arXiv subjects

O. Bienaymé

Publications and source records attributed to O. Bienaymé.

At least 37 records · Page 2Linked to original sources

arXiv2013

The asymmetric drift, the local standard of rest, and implications from RAVE data

The determination of the LSR is still a matter of debate. The classical value of the tangential peculiar motion of the Sun with respect to the LSR was challenged in recent years, claiming a significantly larger value. We show that the RAdial Velocity Experiment (RAVE) sample of dwarf stars is an excellent data set to derive tighter boundary conditions to chemodynamical evolution models of the extended solar neighbourhood. We present an improved Jeans analysis, which allows a better interpretation of the measured kinematics of stellar populations in the Milky Way disc. We propose an improved version of the Strömberg relation with the radial scalelengths as the only unknown. Binning RAVE stars in metallicity reveals a bigger asymmetric drift (corresponding to a smaller radial scalelength) for more metal-rich populations. With the standard assumption of velocity-dispersion independent radial scalelengths in each metallicity bin, we redetermine the LSR. The new Strömberg equation yields a joint LSR value of V_\sun=3.06 \pm 0.68 km/s, which is even smaller than the classical value based on Hipparcos data. The corresponding radial scalelength increases from 1.6 kpc for the metal-rich bin to 2.9 kpc for the metal-poor bin, with a trend of an even larger scalelength for young metal-poor stars. When adopting the recent Schönrich value of V_\sun=12.24 km/s for the LSR, the new Strömberg equation yields much larger individual radial scalelengths of the RAVE subpopulations, which seem unphysical in part. The new Strömberg equation allows a cleaner interpretation of the kinematic data of disc stars in terms of radial scalelengths. Lifting the LSR value by a few km/s compared to the classical value results in strongly increased radial scalelengths with a trend of smaller values for larger velocity dispersions.

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arXiv2011

A spectroscopic survey of thick disc stars outside the solar neighbourhood

We performed a spectroscopic survey of nearly 700 stars probing the galactic thick disc far from the solar neighbourhood towards the galactic coordinates (l~277, b~47). The derived effective temperatures, surface gravities and overall metallicities were then combined with stellar evolution isochrones, radial velocities and proper motions to derive the distances, kinematics and orbital parameters of the sample stars. The targets belonging to each galactic component (thin disc, thick disc, halo) were selected either on their kinematics or according to their position above the galactic plane, and the vertical gradients were also estimated. We present here atmospheric parameters, distances and kinematics for this sample, and a comparison of our kinematic and metallicity distributions with the Besancon model of the Milky Way. The thick disc far from the solar neighbourhood is found to differ only slightly from the thick disc properties as derived in the solar vicinity. For regions where the thick disc dominates, we measured vertical velocity and metallicity trends of d(V_phi)/dZ = 19 +/- 8 km/s/kpc and d[M/H]/dZ = -0.14 +/- 0.05 dex/kpc, respectively. These trends can be explained as a smooth transition between the different galactic components, although intrinsic gradients could not be excluded. In addition, a correlation d(V_phi)/d[M/H] = -45 +/- 12 km/s/dex between the orbital velocity and the metallicity of the thick disc is detected. This gradient is inconsistent with the SDSS photometric survey analysis, which did not detect any such trend, and challenges radial migration models of thick disc formation. Estimations of the scale heights and scale lengths for different metallicity bins of the thick disc result in consistent values, with hR~3.4 \pm 0.7 kpc, and hZ~694 \pm 45 pc, showing no evidence of relics of destroyed massive satellites.

