RAVE The Radial Velocity Experiment

Transcription

1 RAVE The Radial Velocity Experiment Draft October 2002 The RAVE Science Working Group Matthias Steinmetz (AIP, Germany), James Binney (Oxford, UK), Brian Boyle (AAO, Australia/UK), Walter Dehnen (AIP, Germany), Daniel Eisenstein (Steward, USA), Ken Freeman (RSAA, Australia), Sabine Frink (ARI, Germany), Gerry Gilmore (Cambridge, UK), Joss Hawthorn (AAO, Australia/UK), Amina Helmi (Utrecht, Netherlands) Hiroshi Karoji (NAOJ, Japan), Julio Navarro (Victoria, Canada), Fred Watson (AAO, Australia/UK), Rosie Wyse (JHU, USA)

2 Executive Summary RAVE (Radial Velocity Experiment) is an ambitious program to conduct an all-sky survey (complete to V = 16) to measure the radial velocities, metallicities and abundance ratios of 50 million stars using the 1.2-m UK Schmidt Telescope of the AAO, together with a northern counterpart, over the period The survey would represent a giant leap forward in our understanding of our own Milky Way galaxy, providing a vast stellar kinematic database three orders of magnitude larger than any other survey proposed for this coming decade. The main data product will be a magnitude-limited survey of 26 million thin disk main sequence stars, 9 million thick disk stars, 2 million bulge stars, 1 million halo stars, and a further 12 million giant stars including some out to 60 kpc from the Sun. RAVE will offer the first truly representative inventory of stellar radial velocities for all major components of the Galaxy. Its completeness and homogeneity will make it an invaluable stand-alone resource, but its full potential will be realised when the radial velocities are combined with proper motions and parallaxes from other sources (USNO, Tycho, DIVA). The survey is made possible by recent technical innovations in multi-fibre spectroscopy; specifically the development of the Echidna concept at the AAO for positioning fibres using piezo-electric ball/spines. A 1m-class Schmidt telescope equipped with an Echidna fibre-optic positioner and suitable spectrograph would be able to obtain spectra for over stars per clear night. The cost of such a system is relatively modest (US$ M), and could be commissioned by the start of Including operational costs, a five year survey would cost approximately US$3.5M per telescope, a fraction of the cost of space-based missions with similar objectives. Although the main survey cannot begin until 2006, a key component of the RAVE survey is a pilot program of 10 5 stars which may be carried out using the existing 6dF facility in unscheduled bright time over the period This will not only provide invaluable information on the local spiral arm but also help determine the best observing strategies for the main survey in order to maximise the RAVE science outcomes. ii

3 RAVE The Radial Velocity Experiment... i 1 INTRODUCTION STRATEGIC CONTEXT INTERNATIONAL COLLABORATION TIMELINE SURVEY FIGURES OF MERIT SCIENCE CASE PILOT STUDY MAIN STUDY BENEFITS OF RAVE IN OTHER AREAS OF ASTRONOMY RAVE IN THE CONTEXT OF FUTURE SATELLITE MISSIONS RAVE TECHNICAL CASE BACKGROUND PILOT STUDY MAIN STUDY DATA PROCESSING, ARCHIVING AND MANAGEMENT (TO BE ADDED) FINANCIAL ISSUES OVERVIEW PPARC RAVE AND THE GEMINI WIDE-FIELD OTHER OPTIONS CODA APPENDIX A. LIST OF COLLABORATORS (AS OF SEPTEMBER 1 ST, 2002) B. LIST OF SCIENTIST EXPRESSED INTEREST IN JOINING THE PROJECT iii

4 1 Introduction 1.1 Strategic Context In the first decade of the 21 st century, it is being increasingly recognised (see e.g. Freeman & Bland-Hawthorn 2002, ARAA, 40, 487) that many of the clues to the fundamental problem of galaxy formation in the early Universe lie locked up in the motions and chemical composition of stars in our Milky Way galaxy. Consequently, significant effort has been placed into planning the next generation of large-scale astrometric surveys e.g. DIVA (launch March 2006) and GAIA (launch 2012). These two European missions are expected to measure about 40 million and one billion Galactic stars respectively, with GAIA also providing radial velocities for up to 10% of targets. Stellar spectroscopy plays a crucial role in these studies, not only providing a key component of the 6-dimensional phase space of stellar positions and velocities, but also providing much-needed information on the chemical composition of individual stars. Taken together, information on space motion and composition can be used to unravel the formation process of the Galaxy. 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 (e.g. Gilmore et al. 2002, ApJ, 574, 39) provide important information, a complete overall picture of the Galaxy is lacking. Moreover deep pencilbeam spectroscopic surveys provide only three of the six phase space coordinates. The remaining three coordinates of phase space can be determined from astrometric surveys, e.g. Hipparcos. However, Hipparcos did not measure radial velocities and the follow-up observations have been largely ad hoc. Only a few tens of thousands of radial velocities of the complete catalogue (118,000 stars) have been acquired to date. However, with the successful demonstration of ultra-wide-field (30deg 2 ) multi-object spectroscopy on the UK Schmidt telescope, we have entered the era of all-sky spectroscopic surveys and a major opportunity beckons to generate the first large-scale spectroscopic survey of Galactic stars with a radial velocity precision better than 2 kms -1. In this paper we map out the case for RAVE (RAdial Velocity Experiment), an ambitious plan to measure radial velocities and chemical compositions for up to 50 million stars by 2010 using novel instrumentation techniques on the UK Schmidt telescope and a Northern counterpart. Although providing a database of unparalleled power in its own right, RAVE can also enhance the science outcomes of missions such as DIVA over the coming decade. DIVA will obtain parallaxes and proper motions for a large fraction of RAVE program stars down to V=15, along with spectrophotometry for the brighter stars in 12 intermediate bands. 1

