Publications by James Binney
Analysing surveys of our Galaxy - I. Basic astrometric data
Monthly Notices of the Royal Astronomical Society (2011)
Supernova-driven gas accretion in the Milky Way
Monthly Notices of the Royal Astronomical Society (2011)
The detection and treatment of distance errors in kinematic analyses of stars
Monthly Notices of the Royal Astronomical Society (2011)
Testing formation mechanisms of the Milky Way's thick disc with RAVE
Monthly Notices of the Royal Astronomical Society 413 (2011) 2235-2241
We study the eccentricity distribution of a thick-disc sample of stars (defined as those withVy > 50kms-1and1 < |z|/kpc < 3) observed in the Radial Velocity Experiment (RAVE). We compare this distribution with those obtained in four simulations of galaxy formation taken from the literature as compiled by Sales et al. Each simulation emphasizes different scenarios for the origin of such stars (satellite accretion, heating of a pre-existing thin disc during a merger, radial migration, and gas-rich mergers). We find that the observed distribution peaks at low eccentricities and falls off smoothly and rather steeply to high eccentricities. This finding is fairly robust to changes in distances and to plausible assumptions about thin-disc contamination. Our results favour models where the majority of stars formed in the Galaxy itself on orbits of modest eccentricity and disfavour the pure satellite accretion case. A gas-rich merger origin where most of the stars form 'in situ' appears to be the most consistent with our data. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.
Detection of a radial velocity gradient in the extended local disc with RAVE
Monthly Notices of the Royal Astronomical Society 412 (2011) 2026-2032
Using a sample of 213713 stars from the Radial Velocity Experiment (RAVE) survey, limited to a distance of 2kpc from the Sun and to |z| < 1kpc, we report the detection of a velocity gradient of disc stars in the fourth quadrant, directed radially from the Galactic Centre. In the direction of the Galactic Centre, we apply a simple method independent of stellar proper motions and of Galactic parameters to assess the existence of this gradient in the RAVE data. This velocity gradient corresponds to |K+C| > rsim 3 km s-1 kpc-1, where K and C are the Oort constants measuring the local divergence and radial shear of the velocity field, respectively. In order to illustrate the effect, assuming a zero radial velocity of the local standard of rest we then reconstruct the two-dimensional Galactocentric velocity maps using two different sets of proper motions and photometric distances based either on isochrone fitting or on K-band magnitudes, and considering two sets of values for the Galactocentric radius of the Sun and local circular speed. Further observational confirmation of our finding with line-of-sight velocities of stars at low latitudes, together with further modelling, should help constrain the non-axisymmetric components of the Galactic potential, including the bar, the spiral arms and possibly the ellipticity of the dark halo. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.
A search for new members of the βPictoris, Tucana-Horologium and εCha moving groups in the RAVE data base
Monthly Notices of the Royal Astronomical Society 411 (2011) 117-123
We report on the discovery of new members of nearby young moving groups, exploiting the full power of combining the Radial Velocity Experiment (RAVE) survey with several stellar age diagnostic methods and follow-up high-resolution optical spectroscopy. The results include the identification of one new and five likely members of the βPictoris moving group, ranging from spectral types F9 to M4 with the majority being M dwarfs, one K7 likely member of the εCha group and two stars in the Tucana-Horologium association. Based on the positive identifications, we foreshadow a great potential of the RAVE data base in progressing towards a full census of young moving groups in the solar neighbourhood. © 2010 The Authors. Journal compilation © 2010 RAS.
Galaxia: A code to generate a synthetic survey of the Milky way
Astrophysical Journal 730 (2011)
We present here a fast code for creating a synthetic survey of the Milky Way. Given one or more color-magnitude bounds, a survey size, and geometry, the code returns a catalog of stars in accordance with a given model of the Milky Way. The model can be specified by a set of density distributions or as an N-body realization. We provide fast and efficient algorithms for sampling both types of models. As compared to earlier sampling schemes which generate stars at specified locations along a line of sight, our scheme can generate a continuous and smooth distribution of stars over any given volume. The code is quite general and flexible and can accept input in the form of a star formation rate, age-metallicity relation, age-velocity-dispersion relation, and analytic density distribution functions. Theoretical isochrones are then used to generate a catalog of stars, and support is available for a wide range of photometric bands. As a concrete example, we implement the Besançon Milky Way model for the disk. For the stellar halo we employ the simulated stellar halo N-body models of Bullock & Johnston. In order to sample N-body models, we present a scheme that disperses the stars spawned by an N-body particle, in such a way that the phase-space density of the spawned stars is consistent with that of the N-body particles. The code is ideally suited to generating synthetic data sets that mimic near future wide area surveys such as GAIA, LSST, and HERMES. As an application we study the prospect of identifying structures in the stellar halo with a simulated GAIA survey. We plan to make the code publicly available. © 2011. The American Astronomical Society. All rights reserved.
