Protostellar collapse: rotation and disk formation

We present some important conclusions from recent calculations pertaining to the collapse of rotating molecular cloud cores with axial symmetry, corresponding to evolution of young stellar objects through classes 0 and begin of class I. Three main issues have been addressed: (1) The typical timescale for building up a preplanetary disk - once more it turned out that it is of the order of one free-fall time which is decisively shorter than the widely assumed timescale related to the so-called 'inside-out collapse'; (2) Redistribution of angular momentum and the accompanying dissipation of kinetic (rotational) energy - together these processes govern the mechanical and thermal evolution of the protostellar core to a large extent; (3) The origin of calcium-aluminium-rich inclusions (CAIs) - due to the specific pattern of the accretion flow, material that has undergone substantial chemical and mineralogical modifications in the hot (exceeding 900 K) interior of the protostellar core may have a good chance to be advectively transported outward into the cooler remote parts (beyond 4 AU, say) of the growing disk and to survive there until it is incorporated into a meteoritic body.Comment: 4 pages, 4 figure

Using zeta-potential measurements to quantify peptide partition to lipid membranes

© The Author(s) 2011. This article is published with open access at Springerlink.com.Open Access: This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.Many cellular phenomena occur on the biomembranes. There are plenty of molecules (natural or xenobiotics) that interact directly or partially with the cell membrane. Biomolecules, such as several peptides (e.g., antimicrobial peptides) and proteins, exert their effects at the cell membrane level. This feature makes necessary investigating their interactions with lipids to clarify their mechanisms of action and side effects necessary. The determination of molecular lipid/water partition constants (Kp) is frequently used to quantify the extension of the interaction. The determination of this parameter has been achieved by using different methodologies, such as UV-Vis absorption spectrophotometry, fluorescence spectroscopy and ζ-potential measurements. In this work, we derived and tested a mathematical model to determine the Kp from ζ-potential data. The values obtained with this method were compared with those obtained by fluorescence spectroscopy, which is a regular technique used to quantify the interaction of intrinsically fluorescent peptides with selected biomembrane model systems. Two antimicrobial peptides (BP100 and pepR) were evaluated by this new method. The results obtained by this new methodology show that ζ-potential is a powerful technique to quantify peptide/lipid interactions of a wide variety of charged molecules, overcoming some of the limitations inherent to other techniques, such as the need for fluorescent labeling.This work was partially supported by project PTDC/QUI/ 69937/2006 from Fundação para a Ciência e Tecnologia-Ministério da Ciência, Tecnologia e Ensino Superior (FCT-MCTES, Portugal), and by Fundação Calouste Gulbenkian (Portugal). JMF and MMD also thank FCT-MCTES for grants IMM/BT/37-2010 and SFRH/BD/41750/2007, respectively

Magnetically Controlled Spasmodic Accretion During Star Formation. I. Formulation of the Problem and Method of Solution

We formulate the problem of the late accretion phase of the evolution of an isothermal magnetic disk surrounding a forming star. The evolution is described by the six-fluid MHD equations, accounting for the presence of neutrals, atomic and molecular ions, electrons, and neutral, positively, and negatively charged grains. Only the electron fluid is assumed to be attached to the magnetic field, in order to investigate the effect of the detachment of the ions from the magnetic field lines that begins at densities as low as 10^8 cm^-3. The "central sink approximation" is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 10^11 cm^-3. In this way, the structure and evolution of the isothermal disk surrounding the forming star can be studied at late times without having to implement the numerically costly radiative transfer required by the physics of the opaque core. The mass and magnetic flux accumulating in the forming star arecalculated, as are their effects on the structure & evolution of the surrounding disk. The numerical method of solution first uses an adaptive grid and later, after a central region a few AU in radius becomes opaque, switches to a stationary but nonuniform grid with a central sink cell. It also involves an implicit time integrator, an advective difference scheme that possesses the transportive property, a second-order difference approximation of forces inside a cell, an integral approximation of the gravitational and magnetic fields, and tensor artificial viscosity that permits an accurate investigation of the formation and evolution of shocks in the neutral fluid.Comment: Astrophysical Journal, in press. 32 page