5 The RAVE survey also 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. 1.2 International Collaboration RAVE will be undertaken as a major international collaboration. Initial planning is the responsibility of a Science Working Group chaired by Matthias Steinmetz (Astrophysical Institute Potsdam). Other members are at AAO, AIP, ANU, Strasbourg, Heidelberg, IoA, Johns Hopkins, NAOJ, Oxford, Steward and Victoria. A detail list of collaborators and scientist who expressed interest in joining the project is given in appendix A and B. In the currently envisaged programme, the equatorial and southern RAVE survey would be carried out by the AAO using the UK Schmidt (see Figure 1) equipped with the UKidna system (UKST Echidna), which would be built by AAO. It would use a purpose-built spectrograph that could be a development of PMAS (the Potsdam Multi-Aperture Spectrograph) or the AAO spectrograph. The northern RAVE survey would require collaboration with a northern-hemisphere Schmidt telescope. Possibilities include the 1.05-m Kiso Schmidt of the University of Tokyo or the Chinese 4-m LAMOST. The other institutions represented on the Science Working Group will support the input catalogue, data reduction and database preparation. Figure 1: The UK Schmidt Telescope (UKST) at the Siding Spring Observatory, New South Wales, Australia. The UKST is operated by the Anglo-Australian Observatory. 1.3 Timeline The RAVE survey is split into two components: a pilot survey and a main survey. The Pilot Survey utilises the existing spectroscopic facilities (6dF positioner and VPH spectrograph) at the UK Schmidt and could begin as soon as the input catalogues are available (March 2003), provided additional resources (A$250k) are made available to support bright-time operation of the UK Schmidt. The pilot survey would last until the UK Schmidt is upgraded with the UKidna system required for the main RAVE survey in mid-2005 (see below). Over this period it will be possible to observe 10 5 stars (see below). 2

6 The Main Survey requires the development of a spectrograph and an Echidna-style positioner for the UK Schmidt facility and similar instrumentation for the Northernhemisphere facility. Equipped in this fashion, each telescope would be able observe 25 million stars over the proposed 5-year lifetime of the survey (see below). It is currently envisaged that the positioner for the Main Survey (UKidna) could be built as a prototype for the proposed Gemini wide-field facility. This would enable the cost for the positioner to be resourced from the AATB s in-kind contribution of $US 1.4M to the Australian Major National Research Facility (MNRF) grant earmarked for Gemini and SKA development. Using the AATB s in-kind resources in this fashion would require the approval of both the AATB and the Australian Astronomy Board of Management (AABoM); the managing organisation for the MNRF. The current project plan for UKidna has the instrument on the telescope by mid-2005 and commissioned by the end of This is also compatible with the time scales for the completion of the existing 6dF galaxy survey (early-mid 2005). Provided additional resources could be found to design and build a spectrograph (a early concept for which already exists) over this same time scale, the RAVE main survey on the UK Schmidt could begin by the start of The time scales for the main survey on the Northern hemisphere are much more uncertain and largely depend on the facility which would carry out the survey. In principle, if a decision was made on the choice of facility by mid and resources were made available to build the fibre positioner (if required) and spectrograph, the Northern Hemisphere component of the RAVE survey could begin contemporaneously with the Southern Hemisphere. The following provides an outline of the timeline for the survey on the UK Schmidt telescope: May 2002: June 2002: Sept 2002: RAVE Kick-off meeting: Science Working Group formed RAVE Statement of Interest submitted to PPARC Draft RAVE White Paper submitted to AATB Sep 2002: Approval to proceed from AATB Oct 2002: AABoM approval to use in-kind MNRF contribution for UKidna Mar 2003: Pilot survey begins mid-2003: UKidna final design begins mid-2003: Spectrograph design commences mid-2004: UKidna build commences mid-2004: Spectrograph build commences mid-2005: Pilot survey completes, UKidna commissioning begins start-2006: UKidna commissioning complete, main survey begins start-2006 DIVA launched end-2010: Main survey completed end-2011: Full data release 2012: GAIA lanched The attached project plan outlines the dependencies of these tasks. 3

7 Figure 2: Project plan for the RAdial Velocity Experiment. 1.4 Survey Figures of Merit The numbers of objects in the RAVE surveys are based on the following figures-of-merit appropriate for the UK Schmidt Telescope on Siding Spring Mountain (numbers in parentheses refer to current 6dF capability in relation to pilot survey) : Nights per year 330 (120) Fraction of clear nights: 0.6 Hours/night: 9 Minutes/field (incl. overheads): 30 (120) Fibres/field: 2250 (120) Fibres on stars: 90% Objects with target SNR: 95% In a 330-night year (allowing for maintenance stand-downs), it should be possible to observe approximately 5 million stars. A five-year program on a pair of telescopes in the Northern and Southern Hemisphere could observe 50 million stars. 2. Science Case 2.1 Pilot Study This is a preliminary spectral survey, using the existing 6dF system to observe about 100,000 stars. Such a limited survey already has great scientific potential. One favoured option would be to target the large number of Hipparcos (and Tycho) stars that are currently without accurate radial velocities. The Hipparcos and Tycho catalogues include about 118,000 and 2,539,913 stars, respectively, over the whole sky. The availability of radial velocities for these stars would greatly increase the utility of the catalogues for galactic dynamics; most of these stars are currently missing the vital 6th phase space co-ordinate. After completing the southern Hipparcos catalogue sample, we should then turn to the Tycho catalogue stars. Here we would propose to concentrate on stars in the colour range 0.4 < B-V < 0.8. For these stars, useful photometric parallaxes can be derived if 4

