A Spectroscopic Survey of Subarcsecond Binaries in the Taurus-Auriga Dark Cloud with the Hubble Space Telescope

We report the results of a spectroscopic survey of 20 close T Tauri binaries in the Taurus-Auriga dark cloud where the separations between primaries and their secondaries are less than the typical size of a circumstellar disk around a young star. Analysis of low-resolution and medium-resolution STIS spectra yields the stellar luminosities, reddenings, ages, masses, mass accretion rates, IR excesses, and emission line luminosities for each star in each pair. We examine the ability of IR color excesses, H-alpha equivalent widths, [O I] emission, and veiling to distinguish between weak emission and classical T Tauri stars. Four pairs have one cTTs and one wTTs; the cTTs is the primary in three of these systems. This frequency of mixed pairs among the close T Tauri binaries is similar to the frequency of mixed pairs in wider young binaries. Extinctions within pairs are usually similar; however, the secondary is more heavily reddened than the primary in some systems, where it may be viewed through the primary's disk. Mass accretion rates of primaries and secondaries are strongly correlated, and H-alpha luminosities, IR excesses, and ages also correlate within pairs. Primaries tend to have somewhat larger accretion rates than their secondaries do, and are typically slightly older than their secondaries according to three different sets of modern pre-main-sequence evolutionary tracks. Age differences for XZ Tau and FS Tau, systems embedded in reflection nebulae, are striking; the secondary in each pair is less massive but more luminous than the primary. The stellar masses of the UY Aur and GG Tau binaries measured from their rotating molecular disks are about 30% larger than the masses inferred from the spectra and evolutionary tracks

Design Considerations in the Development of a Modern Cavitation Tunnel

The specification and overall design of the new Australian Maritime College Cavitation Tunnel is presented. This facility has been funded under the Australian Government Major National Research Facilities Program as part of the Australian Maritime Hydrodynamics Research Centre (AMHRC). The AMHRC is a joint venture between the Australian Maritime College, the Defence Science and Technology Organisation and the University of Tasmania. The facility has been developed for naval hydrodynamics research with particular emphasis on the modelling of cavitating and turbulent flow physics. Development of circuit architecture and components are discussed in detail as well as ancillary systems. The facility’s specific capabilities include the ability to strictly control circuit water gas content (both dissolved and free), continuous high-volume injection and separation of incondensable gases, control of the boundary layer on one wall of the test section, and low background noise and vibration levels

GRAVITY: The AO-Assisted, Two-Object Beam-Combiner Instrument

We present the proposal for the infrared adaptive optics (AO) assisted, two-object, high-throughput, multiple-beam-combiner GRAVITY for the VLTI. This instrument will be optimized for phase-referenced interferometric imaging and narrow-angle astrometry of faint, red objects. Following the scientific drivers, we analyze the VLTI infrastructure, and subsequently derive the requirements and concept for the optimum instrument. The analysis can be summarized with the need for highest sensitivity, phase referenced imaging and astrometry of two objects in the VLTI beam, and infrared wavefront-sensing. Consequently our proposed instrument allows the observations of faint, red objects with its internal infrared wavefront sensor, pushes the optical throughput by restricting observations to K-band at low and medium spectral resolution, and is fully enclosed in a cryostat for optimum background suppression and stability. Our instrument will thus increase the sensitivity of the VLTI significantly beyond the present capabilities. With its two fibers per telescope beam, GRAVITY will not only allow the simultaneous observations of two objects, but will also push the astrometric accuracy for UTs to 10 micro-arcsec, and provide simultaneous astrometry for up to six baselines.Comment: 12 pages, to be published in the Proceedings of the ESO Workshop on "The Power of Optical/IR Interferometry: Recent Scientific Results and 2nd Generation VLTI Instrumentation", eds. F. Paresce, A. Richichi, A. Chelli and F. Delplancke, held in Garching, Germany, 4-8 April 200

Single stars in the Hyades open cluster. Fiducial sequence for testing stellar and atmospheric models

Age and mass determinations for isolated stellar objects remain model-dependent. While stellar interior and atmospheric theoretical models are rapidly evolving, we need a powerful tool to test them. Open clusters are good candidates for this role. We complement previous studies on the Hyades multiplicity by Lucky Imaging observations with the AstraLux Norte camera. This allows us to exclude possible binary and multiple systems with companions outside 2--7 AU separation and to create a "single-star sequence" for the Hyades. The sequence encompasses 250 main-sequence stars ranging from A5V to M6V. Using the "Tool for Astrophysical Data Analysis" (TA-DA), we create various theoretical isochrones applying different combinations of interior and atmospheric models. We compare the isochrones with the observed Hyades single-star sequence on J vs. J - K_s, J vs. J - H and K_s vs. H - K_s color-magnitude diagrams. As a reference we also compute absolute fluxes and magnitudes for all stars from X-ray to mid-infrared based on photometric measurements available in the literature(ROSAT X-ray, GALEX UV, APASS gri, 2MASS JHK_s, and WISE W1 to W).We find that combinations of both PISA and DARTMOUTH stellar interior models with BT-Settl 2010 atmospheric models describe the observed sequence well. The full sequence covers the mass range 0.13 to 2.3 Msun, and effective temperatures between 3060 K and 8200 K. Within the measurement uncertainties, the current generation of models agree well with the single-star sequence. The primary limitations are the uncertainties in the measurement of the distance to individual Hyades members, and uncertainties in the photometry. Additionally, a small (~0.05 mag) systematic offset can be noted on J vs. J - K and K vs. H - K diagrams - the observed sequence is shifted to redder colors from the theoretical predictions.Comment: 6 pages, 2 figures, 1 table. The extended version of the table will be available online soon. Accepted for publication in Astronomy & Astrophysic

