http://www.researchonline.mq.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9278 In 2008 January the twenty-fourth Chinese expedition team successfully deployed the Chinese Small Telescope ARray (CSTAR) to Dome A, the highest point on the Antarctic plateau. CSTAR consists of four 14.5 cm optical telescopes, each with a different filter (g, r, i, and open) and has a 4.5° × 4.5° field of view (FOV). It operates robotically as part of the Plateau Observatory, PLATO, with each telescope taking an image every 30 s throughout the year whenever it is dark. During 2008, CSTAR 1 performed almost flawlessly, acquiring more than 0.3 million i-band images for a total integration time of 1728 hr during 158 days of observations. For each image taken under good sky conditions, more than 10,000 sources down to 16th magnitude could be detected. We performed aperture photometry on all the sources in the field to create the catalog described herein. Since CSTAR has a fixed pointing centered on the south celestial pole (decl. = -90°), all the sources within the FOV of CSTAR were monitored continuously for several months. The photometric catalog can be used for studying any variability in these sources, and for the discovery of transient sources such as supernovae, gamma-ray bursts, and minor planets. 2013-05-23T06:01:32.891Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:24770 The purpose of this paper is to quantify the efficiency with which Australian universities utilise their teaching resources. The study estimates the cost efficiency of 36 universities over the period 1995–2002 using stochastic frontier analysis. The present study differs from previous cost and efficiency studies of Australian universities in two respects. First, it employs stochastic frontier analysis for the specification of a cost function for Australian universities which allows for the estimation of cost efficiency for each university under study. Second, a panel data set is utilised in the estimation of the cost function which enables not only comparisons of cost efficiency between universities but also an econometric testing of the assumption of an identical cost function for every university. The main finding is that universities are not operating efficiently as measured by cost efficiency and in relative terms. An efficiency ranking is derived and policy inferences are discussed. 2013-03-13T09:50:51.076Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12158 The unique atmospheric conditions present at sites such as Dome C on the Antarctic plateau are very favorable for high spatial resolution astronomy. At Dome C, the majority of the optical turbulence is confined to a 30 to 40 m thick stable boundary layer that results from the strong temperature inversion created by the heat exchange between the air and the ice-covered ground. To fully realize the potential of the exceptionally calm free atmosphere, this boundary layer must be overcome. In this article we compare the performance of two methods proposed to beat the boundary layer: mounting a telescope on a tower that physically puts it above the turbulent layer, and installing a telescope at ground level with a ground-layer adaptive optics system. A case is also made to combine these two methods to further improve the image quality. 2012-07-26T18:31:55.042Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12157 The PLATeau Observatory (PLATO) is an automated self-powered astrophysical observatory that was deployed to Dome A, the highest point on the Antarctic plateau, in 2008 January. PLATO consists of a suite of site-testing instruments designed to quantify the benefits of the Dome A site for astronomy, and science instruments designed to take advantage of the unique observing conditions. Instruments include CSTAR, an array of optical telescopes for transient astronomy; Gattini, an instrument to measure the optical sky brightness and cloud cover statistics; DASLE, an experiment to measure the statistics of the meteorological conditions within the near-surface layer; Pre-HEAT, a submillimeter tipping radiometer measuring the atmospheric transmission and water vapor content and performing spectral line imaging of the Galactic plane; and Snodar, an acoustic radar designed to measure turbulence within the near-surface layer. PLATO has run completely unattended and collected data throughout the winter 2008 season. Here we present a detailed description of the PLATO instrument suite and preliminary results obtained from the first season of operation. 2012-07-26T18:31:52.606Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14094 We have investigated the nature of flare emission from Sgr A* during multi-wavelength observations of this source that took place in 2004, 2005, and 2006. We present evidence for dimming of submillimeter and radio flux during the peak of near-IR flares. This suggests that the variability of Sgr A* across its wavelength spectrum is phenomenologically related. The model explaining this new behavior of flare activity could be consistent with adiabatically cooling plasma blobs that are expanding but also partially eclipsing the background quiescent emission from Sgr A*. When a flare is launched, the plasma blob is most compact and is brightest in the optically thin regime whereas the emission in radio/submillimeter wavelengths has a higher opacity. Absorption in the observed light curve of Sgr A* at radio/submillimeter flux is due to the combined effects of lower brightness temperature of plasma blobs with respect to the quiescent brightness temperature and high opacity of plasma blobs. This implies that plasma blobs are mainly placed in the magnetosphere of a disk-like flow or further out in the flow. The depth of the absorption being larger in submillimeter than in radio wavelengths implies that the intrinsic size of the quiescent emission increases with increasing wavelength which is consistent with previous size measurements of Sgr A*. Lastly, we believe that occultation of the quiescent emission of Sgr A* at radio/submillimeter by IR flares can be used as a powerful tool to identify flare activity at its earliest phase of its evolution. 2011-11-24T07:33:42.702Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14359 Close binary systems undergoing mass transfer or common envelope interactions can account for the morphological properties of some planetary nebulae. The search for close binary companions in planetary nebulae is hindered by the difficulty of detecting cool, late-type, main-sequence companions in binary systems with hot pre-white-dwarf primaries. However, models of binary planetary nebula progenitor systems predict that mass accretion or tidal interactions can induce rapid rotation in the companion, leading to X-ray-emitting coronae. To test such models, we have searched for, and detected, X-ray emission from three binary central stars within planetary nebulae: the post-common envelope close binaries in HFG 1 and DS 1 consisting of O-type subdwarfs with late-type, main-sequence companions and the binary system in LoTr 5 consisting of O-type subdwarf and rapidly rotating, late-type giant companion. The X-ray emission in each case is best characterized by spectral models consisting of two optically thin thermal plasma components with characteristic temperatures of ~10 MK and 15-40 MK and total X-ray luminosities ~10³⁰ erg s⁻¹. We consider the possible origin of the X-ray emission from these binary systems and conclude that the most likely origin is, in each case, a corona around the late-type companion, as predicted by models of interacting binaries. 2011-11-24T07:31:47.757Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14443 Theories of thick-disk formation can be differentiated by measurements of stellar elemental abundances. We have undertaken a study of metal-poor stars selected from the RAVE spectroscopic survey of bright stars to establish whether or not there is a significant population of metal-poor thick-disk stars ([Fe/H] ≲ –1.0) and to measure their elemental abundances. In this Letter, we present abundances of four α-elements (Mg, Si, Ca, and Ti) and iron for a subsample of 212 red giant branch and 31 red clump/horizontal branch stars from this study. We find that the [α/Fe] ratios are enhanced, implying that enrichment proceeded by purely core-collapse supernovae. This requires that star formation in each star-forming region had a short duration. The relative lack of scatter in the [α/Fe] ratios implies good mixing in the interstellar medium prior to star formation. In addition, the ratios resemble that of the halo, indicating that the halo and thick disk share a similar massive star initial mass function. We conclude that the α-enhancement of the metal-poor thick disk implies that direct accretion of stars from dwarf galaxies similar to surviving dwarf galaxies today did not play a major role in the formation of the thick disk. 2011-11-24T07:31:27.199Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14474 Very metal-poor stars are of obvious importance for many problems in chemical evolution, star formation, and galaxy evolution. Finding complete samples of such stars which are also bright enough to allow high-precision individual analyses is of considerable interest. We demonstrate here that stars with iron abundances [Fe/H] <–2 dex, and down to below –4 dex, can be efficiently identified within the Radial Velocity Experiment (RAVE) survey of bright stars, without requiring additional confirmatory observations. We determine a calibration of the equivalent width of the calcium triplet lines measured from the RAVE spectra onto true [Fe/H], using high spectral resolution data for a subset of the stars. These RAVE iron abundances are accurate enough to obviate the need for confirmatory higher-resolution spectroscopy. Our initial study has identified 631 stars with [Fe/H] ≤–2, from a RAVE database containing approximately 200,000 stars. This RAVE-based sample is complete for stars with [Fe/H] ≲–2.5, allowing statistical sample analysis. We identify three stars with [Fe/H] ≲–4. Of these, one was already known to be "ultra metal-poor," one is a known carbon-enhanced metal-poor star, but we obtain [Fe/H] = –4.0, rather than the published [Fe/H] = –3.3, and derive [C/Fe] = +0.9, and [N/Fe] = +3.2, and the third is at the limit of our signal-to-noise ratio. RAVE observations are ongoing and should prove to be a rich source of bright, easily studied, very metal-poor stars. 