Cosmic abundances: The impact of stellar duplicity

The mass-transfer scenario links chemical peculiarities with stellar duplicity for an increasing number of stellar classes (classical and dwarf barium stars, subgiant and giant CH stars, S stars without technetium, yellow symbiotic stars, WIRRING stars, Abell-35-like nuclei of planetary nebulae...). Despite these successes, the mass-transfer scenario still faces several problems: What is the mass-transfer mode? Why orbital elements of dwarf barium stars do not fully match those of the classical barium stars? What is the origin of the few non-binary stars among dwarf barium stars? The paper reviews these open questions.Comment: 14 pages, 4 figures, to appear in `Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis', edited by F.N. Bash, T.G. Barnes, ASP Conf. Ser., in pres

The Henize sample of S stars. I. The technetium dichotomy

This paper is the first one in a series investigating the properties of the S stars belonging to the Henize sample (205 S stars with delta<-25 deg. and R<10.5) in order to derive the respective properties (like galactic distribution and relative frequencies) of intrinsic (i.e. genuine asymptotic giant branch) S stars and extrinsic (i.e. post mass-transfer binary) S stars. High-resolution (R=30 000 to 60 000) spectra covering the range 4230-4270AA have been obtained for 76 S stars, 8 M stars and 2 symbiotic stars. The 4262AA and 4238AA blends involving a Tc I line were analysed separately and yield consistent conclusions regarding the presence or absence of technetium. Only one `transition' case (Hen 140 = HD 120179, a star where only weak lines of technetium are detectable) is found in our sample. A resolution greater than R =30 000 is clearly required in order to derive unambiguous conclusions concerning the presence or absence of technetium. The Tc/no Tc dichotomy will be correlated with radial velocity and photometric data in a forthcoming paper.Comment: 10 pages, 5 figures, Latex, accepted for publication in Astronomy and Astrophysics main journal. Also available at http://astro.ulb.ac.be

A CORAVEL radial-velocity monitoring of giant Ba and S stars: spectroscopic orbits and intrinsic variations

This paper provides orbital parameters for 38 barium stars and 10 extrinsic S stars derived from a decade-long CORAVEL monitoring. Lower bounds on the orbital period (generally exceeding 10 y) have been obtained for 10 more systems. Mira S, SC and (Tc-poor) C stars have also been monitored and show intrinsic radial-velocity variations due to atmospheric phenomena. Tentative orbital solutions are proposed for 3 stars (S UMa, X Cnc, BD-08:1900) where the velocity and photometric periods are different. Three stars (RZ Peg, SS Vir and R CMi) exhibit radial-velocity variations synchronous with the light variations. Pseudo-orbital solutions have been derived for those stars. In the case of RZ Peg, a line-doubling phenomenon is observed near maximum light, and probably reflects the shock wave propagating through the photosphere.Comment: Astronomy & Astrophysics Supplements, 20 pages, 8 figures, 8 tables (LaTeX). Also available at: http://obswww.unige.ch/~udry/cine/barium/barium.htm

The Henize sample of S stars: IV. New symbiotic stars

The properties of the few symbiotic stars detected among the 66 binary S stars from the Henize sample are discussed. Two stars (Hen 18 and Hen 121) exhibit both a strong blue-violet continuum and strong H_alpha emission (FWHM of 70 km/s), whereas Hen 134 and 137 exhibit weak H_alpha emission. The H_alpha profiles are typical of non-dusty symbiotic stars belonging to class S-3 as defined by Van Winckel et al. (1993, A&AS 102, 401). In that class as in the Henize symbiotic S stars, He I, [N II] or [S II] emission lines are absent, suggesting that the nebular density is high but the excitation rather low. The radial velocity of the centre of the H_alpha emission is identical to that of the companion star (at least for Hen 121 where this can be checked from the available orbital elements), thus suggesting that the H_alpha emission originates from gas moving with the companion star. For Hen 121, this is further confirmed by the disappearance of the ultraviolet Balmer continuum when the companion is eclipsed by the S star. Hen 121 is thus the second eclipsing binary star discovered among extrinsic S stars (the first one is HD 35155). A comparison of the available data on orbital periods and H_alpha emission leads to the conclusion that H_alpha emission in S stars seems to be restricted to binary systems with periods in the range 600 - 1000 d, in agreement with the situation prevailing for red symbiotic stars (excluding symbiotic novae). Symbiotic S stars are found among the most evolved extrinsic S stars.Comment: 10 pages, 9 figures, accepted for publication in Astronomy and Astrophysic

Temporary Employment Services (Labour Brokers) in South Africa and Namibia

South Africa currently allows labour broking although this area of commerce is problematic. The trade union movement, government and organised business are presently debating the future regulation of this industry. Namibia has experimented with, and failed, to place a legislative ban on labour broking. The Supreme Court of Appeal of Namibia considered International Labour Organisation conventions and provisions of their Constitution before concluding that labour broking should be regulated but not prohibited. In this article it is argued that South African policy makers can gain valuable insights from the Namibian experience. It is submitted that it would be appropriate for Parliament to take cognisance of international and foreign principles and to accept amendments that would provide for stricter regulation for labour broking, rather than placing an outright ban on this economic activity

Spectroscopic binaries among AGB stars from HERMES/Mercator: the case of V Hya

We report on our search for spectroscopic binaries among a sample of AGB stars. Observations were carried out in the framework of the monitoring of radial velocities of (candidate) binary stars performed at the Mercator 1.2m telescope, using the HERMES spectrograph. We found evidence for duplicity in UV Cam, TU Tau, BL Ori, VZ Per, T Dra, and V Hya. This short communication focus on V Hya, found to behave like RV Tau of the b subtype, which are binaries surrounded by a disc.Comment: Poster presented at IAU Symp. 343 "Why Galaxies Care About AGB Stars", Vienna, august 201

The temperature and chronology of heavy-element synthesis in low-mass stars

Roughly half of the heavy elements (atomic mass greater than that of iron) are believed to be synthesized in the late evolutionary stages of stars with masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly iron) capture neutrons and progressively build up (through the slow-neutron-capture process, or s-process) heavier elements that are subsequently brought to the stellar surface by convection. Two neutron sources, activated at distinct temperatures, have been proposed: 13C and 22Ne, each releasing one neutron per alpha-particle (4He) captured. To explain the measured stellar abundances, stellar evolution models invoking the 13C neutron source (which operates at temperatures of about one hundred million kelvin) are favoured. Isotopic ratios in primitive meteorites, however, reflecting nucleosynthesis in the previous generations of stars that contributed material to the Solar System, point to higher temperatures (more than three hundred million kelvin), requiring at least a late activation of 22Ne. Here we report a determination of the s-process temperature directly in evolved low-mass giant stars, using zirconium and niobium abundances, independently of stellar evolution models. The derived temperature supports 13C as the s-process neutron source. The radioactive pair 93Zr-93Nb used to estimate the s-process temperature also provides, together with the pair 99Tc-99Ru, chronometric information on the time elapsed since the start of the s-process, which we determine to be one million to three million years.Comment: 30 pages, 10 figure