On the accretion mode of the intermediate polar V1025 Centauri

The long white-dwarf spin periods in the magnetic cataclysmic variables EX Hya and V1025 Cen imply that if the systems possess accretion discs then they cannot be in equilibrium. It has been suggested that instead they are discless accretors in which the spin-up torques resulting from accretion are balanced by the ejection of part of the accretion flow back towards the secondary. We present phase-resolved spectroscopy of V1025 Cen aimed at deducing the nature of the accretion flow, and compare this with simulations of a discless accretor. We find that both the conventional disc-fed model and the discless-accretor model have strengths and weaknesses, and that further work is needed before we can decide which applies to V1025 Cen.Comment: 9 pages, 8 figures, To appear in MNRAS, includes low-res figures to reduce siz

Wetland mapping from digitized aerial photography

Computer assisted interpretation of small scale aerial imagery was found to be a cost effective and accurate method of mapping complex vegetation patterns if high resolution information is desired. This type of technique is suited for problems such as monitoring changes in species composition due to environmental factors and is a feasible method of monitoring and mapping large areas of wetlands. The technique has the added advantage of being in a computer compatible form which can be transformed into any georeference system of interest

On a Site of X-ray Emission in AE Aquarii

An analysis of recently reported results of XMM-Newton observations of AE Aqr within a hypothesis that the detected X-ray source is located inside the Roche lobe of the white dwarf is presented. I show this hypothesis to be inconsistent with the currently adopted model of mass-transfer in the system. Possible solutions of this problem are briefly discussed.Comment: 4 pages, accepted for publication in ApJ Letter

WD0837+185:the formation and evolution of an extreme mass ratio white dwarf-brown dwarf binary in Praesepe

There is a striking and unexplained dearth of brown dwarf companions in close orbits (< 3AU) around stars more massive than the Sun, in stark contrast to the frequency of stellar and planetary companions. Although rare and relatively short-lived, these systems leave detectable evolutionary end points in the form of white dwarf - brown dwarf binaries and these remnants can offer unique insights into the births and deaths of their parent systems. We present the discovery of a close (orbital separation ~ 0.006 AU) substellar companion to a massive white dwarf member of the Praesepe star cluster. Using the cluster age and the mass of the white dwarf we constrain the mass of the white dwarf progenitor star to lie in the range 3.5 - 3.7 Msun (B9). The high mass of the white dwarf means the substellar companion must have been engulfed by the B star's envelope while it was on the late asymptotic giant branch (AGB). Hence, the initial separation of the system was ~2 AU, with common envelope evolution reducing the separation to its current value. The initial and final orbital separations allow us to constrain the combination of the common envelope efficiency (alpha) and binding energy parameters (lambda) for the AGB star to alpha lambda ~3. We examine the various formation scenarios and conclude that the substellar object was most likely to have been captured by the white dwarf progenitor early in the life of the cluster, rather than forming in situ.Comment: Accepted for publication in ApJ

High Resolution Infrared Imaging and Spectroscopy of the Pistol Nebula: Evidence for Ejection

We present new NICMOS/HST infrared images and CGS4/UKIRT Br-alpha (4.05 um) spectroscopy of the Pistol Star and its associated nebula, finding strong evidence to support the hypothesis that the Pistol Nebula was ejected from the Pistol Star. The Pa-alpha NICMOS image shows that the nebula completely surrounds the Pistol Star, although the line intensity is much stronger on its northern and western edges. The Br-alpha spectra show the classical ring-like signature of quasi-spherical expansion, with weak blueshifted emission (V_max approx -60 km/s) and strong redshifted emission (V_max approx +10 km/s), where the velocities are with respect to the velocity of the Pistol Star; further, the redshifted emission appears to be "flattened" in the position-velocity diagram. These data suggest that the nebula was ejected from the star several thousand years ago, with a velocity between the current terminal velocity of the stellar wind (95 km/s) and the present expansion velocity of gas in the outer shell of the nebula (60 km/s). The Pa-alpha image reveals several emission-line stars in the region, including two newly-identified emission-line stars north of the Pistol Star with spectral types earlier than WC8 (T_eff > 50,000 K). The presence of these stars, the morphology of the Pa-alpha emission, and the velocity field in the gas suggest that the side of the nebula furthest from us is approaching, and being ionized by, the hot stars of the Quintuplet, and that the highest velocity redshifted gas has been decelerated by winds from the Quintuplet stars. We also discuss the possibility that the nebular gas might be magnetically confined by the ambient magnetic field delineated by the nearby nonthermal filaments.Comment: Figure 1 is included as a JPG file. Figure 1 and 2 also available at ftp://quintup.astro.ucla.edu/pistol2

