Binaries among Ap and Am stars

The results of long-term surveys of radial velocities of cool Ap and Am stars are presented. There are two samples, one of about 100 Ap stars and the other of 86 Am stars. Both have been observed with the CORAVEL scanner from Observatoire de Haute-Provence (CNRS), France. The conspicuous lack of short-period binaries among cool Ap stars seems confirmed, although this may be the result of an observational bias; one system has a period as short as 1.6 days. A dozen new orbits could be determined, including that of one SB2 system. Considering the mass functions of 68 binaries from the literature and from our work, we conclude that the distribution of the mass ratios is the same for the Bp-Ap stars than for normal G dwarfs. Among the Am stars, we found 52 binaries, i.e. 60%; an orbit could be computed for 29 of them. Among these 29, there are 7 SB2 systems, one triple and one quadruple system. The 21 stars with an apparently constant radial velocity may show up later as long-period binaries with a high eccentricity. The mass functions of the SB1 systems are compatible with cool main-sequence companions, also suggested by ongoing spectral observations.Comment: 5 pages, 2 figures, to appear in: Proc. of the 26th workshop of the European Working Group on CP stars, Contrib. Astr. Obs. Skalnate Pleso Vol. 27, No

A CORAVEL radial-velocity monitoring of S stars: symbiotic activity vs. orbital separation

Orbital elements are presented for the Tc-poor S stars HR 363 (= HD 7351) and HD 191226. With an orbital period of 4592 d (=12.6 y), HR 363 has the longest period known among S stars, and yet it is a strong X-ray source. Its X-ray flux is similar to that of HD 35155, an S star with one of the shortest orbital periods (640 d). This surprising result is put in perspective with other diagnostics of binary interaction observed in binary S stars. They reveal that there is no correlation between the level of binary interaction and the orbital period. This situation may be accounted for if the wind mass-loss rate from the giant is the principal factor controlling the activity level in these (detached) systems, via a stream of matter funneled through the inner Lagragian point.Comment: Astronomy & Astrophysics Supplements, 6 pages, 2 figures, 4 tables (LaTeX A&A). Also available at: http://obswww.unige.ch/~udry/cine/barium/barium.htm

Contribution to the search for binaries among Am stars - VIII. New spectroscopic orbits of eight systems and statistical study of a sample of 91 Am stars

International audienceThis paper is the last of a series devoted to the study of Am stars, with the monitoring of radial velocities of a sample of 91 objects during more than 20 yr. The purpose was to determine which stars were members of spectroscopic binaries (SBs) and study in detail those systems in order to obtain observational constraints on the origin of the Am phenomenon. In the first part, we present the results of a detailed study of eight Am stars (HD 32893, 60489, 109762, 111057, 113697, 204918, 219675 and BD+44° 4512) observed at the Haute-Provence and Cambridge observatories with CORAVEL instruments. We find that these objects are single-lined SBs whose orbital elements are determined for the first time. HD 32893 is found to be a triple spectroscopic system whose third body might be detected by speckle interferometry. Physical parameters are inferred for the primaries of those SBs. We then investigate the influence of tidal interaction and find that it has already led to the synchronism of the primaries and to the circularization of the orbits of four of those systems. In the second part of this paper, we present the main results of our whole programme and derive some statistical properties of Am stars. We give the recapitulating table of the orbital parameters found for the SBs of our whole sample and the list of those for which no evidence for radial velocity variations could be found during our monitoring. Our study shows that at least 64 per cent of Am stars are members of SBs. This rate is significantly greater than that of normal stars. Although some SBs may have been not detected, this study shows that a substantial fraction of Am stars do not belong to SBs: they are either isolated stars or members of wide binary systems. We then present some statistical properties of the orbital parameters of the SBs whose primary is an Am star, on an extended sample obtained by adding 29 Am SB orbits published by other authors. The corresponding e versus logP diagram shows a cut-off between the circular and the eccentric systems at P ~ 5.6 +/- 0.5 d, which indicates a typical age of 0.5 - 1 × 109 yr for the Am stars, which is in agreement with the values found in our previous detailed studies. A Monte Carlo analysis shows that the distribution of the mass function values f(m) is compatible with a power-law distribution N(m) ~ m-? of the masses m of the companions with ? = 0.3 +/- 0.2 or with a Gaussian distribution centred on 0.8 +/- 0.5Msolar, which indicates that the companions of Am SBs are mostly dwarf stars of type G-K-M

