The Radial Velocity Experiment (RAVE): second data release
ABSTRACT We present the second data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities (RVs) and stellar atmosphere parameters of up to one million stars using the 6dF multi-object spectrograph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO). It is obtaining medium resolution spectra (median R=7,500) in the Ca-triplet region (8,410--8,795 \AA) for southern hemisphere stars in the magnitude range 9<I<12. Following the first data release (Steinmetz et al. 2006) the current release doubles the sample of published RVs, now containing 51,829 RVs for 49,327 individual stars observed on 141 nights between April 11 2003 and March 31 2005. Comparison with external data sets shows that the new data collected since April 3 2004 show a standard deviation of 1.3 km/s, about twice better than for the first data release. For the first time this data release contains values of stellar parameters from 22,407 spectra of 21,121 individual stars. They were derived by a penalized \chi^2 method using an extensive grid of synthetic spectra calculated from the latest version of Kurucz models. From comparison with external data sets, our conservative estimates of errors of the stellar parameters (for a spectrum with S/N=40) are 400 K in temperature, 0.5 dex in gravity, and 0.2 dex in metallicity. We note however that the internal errors estimated from repeat RAVE observations of 822 stars are at least a factor 2 smaller. We demonstrate that the results show no systematic offsets if compared to values derived from photometry or complementary spectroscopic analyses. The data release includes proper motion and photometric measurements. It can be accessed via the RAVE webpage: http://www.rave-survey.org and through CDS. Comment: 85 pages, 23 figures, 14 tables, accepted for publication in the Astronomical Journal
arXiv:0806.0546v1 [astro-ph] 3 Jun 2008
The Radial Velocity Experiment (RAVE): second data release
T. Zwitter1, A. Siebert2,3, U. Munari4, K. C. Freeman5, A. Siviero4, F. G. Watson6, J. P.
Fulbright7, R. F. G. Wyse7, R. Campbell8,2, G. M. Seabroke9,10, M. Williams5,2, M.
Steinmetz2, O. Bienaym´ e3, G. Gilmore9, E. K. Grebel11, A. Helmi12, J. F. Navarro13, B.
Anguiano2, C. Boeche2, D. Burton6, P. Cass6, J. Dawe†,6, K. Fiegert6, M. Hartley6, K.
Russell6, L. Veltz3,2, J. Bailin14, J. Binney15, J. Bland-Hawthorn16, A. Brown17, W.
Dehnen18, N. W. Evans9, P. Re Fiorentin1, M. Fiorucci4, O. Gerhard19, B. Gibson20, A.
Kelz2, K. Kujken12, G. Matijeviˇ c1, I. Minchev21, Q. A. Parker8, J. Pe˜ narrubia13, A.
Quillen21, M. A. Read22, W. Reid8, S. Roeser11, G. Ruchti7, R.-D. Scholz2, M. C. Smith9,
R. Sordo4, E. Tolstoi12, L. Tomasella4, S. Vidrih11,9,1, E. Wylie de Boer5
– 2 –
We present the second data release of the Radial Velocity Experiment
(RAVE), an ambitious spectroscopic survey to measure radial velocities and stel-
lar atmosphere parameters (temperature, metallicity, surface gravity, and rota-
tional velocity) of up to one million stars using the 6dF multi-object spectro-
graph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory
(AAO). The RAVE program started in 2003, obtaining medium resolution spec-
1University of Ljubljana, Faculty of Mathematics and Physics, Ljubljana, Slovenia
2Astrophysikalisches Institut Potsdam, Potsdam, Germany
3Observatoire de Strasbourg, Strasbourg, France
4INAF, Osservatorio Astronomico di Padova, Sede di Asiago, Italy
5RSAA, Australian national University, Canberra, Australia
6Anglo Australian Observatory, Sydney, Australia
7Johns Hopkins University, Baltimore MD, USA
8Macquarie University, Sydney, Australia
9Institute of Astronomy, University of Cambridge, UK
10e2v Centre for Electronic Imaging, School of Engineering and Design, Brunel University, Uxbridge, UK
11Astronomisches Rechen-Institut, Center for Astronomy of the University of Heidelberg, Heidelberg,
12Kapteyn Astronomical Institute, University of Groningen, Groningen, the Netherlands
13University of Victoria, Victoria, Canada
14Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Australia
15Rudolf Pierls Center for Theoretical Physics, University of Oxford, UK
16Institute of Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
17Sterrewacht Leiden, University of Leiden, Leiden, the Netherlands
18University of Leicester, Leicester, UK
19MPI fuer extraterrestrische Physik, Garching, Germany
20University of Central Lancashire, Preston, UK
21University of Rochester, Rochester NY, USA
22University of Edinburgh, Edinburgh, UK
– 3 –
tra (median R=7,500) in the Ca-triplet region (λλ 8,410–8,795˚ A) for southern
hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogues, in the
magnitude range 9 < I < 12. Following the first data release (Steinmetz et al.