Numerical Thermal Analysis and 2-D CFD Evaluation Model for An Ideal Cryogenic Regenerator

Regenerative cryocoolers such as Stirling, Gifford–McMahon, and pulse tube cryocoolers possess great merits such as small size, low cost, high reliability, and good cooling capacity. These merits led them to meet many IR and superconducting based application requirements. The regenerator is a vital element in these closed-cycle cryocoolers, but the overall performance depends strongly on the effectiveness of the regenerator. This paper presents a one-dimensional numerical analysis for the idealized thermal equations of the matrix and the working gas inside the regenerator. The algorithm predicts the temperature profiles for the gas during the heating and cooling periods, along with the matrix nodal temperatures. It examines the effect of the regenerator’s length and diameter, the matrix’s geometric parameters, the number of heat transfer units, and the volumetric flow rate, on the performance of an ideal regenerator. This paper proposes a 2D axisymmetric CFD model to evaluate the ideal regenerator model and to validate its findings

Pulse Tube Cryocooler: Phasor Analysis and One-Dimensional Numerical Simulation

An analysis of the detailed operation for the tube element is proposed for an orifice pulse tube cryocooler. This is achieved through phasor analysis using basic thermodynamic relations to estimate the approximated cooling power associated with this machine. Moreover, the effect of the phase shift angle is illustrated by forming an analogy between the phase shift mechanism and a series RLC circuit model. Next, a one-dimensional model based on the conservation equations of mass and energy is presented; the reduced model is solved numerically, for the temperature and velocity of the gas along the tube, to determine the mass flow and time-averaged enthalpy flows at the cold and hot ends of the tube. The findings from the one-dimensional analysis are compared with the phasor analysis results and validated by comparison with similar studies in the literature

Computational and Experimental Investigation of Flow Around a 3-1 Prolate Spheroid

The flow around a 3-1 prolate spheroid near the critical Reynolds number is investigated experimentally and numerically. This work was conducted as part of a larger project to examine the flow around Unmanned Underwater Vehicles. The experimental investigation has been performed in a water tunnel at the Australian Maritime College. Fast response pressure probes and a 3-D automated traverse have been developed to investigate the state of the boundary layer. A commercial CFD code has been modified to allow the experimentally determined boundary layer state to be included in the computation. Qualitative and quantitative comparisons between the measured and calculated results are discussed. The tests on the spheroid were conducted within a Reynolds numbers range of 0 .6 ×106 to 4×106. The results presented here are for an incidence of 10

A Force Balance to Measure the Total Drag of Biofilms on Test Plates

A floating force balance has been designed and integrated into the working section of a to enable the measurement of total drag on test plates, which form part of the tunnel wall. Measurements completed include a calibration of the rig using a smooth acrylic plate, a smooth painted plate, and an artificially roughened plate. The painted plate and rough plate have also been studied with biofilms attached to their surface. The water tunnel and total drag rig have been built specifically to allow the detailed investigation of freshwater biofilm effects have on the flow through hydraulic conduits. Calibration results show that useful information can be obtained by using the force balance, particularly in association with other measurement techniques. Research into the effects of biofilms showsthat large increases in friction and effective roughness can be expected

The GRAVITY fringe tracker: correlation between optical path residuals and atmospheric parameters

After the first year of observations with the GRAVITY fringe tracker, we compute correlations between the optical path residuals and atmospheric and astronomical parameters. The median residuals of the optical path residuals are 180 nm on the ATs and 270 nm on the UTs. The residuals are uncorrelated with the target magnitudes for Kmag below 5.5 on ATs (9 on UTs). The correlation with the coherence time is however extremely clear, with a drop-off in fringe tracking performance below 3 ms.Comment: submitted to SPIE Astronomical Telescopes & Instrumentation 201

The Peak Brightness and Spatial Distribution of AGB Stars Near the Nucleus of M32

The bright stellar content near the center of the Local Group elliptical galaxy M32 is investigated with 0.12 arcsec FWHM H and K images obtained with the Gemini Mauna Kea telescope. Stars with K = 15.5, which are likely evolving near the tip of the asymptotic giant branch (AGB), are resolved to within 2 arcsec of the nucleus, and it is concluded that the peak stellar brightness near the center of M32 is similar to that in the outer regions of the galaxy. Moreover, the projected density of bright AGB stars follows the visible light profile to within 2 arcsec of the nucleus, indicating that the brightest stars are well mixed throughout the galaxy. Thus, there is no evidence for an age gradient, and the radial variations in spectroscopic indices and ultraviolet colors that have been detected previously must be due to metallicity and/or some other parameter. We suggest that either the bright AGB stars formed as part of a highly uniform and coherent galaxy-wide episode of star formation, or they originated in a separate system that merged with M32.Comment: 9 pages of text, 3 figures. ApJ (Letters) in pres