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arXiv2011

The RAVE catalogue of stellar elemental abundances: first data release

We present chemical elemental abundances for $36,561$ stars observed by the RAdial Velocity Experiment (RAVE), an ambitious spectroscopic survey of our Galaxy at Galactic latitudes $|$b$|>25^{\circ}$ and with magnitudes in the range 9$<I_{DENIS}<$13. RAVE spectra cover the Ca-triplet region at 8410--8795Å with resolving power R$\sim$7500. This first data release of the RAVE chemical catalogue is complementary to the third RAVE data release of radial velocities and stellar parameters, and it contains chemical abundances for the elements Mg, Al, Si, Ca, Ti, Fe and Ni, with a mean error of $\sim$0.2 dex, as judged from accuracy tests performed on synthetic and real spectra. Abundances are estimated through a dedicated processing pipeline in which the curve of growth of individual lines is obtained from a library of absorption-line equivalent widths to construct a model spectrum that is then matched to the observed spectrum via a $χ^2$-minimization technique. We plan to extend this pipeline to include estimates for other elements, such as oxygen and sulfur, in future data releases.

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arXiv2011

Chemically tagging the Hyades stream: Does it partly originate from the Hyades cluster?

The Hyades stream has long been thought to be a dispersed vestige of the Hyades cluster. However, recent analyses of the parallax distribution, of the mass function, and of the action-space distribution of stream stars have shown it to be rather composed of orbits trapped at a resonance of a density disturbance. This resonant scenario should leave a clearly different signature in the element abundances of stream stars than the dispersed cluster scenario, since the Hyades cluster is chemically homogeneous. Here, we study the metalllicty as well as the element abundances of Li, Na, Mg, Fe, Zr, Ba, La, Ce, Nd, and Eu for a random sample of stars belonging to the Hyades stream, and compare them with those of stars from the Hyades cluster. From this analysis: (i) we independently confirm that the Hyades stream cannot be solely composed of stars originating in the Hyades cluster; (ii) we show that some stars from the Hyades stream nevertheless have abundances compatible with an origin in the cluster; (iii) we emphasize that the use of Li as a chemical tag of the cluster origin of main-sequence stars is very efficient in the range 5500 - 6200 K, since the Li sequence in the Hyades cluster is very tight; (iv) we show that, while this evaporated population has a metallicity excess of ~0.2 dex w.r.t. the local thin disk population, identical to that of the Hyades cluster, the remainder of the Hyades stream population has still a metallicity excess of ~0.06 to 0.15 dex, consistent with an origin in the inner Galaxy; (v) we show that the Hyades stream can be interpreted as an inner 4:1 resonance of the spiral pattern: this then also reproduces an orbital family compatible with the Sirius stream, and places the origin of the Hyades stream up to 1 kpc inwards from the solar radius, which might explain the observed metallicity excess of the stream population.

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arXiv2010

Distance determination for RAVE stars using stellar models II: Most likely values assuming a standard stellar evolution scenario

The RAdial Velocity Experiment (RAVE) is a spectroscopic survey of the Milky Way. We use the subsample of spectra with spectroscopically determined values of stellar parameters to determine the distances to these stars. The list currently contains 235,064 high quality spectra which show no peculiarities and belong to 210,872 different stars. The numbers will grow as the RAVE survey progresses. The public version of the catalog will be made available through the CDS services along with the ongoing RAVE public data releases. The distances are determined with a method based on the work by Breddels et al.~(2010). Here we assume that the star undergoes a standard stellar evolution and that its spectrum shows no peculiarities. The refinements include: the use of either of the three isochrone sets, a better account of the stellar ages and masses, use of more realistic errors of stellar parameter values, and application to a larger dataset. The derived distances of both dwarfs and giants match within ~21% to the astrometric distances of Hipparcos stars and to the distances of observed members of open and globular clusters. Multiple observations of a fraction of RAVE stars show that repeatability of the derived distances is even better, with half of the objects showing a distance scatter of \simlt 11%. RAVE dwarfs are ~300 pc from the Sun, and giants are at distances of 1 to 2 kpc, and up to 10 kpc. This places the RAVE dataset between the more local Geneva-Copenhagen survey and the more distant and fainter SDSS sample. As such it is ideal to address some of the fundamental questions of Galactic structure and evolution in the pre-Gaia era. Individual applications are left to separate papers, here we show that the full 6-dimensional information on position and velocity is accurate enough to discuss the vertical structure and kinematic properties of the thin and thick disks.

