arXiv subjects

S. Udry

Publications and source records attributed to S. Udry.

At least 613 records · Page 34Linked to original sources

arXiv2002

The CORALIE survey for southern extra-solar planets IX. A 1.3-day period brown dwarf disguised as a planet

In this article we present the case of HD 41004 AB, a system composed of a K0V star and a 3.7-magnitude fainter M-dwarf companion separated by only 0.5 arcsec. An analysis of CORALIE radial-velocity measurements has revealed a variation with an amplitude of about 50m/s and a periodicity of 1.3days. This radial-velocity signal is consistent with the expected variation induced by the presence a very low mass giant planetary companion to HD 41004 A, whose light dominates the spectra. The radial-velocity measurements were then complemented with a photometric campaign and with the analysis of the bisector of the CORALIE Cross-Correlation Function (CCF). While the former revealed no significant variations within the observational precision of 0.003-0.004 mag (except for an observed flare event), the bisector analysis showed that the line profiles are varying in phase with the radial-velocity. This latter result, complemented with a series of simulations, has shown that we can explain the observations by considering that HD 41004 B has a brown-dwarf companion orbiting with the observed 1.3-day period. If confirmed, this detection represents the first discovery of a brown dwarf in a very short period (1.3-day) orbit around an M dwarf. Finally, this case should be taken as a serious warning about the importance of analyzing the bisector when looking for planets using radial-velocity techniques.

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arXiv2002

The CORALIE survey for southern extra-solar planets VIII. The very low-mass companions of HD141937, HD162020, HD168443, HD202206: brown dwarfs or superplanets?

Doppler CORALIE measurements of the solar-type stars HD141937, HD162020, HD168443 and HD202206 show Keplerian radial-velocity variations revealing the presence of 4 new companions with minimum masses close to the planet/brown-dwarf transition, namely with m_2sin(i) = 9.7, 14.4, 16.9, and 17.5 M_Jup, respectively. The orbits present fairly large eccentricities (0.22<e<0.43). Except for HD162020, the parent stars are metal rich compared to the Sun, as are most of the detected extra-solar planet hosts. Considerations of tidal dissipation in the short-period HD162020 system points towards a brown-dwarf nature for the low-mass companion. HD168443 is a multiple system with two low-mass companions being either brown dwarfs or formed simultaneously in the protoplanetary disks as superplanets. For HD202206, the radial velocities show an additional drift revealing a further outer companion, the nature of which is still unknown. Finally, the stellar-host and orbital properties of massive planets are examined in comparison to lighter exoplanets. Observed trends include the need of metal-rich stars to form massive exoplanets and the lack of short periods for massive planets. If confirmed with improved statistics, these features may provide constraints for the migration scenario.

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arXiv2002

The CORALIE survey for southern extra-solar planets VII. Two short-period Saturnian companions to HD108147 and HD168746

We present the discovery of two Saturn-mass companions to HD108147 and HD168746. Both belong to the lightest ever discovered planets. The minimum mass of the companion to HD168746 is of only 0.77 the mass of Saturn and its orbital period is 6.4 days. The companion to HD108147 orbits its parent star in 10.9 days and its minimum mass is 1.34 that of Saturn. Its orbit is characterized by a high eccentricity, e=0.50, indicating possibly the presence of a second companion. The detection of Saturn-mass planets by means of the Doppler technique demands high radial-velocity measurement precision. The two new candidates were discovered by means of the CORALIE echelle spectrograph. The instrumental accuracy of CORALIE combined with the simultaneous ThAr-reference technique has reached a level better than 3m/s. On many observed objects the precision is now limited by photon noise. We present in this paper the weighted cross-correlation technique, which leads to an improvement in the photon noise of the computed radial velocity. We discuss as well a modification of the numerical cross-correlation mask which reduces significantly the residual perturbation effects produced by telluric absorption lines.

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arXiv2001

A Planet Candidate in the Stellar Triple System HD178911

We report the detection of a low-mass companion orbiting the solar-type star HD178911B, the distant component of the stellar triple system HD178911. The variability of HD178911B was first detected using radial-velocity measurements obtained with the HIRES spectrograph mounted on the 10-m Keck1 telescope at the W.M. Keck Observatory (Hawaii, USA). We then started an intense radial-velocity follow up of the star with the ELODIE echelle spectrograph mounted on the 1.93-m telescope at the Observatoire de Haute-Provence (France) in order to derive its orbital solution. The detected planet candidate has an orbital period of 71.5 days and a minimum mass of 6.3 Jupiter masses. We performed a spectral analysis of the star, which shows that the lithium abundances in the system are similar to those in another known planet-hosting wide binary stellar system, 16 Cyg. In both systems the lithium is undetected in the atmosphere of the visual secondary harboring the planetary companion, but is easily detected in the spectrum of the visual primary. We discuss this similarity and its ramifications.