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

Massive zero-metal stars: Energy production and mixing

Time-dependent nuclear network calculations at constant temperature show that for zero-metal stars ≳ $20 M_{\odot}$ (i) β-decay reactions and (ii) the 13N(p,γ)14O reaction must be included. It is also shown that the nuclear timescale in these zero-metal stars is shorter than the mixing timescale and therefore the assumption of instantaneous mixing across convective regions is not fulfilled. We conclude that proper modeling of these processes may alter the evolution of massive zero-metal stars

From clouds to stars - Protostellar collapse and the evolution to the pre-main sequence - 1. Equations and evolution in the Hertzsprung-Russell diagram

We present the first study of early stellar evolution with "cloud" initial conditions utilizing a system of equations that comprises a solar model solution. All previous studies of protostellar collapse either make numerous assumptions specifically tailored for different parts of the flow and different parts of the evolution or they do not reach the pre- main sequence phase. We calculate the pre-main sequence properties of marginally gravitationally unstable, isothermal, equilibrium "Bonnor-Ebert" spheres with an initial temperature of 10 K and masses of 0.05 to 10 M. The mass accretion rate is determined by the solution of the flow equations rather than being prescribed or neglected. In our study we determine the protostar's radii and the thermal structure together with the mass and mass accretion rate, luminosity and effective temperature during its evolution to a stellar pre-main sequence object. We calculate the time needed to accrete the final stellar masses, the corresponding mean mass accretion rates and median luminosities, and the corresponding evolutionary tracks in the theoretical Hertzsprung-Russell diagram. We derive these quantities from the gas flow resulting from cloud collapse. We do not assume a value for an "initial" stellar radius and an "initial" stellar thermal structure at the "top of the track", the Hayashi-line or any other instant of the evolution. Instead we solve the flow equations for a cloud fragment with spherical symmetry. The system of equations we use contains the equations of stellar structure and evolution as a limiting case and has been tested by a standard solar model and by classical stellar pulsations (Wuchterl & Feuchfinger 1998; Feuchfinger 1999; Dorfi & Feuchfinger 1999). When dynamical accretion effects have become sufficiently small so that a comparison to existing hydrostatic stellar evolution calculations for corresponding masses can be made, young stars of 2 M. appear close to the location of the Henyey part of the respective classical evolutionary track and at substantially larger ages for given luminosities than those inferred from previous calculations. 1 M. stars appear at lower luminosities, to the left of the corresponding Hayashi-tracks and are about 1 Myr older than an a-priori hydrostatic stellar evolution model at the same luminosity. They burn most of their deuterium during the main accretion phase before mass accretion halts and they become visible. They do not become fully convective during the entire evolution calculated, i.e., up to 1.5 Myr. Altogether the structure of our solar mass star at 1 Myr, with its raditive core and convective envelope, resembles the present Sun rather then a fully convective object. Very low mass stars and proto brown dwarfs close to the substellar limit appear with luminosities close to those at the "top of the tracks", giving ages roughly in accordance with classical values, tentatively at 0.05 to 0.09 dex higher effective temperatures