Contribution to the search for binaries among Am stars - VII. Orbital elements of seven new spectroscopic binaries, implications on tidal effects

International audienceWe present the results of a radial-velocity study of seven Am stars (HD 3970, 35035, 93946, 151746, 153286, 204751 and 224002) observed at the Observatoire de Haute-Provence (OHP) and the Cambridge Observatories with CORAVEL instruments. We find that these systems are single-lined spectroscopic binaries whose orbital elements are determined for the first time. Among this sample, HD 35035 and 153286 have long periods, with P = 2.8 and 9.5yr, respectively, which is rather unusual for Am stars. Four systems have orbits with large eccentricities (with e >= 0.4). Physical parameters are inferred from this study for the primaries of those systems. We then investigate the influence of tidal interaction, which has already led to the synchronism of the primaries and/or to the circularization of the orbits of some systems belonging to this sample. We extend this study to the list of 33 objects studied in this series of papers and derive values of the critical fractional radii r = R/a for circularization and synchronization of Am-type binaries. We find that the stars with r >~ 0.15 are orbiting on circular orbits and that synchronism is likely for all components with r >~ 0.20

Contribution to the search for binaries among Am stars — V. Orbital elements of eight short-period spectroscopic binaries

We present the results of a radial-velocity study of eight Am stars (HD 341, 55822, 61250, 67317, 93991, 162950, 224890 and 225137) observed at Observatoire de Haute-Provence with the CORAVEL instrument. We find that these systems are single-line spectroscopic binaries whose orbital elements are determined for the first tim

Contribution to the search for binaries among Am stars - III. HD 7119: a double-lined spectroscopic binary and a triple system

Radial velocity observations of HD 7119 with the CORAVEL instrument at Observatoire de Haute-Provence are reported. Known as an Amδδ Del metallic-line star, HD 7119 was included in our spectroscopic survey of Am-type stars, the purpose of which was to determine the frequency of binaries in this stellar family. This object is found to be a double-lined spectroscopic binary with a variable value of V0, the systematic velocity of the centre of gravity of the pair. The variation of this parameter is interpreted in terms of the orbital motion of an unseen third body with a much longer period. The orbital elements were derived for both the short- and the long-period orbits. These orbits can be considered to be well determined since these observations were performed on a regular basis over the 1992-1998 period, covering more than 320 orbital cycles for the short-period (P= 6.76 d) and 1.3 cycle for the long-period orbit (P∼ 1700 d). As deduced from the ratio of the correlation dip areas, the magnitude difference of the components of the short-period system is 0.7 mag. Combined with the Hipparcos data, this value leads to visual absolute magnitudes of 0.5 and 1.2 for the primary and secondary components, respectively. Such magnitudes are consistent with evolved δ Del-type stars. The third body could be a cool dwarf star with a minimum mass of 0.5 M⊙, located at ∼ 0.016 arcsec of the main system. Consequently, it cannot be the visual companion detected by Couteau with a separation of 3.35 arcsec. If this latter visual component were a physical component (rather than an optical one), HD 7119 would be a quadruple syste

Multiplicity among peculiar A stars I. The Ap stars HD 8441 and HD 137909, and the Am stars HD 43478 and HD 96391

We present the first results of a radial-velocity survey of cool Ap and Am stars. HD 8441 is not only a double system with P = 106.357 days, but is a triple one, the third companion having an orbital period larger than 5000 days. Improved orbital elements are given for the classical Ap star HD 137909 = beta CrB by combining our radial velocities with published ones. We yield new orbital elements of the two Am, SB2 binaries HD 43478 and HD 96391. Good estimates of the individual masses of the components of HD 43478 can be given thanks to the eclipses of this system, for which an approximate photometric solution is also proposed.Comment: 10 pages, 10 figures, accepted for publication in A&A

