Protostellar disks and the primitive solar nebula

The objective is to obtain quantitative information on the turbulent transport of mass, angular momentum, and energy under the conditions that characterize the solar nebula, by direct numerical calculations. These calculations were made possible by research conducted on supercomputers (Cray XMP and Cray 2) by the Ames Computational Fluid Dynamics Branch. Techniques were developed that permitted the accurate representation of turbulent flows over the full range of important eddy sizes. So far, these techniques were applied (and verified) primarily in mundane laboratory situations, but they have a strong potential for astrophysical applications. A sequence of numerical experiments were conducted to evaluate the Reynold's stress tensor, turbulent heat transfer rate, turbulent dissipation rate, and turbulent kinetic energy spectrum, as functions of position, for conditions relevant to the solar nebula. Emphasis is placed on the variation of these properties with appropriate nondimensional quantities, so that relations can be derived that will be useful for disk modeling under a variety of hypotheses and initial conditions

High potential x-ray tube

An account is given of further development work on the high potential x-ray tube at the California Institute of Technology. Details of the construction of the tube and its housing are presented. The housing, which is a concrete structure erected on the floor of the high potential laboratory, makes it possible to operate and make observations at close range. The tube has been equipped with a hot cathode and a tungsten target, thus rendering it more suitable for spectrographic work. High speed cathode rays outside of the tube have been obtained by replacing the target by thin windows of mica or metal. Continuous operation is possible over a period of several hours at six hundred kilovolts and with a space current of three to four milliamperes. A comparison between different types of high potential x-ray tubes and of different methods of operation is contained in the discussion

Sustainability, Globalization, and the Energy Sector Europe in a Global Perspective

International audienceThis paper analyzes the socio-economic effects of energy sustainability challenges raised by oil depletion and climate change at the European and global level. We assess macroeconomic impacts at different time horizons over 2010-2100 and under different visions of the future of globalization . Fragmented capital markets affect the pace and direction of change and induce additional economic losses in the long term. Regionalized good markets have a positive effect in the long term since less intense international trade moderates the effects of fossil fuel constraints. A sustainable energy future will require implementing policies and measures that are able to (i) provide correct incentives for long-term investments by resorting to other signals than current market prices, (ii) incorporate sectoral measures that act complementarily to pricing schemes measures for sectors confronted with biased agents’ behaviors or strong inertias, (iii) foster globalization patterns that are consistent with energy sustainability objectives. The challenge consists in articulating the objectives and the instruments of these different policy and measures triggering the transition towards sustainable future

Nonlinear Outcome of Gravitational Instability in Disks with Realistic Cooling

We consider the nonlinear outcome of gravitational instability in optically thick disks with a realistic cooling function. We use a numerical model that is local, razor-thin, and unmagnetized. External illumination is ignored. Cooling is calculated from a one-zone model using analytic fits to low temperature Rosseland mean opacities. The model has two parameters: the initial surface density Sigma_0 and the rotation frequency Omega. We survey the parameter space and find: (1) The disk fragments when t_c,eff Omega = 1, where t_c,eff is an effective cooling time defined as the average internal energy of the model divided by the average cooling rate. This is consistent with earlier results that used a simplified cooling function. (2) The initial cooling time t_c0 or a uniform disk with Q = 1 can differ by orders of magnitude from t_c,eff in the nonlinear outcome. The difference is caused by sharp variations in the opacity with temperature. The condition t_c0 Omega = 1 therefore does not necessarily indicate where fragmentation will occur. (3) The largest difference between t_c,eff and t_c0 is near the opacity gap, where dust is absent and hydrogen is largely molecular. (4) In the limit of strong illumination the disk is isothermal; we find that an isothermal version of our model fragments for Q < 1.4. Finally, we discuss some physical processes not included in our model, and find that most are likely to make disks more susceptible to fragmentation. We conclude that disks with t_c,eff Omega < 1 do not exist.Comment: 30 pages, 12 figure