2006) the current release doubles the sample of published radial velocities, now
containing 51,829 radial velocities for 49,327 individual stars observed on 141
nights between April 11 2003 and March 31 2005. Comparison with external
data sets shows that the new data collected since April 3 2004 show a standard
deviation of 1.3 km s−1, about twice better than for the first data release. For
the first time this data release contains values of stellar parameters from 22,407
spectra of 21,121 individual stars. They were derived by a penalized χ2method
using an extensive grid of synthetic spectra calculated from the latest version
of Kurucz stellar atmosphere models. From comparison with external data sets,
our conservative estimates of errors of the stellar parameters for a spectrum with
an average signal to noise ratio of ∼40 are 400 K in temperature, 0.5 dex in
gravity, and 0.2 dex in metallicity. We note however that, for all three stel-
lar parameters, the internal errors estimated from repeat RAVE observations of
822 stars are at least a factor 2 smaller. We demonstrate that the results show
no systematic offsets if compared to values derived from photometry or comple-
mentary spectroscopic analyses. The data release includes proper motions from
Starnet2, Tycho2, and UCAC2 catalogs and photometric measurements from
Tycho-2 USNO-B, DENIS and 2MASS. The data release can be accessed via the
RAVE webpage: http://www.rave-survey.org and through CDS.
Subject headings: catalogs, surveys, stars: fundamental parameters
This paper presents the second data release from the Radial Velocity Experiment (RAVE),
an ambitious spectroscopic survey of the southern sky which has already observed over
200,000 stars away from the plane of the Milky Way (|b| > 25o) and with apparent magni-
tudes 9 < IDENIS< 13. The paper follows the first data release, described in Steinmetz et al.
(2006), hereafter Paper I. It doubles the number of published radial velocities. For the first
time it also uses spectroscopic analysis to provide information on values of stellar parame-
ters: temperature, gravity, and metallicity. Note that the latter in general differs from iron
abundance, because metallicity is the proportion of matter made up of all chemical elements
other than hydrogen and helium in the stellar atmosphere. Stellar parameters are given for
the majority of the newly published stars. This information is supplemented by additional
– 4 –
data from the literature: stellar position, proper motion, and photometric measurements
from DENIS, 2MASS and Tycho surveys.
Scientific uses of such a data set were described in Steinmetz (2003). They include
the identification and study of the current structure of the Galaxy and of remnants of its
formation, recent accretion events, as well as discovery of individual peculiar objects and
spectroscopic binary stars. Kinematic information derived from the RAVE dataset has been
used (Smith et al. 2007) to constrain the Galactic escape speed at the Solar radius to vesc=
(where vcirc= 220 km s−1is the local circular velocity) is a model-independent confirmation
that there must be a significant amount of mass exterior to the Solar circle, i.e. it convincingly
demonstrates the presence of a dark halo in the Galaxy. A model-dependent estimate yields
the virial mass of the Galaxy of 1.31+0.97
per cent confidence). Veltz et al. (2008) discussed kinematics towards the Galactic poles
and identified discontinuities that separate thin disk, thick disk and a hotter component.
Seabroke et al. (2008) searched for in-falling stellar streams on to the local Milky Way disc
and found that it is devoid of any vertically coherent streams containing hundreds of stars.