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arXiv2001

The distribution of exoplanet masses

The present study derives the distribution of secondary masses M2 for the 67 exoplanets and very low-mass brown dwarf companions of solar-type stars, known as of April 4, 2001. This distribution is related to the distribution of M2 sin i through an integral equation of Abel's type. Although a formal solution exists for this equation, it is known to be ill-behaved, and thus very sensitive to the statistical noise present in the input M2 sin i distribution. To overcome that difficulty, we present two robust, independent approaches: (i) the formal solution of the integral equation is numerically computed after performing an optimal smoothing of the input distribution, (ii) the Lucy-Richardson algorithm is used to invert the integral equation. Both approaches give consistent results. The resulting statistical distribution of exoplanet true masses reveals that there is no reason to ascribe the transition between giant planets and brown dwarfs to the threshold mass for deuterium ignition (about 13 MJ). The M2 distribution shows instead that all the objects have M2 < 10 MJ, except the heavier candidates which cluster around 15 MJ.

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arXiv2001

No planet for HD 166435

The G0V star HD166435 has been observed by the fiber-fed spectrograph ELODIE as one of the targets in the large extra-solar planet survey that we are conducting at the Observatory of Haute-Provence. We detected coherent, low-amplitude, radial-velocity variations with a period of 3.7987days, suggesting a possible close-in planetary companion. Subsequently, we initiated a series of high-precision photometric observations to search for possible planetary transits and an additional series of CaII H and K observations to measure the level of surface magnetic activity and to look for possible rotational modulation. Surprisingly, we found the star to be photometrically variable and magnetically active. A detailed study of the phase stability of the radial-velocity signal revealed that the radial-velocity variability remains coherent only for durations of about 30days. Analysis of the time variation of the spectroscopic line profiles using line bisectors revealed a correlation between radial velocity and line-bisector orientation. All of these observations, along with a one-quarter cycle phase shift between the photometric and the radial-velocity variationss, are well explained by the presence of dark photospheric spots on HD166435. We conclude that the radial-velocity variations are not due to gravitational interaction with an orbiting planet but, instead, originate from line-profile changes stemming from star spots on the surface of the star. The quasi-coherence of the radial-velocity signal over more than two years, which allowed a fair fit with a binary model, makes the stability of this star unusual among other active stars. It suggests a stable magnetic field orientation where spots are always generated at about the same location on the surface of the star.

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arXiv2001

The CORALIE survey for southern extrasolar planets V: 3 new extrasolar planets

We report the detection of 3 new planetary companions orbiting the solar-type stars GJ 3021, HD 52265 and HD 169830 using radial-velocity measurements taken with the CORALIE echelle spectrograph. All these planetary companions have longer orbital periods than the 51 Peg-like objects. The orbits are fairly eccentric. The minimum masses of these planets range from 1 to 3.3 M_Jup. The stars have spectral types from F8V to G6V. They are metal-rich. We also present our radial-velocity measurements for three solar-type stars known to host planetary companions iota Hor (HD 17051), HD 210277 and HD 217107.

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arXiv2001

HD 80606 b, a planet on an extremely elongated orbit

We report the detection of a planetary companion orbiting the solar-type star HD 80606, the brighter component of a wide binary with a projected separation of about 2000 AU. Using high-signal spectroscopic observations of the two components of the visual binary, we show that they are nearly identical. The planet has an orbital period of 111.8 days and a minimum mass of 3.9 M_Jup. With e=0.927, this planet has the highest orbital eccentricity among the extrasolar planets detected so far. We finally list several processes this extreme eccentricity could result from.

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arXiv2001

Bisector analysis as a diagnostic of intrinsic radial-velocity variations

In this contribution we present the results of the application of the bisector of the cross-correlation function as a diagnostic of activity-related radial-velocity variations. The results show that the technique is very effective. We present examples for which the application of the bisector analysis was essential to establish the planetary nature of the candidate or to exclude an orbital signature. An analysis of the behaviour of the bisector for active dwarfs of different spectral types shows that the relation between the bisector and the radial-velocity variation depends in a great extent on the v sin(i) of the star. The results may shed a new light on the intrinsic sources of radial-velocity variation for different types of solar-type dwarfs.

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arXiv2000

Accurate masses of very low mass stars: III 16 new or improved masses

We have obtained adaptive optics images and accurate radial velocities for 7 very low mass systems, in the course of a long term effort to determine accurate masses for very low mass stars (M<0.6 Solar Mass). We use the new data, together with measurements from the litterature for some stars, to determine new or improved orbits for these 7 systems. They provide masses for 16 very low mass stars with accuracies that range between 0.2% and 5%, and in some cases a very accurate distance as well. This information is used in a companion paper to discuss the Mass-Luminosity relation for the V, J, H and K photometric bands.

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arXiv2000

Accurate masses of very low mass stars: IV Improved mass-luminosity relations

We present improved visual and near-infrared empirical mass-luminosity relations for very low mass stars (M$<$0.6~\Msol). These relations make use of all stellar masses in this range known with better than 10% accuracy, most of which are new determinations with 0.2 to 5% accuracy from our own programme, presented in a companion paper. As predicted by stellar structure models, the metallicity dispersion of the field populations induces a large scatter around the mean V band relation, while the infrared relations are much tighter. The agreement of the observed infrared mass-luminosity relations with the theoretical relations of Baraffe et al. (1998) and Siess et al. (2000) is impressive, while we find an increasingly significant discrepancy in the V band for decreasing masses. The theoretical mass-luminosity relation which is insufficiently steep, and has introduced some errors in the local stellar mass functions derived from V band luminosity functions.

