A rival for Babcock's star: the extreme 30-kG variable magnetic field in the Ap star HD 75049

The extraordinary magnetic Ap star HD 75049 has been studied with data obtained with the European Southern Observatory Very Large Telescope and 2.2-m telescopes. Direct measurements reveal that the magnetic field modulus at maximum reaches 30 kG. The star shows photometric, spectral and magnetic variability with a rotation period of 4.049 d. Variations of the mean longitudinal magnetic field can be described to first order by a centred dipole model with an inclination i= 25°, an obliquity β= 60° and a polar field Bp= 42 kG. The combination of the longitudinal and surface magnetic field measurements implies a radius of R= 1.7 R⊙, suggesting that the star is close to the zero-age main sequence. HD 75049 displays moderate overabundances of Si, Ti, Cr, Fe and large overabundances of rare earth elements. This star has the second strongest magnetic field of any main-sequence star after Babcock's star, HD 215441, which it rivals

Mechanical properties of thermally-treated and recycled glass fibres

This paper investigates the effects of temperature, heating time and atmosphere on the tensile modulus and strength of thermally-treated E-glass fibres. The heating conditions that were investigated are identical to those used in thermal recycling of waste polymer matrix composite materials, and therefore this study determines the effects of the recycling process conditions on the properties of reclaimed fibreglass. The loss in fibre strength is dependent on the temperature and time of the thermal process, and large strength loss occurs under the heating conditions used for high temperature incineration of polymer composites. A phenomenological model is presented for the residual fibre strength for the temperatures and heating time of the thermal recycling process. The reduction in fibre strength is dependent on the thermal recycling atmosphere under low temperature or short heating time conditions, but at high temperatures the strength loss is the same, regardless of furnace atmosphere (ambient air, dry air or inert gas). Quantitative fractographic analysis of the fibres shows that fracture for all heat treatments is caused by surface flaws. The strength loss is most probably due to structural relaxation during thermal annealing and a secondary effect of adsorbed surface water attacking the glass by thermally-activated stress-corrosion. It is shown that large reductions in fibre strength due to thermal recycling are not recovered during composite manufacture, therefore resulting in composite materials with significantly lower strength. The reduced strength of the composite matches the reduced fibre strength following thermal recycling

The Core-Wing Anomaly of Cool Ap Stars: Abnormal Balmer Profiles

Paper by Cowley et al. The Core-Wing Anomaly Etc. The profiles of H$\alpha$ in a number of cool Ap stars are anomalous. Broad wings, indicative of temperatures in the range 7000-8000K end abruptly in narrow cores. The widths of these cores are compatible with those of dwarfs with temperatures of 6000K or lower. This profile has been known for Przybylski's star, but it is seen in other cool Ap's. The H$\beta$ profile in several of these stars shows a similar core-wing anomaly (CWA). In Przybylski's star, the CWA is probably present at higher Balmer members. We are unable to account for these profiles within the context of LTE and normal dwarf atmospheres. We conclude that the atmospheres of these stars are not ``normal.'' This is contrary to a notion that has long been held.Comment: 4 Pages 5 Figures. Submitted to Astronomy and Astrophysics 4 Dec. 200

Magnetic chemically peculiar stars

Chemically peculiar (CP) stars are main-sequence A and B stars with abnormally strong or weak lines for certain elements. They generally have magnetic fields and all observables tend to vary with the same period. Chemically peculiar stars provide a wealth of information; they are natural atomic and magnetic laboratories. After a brief historical overview, we discuss the general properties of the magnetic fields in CP stars, describe the oblique rotator model, explain the dependence of the magnetic field strength on the rotation, and concentrate at the end on HgMn stars.Comment: 9 pages, 4 figures, 2 tables, chapter in "Determination of Atmospheric Parameters of B-, A-, F- and G-Type Stars", Springer (2014), eds. E. Niemczura, B. Smalley, W. Pyc

Discovery of kilogauss magnetic fields in three DA white dwarfs

We have detected longitudinal magnetic fields between 2 and 4 kG in three (WD 0446$-$790, WD 1105$-$048, WD 2359$-$434) out of a sample of 12 normal DA white dwarfs by using optical spectropolarimetry done with the VLT Antu 8 m telescope equipped with FORS1. With the exception of 40 Eri B (4 kG) these are the first positive detections of magnetic fields in white dwarfs below 30 kG. Although suspected, it was not clear whether a significant fraction of white dwarfs contain magnetic fields at this level. These fields may be explained as fossil relics from magnetic fields in the main-sequence progenitors considerably enhanced by magnetic flux conservation during the shrinkage of the core. A detection rate of 25 % (3/12) may indicate now for the first time that a substantial fraction of white dwarfs have a weak magnetic field. This result, if confirmed by future observations, would form a cornerstone for our understanding on the evolution of stellar magnetic fields. Keywords: stars: white dwarfs - stars: magnetic fields - stars: individual: WD0446-790, WD1105-048, WD2359-434Comment: 15 pages, 7 figures, Astronomy and Astrophysics, in pres

The magnetic fields of hot subdwarf stars

Detection of magnetic fields has been reported in several sdO and sdB stars. Recent literature has cast doubts on the reliability of most of these detections. We revisit data previously published in the literature, and we present new observations to clarify the question of how common magnetic fields are in subdwarf stars. We consider a sample of about 40 hot subdwarf stars. About 30 of them have been observed with the FORS1 and FORS2 instruments of the ESO VLT. Here we present new FORS1 field measurements for 17 stars, 14 of which have never been observed for magnetic fields before. We also critically review the measurements already published in the literature, and in particular we try to explain why previous papers based on the same FORS1 data have reported contradictory results. All new and re-reduced measurements obtained with FORS1 are shown to be consistent with non-detection of magnetic fields. We explain previous spurious field detections from data obtained with FORS1 as due to a non-optimal method of wavelength calibration. Field detections in other surveys are found to be uncertain or doubtful, and certainly in need of confirmation. There is presently no strong evidence for the occurrence of a magnetic field in any sdB or sdO star, with typical longitudinal field uncertainties of the order of 2-400 G. It appears that globally simple fields of more than about 1 or 2 kG in strength occur in at most a few percent of hot subdwarfs, and may be completely absent at this strength. Further high-precision surveys, both with high-resolution spectropolarimeters and with instruments similar to FORS1 on large telescopes, would be very valuable

Tensile strength modelling of glass fiber-polymer composites in fire

A thermal-mechanical model is presented to calculate the tensile strength and time-to-failure of glass fiber reinforced polymer composites in fire. The model considers the main thermal processes and softening (mechanical) processes of fiberglass composites in fire that ensure an accurate calculation of tensile strength and failure time. The thermal component of the model considers the effects of heat conduction, matrix decomposition and volatile out-gassing on the temperature-time response of composites. The mechanical component of the model considers the tensile softening of the polymer matrix and glass fibers in fire, with softening of the fibers analyzed as a function of temperature and heating time. The model can calculate the tensile strength of a hot, decomposing composite exposed to fire up to the onset of flaming combustion. The thermal-mechanical model is confined to hot, smoldering fiberglass composites prior to ignition. Experimental fire tests are performed on dry fiberglass fabric and fiberglass/vinyl ester composite specimens to validate the model. It is shown that the model gives an approximate estimate of the tensile strength and time-to-failure of the materials when exposed to one-sided heating at a constant heat flux. It is envisaged the model can be used to calculate the tensile softening and time-to-failure of glass-polymer composite structures exposed to fire