Casagrande L.,The Australian National University |
VandenBerg D.A.,University of Victoria
Monthly Notices of the Royal Astronomical Society | Year: 2014
After a pedagogical introduction to the main concepts of synthetic photometry, colours and bolometric corrections in the Johnson-Cousins, 2MASS, and HST-ACS/WFC3 photometric systems are generated from MARCS synthetic fluxes for various [Fe/H] and [α/Fe] combinations, and virtually any value of E(B - V) ≤ 0.7. The successes and failures of model fluxes in reproducing the observed magnitudes are highlighted. Overall, extant synthetic fluxes predict quite realistic broad-band colours and bolometric corrections, especially at optical and longer wavelengths: further improvements of the predictions for the blue and ultraviolet spectral regions await the use of hydrodynamic models where the microturbulent velocity is not treated as a free parameter. We show how the morphology of the colour-magnitude diagram (CMD) changes for different values of [Fe/H] and [α/Fe]; in particular, how suitable colour combinations can easily discriminate between red giant branch and lower main-sequence populations with different [α/Fe], due to the concomitant loops and swings in the CMD. We also provide computer programs to produce tables of synthetic bolometric corrections as well as routines to interpolate in them. These colour-Teff-metallicity relations may be used to convert isochrones for different chemical compositions to various bandpasses assuming observed reddening values, thus bypassing the standard assumption of a constant colour excess for stars of different spectral type. We also show how such an assumption can lead to significant systematic errors. The MARCS transformations presented in this study promise to provide important constraints on our understanding of the multiple stellar populations found in globular clusters (e.g. the colours of lower main-sequence stars are predicted to depend strongly on [α/Fe]) and of those located towards/in the Galactic bulge. © 2014 The Authors.
Engerer N.A.,The Australian National University |
Engerer N.A.,Canberra Research LaboratoryACT
Solar Energy | Year: 2015
Separating global horizontal irradiance measurements into direct and diffuse components has been vigorously discussed over the past half-century of solar radiation research leading to the creation of many models which attempt to compute these components with varying degrees of success. However, over the course of this discussion, nearly all studies have focused on hourly values, with no studies that have proposed a model for minute-level values of irradiance. As data-logging technologies have become much more prolific and their storage capabilities much larger, solar radiation monitoring sites are more commonly logging data at intervals much less than one hour, but no models exists that are designed to separate these measurements into direct and diffuse components. In Australia, the Australian Bureau of Meteorology and the Australian Solar Institute have compiled a dataset of tens of millions of one-minute global, direct and diffuse solar irradiance observations, comprising data from regions all around Australia. This dataset provides a unique opportunity to investigate the relationships between global irradiance and its direct and diffuse components at higher resolution than has previously been possible. Herein, the largest and most complete diffuse fraction model analysis yet undertaken for Australian solar radiation data, and the first ever to focus on minute resolution data is reported. Nine of the most prominent diffuse fraction, or "separation", models are tested against minute resolution radiation data from three datasets. The first removed cloud enhancement events in accordance with practices undertaken by the majority of studies in the literature. The second retains these events in order to assess which model would be best suited for operational purposes. The third consisted of only clear sky observations, in order to assess the performance of diffuse fraction models under clear skies. Through the course of this study only the Perez model was found to perform satisfactorily for minute resolution data at sites in southeastern Australia. Three new diffuse models proposed in this study, one trained for each of the three datasets, were found to greatly exceed the performance of existing modeling techniques, with slight improvements over the Perez model. © 2015 Elsevier Ltd.