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arXiv2000

Detection of a spectroscopic transit by the planet orbiting the star HD209458

We report the first detection of a planetary transit by spectroscopic measurements. We have detected the distortion of the stellar line profiles during a planetary transit. With the ELODIE spectrograph we took a sequence of high precision radial velocities of the star HD209458 at time of a transit of its planet. We detected an anomaly in the residuals of the orbit. The shape and the amplitude of the anomaly are modeled as a change of the mean stellar line profile resulting from the planet crossing the disk of the rotating star. The planetary orbit is in the same direction as the stellar rotation. Using the photometric transit to constrain the timing and the impact parameters of the transit, we measure an angle alpha=3.9d between the orbital plane and the apparent equatorial plane as well as a vsini=3.75(+-)1.25 kms-1. With additional constrains on the inclination of the star and on the statistics of the line of sight distribution, we can set an upper limit of 30d to the angle between the orbital plane and the stellar equatorial plane.

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arXiv2000

The Spectroscopic Orbit of the Planetary Companion Transiting HD209458

We report a spectroscopic orbit with period P = 3.52433 +/- 0.00027 days for the planetary companion that transits the solar-type star HD209458. For the metallicity, mass, and radius of the star we derive [Fe/H] = 0.00 +/- 0.02, M = 1.1 +/- 0.1 solar masses, and R = 1.3 +/- 0.1 solar radii. This is based on a new analysis of the iron lines in our HIRES template spectrum, and also on the absolute magnitude and color of the star, and uses isochrones from four different sets of stellar evolution models. Using these values for the stellar parameters we reanalyze the transit data and derive an orbital inclination of i = 85.2 +/- 1.4 degrees. For the planet we derive a mass of Mp = 0.69 +/- 0.05 Jupiter masses, a radius of Rp = 1.54 +/- 0.18 Jupiter radii, and a density of 0.23 +/- 0.08 grams per cubic cm.

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arXiv1999

A planet orbiting the star Gliese 86

A 4MJ planet with a 15.8day orbital period has been detected from very precise radial velocity measurements with the CORALIE echelle spectrograph. A second remote and more massive companion has also been detected. All the planetary companions so far detected in orbit closer than 0.08 AU have a parent star with a statistically higher metal content compared to the metallicity distribution of other stars with planets. Different processes occuring during their formation may provide a possible explanation for this observation.

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arXiv1999

Accurate masses of very low mass stars: II The very low mass triple system Gl 866

We present very accurate orbital parameters and mass measurements (2.4% accuracy) for the well known very low mass triple system Gl 866. We obtain first orbital elements for the short-period orbit and greatly improve the long period orbit. All three stars have masses close to 0.1 solar masses, and the system thus provides the strongest constraints to date on the mass-luminosity relation close to the brown dwarf limit.

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arXiv1998

New neighbours. I. 13 new companions to nearby M dwarfs

We present preliminary results of a radial-velocity search for companions to a volume limited sample of M dwarfs. Typical accuracy range between 10m/s for the brighter stars and 70m/s at our limiting magnitude. We complement the radial velocity monitoring with adaptive optics imaging to identify close visual companions in a larger volume-limited sample. For stellar companions the two techniques together cover the full separation range, to beyond the limiting distance of the sample. We will therefore eventually obtain a statistically meaningful inventory of the stellar multiplicity of nearby M-dwarf systems. We also have useful sensitivity to giant planets, as illustrated by our recent detection of a planetary companion to Gl 876. After 2.5 years, we have discovered 12 previously unknown components in this 127 stars sample. 7 of these are actually beyond our 9 pc limit, and were included on the basis of photometric parallaxes which were biased-down by the unrecognized companion. The remaining 5 are true additions to the 9 pc inventory. More are certainly forthcoming, given our present selection bias towards short periods and relatively massive companions. We have derived orbital elements for 7 of the new systems, and for some previously known binaries. One system is a Post-Common-Envelope WD+M binary. Some of the new binaries have excellent prospects for an accurate mass determination over the next few years. Preliminary masses have accuracies of 2% at 0.4-0.6 \Msol and 10% at 0.1 \Msol.

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arXiv1998

Evidence for a signature of the galactic bar in the solar neighbourhood

Using available kinematical data for a subsample of NLTT stars from the HIPPARCOS mission we confirm the existence of a previously reported local anomaly in the (u,v) plane: the mean motion u for old disc stars, with v < -30 km/s, is largely positive (+19 +/- 9 km/s w.r.t. the Galactic Center). With the use of the newest global self-consistent numerical models of our Galaxy, we show that a bar could be responsible for this observed velocity anomaly. A fraction of our stars have bar perturbed ``hot'' orbits, allowing them to erratically wander from inside the bar to regions outside the corotation, in particular through the solar neighbourhood.

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arXiv1998

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.

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