The passage of the disrupting Sagittarius dwarf galaxy leading tidal stream through the
Solar neighborhood is therefore ruled out. Additional ongoing studies have been listed in
−44km s−1(90 percent confidence). The fact that v2
escis significantly greater than 2v2
−0.49× 1012M⊙and the virial radius of 297+60
The structure of this paper is as follows: Section 2 is a description of the observations,
which is followed by a section on data reduction and processing. Data quality is discussed
in Section 4, with a particular emphasis on a comparison of the derived values of stellar
parameters with results from an analysis of external data sets. Section 5 is a presentation
of the data product, followed by concluding remarks on the results in the context of current
large spectroscopic surveys.
RAVE is a magnitude limited spectroscopic survey. For this reason it avoids any kine-
matic bias in the target selection. The wavelength range of 8410 to 8795˚ A overlaps with
the photometric Cousins I band. However the DENIS and 2MASS catalogs were not yet
available at the time of planning of the observations we present here. So this data release
uses the same input catalog as Paper I: the bright stars were selected using I magnitudes
estimated from the Tycho-2 VTand BTmagnitudes (Høg et al. 2000), and the faint ones
were chosen by their I magnitudes in the SuperCOSMOS Sky Survey (Hambly et al. 2001),
hereafter SSS. Transformations to derive the I magnitude and its relation to the DENIS I
– 5 –
magnitude values are discussed in Paper I. There we also comment on the fact that Super-
COSMOS photographic I magnitudes show an offset with respect to DENIS I magnitudes
(Fig. 1). So, although the initial magnitude limit of the survey was planned to be 12.0, the
actual limit is up to one magnitude fainter.
The survey spans a limited range in apparent magnitude, still it 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.
Fig. 1.— Cousins I-band magnitudes of RAVE spectra in the 2nd data release. The smooth
line denotes magnitudes derived from Tycho-2 and SSS survey photometry which were used
as an input catalog for RAVE. The solid line histogram depicts DENIS I magnitudes for the
77% of stars which are also in the 2nd release of the DENIS catalog. Short and long dashed
lines are histograms of DENIS I magnitudes for stars from the Tycho-2 and SSS surveys,
respectively. Test fields close to the Galactic plane (|b| < 25o) are not plotted.
The instrumental setup is similar to the one used in Paper I. Two field plates with
robotically positioned fibers are used in turn in the focus of the UK Schmidt telescope at
the Anglo-Australian Observatory. A field plate covers a 5.7ofield of view and feeds light to
up to 150 fibers each with an angular diameter of 6.7” on the sky. One should be careful to
avoid chance superpositions with target stars when using such wide fibers. As a precaution
we avoid regions close to the Galactic plane (|b| < 25o) or dense stellar clusters. Also, all
candidate stars are visually checked for possible contamination prior to observing using the
– 6 –
1-arcmin SSS thumbnails from the on-line SSS R-band data.
Each field plate contains 150 science fibers, with additional bundles used for guiding. A
robot positioner configures the plate for each field by moving each fiber end to the desired
position. The associated mechanical stress occasionally causes the fiber to break, so it needs
to be repaired. A typical fiber is broken after every 2 years of use on average, and is repaired
in the next 8 months. Figure 2 shows the number of fibers which were used successfully to
collect star light for each of the 517 pointings. The number varies with time. A period of
decline is followed by a sharp rise after the repair of broken fibers on the corresponding field
plate. Each pointing was typically used to successfully observe 106 stars. An additional 9
or 10 fibers were used to monitor the sky background.
The light is dispersed by a bench-mounted Schmidt-type spectrograph to produce spec-
tra with a resolving power of R ∼ 7500. The main improvement introduced since the first
data release is the use of a blue light blocking filter (Schott OG531) which blocks the second
order spectrum. This allows for an unambiguous placement of the continuum level and so
permits the derivation of values of stellar parameters, in addition to the radial velocity. The
introduction of the blocking filter lowers the number of collected photons by only ∼ 25%,
so we decided to keep the same observing routine as described in Paper I. The observation
of a given field consists of 5 consecutive 10-minute exposures, which are accompanied by
flat-field and Neon arc calibration frames.
