The ELODIE Archive¹

Archiving astronomical data has always been a traditional activity of ground‐based observatories. However, in the last 20 years this traditional responsibility of data producers has been significantly weakened. While photographic plates have been carefully stored and archived in the past, magnetic tapes have not always been kept for more than a few years, and much data have thus disappeared. Photographic plates, and in particular spectra, are often still safely kept, and efforts to make them publicly available are being deployed (Griffin 2004), but serious archiving of electronic data was only developed in the wake of the Hubble Space Telescope archive.

Now, with the upcoming Virtual Observatory (VO) and multiwavelength, multi‐epoch, or multiparametric analysis, there is renewed interest in this material, and consequently there is a large demand for online availability of observational data. Archiving is also a way to optimize observing time (it avoids redundancy in observing programs) and ensures a better scientific efficiency for an instrument.

In this paper we present the online archive of the data obtained with the ELODIE spectrograph, which has been in operation at the Observatoire de Haute‐Provence (OHP) 1.93 m telescope since mid‐1993. The main characteristics of the instrument are described in Baranne et al. (1996), and some of them are listed in Table 1. The online archive has been available since 2003 September and allows access to the FITS data and to the pipeline that reprocesses them on‐the‐fly.

In § 2 we present the content of the archive. The original archived data are described in § 3, and the organization of the archive in § 4. Section 5 describes the user interface, the data processing, and the different data products. Plans for future development are discussed in § 6.

The archive presently contains more than 18,000 spectra obtained in the interval 1994–2001. The data that are accessible through the archive have passed the period of exclusivity (of 2 yr). This period is imposed by OHP for all observers except those who requested a longer period of protection for their data. After this period, data become available to the astronomical community.

The archived spectra are only those that have a signal‐to‐noise ratio (S/N) per pixel at 5500 Å larger than 10.

The characteristics of the content are shown in Table 1.

The scientific content of the archive is very rich, since different kinds of objects have been observed with ELODIE for different purposes. The high accuracy of the radial velocities obtained with this instrument (now reaching ∼7 ms⁻¹ in the scrambled‐fiber, simultaneous‐thorium mode) allowed the discovery of the first exoplanet around 51 Peg by Mayor & Queloz (1995). The search for exoplanets is continuing with ELODIE. The discovery of spectroscopic and eclipsing binaries, as well as M‐dwarf systems and the determination of stellar masses (Delfosse et al. 1999), has also been made with ELODIE thanks to the accurate radial velocities.

The spectrograph has been used as well for asteroseismology (Bertaux et al. 2003, etc.) and to determine stellar abundances and stellar parameters (Erspamer & North 2002, 2003; Kovtyukh et al. 2003; Soubiran et al. 2003; Santos et al. 2003; Katz et al. 1998; Oblak et al. 2002; Monier et al. 2002, etc.). Research using the off‐line archive allowed the development of the software TGMET for automatic determination of atmospheric stellar parameters T_eff, log (g), and [Fe/H] (Katz et al. 1998).

The compilation of the reference library for the TGMET software by Soubiran et al. (1998) was the first step to make reduced ELODIE spectra available through the World Wide Web. This opportunity was used for instance by Mishenina & Kovtyukh (2001), who selected in the library a large set of metal‐poor stars to measure abundances of various elements.

In addition, different studies of open clusters (Soubiran et al. 2000), planetary nebulae (Neiner et al. 2000), RR Lyrae (Chadid et al. 1999), Herbig Ae/Be stars (Corporon & Lagrange‐Henri 1999), comets (The Hale‐Bopp comet by Rauer et al. 1997), long‐period variable stars (Mira variables; e.g., Alvarez et al. 2000), and X‐ray source counterparts (Chevalier & Ilovaisky 1997) have also been made with ELODIE.

Moreover, flux‐calibrated stellar libraries have been created (Prugniel & Soubiran 2001; Imbert 2002) and used for stellar population synthesis at high spectral resolution (Le Borgne et al. 2002).

Finally, observations with ELODIE in the Milky Way allowed the determination of the distribution of stars in the Galactic Disk (Siebert et al. 2003) and the distribution of widths and intensities of the diffuse interstellar bands (DIB; Tuairisg et al. 2000).

The unique characteristic of both ELODIE and its twin spectrograph CORALIE at ESO (La Silla) in Chile is that the special cross‐disperser combining a prism and a grism yields near‐constant interorder spacing, allowing 67 orders to fit into a compact CCD 1024 × 1024 data format.

Two entrance apertures (each 2^'' wide) in the focal plane of the telescope are dedicated respectively to the simultaneous observation of the target and of either the sky or the thorium lamp that is used to derive the wavelength calibration. The continuum lamp frames (from a tungsten lamp) are used to derive the location of the 67 orders and to extract both the spectrum of the flat field and the blaze function.

