Dimitri Pourbaix

A Planet at 5 AU Around 55 Cancri

We report precise Doppler-shift measurements of 55 Cancri (G8 V) obtained from 1989 to 2002 at Lick Observatory. The velocities reveal evidence for an outer planetary companion to 55 Cancri orbiting at 5.5 AU. The velocities also conirm a second, inner planet at 0.11 AU. The outer planet is the irst extrasolar planet found that orbits near or beyond the orbit of Jupiter. It was drawn from a sample of 50 stars observed with suci ent duration and quality to detect a giant planet at 5 AU, implying that such planets are not rare. The properties of this Jupiter analog may be compared directly to those of the Jovian planets in our solar system. Its eccentricity is modest, e … 0:16, compared with e … 0:05 for both Jupiter and Saturn. Its mass is at least 4.0 MJUP (M sini). The two planets do not perturb each other signiicantly. Moreover, a third planet of sub-Jupiter mass could easily survive between these two known planets. Indeed, a third periodicity remains in the velocity measurements with P … 44:3 days and a semiamplitude of 13 m s 1 . This periodicity is caused either by a third planet at a … 0:24 AU or by inhomogeneities on the stellar surface that rotate with period 42 days. The planet interpretation is more likely, as the stellar surface is quiet both chromospherically [log﷿R 0 fi… 5 :0] and photospherically (brightness variations less than 1 mmag). Moreover, any hypothetical surface inhomogeneity would have to persist in longitude for 14 yr. Even with all three planets, an additional planet of terrestrial mass could orbit stably at 1 AU. The star 55 Cancri is apparently a normal, middle-aged main-sequence star with a mass of 0.95 M , rich in heavy elements (‰Fe=H …˛ 0 :27). This high metallicityraises the issue of the precise relationship between its age, rotation, and chromosphere. Subject headings: planetary systems N stars:individual (55 Cancri,HIP 43587,HD 75732, HR 3522, 1 Cancri)

Reprocessing the Hipparcos data of evolved stars III. Revised Hipparcos period-luminosity relationship for galactic long-period variable stars ?;??

We analyze the K band luminosities of a sample of galactic long-period variables using parallaxes measured by the Hipparcos mission. The parallaxes are in most cases re-computed from the Hipparcos Intermediate Astrometric Data using improved astrometric fits and chromaticity corrections. The K band magnitudes are taken from the literature and from measure- ments by COBE, and are corrected for interstellar and circumstellar extinction. The sample contains stars of several spectral types: M, S and C, and of several variability classes: Mira, semiregular SRa, and SRb. We find that the distribution of stars in the period-luminosity plane is independent of circumstellar chemistry, but that the dierent variability types have dierent P-L distributions. Both the Mira variables and the SRb variables have reasonably well-defined period-luminosity relationships, but with very dierent slopes. The SRa variables are distributed between the two classes, suggesting that they are a mixture of Miras and SRb, rather than a separate class of stars. New period-luminosity re- lationships are derived based on our revised Hipparcos parallaxes. The Miras show a similar period-luminosity relationship to that found for Large Magellanic Cloud Miras by Feast et al. (1989). The maximum absolute K magnitude of the sample is about 8:2 for both Miras and semi-regular stars, only slightly fainter than the expected AGB limit. We show that the stars with the longest periods (P> 400 d) have high mass loss rates and are almost all Mira variables.

Ten Low-Mass Companions from the Keck Precision Velocity Survey

Ten new companions have emerged from the Keck precision Doppler velocity survey, with minimum (M sin i) masses ranging from 0.8 MJUP to 0.34 M☉. Five of these are planet candidates with M sin i < 12 MJUP, two are brown dwarf candidates with M sin i ~ 30 MJUP, and three are low-mass stellar companions. Hipparcos astrometry reveals the orbital inclinations and masses for three of the (more massive) companions, and it provides upper limits to the masses for the rest. A new class of extrasolar planet is emerging, characterized by nearly circular orbits and orbital radii greater than 1 AU. The planet HD 4208b appears to be a member of this new class. The mass distribution of extrasolar planets continues to exhibit a rapid rise from 10 MJUP toward the lowest detectable masses near 1 MSAT.

Constraining the difference in convective blueshift between the components of alpha Centauri with precise radial velocities

New radial velocities of Cen A & B obtained in the framework the Anglo-Australian Planet Search programme as well as in the CORALIE programme are added to those by Endl et al. (2001) to improve the pre- cision of the orbital parameters. The resulting masses are 1:105 0:0070 M and 0:934 0:0061 M for A and B respectively. The factors limiting how accurately these masses can be derived from a combined visual-spectroscopic solution are investigated. The total eect of the convective blueshift and the gravitational redshift is also inves- tigated and estimated to dier by 215 8m s 1 between the components. This suggests that the dierence in convective blueshift between the components is much smaller than predicted from current hydrodynamical model atmosphere calculations.

