The Decay of Accreting Triple Systems as Brown Dwarf Formation Scenario

We investigate the dynamical decay of non-hierarchical accreting triple systems and its implications on the ejection model as Brown Dwarf formation scenario. A modified chain-regularization scheme is used to integrate the equations of motion, that also allows for mass changes over time as well as for momentum transfer from the accreted gas mass onto the bodies. We integrate an ensemble of triple systems within a certain volume with different accretion rates, assuming several prescriptions of how momentum is transferred onto the bodies. We follow their evolution until the systems have decayed. We analyze the end states and decay times of these systems and determine the fraction of Brown Dwarfs formed, their escape speeds as well as the semi-major axis distribution of the formed Brown Dwarf binaries. We find that the formation probability of Brown Dwarfs depends strongly on the assumed momentum transfer which is related to the motion of the gas. Due to ongoing accretion and consequent shrinkage of the systems, the median escape velocity is increased by a factor of 2 and the binary separations are decreased by a factor of 5 compared with non-accreting systems. Furthermore, the obtained semi-major axis distribution drops off sharply to either side of the median, which is also supported by observations. We conclude that accretion and momentum transfer of accreted gas during the dynamical decay of triple systems is able to produce the observed distribution of close binary Brown Dwarfs, making the ejection model a viable option as Brown Dwarf formation scenario.Comment: 31 pages, 8 figures, accepted for publication in Ap

A Survey for Circumstellar Disks Around Young Substellar Objects

(Abridged) We have completed the first systematic survey for disks around spectroscopically identified young brown dwarfs and very low mass stars. We have obtained L'-band (3.8 um) imaging for 38 very cool objects in IC 348 and Taurus. Our targets span spectral types from M6 to M9.5 (~100 to ~15 Mjup). Using the objects' measured spectral types and extinctions, we find that most of our sample (77%+/-15%) possess intrinsic IR excesses, indicative of disks. Because the excesses are modest, conventional analyses using only IR colors would have missed most of the sources with excesses. The observed IR excesses are correlated with Halpha emission, consistent with a common accretion disk origin. The excesses can be explained by disk reprocessing of starlight alone; the implied accretion rates are at least an order of magnitude below typical values for classical T Tauri stars. The observed distribution of IR excesses suggests the presence of inner disk holes. The disk frequency appears to be independent of the mass and age. In the same star-forming regions, disks around brown dwarfs are at least as long-lived (~3 Myr) as disks around the T Tauri stars. Altogether, the frequency and properties of young circumstellar disks appear to be similar from the stellar regime down to the substellar and planetary-mass regime. This provides prima facie evidence of a common origin for most stars and brown dwarfs.Comment: ApJ, in press, 28 pages. Minor change to the online, abridged version of the abstract. No change to the actual pape

First Detection of Millimeter Dust Emission from Brown Dwarf Disks

We report results from the first deep millimeter continuum survey targeting Brown Dwarfs (BDs). The survey led to the first detection of cold dust in the disks around two young BDs (CFHT-BD-Tau 4 and IC348 613), with deep JCMT and IRAM observations reaching flux levels of a few mJy. The dust masses are estimated to be a few Earth masses assuming the same dust opacities as usually applied to TTauri stars.Comment: 5 pages, accepted for ApJ

Effects of Spin on Tennis Ball Aerodynamics: An Experimental and Computational Study

The aerodynamic behaviour of a tennis ball is very complex and significantly differs from other sports balls due to its surface structures (fuzz, seam orientation etc). Relatively high rotational speeds (spin) make the aerodynamic properties of tennis balls even more complex. Although several studies have been conducted on drag and lift in steady state condition (no spin involved) by the author and others, little or no studies have been conducted on spin effects. The so called Magnus effect on a sphere is well known in fluid mechanics. It is believed that the spinning can affect aerodynamic drag and lift of a tennis ball thus the motion and flight path of the ball. Therefore, the primary objectives of this work are to study the spin effects using both experimental and computational methods. In order to achieve these objectives, a series of tennis balls were used to measure their aerodynamics forces as a function of wind speeds, seam orientation and spins. The experimental study was conducted in the RMIT Industrial Wind Tunnel. A computational study of a simplified tennis ball was also studied using commercial software ‘FLUENT’. The CFD results were compared with the experimental findings. Flow around the ball was visualised with smoke

Local magnetometry at high fields and low temperatures using InAs Hall sensors

We characterize the temperature (0.3⩽T⩽300 K), magnetic field(0⩽H⩽80 kOe), and thickness (0.1, 0.5, and 2.5 μm) dependence of the Hall response of high purity InAs epilayers grown using metalorganic chemical vapor deposition. The high sensitivity, linearity, and temperature independence of the response make them attractive for local Hall probe magnetometry, and uniquely qualified for high field applications below liquid helium temperatures. As a stringent test of performance, we use a six element micron-sized array to monitor the internal field gradient during vortex avalanches at milliKelvin temperatures in a single crystal of YBa_2Cu_3O_(7−δ)

Ventilation rates in recently constructed U.S. school classrooms

Low ventilation rates (VRs) in schools have been associated with absenteeism, poorer academic performance, and teacher dissatisfaction. We measured VRs in 37 recently constructed or renovated and mechanically ventilated U.S. schools, including LEED and EnergyStarâ certified buildings, using CO2 and the steadyâ state, buildâ up, decay, and transient mass balance methods. The transient mass balance method better matched conditions (specifically, changes in occupancy) and minimized biases seen in the other methods. During the school day, air change rates (ACRs) averaged 2.0±1.3 hourâ 1, and only 22% of classrooms met recommended minimum ventilation rates. HVAC systems were shut off at the school day close, and ACRs dropped to 0.21±0.19 hourâ 1. VRs did not differ by building type, although costâ cutting and comfort measures resulted in low VRs and potentially impaired IAQ. VRs were lower in schools that used unit ventilators or radiant heating, in smaller schools and in larger classrooms. The steadyâ state, buildâ up, and decay methods had significant limitations and biases, showing the need to confirm that these methods are appropriate. Findings highlight the need to increase VRs and to ensure that energy saving and comfort measures do not compromise ventilation and IAQ.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138411/1/ina12384.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138411/2/ina12384_am.pd