Project description

Introduction

Strategic Context

The Milky Way galaxy is an excellent laboratory to study the formation and evolution of galaxies. With past and ongoing large surveys such as Hipparcos, SDSS, 2MASS or DENIS, we have access to data which allow us to refine our knowledge of Galaxy formation. However, with the exception of the SDSS survey, which mainly samples the halo of the Galaxy, the full description of the 6D phase space, i.e. the combination of the position and velocity spaces, is not available due to the missing radial velocity and/or distance.

The fact that so few large-scale stellar spectroscopic programs have been undertaken is largely due to the scale of the problem. The Milky Way galaxy is all around us, requiring all-sky surveys to provide a complete picture. Although pencil-beam stellar spectroscopic surveys provide important information, a complete overall picture of the Galaxy is lacking. Moreover deep pencil-beam spectroscopic surveys provide only three of the six phase space coordinates.

With the advent of multi-fiber spectroscopy, in combination with the large field of view of Schmidt telescopes, it is now possible to acquire in a reasonable amount of time spectra for a large sample of stars that is representative for the different stellar populations of the Galaxy. Spectroscopy enables us to measure the radial velocity, which in turn allows us to study in detail the dynamics of the Galaxy. Spectroscopy also permits to measure the stellar parameters as well as the abundance of chemical elements in a stellar atmosphere which holds important clues on the initial chemical composition and its subsequent metal enrichment. The measurement of the radial velocity and of the chemical abundances as well as the derivation of stellar temperature and gravity in order to complement existing catalogues is the main purpose of the RAVE project.

The UK Schmidt Telescope (UKST) at the Siding Spring Observatory, New South Wales, Australia. The UKST is operated by the Anglo-Australian Observatory.

The UK Schmidt Telescope (UKST) at the Siding Spring Observatory, New South Wales, Australia. The UKST is operated by the Anglo-Australian Observatory.

Although providing a database of unparalleled power in its own right, RAVE can also enhance the science outcomes of missions such as ESA’s GAIA over the coming decades. RAVE provides an opportunity to pre-empt some of the spectral work in the GAIA mission, providing results up to one decade earlier than that planned for the GAIA final release. It will also serve as a real case study to fine tune the GAIA procedures.

International Collaboration

RAVE is an international collaboration with over 30 active members. The PI of RAVE is Prof. Matthias Steinmetz at Leibniz Institut für Astrophysik Potsdam, where the final data processing also takes place. The data acquisition takes place at the AAO using the UK
Schmidt telescope (see Figure 1) equipped with the 6dF system. Initial checks on RAVE data are perfomed at Macquarie University and data reduction at the Istituto Nazionale di Astrofisica at Padova. Other members are at ANU, Strasbourg, Heidelberg, Groningen, John Hopkins and Sydney Universities.

Survey description

Observations

RAVE was carried out using the 6dF multi-fiber spectroscopic facility at the UK Schmidt telescope of the Anglo-Australian Observatory in Siding Spring, Australia. RAVE is a magnitude limited spectroscopic survey in the range 9< I< 13. The wavelength range of 8410 to 8795 Angstrom overlaps with the photometric Cousins I band. The survey probes both the nearby and more distant Galaxy. Typical distances for K0 dwarfs are between 50 and 250 pc, while the K0 giants are located at distances of 0.7 to 3 kpc. The most distant RAVE stars, a few luminous blue variable stars, are actually members of the Large Magellanic Cloud.

Heliocentric radial Velocity as a function of Galactic longitude for the Galactic latitude slice -45° < b < -30°. Large Magellanic Cloud stars can be seen as a group of stars around RV=300 km/s, where the trend in RVs for these stars is due to the rotation of the LMC.

Heliocentric radial Velocity as a function of Galactic longitude for the Galactic latitude slice -45° < b < -30°. Large Magellanic Cloud stars can be seen as a group of stars around RV=300 km/s, where the trend in RVs for these stars is due to the rotation of the LMC.