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arXiv2010

GYES, a multifibre spectrograph for the CFHT

We have chosen the name of GYES, one of the mythological giants with one hundred arms, offspring of Gaia and Uranus, for our instrument study of a multifibre spectrograph for the prime focus of the Canada-France-Hawaii Telescope. Such an instrument could provide an excellent ground-based complement for the Gaia mission and a northern complement to the HERMES project on the AAT. The CFHT is well known for providing a stable prime focus environment, with a large field of view, which has hosted several imaging instruments, but has never hosted a multifibre spectrograph. Building upon the experience gained at GEPI with FLAMES-Giraffe and X-Shooter, we are investigating the feasibility of a high multiplex spectrograph (about 500 fibres) over a field of view 1 degree in diameter. We are investigating an instrument with resolution in the range 15000 to 30000, which should provide accurate chemical abundances for stars down to 16th magnitude and radial velocities, accurate to 1 km/s for fainter stars. The study is led by GEPI-Observatoire de Paris with a contribution from Oxford for the study of the positioner. The financing for the study comes from INSU CSAA and Observatoire de Paris. The conceptual study will be delivered to CFHT for review by October 1st 2010.

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arXiv2010

Origins of the Thick Disk as Traced by the Alpha-Elements of Metal-Poor Giant Stars Selected from RAVE

Theories of thick disk formation can be differentiated by measurements of stellar elemental abundances. We have undertaken a study of metal-poor stars selected from the RAVE spectroscopic survey of bright stars to establish whether or not there is a significant population of metal-poor thick-disk stars ([Fe/H] <~ -1.0) and to measure their elemental abundances. In this paper, we present abundances of four alpha-elements (Mg, Si, Ca, Ti) and iron for a subsample of 212 RGB and 31 RC/HB stars from this study. We find that the [alpha/Fe] ratios are enhanced implying that enrichment proceeded by purely core-collapse supernovae. This requires that star formation in each star forming region had a short duration. The relative lack of scatter in the [alpha/Fe] ratios implies good mixing in the ISM prior to star formation. In addition, the ratios resemble that of the halo, indicating that the halo and thick disk share a similar massive star IMF. We conclude that the alpha enhancement of the metal-poor thick disk implies that direct accretion of stars from dwarf galaxies similar to surviving dwarf galaxies today did not play a major role in the formation of the thick disk.

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arXiv2009

RAVE spectroscopy of luminous blue variables in the Large Magellanic Cloud

CONTEXT: The RAVE spectroscopic survey for galactic structure and evolution obtains 8400-8800 Ang spectra at 7500 resolving power at the UK Schmidt Telescope using the 6dF multi-fiber positioner. More than 300,000 9 25 deg southern stars have been observed to date. AIMS: This paper presents the first intrinsic examination of stellar spectra from the RAVE survey, aimed at evaluating their diagnostic potential for peculiar stars and at contributing to the general understanding of Luminous Blue Variables (LBVs). METHODS: We used the multi-epoch spectra for all seven LBVs observed, between 2005 and 2008, in the Large Magellanic Cloud (LMC) by the RAVE survey. RESULTS: We demonstrate that RAVE spectra possess significant diagnostic potential when applied to peculiar stars and, in particular, LBVs. The behaviour of the radial velocities for both emission and absorption lines, and the spectral changes between outburst and quiescence states are described and found to agree with evidence gathered at more conventional wavelengths. The wind outflow signatures and their variability are investigated, with multi-components detected in S Doradus. Photoionisation modelling of the rich emission line spectrum of R 127 shows evidence of a massive detached ionised shell that was ejected during the 1982-2000 outburst. Surface inhomogeneities in the nuclear-processed material, brought to the surface by heavy mass loss, could have been observed in S Doradus, even if alternative explanations are possible. We also detect the transition from quiescence to outburst state in R 71. Finally, our spectrum of R 84 offers one of the clearest views of its cool companion.

