Parallax and masses of alpha Centauri revisited

Context. Despite the thorough work of van Leeuwen (2007), the parallax of alpha Centauri is still far from being carved in stone. Any derivation of the individual masses is therefore uncertain, if not questionable. And yet, that does not prevent this system from being used for calibration purpose in several studies. Aims. Obtaining more accurate model-free parallax and individual masses of this system. Methods. With HARPS, the radial velocities are not only precise but also accurate. Ten years of HARPS data are enough to derive the complement of the visual orbit for a full 3D orbit of alpha Cen. Results. We locate alpha Cen (743 mas) right where Hipparcos (ESA 1997) had put it, i.e. slightly further away than derived by Soderhjelm (1999). The components are thus a bit more massive than previously thought (1.13 and 0.97 Msun for A and B respectively). These values are now in excellent agreement with the latest asteroseismologic results.Comment: 4 pages, 3 figures, accepted in Astronomy & Astrophysic

Precision and accuracy of the orbital parameters derived from 2D & 1D space observations of visual or astrometric binaries

Recent investigations (e.g. Han et al. 2001) have shown that fitting the Hipparcos observations with an orbital model when the astrometric wobble caused by the companion is below the noise level can have rather unexpected consequences. With new astrometric missions coming out within the next ten years, it is worth investigating the orbit reconstruction capabilities of such instruments at low signal-to-noise ratio. This is especially important because some of them will have no input catalogue thus meaning that all the orbital parameters will have to be derived from scratch. The puzzling case of almost parabolic orbits is also investigated.Comment: 6 pages, 7 figures, accepted by A&

The multiplicity of \phi\ Phe revisited

The chemically peculiar B star $\phi$ Phe was, until very recently, considered a triple system, even though the data were not conclusive and the orbits rather uncertain. Very recent results by Korhonen et al. (2013) provided a revised orbit, different from the then available astrometric Hipparcos orbit. Additional spectroscopic data, obtained with the BESO spectrograph at Cerro Armazones, confirm the newly found orbit, even though the resulting radial velocities do not allow to improve on the recent orbit. We combine the latter with the Hipparcos measurements to secure the astrometric orbit, and derive the inclination of the system. Using evolutionary tracks, we can finally constrain all the parameters of the two components in this system. We confirm the mass of the primary, 3 M$_\odot$, and find that the companion has a mass of 0.9 M$_\odot$. The inclination of the system is $i=93^{\circ} \pm 4.7^{\circ}$, and is potentially eclipsing; we predict the time of the next conjunction. Given that the eccentricity of the orbit and the exact value of the semi-amplitude of the radial velocity relies on just one set of points, we also urge observers to measure radial velocities at the next periastron passage in April 2015.Comment: 5 papes, accepted as Research Note in Astronomy and Astrophysic

Astrometric orbits of SB9 stars

Hipparcos Intermediate Astrometric Data (IAD) have been used to derive astrometric orbital elements for spectroscopic binaries from the newly released Ninth Catalogue of Spectroscopic Binary Orbits (SB9). Among the 1374 binaries from SB9 which have an HIP entry, 282 have detectable orbital astrometric motion (at the 5% significance level). Among those, only 70 have astrometric orbital elements that are reliably determined (according to specific statistical tests discussed in the paper), and for the first time for 20 systems, representing a 10% increase relative to the 235 DMSA/O systems already present in the Hipparcos Double and Multiple Systems Annex. The detection of the astrometric orbital motion when the Hipparcos IAD are supplemented by the spectroscopic orbital elements is close to 100% for binaries with only one visible component, provided that the period is in the 50 - 1000 d range and the parallax is larger than 5 mas. This result is an interesting testbed to guide the choice of algorithms and statistical tests to be used in the search for astrometric binaries during the forthcoming ESA Gaia mission. Finally, orbital inclinations provided by the present analysis have been used to derive several astrophysical quantities. For instance, 29 among the 70 systems with reliable astrometric orbital elements involve main sequence stars for which the companion mass could be derived. Some interesting conclusions may be drawn from this new set of stellar masses, like the enigmatic nature of the companion to the Hyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but the companion remains elusive.Comment: Astronomy & Astrophysics, in press (16 pages, 12 figures); also available at http://www.astro.ulb.ac.be/Html/ps.html#Astrometr

A probable close brown dwarf companion to GJ 1046 (M2.5V)