2011-11-24T07:31:19.250Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14461 We devise a new method for the detection of double-lined binary stars in a sample of the Radial Velocity Experiment (RAVE) survey spectra. The method is both tested against extensive simulations based on synthetic spectra and compared to direct visual inspection of all RAVE spectra. It is based on the properties and shape of the cross-correlation function, and is able to recover ~80% of all binaries with an orbital period of order 1 day. Systems with periods up to 1 yr are still within the detection reach. We have applied the method to 25,850 spectra of the RAVE second data release and found 123 double-lined binary candidates, only eight of which are already marked as binaries in the SIMBAD database. Among the candidates, there are seven that show spectral features consistent with the RS CVn type (solar type with active chromosphere) and seven that might be of W UMa type (over-contact binaries). One star, HD 101167, seems to be a triple system composed of three nearly identical G-type dwarfs. The tested classification method could also be applicable to the data of the upcoming Gaia mission. 2011-11-14T19:47:36.424Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15207 The emission nebula around the subdwarf B (sdB) star PHL 932 is currently classified as a planetary nebula (PN) in the literature. Based on a large body of multi-wavelength data, both new and previously published, we show here that this low-excitation nebula is in fact a small Strömgren sphere (Hii region) in the interstellar medium around this star. We summarise the properties of the nebula and its ionizing star, and discuss its evolutionary status. We find no compelling evidence for close binarity, arguing that PHL 932 is an ordinary sdB star. We also find that the emission nebulae around the hot DO stars PG 0108 + 101 and PG 0109 + 111 are also Strömgren spheres in the ISM, and along with PHL 932, are probably associated with the same extensive region of high-latitude molecular gas in Pisces–Pegasus. 2011-10-05T13:11:51.465Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15209 We present the results of the search for candidate Planetary Nebulae interacting with the interstellar medium (PN–ISM) in the framework of the INT Photometric Hα Survey (IPHAS) and located in the right ascension range 18–20 h. The detection capability of this new Northern survey, in terms of depth and imaging resolution, has allowed us to overcome the detection problem generally associated to the low surface brightness inherent to PNe-ISM. We discuss the detection of 21 IPHAS PN–ISM candidates. Thus, different stages of interaction were observed, implying various morphologies i.e. from the unaffected to totally disrupted shapes. The majority of the sources belong to the so-called WZO2 stage which main characteristic is a brightening of the nebula’s shell in the direction of motion. The new findings are encouraging as they would be a first step into the reduction of the scarcity of observational data and they would provide new insights into the physical processes occurring in the rather evolved PNe. 2011-10-05T13:11:46.364Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15212 The total number of true, likely and possible planetary nebulae (PN) now known in the Milky Way is nearly 3000, double the number known a decade ago. The new discoveries are a legacy of the recent availability of wide field, narrowband imaging surveys, primarily in the light of H-alpha. In this paper, we summarise the various PN discovery techniques, and give an overview of the many types of objects which mimic PN and which appear as contaminants in both Galactic and extragalactic samples. Much improved discrimination of classical PN from their mimics is now possible based on the wide variety of high-quality multiwavelength data sets that are now available. We offer improved taxonomic and observational definitions for the PN phenomenon based on evaluation of these better diagnostic capabilities. However, we note that evidence is increasing that the PN phenomenon is heterogeneous, and PN are likely to be formed from multiple evolutionary scenarios. In particular, the relationships between some collimated symbiotic outflows and bipolar PN remain uncertain. 2011-10-05T13:11:42.502Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:15210 Recent Hα surveys such as SHS and IPHAS have improved the completeness of the Galactic planetary nebula (PN) census. We now know of ∼3000 PNe in the Galaxy, but this is far short of most estimates, typically ∼25 000 or more for the total population. The size of the Galactic PN population is required to derive an accurate estimate of the chemical enrichment rates of nitrogen, carbon, and helium. In addition, a high PN count (>20 000) is strong evidence that most main-sequence stars of mass 1-8M⊙ will go through a PN phase, while a low count (<10 000) argues that special conditions (e.g. close binary interactions) are required to form a PN. We describe a technique for finding hundreds more PNe using the existing data collections of the digital sky surveys, thereby improving the census of Galactic PNe. 2011-10-05T13:11:42.320Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9516 5 page(s) 2011-08-26T04:41:48.307Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14465 Context. Knowledge of the total population of symbiotic stars in the Galaxy is important for understanding basic aspects of stellar evolution in interacting binaries and the relevance of this class of objects in the formation of supernovae of type Ia. Aims. In a previous paper, we presented the selection criteria needed to search for symbiotic stars in IPHAS, the INT Hα survey of the Northern Galactic plane. IPHAS gives us the opportunity to make a systematic, complete search for symbiotic stars in a magnitude-limited volume. Methods. Follow-up spectroscopy at different telescopes worldwide of a sample of sixty two symbiotic star candidates is presented. Results. Seven out of nineteen S-type candidates observed spectroscopically are confirmed to be genuine symbiotic stars. The spectral type of their red giant components, as well as reddening and distance, were computed by modelling the spectra. Only one new D-type symbiotic system, out of forty-three candidates observed, was found. This was as expected (see discussion in our paper on the selection criteria). The object shows evidence for a high density outflow expanding at a speed ≥65 km s-1.