Note that we use two field plates on an alternating basis (fibers from one fiber plate are
being configured while we observe with the other field plate). So fibers from a given field
plate are mounted to the spectrograph slit prior to observation of each field. To do this the
cover of the spectrograph needs to be removed, so its temperature may change abruptly.
The associated thermal stress implies that it is best to use the flatfield and Neon arc lamp
exposures obtained immediately after the set of scientific exposures when the spectrograph
is largely thermally stabilized. For all data new to this data release we ensured that such
flatfield and arc lamp exposures have been obtained and used in the data reduction.
Observations were obtained between April 11 2003 and March 31 2005. The observations
obtained since April 3 2004 yielded data which were not published in Paper I, so they are
new to this data release. Statistics on the number of useful nights, of field centers and of
stellar spectra are given in Table 1. These numbers make the present, second data release
about twice as large as the one presented in Paper I. Stars were mostly observed only once,
but 75 stars from the field centered on R.A. = 16h07m, Dec. = −49owere deliberately
observed 8 times to study their variability.
Observations are limited to the southern hemisphere and have a distance of at least 25
– 7 –
Fig. 2.— Number of fibers observing stars (circles) and sky background (triangles) for fields
in the 2nd data release. Filled symbols mark observations obtained with fiber plate 1 and
open symbols those with plate 2. Test fields close to the Galactic plane (|b| < 25o) have been
omitted from the graph.
– 8 –
degrees from the Galactic plane (except for a few test fields). Their distribution is plotted in
Figure 21. The unvisited area is concentrated around the Galactic plane and in the direction
of the Magellanic clouds.
3.Data reduction and processing
The data reduction is performed in several steps:
1. Quality control of the acquired data.
2. Spectra reduction.
3. Radial velocity determination and estimation of physical stellar parameters.
In the first step the RAVEdr software package and plotting tools are used to make a pre-
liminary estimate of data quality in terms of signal levels, focus quality and of possible
interference patterns. This serves two goals: to quickly determine which observations need
to be repeated because of unsatisfactory data quality, and to exclude any problematic data
from further reduction steps. For the first data release 17% of all pointings were classified
as problematic, while in this data release the overall dropout rate fell to 13%. Problematic
data are kept separately and are not part of this data release. The next two steps of the
data reduction process are described below.
We use a custom set of IRAF routines which have been described in detail in Paper I.
Here we highlight only the improvements introduced for reduction of data new to this data
The use of the blue light blocking filter permits a more accurate flatfielding of the data.
The spectra have a length of 1031 pixels, and are found to cover a wavelength interval of
384.6 ± 1.7˚ A. The resolving power is the same as estimated in Paper I, we use the value of
R ≃ 7,500 throughout. The camera of the spectrograph has a very fast focal ratio (F/1).
The associated optical aberrations at large off-axis angles imply that the central wavelength
of the spectrograph is not constant, but depends on the fiber number (Figure 3). This
means that the wavelengths covered by a spectrum depend on its fiber number. Also any
residual cross–talk between the spectra in adjacent fibers is generally shifted in wavelength.
– 9 –
Table 1.Observing statistics
in this DRData
Number of nights of observation
Number of fields (incl. repeats)
Sky area covered (square degrees)
Number of different stars
Number of stars observed once
Number of stars observed twice
Number of stars observed 3 times
Number of stars observed 4 times
Number of stars observed 5 times
Number of stars observed 6 times
Number of stars observed 7 times
Number of stars observed 8 times
present data release, the right one only data obtained after
April 3 2004, i.e. new in this data release.
— The middle column counts all data in the
– 10 –
This makes an iterative procedure to remove illumination from adjacent fibers even more
important (see Paper I for details). The peak of central wavelengths around the half-point
of their distribution shows that our instrumental setup remained quite stable for one year
when the data new to this data release were obtained.