Resolved double-lined spectroscopic binaries: A neglected source of hypothesis-free parallaxes and stellar masses

Double-lined spectroscopic binaries, once visually resolved (VB-SB2), provide hypothesis-free orbital parallaxes and masses of both components. Unlike eclipsing-spectroscopic binaries for which many accurate masses are already known (Andersen 1991; Andersen 1997), the number of VB-SB2 remains rather small. This paper presents 40 such systems for which published visual observations and radial velocities allow a simultaneous adjustment of both data sets. The precision of the indi- vidual masses as well as the evolution of that precision with respect to the published precision is investigated.

Asteroseismology and interferometry

Asteroseismology provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Recent developments, including the first systematic studies of solar-like pulsators, have boosted the impact of this field of research within astrophysics and have led to a significant increase in the size of the research community. In the present paper we start by reviewing the basic observational and theoretical properties of classical and solar-like pulsators and present results from some of the most recent and outstanding studies of these stars. We centre our review on those classes of pulsators for which interferometric studies are expected to provide a significant input. We discuss current limitations to asteroseismic studies, including difficulties in mode identification and in the accurate determination of global parameters of pulsating stars, and, after a brief review of those aspects of interferometry that are most relevant in this context, anticipate how interferometric observations may contribute to overcome these limitations. Moreover, we present results of recent pilot studies of pulsating stars involving both asteroseismic and interferometric constraints and look into the future, summarizing ongoing efforts concerning the development of future instruments and satellite missions which are expected to have an impact in this field of research.

A Planetary Companion to HD 40979 and Additional Planets Orbiting HD 12661 and HD 38529

We report the detection of three extrasolar planets from the Lick and Keck observatories. The F8 V star HD 40979 has a companion with orbital period P = 263.1 ± 3 days, eccentricity e = 0.25 ± 0.05, and velocity semiamplitude K = 101.2 ± 5.6 m s-1. The inferred semimajor axis is 0.83 AU and M sin i = 3.28MJup. Observations of planetary companions orbiting the G6 V star HD 12661 and the G4 IV star HD 38529 have already been published, and here we report additional, longer period companions for both of these stars. The outer companion to HD 12661 has Pc = 1444.5 ± 12.5 days, ec = 0.20 ± 0.04, and Kc = 27.6 ± 2.5 m s-1. Adopting a stellar mass of 1.07 M☉, we find Mc sin i = 1.57MJup and a semimajor axis of 2.56 AU. The second companion to HD 38529 has Pc = 2174 ± 30 days, ec = 0.36 ± 0.05, and Kc = 170.5 ± 9 m s-1. The assumed mass of 1.39 M☉ for HD 38529 yields Mc sin i = 12.7MJup and a semimajor axis of 3.68 AU. Photometric observations at Fairborn Observatory reveal low-amplitude brightness variations in HD 40979 and HD 38529 due to rotational modulation in the visibility of photospheric starspots, and they yield rotation periods of 7.0 and 35.7 days, respectively, very different from the planetary orbital periods. The orbital parameters of these two systems are compared with updated parameters for all of the known multiple-planet systems. Updated velocities are provided for the Andromedae system.

A Redetermination of the Mass of Procyon

The parallax and astrometric orbit of Procyon have been redetermined from PDS measurements of over 250 photographic plates spanning 83 years, with roughly 600 exposures used in the solution. These data are combined with two modern measurements of the primary–white dwarf separation, one utilizing a ground-based coronagraph, the other, the Planetary Camera (PC) of the Hubble Space Telescope. Together with the redetermined astrometric orbit and parallax, these yield new estimates of the component masses. The derived masses are 1.497 ± 0.037 M⊙ for the primary and 0.602 ± 0.015M⊙ for the white dwarf secondary. These mass values are heavily weighted by the PC separation measurement, which, while being somewhat discordant with the ground-based measures, we argue is more precise and more accurate and thus deserving of its greater weight. This stated, the long-standing discrepancy between previous determinations of the observed mass of Procyon A (1.75 M⊙) and the value supported by stellar evolution models (1.50 M⊙) appears to be reconciled.

Astrometric orbits of S B 9 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 ). This endeavour is justified by the fact that (i) the astrometric orbital motion is often difficult to detect without the prior knowledge of the spectroscopic orbital elements, and (ii) such knowledge was not available at the time of the construction of the Hipparcos Catalogue for the spectroscopic binaries which were recently added to the SB9 catalogue. Among the 1374 binaries from SB9 which have an HIP entry (excluding binaries with visual companions, or DMSA/C in the Double and Multiple Stars Annex), 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), and for the first time for 20 systems. This represents a 8.5% increase of the number of astrometric systems with known orbital elements (The Double and Multiple Systems Annex contains 235 of those DMSA/ Os ystems). 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 > 5m as. 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.

Screening the Hipparcos-based astrometric orbits of sub-stellar objects

The combination of Hipparcos astrometric data with the spectroscopic data of putative extrasolar planets seems to indicate that a significant fraction of these low-mass companions could be brown or M dwarfs (Han et al. 2001). We show that this is due to the adopted reduction procedure, and consequently that the Hipparcos data do not reject the planetary mass hypothesis in all but one cases. Additional companions, undetected so far, might also explain the large astrometric residuals of some of these stars.