A Ca ii H&K survey of γ Doradus candidates

In an investigation of the starspot hypothesis as it applies to the 'slowly variable' F-type dwarfs, we spectroscopically observed eight promising γ Doradus candidates to search for Ca  ii H&K emission. We found that there are no significant emission reversals in the cores of these resonance lines. Based on the ceiling flux calculations of the Ca  ii K line and on calculations of the Rossby number, we conclude that there is no support for the presence of strong magnetic activity and the starspot hypothesis in these objects.     

On the galactic motion

A certain potential function is studied as a possible model for the galactic potential. Some solutions are obtained. Also the numerical study of some of the orbits and their stability is carried out.     

White dwarf + red dwarf binary systems and the period gap of cataclysmic variables

Cataclysmic variables (CV) and pre-cataclysmic binaries (PCB) are discussed. The main difference between them (accretion or its absence) is shown to be a consequence of the evolutionary process and the properties of their progenitors. Both types of system have a bimodal distribution of their periods, but the extrema are in counterphase. Luminosity-effective temperature diagrams for the primary components are used to show that both systems have approximately the same age, which conflicts with the notion of PCBs as precursors of CVs. Calculations of the maximum distance between components for which the system remains stable show that CVs have passed this limit, while PCBs maintain their stability during this evolution. It is suggested that, after ejecting a common shell, future CVs immediately become semi-detached systems. It this is so, then there must be cataclysmic variables which are the central stars of planetary nebulae. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 427–439 (August 2007).     

Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

Due to the knowledge of the rotation period and the presence of a rather strong surface magnetic field, the sharp‐lined young Herbig Ae star HD 101412 with a rotation period of 42 d has become one of the most well‐studied targets among the Herbig Ae stars. High‐resolution HARPS polarimetric spectra of HD 101412 were recently obtained on seven different epochs. Our study of the spectral variability over the part of the rotation cycle covered by HARPS observations reveals that the line profiles of the elements Mg, Si, Ca, Ti, Cr, Mn, Fe, and Sr are clearly variable while He exhibits variability that is opposite to the behaviour of the other elements studied. Since classical Ap stars usually show a relationship between the magnetic field geometry and the distribution of element spots, we used in our magnetic field measurements different line samples belonging to the three elements with the most numerous spectral lines, Ti, Cr, and Fe. Over the time interval covered by the available spectra, the longitudinal magnetic field changes sign from negative to positive polarity. The distribution of field values obtained using Ti, Cr, and Fe lines is, however, completely different compared to the magnetic field values determined in previous low‐resolution FORS 2 measurements, where hydrogen Balmer lines are the main contributors to the magnetic field measurements, indicating the presence of concentration of the studied iron‐peak elements in the region of the magnetic equator. Further, we discuss the potential role of contamination by the surrounding warm circumstellar matter in the appearance of Zeeman features obtained using Ti lines. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

We look for high‐amplitude variable young stars in the open clusters and associations of the Orion Belt. We use public data from the ASAS‐3 Photometric V ‐band Catalogue of the All Sky Automated Survey, infrared photometry from the 2MASS and IRAS catalogues, proper motions, and the Aladin sky atlas to obtain a list of the most variable stars in a survey area of side 5° centred on the bright star Alnilam (ε Ori) in the centre of the Orion Belt. We identify 32 highly variable stars, of which 16 had not been reported to vary before. They are mostly variable young stars and candidates (16) and background giants (8), but there are also field cataclysmic variables, contact binaries, and eclipsing binary candidates. Of the young stars, which typically are active Herbig Ae/Be and T Tauri stars with Hα emission and infrared flux excess, we discover four new variables and confirm the variability status of another two. Some of them belong to the well known σ Orionis cluster. Besides, six of the eight giants are new variables, and three are new periodic variables (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Recent discoveries of supersoft X‐ray sources in M 31

Classical novae (CNe) have recently been reported to represent the major class of supersoft X‐ray sources (SSSs) in the central area of our neighbouring galaxy M 31. This paper presents a review of results from recent X‐ray observations of M 31 with XMM‐Newton and Chandra. We carried out a dedicated optical and X‐ray monitoring program of CNe and SSSs in the central area ofM 31. We discovered the first SSSs in M 31 globular clusters (GCs) and their connection to the very first discovered CN in a M 31 GC. This result may have an impact on the CN rate in GCs. Furthermore, in our optical and X‐ray monitoring data we discovered the CN M3 1N 2007‐11a, which shows a very short SSS phase of 29–52 days. Short SSS states (durations ≤ 100 days) of CNe indicate massive white dwarfs (WDs) that are candidate progenitors of supernovae type Ia. In the case of M31N 2007‐11a, the optical and X‐ray light curves suggest a binary containing a WD with M_WD > 1.0 M_⊙. Finally, we present the discovery of the SSS counterpart of the CN M31N 2006‐04a. The X‐ray light curve of M31N 2006‐04a shows short‐time variability, which might indicate an orbital period of about 2 hours (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spatiotemporal variations of maximum seasonal freeze depth in 1950s–2007 over the Heihe River Basin, Northwest China

Investigation on spatiotemporal variations of maximum seasonal freeze depth （MSFD） over the Heihe River Basin is of great importance for systematic understanding of regional climate and environmental change, ecological-hydrological processes, water resources assessment, construction and resource development. Based on soil and air temperatures at the meteorological stations of the China Meteorological Administration （CMA） over the Heihe River Basin, MSFDs time series are structured into a composite time series over the 1960-2007 period. Evaluating the averaged MSFD time series for 1960 2007 reveals a statistically significant trend of 4.0 cm/decade or a net change of-19.2 cm for the 48-year period over the basin. The MSFD had significantly negative correlation with mean annual air temperature （MAAT）, winter air temperature, mean annual ground surface temperature （MAGST）, degree days of thawing for the air （DDTa） as well as for the surface （DDTs）, and degree days of freezing for the surface （DDFs）. While there was significantly positive correlation between DDF,. and MSFD time series, MSFD was deeper and changed greatly in the Heihe River source area. It was shallower in the east-central basin and gradually deepened in other sections of the basin. The MSFD distribution pattern in 2003-2005 is consistent with that of averaged degree days of freezing for air （DDFa） in 1960-2007. However, the maximum of MSFD may not be accurate, because there is no long term observation data in the deep seasonally frozen ground regions near the lower boundary of permafrost. With increasing elevation, averaged DDFa increased at a rate of 51.6 ℃-day/100m, therefore, the MSFG and the date reaching MSFG became deeper and later, respectively.     

On the properties of strange modes

Properties of the so-called strange modes occurring in linear stability calculations of stellar models are discussed. The behaviour of these modes is compared for two different sets of stellar models, for very massive zero-age main-sequence stars and for luminous hydrogen-deficient stars, both with high luminosity-to-mass ratios. We have found that the peculiar behaviour of the frequencies of the strange modes with the change of a control parameter is caused by the pulsation amplitude of a particular eigenmode being strongly confined to the outer part of the envelope, around the density inversion zone. The frequency of a strange mode changes because the depth of the confinement zone changes with the control parameter. Weakly non-adiabatic strange modes tend to be overstable because the amplitude confinement quenches the effect of radiative damping. On the other hand, extremely non-adiabatic strange modes become overstable because the perturbation of radiation force (gradient of radiation pressure) provides a restoring force that can be out of phase with the density perturbation. We discuss this mechanism by using a plane-parallel two-zone model.