8 the trigonometric parallaxes are not available. Furthermore, this subset includes the evolved subgiant stars, and for these stars it is possible to estimate evolutionary ages directly from their luminosities and metallicities. The 6dF spectra will also provide useful estimates of the [Fe/H] value and the ratio [α/fe] of the alpha elements to iron. In combination with the data from the Hipparcos satellite, we can then use the velocity and metallicity data to improve the age-metallicity-velocity relation for the nearby stars. This AMVR is the crucial input data for our understanding of the chemical and dynamical evolution of the galactic disk. These data can be used to weigh the local spiral arm. The pilot survey of the Hipparcos catalogue stars will provide a large sample of disk stars extending through and beyond the nearby spiral arm. A straightforward dynamical analysis of the distribution and kinematics of these disk stars will allow us to measure the perturbation to the local velocity field from the spiral arm, and so estimate the mass associated with the spiral arm itself. This would be the first non-local estimate of the density of the galactic disk and the dark matter contribution to the disk. 2.2 Main Study Introduction The main RAVE survey will provide a vast stellar kinematic database three orders of magnitude larger than any other survey proposed for this decade. The main data product will be a magnitude-limited survey of 26 million thin disk main sequence stars, 9 million thick disk stars, 2 million bulge stars, 1 million halo stars, and a further 12 million giant stars including some out to 60 kpc from the Sun. RAVE will offer the first truly representative inventory of stellar radial velocities for all major components of the Galaxy. Its completeness and homogeneity will make it an invaluable stand-alone resource, but its full potential will be realised when the radial velocities are combined with DIVA proper motions and parallaxes. RAVE has a number of science goals addressing a wide range of priority areas in galactic structure and dynamics in both the UK and international communities. They include: A search for unique chemical and kinematic signatures of stellar streams in the halo, outer bulge and thick disk due to satellite accretion The dynamical influence of the local spiral arms and inner bar The degree of ellipticity, warping and lop-sidedness of the disk The first non-local measurement of surface density in the disk Detailed structure of the spiral arms and stellar associations A complete survey of the brightest 50 million stars would reach down to a completeness limit of V=16 (or equivalently I=15). This is sufficiently deep to allow for kinematic and chemical studies of all major stellar components of the Galaxy RAVE and the Milky Way Galaxy Within the cold dark matter (CDM) paradigm, the Galaxy built up through a process of accretion over billions of years from the outer halo. Sophisticated computer simulations 5

9 of structure growth within a CDM universe have now begun to shed light on how this process may have taken place (see e.g. Steinmetz & Navarro 2002; New Astronomy 7/4, , Figure 3). These advanced computer models do not only provide information about the structure and kinematics of the major stellar components of a galaxy but also on their chemical signatures and their stellar age distribution. In the context of these simulations, the proposed RAVE survey will revolutionize our understanding of the formation and evolution of all major components of the Galaxy: the disk, the bulge and the halo. Figure 3: Result of a high resolution cosmological simulation designed to resolve the structure and kinematics of individual galaxies. Thin disk, thick disk and spheroid (top row) can be clearly identified by analyzing the eccentricity of particle orbits (bottom left). Each of these three components shows its characteristic star formation history (bottom right). Halo sub-structure. The details of galaxy formation are not well understood. In particular, CDM simulations actually predict far more infalling satellites than are currently observed. The orbital timescales of stars in the outer parts of galaxies are several billion years and it is here we would expect to find surviving remnants of accretion. In 1994, the disrupting Sagittarius dwarf spheroidal galaxy was discovered by Ibata and collaborators from a multi-fibre radial velocity survey. Five years later, Helmi discovered a stellar stream within 1 kpc of the Sun after combining a radial velocity catalogue with the Hipparcos database. 6

10 Figure 4: left: model simulation illustrating the formation of tidal streams from disrupted infalling satellites (from Harding, PhD thesis, 2001); right: comparison of the velocity correlation signal (i.e. a measure of clumping in velocity space) of simulated tidal streams within a 1kpc circle around the Sun based on HIPPARCOS data (right) and from a combination of anticipated DIVA and RAVE data sets (left). Different symbols representing different streams. DIVA+RAVE give rise to a significant correlation for a number of streams at small velocity differences while such a signal is hardly visible in the HIPPARCOS data Both of these studies demonstrate the power of accurate radial velocities and proper motions in identifying cold stellar streams. We can expect the RAVE survey to reveal evidence of many tens of similar streams both in the halo, in the outer bulge and within the thick disk. When combined with the DIVA survey, this tally may extend to many hundreds of infalling systems (Figure 4). The evolution in phase space of a disrupting satellite is well behaved, as its stars become phase mixed. It should be possible to recognize partially phase-mixed structures that cover the observed space, although special methods (e.g. use of integrals of motion) are needed to find them. If the cold stream is more than 20 kpc from the galactic centre, these structures provide important probes of the dark halo potential. Chemical signatures. A key aspect of RAVE will be the availability of chemical signatures like [α/fe] and [Fe/H], in addition to accurate kinematics. The α elements arise from massive stars and the bulk of their mass is released in Type II supernova explosions. The Fe-peak elements are produced primarily by Type Ia supernovae which begin to dominate after a billion years or more. Unique signatures from abundance ratio pairs like ([α/fe], [Fe/H]) may help to identify a common site of formation among widely separated stars. The use of chemical signatures can be extended to other components of the Galaxy, in particular, the halo and the outer bulge and the thick disk (see Figure 5,6) From the RAVE sample of a million halo stars, we can expect to find about a thousand stars with [Fe/H] < -4, and ten thousand stars with [Fe/H] < -3. The most metal poor stars are known to exhibit a very large scatter in chemical abundance ratios indicating that their formation sites in the early universe were enriched by only a few supernovae. This tags individual stars to particular fragments of the protocloud. 7

