The rapid evolution of the exciting star of the Stingray Nebula

SAO244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed UV and optical spectra, taken during 1988-2013, allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. We find that the central star has steadily increased its effective temperature from 38kK in 1988 to a peak value of 60kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log (dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from 1800km/s to 2800km/s. Since around 2002, the star stopped heating and has cooled down again to 55kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. By comparison with stellar-evolution calculations, we confirm that SAO244567 must be a low-mass star (M < 0.55 Msun). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the common-envelope phase, during which the planetary nebula was ejected.Comment: 16 pages, 13 figures, accepted for publication in A&

A Five-year Spectroscopic and Photometric Campaign on the Prototypical alpha Cygni Variable and A-type Supergiant Star Deneb

Deneb is often considered the prototypical A-type supergiant, and is one of the visually most luminous stars in the Galaxy. A-type supergiants are potential extragalactic distance indicators, but the variability of these stars needs to be better characterized before this technique can be considered reliable. We analyzed 339 high resolution echelle spectra of Deneb obtained over the five-year span of 1997 through 2001 as well as 370 Stromgren photometric measurements obtained during the same time frame. Our spectroscopic analysis included dynamical spectra of the H-alpha profile, H-alpha equivalent widths, and radial velocities measured from Si II 6347, 6371. Time-series analysis reveals no obvious cyclic behavior that proceeds through multiple observing seasons, although we found a suspected 40 day period in two, non-consecutive observing seasons. Some correlations are found between photometric and radial velocity data sets, and suggest radial pulsations at two epochs. No correlation is found between the variability of the H-alpha profiles and that of the radial velocities or the photometry. Lucy (1976) found evidence that Deneb was a long period single-lined spectroscopic binary star, but our data set shows no evidence for radial velocity variations caused by a binary companion.Comment: 49 pages, 9 figures, 5 tables, accepted for publication in the Astronomical Journa

Synthesis of bulk, dense, nanocrystalline yttrium aluminum garnet from amorphous powders

Amorphous powders of Al2O3x2014;37.5 mol% Y2O3 (yttrium aluminum garnet (YAG)) were prepared by coprecipitation, decomposed at 800xB0;C, and hot-pressed uniaxally at low temperature (600xB0;C) and a moderate pressure (750 MPa). Optimum conditions yielded microstructures with only 2% porosity and partial crystallization of YAG. Further processing using high quasi-hydrostatic pressure (1 GPa) at 1000xB0;C enabled the production of fully crystallized YAG with gt;96% relative density and a nanocrystalline grain size of x223C;70 nm. 13

A review of wet air oxidation and thermal hydrolysistechnologies in sludge treatment

With rapid world population growth and strict environmental regulations, increasingly large volumes of sludge are being produced in today&#039;s wastewater treatment plants (WWTP) with limited disposal routes. Sludge treatment has become an essential process in WWTP, representing 50% of operational costs. Sludge destruction and resource recovery technologies are therefore of great ongoing interest. Hydrothermal processing uses unique characteristics of water at elevated temperatures and pressures to deconstruct organic and inorganic components of sludge. It can be broadly categorized into wet oxidation (oxidative) and thermal hydrolysis (non-oxidative). While wet air oxidation (WAO) can be used for the final sludge destruction and also potentially producing industrially useful by-products such as acetic acid, thermal hydrolysis (TH) is mainly used as a pre-treatment method to improve the efficiency of anaerobic digestion. This paper reviews current hydrothermal technologies, roles of wet air oxidation and thermal hydrolysis in sludge treatment, and challenges faced by these technologies

Far-infrared imaging of post-AGB stars and (proto)-planetary nebulae with the AKARI Far-Infrared Surveyor

By tracing the distribution of cool dust in the extended envelopes of post-AGB stars and (proto)-planetary nebulae ((P)PNe) we aim to recover, or constrain, the mass loss history experienced by these stars in their recent past. The Far-Infrared Surveyor (FIS) instrument on board the AKARI satellite was used to obtain far-infrared maps for a selected sample of post-AGB stars and (P)PNe. We derived flux densities (aperture photometry) for 13 post-AGB stars and (P)PNe at four far-infrared wavelengths (60, 90, 140, and 160 um). Radial (azimuthally averaged) profiles are used to investigate the presence of extended emission from cool dust. No (detached) extended emission is detected for any target in our sample at levels significant with respect to background and cirrus emission. Only IRAS 21046+4739 reveals tentative excess emission between 30 and 130". Estimates of the total dust and gas mass from the obtained maps indicate that the envelope masses of these stars should be large in order to be detected with the AKARI FIS. Imaging with higher sensitivity and higher spatial resolution is needed to detect and resolve, if present, any cool compact or extended emission associated with these evolved stars.Comment: accepted for publication in The Astronomical Journal (16 pages, 3 figures and 4 tables

Radiation and String Atmosphere for Relativistic Stars

We extend the Vaidya radiating metric to include both a radiation field and a string fluid. Assuming diffusive transport for the string fluid, we find new analytic solutions of Einstein's field equations. Our new solutions represent an extention of Xanthopoulos superposition.Comment: To appear in Phys. Rev. D, Rapid Communicatio

Birth and early evolution of a planetary nebula

The final expulsion of gas by a star as it forms a planetary nebula --- the ionized shell of gas often observed surrounding a young white dwarf --- is one of the most poorly understood stages of stellar evolution. Such nebulae form extremely rapidly (about 100 years for the ionization) and so the formation process is inherently difficult to observe. Particularly puzzling is how a spherical star can produce a highly asymmetric nebula with collimated outflows. Here we report optical observations of the Stingray Nebula which has become an ionized planetary nebula within the past few decades. We find that the collimated outflows are already evident, and we have identified the nebular structure that focuses the outflows. We have also found a companion star, reinforcing previous suspicions that binary companions play an important role in shaping planetary nebulae and changing the direction of successive outflows.Comment: 9 pages + 3 figures. To appear in Nature, 2 April 199

Temperature and ac Effects on Charge Transport in Metallic Arrays of Dots

We investigate the effects of finite temperature, dc pulse, and ac drives on the charge transport in metallic arrays using numerical simulations. For finite temperatures there is a finite conduction threshold which decreases linearly with temperature. Additionally we find a quadratic scaling of the current-voltage curves which is independent of temperature for finite thresholds. These results are in excellent agreement with recent experiments on 2D metallic dot arrays. We have also investigated the effects of an ac drive as well as a suddenly applied dc drive. With an ac drive the conduction threshold decreases for fixed frequency and increasing amplitude and saturates for fixed amplitude and increasing frequency. For sudden applied dc drives below threshold we observe a long time power law conduction decay.Comment: 6 pages, 7 postscript figure