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

Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk

We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau de Bure interferometer in the isotopologues of CO, and in continuum at 3 and 1.3 mm. Instead of being centrally peaked, the continuum emission is dominated by a bright, asymmetric (spiral-like) feature at about 140 AU from the central star. The large scale molecular structure suggests the AB Aur disk is inclined between 23 and 43 degrees, but the strong asymmetry of the continuum and molecular emission prevents an accurate determination of the inclination of the inner parts. We find significant non-Keplerian motion, with a best fit exponent for the rotation velocity law of 0.41 +/- 0.01, but no evidence for radial motions. The disk has an inner hole about 70 AU in radius. The disk is warm and shows no evidence of depletion of CO. The dust properties suggest the dust is less evolved than in typical T Tauri disks. Both the spiral-like feature and the departure from purely Keplerian motions indicates the AB Aur disk is not in quasi-equilibrium. Disk self-gravity is insufficient to create the perturbation. This behavior may be related either to an early phase of star formation in which the Keplerian regime is not yet fully established and/or to a disturbance of yet unknown origin. An alternate, but unproven, possibility is that of a low mass companion located about 40 AU from AB Aur.Comment: 10 pages, 5 figures, accepted for publication in Astronomy & Astrophysic

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

Substellar companions and isolated planetary mass objects from protostellar disc fragmentation

Self-gravitating protostellar discs are unstable to fragmentation if the gas can cool on a time scale that is short compared to the orbital period. We use a combination of hydrodynamic simulations and N-body orbit integrations to study the long term evolution of a fragmenting disc with an initial mass ratio to the star of M_disc/M_star = 0.1. For a disc which is initially unstable across a range of radii, a combination of collapse and subsequent accretion yields substellar objects with a spectrum of masses extending (for a Solar mass star) up to ~0.01 M_sun. Subsequent gravitational evolution ejects most of the lower mass objects within a few million years, leaving a small number of very massive planets or brown dwarfs in eccentric orbits at moderately small radii. Based on these results, systems such as HD 168443 -- in which the companions are close to or beyond the deuterium burning limit -- appear to be the best candidates to have formed via gravitational instability. If massive substellar companions originate from disc fragmentation, while lower-mass planetary companions originate from core accretion, the metallicity distribution of stars which host massive substellar companions at radii of ~1 au should differ from that of stars with lower mass planetary companions.Comment: 5 pages, accepted for publication in MNRA

Factors deterring schools from mixed attainment teaching practice

Mixed-attainment teaching has strong support from research and yet English schools are far more likely to teach students in ‘ability’ groups. Although research has considered some of the specific benefits of mixed-attainment grouping, there has been little attention to the reasons schools avoid it. This article explores data from the pilot and recruitment phases of a large-scale study into grouping practices and seeks to identify reasons for the low rate of mixed attainment grouping in English secondary schools. We report on our struggle to recruit schools, and explore the different explanations provided by teachers as to why mixed attainment practice is seen as problematic. The difficulties are characterised as a vicious circle where schools are deterred by a paucity of exemplars and resources and the educational climate is characterised as fearful, risk-averse and time-poor. Suggestions are made as to strategies to support schools in taking up mixed attainment practices

Globalisation from Above? Corporate Social Responsibility, the Workers' Party and the Origins of the World Social Forum

In its assessment of the origins and early development of the World Social Forum this article challenges traditional understandings of the Forum as representing ‘globalisation from below’. By tracing the intricate relations among elements of business, civil society, and the Workers’ Party in the first years of the Forum, this article reveals the major role played by a corporate movement stemming from the Brazilian democratisation process in the 1980s, and how this combined with the transformed agenda of the Workers’ Party as it gained higher political offices to constrain the Forum’s activities from the outset. In so doing, this article challenges not only widespread conceptions of the Forum as a counter‐hegemonic alternative but also current critiques concerning its subsequent limitations. Furthermore, it reveals how traditional understandings of the World Social Forum and of global civil society are underpinned by flawed assumptions which typecast political activities in the global ‘South’

'Another World is Possible': A Study of Participants at Australian Alter-Globalisation Social Forums

The past decade has seen the emergence of a mass 'alter-globalisation' movement in many regions of the world. One element in this movement has been the World Social Forum and its continental, regional, national and local spin-offs. In the first half of this article, I provide a critical analysis of the social forum experience, particularly the World Social Forum, and outline both those aspects of the experience that are commonly agreed as successes as well as those that are frequently held to be their failings or limitations. In the second half of the article, I report on a survey of the participants at two Australian social forums in 2004 which details their backgrounds, motivations, attitudes, experience, and ambitions. Comparison is made with their closest parallels - the activists from the new social movements of the 1970s and 1980s previously examined by Offe, Touraine, Melucci and others