A Far-Infrared Survey of Molecular Cloud Cores

We present a catalogue of molecular cloud cores drawn from high latitude, medium opacity clouds, using the all-sky IRAS Sky Survey Atlas (ISSA) images at 60 and 100~$\mu$m. The typical column densities of the cores are $N(H_2)\sim 3.8 \times 10^{21}$cm$^{-2}$ and the typical volume densities are $n(H_2) \sim 2 \times 10^3$cm$^{-3}$. They are therefore significantly less dense than many other samples obtained in other ways. Those cloud cores with IRAS point sources are seen to be already forming stars, but this is found to be only a small fraction of the total number of cores. The fraction of the cores in the protostellar stage is used to estimate the prestellar timescale - the time until the formation of a hydrostatically supported protostellar object. We argue, on the basis of a comparison with other samples, that a trend exists for the prestellar lifetime of a cloud core to decrease with the mean column density and number density of the core. We compare this with model predictions and show that the data are consistent with star formation regulated by the ionisation fraction.Comment: 13 pages, 7 figure

How Big is Big Enough? Firm Size as a Barrier to Exporting in South Carolina's Manufacturing Sector

International Relations/Trade,

Tidal Barrier and the Asymptotic Mass of Proto Gas-Giant Planets

Extrasolar planets found with radial velocity surveys have masses ranging from several Earth to several Jupiter masses. While mass accretion onto protoplanetary cores in weak-line T-Tauri disks may eventually be quenched by a global depletion of gas, such a mechanism is unlikely to have stalled the growth of some known planetary systems which contain relatively low-mass and close-in planets along with more massive and longer period companions. Here, we suggest a potential solution for this conundrum. In general, supersonic infall of surrounding gas onto a protoplanet is only possible interior to both of its Bondi and Roche radii. At a critical mass, a protoplanet's Bondi and Roche radii are equal to the disk thickness. Above this mass, the protoplanets' tidal perturbation induces the formation of a gap. Although the disk gas may continue to diffuse into the gap, the azimuthal flux across the protoplanets' Roche lobe is quenched. Using two different schemes, we present the results of numerical simulations and analysis to show that the accretion rate increases rapidly with the ratio of the protoplanet's Roche to Bondi radii or equivalently to the disk thickness. In regions with low geometric aspect ratios, gas accretion is quenched with relatively low protoplanetary masses. This effect is important for determining the gas-giant planets' mass function, the distribution of their masses within multiple planet systems around solar type stars, and for suppressing the emergence of gas-giants around low mass stars

Potential climate forcing of land use and land cover change

Pressure on land resources is expected to increase as global population continues to climb and the world becomes more affluent, swelling the demand for food. Changing climate may exert additional pressures on natural lands as present-day productive regions may shift, or soil quality may degrade, and the recent rise in demand for biofuels increases competition with edible crops for arable land. Given these projected trends there is a need to understand the global climate impacts of land use and land cover change (LULCC). Here we quantify the climate impacts of global LULCC in terms of modifications to the balance between incoming and outgoing radiation at the top of the atmosphere (radiative forcing, RF) that are caused by changes in long-lived and short-lived greenhouse gas concentrations, aerosol effects, and land surface albedo. We attribute historical changes in terrestrial carbon storage, global fire emissions, secondary organic aerosol emissions, and surface albedo to LULCC using simulations with the Community Land Model version 3.5. These LULCC emissions are combined with estimates of agricultural emissions of important trace gases and mineral dust in two sets of Community Atmosphere Model simulations to calculate the RF of changes in atmospheric chemistry and aerosol concentrations attributed to LULCC. With all forcing agents considered together, we show that 40% (+/- 16 %) of the present-day anthropogenic RF can be attributed to LULCC. Changes in the emission of non-CO2 greenhouse gases and aerosols from LULCC enhance the total LULCC RF by a factor of 2 to 3 with respect to the LULCC RF from CO2 alone. This enhancement factor also applies to projected LULCC RF, which we compute for four future scenarios associated with the Representative Concentration Pathways. We attribute total RFs between 0.9 and 1.9 W m(-2) to LULCC for the year 2100 (relative to a preindustrial state). To place an upper bound on the potential of LULCC to alter the global radiation budget, we include a fifth scenario in which all arable land is cultivated by 2100. This theoretical extreme case leads to a LULCC RF of 3.9 W m(-2) (+/- 0.9 W m(-2)), suggesting that not only energy policy but also land policy is necessary to minimize future increases in RF and associated climate changes

The initial conditions of isolated star formation: IV - C18O observations and modelling of the pre-stellar core L1689B

We present C18O observations of the pre-stellar core L1689B, in the (J=3-2) and (J=2-1) rotational transitions, taken at the James Clerk Maxwell Telescope in Hawaii. We use a lambda-iteration radiative transfer code to model the data. We adopt a similar form of radial density profile to that which we have found in all pre-stellar cores, with a `flat' inner profile, steepening towards the edge, but we make the gradient of the `flat' region a free parameter. We find that the core is close to virial equilibrium, but there is tentative evidence for core contraction. We allow the temperature to vary with a power-law form and find we can consistently fit all of the CO data with an inverse temperature gradient that is warmer at the edge than the centre. However, when we combine the CO data with the previously published millimetre data we fail to find a simultaneous fit to both data-sets without additionally allowing the CO abundance to decrease towards the centre. This effect has been observed qualitatively many times before, as the CO freezes out onto the dust grains at high densities, but we quantify the effect. Hence we show that the combination of mm/submm continuum and spectral line data is a very powerful method of constraining the physical parameters of cores on the verge of forming stars.Comment: 10 pages. Accepted for publication in MNRA

On The Orbital Evolution of Jupiter Mass Protoplanet Embedded in A Self-Gravity Disk

We performed a series of hydro-dynamic simulations to investigate the orbital migration of a Jovian planet embedded in a proto-stellar disk. In order to take into account of the effect of the disk's self gravity, we developed and adopted an \textbf{Antares} code which is based on a 2-D Godunov scheme to obtain the exact Reimann solution for isothermal or polytropic gas, with non-reflecting boundary conditions. Our simulations indicate that in the study of the runaway (type III) migration, it is important to carry out a fully self consistent treatment of the gravitational interaction between the disk and the embedded planet. Through a series of convergence tests, we show that adequate numerical resolution, especially within the planet's Roche lobe, critically determines the outcome of the simulations. We consider a variety of initial conditions and show that isolated, non eccentric protoplanet planets do not undergo type III migration. We attribute the difference between our and previous simulations to the contribution of a self consistent representation of the disk's self gravity. Nevertheless, type III migration cannot be completely suppressed and its onset requires finite amplitude perturbations such as that induced by planet-planet interaction. We determine the radial extent of type III migration as a function of the disk's self gravity.Comment: 19 pages, 13 figure

Scalar--Flat Lorentzian Einstein--Weyl Spaces

We find all three-dimensional Einstein--Weyl spaces with the vanishing scalar curvatureComment: 4 page