Most of the other candidates are lightly reddened classical T Tauri stars and more highly reddened young stellar objects that may be either more massive young stars of HAeBe type or classical Be stars. In addition, a few notable objects have been found, such as three new Wolf-Rayet stars and two relatively high-luminosity evolved massive stars. We also found a helium-rich source, possibly a dense ejecta hiding a WR star, which is surrounded by a large ionized nebula. Conclusions. These spectroscopic data allow us to refine the selection criteria for symbiotic stars in the IPHAS survey and, more generally, to better understand the behaviour of different Hα emitters in the IPHAS and 2MASS colour-colour diagrams. 2011-08-11T23:20:16.777Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14460 Our previous identification and spectroscopic confirmation of 431 faint, new planetary nebulae (PNe) in the central 25 deg2 region of the Large Magellanic Cloud (LMC) permit us to now examine the shape of the LMC planetary nebula luminosity function (PNLF) through an unprecedented 10 mag range. The majority of our newly discovered and previously known PNe were observed using the 2dF, multi-object fibre spectroscopy system on the 3.9-m Anglo-Australian Telescope and the FLAMES multi-object spectrograph on the 8-m Very Large Telescope. We present reliable [O iii] 5007 Å and Hβ flux estimates based on calibrations to well-established PN fluxes from previous surveys and spectroscopic standard stars. The bright cut-off (M*) of the PNLF is found by fitting a cumulative function to the bright end of the PNLF over a 3.4 mag range. This cut-off is used to estimate a new distance modulus of 18.46 to the LMC, in close agreement with previous PNLF studies and the best estimates by other indicators. The bright-end cut-off is robust to small samples of bright PNe since significantly increased PN samples do not change this fiducial. We then fit a truncated exponential curve directly to the bright end of the function over a 6 mag range and test the curve's ability to indicate the position of M*. Because of the significant increase in the number of LMC PNe, the shape of the PNLF is now examined in greater detail than has previously been possible. Newly discovered features include a small increase in the number of PNe over the brightest 4 mag followed by a steep rise over 2 mag, a peak at 6 mag below the bright cut-off and an almost linear drop-off to the faint end. Dips at the bright end of the PNLF are examined in relation to the overall shape of the PNLF and the exponential increase in the number of PNe. Through cumulative functions, the new LMC PNLF is compared to those from the Small Magellanic Cloud and a new deep local Galactic sample revealing the effects of incompleteness. The new [O iii] 5007 Å LMC PNLF is then compared to our new Hβ LMC PNLF using calibrated and measured fluxes for the same objects, revealing the effects of metallicity on the [O iii] 5007 Å line. 2011-08-11T04:50:14.433Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14446 Aims. We develop a method for deriving distances from spectroscopic data and obtaining full 6D phase-space coordinates for the RAVE survey's second data release. Methods. We used stellar models combined with atmospheric properties from RAVE (effective temperature, surface gravity and metallicity) and (J-Ks) photometry from archival sources to derive absolute magnitudes. In combination with apparent magnitudes, sky coordinates, proper motions from a variety of sources and radial velocities from RAVE, we are able to derive the full 6D phase-space coordinates for a large sample of RAVE stars. This method is tested with artificial data, Hipparcos trigonometric parallaxes and observations of the open cluster M 67. Results. When we applied our method to a set of 16 146 stars, we found that 25% (4037) of the stars have relative (statistical) distance errors of <35%, while 50% (8073) and 75% (12 110) have relative (statistical) errors smaller than 45% and 50%, respectively. Our various tests show that we can reliably estimate distances for main-sequence stars, but there is an indication of potential systematic problems with giant stars owing to uncertainties in the underlying stellar models. For the main-sequence star sample (defined as those with log(g) > 4), 25% (1744) have relative distance errors <31%, while 50% (3488) and 75% (5231) have relative errors smaller than 36% and 42%, respectively. Our full dataset shows the expected decrease in the metallicity of stars as a function of distance from the Galactic plane. The known kinematic substructures in the U and V velocity components of nearby dwarf stars are apparent in our dataset, confirming the accuracy of our data and the reliability of our technique. We provide independent measurements of the orientation of the UV velocity ellipsoid and of the solar motion, and they are in very good agreement with previous work. Conclusions. The distance catalogue for the RAVE second data release is available at http://www.astro.rug.nl/~rave, and will be updated in the future to include new data releases. 2011-08-10T08:32:41.918Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14355 Close-binary central stars of planetary nebulae (CSPNe) provide an opportunity to explore the evolution of PNe, their shaping, and the evolution of binary systems undergoing a common-envelope phase. Here, we present the results of time-resolved photometry of the binary central stars (CSs) of the PNe NGC 6026 and NGC 6337 as well as time-resolved spectroscopy of the CS of NGC 6026. The results of a period analysis give an orbital period of 0.528086(4) days for NGC 6026 and a photometric period of 0.1734742(5) days for NGC 6337. In the case of NGC 6337, it appears that the photometric period reflects the orbital period and that the variability is the result of the irradiated hemisphere of a cool companion. The inclination of the thin PN ring is nearly face-on. Our modeled inclination range for the close central binary includes nearly face-on alignments and provides evidence for a direct binary-nebular shaping connection. For NGC 6026, however, the radial-velocity curve shows that the orbital period is twice the photometric period. In this case, the photometric variability is due to an ellipsoidal effect in which the CS nearly fills its Roche lobe and the companion is most likely a hot white dwarf. NGC 6026 then is the third PN with a confirmed central binary where the companion is compact. Based on the data and modeling using a Wilson-Devinney code, we discuss the physical parameters of the two systems and how they relate to the known sample of close-binary CSs, which comprise 15%-20% of all PNe. 2011-08-06T02:41:01.572Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14230 We present narrow band AAO/UKST Hα images and medium and low resolution optical spectra of a nebula shell putatively associated with the Wolf-Rayet star WR 60. We also present the first identification of this shell in the radio regime at 843 MHz and at 4850 MHz from the Sydney University Molonglo Sky Survey (SUMSS), and from the Parkes-MIT-NRAO (PMN) survey respectively. This radio emission closely follows the optical emission. The optical spectra from the shell exhibits the typical shock excitation signatures sometimes seen in Wolf-Rayet stellar ejecta but also common to supernova remnants. A key finding however, is that the WR 60 star, is not, in fact, anywhere near the geometrical centre of the putative arcuate nebula ejecta as had been previously stated. This was due to an erroneous positional identification for the star in the literature which we now correct. This new identification calls into serious question any association of the nebula with WR 60 as such nebula are usually quite well centred on the WR stars themselves. We now propose that this fact combined with our new optical spectra, deeper Hα imaging and newly identified radio structures actually imply that the WR 60 nebula should be reclassified as an unassociated new supernova remnant which we designate G310.5+0.8. 2011-08-01T06:24:33.100Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14068 We use N-body simulations to explore the possibility that the Sagittarius (Sgr) dwarf galaxy was originally a late-type, rotating disc galaxy, rather than a non-rotating, pressure-supported dwarf spheroidal galaxy, as previously thought. We find that bifurcations in the leading tail of the Sgr stream, similar to those detected by the SDSS survey, naturally arise in models where the Sgr disc is misaligned with respect to the orbital plane. Moreover, we show that the internal rotation of the progenitor may strongly alter the location of the leading tail projected on the sky, and thus affect the constraints on the shape of the Milky Way dark matter halo that may be derived from modelling the Sgr stream. Our models provide a clear, easily tested prediction; although tidal mass stripping removes a large fraction of the original angular momentum in the progenitor dwarf galaxy, the remnant core should still rotate with a velocity amplitude ∼20 km s−1 that could be readily detected in future, wide-field kinematic surveys of the Sgr dwarf. 2011-07-08T12:11:24.877Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13894 Recycling of oceanic crust into the deep mantle via subduction is a widely accepted mechanism for creating compositional heterogeneity in the upper mantle and for explaining the distinct geochemistry of mantle plumes. The oxygen isotope ratios (δ¹⁸⁰) of some ocean island basalts (OIB) span values both above and below that of unmetasomatised upper mantle (5.5 ± 0.4‰) and provide support for this hypothesis, as it is widely assumed that most variations in δ¹⁸⁰ are produced by near-surface low-temperature processes. Here we show a significant linear relationship between δ¹⁸⁰ and stable iron isotope ratios (δ⁵⁷Fe) in a suite of pristine eclogite xenoliths. The δ¹⁸⁰ values of both bulk samples and garnets range from values within error of normal mantle to significantly lighter values. The observed range and correlation between δ¹⁸⁰ and δ⁵⁷Fe is unlikely to be inherited from oceanic crust, as δ⁵⁷Fe values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with δ¹⁸⁰. It is proposed that the correlated δ⁵⁷Fe and δ¹⁸⁰ variations in this particular eclogite suite are predominantly related to isotopic fractionation by disequilibrium partial melting although modification by melt percolation processes cannot be ruled out. Fractionation of Fe and O isotopes by removal of partial melt enriched in isotopically heavy Fe and O is supported by negative correlations between bulk sample δ⁵⁷Fe and Cr content and bulk sample and garnet δ¹⁸⁰ and Sc contents, as Cr and Sc are elements that become enriched in garnet- and pyroxene-bearing melt residues. Melt extraction could take place either during subduction, where the eclogites represent the residues of melted oceanic lithosphere, or could take place during long-term residence within the lithospheric mantle, in which case the protoliths of the eclogites could be of either crustal or mantle origin. This modification of both δ⁵⁷Fe and δ¹⁸⁰ by melting processes and specifically the production of low-δ¹⁸⁰ signatures in mafic rocks implies that some of the isotopically light δ¹⁸⁰ values observed in OIB and eclogite xenoliths may not necessarily reflect near-surface processes or components. 