The determination of radial velocity and stellar parameters is based on the 788 pixels
of the central part of the wavelength range only (8449.77˚ A < λ < 8746.84˚ A). This avoids
telluric absorption lines and a ghost image caused by internal reflections of non-dispersed
light at the borders of the wavelength range which are occasionally present and could jeop-
ardize the results, as described in Paper I. The edges of the spectral interval are avoided
also because of a poorer focus, lower resolving power and a lower quality of the wavelength
Figure 4 plots the average ADU count level of the central part of the final 1-D spectrum,
and per one hour of exposure time, as a function of Denis I magnitude. Only data new to
this data release are plotted. The line follows the relation
where the constant term is the mode of the magnitude corrected count distribution. These
count levels are 0.25 mag below those in Paper I. The difference is due to the 2nd order
blocking filter. Note however that the filter allowed for a more accurate flatfielding, and so
better determined count levels. This information has been used in data quality control.
3.2. Radial velocity determination
The general routine stayed the same as described in detail in Paper I. Radial velocities
are computed from sky-subtracted normalized spectra, while sky unsubtracted spectra are
used to compute the zero-point correction. The latter is needed because of thermal variations
of the spectrograph which cause a shift of the order of one tenth of a pixel or 1.5 km s−1.
Radial velocities are computed from 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 is based on the latest generation of Kurucz models. 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.
Most spectra have a microturbulent velocity of 2 km s−1(with additional entries for 1 and 4
km s−1), while the α enhancements of [α/Fe] = 0.0 and +0.4 are used. The use of the blue
blocking filter simplifies the computations, as no contribution from the 2nd order spectrum
needs to be considered. Both the observed spectra and theoretical templates are normalized
prior to the radial velocity measurement. We use IRAF’s task continuum with a two-piece
– 11 –
Fig. 3.— Variation of central wavelength as a function of fiber number for data new to this
release. Shades of gray code the number of spectra in a certain bin, as given in the key. The
line follows half-point central wavelengths as a function of fiber number.
– 12 –
Fig. 4.— Average number of counts per pixel per hour of exposure time as a function of
DENIS I magnitude. Shades of grey code the number of spectra in a certain bin, as given
in the key. The average count level is calculated from the central part of the spectrum only
(8449.77˚ A ≤ λ ≤ 8746.84˚ A). The inclined line follows eq. 1.
– 13 –
cubic spline. The rejection criteria used in 10 consecutive iterations of the continuum level
are asymmetric (1.5-σ low and 3-σ high).
Kurucz synthetic spectra used in cross-correlation do not include corrections of radial
velocity due to convective motions in the stellar atmosphere or due to a gravitational redshift
of light leaving the star (F. Castelli, private communication). The combined shift is in the
range of –0.4 km s−1for F dwarfs to +0.4 km s−1for K dwarfs (Gullberg & Lindegren 2002),
while the near absence of gravitational redshift in giants causes a ∼ 0.4 km s−1shift between
giants and dwarfs. The exact value of these corrections is difficult to calculate, so we follow
the Resolution C1 of the IAU General Assembly in Manchester (Rickman 2002) and report
the heliocentric radial velocities without corrections for gravitational or convective shifts in
the stellar atmosphere. Note however that these values may be different from the line-of-sight
component of the velocity of the stellar center of mass (Lindegren 1999; Latham 2001).
In the final data product we report the heliocentric radial velocity and its error, together
with the value of the applied zero-point velocity correction, the radial velocity of sky lines and
their correlation properties. A detailed description of the data release is given in Section 5.
3.3.Stellar parameter determination
The name of the survey suggests that RAVE is predominantly a radial velocity survey.
However, the spectral type of the survey stars is generally not known and the input catalog
does not use any color criterion, so RAVE stars are expected to include all evolutionary stages
and a wide range of masses in the H-R diagram. The properties of the stellar spectra in the
wavelength interval used by RAVE strongly depend on the values of the stellar parameters
(Munari et al. 2001). While the Ca II IR triplet is almost always present, the occurrence
and strength of Paschen, metallic and molecular lines depends on temperature, gravity and
metallicity (see e.g. Figure 4 in Zwitter et al. (2004)). So we cannot adopt the common
practice of using a small number of spectral templates to derive the radial velocity alone,
as it has been commonly done at, e.g., the ELODIE spectrograph at OHP. We therefore
construct the best matching template from a large library of synthetic Kurucz spectra (see
Sec. 3.2). The parameters of the best matching spectrum are assumed to present the true
physical parameters in the stellar atmosphere.
