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Study of the solar interior through analysis of solar oscillations, a field now referred to as helioseismology, has generated interest in the development of asteroseismology, the study of the interior of pulsating stars through the analysis of their oscillations. Some progress has been made in the study of the Rapidly Oscillating Ap stars, cool magnetic Ap stars which pulsate in high overtone, low degree non-radial p-modes with periods between 4 and 15 minutes. Since all of these stars found so far are bright (V<10), their light variations can be studied with small telescopes; most of the work done has been with a 0.5-m telescope. Their light variations are extremely complex, however, which means that they must be intensively observed, something which can only be done with small telescopes. Due to aliasing problems, multisite observing programs of the Rapidly Oscillating Ap stars are often necessary in order to decipher their frequency patterns. A collaborative observing program is suggested.
Using in situ data from 2011 and 2013, we evaluate the ability of CryoSat-2 (CS-2) to retrieve sea-ice freeboard over fast ice in McMurdo Sound. This provides the first systematic validation of CS-2 in the coastal Antarctic and offers insight into the assumptions currently used to process CS-2 data. European Space Agency Level 2 (ESAL2) data are compared with results of a Waveform Fitting (WfF) procedure and a Threshold-First-Maximum-Retracker-Algorithm employed at 40% (TFMRA40). A supervised freeboard retrieval procedure is used to reduce errors associated with sea surface height identification and radar velocity in snow. We find ESAL2 freeboards located between the ice and snow freeboard rather than the frequently assumed snow/ice interface. WfF is within 0.04 m of the ice freeboard but is influenced by variable snow conditions causing increased radar backscatter from the air/snow interface. Given such snow conditions and additional uncertainties in sea surface height identification, a positive bias of 0.14 m away from the ice freeboard is observed. TFMRA40 freeboards are within 0.03 m of the snow freeboard. The separation of freeboard estimates is primarily driven by the different assumptions of each retracker, although waveform alteration by variations in snow properties and surface roughness is evident. Techniques are amended where necessary, and automatic freeboard retrieval procedures for ESAL2, WfF and TFMRA40 are presented. CS-2 detects annual fast-ice freeboard trends using all three automatic procedures that are in line with known sea-ice growth rates in the region.
The day before the start of IAU Symposium 185 Wojtek Dziembowski and I were sitting in the beautiful 300-year-old garden of Shisendo in Kyoto discussing asteroseismology. Wojtek put his fingertips together to make a little temple of his hands in front of him. Then, slowly moving this temple up and down in front of him, while staring off at the green Japanese mosses in the dappled sun and shade of Shisendo‘s garden forest he said to me in his studied way, “Don, I am not sure that there is any asteroseismology yet”. I thought, “Uh-oh. If others believe that is true, then they may not think I have much to say in my review of new developments in asteroseismology at S185.” Yet I do have much to say, so I think I had better define what I mean by asteroseismology.
Sediment-laden icebergs are rarely sighted in Antarctic waters. However, during the recent Deep Freeze 79-USCGC Glacier expedition to the George V Coast and the south-western Ross Sea, nine sediment-laden icebergs and several pieces of pack ice with surficial sediment layers were observed. These observations include basal debris zones, debris slumped on to glaciers and floating ice, and englacial debris believed to have been incorporated along shear zones.
Sediment samples collected from icebergs were texturally and mineralogically variable. Some were unsorted mixtures consisting of a wide variety of angular minerals and rock fragments; others consisted primarily of slate clasts, quartz sand, and rock flour.
For Commission 27 these triennial reports have traditionally been very compact literature reviews of all the fields relevant to our commission. For several triennia we have been discussing the relevance of them, and asking just who their readership is. It seems that only a few people read them, and fewer use them as introductions to the subject – supposedly one of their prime purposes. The major beneficiaries have been the writers, who have been forced to do three-year reviews of their subjects. The IAU EC gave us the option this triennium of a shorter, four-page report to which the majority of the SOC agreed.
The rapidly oscillating Ap stars pulsate in high-overtone, low degree p-modes with their pulsation axes aligned with their oblique magnetic axes. They show non-linearity in their pulsation in three ways:
1)The harmonics of the basic pulsation frequency are detectable.
2)The pulsation phase seems to vary stochastically on a time scale of days to years depending on the star.
3)The form of the nonradial surface distortion is not constant with time.