2011-06-30T15:11:03.654Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13882 Most peridotite massifs and mantle xenoliths show a wide range of isotopic variations, often involving significant decoupling between Hf, Nd and Sr isotopes. These variations are generally ascribed either to mingling of individual components of contrasted isotopic compositions or to time integration of parent-element enrichment by percolating melts/fluids, superimposed onto previous depletion event(s). However, strong isotopic decoupling may also arise during porous flow as a result of daughter-elements fractionation during solid–liquid interaction. Although porous flow is recognized as an important process in mantle rocks, its effects on mantle isotopic variability have been barely investigated so far. The peridotites of the Lherz massif (French Pyrenees) display a frozen melt percolation front separating highly refractory harzburgites from refertilized lherzolites. Isotopic signatures observed at the melt percolation front show a strong decoupling of Hf from Nd and Sr isotopes that cannot be accounted for by simple mixing involving the harzburgite protolith and the percolating melt. Using one dimensional percolation–diffusion and percolation–reaction modeling, we show that these signatures represent transient isotopic compositions generated by porous flow. These signatures are governed by a few critical parameters such as daughter element concentrations in melt and peridotite, element diffusivity, and efficiency of isotopic homogenization rather than by the chromatographic effect of melt transport and the refertilization reaction. Subtle variations in these parameters may generate significant inter-isotopic decoupling and wide isotopic variations in mantle rocks. 2011-06-29T19:30:23.725Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13439 We use thermodynamically self-consistent and hybrid methods to analyze the correlation of important physical parameters (e.g. bulk density, elastic moduli) with bulk Mg# and modal composition in mantle peridotites at upper mantle conditions. Temperature (anharmonic and anelastic), pressure and compositional derivatives for all these parameters are evaluated. The results show that the widely used correlations between Vp/Vs and Mg# in peridotites are strictly valid only for garnet-bearing assemblages at temperatures < 900 °C. The correlation breaks down when: i) spinel is the stable Al-rich phase in the assemblage and ii) when anelastic attenuation of seismic velocities becomes important (T≥900 °C). This implies that the range of applicability of published Vp/Vs–Mg# correlations for the upper mantle is limited to a depth interval between the spinel–garnet phase transition and the 900 °C isotherm. We use numerical simulations to show that this depth interval is virtually nonexistent in lithospheres thinner than not, vert, similar 140 km and can comprise up to ∼ 50% of the lithospheric mantle in thick (> 220 km) lithospheric domains. In addition, we show that for most of the upper mantle the expected Δ(Vp/Vs) values associated with compositional variations are smaller than the resolution limit of current seismological methods. All these considerations suggest that the Vp/Vs ratio is not a reliable measure of compositional variations and that for large parts of the upper mantle compositional anomalies cannot be separated from thermal anomalies on the basis of seismological studies only. We further confirm that the only reliable indicator of compositional anomalies in a peridotitic mantle is the ratio of density to shear wave velocities (ρ/Vs). Our results demonstrate that geophysical–petrological models (forward or inverse) that model these two fields (i.e. density and Vs) self-consistently within a robust thermodynamic framework are necessary for characterizing the small-scale thermal and compositional structure of the lithosphere and sublithospheric upper mantle. 2011-05-31T09:20:16.925Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12969 We present molecular-line observations at millimetre, centimetre and infrared wavelengths of the region containing OH(1720 MHz) masers in the supernova remnant (SNR) G349.7+0.2, using the Australia Telescope (AT) Mopra antenna, the Swedish–ESO Submillimeter Telescope, the AT Compact Array and the UNSW Infrared Fabry–Perot narrow-band filter installed on the Anglo-Australian Telescope. Several molecular transitions were observed between 1.6 and 3 mm to constrain the physical parameters of the molecular cloud interacting with the SNR and to investigate the effects of the SNR shock on the gas chemistry. We detected shock-excited near-infrared H₂ emission towards the centre of the SNR, revealing highly clumped molecular gas and a good correlation with published mid-infrared images from the Spitzer Space Telescope. An excellent correlation between the H2 clumps and OH(1720 MHz) maser positions supports the shock excitation of the OH(1720 MHz) maser emission. Furthermore, we detected OH absorption at 1665 and 1667 MHz which shows a good correlation with the shocked H₂ emission and the masers. We found maser emission at 1665 MHz near the OH(1720 MHz) masers in this SNR, which is found to be associated with a GLIMPSE source SSTGLMC G349.7294+00.1747. We also detected 1665 and 1667 MHz OH masers, and weak 4.8 GHz H₂CO absorption towards the ultracompact H ii region IRAS 17147–3725 located to the southeast of the SNR. We found no 4.7- or 6-GHz excited-state OH masers or 6-GHz CH₃OH maser towards either the SNR or the H ii region. 2011-05-25T21:53:23.009Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12976 We present new spectroscopic data for 26 stars in the recently discovered Canes Venatici I (CVn I) dwarf spheroidal galaxy, obtained with the Gemini Multi-Object Spectrograph North (GMOS-N) on the Gemini North telescope. We use these data to investigate the recent claim of the presence of two dynamically inconsistent stellar populations in this system. While we find no evidence for kinematically distinct subpopulations in our sample, we also show that the available kinematic data set in CVnI is likely too small to draw robust conclusions about its subpopulations. We are, however, able to obtain a mass estimate for CVn I that is consistent with all available data, including previously published data. We discuss possible differences between our sample and the earlier data set, and study the general detectability of subpopulations in small kinematic samples. We conclude that, in the absence of additional supporting observational evidence (e.g. metallicity gradients), subpopulations in small kinematic samples (typically fewer than 100 stars) should be treated with caution, as their identification depends on multiple parameters and rarely produces a signal at a high confidence level. It is therefore essential to explicitly determine the statistical significance of any suggested subpopulation. 