The research found the females are actually searching for a safe haven from birds and other predators rather than hunting for the perfect match. Researcher Professor Patricia Backwell from The Australian National University (ANU) said the findings overturn previous theories and helped scientists better understand the breeding habits of fiddler crabs, which are crucial to the ecological health of mangroves, salt marshes and muddy beaches around the world. "This behaviour of visiting and supposedly rejecting successive males has always been taken as a defining feature of female choosiness, but this study shows that things are not always what they seem," said Professor Backwell, from the ANU Research School of Biology. Male fiddler crabs are known for having one claw that is considerably larger than the other. Fiddler crabs are found in mangroves and salt marshes and on sandy or muddy beaches of West Africa, the Western Atlantic, Eastern Pacific and Indo-Pacific. Professor Backwell said female fiddler crabs visited successive displaying males in their burrows to identify safe places to hide in the event of predator attacks, and not because they were searching for a perfect mating partner. "If a bird attacks, female fiddler crabs can move quickly and directly back to the last burrow it visited," said Professor Backwell, who is based in Darwin at the moment to conduct field work. "Having this map of burrow positions is essential if they are to survive a bird attack, and this is true for females who are looking for a mate and those who are looking for a burrow." Co-lead researcher Dr Marianne Peso said the team conducted experiments to observe and compare the behaviour of mate- and burrow-searching females. The team noticed female fiddler crabs not seeking a mate visited successive males before settling in a new burrow in the same manner as mate-searching females. "We watched displaced females move across the mudflat, testing mate preferences with male-mimicking robotic crabs, examining male reactions to the females and testing the females' response to a simulated bird predator," said Dr Peso, who was based at ANU at the time of the study and is now at Macquarie University Department of Biological Sciences. "In all experiments, mate-searching and burrow-searching females behaved identically. "They all visited courting males, they found the same robotic males attractive, males treated them in the same way as potential mates and all the females retreated to the last burrow they visited when swooped by the plastic bird." More information: M. Peso et al, Not what it looks like: mate-searching behaviour, mate preferences and clutch production in wandering and territory-holding female fiddler crabs: Table 1., Royal Society Open Science (2016). DOI: 10.1098/rsos.160339
Lead researcher Dr Yuerui (Larry) Lu from The Australian National University (ANU) said the discovery hinged on the remarkable potential of the molybdenum disulphide crystal. "This type of material is the perfect candidate for future flexible displays," said Dr Lu, leader of Nano-Electro-Mechanical System (NEMS) Laboratory in the ANU Research School of Engineering. "We will also be able to use arrays of micro lenses to mimic the compound eyes of insects." The 6.3-nanometre lens outshines previous ultra-thin flat lenses, made from 50-nanometre thick gold nano-bar arrays, known as a metamaterial. "Molybdenum disulphide is an amazing crystal," said Dr Lu "It survives at high temperatures, is a lubricant, a good semiconductor and can emit photons too. "The capability of manipulating the flow of light in atomic scale opens an exciting avenue towards unprecedented miniaturisation of optical components and the integration of advanced optical functionalities." Molybdenum disulphide is in a class of materials known as chalcogenide glasses that have flexible electronic characteristics that have made them popular for high-technology components. Dr Lu's team created their lens from a crystal 6.3-nanometres thick - 9 atomic layers - which they had peeled off a larger piece of molybdenum disulphide with sticky tape. They then created a 10-micron radius lens, using a focussed ion beam to shave off the layers atom by atom, until they had the dome shape of the lens. The team discovered that single layers of molybdenum disulphide, 0.7 nanometres thick, had remarkable optical properties, appearing to a light beam to be 50 times thicker, at 38 nanometres. This property, known as optical path length, determines the phase of the light and governs interference and diffraction of light as it propagates. "At the beginning we couldn't imagine why molybdenum disulphide had such surprising properties," said Dr Lu. Collaborator Assistant Professor Zongfu Yu at the University of Wisconsin, Madison, developed a simulation and showed that light was bouncing back and forth many times inside the high refractive index crystal layers before passing through. Molybdenum disulphide crystal's refractive index, the property that quantifies the strength of a material's effect on light, has a high value of 5.5. For comparison, diamond, whose high refractive index causes its sparkle, is only 2.4, and water's refractive index is 1.3. This study is published in the Nature serial journal Light: Science and Applications. Explore further: New material allows for ultra-thin solar cells
News Article | September 7, 2016
The Australian National University will establish an international research program to improve ways to store renewable energy under a new $8 million partnership with the ACT Government. ANU Vice-Chancellor Professor Brian Schmidt thanked the ACT Government for contributing up to $5 million to support the program, which would help to establish Australian research leadership in the integration of battery material technology with electricity network storage.