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

Vertically Self-Gravitating ADAFs in the Presence of Toroidal Magnetic Field

Force due to the self-gravity of the disc in the vertical direction is considered to study its possible effects on the structure of a magnetized advection-dominated accretion disc. We present steady-sate self similar solutions for the dynamical structure of such a type of the accretion flows. Our solutions imply reduced thickness of the disc because of the self-gravity. It also imply that the thickness of the disc will increase by adding the magnetic field strength.Comment: Accepted for publication in Astrophysics and Space Science

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

Exploratory investigation of drivers of attainment in ethnic minority adult learners

There is evidence that ethnic minority learners in further education in England either under-achieve or are under-represented because they face various inhibitors connected to their ethnicity. Motivators may be in place, however, which increase attainment specifically for some ethnic groups. This exploratory study intends to examine what works and what does not among South Asian (Pakistani and Bangladeshi heritage) females and black male adult learners in FE. A mixed-method study was carried out using questionnaires and focus groups with 68 ethnic minority students in three further education colleges in England. The combination of the results showed that being a member of a minority culture and/or religion may increase feelings of isolation in academic settings; teaching staff who are knowledgeable about the student’s culture increase feelings of inclusion; and role models are crucially important. Results are discussed in light of British data of school experiences of ethnic minority learners

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&

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

Evolution of oxygen isotopic composition in the inner solar nebula

Changes in the chemical and isotopic composition of the solar nebula with time are reflected in the properties of different constituents that are preserved in chondritic meteorites. CR carbonaceous chondrites are among the most primitive of all chondrite types and must have preserved solar nebula records largely unchanged. We have analyzed the oxygen and magnesium isotopes in a range of the CR constituents of different formation temperatures and ages, including refractory inclusions and chondrules of various types. The results provide new constraints on the time variation of the oxygen isotopic composition of the inner (<5 AU) solar nebula - the region where refractory inclusions and chondrules most likely formed. A chronology based on the decay of short-lived 26Al (t1/2 ~ 0.73 Ma) indicates that the inner solar nebula gas was 16O-rich when refractory inclusions formed, but less than 0.8 Ma later, gas in the inner solar nebula became 16O-poor and this state persisted at least until CR chondrules formed ~1-2 Myr later. We suggest that the inner solar nebula became 16O-poor because meter-size icy bodies, which were enriched in 17,18O due to isotopic self-shielding during the ultraviolet photo dissociation of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated outside the snowline, drifted rapidly towards the Sun, and evaporated at the snowline. This led to significant enrichment in 16O-depleted water, which then spread through the inner solar system. Astronomical studies of the spatial and/or temporal variations of water abundance in protoplanetary disks may clarify these processes.Comment: 27 pages, 5 figure

Sub-Keplerian accretion onto circumstellar disks

Models of the formation, evolution and photoevaporation of circumstellar disks are an essential ingredient in many theories of the formation of planetary systems. The ratio of disk mass over stellar mass in the circumstellar phase of a disk is largely determined by the angular momentum of the original cloud core from which the system was formed. While full 3D or 2D axisymmetric hydrodynamical models of accretion onto the disk automatically treat all aspects of angular momentum, this is not so trivial for 1D and semi-2D viscous disk models. Since 1D and semi-2D disk models are still very useful for long-term evolutionary modelling of disks with relatively little numerical effort, we investigate how the 2D nature of accretion affects the formation and evolution of the disk in such models. A proper treatment of this problem requires a correction for the sub-Keplerian velocity at which accretion takes place. We develop an update of our semi-2D time-dependent disk evolution model to properly treat the effects of sub-Keplerian accretion. The new model also accounts for the effects of the vertical extent of the disk on the accretion streamlines from the envelope. The disks produced with the new method are smaller than those obtained previously, but their mass is mostly unchanged. The new disks are a few degrees warmer in the outer parts, so they contain less solid CO. Otherwise, the results for ices are unaffected. The 2D treatment of the accretion results in material accreting at larger radii, so a smaller fraction comes close enough to the star for amorphous silicates to be thermally annealed into crystalline form. The lower crystalline abundances thus predicted correspond more closely to observed abundances than did earlier model predictions. We argue that thermal annealing followed by radial mixing must be responsible for at least part of the observed crystalline material.Comment: Accepted by A&A; 10 pages, 6 figure