The Kinematic and Spatial Deployment of Compact, Isolated High-Velocity Clouds

We have identified a class of high-velocity clouds which are compact and apparently isolated. The clouds are compact in that they have angular sizes less than 2 degrees FWHM. They are isolated in that they are separated from neighboring emission by expanses where no emission is seen to the detection limit of the available data. Candidates for inclusion in this class were extracted from the Leiden/Dwingeloo HI survey of Hartmann & Burton and from the Wakker & van Woerden catalogue of high-velocity clouds. The candidates were subject to independent confirmation using either the 25-meter telescope in Dwingeloo or the 140-foot telescope in Green Bank. We argue that the resulting list, even if incomplete, is sufficiently representative of the ensemble of compact, isolated HVCs - CHVCs - that the characteristics of their disposition on the sky, and of their kinematics, are revealing of some physical aspects of the class. The CHVCs are in fact distributed quite uniformly across the sky. A global search for the reference frame which minimizes the velocity dispersion of the ensemble returns the Local Group Standard of Rest with high confidence. The CHVCs are not stationary with respect to this reference frame but have a mean infall velocity of 100 km/s. These properties are strongly suggestive of a population which has as yet had little interaction with the more massive Local Group members. At a typical distance of about 1 Mpc these objects would have sizes of about 15 kpc and gas masses, M_HI, of a few times 10^7 M_Sun, corresponding to those of (sub-)dwarf galaxies. (abridged)Comment: 13 page LaTeX, requires aa.cls and rotate.sty, 5 GIF figures. Accepted for publication in A&

Validation of the communications link analysis and simulation system (CLASS)

CLASS (Communication Link Analysis and Simulation System) is a software package developed for NASA to predict the communication and tracking performance of the Tracking and Data Relay Satellite System (TDRSS) services. The methods used to verify CLASS are described. The usefulness of a software tool such as CLASS depends strongly on the reliability and accuracy of the results it produces. For this reason, considerable attention was paid to validation throughout the CLASS development

Orbiter global positioning system design and Ku-band problems investigation, exhibit B, revision 1

The LinCom effort in supporting the JSC study of the use of the Global Positioning System (GPS) on the space shuttle and in Ku-band problem investigation is documented. LinCom was tasked to evaluate system implementation, performance, and integration aspects of the shuttle GPS and to provide independent technical assessment of reports submitted to JSC regarding integration studies, system studies and navigation analyses

Shuttle Ku-band signal design study

Carrier synchronization and data demodulation of Unbalanced Quadriphase Shift Keyed (UQPSK) Shuttle communications' signals by optimum and suboptimum methods are discussed. The problem of analyzing carrier reconstruction techniques for unbalanced QPSK signal formats is addressed. An evaluation of the demodulation approach of the Ku-Band Shuttle return link for UQPSK when the I-Q channel power ratio is large is carried out. The effects that Shuttle rocket motor plumes have on the RF communications are determined also. The effect of data asymmetry on bit error probability is discussed

QSO Absorption Line Constraints on Intragroup High-Velocity Clouds

We show that the number statistics of moderate redshift MgII and Lyman limit absorbers may rule out the hypothesis that high velocity clouds are infalling intragroup material.Comment: 4 pages, no figures; submitted to Astrophysical Journal Letters; revised version, more general and includes more about Braun and Burton CHVC

A search for radio pulsars and fast transients in M31 using the WSRT

We present the results of the most sensitive and comprehensive survey yet undertaken for radio pulsars and fast transients in the Andromeda galaxy (M31) and its satellites, using the Westerbork Synthesis Radio Telescope (WSRT) at a central frequency of 328 MHz. We used the WSRT in a special configuration called 8gr8 (eight-grate) mode, which provides a large instantaneous field-of-view, about 5 square degrees per pointing, with good sensitivity, long dwell times (up to 8 hours per pointing), and good spatial resolution (a few arc minutes) for locating sources. We have searched for both periodicities and single pulses in our data, aiming to detect bright, persistent radio pulsars and rotating radio transients (RRATs) of either Galactic or extragalactic origin. Our searches did not reveal any confirmed periodic signals or bright single bursts from (potentially) cosmological distances. However, we do report the detection of several single pulse events, some repeating at the same dispersion measure, which could potentially originate from neutron stars in M31. One in particular was seen multiple times, including a burst of six pulses in 2000 seconds, at a dispersion measure of 54.7 pc cm^-3, which potentially places the origin of this source outside of our Galaxy. Our results are compared to a range of hypothetical populations of pulsars and RRATs in M31 and allow us to constrain the luminosity function of pulsars in M31. They also show that, unless the pulsar population in M31 is much dimmer than in our Galaxy, there is no need to invoke any violation of the inverse square law of the distance for pulsar fluxes.Comment: 18 pages, 14 figures, 8 tables. Accepted for publication in the main journal of MNRA

The 24-Cell and Calabi-Yau Threefolds with Hodge Numbers (1,1)

Calabi-Yau threefolds with h^11(X)=h^21(X)=1 are constructed as free quotients of a hypersurface in the ambient toric variety defined by the 24-cell. Their fundamental groups are SL(2,3), a semidirect product of Z_3 and Z_8, and Z_3 x Q_8.Comment: 22 pages, 3 figures, 3 table

Yukawa Couplings in Heterotic Standard Models

In this paper, we present a formalism for computing the Yukawa couplings in heterotic standard models. This is accomplished by calculating the relevant triple products of cohomology groups, leading to terms proportional to Q*H*u, Q*Hbar*d, L*H*nu and L*Hbar*e in the low energy superpotential. These interactions are subject to two very restrictive selection rules arising from the geometry of the Calabi-Yau manifold. We apply our formalism to the "minimal" heterotic standard model whose observable sector matter spectrum is exactly that of the MSSM. The non-vanishing Yukawa interactions are explicitly computed in this context. These interactions exhibit a texture rendering one out of the three quark/lepton families naturally light.Comment: 21 pages, LaTe

Shuttle/TDRSS modelling and link simulation study

A Shuttle/TDRSS S-band and Ku-band link simulation package called LinCsim was developed for the evaluation of link performance for specific Shuttle signal designs. The link models were described in detail and the transmitter distortion parameters or user constraints were carefully defined. The overall link degradation (excluding hardware degradations) relative to an ideal BPSK channel were given for various sets of user constraint values. The performance sensitivity to each individual user constraint was then illustrated. The effect of excessive Spacelab clock jitter on the return link BER performance was also investigated as was the problem of subcarrier recovery for the K-band Shuttle return link signal