2011-05-25T21:53:02.916Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12983 23 page(s) 2011-05-25T21:52:41.097Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13093 We study a model of weakly ionized, protostellar accretion discs that are threaded by a large-scale, ordered magnetic field and power a centrifugally driven wind. We consider the limiting case where the wind is the main repository of the excess disc angular momentum and generalize the radially localized disc model of Wardle & Königl, which focused on the ambipolar diffusion regime, to other field diffusivity regimes, notably Hall and Ohm. We present a general formulation of the problem for nearly Keplerian, vertically isothermal discs using both the conductivity-tensor and the multifluid approaches and simplify it to a normalized system of ordinary differential equations in the vertical space coordinate. We determine the relevant parameters of the problem and investigate, using the vertical-hydrostatic-equilibrium approximation and other simplifications, the parameter constraints on physically viable solutions for discs in which the neutral particles are dynamically well coupled to the field already at the mid-plane. When the charged particles constitute a two-component ion–electron plasma, one can identify four distinct sub-regimes in the parameter domain where the Hall diffusivity dominates and three sub-regimes in the Ohm-dominated domain. Two of the Hall sub-regimes can be characterized as being ambipolar diffusion-like and two as being Ohm-like: the properties of one member of the first pair of sub-regimes are identical to those of the ambipolar diffusion regime, whereas one member of the second pair has the same characteristics as one of the Ohm sub-regimes. All the Hall sub-regimes have Brb/|Bφb| (ratio of radial-to-azimuthal magnetic field amplitudes at the disc surface) >1, whereas in two Ohm sub-regimes this ratio is <1. When the two-component plasma consists, instead, of positively and negatively charged grains of equal mass, the entire Hall domain and one of the Ohm sub-regimes with Brb/|Bφb| < 1 disappear. All viable solutions require the mid-plane neutral–ion momentum exchange time to be shorter than the local orbital time. We also infer that vertical magnetic squeezing always dominates over gravitational tidal compression in this model. In a follow-up paper we will present exact solutions that test the results of this analysis in the Hall regime. 2011-05-25T21:48:27.903Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13229 10 page(s) 2011-05-25T21:42:59.153Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13232 The three main methods currently in use for estimating the excitation class of planetary nebulae (PNe) central stars are compared and evaluated using 586 newly discovered and previously known PNe in the Large Magellanic Cloud (LMC). In order to achieve this we ran a series of evaluation tests using line ratios derived from de-reddened, flux-calibrated spectra. Pronounced differences between the methods are exposed after comparing the distribution of objects to their derived excitation. Line ratio comparisons show that each method’s input parameters have a strong effect on the estimated excitation of a central star. Diagrams were created by comparing excitation classes with Hβ line fluxes. The best methods are then compared to published temperatures using the Zanstra method and assessed for their ability to reflect central star effective temperatures and evolution. As a result we call for a clarification of the term ‘excitation class’ according to the different input parameters used. The first method, which we refer to as Exneb relies purely on the ratios of certain key emission lines. The second method, which we refer to as Ex* includes modeling to create a continuous variable and, for optically thick PNe in the Magellanic Clouds, is designed to relate more closely to intrinsic stellar parameters. The third method, we refer to as Ex[Oiii]/Hβ since the [Oiii]/Hβ ratio is used in isolation to other temperature diagnostics. Each of these methods is shown to have serious drawbacks when used as an indicator for central star temperature. Finally, we suggest a new method (Exρ) for estimating excitation class incorporating both the [Oiii]/Hβ and the Heii λ4686/Hβ ratios. Although any attempt to provide accurate central star temperatures using the excitation class derived from nebula lines will always be limited, we show that this new method provides a substantial improvement over previous methods with better agreement to temperatures derived through the Zanstra method. 2011-05-25T21:42:48.690Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12417 During searches for new optical Galactic supernova remnants (SNRs) in the high-resolution, high-sensitivity Anglo-Australian Observatory/United Kingdom Schmidt Telescope Hα survey of the southern Galactic plane, we uncovered a variety of filamentary and more diffuse, extensive nebular structures in the vicinity of Wolf–Rayet (WR) star 48 (Θ Muscae), only some of which were previously recognized. We used the double-beam spectrograph of the Mount Stromlo and Siding Spring Observatory 2.3-m to obtain low- and mid-resolution spectra of selected new filaments and structures in this region. Despite spectral similarities between the optical spectra of WR star shells and SNRs, a careful assessment of the new spectral and morphological evidence from our deep Hα imagery suggests that the putative shell of Θ Mus is not a WR shell at all, as has been commonly accepted, but is rather part of a more complex area of large-scale overlapping nebulosities in the general field of the WR star. The emission comprises a possible new optical SNR and a likely series of complex H ii regions. Finally, we present the intriguing detection of apparent collimated, directly opposing, ionized outflows close to Θ Mus itself which appears unique among such stars. 2011-04-05T02:50:24.909Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12153 The Antarctic plateau contains the best sites on earth for many forms of astronomy, but none of the existing bases was selected with astronomy as the primary motivation. In this article, we try to systematically compare the merits of potential observatory sites. We include South Pole, Domes A, C, and F, and also Ridge B (running northeast from Dome A), and what we call “Ridge A” (running southwest from Dome A). Our analysis combines satellite data, published results, and atmospheric models, to compare the boundary layer, weather, aurorae, airglow, precipitable water vapor, thermal sky emission, surface temperature, and the free atmosphere, at each site. We find that all Antarctic sites are likely to be compromised for optical work by airglow and aurorae. Of the sites with existing bases, Dome A is easily the best overall; but we find that Ridge A offers an even better site. We also find that Dome F is a remarkably good site. Dome C is less good as a thermal infrared or terahertz site, but would be able to take advantage of a predicted “OH hole” over Antarctica during spring. 2011-03-10T12:50:51.605Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12163 Conodonts, microfossils composed of carbonate-fluor apatite, are abundant in Palaeozoic–Triassic sediments and have a high potential to preserve primary oxygen isotope signals. In order to reconstruct the palaeotemperature history of the Devonian, the oxygen isotope composition of apatite phosphate was measured on 639 conodont samples from sequences in Europe, North America and Australia. The Early Devonian (Lochkovian; 416–411 Myr) was characterized by warm tropical temperatures of around 30 °C. A cooling trend started in the Pragian (410 Myr) with intermediate temperatures around 23 to 25 °C reconstructed for the Middle Devonian (397–385 Myr). During the Frasnian (383–375 Myr), temperatures increased again with temperatures to 30 °C calculated for the Frasnian–Famennian transition (375 Myr). During the Famennian (375–359 Myr), surface water temperatures slightly decreased. Reconstructed Devonian palaeotemperatures do not support earlier views suggesting the Middle Devonian was a supergreenhouse interval, an interpretation based partly on the development of extensive tropical coral–stromatoporoid communities during the Middle Devonian. Instead, the Devonian palaeotemperature record suggests that Middle Devonian coral–stromatoporoid reefs flourished during cooler time intervals whereas microbial reefs dominated during the warm to very warm Early and Late Devonian. 2011-03-10T12:50:19.868Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:11988 Isotopes of the U-decay series, such as ²³⁸U-²³⁰Th-²²⁶Ra, have been used successfully to study timescales of magmatic processes. This study provides an overview of a shorter lived isotope pair, ²¹⁰Pb-²²⁶Ra, which allows study of processes occuring during a crucial time window for magma ascent and eruption. ²¹⁰Pb is fractionated from its great-grandparent ²²⁶Ra by both Pb-Ra fractionation between crystal and melt and via its intermediate parent ²²²Rn during degassing. ²¹⁰Pb-²²⁶Ra activity ratios have been measured in volcanic rocks of various settings for the last 40 years. Here we review published data and their implications for magmatic processes. In particular it is observed that ²¹⁰Pb-²²⁶Ra fractionation is larger at subduction zone volcanoes than in Mid Ocean Ridge Basalts and Ocean Island Basalts. The larger fractionations, which include both ²¹⁰Pb deficits and excesses, are the result of recent fractionation of ²¹⁰Pb-²²⁶Ra during magma degassing and gas streaming. MORB and OIB suffer less from overprinting of prior ²¹⁰Pb-²²⁶Ra signals by degassing. These magmas preserve ²¹⁰Pb-²²⁶Ra fractionation induced by partial melting of the mantle placing tight constraints upon magma ascent rates. 