Pour quelles raisons, le taux de transmission verticale du Virus de l'Immunodéficience Humaine (VIH) durant l'accouchement et la période du post-partum reste élevé en Afrique de l'Est, malgré les programmes de prévention actuels ?: travail de Bachelor

Introduction : Des progrès énormes ont été réalisés en matière de prévention de la transmission mère-enfant (TME) du VIH grâce à la mise en place de différents programmes en Afrique. En 2016, le taux de TME restait toutefois élevé en Afrique de l’Est. Objectif : Effectuer une revue de littérature pour déterminer quels facteurs influencent l'adhérence et l'accès aux traitements antirétroviraux. Et cela, dans le but de mieux comprendre pourquoi le taux de TME demeure élevé en Afrique de l’Est malgré les programmes de prévention actuels. Méthode : La dernière recherche a été réalisée le 10 janvier 2018 avec le moteur de recherche PubMed. Les mots-clés suivants ont été utilisés (((HIV[MeSH Terms]) ET pregnancies[MeSH Terms]) ET africa, eastern[MeSH Terms]) ET prenatal care[MeSH Terms]. Cinq études ont été sélectionnées, à l’aide de de bases de données scientifiques, pour répondre à la question de recherche. Les articles ont été analysés de manière descriptive, puis critique avant d’être discutés selon six hypothèses. Résultat : La stigmatisation et la spiritualité ne semblent pas influencer avec l’adhérence au traitement. Toutefois, les femmes reportent des expériences de vie négatives à l'annonce de leur séropositivité. La distance de l’habitation au centre de soins n'a pas été décrite comme facteur influençant. Cependant, différents aspects liés à celle-ci tels que la sécurité, la disponibilité des transports ou leurs coûts le sont clairement. La mauvaise répartition des ressources financières est un facteur déterminant. Conclusion : Les recommandations liées aux soins anténataux sont applicables dans les centres de soins. Malgré tout, le suivi post-partum reste superficiel. Or, le risque de transmission par l'allaitement maternel est établi. Afin de réduire ce risque, il serait intéressant de mener une étude sur les programmes de prévention actuels ciblés sur cette période

A Renaissance study of Am stars. I. The mass ratio distribution

Triggered by the study of Carquillat & Prieur (2007, MNRAS, 380, 1064) of Am binaries, I reanalyse their sample of 60 orbits to derive the mass ratio distribution (MRD), assuming as they did a priori functional forms, i.e. a power law or a Gaussian. The sample is then extended using orbits published by several groups and a full analysis of the MRD is made, without any assumption on the functional form. I derive the MRD using a Richardson-Lucy inversion method, assuming a fixed mass of the Am primary and randomly distributed orbital inclinations. Using the large sub-sample of double-lined spectroscopic binaries, I show that this methodology is indeed perfectly adequate. Using the inversion method, applied to my extended sample of 162 systems, I find that the final MRD can be approximated by a uniform distribution.Comment: 7 pages, 9 figures; Accepted by A&

Abundance Analysis of Barium Stars

We obtain the chemical abundances of six barium stars and two CH subgiant stars based on the high signal-to-noise ratio and high resolution Echelle spectra. The neutron capture process elements Y, Zr, Ba, La, Eu show obvious overabundance relative to the Sun, for example, their [Ba/Fe] values are from 0.45 to 1.27. Other elements, including Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Ni, show comparable abundances to the Solar ones, and their [Fe/H] cover a range from $-$0.40 to 0.21, which means they belong to Galactic disk. The predicts of the theoretical model of wind accretion for binary systems can explain the observed abundance patterns of the neutron capture process elements in these stars, which means that their overabundant heavy-elements could be caused by accreting the ejecta of AGB stars, the progenitors of the present white dwarf companions in the binary systems.Comment: 24 pages, 6 figures, accepted for publication in ChJA