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arXiv2009

Galactic kinematics with modified Newtonian dynamics

We look for observational signatures that could discriminate between Newtonian and modified Newtonian (MOND) dynamics in the Milky Way, in view of the advent of large astrometric and spectroscopic surveys. Indeed, a typical signature of MOND is an apparent disk of "phantom" dark matter, which is uniquely correlated with the visible disk-density distribution. Due to this phantom dark disk, Newtonian models with a spherical halo have different signatures from MOND models close to the Galactic plane. The models can thus be differentiated by measuring dynamically (within Newtonian dynamics) the disk surface density at the solar radius, the radial mass gradient within the disk, or the velocity ellipsoid tilt angle above the Galactic plane. Using the most realistic possible baryonic mass model for the Milky Way, we predict that, if MOND applies, the local surface density measured by a Newtonist will be approximately 78 Msun/pc2 within 1.1 kpc of the Galactic plane, the dynamically measured disk scale-length will be enhanced by a factor of 1.25 with respect to the visible disk scale-length, and the local vertical tilt of the velocity ellipsoid at 1 kpc above the plane will be approximately 6 degrees. None of these tests can be conclusive for the present-day accuracy of Milky Way data, but they will be of prime interest with the advent of large surveys such as GAIA.

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arXiv2009

Potential-density pairs and vertical tilt of the stellar velocity ellipsoid

We define new potential-density pairs and examine the impact of the potential flattening on the vertical velocity ellipsoid tilt, $δ$. By means of numerical integrations and analytical calculations, we estimate $δ$ in a variety of galactic potentials. We show that at 1 kpc above the Galactic plane at the solar radius, $δ$ can differ by 5 degrees, depending on whether the dark matter halo is flat or spherical. This result excludes the possibility of an extremely flattened Galactic dark halo.

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arXiv2008

Estimation of the Tilt of the Stellar Velocity Ellipsoid from RAVE and Implications for Mass Models

We present a measure of the inclination of the velocity ellipsoid at 1 kpc below the Galactic plane using a sample of red clump giants from the RAVE DR2 release. We find that the velocity ellipsoid is tilted towards the Galactic plane with an inclination of 7.3 +/-1.8 degree. We compare this value to computed inclinations for two mass models of the Milky Way. We find that our measurement is consistent with a short scale length of the stellar disc (Rd ~2 kpc) if the dark halo is oblate or with a long scale length (Rd~3 kpc) if the dark halo is prolate. Once combined with independent constraints on the flattening of the halo, our measurement suggests that the scale length is approximately halfway between these two extreme values, with a preferred range [2.5-2.7] kpc for a nearly spherical halo. Nevertheless, no model can be clearly ruled out. With the continuation of the RAVE survey, it will be possible to provide a strong constraint on the mass distribution of the Milky Way using refined measurements of the orientation of the velocity ellipsoid.

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arXiv2008

Galactic kinematics with RAVE data: I. The distribution of stars towards the Galactic poles

We analyze the distribution of G and K type stars towards the Galactic poles using RAVE and ELODIE radial velocities, 2MASS photometric star counts, and UCAC2 proper motions. The combination of photometric and 3D kinematic data allows us to disentangle and describe the vertical distribution of dwarfs, sub-giants and giants and their kinematics. We identify discontinuities within the kinematics and magnitude counts that separate the thin disk, thick disk and a hotter component. The respective scale heights of the thin disk and thick disk are 225$\pm$10 pc and 1048$\pm$36 pc. We also constrain the luminosity function and the kinematic distribution function. The existence of a kinematic gap between the thin and thick disks is incompatible with the thick disk having formed from the thin disk by a continuous process, such as scattering of stars by spiral arms or molecular clouds. Other mechanisms of formation of the thick disk such as `created on the spot' or smoothly `accreted' remain compatible with our findings.