These three effects are illustrated with HR 3831, the best studied of the roAp stars. HR 3831 pulsates in distorted dipole mode which can be modelled as a linear sum of axisymmetric l = 0, 1, 2, and 3 spherical harmonics aligned with the magnetic axis. This gives rise to a 7-frequency multiplet split by exactly the rotation frequency. The form of the distortion shows small changes on a time-scale of years. HR 3831 shows a 5-frequency rotationally split first harmonic multiplet, a 3-frequency rotationally split second harmonic multiplet, and a single third harmonic frequency has probably been detected at an amplitude of 0.065 mmag. The first harmonic has changed its form significantly over the last 10 years. A technique for decomposing the fundamental frequency septuplet into its component spherical harmonics is used to fit the pulsation phase as a function of rotation phase. This allows a unique O-C to be defined for any length of light curve. The long term behaviour of the O-C diagram cannot be modelled adequately with a combination of periodic (Doppler shift) and quadratic (evolution) terms; there seems to be a significant stochastic component. The direction of the pulsation phase reversal at rotational phase 0.747 is indeterminate; sometimes it is a positive-going reversal, sometimes negative-going. At present it is not known whether this is a numerical artifact, or a physical effect in the star. If it is a physical effect, it means that small non-periodic differences in pulsation amplitude between the bipolar hemispheres have been detected.
For several of the rapidly oscillating Ap stars the best luminosity estimates available come from the asteroseismological interpretation of their pulsational frequency spectra. We give a list of the 23 currently known roAp stars along with their Strömgren photometric indices, Teff estimated from the Hβ index, and luminosity estimated asteroseismologically. In one case, ϒ Equ, we have an asteroseismological luminosity and a parallax luminosity which are in good agreement. Some of the roAp stars pulsate with frequencies greater than the critical frequency calculated for standard A-star models. This plus multi-colour high-speed photometry of HR 3831 indicate that the temperature gradients in the atmospheres of these stars are substantially steeper than in standard A-star models. We advocate a fine analysis of HR 3831 to see if there is consistency with the pulsational conclusions about T-τ in this star. Further fine analyses and multi-colour pulsational analyses on other roAp stars are then called for. The pulsation mode in HR 3831 can be decomposed into primarily an axisymmetric dipole mode with small radial, quadrupole and octupole perturbations. If the magnetic field is governing the distortion of this mode from a purely dipole mode, then the pulsation can be used to infer the magnetic field geometry. Comments on our current knowledge of all 23 roAp stars are made.
We present the first results from multi-site observations of the δ Scuti star XX Pyx (CD–24°7599). The observations were carried out as the 17th run of the Delta Scuti Network. We collected 583 hr of B, V time-series photometry, resulting in a detection level (4σ) in the amplitude spectrum of 0.5 mmag. We detect 6 new pulsation frequencies, bringing the total number of frequencies known in this star up 19.
We present the first results of the analysis of 22 Blazhko stars. We find: 1) Blazhko RRab stars that are nearly pure amplitude modulators; 2) Blazhko RRab stars that have both amplitude and phase modulation; 3) A Blazhko RRab star that has an abrupt period change; 4) Proof of the Blazhko effect in RRc stars. Our data show the character of the amplitude and phase modulations of the light curves over the Blazhko cycles far better than has been previously possible.
A code for solving non-adiabatic radial pulsation equations with consistent treatment of radiative transfer is used to study the oscillations in the atmospheres of the rapidly oscillating Ap stars. We find that our results are consistent with the presence of an observable node in the temperature eigenfunction.
Pulsation is ubiquitous among chemically normal A-type stars, but comparatively rare among chemically peculiar Am and Ap stars of the same temperature range. The conventional explanation for this is that diffusion produces the surface abundance anomalies in the Am and Ap stars, and also drains He from the He-II ionisation zone, thus quenching the κ-mechanism that drives δ Scuti pulsation. The pulsating Am and Ap stars exhibit dichotomous pulsation characteristics. The Am stars (and related stars) exhibit low-overtone δ Scuti pulsation, with amplitudes ranging from a few mmag to 0.1 mag. The pulsating Ap stars exhibit high-overtone pulsation with periods in the range 6-16 min and Johnson B semi-amplitudes typically ≤ 5 mmag. These stars are referred to as rapidly oscillating Ap stars, or ‘roAp’ stars (the see review by Martinez & Kurtz 1995).
