Circumstellar discs in the young σ Orionis cluster

We present new K - and L '-band imaging observations for members of the young (3–5 Myr) σ Orionis cluster, obtained at the United Kingdom Infrared Telescope (UKIRT) with the UKIRT 1–5 micron Imager Spectrometer (UIST). We determine ( K − L ') colour excesses with respect to the photospheres, finding evidence for warm circumstellar dust around 27 out of 83 cluster members that have masses between 0.04 and  1.0 M_⊙  . This indicates a circumstellar disc frequency of at least (33 ± 6) per cent for this cluster, consistent with previous determinations from smaller samples and also consistent with the 3-Myr disc half-life suggested by Haisch et al. There is marginal evidence that the disc frequency declines towards lower masses, but the data are also consistent with no mass dependence at all. There is no evidence for spatial segregation of objects with and without circumstellar discs.     

Optical and infrared diagnostics of SDSS galaxies in the SWIRE survey

We present the rest-frame optical and infrared colours of a complete sample of  1114 z < 0.3  galaxies from the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy Survey and the Sloan Digital Sky Survey (SDSS). We discuss the optical and infrared colours of our sample and analyse in detail the contribution of dusty star-forming galaxies and active galactic nuclei (AGN) to optically selected red sequence galaxies.
We propose that the optical  ( g − r )  colour and infrared  log( L ₂₄/ L _3.6)  colour of galaxies in our sample are determined primarily by a bulge-to-disc ratio. The  ( g − r )  colour is found to be sensitive to the bulge-to-disc ratio for disc-dominated galaxies, whereas the  log( L ₂₄/ L _3.6)  colour is more sensitive for bulge-dominated systems.
We identify ∼18 per cent (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star-forming galaxies with blue optical colours. Using emission-line diagnostic diagrams, 78 are found to have an AGN contribution and 117 are identified as star-forming systems. The red  ( g − r )  colour of the star-forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component.
The number densities of optically red star-forming galaxies are found to correspond to ∼13 per cent of the total number density of our sample. In addition, these systems contribute ∼13 per cent of the total optical luminosity density, and 28 per cent of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for 'dry mergers'.     

Molecular line intensities as measures of cloud masses – I. Sensitivity of CO emissions to physical parameter variations

A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H₂, the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the  ¹²CO( J = 1 → 0)  emission line, using a CO-to-H₂ conversion factor, X . Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different from those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor.     

InFOCμS Hard X-ray Imaging Telescope

InFOCμS is a new generation balloon-borne hard X-ray telescope with focusing optics and spectroscopy. We had a successful 22.5-hour flight from Fort Sumner, NM on September 16,17, 2004. In this paper, we present the performance of the hard X-ray telescope, which consists of a depth-graded platinum/carbon multilayer mirror and a CdZnTe detector. The telescope has an effective area of 49 cm² at 30 keV, an angular resolution of 2.4 arcmin (HPD), and a field of view of 11 arcmin (FWHM) depending on energies. The CdZnTe detector is configured with a 12 × 12 segmented array of detector pixels. The pixels are 2 mm square, and are placed on 2.1 mm centers. An averaged energy resolution is 4.4 keV at 60 keV and its standard deviation is 0.36 keV over 128 pixels. The detector is surrounded by a 3-cm thick CsI anti coincidence shield to reduce background from particles and photons not incident along the mirror focal direction. The inflight background is 2.9 × 10⁻⁴ cts cm⁻² sec⁻¹ keV⁻¹ in the 20–50 keV band.     

Results from the Whole Earth Telescope — Indian contributions

The Indian team at ISRO has been part of the Whole Earth Telescope (WET) team since 1988 when we first participated in the campaign on V471 Tau. We have been part of many other observing campaigns over the last decade. This presentation traces the circumstances leading to our joining the WET team and how useful the coverage from the Indian longitude has been. The results of several pulsators from the WET runs during which we participated are also described. These include PG1159-035 the prototype of the GW Vir type of stars, RE J 0751+14 a cataclysmic variable, PG 1336-018 a binary with an sdB pulsator and finally HR 1217 a roAp star. The paper concludes with what the limitations are in our observations and how we can overcome them in the future.     

Impact of the Mass Parameter on Particle Dynamics in a Ring Configuration of N Bodies

Parameters play a very important and determinative role in the dynamics of a dynamical system as well as in the formation of its particular characteristics. In this paper we investigate the way in which a large scale variation of the mass parameter, influences the behavior of a mass-less particle which moves in the vicinity of a ring arrangement of N-bodies. More precisely, we study the impact of this parameter on periodic motions and their characteristics.     

Analytical Study of Supernova Remnant Non-Stationary Expansions

We study analytically the Rayleigh–Taylor instability in expanding supernova gas shell. The instability appears at the inner shell surface accelerated by blowing pulsar wind. The most dangerous perturbations correspond to wavelengths comparable to the shell thickness. We analyze the fragility of the supernova remnant shell in function of the initial perturbation amplitude and the shell thickness.     

The age of Wolfe Creek meteorite crater (Kandimalal), Western Australia

Wolfe Creek crater lies in northwestern Australia at the edge of the Great Sandy Desert. Together with Meteor Crater, it is one of the two largest craters on Earth from which meteorite fragments have been recovered. The age of the impact is poorly constrained and unpublished data places the event at about 300,000 years ago. In comparison, Meteor Crater is well constrained by exposure dating. In this paper, we present new ages for Wolfe Creek Crater from exposure dating using the cosmogenic nuclides ¹⁰Be and ²⁶Al, together with optically stimulated luminescence ages (OSL) on sand from a site created by the impact. We also present a new topographic survey of the crater using photogrammetry. The exposure ages range from ~86 to 128 ka. The OSL ages indicate that the age of the impact is most likely to be ~120 ka with a maximum age of 137 ka. Considering the geomorphic setting, the most likely age of the crater is 120 ± 9 ka. Last, we review the age of Meteor Crater in Arizona. Changes in production rates and scaling factors since the original dating work revise the impact age to 61.1 ± 4.8 ka, or ~20% older than previously reported.