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

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

SB9: The Ninth Catalogue of Spectroscopic Binary Orbits

The Ninth Catalogue of Spectroscopic Binary Orbits (http://sb9.astro.ulb.ac.be) continues the series of compilations of spectroscopic orbits carried out over the past 35 years by Batten and collaborators. As of 2004 May 1st, the new Catalogue holds orbits for 2,386 systems. Some essential differences between this catalogue and its predecessors are outlined and three straightforward applications are presented: (1) Completeness assessment: period distribution of SB1s and SB2s; (2) Shortest periods across the H-R diagram; (3) Period-eccentricity relation.Comment: Accepte for publication in A&A, 6 pages, 6 figure

Report of IAU Commission 30 on Radial Velocities (2006-2009)

Brief summaries are given on the following subjects: Radial velocities and exoplanets (Toward Earth-mass planets; Retired A stars and their planets; Current status and prospects); Toward higher radial velocity precision; Radial velocities and asteroseismology; Radial velocities in Galactic and extragalactic clusters; Radial velocities for field giants; Galactic structure -- Large surveys (The Geneva-Copenhagen Survey; Sloan Digital Sky Survey; RAVE); Working groups (WG on radial velocity standards; WG on stellar radial velocity bibliography; WG on the catalogue of orbital elements of spectroscopic binaries [SB9]).Comment: 11 pages, to appear in the IAU Transactions Vol. XXVIIA, Reports on Astronomy 2006-2009, ed. Karel van der Hucht. Editor: G. Torre

Hertzsprung-Russell diagram and mass distribution of barium stars

With the availability of parallaxes provided by the Tycho-Gaia Astrometric Solution, it is possible to construct the Hertzsprung-Russell diagram (HRD) of barium and related stars with unprecedented accuracy. A direct result from the derived HRD is that subgiant CH stars occupy the same region as barium dwarfs, contrary to what their designations imply. By comparing the position of barium stars in the HRD with STAREVOL evolutionary tracks, it is possible to evaluate their masses, provided the metallicity is known. We used an average metallicity [Fe/H] = -0.25 and derived the mass distribution of barium giants. The distribution peaks around 2.5 Msun with a tail at higher masses up to 4.5 Msun. This peak is also seen in the mass distribution of a sample of normal K and M giants used for comparison and is associated with stars located in the red clump. When we compare these mass distributions, we see a deficit of low-mass (1 - 2 Msun) barium giants. This is probably because low-mass stars reach large radii at the tip of the red giant branch, which may have resulted in an early binary interaction. Among barium giants, the high-mass tail is however dominated by stars with barium indices of less than unity, based on a visual inspection of the barium spectral line; that is, these stars have a very moderate barium line strength. We believe that these stars are not genuine barium giants, but rather bright giants, or supergiants, where the barium lines are strengthened because of a positive luminosity effect. Moreover, contrary to previous claims, we do not see differences between the mass distributions of mild and strong barium giants.Comment: 14 pages, 17 figure

The Empirical Mass-Luminosity Relation for Low Mass Stars

This work is devoted to improving empirical mass-luminosity relations and mass-metallicity-luminosity relation for low mass stars. For these stars, observational data in the mass-luminosity plane or the mass-metallicity-luminosity space subject to non-negligible errors in all coordinates with different dimensions. Thus a reasonable weight assigning scheme is needed for obtaining more reliable results. Such a scheme is developed, with which each data point can have its own due contribution. Previous studies have shown that there exists a plateau feature in the mass-luminosity relation. Taking into account the constraints from the observational luminosity function, we find by fitting the observational data using our weight assigning scheme that the plateau spans from 0.28 to 0.50 solar mass. Three-piecewise continuous improved mass-luminosity relations in K, J, H and V bands, respectively, are obtained. The visual mass-metallicity-luminosity relation is also improved based on our K band mass-luminosity relation and the available observational metallicity data.Comment: 8 pages, 2 figures. Accepted for publication in Astrophysics & Space Scienc

On metal-deficient barium stars and their link with yellow symbiotic stars

This paper addresses the question of why metal-deficient barium stars are not yellow symbiotic stars (YSyS). Samples of (suspected) metal-deficient barium (mdBa) stars and YSyS have been collected from the literature, and their properties reviewed. It appears in particular that the barium nature of the suspected mdBa stars needs to be ascertained by detailed abundance analyses. Abundances are therefore derived for two of them, HD 139409 and HD 148897, which reveal that HD 148897 should not be considered a barium star. HD 139409 is a mild barium star, with overabundances observed only for elements belonging to the first s-process peak (Y and Zr). The evidence for binarity among mdBa stars is then reviewed, using three different methods: (i) radial-velocity variations (from CORAVEL observations), (ii) Hipparcos astrometric data, and (iii) a method based on the comparison between the Hipparcos and Tycho-2 proper motions. A first-time orbit is obtained for HIP 55852, whereas evidence for the (so far unknown) binary nature of HIP 34795, HIP 76605, HIP 97874 and HIP 107478 is presented. Two stars with no evidence for binarity whatsoever (HIP 58596 and BD +3 2688) are candidates low-metallicity thermally-pulsing asymptotic giant branch stars, as inferred from their large luminosities. The reason why mdBa stars are not YSyS is suggested to lie in their different orbital period distributions: mdBa stars have on average longer orbital periods than YSyS, and hence their companion accretes matter at a lower rate, for a given mass loss rate of the giant star. The definite validation of this explanation should nevertheless await the determination of the orbital periods for the many mdBa stars still lacking periods, in order to make the comparison more significant.Comment: Astronomy & Astrophysics, in press; 16 pages, 14 figures; also available at http://www.astro.ulb.ac.be/Html/ps.html#PR

Search for brown-dwarf companions of stars

The discovery of 9 new brown-dwarf candidates orbiting stars in the CORALIE and HARPS radial-velocity surveys is reported. New CORALIE radial velocities yielding accurate orbits of 6 previously-known hosts of potential brown-dwarf companions are presented. Including targets selected from the literature, 33 hosts of potential brown-dwarf companions are examined. Employing innovative methods, we use the new reduction of the Hipparcos data to fully characterise the astrometric orbits of 6 objects, revealing M-dwarf companions with masses between 90 M_Jup and 0.52 M_Sun. Additionally, the masses of two companions can be restricted to the stellar domain. The companion to HD 137510 is found to be a brown dwarf. At 95 % confidence, the companion of HD 190228 is also a brown dwarf. The remaining 23 companions persist as brown-dwarf candidates. Based on the CORALIE planet-search sample, we obtain an upper limit of 0.6 % for the frequency of brown-dwarf companions around Sun-like stars. We find that the companion-mass distribution function is rising at the lower end of the brown-dwarf mass range, suggesting that in fact we are detecting the high-mass tail of the planetary distribution.Comment: 24 pages, 21 figures, 10 tables. Accepted for publication in Astronomy and Astrophysics. Abridged abstrac

Testing Planet Formation Models with Gaia μ\muas Astrometry

In this paper, we first summarize the results of a large-scale double-blind tests campaign carried out for the realistic estimation of the Gaia potential in detecting and measuring planetary systems. Then, we put the identified capabilities in context by highlighting the unique contribution that the Gaia exoplanet discoveries will be able to bring to the science of extrasolar planets during the next decade.Comment: 4 pages, 1 figure. To appear in the proceedings of "IAU Symposium 248 - A Giant Step: from Milli- to Micro-arcsecond Astrometry", held in Shanghai, China, 15-19 Oct. 200