The dawning of the stream of Aquarius in RAVE
Astrophysical Journal Letters 728 (2011)
We identify a new, nearby (0.5 kpc ≲ d ≲ 10 kpc) stream in data from the RAdial Velocity Experiment (RAVE). As the majority of stars in the stream lie in the constellation of Aquarius, we name it the Aquarius Stream. We identify 15 members of the stream lying between 30° < l < 75° and -70° < b < -50°, with heliocentric line-of-sight velocities V ∼ -200 km s. The members are outliers in the radial velocity distribution, and the overdensity is statistically significant when compared to mock samples created with both the Besançon Galaxy model and newly developed code Galaxia. The metallicity distribution function and isochrone fit in the log g-T plane suggest that the stream consists of a 10Gyr old population with [M/H] ∼ -1.0. We explore relations to other streams and substructures, finding that the stream cannot be identified with known structures: it is a new, nearby substructure in the Galaxy's halo. Using a simple dynamical model of a dissolving satellite galaxy, we account for the localization of the stream. We find that the stream is dynamically young and therefore likely the debris of a recently disrupted dwarf galaxy or globular cluster. The Aquarius stream is thus a specimen of ongoing hierarchical Galaxy formation, rare for being right in the solar suburb. © 2011. The American Astronomical Society.
Distance determination for RAVE stars using stellar models
Astronomy and Astrophysics 511 (2010)
Aims: We develop a method for deriving distances from spectroscopic data and obtaining full 6D phase-space coordinates for the RAVE survey's second data release. Methods: We used stellar models combined with atmospheric properties from RAVE (effective temperature, surface gravity and metallicity) and (J - Ks) photometry from archival sources to derive absolute magnitudes. In combination with apparent magnitudes, sky coordinates, proper motions from a variety of sources and radial velocities from RAVE, we are able to derive the full 6D phasespace coordinates for a large sample of RAVE stars. This method is tested with artificial data, Hipparcos trigonometric parallaxes and observations of the open cluster M67. Results: When we applied our method to a set of 16 146 stars, we found that 25% (4037) of the stars have relative (statistical) distance errors of <35%, while 50% (8073) and 75% (12 110) have relative (statistical) errors smaller than 45% and 50%, respectively. Our various tests show that we can reliably estimate distances for main-sequence stars, but there is an indication of potential systematic problems with giant stars owing to uncertainties in the underlying stellar models. For the main-sequence star sample (defined as those with log(g) > 4), 25% (1744) have relative distance errors <31%, while 50% (3488) and 75% (5231) have relative errors smaller than 36% and 42%, respectively. Our full dataset shows the expected decrease in the metallicity of stars as a function of distance from the Galactic plane. The known kinematic substructures in the U and V velocity components of nearby dwarf stars are apparent in our dataset, confirming the accuracy of our data and the reliability of our technique. We provide independent measurements of the orientation of the UV velocity ellipsoid and of the solar motion, and they are in very good agreement with previous work. Conclusions: The distance catalogue for the RAVE second data release is available at http://www.astro.rug.nl/~rave, and will be updated in the future to include new data releases. © 2010 ESO.
Galactic fountains and gas accretion
AIP Conference Proceedings 1240 (2010) 166-168
Star-forming disc galaxies such as the Milky Way need to accrete ≳1 M⊙ of gas each year to sustain their star formation. This gas accretion is likely to come from the cooling of the hot corona, however it is still not clear how this process can take place. We present simulations supporting the idea that this cooling and the subsequent accretion are caused by the passage of cold galactic-fountain clouds through the hot corona. The Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas causes coronal gas to condense in the cloud's wake. For likely parameters of the Galactic corona and of typical fountain clouds we obtain a global accretion rate of the order of that required to feed the star formation. © 2010 American Institute of Physics.
ORIGINS OF THE THICK DISK AS TRACED BY THE ALPHA ELEMENTS OF METAL-POOR GIANT STARS SELECTED FROM RAVE
ASTROPHYSICAL JOURNAL LETTERS 721 (2010) L92-L96
Stellar distances from spectroscopic observations: A new technique
Monthly Notices of the Royal Astronomical Society 407 (2010) 339-354
A Bayesian approach to the determination of stellar distances from photometric and spectroscopic data is presented and tested both on pseudo-data, designed to mimic data for stars observed by the Radial Velocity Experiment survey, and on the real stars from the Geneva-Copenhagen survey. It is argued that this method is optimal in the sense that it brings to bear all available information and that its results are limited only by observational errors and the underlying physics of stars. The method simultaneously returns the metallicities, ages and masses of programme stars. Remarkably, the uncertainty in the output metallicity is typically 44 per cent smaller than the uncertainty in the input metallicity. © 2010 The Authors. Journal compilation © 2010 RAS.
