On the frequency and amplitude variations of the δ Scuti star CD−24 7599 (=XX Pyx)

We present 132 h of new time-series photometric observations of the δ Scuti star CD−24 7599 acquired during 86 nights from 1993 to 1996 to study its frequency and amplitude variations. By using all published observations we demonstrate that the three dominating pulsation modes of the star can change their photometric amplitudes within one month at certain times, while the amplitudes can remain constant within the measurement errors at other times. CD−24 7599 also exhibits frequency variations, which do not show any correspondence between the different modes.   The typical time-scale for the amplitude variations is found to be several hundred days, which is of the same order of magnitude as the inverse linear growth rates of a selected model. We find no evidence for periodic amplitude modulation of two of the investigated modes ( f ₂ and f ₃), but f ₁ may exhibit periodic modulation. The latter result could be spurious and requires confirmation. The observed frequency variations may either be continuous or reflect sudden frequency jumps. No evidence for cyclical period changes is obtained.   We exclude precession of the pulsation axis and oblique pulsation for the amplitude variations. Beating of closely spaced frequencies cannot explain the amplitude modulations of two of the modes, while it is possible for the third. Evolutionary effects, binarity, magnetic field changes or avoided crossings cannot be made responsible for the observed period changes. Only resonance between different modes may be able to explain the observations. However, at this stage a quantitative comparison is not possible. More observations, especially data leading to a definite mode identification and further measurements of the temporal behaviour of the amplitudes and frequencies of CD−24 7599, are required.     

Structural and magnetic microinhomogeneities in accessory spinel of the system Fe2+(Cr2–xFex3+)O4 from the Kytlym massif (Urals platinum-bearing belt)

The microstructure and magnetic properties of accessory Fe–Cr-spinels from the Kytlym massif of the Urals platinum-bearing belt were studied. Atypical Fe–Cr-spinels in the form of magnetic microareas in grains of primary nonmagnetic Fe–Cr-spinel have been revealed for the first time in the bed dunites of the Kytlym multiphase concentrically zoned massif, North Urals. These spinels are responsible for the magnetic properties of the dunites. It has been established that the microareas are separations in solid solution Fe²⁺(Cr_2–xFe_x³⁺)O₄, which are enriched in Fe^3 + and are probably an intermediate product of the transformation of primary accessory Fe-Cr-spinel during the formation of the dunite massif. These are magnetic microphases with particular chemical composition, cation distribution, and corresponding reversed crystal lattice, which determine the main magnetic properties of the microarea: the magnitude and direction of magnetization vector and Curie temperature. The formation of this earlier unknown type of magnetic Fe–Cr-spinel is probably conjugate with the formation and concentration of PGE mineralization in the bed dunites of the Kytlym platinum-bearing massif. The presence of such magnetization carriers in rocks and ores must be taken into account in geophysical research at the Urals chromite and platinum–chromite deposits.     

Excitation and visibility of slow modes in rotating B-type stars

We use the traditional approximation to describe oscillations with frequencies comparable to the angular rotation rate. Validity of this approximation in application to main-sequence B stars is discussed. Numerical results regarding mode stability and visibility are presented for a model of the Be star HD 163868. For this object, Walker et al. detected a record number of mode frequencies using data from the small space telescope MOST . Our interpretation of these data differs from that of Walker et al. In particular, we interpret peaks in the lowest frequency range as retrograde g modes. We find instability in a large number of modes that remain undetectable because of unfavourable aspect and/or effect of cancellation. There is no clear preference to excitation of prograde modes.     

Towards mode selection in δ Scuti stars: regularities in observed and theoretical frequency spectra

Only a fraction of the theoretically predicted non-radial pulsation modes have so far been observed in δ Scuti stars. Nevertheless, the large number of frequencies detected in recent photometric studies of selected δ Scuti stars allow us to look for regularities in the frequency spacing of modes. Mode identifications are used to interpret these results.
Statistical analyses of several δ Scuti stars (FG Vir, 44 Tau, BL Cam and others) show that the photometrically observed frequencies are not distributed at random, but that the excited non-radial modes cluster around the frequencies of the radial modes over many radial orders.
The observed regularities can be partly explained by modes trapped in the stellar envelope. This mode selection mechanism was proposed by Dziembowski & Królikowska and shown to be efficient for  ℓ= 1  modes. New pulsation model calculations confirm the observed regularities.
We present the s – f diagram, which compares the average separation of the radial frequencies ( s ) with the frequency of the lowest frequency unstable radial mode ( f ). This provides an estimate for the  log  g   value of the observed star, if we assume that the centres of the observed frequency clusters correspond to the radial mode frequencies. This assumption is confirmed by examples of well-studied δ Scuti variables in which radial modes were definitely identified.     

Magnetic microphases in chrome-spinels from alpine-type ultramafic rocks,Central Urals

The ore and accessory chrome-spinels from metamorphosed dunites of the Cr-bearing Klyuchevskoi alpine-type ultramafic massif are studied. As a result of use of thermomagnetic analysis in the range of 4–900 K, magnetic resonance spectroscopy, and magnetic-force microscopy, secondary magnetic Fe³⁺-enriched microphases chaotically distributed in the primary nonmagnetic mineral were revealed for the first time in accessory chrome-spinels. It was established that the metamorphosed accessory chrome-spinels produce the magnetic properties of the host rocks and the primary nonmagnetic chrome-spinels forming ore bodies remains almost unaltered. This originates the contrast of magnetic properties between the ore body and host rocks and provides the geomagnetic anomaly in the ore-hosting zone.     

Solar models with low opacity

Solar Physics - Evolutionary models of the present Sun with standard and artifically low opacity of stellar matter are obtained and adiabatic nonradial oscillations of the models are computed. The...