We present the first massive frequency analysis of the 1200 first overtone RR Lyrae stars in the Large Magellanic Cloud observed in the first 4.3 yr of the MACHO project. Besides the many new double-mode variables, we also discovered stars with closely spaced frequencies. These variables are most probably nonradial pulsators.
With the resounding success of Helioseismology in determining the interior structure and rotation of the Sun, and in providing unprecedented studies of the interaction of pulsation and magnetic fields in the solar atmosphere, astronomers have been delighted, after decades of disappointing attempts, with the recent discovery of solar-like oscillations in ξHya, β Hyi, αCen, η Boo and νInd. There is now true seismology of a variety of solar-like stars. Asteroseismology also studies stars with a wide variety of interior and surface conditions. For two decades asteroseismic techniques have been applied to many pulsating stars across the HR Diagram. This review describes for non-specialists pulsation modes in stars and discusses a selection of some of the successes already accomplished in asteroseismology.
The present state of our knowledge of non-radial oscillations in δ Scuti stars and Rapidly Oscillating Ap stars is discussed primarily from an observational point of view. For the δ Scuti stars the need for complete frequency solutions for multi-periodic non-radial oscillating stars is emphasized in order for stellar seismology of these stars to be possible. An introduction to the Rapidly Oscillating Ap stars is given along with reference to a more complete recent review of those stars.
We present experimental evidence for the spontaneous breaking of cubic symmetry in the band structure of films of Ga0 52 1n0 48P grown by organomeltallic vapor phase epitaxy on (100) GaAs substrates. We show how this effect is related to the spontaneous ordering of the alloy, and its correlation with the anomalous lowering of the band-gap observed in these films.
A reduction in the optical energy gap of more than 65 meV has been observed in In0.53Ga0.47 As grown on (100) InP by atmospheric pressure metalorganic vapor phase epitaxy. The band gap energies were deduced from room temperature photocurrent spectroscopic measurements, accounting for differences in composition and strain. Spontaneous CuPt type ordering of In and Ga atoms on the (111) subplanes of the InGaAs2 was confirmed by transmission electron microscopy. Superlattice signatures in the transmission micrographs were observed only for samples with associated reduced band gap energies, and were confirmed by visible double periodicity in high resolution images. In0.53Ga0.47 As was grown under a variety of conditions, some which promoted ordering. In general, lower growth temperatures and moderate (∼4 μ/hr) growth rates promoted a greater degree of ordering and reduction of the band gap energy. The influence of growth conditions on the ordered structure is considered within the context of current theories.
The band gap of Ga0.51n0.5P is studied as a function of phosphine pressure, B-type substrate misorientation, growth rate, and growth temperature, with emphasis placed on the effect of the phosphine pressure. Over most of the parameter space explored (high temperatures, large substrate misorientations, and low growth rates), the band gap increases with decreasing phosphine. This increase is proposed to be caused by lower phosphorus coverage of the surface, resulting in a different surface structure that doesn't promote ordering. The implications of this effect on the observed variations of band gap with growth temperature, substrate misorientation, and growth rate are discussed. For regions of parameter space in which the ordering appears to be kinetically limited by surface diffusion, the band gap increases slightly with phosphine pressure, consistent with observations that increased group-V pressure decreases the group-III surface diffusion length.
The pore size distribution in cement paste over the range of pore sizes interrogated by high pressure mercury intrusion porosimetry may be described by a mixture of two lognormal distributions. The compound distribution of pore sizes may be given as:
where p(x) is the probability density function of pores of size x, f and (1−f) are the weights of sub-distributions, μ1 and μ2 are the location parameters of sub-distributions, and σ1 and σ2 are the shape parameters of sub-distributions. These two sub-distributions may represent the larger and smaller capillary pores respectively. The changes in the sub-distributions and the compound distribution as functions of curing age and water-to-cement ratio are discussed.
A panel of 10 monoclonal antibodies was used to subgroup 326 strains of Legionella pneumophila serogroup 1. All but two strains could be classified into three major subgroups named after their representative strains Pontiac 1, Olda and Bellingham 1. Of the 50 isolates from patients, 44 representing 32 separate incidents were of the Pontiac subgroup. This subgroup was also found in 16 of 18 buildings epidomiologically associated with Legionnaires' Disease. In contrast, strains of the Olda subgroup predominated in buildings where no infections had occurred. In 9 of the 11 incidents where isolates were available from at least one patient as well as from the suspected environmental source, the monoclonal antibody reaction patterns of strains from patients were identical to those of one or more of their environmental counterparts.