11 Figure 5: [Mg/Fe] vs [Fe/H] for halo stars (open circles) and disk stars (filled circles) (from Gilmore and Wyse 1998, AJ 116, 748). The disk stars show a well-defined trend, contrasted with the apparent scatter of the halo stars. The shaded region marks the locus of normal metal-poor halo stars in this plane. The star symbol represents the anomalous metal-poor halo subgiant of Carney et al. (1997), and the open triangle the anomalous common proper-motion pair of halo dwarfs from King (1997). The solid lines schematically illustrate the expected pattern of element ratios in self-enriching systems of fixed IMF, but differing star formation rates. Bulges. The formation of stellar bulges, a major element of galaxy classification schemes, is not well understood. The Galactic bulge stars are almost as metal rich as the thin disk but as old as the thick disk and parts of the halo. Our current picture is that large bulges are formed from a rapid collapse of a spherical cloud, and that the small bulges are either formed from accretion or from the action of the central bar after the disk formed. In an alternative model, as favoured by the CDM model of structure formation, bulges are the remnants of early gas-rich mergers between some of the first building blocks of a galaxy. A key constraint is the [α/fe] abundance ratio, which has been determined for only a few dozen bulge stars to date. A short star formation epoch either during the collapse of the bulge or, as favoured in the CDM model, in the progenitors form which a bulge is assembled is expected to lead to enhanced [α/fe] for most of the stars; an extended star formation period during the bulge assembly would imply [α/fe] = 0. 8

12 Figure 6: left: Metallicity distributions for different stellar components. From top to bottom: local stellar halo, outer bulge K-giants, local thin disk F/G stars, local thick disk F/G stars, and the solar cylinder, i.e. all stars integrated from the vertical disk plane to infinity; right: scatter plot from iron abundance vs B-V color for thick disk F/G stars, selected in situ in the South Galactic Pole at 1-2kpc above the Galactic Plane (stars), together with the 14 Gyr turnoff colors (crosses) and 15 Gyr turnoff colors (asterisks). The open circle represents the turnoff color (dereddened) and metallicity of 47 Tuc. The vast majority of thick disk stars lie to the red of these turnoff points, indicating that few, if any, stars in this population are younger than this globular cluster (from Wyse, 2000 astroph/ ). Most of the baryons reside in the disk. Freeman & Bland-Hawthorn (2002; ARAA 40, ) have argued that establishing a theory of galaxy formation is largely about understanding the formation and evolution of the disk. The RAVE survey will greatly increase our knowledge about the origin of the thick disk, and the current dynamical state of the thin disk. Thick disk. The thick disk, which is at least 12 Gyr old, is widely believed to be a `snap frozen relic of the early disk shortly after the onset of disk dissipation. In this picture, an infalling satellite vertically heated the early disk to a scale height of 1 kpc. Another possibility is that the thick disk is made up of tidal debris from roughly ten infalling satellites. A third scenario, recently suggested by Kroupa, is that the thick disk arose from 10 6 M star clusters that have long since been disrupted. From the combined chemical and kinematic signatures, the RAVE survey should cleanly distinguish between competing models for the thick disk and thus end a many-year old debate on the origin of the thick disk. A major unknown in disk formation is whether the extent of the stellar disk is laid down during the major epoch of dissipation, or whether it grows with time. The RAVE survey will clearly establish whether the radial extent of the thick disk is comparable to or less than the thin disk. The chemical information will also be very important. The existence of an abundance gradient in the thick disk, as we observe in the thin disk, would argue against an infall origin; unique chemical signatures in the thick disk would argue for an infall origin or maybe even the formation scenario proposed by Kroupa. 9