2011-03-01T04:31:19.398Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:11991 The occurrence in arcs of high-magnesium andesites with Mg#'s of around 0.7 implies that such rocks are generated in, or equilibrated with, mantle peridotite under conditions which do not pertain beneath ridges. In this paper we use a review of experimental data on anhydrous peridotite melting to show that these primitive high-Mg andesites cannot be generated by anhydrous melting of lherzolite. Addition of H₂O to peridotitic compositions has been shown, in numerous studies, to displace partial melts from basaltic towards more andesitic compositions leading to the possibility that high-Mg andesites are “wet” melts of lherzolite. Our review of the experimental data demonstrates however, that addition of H₂O alone cannot explain the increases in both SiO₂ and MgO contents (on an anhydrous basis) required to shift from basaltic to high-Mg andesitic melts in equilibrium with lherzolite residue. A much more plausible alternative is that these melts are extracted from a harzburgite residue, a model which we develop in more detail. We performed experiments at 0.6 GPa in which high Mg-andesitic melts were equilibrated with an olivine + orthopyroxene residue and find (in the absence of H₂O) that MgO and SiO₂ contents increase in the ratio 2:1 as the degree of undersaturation in clinopyroxene increases. This is the right sign and magnitude of effect to explain the compositions of primitive high-Mg andesites. Data from earlier studies of the CaO–MgO–SiO₂ system at 1 atm and natural compositions at 1.5–2 GPa are in excellent agreement with our observations. When the effects of H₂O and clinopyroxene undersaturation are added together we obtain a line in Pressure–H₂O space which describes the conditions under which a given high Mg-andesite could be in equilibrium with a harzburgite residue. Application to rocks from White Island (New Zealand), Amphlett Island (Papua New Guinea), Setouchi Belt (Japan), Mt. Shasta (USA), Adak Island and Piip volcano (Aleutians, USA) yield, for crustal thicknesses > 20 km, H₂O contents of the melts of 2–7%, in generally good agreement with the available compositions of melt inclusions. 2011-03-01T04:31:12.329Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:11800 Our R-band data show that the optical light from Malin 1 corresponds well with the >2-arcmin extent of the galaxy's HIi content and continues well beyond previously published V-band optical light radial profiles. Analysis of our image yields improved understanding of the galaxy's properties. We measure ellipticity of 0.20 ± 0.03, implying inclination of 38 ± 3°, and we trace the radial profile to 77 arcsec. A single dusty spiral arm is also weakly discernable, and is consistent with the rotation direction of the Hi and spiral structure of the inner disk. Possible scenarios for the origin of the spiral structure are discussed. 2011-02-14T10:40:58.268Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:11375 We present an empirical analysis of the effectiveness of frame selection (also known as lucky imaging) techniques for high-resolution imaging. A high-speed image recording system has been used to observe a number of bright stars. The observations were made over a wide range of values of D/r₀ and exposure time. The improvement in Strehl ratio of the stellar images due to aligning frames and selecting the best frames was evaluated as a function of these parameters. We find that improvement in Strehl ratio by factors of 4–6 can be achieved over a range of D/r₀ from 3 to 12, with a slight peak at D/r₀∼ 7 . The best Strehl improvement is achieved with exposure times of 10 ms or less, but significant improvement is still obtained at exposure times as long as 640 ms. Our results are consistent with previous investigations but cover a much wider range of parameter space. We show that Strehl ratios of >0.7 can be achieved in appropriate conditions whereas previous studies have generally shown maximum Strehl ratios of ∼0.3. The results are in reasonable agreement with the simulations of Baldwin, Warner & Mackay. 2011-01-20T23:41:32.719Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:11381 We present results from a time-dependent gas-phase chemical model of a hot core based on the physical conditions of G305.2+0.2. While the cyanopolyyne HC₃N has been observed in hot cores, the longer chained species, HC₅N, HC₇N and HC₉N, have not been considered as the typical hot-core species. We present results which show that these species can be formed under hot core conditions. We discuss the important chemical reactions in this process and, in particular, show that their abundances are linked to the parent species acetylene which is evaporated from icy grain mantles. The cyanopolyynes show promise as 'chemical clocks' which may aid future observations in determining the age of hot core sources. The abundance of the larger cyanopolyynes increases and decreases over relatively short time-scales, ∼10²·⁵ yr. We present results from a non-local thermodynamic equilibrium statistical equilibrium excitation model as a series of density, temperature and column density dependent contour plots which show both the line intensities and several line ratios. These aid in the interpretation of spectral-line data, even when there is limited line information available. In particular, non-detections of HC₅N and HC₇N in Walsh et al. are analysed and discussed. 2011-01-20T23:41:20.634Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9015 The unique atmospheric characteristics found at Dome C on the Antarctic plateau offer significant advantages for the operation of adaptive optics systems. An analysis is presented here comparing the performance of adaptive optics systems on telescopes located at Dome C with similar systems located at a mid-latitude site. The large coherence length, wide isoplanatic angle, and long coherence time of the Dome C atmosphere allow an adaptive optics system located there to correct to high order, observe over wide fields and use faint guide stars, resulting in a lower total wavefront error and a significant increase in sky coverage factor than can be achieved at a typical mid-latitude site. While the same performance could in principle be achievable at mid-latitude sites, this would only occur under exceptionally stable atmospheric conditions that are likely to occur on only a few nights per year. 2010-11-26T12:51:16.046Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:10612 Shock-crossing data obtained from spacecraft are used to test the shock location models derived by Chapman and Cairns [2003] . Three sets of data are considered: (1) ISEE 1 for 24–25 September 1987, (2) Wind, Geotail, IMP 8, and Interball for the intervals 26–27 April and 10–13 May 1999, and (3) IMP 8, Geotail, Magion-4, and Cluster during the period 1973–2003 from the bow shock database (available at http://nssdc.gsfc.nasa.gov/ftphelper/bowshock.html). Derived from MHD simulations, the two shock models are for angles θ IMF = 45° and 90° between the upstream magnetic field B IMF and solar wind velocity v sw . These models have azimuthal asymmetries, and they depend explicitly on the upstream Mach number M A and ram pressure P ram. We also test Cairns et al.'s [1995] rotationally symmetric shock model. The new models perform better on average than the rotationally symmetric model, providing some evidence and support for the shock's shape being strongly dependent upon M A and θ IMF. We also compare our analyses here with model/spacecraft comparisons performed by Merka et al. [2003a] and discuss the importance of filtering on the model predictions. 2010-11-17T04:30:32.467Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:10338 Earth's bow shock changes its three-dimensional (3-D) location in response to changes in the solar wind ram pressure Pram, Alfvén Mach number MA, magnetic field orientation, fast mode Mach number Mms, and sonic Mach number MS. Using shock locations from global 3-D ideal MHD simulations [ Cairns and Lyon, 1995 ], empirical models are derived for the 3-D shape and location of Earth's bow shock in the near-Earth regime as a function of solar wind conditions. Multiple simulations with different MA and Pram but two orientations of the interplanetary magnetic field BIMF are analyzed: θIMF = 45° and 90° with respect to the solar wind direction vsw. Models for the (paraboloid) flaring parameter bs as a function of MA, azimuthal angle ϕ, and θIMF = 45° or 90°, show bs decreasing with MA, corresponding to the shock becoming blunter and less swept back (with a larger cross section), as expected. Together with models for the shock's standoff distance (which increases with decreasing MA) the models for bs(MA, ϕ) predict the shock's 3-D location. Variations of bs with ϕ represent eccentricities in the shock's cross section (i.e., a departure from circularity), with the shock extending further perpendicular to vms (the fast mode speed) than parallel, as MA → 1. An additional effect is observed in which the shock shape is “skewed” for θIMF = 45° (but not for θIMF = 90°) in the plane containing BIMF and vsw. These latter two effects are consistent with the fast mode velocity varying with propagation direction relative to BIMF. 2010-10-29T02:42:08.481Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:10337 The location and geometry of Earth's bow shock vary considerably with the solar wind conditions. More specifically, Earth's bow shock is formed by