RAVE observation commenced in April 2003 at a rate of 7 nights per lunation. With the completion of the 6dF Galaxy Redshift Survey (Jones et al. 2004) in August 2005, this rate increase to 20-25 scheduled nights per lunation. The original setup consists of two field plates (complemented by a third plate in 2009) with robotically positioned fibres in the focus of the UK Schmidt telescope at the Anglo-Australian Observatory. A field plate covers a 5.7 degree field of view and feeds light to up to 150 fibers each with an angular diameter of 6.7 arcsec on the sky. In order to avoid chance superpositions with target stars when using such wide fibers, we avoid regions close to the Galactic plane, near the galactic bulge, or dense stellar clusters.

As of April 2013, a total of 574,630 spectra had been amassed on 483,330 unique objects. In order to statistically account for the contamination effects of unresolved spectroscopic binaries, about 20000 stars from the RAVE input catalogue where observed in repeat sequence following a logarithmic cadence. The RAVE data are consecutively published in a series of public data releases. DR1 (Steinmetz et al, 2006) provided 25,274 radial velocities measurements for 24,748 individual stars. DR2 (Zwitter et al, 2008) increased these number to 51,829 radial velocities for 49,327 individual stars and provided furthermore stellar parameters derived from 22,407 spectra of 21,121 individual stars. The release of DR3 (Siebert et al, 2011) featured ~86000 radial velocities and ~45000 stellar parameters.  DR4 (Kordopatis et al, 2013), extends by an order of magnitude the size of the previous catalogues, where the e ffective temperatures, surface gravities, overall metallicities ([M/H]), chemical abundances of six alpha-elements, line-of-sight velocities and distances are published.  Additional cross-correlations with photometric (2MASS, DENIS, APASS) and proper motion catalogues (UCAC4, PPMXL) are also provided in order to complete the available information on each star.  Finally, quality flags such as the convergence of the parameterisation algorithm (see Kordopatis et al. 2011, 2013), spectral morphological flags (for example to identify spectroscopic binary stars, Matijevic et al. 2012), chromospheric activity information (Žerjal et al. 2013), 2 of the fit of each line (see Boeche et al. 2011), and signal-to-noise ratio allow the user to select sub-samples according to the requirements of their study.

RAVE Data Products

Radial Velocity

The RAVE data reduction pipeline derives radial velocities from sky-subtracted normalized spectra via cross-correlation with an extensive library of synthetic spectra. A set of 57,943 spectra degraded to the resolving power of RAVE from Munari et al. (2005) is used. It covers all loci of non-degenerate stars in the H-R diagram, with metallicities in the range of -2.5 < [M/H] < +0.5, and enhancements of [alpha/Fe] = 0.0 and +0.4. A microturbulent velocity of 2 km/s is assumed. Comparison with external data sets shows that the RAVE radial velocities are on average more accurate than 2 km/s.

Stellar Parameters

To derive stellar parameters, temperature, gravity and metallicity, the RAVE DR4 pipeline employs a hybrid of a decision-tree algorithm called DEGAS (Bijaoui et al. 2012) and a projection method called MATISSE (Recio-Blanco et al. 2006).  Our estimates of errors of the stellar parameters for a spectrum with a signal-to-noise ratio (S/N) of ~50 are 150 K in temperature, 0.3 dex in gravity, and 0.12 dex in metallicity (Kordopatis et al, 2013).

Chemical Abundances

Abundances for individual chemical elements are determined for the elements Mg, Al, Si, Ti, Ni and Fe based on a curve of growth analysis (Boeche et al. 2011). The chemical pipeline relies on an equivalent widths (EWs) library which contains the expected EWs of the lines visible in the RAVE wavelength range (604 atomic and molecule lines). These EWs are computed for a grid of stellar parameters values covering the range [4000, 7000] K in temperature, [0.0, 5.0] dex in gravity (log g) and [-2.5, +0.5] dex in [M/H] and five levels of abundances in the range[-0.4, +0.4] dex.  The chemical pipeline constructs on-the-fly spectrum models by adopting the e ffective temperatures and surface gravities obtained by the DR4 atmospheric parameters pipeline. It then searches for the best fi tting model by minimizing the Chi2 between the models and the observational data. The estimated errors in abundance, based on a comparison with reference stars, depend on the element and range from 0.17 dex for Mg, Al and Ti to 0.3 dex for Ti and Ni. The error for Fe is estimated as 0.23 dex.