13 Thin disk. The largest fraction of the RAVE targets will be thin disk dwarfs and giants. Little is known about the dynamical state of the thin disk beyond 2 kpc of the Sun. The existence of the inner bar and outer stellar warp is firmly established but many areas of astrophysics would benefit from their influence being understood in far more detail. Some external galaxies have optical disks, which appear to be lop-sided with respect to the dark halo. Whether this is the case for the Galaxy is not known. The intrinsic brightness of the giants allows these important tracers to be observed throughout the entire optical extent of the Galaxy. The giants probe the large-scale dynamical state of the Galaxy, in particular the influence of the inner bar, the outer warp and the degree of eccentricity and lop-sidedness of the optical disk. The vast number of dwarf stars in the RAVE survey will reveal the dynamical state of the thin disk and neighbouring spiral arms within a few kiloparsecs of the Sun s position. This is crucial information if we are to construct an accurate model of the gravitational potential of the disk, and its distribution function. Recently, it has become clear that even the old stellar populations appear to show sub-structure. The RAVE survey will provide fundamental information on how different stellar populations deviate from dynamical equilibrium, and therefore constrain the formation history of the disk and its different components (e.g. spiral arms, stellar associations, etc) Observations UKidna will target the Ca triplet ( nm) window favoured by the GAIA instrument definition team. This window has almost no atmospheric absorption features; the sky lines are well separated at R=10,000 resolution with fibre spectroscopy, occupy only a few percent of the full spectral band, and are easily subtracted. Since the OH lines are sharply defined, the problem of adjusting the relative throughputs of fibres becomes trivial. RAVE only needs a few sky fibres in each pointing to achieve good subtraction of the total sky (atmosphere + moonlight + zodiacal light). This wavelength region provides a rich forest of metal lines dominated by Ca II, Fe I, Ti I, CN, Mg I, Si I, Cr I, N I, Co I, Ni I, Mn I, S I, TiO and weaker features (see Figure 7). For V magnitudes of (13,14,15,16), UKidna will deliver signal to noise ratios of (90,55,35,20) per resolution element at R=10,000 in the standard 30 min exposure. This will provide differential [α/fe] abundance ratios for the bright half of the survey (V<15) to better than 0.1 dex, and [Fe/H] for all stars to a similar precision. The Ca triplet region is ideal for kinematic surveys for stars later than type B. It should be possible to achieve radial velocity precision better than 1 km/s for all stars in the survey. 10

14 Figure 7: Spectral features in the Ca-triplet region for stars of different spectral type (from Munari & Castelli 2000). 2.3 Benefits of RAVE in other areas of Astronomy Stellar structure and evolution. Even though the spectral range is limited to the Ca-triplet region, RAVE will provide by far the largest catalogue of stellar spectra ever collected. Together with the DIVA multiband photometry RAVE will establish a large catalogue of metallicities and abundance ratios for 40 million stars together with valuable constraints on their surface properties (surface temperature, gravity). This is an unequaled data set for studies of stellar structure and evolution. Furthermore, central emission in the Ca-triplet for some stars would be an indication of magnetic activity. RAVE would thus also establish the largest database on active F-M stars. Such a catalogue is a valuable resource for follow-up high-resolution spectroscopy e.g. using UVES on the VLT. Depending on the detailed survey design (multiple observations), RAVE will also discover a large number of spectroscopic binaries. RAVE's impact on nearby OB-association studies. The RAVE survey will also contribute to our understanding of nearby young stellar associations, in terms of stellar membership and kinematics. OB associations are unbound aggregates of young stars which are typically less than a few 10 7 years, tens of parsec in size, with a small velocity dispersion (~1-3 km/s). The pilot will allow the study of FG dwarfs in extended OB associations out to ~200pc, and ~400 pc for F stars only. Two OB associations lie within 200 pc (Sco-Cen and Cas-Tau), with three more within 400 pc (Trumpler 10, Per OB2, Lac OB1). Proper motions complemented by radial velocities can be used to select probable members of these groups, whose memberships are poorly constrained below the OB population. Identification of FG-type members through radial velocities, proper motions, and color-magnitude diagram 11

15 position, will allow us to calculate the expansion speeds of nearby OB associations. These expansion rates provide an age estimate for these stellar populations independent of stellar evolutionary models. The membership studies will also allow some comparison of the initial mass function in OB associations to those in well-studied open clusters (e.g. Pleiades) and T associations (e.g. Taurus molecular clouds). RAVE's benefits to studying the ROSAT field star population Approximately 6000 HIPPARCOS stars have X-ray counterparts in the ROSAT All-Sky Survey (RASS-HIP), along with ~14,000 Tycho-1 stars (RASS-Tycho). The status of these presumably coronally-active stars is open to debate, but they are likely dominated by young main sequence and pre-main sequence stars, as well as older chromospherically active binaries. The nature, kinematics, and origin of these objects is of considerable interest in terms of the recent star-formation in the solar neighborhood. The distribution of the RASS-Tycho population lies preferentially in a plane tilted 20 degrees to the Galactic plane, apparently representing the low-mass population of the "Gould Belt" OB stellar population. Most of the RASS-HIP and RASS-Tycho stars have B-V colors consistent with being G stars. The RAVE pilot study will survey the RASS- HIP stars in an unbiased manner. The RAVE main survey will include all FG-type counterparts to RASS sources in an unbiased manner. 2.4 RAVE in the context of future satellite missions RAVE and DIVA. DIVA is a small astronomy satellite funded by the DLR (Deutsches Zentrum für Luft- und Raumfahrt e.v.), planned for launch in spring of In 2000 it has been selected as prime candidate for the next German national space science mission. It will measure positions, proper motions and parallaxes, magnitudes and colors of about 35 million stars down to 15 th magnitude in V. Broad-band photometry of all stars of the DIVA survey and spectrophotometry of all stars brighter than V = 13.3 will be provided by DIVA. Additionally, it will perform a photometric survey in two channels in the UV between 300 and 400 nm to a limiting magnitude of about V = 14. DIVA will perform a survey of the whole sky only limited by the sensitivity of its instruments. It will surpass the performance of ESA's HIPPARCOS in all relevant aspects: by the number of objects observed, the measurement accuracy and by its vast number of photometric and spectrophotometric data. In a sense it is a precursor mission for the technology of the upcoming cornerstone GAIA (see below) in the ESA Horizon programme. DIVA has passed its preliminary design review in April The implementation phase (Phase C/D) is envisioned for spring of 2003 subject to successful funding negotiation with ESA or NASA. DIVA and RAVE ideally complement each other. Both are magnitude limited all-sky surveys with a similar magnitude limit (V=15-16) and similar number of sources (~40 million). The DIVA schedule (data collection , data reduction ) nicely coincides with the prospected time frame for the RAVE main survey ( ). The broadband and UV photometry of DIVA complements the RAVE spectra in the Catriplet region and enhances the use of the database for stellar structure and activity studies. Already after its first scan of the sky in 2006, DIVA will provide a very homogeneous data set that can be used as an input catalogue for RAVE. Also the performance of DIVA is well matched to that of RAVE. Proper motions can be 12