The original archived data include the results of a reduction process performed automatically at the telescope immediately after data acquisition. This reduction includes correction for bias, dark, or scattered‐light subtractions, cosmic‐ray rejection, and optimal extraction of the data along the orders (using Horne's algorithm). It includes as well the division by the spectrum of the flat field and by the blaze function (both normalized to unity in each order), followed by wavelength calibration. The results of this original processing are not entirely satisfactory (as explained below), and consecutive echelle orders could not be reconnected.

The ELODIE archive is organized in three main structures:

• 1.  
  The first one contains the original data copied onto a dedicated machine and itemized in a PostgreSQL² database managed by the SQL language.
• 2.  
  The second one holds the code package written in C or Fortran 77 proper to the ELODIE archive. These programs also use routines from the Pleinpot software package distributed by the MIGALE project³ and developed by Ph. Prugniel and the MIGALE group.
• 3.  
  The third structure is the Web interface written in HTML and Javascript.

In § 5 we describe the user interface of the archive, the data products that are accessible through the Web pages, and the available data‐processing functions. The general organization of the archive, as well as the online facilities, are summarized in the diagram shown in Figure 1.

Zoom In Zoom Out Reset image size

5.1. The User Interface

5.2. Data Products

As explained in the previous section, the second Web page allows the user to access three different products related to an observation:

• 1.  
  The reconnected spectrum resampled in wavelength with a constant step of 0.05 Å covering the range 4000–6800 Å is given in "instrumental" flux. A procedure developed by Prugniel & Soubiran (2001) has been implemented in the archive software in order to correct the reconnected spectra for the sawtooth shape due to the incomplete removal of the blaze function. Application of this procedure results in an "instrumental" flux distribution that is relative to the tungsten lamp and thus appears "too blue" (see example in Fig. 2). Details on the three FITS file extensions used are given on the Web page of the archive.The spectra are at present in the topocentric reference frame, i.e., "as observed" (not corrected for the radial velocity of the star, nor for the Earth's velocity around the Sun as projected in the line of sight to the star). These corrections will be made available in the next version of the archive software (see § 6).
• 2.  
  The header of the s2d FITS image provides useful information, such as the name of the observer, the telescope position, the S/N ratio, wavelength calibration, etc.
• 3.  
  The s2d image is composed of 67 lines of 1024 pixels (see Fig. 3) holding the extracted spectra for all orders (covering the full spectral range), corrected for the blaze problem (i.e., reconnectable), where a wavelength is assigned to each pixel (the coefficients are given in the FITS header). The s2d spectrum is corrected for the curvature of the echelle orders, but because of a bad correction for scattered light in the online reduction, a residual curvature can still remain. Such a correction is made in the wavelength‐resampled spectra (see above). The advantage of a spectrum in the s2d format is that the original (variable) pixel sampling is preserved across the entire wavelength interval, as no rebinning has taken place; but special software is needed to use this format. Details on the two FITS file extensions used are given on the Web page.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

5.3. The Pipeline Interface

We plan to proceed to the development of phase 2 of the archive software in the summer of 2004, aiming principally to provide additional options for selecting and retrieving the data. New processing functions may also become available during this phase (such as a flux‐calibration process), and links with other projects will be developed.

We list here the main improvements that are planned for the phase 2 development of the archive:

• 1.  
  Searching for data by observational characteristics.—The observational parameters, generated when the observations are obtained at the telescope and included in the FITS header (the S/N ratio, the date of observation, the name of the observer, etc.) are already stored in a table of the database. An advanced selection menu that will put constraints on these parameters will be developed.
• 2.  
  Searching for data by stellar characteristics.—A "server of fundamental parameters of stars" is planned to be installed at Bordeaux in 2004. It will provide the effective temperature, surface gravity and metallicity of stars (extension of the Cayrel de Strobel et al. [2001] catalog) using the TGMET code of Katz et al. (1998). In addition, it will also provide estimates of the absolute magnitude, the spectral classification, the magnitude and the distance.The latter parameters are available at present from the SIMBAD database and we are currently studying improvements of cross‐access with SIMBAD. Possibly in the future, SIMBAD will give direct access to the entries in the ELODIE archive. Thanks to the Bordeaux server, the search for data in the ELODIE archive according to physical characteristics will be possible.
• 3.  
  Searching data using predefined lists of samples.—The list of predefined samples will also be extended.
• 4.  
  Searching data from a list of objects provided by the user.—In order to facilitate the use of the archive, we intend to provide the possibility for the user to upload a list of objects in order to search for spectra in the archive.
• 5.  
  Downloading a whole sample.—We intend to provide a script that allows the user to download a list of spectra. This option is preferable to the use of the Web interface, although the latter option is still a possibility.

In the meantime, the content of the ELODIE archive is continuously updated, and efforts are underway to improve the quality assessment of the material and the quality of the cross‐identifications.

The ELODIE spectrograph will be replaced in late 2005 by a new instrument, SOPHIE, which will have improved capabilities in terms of stability, limiting magnitude, and accuracy of the radial velocities (an improvement of a factor of 2.5 on the magnitude and of 2.5–3 on the radial velocities). We plan to extend the present archive to host the spectra from SOPHIE.