ASTEROIDS IN DFBS

The project is aimed at collection and study of massive physical and dynamical characteristics of asteroids. It was started on the initiative of J. Berthier and W. Thuillot (IMCCE, Paris), and A. Sarkissian (IPSL, Paris). It aimed at discovery and study of the low-dispersion spectra (5 nm resolution spectra ranging from 340 to 690 nm) of the known solar system objects, mainly asteroids, in the DFBS fields (Thuillot et al. 2007). With a limiting V magnitude closed to 18 for the fainter sources in the FBS, we roughly estimate to a few hundreds the number of spectra of asteroids that can be detectable in the 1667 plates of 4 square degrees of the DFBS. The spectral characterization of asteroids is important for understanding the evolution of their compositional and mineralogical properties. This knowledge is also important to study and quantify the physical properties of the interior of asteroids (e.g. composition, structure, bulk density, ...). Nowadays, the number of asteroids for which spectra have been acquired is about few thousands. Most of them have been recorded in the past 20 years during dedicated surveys (e.g. Xu et al., 1995, Bus & Binzel, 2002, Lazzaro et al., 2004). Even if these surveys have already measured most of the brighter asteroids which may be detectable in the DFBS, it remains useful to discover spectra in the DFBS in order to carry on the building up of the collections of spectra of asteroids in the visible wavelength. It could also offer a unique opportunity to study the time-dependent modification of the surface reflectivity of asteroids by comparing the FBS spectra (acquired between 1965 and 1980) and recent ones (post 1990).

The project includes:

By the width of their spectra (connected to the speed of their motion) asteroids can be roughly divided into two types:

    1. "fast" asteroids, when the spectrum is stretched due to the motion of asteroid during the exposure time (typically 20 minutes; in rare cases it can be also stretched in the direction of the dispersion). 

    2. "slow" asteroids, when the spectrum is stellar-like and does not exceed 5 pixels in its width.

    a) Cross correlation of DFBS positions with positions of known objects from catalogues of stars and galaxies;

    b) Search for stretched spectra in DFBS (for fast asteroids);

    c) analysis of spectra and search for solar-like spectra.

Search and extraction of asteroids by ALADIN and SkyBoT

The extraction of asteroid spectra in the DFBS plates requires to solve two main issues: the identification of the targets in the plates and the calibration of their spectra. The localization and the identification of the solar system objects in the field of views are performed using the SkyBoT web-service (Berthier et al., 2006). This Virtual Observatory (IVOA) tool makes easy to know which asteroids are located in any field of view at any epoch. Then, by looking among the asteroids of magnitude lesser than 16 located in each plate, we are able to cross match them with the sources taking into account the known stars. As the aspect of each spectrum is particular, we need to inspect manually each extracted spectrum to ensure it is really the spectrum of a solar system object.

Extraction and analysis of spectra by EXATODS 

For the extraction and analysis of the asteroid spectra from the DFBS plates, dedicated software has been developed by Alain Sarkissian "EXATODS" -    Extraction and Analysis TOol of DFBS Spectra. 

It is developed to overcome the main difficulty to analyze DFBS spectra conneceted with their wavelength and photometric calibrations.

It scans full plate to find bright spectra than 2 scan each individual spectra and measures the angle of the rotation of each individual spectrum and follows the direction fo the dispersion to obtain the correct wavelength calibration. More, alignment of the objective prism, the plate and the scanning direction is variable from plate to plate and within the same plate (see Fig. 2, spectra of plate #703 looks vertical and those of plate #1250 are slightly inclined). To solve these issues, we have developed a dedicated workflow in the Virtual Observatory framework.

The FBS is composed of 16x16 cm plates covered by various photographic emulsions (mainly Kodak IIAF, IIaF, IIF and 103aF) which have been digitized. In order to calibrate each spectrum, we have to take into account the sensitivity function of the emulsion combined with the transmission function of the optics (which is important in the UV). However, as it was common at the time to modify the chemistry of the emulsion to improve the sensitivity of the plates to calibrate spectra, we cannot directly use the theoretical sensitivity function as it is, because it varies from plate to plate. All spectra are extracted from plates taking the middle and the width of the best Gaussian fitted to each horizontal line of the spectrum (see Fig. 3). The complete spectrum is corrected of inclination by linear interpolation in pixel space. Note that correction can reach up to 4 pixels from Red to UV parts (Fig. 3, upper left panel). Then, the spectrum of a sun-like star, closed to the target, is picked up and is compared to its reference spectrum.. That provides the component of the sun light and an estimation of the emulsion correction in the neighborhood of the target. The spectra of one or more reference stars are also extracted and compared to their reference spectra in order to ensure the goodness of the estimation of the sensitivity function of the emulsion in the plate. Finally, the asteroid spectrum is calibrated in wavelength by associating i) a chemical specie identified in its spectra to the corresponding wavelength (usually H, K, Ca lines in the UV, and Hβ and Hα) and ii) extreme wavelength in the sensitivity functions at approximately 455, 500 and 660 nm (see Fig. 3, lower left panel). After calibrating, a full analysis of the asteroid spectra is performed by means of the classical methods used to analyze planetary spectra in order to provide physical characterizations of the objects, such as the surface spectral reflectance in the visible and, therefore, an estimation of the composition of the asteroid surface.

Participating  Institutions

Related Publications

Berthier J., Thuillot W., Mickaelian A., Sarkissian A., et al. – Asteroid Search with the DFBS // Science with Virtual Observatories: JENAM-2007, Special Session #8, Yerevan 2007, in press.

  Thuillot W., Berthier J., Sarkissian A., Mickaelian A., Sargsyan L., Iglesias J., Vachier F., Birlan M., Simon, G. – Massive Physical and Dynamical Characterization of Asteroids // The Virtual Observatory in Action: New Science, New Technology, and Next Generation Facilities, 26th meeting of the IAU, Special Session 3, 17-18, 21-22 August, 2006 in Prague, Czech Republic, SPS3, 2007, in press.

Berthier J., Vachier F., Thuillot W., Fernique P., Ochsenbein F., Genova F., Lainey V., Arlot, J.-E, SkyBoT, a new VO service to identify Solar System objects. Astronomical Data Analysis Software and Systems XV, 2006, p. 351.

 Lazzaro, D., C.A. Angeli, J.M. Carvano, T. Mothe-Diniz, R. Duffard, and M. Florczak, S3OS2: The visible spectroscopic survey of 820 asteroids, Icarus 172, 179-220, 2004.

 Bonnarel F., Fernique P., Bienayme O., Egret D., Genova F., Louys M., Ochsenbein F., Wenger M., Bartlett J. G., The Aladin interactive sky atlas. A reference tool for identification of astrojnomical sources, Astron. Astrophys. Supll. Ser., 143, 2000.

 Bus, S. J. and Binzel, R. P. (2002). Phase II of the Small Main-Belt Asteroid Spectroscopic Survey: The Observations, Icarus 158, 106-145.

 IVOA, International Virtual Observatory Alliance , http://www.ivoa.net

 Xu, S., Binzel, R. P., Burbine, T. H., and Bus, S. J. (1995). Small Main-Belt Asteroid Spectroscopic Survey: Initial Results, Icarus 115, 1-35.