Photometric distances to nine dark globules

Distances to nine dark globules are determined by a method using optical ( VRI ) and near-infrared (near-IR) ( JHK ) photometry of stars projected towards the field containing the globules. In this method, we compute intrinsic colour indices of stars projected towards the direction of the globule by dereddening the observed colour indices using various trial values of extinction   A _V   and a standard extinction law. These computed intrinsic colour indices for each star are then compared with the intrinsic colour indices of normal main-sequence stars and a spectral type is assigned to the star for which the computed colour indices best match with the standard intrinsic colour indices. Distances ( d ) to the stars are determined using the   A _V   and absolute magnitude  ( M_V )  corresponding to the spectral types thus obtained. A distance versus extinction plot is made and the distance at which   A _V   undergoes a sharp rise is taken to be the distance to the globule. All the clouds studied in this work are in the distance range 160–400 pc. The estimated distances to dark globules LDN 544, LDN 549, LDN 567, LDN 543, LDN 1113, LDN 1031, LDN 1225, LDN 1252 and LDN 1257 are  180 ± 35, 200 ± 40, 180 ± 35, 160 ± 30, 350 ± 70, 200 ± 40, 400 ± 80, 250 ± 50  and 250 ± 50 pc, respectively. Using the distances determined, we have estimated the masses of the globules and the far-IR luminosity of the IRAS sources associated with them. The mass of the clouds studied are in the range  10–200 M_⊙  .     

Numerical simulation of comet nuclei I. Water-ice comets

A one-dimensional simulation of pure water-ice cometary nuclei is presented, and the effect of the nucleus as a heat reservoir is considered. The phase transition from amorphous to crystalline ice is studied for two cases: (1) where the released latent heat goes entirely into heating adjacent layers and (2) where the released latent heat goes entirely into sublimation. For a Halley-like orbit it was found that for case 1 the phase boundary penetrates about 15 m on the first orbit and does not advance until sublimation brings the surface to some 10 m from the phase boundary. For case 2 the phase boundary penetrates about 1 m below the surface and remains at this depth as the surface sublimates. For an orbit like that of Schwassmann-Wachmann 1 the phase boundary penetrates about 50 m initially for case 1 and about 1 m for case 2. There is no further transformation until the entire comet is heated slowly to near the transition temperature, after which the entire nucleus is converted to crystalline ice. For an Encke-type orbit case 1 gives a nearly continuous transition of the entire nucleus to crystalline ice, while for case 2 the initial penetration is about 8 m and remains at this depth relative to the surface as sublimation decreases the cometary radius. Thus the entire comet is converted to crystalline ice just before it is completely dissipated.     

The 550-au Mission: a critical discussion

We have studied the science rationale, goals and requirements for a mission aimed at using the gravitational lensing from the Sun as a way of achieving high angular resolution and high signal amplification. We find that such a mission concept is compromised by several practical problems. Most severe are the effects due to the plasma in the solar atmosphere which cause refraction and scattering of the propagating rays. These effects either limit the frequencies that can be observed to those above ∼1 THz, or they move the optical point outwards beyond the vacuum value of ≥550 au. (Thus for observing frequency of 300 GHz the optical point is moved outwards to ∼ 680 au.) Density fluctuations in the inner solar atmosphere will further cause random pathlength differences for different rays. The corrections for the radiation from the Sun itself will also be a major challenge at any wavelength used, but could be mitigated with coronographic techniques. Given reasonable constraints on the spacecraft (particularly in terms of size and propulsion), source selection as well as severe navigational constraints further add to the difficulties for a potential mission. Nevertheless, unbiased surveys of small-scale structure on the sky at short wavelengths might be the most promising application of such a mission.     

A one-dimensional model for the interaction between continental-scale ice sheets and atmospheric stationary waves

 The great continental ice sheets of the Pleistocene represented a significant topographic obstacle to the westerly winds in northern midlatitudes. This work explores how consequent changes in the atmospheric stationary wave pattern might have affected the shape and growth of the ice sheets themselves. A one dimensional (1-D) model is developed which permits an examination of the types and magnitudes of the feedbacks that might be expected. When plausible temperature perturbations are introduced at the ice-sheet margin which are proportional to the stationary wave amplitude, the equilibrium shape of the ice sheet is significantly altered, and depends on the sign of the perturbation. The proposed feedback also affects the response of the ice sheet to time-varying climate forcing. The results suggest that the evolution of a continental-scale ice sheet with a land-based margin may be significantly determined by the changes it induces in the atmospheric circulation. Received: 1 October 1999 / Accepted: 17 July 2000     

Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization

An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (X_CO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al₂O₃–SiO₂–H₂O–CO₂, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over-thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low-P high-T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper-earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper-later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid-crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower X_CO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.     