Two comments are in order before we outline the template spectrum construction
method. First, the template library only covers normal stars. So peculiar objects cannot
be classified correctly. Such objects include double lined spectroscopic binaries and emission
line objects. Sometimes a peculiar nature of the spectrum can be inferred from a poor match
– 14 –
Fig. 5.— Sensitivity of synthetic spectra to stellar parameters. These are synthetic spectra
of non-rotating stars with Solar metallicity and microturbulent velocity of 2 km s−1. Inter-
mittent lines mark regions where a change in one of the parameters causes a change of at
least 3% in normalized flux. From bottom up the intermittent lines mark flux changes for:
a 500 K decrease in temperature (red), a 0.5 dex decrease in metallicity (green), a 0.5 dex
decrease in gravity (blue), a 30 km s−1increase in rotational velocity (cyan). The wavelength
range of the spectra is the one actually used for the determination of stellar parameters.
– 15 –
of the templates, despite a high S/N ratio of the observed spectrum.
The second important point concerns the non-orthogonality of the physical parameters
we use. This is demonstrated in Figure 5: the wavelength ranges with flux levels sensitive
to a change in temperature overlap with those sensitive to metallicity and the rotational
velocity. On the other hand sensitivity to changes in both gravity and temperature depend
on spectral type and class. The intermittent lines in Figure 5 mark wavelengths where
the normalized flux level changes for at least 3% if the value of one of the parameters is
modified by a given amount (temperature by 500 K, gravity or metallicity by 0.5 dex, or
rotational velocity by 30 km s−1). We note that a 3% change is marginally detectable in a
typical RAVE spectrum with S/N = 40, but the non-orthogonality of individual parameters
can present a serious problem (see also Figure 1 in Zwitter (2002)). If the temperature or
gravity would be known a priori, the ambiguities would be largely resolved. An obvious idea
is to use photometric colors to constrain the value of stellar temperature. Unfortunately
the errors of current photometric surveys are too large: a change of 0.03 mag in J − K
corresponds to a shift of 230 K in temperature in a mid-G main sequence star. Also, stellar
colors may be seriously compromised by interstellar extinction or by stellar binarity. We
therefore decided not to use any outside information but to base our estimates of stellar
parameters exclusively on spectral matching. This may change in the future when results
of multicolor and multi-epoch all-sky photometric surveys such as SkyMapper (Keller et al.
2007) will become available.
Our parameter estimation procedure makes use of a full set of theoretical templates.
They span a grid in 6 parameters: temperature, gravity, metallicity, α enhancement, micro-
turbulent and rotational velocity. The sampling in gravity, metallicity, and temperature is
very good, with
On the other hand the current synthetic library contains only one non-Solar α enhancement
value ([α/Fe] = +0.4) and only up to 3 values of microturbulent velocity (1, 2, 4 km s−1,
but only 2 km s−1is available for the whole grid). So we decided to publish values of tem-
perature, gravity and metallicity. The alpha enhancement values are also listed but they
should be interpreted with caution, as they are derived from 2 grid values only. These two
values may not span the whole range of α enhancement which is present in nature. Also the
error of α enhancement can be comparable to the whole range of the grid in this parameter
(see Sec. 3.3.5). Microturbulent velocity values are not published, because their errors are
typically much larger than the range of microturbulent velocities in the grid. Similarly, the
rather low resolving power of RAVE spectra does not allow the determination of rotational
velocities (Vrot) for slow rotators which represent the vast majority of RAVE stars. Hence the
rotational velocity is not published, but fast rotators will be discussed in a separate paper.
So we aim at the estimation of three stellar parameters: effective temperature (Teff), gravity
>∼9 tabulated values for the former two and even more for the temperature.