the steepening of fast mode waves, whose speed v ms depends upon the angle θ bn between the local shock normal n and the magnetic field vector B IMF, as well as the Alfvén and sound speeds (v A and c S ). Since v ms is a minimum for θ bn = 0° and low Alfvén Mach number M A , and maximum for θ bn = 90° and high M A , this implies that as θ IMF (the angle between B IMF and v sw ) varies, the magnitude of v ms should vary also across the shock, leading to changes in shape. This paper presents 3-D MHD simulation data which illustrate the changes in shock location and geometry in response to changes in θ IMF and M A , for 1.4 ≤ M A ≤ 9.7 and 0° ≤ θ IMF ≤ 90°. Specifically, for oblique IMF the shock's geometry is shown to become skewed in planes containing B IMF (e.g., the x − z plane). This is also emphasized in the terminator plane data, where the shock is best represented by ellipses, with centers translated along the z axis. For the θ IMF = 90° simulations the shock is symmetric about the x axis in both the x − y and x − z planes. Simulations for field-aligned flow (θ IMF = 0°) show a dimpling of the nose of the shock as M A → 1. The simulations also illustrate the general movement of the shock in response to changes in M A ; high M A shocks are found closer to Earth than low M A shocks. Farris et al.'s [1991] magnetopause model is used in the simulations, and we discuss the limitations of this, as well as the expected results using a self-consistent model. 2010-10-28T23:30:05.456Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9689 Astronomical spectra taken with ground‐based telescopes in the near-IR spectral region are affected by strong absorptions due to molecules in the Earth’s atmosphere, particularly CO₂ and H₂O. These features need to be removed in order to reveal the true spectrum of the object being observed. The traditional technique for doing this is to observe a standard star with a relatively featureless spectrum (a smooth spectrum standard) and divide the observed spectrum of the object by that of the standard. This technique has a number of practical difficulties but is also fundamentally flawed. Even with a perfectly smooth spectrum standard observed at an identical air mass to the object, this technique does not, in general, reproduce the spectrum of the object that would have been observed in the absence of the Earth’s atmosphere. The problem arises because of the presence of high‐resolution structure in the molecular absorption features, generally on a finer scale than the resolution of the observed spectrum. The transmission of the Earth’s atmosphere for any spectral bin then depends on the unresolved spectral structure of the light being transmitted and cannot be represented by a unique value derived from the standard star, as the traditional technique assumes. We use high‐resolution line‐by‐line radiative transfer models to quantify these effects for two cases: observations of a solar‐type star, and observations of the planet Mars. For a solar‐type star, application of the traditional technique causes errors of a few percent in the vicinity of strong atmospheric absorption features. The case of the planet Mars is an extreme case, since Mars has the same CO₂ absorption features as the Earth’s atmosphere. Our simulations show that applying the traditional astronomical technique to ground‐based spectra of the planet Mars leads to systematic errors of up to 50% in the vicinity of the strong CO₂ absorption features. High‐quality IR spectroscopy with ground‐based telescopes requires an improved technique to handle the absorption in the Earth’s atmosphere. We outline a possible approach based on the use of radiative transfer models for the Earth’s atmosphere. We note that in general it is not possible to correct observed spectra for atmospheric absorption. However, a forward‐modeling approach can be used in which a model spectrum for the object is generated, atmospheric transmission effects added, and the result compared with the observed spectrum. We present a demonstration of the ability of a model to accurately represent the Earth atmosphere transmission in the J band. 2010-10-11T22:20:18.000Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9629 We present the first direct measurements of the terahertz atmospheric transmission above Dome A, the highest point on the Antarctic plateau at an elevation of 4.1 km. The best-quartile atmospheric transmission during the Austral winter is 80% at a frequency of 661 GHz (453 μm), corresponding to a precipitable water vapor column of 0.1 mm. Daily averages as low as 0.025 mm were observed. The Antarctic atmosphere is very stable, and excellent observing conditions generally persist for many days at a time. The exceptional conditions over the high Antarctic plateau open new far-infrared spectral windows to ground-based observation. These windows contain important spectral-line diagnostics of star formation and the interstellar medium which would otherwise only be accessible to airborne or space telescopes. 2010-10-05T07:10:27.939Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9228 We report on infrared observations of the planetary nebula Hen 2-113 obtained with VLT/NACO, VLTI/MIDI, VLT/ISAAC and TIMMI at the ESO 3.6 m. Hen 2-113 exhibits a clear ring-like structure superimposed to a more diffuse environment visible in the L' (3.8 μm), M' (4.78 μm) and 8.7 μm bands. No clear core at 8.7 μm and no fringes through the N band could be detected for this object with MIDI. A qualitative interpretation of the object structure is proposed using a diabolo-like geometrical model. The PAH content of the nebula was also studied with ISAAC and TIMMI observations. This indicates that the PAHs are mostly concentrated towards the lobes of the diabolo and the bipolar lobes of the nebula. In L' band, a void 0.3″ in diameter was discovered with NACO around the central source. The L' and M' fluxes from the central source were derived from NACO data indicating an important infrared excess with respect to the expected stellar emission based on stellar models and short wavelength data. The observed flux from this source in the L' and M' is about 300 and 800 times respectively than those expected from a model including only the central star. Moreover, the central object appears resolved in L' band with measured FWHM of 155 mas. This infrared excess can be explained by emission from a cocoon of hot dust (T ~ 1000K) with a total mass ~10⁻⁹M⊙. 2010-08-19T09:00:27.307Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9231 Aims: We present high spatial resolution observations of the dusty core of the Planetary Nebula with Wolf-Rayet central star CPD-56°8032, for which indications of a compact disk have been found by HST/SITS observations. Methods: These observations were taken with the mid-infrared interferometer VLTI/MIDI in imaging mode providing a typical 300 mas resolution and in interferometric mode using UT2-UT3 47m baseline providing a typical spatial resolution of 20 mas. We also made use of unpublished HST/ACS images in the F435W and F606W filters. Results: The visible HST images exhibit a complex multilobal geometry dominated by faint lobes. The farthest structures are located at 7″ from the star. The mid-IR environment of CPD-56°8032 is dominated by a compact source, barely resolved by a single UT telescope in a 8.7 μm filter ( Δλ=1.6 μm, contaminated by PAH emission). The infrared core is almost fully resolved with the three 40-45 m projected baselines ranging from -5° to 51° but smooth oscillating fringes at low level have been detected in spectrally dispersed visibilities. This clear signal is interpreted in terms of a ring structure which would define the bright inner rim of the equatorial disk. Geometric models allowed us to derive the main geometrical parameters of the disk. For instance, a reasonably good fit is reached with an achromatic and elliptical truncated Gaussian with a radius of 97±11 AU, an inclination of 28±7° and a PA for the major axis at 345°±7°. Furthermore, we performed some radiative transfer modeling aimed at further constraining the geometry and mass content of the disk, by taking into account the MIDI dispersed visibilities, spectra, and the large aperture SED of the source. These models show that the disk is mostly optically thin in the N band and highly flared. As a consequence of the complex flux distribution, an edge-on inclination is not excluded by the data. 2010-08-19T09:00:16.713Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9196 Shaping axisymmetric planetary nebulae is easier if a companion interacts with a primary at the top of the asymptotic giant branch. To determine the impact of binarity on planetary nebula formation and shaping, we need to determine the central star of planetary nebula binary fraction and period distribution. The short-period binary fraction has been known to be 10-15% from a survey of ~100 central stars for photometric variability indicative of irradiation effects, ellipsoidal variability, or eclipses. This survey technique is known to be biased against binaries with long periods and this fact is used to explain why the periods of all the binaries discovered by this survey are smaller than 3 days. In this paper we assess the status of knowledge of binary central stars discovered because of irradiation effects. We determine that, for average parameters, this technique should be biased against periods longer than 1-2 weeks, so it is surprising that no binaries were found with periods longer than 3 days. Even more puzzling is the fact that 9 out of 12 of the irradiated binaries have periods smaller than 1 day, a fact that is starkly at odds with post-common envelope predictions. We suggest that either all common envelope models tend to overestimate post-common envelope periods or this binary survey might have suffered from additional, unquantified biases. If the latter hypothesis is true, the currently known short-period binary fraction is put in serious doubt. We also introduce a new survey for binary-related variability, which will enable us to better quantify biases and determine an independent value for the short-period binary fraction. 