Techniques d'observation spectroscopique d'astéroïdes

50 CHAPTER 2. WHY SPECTROSCOPY?
Table 2.1: The emission lines identification in spectrum of PG1634 +706. The line labels, their corresponding
laboratory wavelengths, these wavelengths shifted with z=1.34, and the wavelengths observed in the spectrum
are presented.
Line λ 0 [nm] λ [nm] λ obs [nm]
C III] 190.6 446.0 444
Fe III 207.7 485.8 488
Fe II + CII] 232.6 544.3 544
Mg II 280.0 655.2 655
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coefficient equal to 0.5416. The determination is at 3σ compared with the level of noise
(where σ = 0.1987 is the standard deviation of the correlation coefficient values plotted in
Fig. 2.4).
Considering the value found for the redshift - z=1.340, the emission lines of known chemical
elements could be identified in the spectrum of PG1634+706 (Table 2). Based on the emission
line identification the accuracy of z determination can be ascertained: z=1.340±0.008.
Because PG1634+706 is a bright quasar with high redshift of spectral lines, it has been
studied in some papers like [Schmidt & Green, 1983, Trevese et al., 2007]. Our observation for
this object was at the limited magnitude for the type of equipment used. With a robust method,
we succeed to extract the signal from noise and compute the redshift. Our determination of
redshift z = 1.340±0.008, with a small telescope agrees with the value found from observation
with large telescopes.
The result obtained allow to assert that even using a small telescope and the simplest spectrograph
valuable results can be obtained. The developed methods for observations and data
reduction can be used as a starting point for spectroscopy of celestial bodies with small telescopes.
2.4 Spectroscopy for asteroids
The knowledge of the surface mineralogy of individual asteroids and groups of asteroids can be
inferred through the spectroscopy. The solar light reflected from the asteroids contains essential
information regarding the optical properties of the materials found at the asteroids surface. The
spectral interval 0.8 - 2.5 µm is very important to discriminate between different mineralogy
of silicate-based compounds. Silicate minerals identification is based on the presence of broad
bands of absorption around 1 and 2 µm. These bands are due essentially to the presence of
olivine and pyroxene (or mixtures) on the surface of the asteroid.
2.4.1 Reflectance versus emission
The incident flux arriving from an asteroid surface is splitted in two contributions (Fig. 2.5):
the solar radiation passively reflected by the surface material, and the solar radiation which has
been absorbed, converted to heat, an re-emitted as thermal radiation [McCord & Adams, 1977].