Dissolution and depletion of ferromagnesian minerals from Holocene tephra layers in an acid bog,New Zealand,and implications for tephra correlation

This study examines the depletion of ferromagnesian silicate minerals from a sequence of thin, distal, mainly rhyolitic tephra layers of Holocene age preserved in an acid peat bog (Kopouatai), North Island, New Zealand. The rate of such depletion has been fast, as indicated by the complete loss of biotite from one tephra layer (Kaharoa Tephra), in which it is normally dominant, in only ca. 770 yr. Chemical dissolution is advocated as the likely cause for the depletion, with amphiboles and other mineral grains commonly showing etch pits, microcaves, and other characteristic surface solution features. Theoretical thermodynamic and kinetic models show a marked increase in the rate of dissolution of all ferromagnesian minerals under conditions of low pH (< 4), but that where silica concentrations in solution are high the relative proportions of minerals remaining are unaffected. However, where concentrations of dissolved silica are low, as in most bog environments, the relative proportions of ferromagnesian minerals are affected as well as absolute amounts being decreased. Amphiboles are depleted relative to pyroxenes, consistent with kinetic studies. The results show that the identification and correlation of tephras on the basis of relative abundances of ferromagnesian minerals alone may be unreliable, and emphasise the need to use multiple criteria in such studies.     

Real-time difference imaging analysis of MOA Galactic bulge observations during 2000

We describe observations carried out by the MOA group of the Galactic bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration. These events are particularly useful for studies of extrasolar planets and faint stars. Approximately 17 deg² were monitored at a sampling rate of up to six times per night. The images were analysed in real time using a difference imaging technique. 20 microlensing candidates were detected, of which eight were alerted to the microlensing community whilst in progress. Approximately half of the candidates had high magnifications (≳10), at least one had very high magnification (≳50), and one exhibited a clear parallax effect. The details of these events are reported here, together with details of the on-line difference imaging technique. Some nova-like events were also observed and these are described, together with one asteroid.     

Monitoring the Microarcsecond Quasar J1819+3845

We present the results of a year-long monitoring campaign on J1819+3845.We interpret the results of this WSRT campaign to infer critical source parameters such as source lifetime and structure on tens of microarcseconds. The long lifetime of the source at such high brightness temperatures requires continuous energy injection or exotic emission processes. We have previously interpreted the extreme scintillation ofJ1819+3845 as due to a relatively nearby (∼ 20 pc) scattering screen. We show this screen has a velocity w.r.t. the LSR of ∼ 25 kms^-1, as measured by the changing scintillation properties throughout the year: the `velocity parallax'. This revised version was published online in July 2006 with corrections to the Cover Date.     

A sapphirine–phlogopite–cordierite paragenesis in a low-P amphibolite facies terrain, Arunta Inlier, Australia

Summary Silica-undersaturated phlogopite schists from the Cackleberry Metamorphics, Arunta Inlier, central Australia, preserve relatively low-temperature sapphirine-bearing parageneses that developed during low-pressure upper amphibolite facies metamorphism. Peak metamorphic phlogopite–cordierite–sapphirine assemblages are interpreted to have formed during the same event recorded in nearby metapelites, at c.3 kbar and 650–700 °C. Initial cooling of the terrain resulted in the breakdown of sapphirine to corundum–chlorite–phlogopite and corundum–spinel–chlorite assemblages. Further retrogression at greenschist facies conditions resulted in the replacement of sapphirine by diaspore–chlorite intergrowths. The reaction textures are consistent with a near-isobaric heating-cooling path at low-pressure, and provide evidence for the stability of sapphirine at c.700 °C at low pressures in rocks of an appropriate Mg- and Fe³⁺-rich bulk composition. Received August 15, 2001 accepted December 27, 2001