Analysis of Clumps in Molecular Cloud Models: Mass Spectrum, Shapes, Alignment and Rotation

Observations reveal concentrations of molecular line emission on the sky, called ``clumps,'' in dense, star-forming molecular clouds. These clumps are believed to be the eventual sites of star formation. We study the three-dimensional analogs of clumps using a set of self-consistent, time-dependent numerical models of molecular clouds. The models follow the decay of initially supersonic turbulence in an isothermal, self-gravitating, magnetized fluid. We find the following. (1) Clumps are intrinsically triaxial. This explains the observed deficit of clumps with a projected axis ratio near unity, and the apparent prolateness of clumps. (2) Simulated clump axes are not strongly aligned with the mean magnetic field within clumps, nor with the large-scale mean fields. This is in agreement with observations. (3) The clump mass spectrum has a high-mass slope that is consistent with the Salpeter value. There is a low-mass break in the slope at \sim 0.5 \msun, although this may depend on model parameters including numerical resolution. (4) The typical specific spin angular momentum of clumps is $4 \times 10^{22} {\rm cm^2 s^{-1}}$. This is larger than the median specific angular momentum of binary stars. Scaling arguments suggest that higher resolution simulations may soon be able to resolve the scales at which the angular momentum of binary stars is determined.Comment: 14 pages, 13 figures, to appear in 2003 July 20 Ap

Fragmentation Instability of Molecular Clouds: Numerical Simulations

We simulate fragmentation and gravitational collapse of cold, magnetized molecular clouds. We explore the nonlinear development of an instability mediated by ambipolar diffusion, in which the collapse rate is intermediate to fast gravitational collapse and slow quasistatic collapse. Initially uniform stable clouds fragment into elongated clumps with masses largely determined by the cloud temperature, but substantially larger than the thermal Jeans mass. The clumps are asymmetric, with significant rotation and vorticity, and lose magnetic flux as they collapse. The clump shapes, intermediate collapse rates, and infall profiles may help explain observations not easily fit by contemporary slow or rapid collapse models.Comment: 25pp, 20 small eps figures, in press ApJ, April 1, 200

Singular Isothermal Disks: II. Nonaxisymmetric Bifurcations and Equilibria

We review the difficulties of the classical fission and fragmentation hypotheses for the formation of binary and multiple stars. A crucial missing ingredient in previous theoretical studies is the inclusion of dynamically important levels of magnetic fields. As a minimal model for a candidate presursor to the formation of binary and multiple stars, we therefore formulate and solve the problem of the equilibria of isopedically magnetized, singular isothermal disks, without the assumption of axial symmetry. Considerable analytical progress can be made if we restrict our attention to models that are scale-free, i.e., that have surface densities that vary inversely with distance from the rotation axis of the system. In agreement with earlier analysis by Syer and Tremaine, we find that lopsided (M=1) configurations exist at any dimensionless rotation rate, including zero. Multiple-lobed (M = 2, 3, 4, ...) configurations bifurcate from an underlying axisymmetric sequence at progressively higher dimensionless rates of rotation, but such nonaxisymmetric sequences always terminate in shockwaves before they have a chance to fission into M=2, 3, 4, ... separate bodies. On the basis of our experience in this paper, and the preceding Paper I, we advance the hypothesis that binary and multiple star-formation from smooth (i.e., not highly turbulent) starting states that are supercritical but in unstable mechanical balance requires the rapid (i.e., dynamical) loss of magnetic flux at some stage of the ensuing gravitational collapse.Comment: 49 pages, 11 figures, LaTeX, needs aaspp4.sty. The Astrophysical Journal, in pres

Control of star formation by supersonic turbulence

Understanding the formation of stars in galaxies is central to much of modern astrophysics. For several decades it has been thought that stellar birth is primarily controlled by the interplay between gravity and magnetostatic support, modulated by ambipolar diffusion. Recently, however, both observational and numerical work has begun to suggest that support by supersonic turbulence rather than magnetic fields controls star formation. In this review we outline a new theory of star formation relying on the control by turbulence. We demonstrate that although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28 figures, in pres

Large signal HBT model for TECAP

A new heterojunction bipolar transistor (HBT) model has been implemented in TECAP. The paper describing this model, A New Large Signal Model for Heterojunction Bipolar Transistors Including Temperature Effects was presented at the IEEE 1991 CICC. The model provides a non-linear large signal repesentation of the HBT and is derived from the Gummel-Poon model for silicon bipolar transistors. It treats the parasitic resistances in the base and collector as non-linear elements. It also accounts for the negative ouput conductance characteristic of HBTs operating at high power densities. The small signal behavior of the model is accurate to 18 Ghz

Characterization and modeling of integrated photosensors in standard CMOS technology

This paper describes the electrical and spectral characteristics of different layers to construct photosensitive elements in standard 1.2 mu m CMOS technology. The use of commercial CMOS processes guarantees good reproduceability of the integrated sensors and optimized cointegration with sensor signal conditioning circuits. Deep and shallow photosensitive PN diodes and a vertical PNP phototransistor are investigated. Electrical on-wafer measurements with optical stimulation are performed and parameter extraction algorithms are established to create SPICE models. These models are suitable for the development of opto-ASICs. The validity of the model is tested using a 0.3 mm2 photodiode together with a two-stage transimpedance amplifier with a transimpedance of 5 MOhm at a 3 dB bandwidth of 90 kHz. the whole circuit is packaged using an optically transparent window in a plastic package

A new large signal model for heterojunction bipolar transistors including temperature effects

Description of an eight mode heterojunction bipolar transistor model suitable for circuit simulation, for which the Gummel-Poon model was used as a basis. Both the large signal DC characteristics and the small signal behavior up to 18 GHz are accurately modeled. The non-linear current gain associated with HBTs is included as well as non-linear intrinsic base and collector resistances. the negative ouput conductance seen at high current levels is modeled as a reduction in current proportionally to the power ouput of the transistor

A new compact model for the avalanche effect in InAlAs/InGaAs HBTs

This letter presents a new compact model for the avalanche effect in InAlAs/InGaAs heterojunction bipolar transistors. Unlike previous models, it is based on the formulation of the avalanche multiplication by Lee et al. (Phys. Rev., vol. 134, p. A761, 1964), which allows the model to be valid for even larger current densities than before. The description of the ionization coefficient takes into account the anomaly of the ionization coefficient of InGaAs at low electric fields. The new model is successfully verified by measurements in a conventional common base setup

A simple method for the thermal resistance measurement of AlGaAs/GaAs heterojunction bipolar transistors

A novel electrical method to accurately measure the thermal resistance of heterojunction bipolar transistors (HBT's) is presented. The keg advantage of the method is its simplicity, because it requires only the measurement of the device DC output characteristics at two different temperatures. In this brief, the measurement technique is illustrated, applied to multifinger HBT's and compared with other methods

Correcting the output conductance for self-heating in InAlAs/InGaAs HBTs

Two methods to correct the output characteristics of a heterojunction bipolar transistor (HBT) for self-heating, which especially suit material systems with low thermal conductivity and high temperature dependence of the current gain, are presented. The first and more conventional approach uses direct measurements of dc parameters (thermal conductivity and the temperature dependence of the current gain). The second method is based on measurements of small-signal parameters. Both procedures are applied to measurements on InAlAs/InGaAs HBTs. These methods result in reconstructed output characteristics that show a temperature-independent behavior and little gradient in the linear region. The methods presented in this paper may be used to investigate the electric field distribution and the avalanche currents of transistors with low thermal conductivity and high temperature dependence of the current gain