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arXiv2005

Vertical distribution of Galactic disk stars: III. The Galactic disk surface mass density from red clump giants

We used red clump stars to measure the surface mass density of the Galactic disk in the solar neighbourhood. High resolution spectra of red clump stars towards the NGP have been obtained with the ELODIE spectrograph at OHP for Tycho-2 selected stars, and nearby Hipparcos counterparts were also observed. We determined their distances, velocities, and metallicities to measure the gravitational force law perpendicular to the Galactic plane. As in most previous studies, we applied one-parameter models of the vertical gravitational potential. We obtained a disk surface mass density within 1.1kpc of the Galactic plane, Sigma_{1.1kpc}=64+/-5 Msun_pc^{-2}, with an excellent formal accuracy, however we found that such one-parameter models can underestimate the real uncertainties. Applying two-parameter models, we derived more realistic estimates of the total surface mass density within 800pc from the Galactic plane, Sigma_{0.8kpc}=57-66 Msun pc^{-2}, and within 1.1kpc, Sigma{1.1kpc}=57-79 Msun pc^{-2}. This can be compared to literature estimates of \sim40 Msun pc^{-2} in stars and to 13 Msun pc^{-2} in the less accurately measured ISM contribution. We conclude that there is no evidence of large amounts of dark matter in the disk and, furthermore, that the dark matter halo is round or not vey much flattened. A by-product of this study is the determination of the half period of oscillation by the Sun through the Galactic plane, 42+/-2Myr, which cannot be related to the possible period of large terrestrial impact craters \sim 33-37Myr.

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arXiv2003

An active M8.5 dwarf wide companion to the M4/DA binary LHS4039/LHS4040

Low-mass and brown dwarfs have recently been found as wide companions to many nearby stars, formerly believed to be single. Wide binaries are usually found as common proper motion pairs. Sometimes, more than two objects share the same large proper motion, identifying them as nearby systems. We have found a third, low-mass component to a known wide binary at a distance of about 21pc, consisting of a red and a white dwarf (LHS4039 and LHS4040; about 150AU separation). The new companion, APMPMJ2354-3316C separated by about 2200AU, was classified as M8.5 dwarf. In recent spectroscopic observations it shows a very strong $H_α$ emission line and blue continuum. Comparing this event to flares in late-type M dwarfs, we find some similarity with LHS2397a, a nearby M8 dwarf which is so far the only known example of a low-mass star with a tight brown dwarf companion (separation of less than 4AU). The level of the activity as measured by $L_{H_α}/L_{bol}$ is comparable to that of the M9.5 dwarf 2MASSWJ0149+29 both during the flare and in quiescence.

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arXiv2002

Vertical distribution of Galactic disk stars I - Kinematics and metallicity

Nearly 400 Tycho-2 stars have been observed in a 720 square degree field in the direction of the North Galactic Pole with the high resolution echelle spectrograph ELODIE. Absolute magnitudes, effective temperatures, gravities and metallicities have been estimated, as well as distances and 3D velocities. Most of these stars are clump giants and span typical distances from 200pc to 800pc to the galactic mid-plane. This new sample, free of any kinematical and metallicity bias, is used to investigate the vertical distribution of disk stars. The old thin disk and thick disk populations are deconvolved from the velocity-metallicity distribution of the sample and their parameters are determined. The thick disk is found to have a moderate rotational lag of -51+-5 km/s with respect to the Sun with velocity ellipsoid (sigma_U, sigma_V, sigma_W)=(63+-6, 39+-4, 39+-4) km/s, mean metallicity of [Fe/H]=-0.48+-0.05 and a high local normalization of 15+-7%. Combining this NGP sample with a local sample of giant stars from the Hipparcos catalogue, the orientation of the velocity ellipsoid is investigated as a function of distance to the plane and metallicity. We find no vertex deviation for old stars, consistent with an axisymmetric Galaxy. Paper II is devoted to the dynamical analysis of the sample, puting new constraints on the vertical force perpendicular to the galactic plane and on the total mass density in the galactic plane.

