Modelling the advance–retreat cycle of a tidewater glacier with simple sediment dynamics

We present a simple coupled glacier-sediment model to simulate the evolution of a tidewater glacier. The model is based on a consideration of the total mass budget of a glacier, whereas ice mechanics are fully parameterized. The calving rate at the glacier terminus is assumed to be proportional to the water depth. We show that the formation of a morainic shoal has a profound influence on the response of a tidewater glacier to climatic forcing. For a slow periodic forcing of the equilibrium-line altitude, the model glacier advances steadily into the estuary, builds up a submarine terminal moraine, and then retreats rapidly when a critical point is hit. For a slowly increasing equilibrium-line altitude, at a rate as low as 1 m per 5 years, we find terminus retreat rates of up to 10 km in 50 years. Our model suggests that, although the response of tidewater glaciers to external forcing is strongly nonlinear, the onset of retreat is controlled by climate change.     

Probing outflow activity in very low mass stars and brown dwarfs

The discovery that newborn very low mass stars and brown dwarfs have optical forbidden line spectra similar to low mass young stars was a strong indication that these objects can also launch outflows. Forbidden lines are the traditional tracers of outflow activity in young stars and observations at these wavelengths have contributed much to the understanding of outflows. However in the case of brown dwarfs, the forbidden emission line regions observed are not well resolved spatially. Thus, their origin in an outflow could not be confirmed. Here, the technique of spectro-astrometry as a means of spatially probing the forbidden emission line regions of very low mass stars and brown dwarfs is introduced. Indeed spectro-astrometric data presented here demonstrates, for the first time, that young brown dwarfs that are actively accreting can drive outflows. Also discussed is the important role adaptive optics will play when it comes to spatially resolving the forbidden emission line regions of sub-stellar objects and the potential for developing spectro-astrometry to a 2D form through integral field spectroscopy.     

Bayesian foreground analysis with CMB data

The quality of CMB observations has improved dramatically in the last few years, and will continue to do so in the coming decade. Over a wide range of angular scales, the uncertainty due to instrumental noise is now small compared to the cosmic variance. One may claim with some justification that we have entered the era of precision CMB cosmology. However, some caution is still warranted: The errors due to residual foreground contamination in the CMB power spectrum and cosmological parameters remain largely unquantified, and the effect of these errors on important cosmological parameters such as the optical depth τ and spectral index n_s is not obvious. A major goal for current CMB analysis efforts must therefore be to develop methods that allow us to propagate such uncertainties from the raw data through to the final products. Here we review a recently proposed method that may be a first step towards that goal.     

A WIMP detector with two-phase xenon

We describe an important new technique to search for WIMPs. This technique employs a method of background discrimination using double phase xenon as detector target. We describe the construction of a two-phase, 1-kg xenon detector. The detector will be installed at the underground laboratory in the Mt. Blanc tunnel, which provides a low background rate. A comparison between the sensitivity curve of our detector and the theoretical events limit from SUSY calculations is presented.     

Active region magnetic fields

The tilt angles of sunspot groups are defined, using the Mount Wilson data set. It is shown that groups with tilt angles greater than or less than the average value (≈ 5 deg) show different latitude dependences. This effect is also seen in synoptic magnetic field data defining plages. The fraction of the total sunspot group area that is found in the leading spots is discussed as a parameter that can be useful in studying the dynamics of sunspot groups. This parameter is larger for low tilt angles, and small for extreme tilt angles in either direction. The daily variations of sunspot group tilt angles are discussed. The result that sunspot tilt angles tend to rotate toward the average value is reviewed. It is suggested that at some depth, perhaps 50 Mm, there is a flow relative to the surface that results from a rotation rate faster than the surface rate by about 60 m/sec and a meridional drift that is slower than the surface rate by about 5 m/sec. This results in a slanted relative flow at that depth that is in the direction of the average tilt angle and may be responsible for the tendency for sunspot groups (and plages) to rotate their magnetic axes in the direction of the average tilt angle.     

Unsteady mass outflow from Wolf-Rayet stars

We show that hydrostatically equilibrium models for the thin photospheres of helium stars based on new opacities κ_R (OPAL and OP) can be constructed only for masses M<5M _⊙. The parameter Г=κL/4πGMc, defined as the ratio of light pressure to gravity, exceeds a critical value of 1.0 for larger masses, which must result in mass outflow under light pressure. This mass limit matches the observed lower limit for the masses of Wolf-Rayet stars (M _WR>5M _⊙)), which is an additional argument that the Wolf-Rayet stellar cores are actually helium stars. By solving the equation of radiative transfer in extended atmospheres, we construct a semiempirical model for a WN5 star (M _WN5=10M _⊙)) with a helium core and an expanding envelope, whose physical and geometric parameters are known mainly from light-curve solution for the eclipsing binary V444 Cyg (WN5+06): outflow rate $\dot M \approx 1.0 \times 10^{ - 5} M_ \odot yr^{ - 1} $ , terminal velocity V _∞≈2000 km s^?1, and expanding-envelope optical depth τ_env≈25. The temperature at the outer boundary of the photosphere of a helium star surrounded by such an envelope is approximately 130 kK higher than that in the absence of an envelope, being T _ph≈240 kK. Because of the high temperatures, the absorption coefficients at the corresponding photospheric levels are smaller than those in models with no envelope; therefore, the photosphere turns out to be in hydrostatic equilibrium and stable against light pressure (Г_max≈0.9). As a way out of this conflicting situation (an expanding envelope together with a hydrostatically equilibrium photosphere), we propose a model of discrete mass outflow, which is also supported by the observed cloudy structure of the envelopes in this type of stars. To quantitatively estimate parameters of the nonuniform outflow model requires detailed gas-dynamical calculations.     

A study of the shell of Nova V705 Cas

Spectroscopic observations of the DQ Her-type Nova V705 Cas in the wavelength range Hγ-7400 Å at the nebular stage are presented. The Balmer decrement yields E _B–V =0.98. Based on statistical relationships, we estimated E _B–V to be 0.7 and the distance to the Nova to be 1.17 kpc. The discrepancy between the estimates of E _B–V may result from circumstellar reddening after the formation of a dust shell. The helium, oxygen, nitrogen, argon, iron, and calcium abundances in the Nova shell are determined. The shell of V705 Cas is similar in chemical composition to those of other DQ Her-type novae.     

Electron density diagnostics for solar ultraviolet lines of O V

We determine the electron densities for a range of solar features using new calculations for the Ov line ratio, R=I(761.1)/I(760.4), in conjunction with observational data obtained with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on the Solar and Heliospheric Observatory (SOHO). The densities obtained from this diagnostic are in good agreement with earlier measured values. This provides support for the theoretical diagnostics presented in this paper, and hence the atomic data used in their derivation. We conclude from these results that this particular Ov ratio is a useful diagnostic for many types of solar features.     

SECIS: The Solar Eclipse Coronal Eclipse Imaging System

The Solar Eclipse Coronal Imaging System (SECIS) is an instrument designed to search for short-period modulations in the solar corona seen either during a total eclipse or with a coronagraph. The CCD cameras used in SECIS have the capability of imaging the corona at a rate of up to 70 frames a second, with the intensities in each pixel digitised in 12-bit levels. The data are captured and stored on a modified PC. With suitable optics it is thus possible to search for fast changes or short-period wave motions in the corona that will have important implications for the coronal heating mechanism. The equipment has been successfully tested using the Evans Solar Facility coronagraph at National Solar Observatory/Sacramento Peak and during the 11 August 1999 eclipse at a site in north-eastern Bulgaria. The instrument is described and preliminary results are outlined.