High-Resolution Continuum Imaging at 1.3 and 0.7 cm of the W3 IRS 5 Region

High-resolution images of the hypercompact HII regions (HCHII) in W3 IRS 5 taken with the Very Large Array (VLA) at 1.3 and 0.7 cm are presented. Four HCHII regions were detected with sufficient signal-to-noise ratios to allow the determination of relevant parameters such as source position, size and flux density. The sources are slightly extended in our ~0.2 arcsecond beams; the deconvolved radii are less than 240 AU. A comparison of our data with VLA images taken at epoch 1989.1 shows proper motions for sources IRS 5a and IRS 5f. Between 1989.1 and 2002.5, we find a proper motion of 210 mas at a position angle of 12 deg for IRS 5f and a proper motion of 190 mas at a position angle of 50 deg for IRS 5a. At the assumed distance to W3 IRS 5, 1.83 +/- 0.14 kpc, these offsets translate to proper motions of ~135 km/s and ~122 km/s$ respectively. These sources are either shock ionized gas in an outflow or ionized gas ejected from high mass stars. We find no change in the positions of IRS 5d1/d2 and IRS 5b; and we show through a comparison with archival NICMOS 2.2 micron images that these two radio sources coincide with the infrared double constituting W3 IRS 5. These sources contain B or perhaps O stars. The flux densities of the four sources have changed compared to the epoch 1989.1 results. In our epoch 2002.5 data, none of the spectral indicies obtained from flux densities at 1.3 and 0.7 cm are consistent with optically thin free-free emission; IRS 5d1/d2 shows the largest increase in flux density from 1.3 cm to 0.7 cm. This may be an indication of free-free optical depth within an ionized wind, a photoevaporating disk, or an accretion flow. It is less likely that this increase is caused by dust emission at 0.7 cm.Comment: 13 pages, 3 figures To be published in The Astrophysical Journa

A new instability of accretion disks around compact magnetic stars

Aperiodic variability and Quasi Periodic Oscillations (QPOs) are observed from accretion disks orbiting white dwarfs, neutron stars, and black holes, suggesting that the flow is universally broken up into discrete blobs. We consider the interaction of these blobs with the magnetic field of a compact, accreting star, where diamagnetic blobs suffer a drag. We show that when the magnetic moment is not aligned with the spin axis, the resulting force is pulsed, and this can lead to resonance with the oscillation of the blobs around the equatorial plane; a resonance condition where energy is effectively pumped into non--equatorial motions is then derived. We show that the same resonance condition applies for the quadrupolar component of the magnetic field. We discuss the conditions of applicability of this result, showing that they are quite wide. We also show that realistic complications, such as chaotic magnetic fields, buoyancy, radiation pressure, evaporation, Kelvin--Helmholtz instability, and shear stresses due to differential rotation do not affect our results. In accreting neutron stars with millisecond periods, we show that this instability leads to Lense-Thirring precession of the blobs, and that damping by viscosity can be neglected.Comment: Accepted for publication in the Astrophysical Journal. AASTeX LateX needed. Two figure

Ryegrass Seeding Rate Alters Plant Morphology and Size--Possible Implications for Pasture Persistence

Poor persistence of perennial ryegrass (Lolium perenne L.) is a major dairy industry issue in New Zealand and Australia. New ryegrass seed is often drilled at 18-30 kg/ha, although previous research indicated that pastures drilled at 10-12 kg/ha can be just as productive (Frame and Boyd 1986; Praat et al. 1996). High seeding rates increase competition between developing seedlings for light, water and nutrients, reduce plant size (Harris 1990) and potentially survival. The experiment reported here investigated the effect of plant density (created by differences in seeding rate) on plant morphology and survival. The hypothesis was that plants established from high seeding rates will be smaller and, therefore, less likely to survive the first summer; a period of substantial environmental stress (e.g., high temperatures, low soil moisture, insect attack)

METHODS FOR APPLYING TRANSMISSION AND /eta/-vs-E CORRECTIONS IN THE DETERMINATION OF /eta/sub2200sub 2200 BY THE MACKLIN-deSAUSSURE EXPERIMENT

A careful determination of /sub 2200/ from results of the Macklin- deSaussure manganese-bath experiment involves corrections for (1) the weak but nonzero transmission of subcadmium neutrons through the uranium or plutonium foils and (2) the small variation of with energy in the subcadmium region. The derivation of a single general expression for applying both these corrections is presented, and two lBM 704 codes (MTC and GTC) which were written especially to facilitate the numerical evaluation of this combined correction factor are described. (auth