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arXiv1998

The local stellar velocity distribution of the Galaxy. Galactic structure and potential

The velocity distribution of neighbouring stars is deduced from the Hipparcos proper motions. We have used a classical Schwarzschild decomposition and also developed a dynamical model for quasi-exponential stellar discs. This model is a 3-D derivation of Shu's model in the framework of Stäckel potentials with three integrals of motion. We determine the solar motion relative to the local standard of rest (LSR), the density and kinematic radial gradients, as well as the local slope of the velocity curve. From the stellar disc dynamical model, we determine explicitly the link between the tangential-vertical velocity (v_θ, v_z) coupling and the local shape of the potential. Using a restricted sample of 3-D velocity data, we measure $z_o$, the focus of the spheroidal coordinate system defining the best fitted Stäckel potential. The parameter $z_o$ is related to the tilt of the velocity ellipsoid and more fundamentally to the mass gradient in the galactic disc. This parameter is found to be 5.7\pm1.4 kpc. This implies that the galactic potential is not extremly flat and that the dark matter component is not confined in the galactic plane.

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arXiv1998

The distribution of nearby stars in phase space mapped by Hipparcos III. Clustering and streaming among A-F type stars

This paper presents the detailed results obtained in the search of density- velocity inhomogeneities in a volume limited and absolute magnitude limited sample of A-F type dwarfs within 125 parsecs of the Sun. A 3-D wavelet analysis is used to extract inhomogeneities, both in the density and velocity distributions. Having established a real picture of the phase space without assumption we come back to previously known observational facts regarding clusters and associations, superclusters. In the 3-D position space, well known open clusters (Hyades, Coma Berenices and Ursa Major), associations (parts of the Scorpio-Centaurus association) as well as the Hyades evaporation track are retrieved. Three new probably loose clusters are identified (Bootes, Pegasus 1 and 2). The sample is relatively well mixed in the position space since less than 7 per cent of the stars belong to structures with coherent kinematics, most likely gravitationally bound. In the velocity space, the majority of large scale velocity structures ($σ$ ~ 6.3 $km s-1$) are Eggen's superclusters (Pleiades SCl, Hyades SCl and Sirius SCl) with the whole Centaurus association. A new supercluster-like structure is found with a mean velocity between the Sun and Sirius SCl velocities. These structures are all characterized by a large age range which reflects the overall sample age distribution. Moreover, a few old streams of ~ 2 Gyr are also extracted at this scale with high U components. We show that all these large velocity dispersion structures represent 46% of the sample. Smaller scales (σ~ 3.8 and 2.4 $km s-1$) reveal that superclusters are always substructured by 2 or more streams which generally exhibit a coherent age distribution. Percentages of stars in these streams are 38% and 18% respectively.

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arXiv1998

The distribution of nearby stars in phase space mapped by Hipparcos II. Inhomogeneities among A-F type stars

A volume limited and absolute magnitude limited sample of A-F type dwarfs within 125 parsecs of the Sun is searched for inhomogeneities in the density- velocity space, expecting signatures of the cluster evaporation, phase mixing and possible disc heating mechanisms. A 3-D wavelet analysis is used to extract inhomogeneities, both in the density and velocity distributions. Thus, a real picture of the phase space is produced. Not only are some clusters and streams detected, but the fraction of clumped stars can be measured. By estimating individual stellar ages one can relate the streams and clusters to the state of the interstellar medium at star formation time and provide a quantitative view of cluster evaporation and stream mixing. As a result, we propose a coherent interpretation of superclusters and derive some quantitative evolutionary tracers which we expect to serve in the understanding of the large scale evolution of the galactic disc. The sample is relatively well mixed in the position space since less than 7 per cent of the stars are proper motion confirmed cluster members. We also detect star evaporation out of the Hyades open cluster. In the velocity space, the typical scale of so-called Eggen's superclusters ($σ6.3 km/s$) does not seem to correspond to any physical entity. The picture they form, their frequency and their divisions at smaller scales are well compatible with their creation by chance coincidence of physically homogeneous smaller scale structures ($σ3.8 or 2.4 km/s$). The detailed analysis of the phase space structures provides a scenario of kinematical evolution in the solar neighbourhood.

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