Techniques d'observation spectroscopique d'astéroïdes

CHAPTER 7. SPECTRAL PROPERTIES OF MAIN BELT ASTEROIDS 115
Table 7.3: Solar analogs used for data reduction, their airmass at the moment of observations and their relative
distance to the object.
Asteroid Solar Analogue Air mass Distance [ ◦ ]
(9147) Kourakuen HD940 1.280 11.5
(854) Frostia G104-335 1.070 11.3
(1333) Cevenola HD127913 1.055 10.1
(1333) Cevenola HD127913 1.031 10.2
(3623) Chaplin HD73708 1.025 6.6
(3623) Chaplin HD73708 1.018 6.6
(10484) Hecht Land115-271 1.200 19.5
(31569) 1999 FL18 Land115-271 1.200 15.1
tel-00785991, version 1 - 7 Feb 2013
The following stars were observed and used as solar analogs: HD940, G104-335, HD127913,
HD73708 and L115-256 (Table 7.3). Our choice was to observe the solar analogue as close
as possible to the target (bellow 20 o ). The differential airmass between the asteroid and the
standard was usually restricted to less than 0.16. The stars were chosen using the tool available
on IRTF website 1 . An exception was made for (10484) Hecht and (31569) 1999 FL18,
where the data reduction were performed using L115-271, commonly used in NIR spectral
measurements.
G104-335, HD127913, HD73708 are G2V type, while HD940 is a K0 star [Høg et al., 2000,
Cutri et al., 2003, Landolt, 1992]. HD 940 was chosen as trade-off between the spectral type,
airmass, and its relative distance to (9147) Kourakuen.
The data reduction followed the procedure described in chapter 3. For the computation of
the final reflectance (ratio between the asteroid spectrum and the star spectrum) is considered
the similar dynamic regimes of the detector [Vacca et al., 2004, Rayner et al., 2003].
7.2 (9147) Kourakuen - a V-type asteroid outside Vesta family
One of the most interesting asteroid family is the one of (4) Vesta. Located in the inner asteroid
belt, the origins of this family is in a collision event that excavated a large crater in the
surface of asteroid (4) Vesta [Asphaug, 1997]. The presence of such a crater in the south hemisphere
of the asteroid has been confirmed by HST images [Thomas et al., 1997] and recently
by NASA’s Dawn spacecraft. This crater, called Rheasilvia, has 505 Km in diameter and is one
of the largest craters in the Solar System. The latest estimates indicate that the cratering event
occurred at least 1.2 Gyr ago [Carruba et al., 2005].
(4) Vesta is particularly interesting because it is the only large asteroid showing a basaltic
crust [McCord et al., 1970]. Basaltic asteroids are believed to derive from bodies whose interiors
reached the melting temperature of silicate rocks and subsequently differentiated
[Gaffey et al., 2002]. Thus, (4) Vesta is a differentiated object with a basaltic crust and exposed
mantle material [Gaffey, 1997] that survived during the Solar System history. It is considered
1 http://irtfweb.ifa.hawaii.edu/cgi-bin/spex/find_a0v.cgi