16 determined with an accuracy between 0.25 mas/yr (V=10) and 1-2 mas/yr near the magnitude limit. Within the DIVA-sphere (most DIVA stars lie within a 1-2 kpc sphere around the Sun), this corresponds to tangential velocities with 1-5 km/s accuracy, while RAVE will deliver radial velocities at the 1-2 km/s accuracy level. DIVA will measure accurate parallaxes for at least stars (10% accuracy, 0.2mas at V=11-12, see Figure 8), i.e. DIVA+RAVE will permit to determine the full 6d phase space distribution for a significant subset of the stars. For lower magnitudes, larger error parallaxes (1mas at V=15) and photometric distance indicators will at least constrain the 6 th dimension and will provide useful statistical constraints. While RAVE will considerably enhance the performance of DIVA regarding galactic structure studies, the primary science goal of RAVE measuring the local structure of the Milky Way can be achieved even in the case of a fatal failure of the DIVA mission. A number of ground-based catalogues (Tycho-2, USNO, HST guide star catalogue) can provide the survey input data (although in a less homogeneous manner) and proper motions at the 1-2 mas/yr level, corresponding to a tangential velocity accuracy of ~5-10 km/s within the RAVE-sphere. The biggest drawback would be the unavailability of DIVA parallaxes and DIVA photometry, which also would make the [Fe/H] and [α/h] measurements from the RAVE spectra less reliable. Photometry from ongoing and anticipated ground based projects (e.g. USAC, 2MASS, VISTA, LSST) can partially fill this gap. Figure 8: left: accuracy of DIVA parallaxes vs magnitude; right: accuracy of DIVA photometry vs magnitude. RAVE and GAIA Unlike HIPPARCOS and DIVA, GAIA will have a radial velocity spectrograph (RVS) on board, which like RAVE explores the region around the Ca-triplet. Again RAVE and GAIA complement each other. While RAVE will focus in medium resolution (R=10000) at a magnitude limit of V=16, the GAIA-RVS will probe deeper (V<18) at lower resolution (R 5000). Since RAVE will be finished 10 years before the GAIA data release and probably still well ahead of the launch of GAIA (probably 2012), RAVE serves as an ideal real-data training set for the final design of the GAIA data reduction pipeline and may even influence some of the final design decisions. 13

17 Figure 9: Test observation of the radial velocity of stars performed in bright time by K. Freeman on 6dF (R=4000) in the Fall of The figure correlates the radial velocity measured with the same fibre at two different epochs one month apart. The figure illustrates that the envisioned accuracy of 2km/s for the pilot survey (R=4000) and of 1km/s for the main project (R=10000) can be achieved. 3 RAVE Technical Case 3.1 Background The proposed RAVE survey is made possible by instrumentation developments within AAO that have already greatly enhanced the capabilities of the UK Schmidt Telescope (6dF) and offer the promise of further substantial enhancement (UKidna). This section addresses the technical aspects of the two phases of the survey as they relate to the instruments themselves. 3.2 Pilot Study Instrument overview. 6dF is the multi-fibre spectroscopy system currently available at the UKST (see Figure 10). It feeds 150 fibres with 100µm (6.7 arcsec) core diameter to the FISCH spectrograph in the dome. As of September 2002, the spectrograph will be upgraded to allow it to be used with high-efficiency VPH gratings. 14

18 Figure 10: left: the 6dF interchangeable field plate unit featuring 150 fibres; center: closeup view of the 6dF fibre retractor; right: snapshot of the 6dF software configuration tool. Observing strategy. The use of the Ca triplet in the far-red region (8498Å, 8542Å, 8662Å) offers significant advantages in the measurement of stellar radial velocities and metallicities. They include: freedom from telluric absorption lines; effectiveness over a wide range of stellar temperatures for velocity and metallicity measurements; better radial velocity precision than in the blue; lower susceptibility to moonlight; the presence of several other diagnostic lines in the same spectral region. Bright-of-moon test observations during late-2001 carried out with 6dF at a dispersion of 0.77Å pixel -1 (1200R reflectance grating) enabled a qualitative assessment of system stability and resolution in the Ca triplet region Å. The tests were made with high S/N (~50) spectra of bright G and K stars (V~11) over a range of zenith distances. Only the central 2/3 of the detector area yields a resolution better than 3 pixels (2.3Å) FWHM because of optical limitations in the existing spectrograph camera. Within this region, however, the external error in a single velocity measurement was found to be better than 2 kms -1 (even after the grating had removed and replaced, and the spectrograph re-focussed), more than adequate for the proposed survey (see Figure 9). This stability performance was repeatable over a period of a month. Survey model technical aspects. The pilot study requires the measurement of 10 5 stars with V<13 and B V<0.8 in targeted regions over a period of two years. With number-densities ~6 deg -2 at b =90, these objects are well matched to 6dF. The study would make use of the 120 unscheduled bright nights of 6dF time per year, and would use the same spectrograph set-up as the test observations (or its equivalent with a VPH grating). Assuming one-hour exposure times (adequate for the magnitude limit even at full moon), a mean of 4 fields per night and an average of 120 objects per field, the survey could be completed in 27 months assuming 60% of nights are spectroscopic. Technical resources required. The principal AAO requirement to expedite the pilot survey is observational effort at the telescope at the level of 1.5FTE. (This assumes that preparation of the input catalogue, development of software, data analysis and database preparation are all carried out by the survey team in an agreement analogous to that with the 6dFSAG). However, in order to achieve the required rate of 120 objects per field, further remedial work will be needed on the existing spectrograph optics. Both scattered light and imaging characteristics will need improvement. While the new VPH gratings may reduce scattered light, only a redesigned field-flattener lens will extend the 15

