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Astron. Astrophys. 321, 652-659 (1997)

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2. (7) Iris: the current picture

(7) Iris is one of the biggest S-type asteroids in the main belt (Tholen 1989, Tholen & Barucci 1989, Tedesco & Veeder, 1992). It was first discovered by J.R. Hind in 1847 (Pilcher 1979). Its orbit (MPC 24084) was recently improved by Wlodarczyr (1993) using 30 new astrometric positions taken at the Observatory of Chorzow and 15 from three other observatories. The orbital elements are listed in Table 1.


[TABLE]

Table 1. Orbital Elements of (7) Iris


Since the 1950s, (7) Iris has been observed photoelectrically by several authors (Groeneveld & Kuiper 1954, van Houten-Groeneveld & van Houten 1958, Taylor 1971, Taylor 1977, Xing-Hai et al. 1982, Zhou et al. 1982, Zappalà & Di Martino, 1986). The rotation period has been determined, and turns out to be 7.139 hours (Lagerkvist et al. 1989).

In 1987 Lagerkvist & Williams first determined the slope parameter (G) for (7) Iris, which turned out to be [FORMULA]. The presently accepted value of H is 5.510 mag.

Iris was observed by the IRAS infrared satellite, leading to a determination of diameter and albedo. After a recalibration of data, the diameter turns out to be about 200 km (199.83 km according to Tedesco & Veeder 1992) with a [FORMULA] error of about 10%; the visual albedo ([FORMULA]) is 0.2766 [FORMULA] 0.030.

In 1962 Gehrels & Owings first gave an estimate of the orientation of the spin axis, and one year later Chang & Chia-Shiang confirmed the previous measurements. Another, independent determination of the pole has been given also by Zappalà & Di Martino (1986). The most recent pole determination is by Magnusson (1986), who finds an ellipsoidal shape with [FORMULA] and [FORMULA] and a pole direction with ecliptic longitude and latitude ([FORMULA]) of either ([FORMULA]) or ([FORMULA]).

In 1990 Broglia & Manara carried out a polarimetric study of (7) Iris. At a mean phase angle of 11.8 degrees, Iris showed a mean polarization of [FORMULA] with a relative polarization variation [FORMULA], where [FORMULA] is the total amplitude of the fitted curve, equal to [FORMULA]. The authors concluded that this remarkable variation implied a variegate surface.

Also Hoffman & Geyer (1993) reported on variable lightcurve irregularities in a short phase interval, and they concluded that they could be explained by the presence of a large albedo spot in the northern hemisphere of (7) Iris. They also concluded that regions of different materials should cover large but not uniformly distributed areas on the surface.

Very recently, Mitchell et al. (1995) on the basis of radar observations concluded that the surface roughness is not negligible with respect to the asteroid dimensions, and this suggested the presence of either regional or scale-dependent variations in small-scale structure.

Since the 1980s (7) Iris has been spectroscopically observed. In 1982 Feierberg et al. published the first infrared spectrum and after them Butterworth & Meadows (1985), Green et al. 1985, Clark et al. 1992, Hiroi et al. 1993, Roettger & Buratti 1994, Xu et al. 1995 and Gaffey & Gilbert 1996 obtained spectroscopic data. However, none of them performed rotationally-resolved spectroscopy.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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