The mode of gas accretion on to star-forming galaxies
Monthly Notices of the Royal Astronomical Society 404 (2010) 1464-1474
It is argued that galaxies like ours sustain their star formation by transferring gas from an extensive corona to the star-forming disc. The transfer is effected by the galactic fountain - cool clouds that are shot up from the plane to kiloparsec heights above the plane. The Kelvin-Helmholtz instability strips gas from these clouds. If the pressure and the metallicity of the corona are high enough, the stripped gas causes a similar mass of coronal gas to condense in the cloud's wake. Hydrodynamical simulations of cloud-corona interaction are presented. These confirm the existence of a critical ablation rate above which the corona is condensed and imply that for the likely parameters of the Galactic corona this rate lies near the actual ablation rate of clouds. In external galaxies, trails of H i behind individual clouds will not be detectable, although the integrated emission from all such trails should be significant. Parts of the trails of the clouds that make up the Galaxy's fountain should be observable and may account for features in targeted 21-cm observations of individual high-velocity clouds and surveys of Galactic H i emission. Taken in conjunction with the known decline in the availability of cold infall with increasing cosmic time and halo mass, the proposed mechanism offers a promising explanation of the division of galaxies between the blue cloud to the red sequence in the colour-luminosity plane. © 2010 The Authors. Journal compilation © 2010 RAS.
Local kinematics and the local standard of rest
Monthly Notices of the Royal Astronomical Society 403 (2010) 1829-1833
We re-examine the stellar kinematics of the solar neighbourhood in terms of the velocity υ⊙ of the Sun with respect to the local standard of rest. We show that the classical determination of its component V⊙ in the direction of Galactic rotation via Strömberg's relation is undermined by the metallicity gradient in the disc, which introduces a correlation between the colour of a group of stars and the radial gradients of its properties. Comparing the local stellar kinematics to a chemodynamical model which accounts for these effects, we obtain (U, V, W)⊙ = (11.1+0.69-0.75, 12.24+0.47-0.47, 7.25+0.37-0.36) km s-1, with additional systematic uncertainties ∼(1, 2, 0.5) km s-1. In particular, V⊙ is 7 km s-1 larger than previously estimated. The new values of (U, V, W)⊙ are extremely insensitive to the metallicity gradient within the disc. © 2010 The Authors. Journal compilation © 2010 RAS.
The RAVE survey: Rich in very metal-poor stars
Astrophysical Journal Letters 724 (2010)
Very metal-poor stars are of obvious importance for many problems in chemical evolution, star formation, and galaxy evolution. Finding complete samples of such stars which are also bright enough to allow high-precision individual analyses is of considerable interest. We demonstrate here that stars with iron abundances [Fe/H] < -2 dex, and down to below -4 dex, can be efficiently identified within the Radial Velocity Experiment (RAVE) survey of bright stars, without requiring additional confirmatory observations. We determine a calibration of the equivalent width of the calcium triplet lines measured from the RAVE spectra onto true [Fe/H], using high spectral resolution data for a subset of the stars. These RAVE iron abundances are accurate enough to obviate the need for confirmatory higher-resolution spectroscopy. Our initial study has identified 631 stars with [Fe/H] ≤ -2, from a RAVE database containing approximately 200,000 stars. This RAVE-based sample is complete for stars with [Fe/H] ≲ -2.5, allowing statistical sample analysis. We identify three stars with [Fe/H] ≲ -4. Of these, one was already known to be "ultra metal-poor," one is a known carbon-enhanced metal-poor star, but we obtain [Fe/H] = -4.0, rather than the published [Fe/H] = -3.3, and derive [C/Fe] = +0.9, and [N/Fe] = +3.2, and the third is at the limit of our signal-to-noise ratio. RAVE observations are ongoing and should prove to be a rich source of bright, easily studied, very metal-poor stars. © 2010. The American Astronomical Society. All rights reserved.
Distance determination for RAVE stars using stellar models: II. Most likely values assuming a standard stellar evolution scenario
Astronomy and Astrophysics 522 (2010)
The RAdial Velocity Experiment (RAVE) is a spectroscopic survey of the Milky Way which already collected over 400000 spectra of ∼330000 different stars. 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 21872 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, A&A, 511, A16). 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 ≲ 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. © 2010 ESO.
Origins of the thick disk as traced by the alpha elements of metal-poor giant stars selected from RAVE
Astrophysical Journal Letters 721 (2010)
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 Letter, we present abundances of four α-elements (Mg, Si, Ca, and Ti) and iron for a subsample of 212 red giant branch and 31 red clump/horizontal branch stars from this study. We find that the [α/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 [α/Fe] ratios implies good mixing in the interstellar medium 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 initial mass function. We conclude that the α-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. © 2010 The American Astronomical Society. All rights reserved.