On type-I migration near opacity transitions. A generalized Lindblad torque formula for planetary population synthesis

We give an expression for the Lindblad torque acting on a low-mass planet embedded in a protoplanetary disk that is valid even at locations where the surface density or temperature profile cannot be approximated by a power law, such as an opacity transition. At such locations, the Lindblad torque is known to suffer strong deviation from its standard value, with potentially important implications for type I migration, but the full treatment of the tidal interaction is cumbersome and not well suited to models of planetary population synthesis. The expression that we propose retains the simplicity of the standard Lindblad torque formula and gives results that accurately reproduce those of numerical simulations, even at locations where the disk temperature undergoes abrupt changes. Our study is conducted by means of customized numerical simulations in the low-mass regime, in locally isothermal disks, and compared to linear torque estimates obtained by summing fully analytic torque estimates at each Lindblad resonance. The functional dependence of our modified Lindblad torque expression is suggested by an estimate of the shift of the Lindblad resonances that mostly contribute to the torque, in a disk with sharp gradients of temperature or surface density, while the numerical coefficients of the new terms are adjusted to seek agreement with numerics. As side results, we find that the vortensity related corotation torque undergoes a boost at an opacity transition that can counteract migration, and we find evidence from numerical simulations that the linear corotation torque has a non-negligible dependency upon the temperature gradient, in a locally isothermal disk.Comment: Appeared in special issue of "Celestial Mechanics and Dynamical Astronomy" on Extrasolar Planetary System

Eccentricity of radiative discs in close binary-star systems

Discs in binaries have a complex behavior because of the perturbations of the companion star. Planet formation in binary-star systems both depend on the companion star parameters and on the properties of the circumstellar disc. An eccentric disc may increase the impact velocity of planetesimals and therefore jeopardize the accumulation process. We model the evolution of discs in close binaries including the effects of self-gravity and adopting different prescriptions to model the disc's radiative properties. We focus on the dynamical properties and evolutionary tracks of the discs. We use the hydrodynamical code FARGO and we include in the energy equation heating and cooling effects. Radiative discs have a lower disc eccentricity compared to locally isothermal discs with same temperature profile. As a consequence, we do not observe the formation of an internal elliptical low density region as in locally isothermal disc models. However, the disc eccentricity depends on the disc mass through the opacities. Akin to locally isothermal disc models, self-gravity forces the disc's longitude of pericenter to librate about a fixed orientation with respect to the binary apsidal line ($\pi$). The disc's radiative properties play an important role in the evolution of discs in binaries. A radiative disc has an overall shape and internal structure that are significantly different compared to a locally isothermal disc with same temperature profile. This is an important finding both for describing the evolutionary track of the disc during its progressive mass loss, and for planet formation since the internal structure of the disc is relevant for planetesimals growth in binary systems. The non-symmetrical distribution of mass in these discs causes large eccentricities for planetesimals that may affect their growth.Comment: accepted for publication in A&A (abstract truncated to comply with astro-ph rules

Analytic solutions to the accretion of a rotating finite cloud towards a central object - II. Schwarzschild spacetime

We construct a general relativistic model for the accretion flow of a rotating finite cloud of non-interacting particles infalling onto a Schwarzschild black hole. The streamlines start at a spherical shell, where boundary conditions are fixed, and are followed down to the point at which they either cross the black hole horizon or become incorporated into an equatorial thin disc. Analytic expressions for the streamlines and the velocity field are given, in terms of Jacobi elliptic functions, under the assumptions of stationarity and ballistic motion. A novel approach allows us to describe all of the possible types of orbit with a single formula. A simple numerical scheme is presented for calculating the density field. This model is the relativistic generalisation of the Newtonian one developed by Mendoza, Tejeda, Nagel, 2009 and, due to its analytic nature, it can be useful in providing a benchmark for general relativistic hydrodynamical codes and for exploring the parameter space in applications involving accretion onto black holes when the approximations of steady state and ballistic motion are reasonable ones.Comment: 12 pages, 6 figures, references and minor changes added to match version accepted for publication in MNRA