19 required image quality to the corners of the field. It might also be necessary to procure a purpose-designed VPH grating. These improvements have yet to be fully costed, but are likely to amount to $US 50k. Finally, the installation of a new control system for the Schmidt Telescope (which is expected to take place as a spin-off from the Tokyo CCD Camera development) will improve the turn-round time from one field to the next. Figure 11: The Echidna concept; right: single ball-spine of the Echidna MOS; center: illustration of the work principle of an individual echidna spine; left: example of fibre allocation to randomly scattered targets. 3.3 Main Study Instrument overview. The proposal to build an Echidna-style multifibre positioner (see Figure 11) for the Schmidt Telescope was prompted by the scientific potential of an extremely large-scale stellar radial velocity survey and the possibility of such an instrument acting as the Gemini Wide-Field prototype. UKidna consists of a 2250-spine fibre array covering the full field of the Schmidt Telescope (6.4 deg square). The spines are hexagonally-packed on ~7mm centres (each having a 15-arcmin patrol area) and are arranged to follow the focal curvature of the field. The fibres themselves are 50µm (3.3 arcsec) in diameter. In its modular format (see Figure 12), the array follows closely the Echidna concept, and it is expected that reconfiguration will take no longer than 5 minutes. Due to space restrictions inside the telescope, it is not possible to replicate the Echidna focal-plane imager, and the measurement of fibre positions will be carried out by a remote one shot spine-tip imager. Major modifications to the telescope will be required to accommodate UKidna, and there will be no possibility of reverting to another mode of operation (e.g. Tokyo CCD Camera) once it is installed. 16

20 Figure 12: left: the Echidna fibre module; right: Complete Echidna design. UKidna will feed an efficient all-schmidt spectrograph modelled on the AAOmega design. Only the red arm will be needed, and the spectrograph will use a single 2k 4k red-optimised detector (with the long dimension in the spatial direction). In normal survey use, the 2250 fibres will divide into three columns of 750, with the columns separated in the spectral direction. With 250Å coverage required for the Ca triplet region, a dispersion of 0.375Å pixel -1 will yield R~10,000 spectroscopy (with 2-pixel sampling by virtue of the f/2 collimator and f/1.3 camera). Observing strategy. Again, the survey requirements dictate the use of the Ca triplet region for the same reasons as in the pilot study. However, there is also the possibility of operating the system at lower efficiency (but twice the resolution) in the blue at secondorder to obtain r- and s-process abundances without materially changing the spectrograph configuration. Survey model technical aspects. In the main study, up to 25 million stars over the whole southern sky will be measured with a number of different options during the period ~ (see Survey Plan). This period is beyond the lifetime of the current 6dF Galaxy Survey, so the RAVE Main Survey could fully occupy the Schmidt Telescope throughout the year, irrespective of the phase of the Moon. This would yield ~240 clear nights per year. Depending on the survey option finally selected, there could be a demand to observe up to an average of 18 fields per clear night. This would be possible, but would limit integration times to ~30 minutes per field to allow reconfiguration and telescope slewing/acquisition time. (An upgraded telescope control system is essential for main study see note under the pilot study above.) Such short integration times would allow radial velocity measurements of the proposed survey stars, but accurate abundance would be marginal at the fainter levels. Northern Hemisphere Options. The main study would be expected to cover the whole sky, and it is proposed that the northern observations could be made with the Chinese LAMOST Schmidt Telescope being built at the Xinglong Station of the Beijing 17

21 Astronomical Observatory. The completion date of this telescope fits well with the expected start date of the main survey (2005/6) and the proposed observations would be well within its capabilities (4-m aperture, 5-deg field, 4000 fibres). However, it is currently planned to equip LAMOST with low resolution (R~500) spectr0graphs and so higher resolution spectrographs would need to be designed and built. Other options for the Northern RAVE survey include the Kiso and Tautenburg Schmidts. The Kiso Schmidt is slightly smaller in both aperture (1.05m) and field-of-view (6 deg square) to the UK Schmidt. The Tautenburg Schmidt is currently the largest Schmidt in existence (1.34m-diameter), but has a smaller field-of-view (3.4 deg square) than either the Kiso or UK Schmidts. Other options could include the Calar-Alto Schmidt, although it has only a 0.8m aperture, or possibly the Byurakan Schmidt. The Oschin (Palomar) Schmidt is probably not an option, since it is already tied up with a long term NEO CCDimaging program. Technical resources required. The primary requirements for the main study are the procurement of UKidna, the modification of the Schmidt Telescope to accommodate it and the procurement of the spectrograph. To these must be added the effort required in auxiliary (e.g. data-reduction) software. The cost breakdown is approximately as follows: UKidna positioner: $US M Telescope modifications $US M Spectrograph $US M Auxiliary software $US 0.1M Total $US M The observational resources that will be required to carry out the main study are rather more than those for the current AAO operation of the Schmidt Telescope because of the round-the-month operation. With an observer pool of 5.0FTE and a technical support effort of 1.0FTE, the operational cost for a 5-year survey will be $US 1.4M. 3.4 Data Processing, Archiving and Management (to be added) Data reduction pipeline. Archiving Link to the International Virtual Observatory 4 Financial Issues 4.1 Overview Currently there are no resources to support either the pilot survey ($US 140k in operational costs, and a further $US 50k for camera upgrades) or the main RAVE surveys (approximately $US 3.8M per telescope, including operational support). 18