2010-08-13T17:40:16.221Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9020 The best ground‐based astronomical sites in terms of telescope sensitivity at infrared and submillimeter wavelengths are located on the Antarctic Plateau, where high atmospheric transparency and low sky emission are obtained because of the extremely cold and dry air; these benefits are well characterized at the South Pole station. The relative advantages offered by three potentially superior sites, Dome C, Dome F, and Dome A, located higher on the Antarctic Plateau, are quantified here through the development of atmospheric models using the line‐by‐line radiative transfer model code. In the near‐ to mid‐infrared, sensitivity gains relative to the South Pole of up to a factor of 10 are predicted at Dome A, and a factor of 2 for Dome C. In the mid‐ to far‐infrared, sensitivity gains relative to the South Pole up to a factor of 100 are predicted for Dome A and 10 for Dome C. These values correspond to even larger gains (up to 3 orders of magnitude) compared to the best mid‐latitude sites, such as Mauna Kea and the Chajnantor Plateau. 2010-08-10T08:51:17.524Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9016 We present low‐resolution turbulence profiles of the atmosphere above Dome C, Antarctica, measured with the MASS instrument during 25 nights in 2004 March–May. Except for the lowest layer, Dome C has significantly less turbulence than Cerro Tololo and Cerro Pachón. In particular, the integrated turbulence at 16 km is always less than the median values at the two Chilean sites. From these profiles we evaluate the photometric noise produced by scintillation, and the atmospheric contribution to the error budget in narrow‐angle differential astrometry. In comparison with the two midlatitude sites in Chile, Dome C offers a potential gain of about 3.6 in both photometric precision (for long integrations) and narrow‐angle astrometry precision. These gain estimates are preliminary, being computed with average wind‐speed profiles, but the validity of our approach is confirmed by independent data. Although the data from Dome C cover a fairly limited time frame, they lend strong support to expectations that Dome C will offer significant advantages for photometric and astrometric studies. 2010-07-27T09:50:23.503Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9019 A good astronomical site must fulfill several criteria including low atmospheric turbulence and low wind speeds. It is therefore important to have a detailed knowledge of the temperature and wind conditions of a location considered for future astronomical research. Antarctica has unique atmospheric conditions that have already been exploited at the South Pole station. Dome C, a site located on a local maximum of the Antarctic plateau, is likely to have even better conditions. In this paper we present the analysis of two decades of wind speed measurements taken at Dome C by an automated weather station (AWS). We also present temperature and wind speed profiles taken over four Antarctic summers using balloon-borne weather sondes. We will show that as well as having one of the lowest average wind speed ever recorded at an existing or potential observatory, Dome C also has an extremely stable upper atmosphere and a very low inversion layer. 2010-07-26T11:10:12.538Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:8361 Context.The overshooting convective motions in the solar photosphere, resulting in the foot point motion of different magnetic structures in the solar atmosphere, are frequently proposed as the source for the excitation of Alfvén waves, which are assumed to propagate towards the chromosphere and corona resulting finally in the heating of these layers by the dissipation of this wave energy. However, the photosphere is a) very weakly ionized, and, b) the dynamics of the plasma particles in this region is heavily influenced by the plasma-neutral collisions. Aims.The purpose of this work is to check the consequences of these two facts on the above scenario and their effects on the electromagnetic waves. Methods.Standard plasma theory is used and the wave physics of the weakly ionized photosphere is discussed. The magnetization and the collision frequencies of the plasma constituents are quantitatively examined. Results.It is shown that the ions and electrons in the photosphere are both un-magnetized; their collision frequency with neutrals is much larger than the gyro-frequency. This implies that eventual Alfvén-type electromagnetic perturbations must involve the neutrals as well. This has the following consequences: i) in the presence of perturbations, the whole fluid (plasma + neutrals) moves; ii) the Alfvén velocity includes the total (plasma + neutrals) density and is thus considerably smaller compared to the collision-less case; iii) the perturbed velocity of a unit volume, which now includes both plasma and neutrals, becomes much smaller compared to the ideal (collision-less) case; and iv) the corresponding wave energy flux for the given parameters becomes much smaller compared to the ideal case. Conclusions.The wave energy flux through the photosphere becomes orders of magnitude smaller, compared to the ideal case, when the effects of partial ionization and collisions are consistently taken into account. 2010-05-27T07:10:12.732Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:8301 We present elemental abundances of 118 stars (28 of which are known extrasolar planetary host stars) observed as part of the Anglo-Australian Planet Search. Abundances of O, Mg, Cr, Y, Zr, Ba, Nd, and Eu (along with previously published abundances for C and Si) are presented. This study is one of the first to specifically examine planetary host stars for the heavy elements produced by neutron capture reactions. We find that the abundances in host stars are chemically different from both the standard solar abundances and the abundances in non-host stars in all elements studied, with enrichments over non-host stars ranging from 0.06 dex (for O) to 0.11 dex (for Cr and Y). Such abundance trends are in agreement with other previous studies of field stars and lead us to conclude that the chemical anomalies observed in planetary host stars are the result of normal galactic chemical evolution processes. Based on this observation, we conclude that the observed chemical traits of planetary host stars are primordial in origin, coming from the original nebula and not from a "pollution" process occurring during or after formation, and that planet formation occurs naturally with the evolution of stellar material. 2010-05-24T10:11:06.369Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:7524 Sgr A*, the massive black hole at the center of the Galaxy, varies in radio through X-ray emission on hourly timescales. The flare activity is thought to arise from the innermost region of an accretion flow onto Sgr A*. We present simultaneous light curves of Sgr A* in radio, submillimeter and X-rays that show a possible time delay of 110 ± 17 minutes between X-ray and 850 μm suggesting that the submillimeter flare emission is optically thick. At radio wavelengths, we detect time lags of 20.4 ± 6.8, 30 ± 12, and 20 ± 6 minutes between the flare peaks observed at 13 and 7 mm (22 and 43 GHz) in three different epochs using the VLA. Linear polarization of 1% ± 0.2% and 0.7 ± 0.1% is detected at 7 and 13 mm, respectively, when averaged over the entire observation on 2006 July 17. A simple model of a bubble of synchrotron-emitting electrons cooling via adiabatic expansion can explain the time delay between various wavelengths, the asymmetric shape of the light curves, and the observed polarization of the flare emission at 43 and 22 GHz. The derived physical quantities that characterize the emission give an expansion speed of vexp ~ 0.003–0.1 c, magnetic field of B ~ 10–70 G, and particle spectral index p ~ 1–2. These parameters suggest that the associated plasma cannot escape from Sgr A* unless it has a large bulk motion. 2010-03-23T13:41:36.326Z ]]>