From clouds to stars

We present the first study of early stellar evolution with “cloud” initial conditions utilizing a system of equations that comprises a solar model solution. All previous studies of protostellar collapse either make numerous assumptions specifically tailored for different parts of the flow and different parts of the evolution or they do not reach the pre-main sequence phase. We calculate the pre-main sequence properties of marginally gravitationally unstable, isothermal, equilibrium “Bonnor-Ebert” spheres with an initial temperature of $10 \, \rm K$ and masses of 0.05 to 10 $M_\odot$. The mass accretion rate is determined by the solution of the flow equations rather than being prescribed or neglected. In our study we determine the protostar's radii and the thermal structure together with the mass and mass accretion rate, luminosity and effective temperature during its evolution to a stellar pre-main sequence object. We calculate the time needed to accrete the final stellar masses, the corresponding mean mass accretion rates and median luminosities, and the corresponding evolutionary tracks in the theoretical Hertzsprung-Russell diagram. We derive these quantities from the gas flow resulting from cloud collapse. We do not assume a value for an “initial” stellar radius and an “initial” stellar thermal structure at the “top of the track”, the Hayashi-line or any other instant of the evolution. Instead we solve the flow equations for a cloud fragment with spherical symmetry. The system of equations we use contains the equations of stellar structure and evolution as a limiting case and has been tested by a standard solar model and by classical stellar pulsations (Wuchterl & Feuchtinger 1998; Feuchtinger 1999; Dorfi & Feuchtinger 1999). When dynamical accretion effects have become sufficiently small so that a comparison to existing hydrostatic stellar evolution calculations for corresponding masses can be made, young stars of $2\, M_\odot$ appear close to the location of the Henyey part of the respective classical evolutionary track and at substantially larger ages for given luminosities than those inferred from previous calculations. $1\,{M_\odot}$ stars appear at lower luminosities, to the left of the corresponding Hayashi-tracks and are about $1 \, \rm Myr$ older than an a-priori hydrostatic stellar evolution model at the same luminosity. They burn most of their deuterium during the main accretion phase before mass accretion halts and they become visible. They do not become fully convective during the entire evolution calculated, i.e., up to 1.5 Myr. Altogether the structure of our solar mass star at 1 Myr, with its raditive core and convective envelope, resembles the present Sun rather then a fully convective object. Very low mass stars and proto brown dwarfs close to the substellar limit appear with luminosities close to those at the “top of the tracks”, giving ages roughly in accordance with classical values, tentatively at $0.05$ to $0.09\,\rm dex$ higher effective temperatures

Protostellar collapse of rotating cloud cores

Aims. We investigate the influence of turbulent viscosity on the collapse of a rotating molecular cloud core with axial symmetry, in particular, on the first and second collapse phase, as well as the evolution of the second (protostellar) core during its first accretion period. By using extensive numerical calculations, we monitor the intricate interactions between the newly formed protostar and the surrounding accretion disk (the first core) in which the star is embedded. Methods. We use a grid-based radiation-hydrodynamics code with a spatial grid designed to meet the high resolution required to study the second core. The radiative transfer is treated in the flux-limited diffusion approximation. A slightly supercritical Bonnor-Ebert sphere of 1   M⊙ and uniform rotation according to a fixed centrifugal radius of 100 AU serves as the initial condition without exception. In a parameter study, we vary the β-viscosity driving the angular momentum transport. Results. Without viscosity (β = 0), a highly flattened accretion disk forms that fragments into several “cold” rings. For β = 10-4, a single “warm” ring forms that undergoes collapse due to hydrogen dissociation. For β = 10-3, ring formation is suppressed completely. The second collapse proceeds on the local thermal timescale, which is in contrast to the current view of a generally dynamical second collapse. During the first accretion period of the second core, the first core heats up globally and, as a consequence, a nearly spherical outflow occurs, destroying the structure of the former accretion disk completely. Finally, for β = 10-2, we see the classical dynamical second collapse and a shorter but more rapid accretion phase. The impact on the surrounding accretion disk is even more pronounced. We follow the resulting massive outflow up to several kyr after the second collapse, where the central parts (R < 0.7 AU) are now cut out and replaced with an appropriate inner boundary condition. Matter is found to turn back to the center at a radius of 500 AU after about 4 kyr and to reach the protostar again after approximately 7 kyr. The results suggest that the star formation process consists of short and rapid accretion phases (lasting on the order of 100 yr) between long and quiet outflow periods (lasting several kyr)