Robert S. Stobie

The DA+dMe eclipsing binary EC13471–1258: its cup runneth over … just

The optical spectrum and light curve of EC13471-1258 show that it is an eclipsing binary with an orbital period of 3 h 37 m comprising a DA white dwarf and a dMe dwarf. Total eclipses of the white dwarf are observed lasting 14 min, with the partial phases lasting 54 s. On one occasion, two pre-eclipse dips were seen. Timings of the eclipses over 10 yr show jitter of up to 12 s. Flares from the M dwarf are regularly observed. The M dwarf also shows a large-amplitude ellipsoidal modulation in the V-band light curve. The component stars emit almost equal amounts of light at 5500 A. Hubble Space Telescope (HST) STIS spectra show strong Lyman-a absorption with weak metal lines of C I,II and Si II superimposed. Model atmosphere analysis yielded an effective temperature of 14 220 ′ 300 K and log g of 8.34 ′ 0.20 for the white dwarf with these errors being strongly correlated. Its metal abundance is 1/30th solar with an uncertainty of 0.5 dex, and it is rapidly rotating with V 1 sin i = 400 ′ 100 km s - 1 . The white dwarf also shows radial velocity variations with a semi-amplitude of 138 ′ 10 km s - 1 . The gravitational redshift of the white dwarf was measured as 62 km s - 1 . From optical spectroscopy the spectral type of the M dwarf was found to be M3.5-M4, its temperature 3100 ′ 75 K, its rotational velocity 140 ′ 10 km s - 1 , its radial velocity semi-amplitude 266 ′ 5 km s - 1 , its mean V - I colour 2.86 and its absolute V magnitude 11.82. Intriguingly, its metal abundance is normal solar. The Ha emission line shows at least two distinct components, one of which is uniformly distributed around the centre of mass of the M dwarf and provided the estimate of the rotational velocity of the M dwarf. The other arises from the other side of the binary centre of mass, well within the white dwarf Roche lobe. This behaviour is confirmed by Doppler tomography, which shows the presence of two distinct velocity components within the primary Roche lobe. The interpretation of these features is uncertain. Variations in strength of the components with binary phase can be attributed to optical thickness in the Balmer lines. Similar behaviour is seen in the observations of the other Balmer emission lines, although with a poorer signal-to-noise ratio. Flares in Ha were observed and are consistent with arising from the vicinity of the M dwarf. Dynamical solutions for the binary are discussed and yield an inclination of 75.5° ′ 2.0°, a white dwarf mass and radius of 0.78 ′ 0.04 M O . and 0.011 ′ 0.01 R O ., and an M dwarf mass and radius of 0.43 ′ 0.04 M O . and 0.42 ′ 0.02 R O .. These parameters are consistent with the Wood mass-radius relation for white dwarfs and the Clemens et al. mass-radius relation for M dwarfs; we argue that the M dwarf just fills its Roche lobe. The radius of the white dwarf and the model fit imply a distance of 48 ′ 5 pc and an absolute V magnitude of 11.74. The rapid rotation of the white dwarf strongly suggests that the system has undergone mass transfer in the past, and implies that it is a hibernating cataclysmic variable. The M dwarf shows the properties expected of secondaries in cataclysmic variables: chromospheric activity and angular momentum loss.

Design of the Southern African Large Telescope (SALT)

SALT is a 10-m class telescope for optical/infrared astronomy to be sited at Sutherland, the observing state of the South African Astronomical Observatory. This telescope will be based on the principle of the Hobby-Eberly Telescope (HET) at McDonald Observatory, Texas. This cost-effective design is a tilted-Arecibo concept with a segmented spherical primary mirror of diameter 11 meters. The telescope has a fixed gravity vector but with full 360 degrees rotation in azimuth. A spherical aberration corrector mounted on a tracker beam at the prime focus enables a celestial object to be followed for 12 degrees across the sky. The SALT design enables over 70% of the sky to be accessed for about 20% of the cost of a conventional telescope of similar aperture. The telescope will be used primarily for spectroscopic studies of celestial objects with a light-weight low-dispersion imaging spectrograph mounted at the prime focus and other higher-dispersion instruments fiber-fed and mounted in an environmentally controlled basement. The concept design for SALT is presented with emphasis on the design changes between SALT and HET.

Two new hot white dwarfs in a region of exceptionally low hi density

Abstract We report the discovery of two hot white dwarfs which have the lowest line-of-sight neutral hydrogen column densities yet measured. The stars were found independently by the ROSAT EUV, Montreal-Cambridge-Tololo, and Edinburgh-Cape surveys. Follow-up observations made using the Voyager 2 ultraviolet spectrometer reveal strong continua shortward of the 912ALyman limit from which we deduce that the neutral hydrogen column densities are 1.3 × 10 17 and 2.0 × 10 17 atoms cm −2 .

Southern African Large Telescope (SALT) project: progress and status after 2 years

The Southern African Large Telescope (SALT) is a 10-m class optical/IR segmented mirror telescope based on the groundbreaking, low cost, Hobby-Eberly Telescope (HET) design. Approval to construct and operate SALT, which will be the largest single optical telescope in the Southern Hemisphere, was given by the South African Government in November 1999, after sufficient guarantees of matching funding from international partners were secured. Facility construction started in January 2001, and SALT is due to start operations by December 2004. SALT will enable a quantum leap in astronomical research capability in Southern Africa, and indeed the continent, where currently the largest telescope is a modest 1.9-m, dating to the 1940s. A substantial amount of design work for SALT has been completed, sourced from multiple suppliers, with ~60% South African content. South African industry is well equipped to handle the construction of most of the telescope, the exceptions being the glass ceramic mirror blanks (from LZOS in Russia), the polishing and ion figuring of these (Eastman Kodak in the USA), and fabrication of the four-element spherical aberration corrector (SAGEM in France). This paper will present (1) the scientific requirements, (2) the specified performance of SALT, (3) the basic design, with emphasis on the innovative modifications to the HET design that enable significantly improved performance, (4) the progress and status of the project, currently in its construction phase, (5) the first generation instrument suite, (6) the management and organisation of the project and (7) the international partnership in SALT.

The EC 14026 Stars

The EC 14026 stars are a new class of pulsating sdB stars. All members of the class are multiperiodic pulsators with periods in the range 90-500 s and amplitudes in the range ~0.001 to 0.3 mag. Atmospheric parameter determinations indicate Teff ~ 35 000 Κ and log g « 5.9, although at least one star has significantly lower gravity and is cooler than most of the others. A brief review of the properties of this class of stars is given.