Distances

Distances are estimated for spectra with SNR > 10/pixel by projecting the atmospheric parameters onto a the Padova set of theoretical isochrones and obtaining the most likely value of the absolute magnitude of the stars.  DR4 employs an algorithm based on the Bayesian distance- nding method presented in Binney et al. (2014), and takes into account the interstellar extinction, as well as kinematic correction factors obtained by the method of Schonrich et al. (2012). Testing using Hipparcos stars indicates that the inverse of the expectation value of the parallax is the most reliable distance estimator.  Compared to a subset of Hipparcos stars, the method results in an over-estimation of less than 10% for the dwarfs and less than 20% for the giants. The method has also been tested on the open cluster spectra, delivering very satisfactory distances, provided a cluster-specific age prior is used.

Science

Local and Global parameters of the Milky Way

One of the first applications of RAVE was to constrain the local escape speed of the Milky Way based on a sample of high-velocity stars (Smith et al 2007, Piffl et al 2014). We find that the escape velocity lies within the range 498< vesc < 608 km/s, with a median likelihood of 533 km/s. The fact that vesc2 is significantly greater than vcirc2 (where vesc = 220 km/s is the local circular velocity) implies that there must be a significant amount of mass exterior to the solar circle, that is, this convincingly demonstrates the presence of a dark halo in the Galaxy. For an adiabatically contracted NFW halo model this result corresponds to a virial mass of 1.3 x 1012 solar masses and a the circular velocity at the virial radius is 142 km/s, arguing for a, compared to expectations from cosmological models, relatively low total mass of the Milky Way with respect to its total luminosity.

The analysis in Veltz et al (2008) focuses on 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. 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 p/m 10 pc and 1048 p/m 36 pc.

In Siebert et al (2008) 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 RAVE. We find that the velocity ellipsoid is tilted towards the Galactic plane with an inclination of 7 degrees. 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 a preferred value for the scale length in the range 2.5-2.7 kpc for a nearly spherical halo. With the continuation of the RAVE survey, it will thus be possible to provide a strong constraint on the mass distribution of the Milky Way.

Using the distances derived in Breddels et al (2010), the RAVE dataset shows the expected decrease in the metallicity of stars as a function of distance from the Galactic plane. 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.

Substructure as probles of formation

In Seabroke et al (2008) we have searched for in-falling stellar streams on to the local Milky Way disc in the CORAVEL and RAVE surveys. Kuiper statistics have been employed to test the symmetry of the Galactic vertical velocity distribution functions in these samples for evidence of a net vertical flow that could be associated with a (tidal?) stream of stars with vertically coherent kinematics. We find that the solar neighborhood is devoid of any vertically coherent streams containing hundreds of stars. There are no vertical streams in the RAVE samples with stellar densities > 1.5 x 103 stars kpc-3,

In Ruchti et al. (2010) we have undertaken a study of metal-poor stars selected from the RAVE to establish whether or not there is a significant population of metal-poor thick-disk stars and to measure their elemental abundances. 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. 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 of the Milky Way.

Stellar astrophysics

In Munari et al 2009 we used the multi-epoch spectra for seven luminous blue variables in the LMC observed by RAVE between 2005 and 2008. The behavior 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.

In Matejivic et al (2010) we have applied a new method for the detection of double-line binary stars to 25,850 spectra of the RAVE DR2 and found 123 double-lined binary candidates, only eight of which are already marked as binaries in the SIMBAD database. Among the candidates, there are seven that show spectral features consistent with the RS CVn type (solar type with active chromosphere) and seven that might be of W UMa type. One star, HD 101167, seems to be a triple system composed of three nearly identical G-type dwarfs

More with a focus on the interstellar medium is a study on the properties of five diffuse interstellar bands (DIB) in the spectra of RAVE stars (Munari et al., 2008). The DIB at 8620.4 Angstrom is by far the strongest and cleanest of all DIBs occurring within the RAVE wavelength range, with no interference by underlying absorption stellar lines in hot stars. It correlates so tightly with reddening following the relation E(B-V) = 2.72 (p/m 0.03)x EW (Angstrom), valid throughout the general interstellar medium of our Galaxy, providing a powerful tool to derive in a direct way the reddening caused by the general interstellar medium.