22 4.2 PPARC A statement of interest (SoI) has been submitted to the UK PPARC for the RAVE survey, with possible UK funding for the pilot survey highlighted as an option for UK support in the initial phases on the RAVE survey. Note that the UK currently provides no direct operational funding for the UK Schmidt. The SoI is currently being considered by the UK s Astronomy Advisory Panel who will make their recommendations to PPARC s science committee in late October. 4.3 RAVE and the Gemini Wide-Field The cost of the UKidna positioner ($US 1.4M) could be identified as the AATB s $US 1.4M in-kind contribution from to the Australian Major National Research Facilities (MNRF) grant over the period The MNRF was specifically targetted at increasing Australia s engagement in Gemini and SKA programs though an increased share of time (on Gemini) and increased levels of involvement in instrumentation and technology development. The design and build of the UKidna positioner could be considered as a prototype for the positioner (4000 fibres) for the Gemini wide-field; an instrument concept being developed by a consortium of US, Canadian, UK and Australian astronomers to be proposed as part of Gemini s 2 nd generation instrument suite. The UKidna prototype would target key areas of risk (e.g. addressing a large number of fibres, patrolling a large field) in the Gemini wide-field concept. Gemini are planning to have a workshop to address the science requirements for the 2 nd generation instruments in mid-2003 at which the Gemini wide-field proposal will be submitted. However, it is unlikely that Gemini will make a final decision on its 2 nd generation instrumentation before mid-late 2004 by which point a significant portion AATB in-kind contribution needs to have been committed to a specific project. Moreover, early design/prototype work in UKidna can only enhance the chances of the Gemini wide-field being adopted. These two factors suggest that a decision to proceed with UKidna should not wait until any final decision is reached on the Gemini wide-field. In this regard, UKidna may be seen as a strategic investment worth the associated level of risk. Furthermore, the risk may be viewed as minor. Even if the Gemini wide-field does not proceed to fruition, the UK/Australian community will be have access to a new facility on the UK Schmidt. 4.4 Other Options Other funding options for all aspects of the survey should be explored immediately; particularly those relating to the Northern hemisphere survey. This may include funding the construction of a suitable higher-resolution spectrograph for the LAMOST telescope, should it be available to start on the Northern hemisphere survey in It may be possible to identify funds earmarked for Japanese-Chinese scientific collaboration to support this aspect. Equally, given the synergies of RAVE with planned space missions it may be possible to approach space agencies (NASA, ESA) for partial support; despite their historic reluctance to support ground-based programs. Individual Universities or national funding agencies should also be approached; operational support for 6dF in bright time is currently provided by a grant from Michigan State University and the NSF to conduct a survey for metal-poor halo stars (PI: Timothy Beers). The success of this survey (due to complete in March 2003) demonstrates that such a model is entirely feasible for the support of the RAVE survey. 19

23 4.5 Coda Above all, the RAVE survey represents outstanding scientific value-for-money; providing key velocity and abundance information on 50 million stars, at least three orders of magnitude larger than anything attempted before, at a cost of less than 15 US cents per star. Appendix A. List of Collaborators (as of September 30 th, 2002) Jeremy Bailin (Steward, USA), Ulrich Bastian (ARI, Heidelberg), James Binney (Oxford, UK), Brian Boyle (AAO, Australia/UK), Walter Dehnen (AIP, Germany), Daniel Eisenstein (Steward, USA), Ken Freeman (RSAA, Australia), Sabine Frink (ARI, Germany), Gerry Gilmore (Cambridge, UK), Naoteru Gouda (NAOJ, Japan), Joss Hawthorn (AAO, Australia/UK), Amina Helmi (Utrecht, Netherlands), Hiroshi Karoji (NAOJ, Japan), Eric Mamajek (Steward, USA), Julio Navarro (Victoria, Canada), Siegfried Röser (ARI, Germany), Elena Schilbach (ARI, Germany), Ralf Scholz (AIP, Germany), Matthias Steinmetz (AIP, Germany), Fred Watson (AAO, Australia/UK), Rosie Wyse (JHU, US), Yuzuru Yoshii(Tokyo, Japan), Nakada Yoshikazu (Tokyo, Japan) B. List of Scientist expressed interest in joining the project Olivier Bienaymé (Strasbourg, France), Sofia Feltzing (Lund, Sweden), Ortwin Gerhard (Basel, Switzerland), Rodrigo Ibata (Strasbourg, France), Ulisse Munari (Asiago, Italy), Scott Tremaine (Princeton, USA), Tim de Zeeuw (Netherlands), Thomaž Zwitter (Ljubljana, Slovenia) 20