Characterizing the velocity field in hydrodynamical simulations of low-mass star formation using spectral line profiles

When low-mass stars form, the collapsing cloud of gas and dust goes through several stages which are usually characterized by the shape of their spectral energy distributions. Such classification is based on the cloud morphology only and does not address the dynamical state of the object. In this paper we investigate the initial cloud collapse and subsequent disk formation through the dynamical behavior as reflected in the sub-millimeter spectral emission line profiles. If a young stellar object is to be characterized by its dynamical structure it is important to know how accurately information about the velocity field can be extracted and which observables provide the best description of the kinematics. Of particular interest is the transition from infalling envelope to rotating disk, because this provides the initial conditions for the protoplanetary disk, such as mass and size. We use a hydrodynamical model, describing the collapse of a core and formation of a disk, to produce synthetic observables which we compare to calculated line profiles of a simple parameterized model. Because we know the velocity field from the hydrodynamical simulation we can determine in a quantitative way how well our best-fit parameterized velocity field reproduces the original. We use a molecular line excitation and radiation transfer code to produce spectra of both our hydro dynamical simulation as well as our parameterized model. We find that information about the velocity field can reasonably well be derived by fitting a simple model to either single-dish lines or interferometric data, but preferentially by using a combination of the two. Our result shows that it is possible to establish relative ages of a sample of young stellar objects using this method, independently of the details of the hydrodynamical model.Comment: 12 pages, 11 figures, accepted for publication in A&A on June 1

Database model and specification of GermOnline Release 2.0, a cross-species community annotation knowledgebase on germ cell differentiation

Summary: GermOnline is a web-accessible relational database that enables life scientists to make a significant and sustained contribution to the annotation of genes relevant for the fields of mitosis, meiosis, germ line development and gametogenesis across species. This novel approach to genome annotation includes a platform for knowledge submission and curation as well as microarray data storage and visualization hosted by a global network of servers. Availability: The database is accessible at http://www.germonline.org/. For convenient world-wide access we have set up a network of servers in Europe (http://germonline.unibas.ch/; http://germonline.igh.cnrs.fr/), Japan (http://germonline.biochem.s.u-tokyo.ac.jp/) and USA (http://germonline.yeastgenome.org/). Supplementary information: Extended documentation of the database is available through the link ‘About GermOnline' at the website

A deeply embedded young protoplanetary disk around L1489 IRS observed by the submillimeter array

Circumstellar disks are expected to form early in the process that leads to the formation of a young star, during the collapse of the dense molecular cloud core. It is currently not well understood at what stage of the collapse the disk is formed or how it subsequently evolves. We aim to identify whether an embedded Keplerian protoplanetary disk resides in the L1489 IRS system. Given the amount of envelope material still present, such a disk would respresent a very young example of a protoplanetary disk. Using the Submillimeter Array (SMA) we have observed the HCO$^+$ $J=$ 3--2 line with a resolution of about 1$''$. At this resolution a protoplanetary disk with a radius of a few hundred AUs should be detectable, if present. Radiative transfer tools are used to model the emission from both continuum and line data. We find that these data are consistent with theoretical models of a collapsing envelope and Keplerian circumstellar disk. Models reproducing both the SED and the interferometric continuum observations reveal that the disk is inclined by 40$^\circ$ which is significantly different to the surrounding envelope (74$^\circ$). This misalignment of the angular momentum axes may be caused by a gradient within the angular momentum in the parental cloud or if L1489 IRS is a binary system rather than just a single star. In the latter case, future observations looking for variability at sub-arcsecond scales may be able to constrain these dynamical variations directly. However, if stars form from turbulent cores, the accreting material will not have a constant angular momentum axis (although the average is well defined and conserved) in which case it is more likely to have a misalignment of the angular momentum axes of the disk and the envelope.Comment: 11 pages, 13 figures, accepted by A&