Absolute magnitudes of OB and Be stars based on Hipparcos parallaxes – III

The absolute visual magnitudes, M_V , of A–M stars are based on calculated Hipparcos trigonometric parallaxes. The sample used consists of 30 986 unreddened and reddened A–M stars in luminosity classes Ia, Iab, Ib, II, III, IV and V. The colour excesses of the reddened stars were calculated using the mean colour indices, according to the SIMBAD data base and the intrinsic B − V values calibrated for the given spectral types and luminosity classes by Schmidt-Kaler. The values of the total-to-selective extinction,   R_V = A_V / E ( B − V )  , for all the reddened stars were calculated from previously published near-infrared photometric measurements. The calculated visual magnitudes, M_V , of A–M stars compare with the earlier determinations of Schmidt-Kaler. The mean absolute magnitudes published by Schmidt-Kaler are generally brighter (except for the stars in luminosity classes V and IV) than those determined in this paper.     

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.     

The inner Galaxy resolved at IJK using DENIS data

We present the analysis of three-colour optical/near-infrared images, in IJK , taken for the DEep Near Infrared Southern Sky Survey (DENIS) project. The region considered covers 17.4 deg² and lies within <5°, b <1.°5. The adopted methods for deriving photometry and astrometry in these crowded images, together with an analysis of the deficiencies nevertheless remaining, are presented. The numbers of objects extracted in I , J and K are 748 000, 851 000 and 659 000 respectively, to magnitude limits of 17, 15 and 13. Eighty per cent completeness levels typically fall at magnitudes 16, 13 and 10 respectively, fainter by about 2 mag than the usual DENIS limits as a result of the crowded nature of these fields. A simple model to describe the disc contribution to the number counts is constructed, and parameters for the dust layer are derived. We find that a formal fit of parameters for the dust plane, from these data in limited directions, gives a scalelength and scaleheight of 3.4±1.0 kpc and 40±5 pc respectively, and a solar position 14.0±2.5 pc below the plane. This latter value is likely to be affected by localized dust asymmetries. We convolve a detailed model of the systematic and random errors in the photometry with a simple model of the Galactic disc and dust distribution to simulate expected colourmagnitude diagrams. These are in good agreement with the observed diagrams, allowing us to isolate those stars from the inner disc and bulge. After correcting for local dust-induced asymmetries, we find evidence for longitude-dependent asymmetries in the distant J and K sources, consistent with the general predictions of some Galactic bar models. We consider complementary L -band observations in the companion paper.     

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.     

The magnetic field and rotational period of 53 Camelopardalis

The magnetic Ap star 53 Cam has been described as the best object for testing theories of elemental diffusion in the presence of a stellar magnetic field. It is thus vital to determine the rotational period of this very important star as accurately as possible. An improved period (8.026 81±0.000 04 d) is derived here by combining new measurements of the effective magnetic field with previously published values.     

The detection of binary companions to subdwarf B stars

Intermediate dispersion spectroscopy of a sample of 40 hot subdwarf B stars between 5500 and 9000Å has been obtained. The sample includes a large fraction of targets which have been studied photometrically. Seven targets show strong lines arising from the infrared Ca  ii triplet, the unequivocal signature of a cool companion. The positive Ca  ii identifications include known photometric binaries and new targets; all are associated with a photometric red excess. Assuming a canonical value for the subdwarf luminosity, all of the detected companions are overluminous compared with the main sequence. The detection procedure indicates an improved and more secure method for the measurement of the binary frequency of hot subdwarfs.     

Radio emission regions in pulsars

We study the concept of radius-to-frequency mapping using a geometrical method for the estimation of pulsar emission altitudes. The semi-empirical relationship proposed by Kijak &38; Gil is examined over three decades of radio frequency. It is argued that the emission region in a millisecond pulsar occupies the magnetosphere over a distance of up to about 30 per cent of the light-cylinder radius, and that in a normal pulsar occupies up to approximately 10 per cent of the light-cylinder radius.     

HST colour–magnitude diagrams of six old globular clusters in the LMC

We report on HST observations of six candidate old globular clusters in the Large Magellanic Cloud (LMC): NGC 1754, 1835, 1898, 1916, 2005 and 2019. Deep exposures with the F555W and F814W filters provide us with colour–magnitude diagrams that reach to an apparent magnitude in V of ∼25, well below the main-sequence turn-off. These particular clusters are associated with significantly high LMC field star densities and care was taken to subtract the field stars from the cluster colour–magnitude diagrams accurately. In two cases there is significant variable reddening across at least part of the image, but only for NGC 1916 does the differential reddening preclude accurate measurements of the CMD characteristics. The morphologies of the colour–magnitude diagrams match well those of Galactic globular clusters of similar metallicity. All six have well-developed horizontal branches, while four clearly have stars on both sides of the RR Lyrae gap. The abundances obtained from measurements of the height of the red giant branch above the level of the horizontal branch are 0.3 dex higher, on average, than previously measured spectroscopic abundances. Detailed comparisons with Galactic globular cluster fiducials show that all six clusters are old objects, very similar in age to classical Galactic globulars such as M5, with little age spread among the clusters. This result is consistent with ages derived by measuring the magnitude difference between the horizontal branch and main-sequence turn-off. We also find a similar chronology by comparing the horizontal branch morphologies and abundances with the horizontal branch evolutionary tracks of Lee, Demarque &38; Zinn. Our results imply that the LMC formed at the same time as the Milky Way Galaxy.     

HST UV measurements of wind structure and velocities in Local Group OB stars

Archival HST FOS and GHRS data sets have been used to collect ultraviolet evidence for large- and small-scale stellar wind structure in extragalactic Local Group OB stars (i.e. SMC, LMC including R136, M31, M33 and NGC 6822). By comparison with previous studies of Galactic OB stars, wind activity is principally diagnosed in individual spectrograms via the presence of 'narrow absorption components' and saturated 'black' absorption troughs in the resonance line doublets. Their characteristics broadly suggest that these stars share the same physical mechanisms for perturbing the winds as those that act in Galactic stars. Both of these spectral indicators are also used to provide reliable measures of wind terminal velocities. These velocities are directly compared with previously published Galactic values, without reliance on model profile fitting. Relative to Galactic OB stars, the most discrepant terminal velocities (and wind line profiles) result from main-sequence early O-type stars in the SMC.