Context. Brown dwarf companions to stars at separations of a few AU or less are rare objects, and none have been found so far around early-type M dwarfs M0V-M5V). With GJ 1046 (M2.5V), a strong candidate for such a system with a separation of 0.42 AU is presented. Aims. We aim at constraining the mass of the companion in order to decide whether it is a brown dwarf or a low-mass star. Methods. We employed precision RV measurements to determine the orbital parameters and the minimum companion mass. We then derived an upper limit to the companion mass from the lack of disturbances of the RV measurements by a secondary spectrum. An even tighter upper limit is subsequently established by combining the RV-derived orbital parameters with the recent new version of the Hipparcos Intermediate Astrometric Data. Results. For the mass of the companion, we derive m>26.9 MJup from the RV data. Based on the RV data alone, the probability that the companion exceeds the stellar mass threshold is just 6.2%. The absence of effects from the secondary spectrum lets us constrain the companion mass to m <229 MJup. The combination of RV and Hipparcos data yields a 3sigma upper mass limit to the companion mass of 112 MJup with a formal optimum value at m=47.2 MJup. From the combination of RV and astrometric data, the chance probability that the companion is a star is 2.9%. Conclusions. We have found a low-mass, close companion to an early-type M dwarf. While the most likely interpretation of this object is that it is a brown dwarf, a low-mass stellar companion is not fully excluded.Comment: Accepted by A&A, 6 pages, 3 figures with 4 jpg files (Fig. 3 has two panels), original version replaced as sidecaption in Figure 1 did not work. Changes in 2nd replacement: (1) Conclusions: 99.9% --> 99.73% . (2) one sentence below: the the --> the . (3) References: Joergens, V., M\"uller, A.. 2007. ApJL, in press --> ... A., 2007, ApJ 666, L11

The Hipparcos observations and the mass of sub-stellar objects

The Hipparcos Intermediate Astrometric Data have been used lately to estimate the inclination of the orbital plane of candidate extrasolar planets. Whereas most of these investigations derive almost face-on orbits, we show that the astrometric data are seldom precise enough to undertake such studies and that the `face-on' result might be just a spurious effect of the method.Comment: Accepted for publication in A&A Letter

Exploitation démographique d’une liste nominative unique : L’exemple de l’île de Saint-Barthélemy en 1840

Cet article présente une évaluation de l’état d’une population et les principaux paramétres qui le modifient en utilisant les données fournies par une seule liste nominative.Deux approches sont utilisées, qui aboutissent à des résultats compatibles. La première se fonde sur les données agrégées c’est-à-dire la structure par âge et sexe de la population; la seconde a pour point de départ la structure des ménages et les liens entre les individus qui les composent.L’étude est réalisée à partir du recensement de la population effectué en 1840 â l’île Saint-Barthélémy, dans les Petites Antilles

Re-processing the Hipparcos Transit Data and Intermediate Astrometric Data of spectroscopic binaries; 1, Ba, CH and Tc-poor S stars

The Intermediate Astrometric Data (IAD) and Transit Data (TD) made available by ESA after the Hipparcos mission make it possible to re-process the observations of any Hipparcos entry. This paper illustrates how TD and IAD may be used in conjunction with the orbital parameters of spectroscopic binaries to improve the astrometric parameters of binaries considered as single stars in the original reduction process. That approach has been applied to 81 dwarf barium stars, strong and mild barium stars, CH stars, and Tc-poor S stars for which spectroscopic orbits became available recently. Among these 81 systems, 23 yield reliable astrometric orbits, thus making it possible to evaluate on real data the impact of an unrecognized orbital motion on the proper motion (Wielen, 1997, A&A 325, 367). Comparison of the proper motion from the Hipparcos catalogue with that re-derived in the present work indicates that the former are indeed far off the present value for binaries with periods in the range 3 to about 8 years. Hipparcos parallaxes of unrecognized spectroscopic binaries turn out to be reliable, except for systems with periods close to 1 year, as expected. Finally, we show that, even when a complete orbital revolution was observed by Hipparcos, the inclination is unfortunately seldom precise

Color-Induced Displacement double stars in SDSS

We report the first successful application of the astrometric color-induced displacement technique (CID, the displacement of the photocenter between different bandpasses due to a varying contribution of differently colored components to the total light), originally proposed by Wielen (1996) for discovering unresolved binary stars. Using the Sloan Digital Sky Survey (SDSS) Data Release 1 with 2.5 million stars brighter than 21m in the u and g bands, we select 419 candidate binary stars with CID greater than 0.5 arcsec. The SDSS colors of the majority of these candidates are consistent with binary systems including a white dwarf and any main sequence star with spectral type later than ~K7. The astrometric CID method discussed here is complementary to the photometric selection of binary stars in SDSS discussed by Smolcic et al. (2004), but there is considerable overlap (15%) between the two samples of selected candidates. This overlap testifies both to the physical soundness of both methods, as well as to the astrometric and photometric quality of SDSS data.Comment: submitted to A&A, 13 pages, 6 figure

Spectroscopic binaries with elliptical orbits

The radial velocity curves of many spectroscopic binaries (SBs) are perturbed by gas streams or proximity effects. For SBs with circular orbits, these perturbations can give rise to spurious orbital eccentricities of high statistical significance. But tests to identify such anomalous orbits can be constructed since perturbed velocity curves are in general no longer Keplerian. The derived tests are applied both to synthetic and to observed velocity curves.Comment: 9pages,3figures,accepted by A&