophysical Journal, in pres

Pop-Up Potential: The Effect of Regulatory Laws on the Innovation of Pop-Up Restaurants

Pop-up restaurants have risen in popularity over the past decade in the United States. As used in this Comment, a pop-up restaurant is when an existing restaurant space temporarily loans that space to another chef or restaurant for a limited period of time. This Comment seeks to explore the potential benefits and exploit the drawbacks of pop-up restaurants as property solutions in large cities. Pop-up restaurants thrive in the landscape of the sharing economy, and as such, legal scholars pose that it is imperative to understand this new type of economic scheme to efficiently regulate the entities within it. The beneficial services that pop-ups offer must be weighed against the drawbacks in order to determine if they are worthy of creating adaptive permitting regimes in particular cities. Should a city choose to regulate pop-ups, this Comment poses that such regulation would be most effective at the municipal level. Furthermore, this Comment examines two successful regulatory models—those of San Francisco and New York—and suggests that they can serve as a guide to cities that do not currently regulate pop-up restaurants

A Service Learning Model: Meeting NCATE Standards Through Educational Technology

Service learning can be described as learning experiences that serve the needs of a specific community and enable students to draw meaningful connections between theory and practice. Through service learning, students observe and model the behaviors, attitudes, and emotional reactions of others. It is important that education programs are built upon NCATE standards in order to ensure quality. This service learning model is based on an undergraduate level educational technology course. Through their participation in Introduction to Computers in Education, future teachers gain practical experience in a school-based environment that prepares them in the use of educational technology for teaching and learning

Navidad y semana santa : discurso de la Iglesia 50's y 60's Bogotá

Historiador (a)Pregrad