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A&A manuscript no.
(will be inserted by hand later)
Your thesaurus codes are:
10 (07.13.2; 07.19.2; 07.13.1; 07.16.2: 1993 VW)
ASTRONOMY
AND
ASTROPHYSICS
14.10.1996
1993 VW: an ordinary chondrite­like near­Earth asteroid ?
M. Di Martino 1 , A. Manara 2 , and F. Migliorini 3
1 Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy
2 Osservatorio Astronomico di Brera, via Brera, 28, 20121 Milano, Italy
3 Dipartimento di Astronomia, Universit`a di Padova, vicolo dell'Osservatorio, 5, 35122 Padova, Italy
Received ; accepted
Abstract. The optical reflectance spectrum of the near­Earth asteroid 1993 VW was measured over the wavelength
range 4500 -- 9200 š A using the 1.52 m telescope of the European Southern Observatory and a CCD spectrograph. This
object, as supposed, has been found to show spectral properties resembling the rare Q taxonomic type asteroids and
possibly ordinary chondrite meteorites.
Key words: asteroids -- meteorites -- spectroscopy -- 1993 VW
1. Introduction
Investigations of asteroid compositions can identify potential parent bodies of specific meteorites or meteorite types or
objects which have experienced similar evolutionary histories. It has been commonly assumed that most meteorites are
asteroidal fragments ejected from their parent bodies as a consequence of impacts, and channeled into chaotic dynamical
routes, associated with mean motion and secular resonances (Greenberg & Chapman,1983; Wetherill, 1985; Wetherill
& Chapman, 1988; Greenberg & Nolan,1989). The main problem is that the ordinary chondrite (OC) meteorites,
which constitute 73 % of all observed meteorite falls, cannot be matched with the typical reflectance spectra of any
common asteroid taxonomic type, the so called spectrophotometric paradox (Wetherill & Chapman,1988). In fact,
despite many efforts, no unambiguous OC assemblages have yet been identified, although most other meteorite classes
(e.g. irons, stony-- irons, basaltic and olivine achondrites, and the different types of carbonaceous chondrites) have good
analogs among the asteroids. Therefore, where do the OC meteorites come from? The only asteroids which so far have
been spectrally identified as probable OC assemblages are the small Q--type objects in the Earth--approaching minor
planets population (Apollo, Amor, Aten groups). They are good potential sources for Earth destined OC meteorites
(Wetherill, 1987), but they are dynamically short lived and the population must be replenished, on time scales which
are very short compared to the age of the solar system, from some long­lived source regions. The Q--class so far consists
for certain of only a single member, (1862) Apollo (Bell et al., 1989; Pieters & McFadden, 1994), even though several
Earth--crossing asteroids may belong to this class. This apparent uniqueness provided little information about the
significance to the structure of the solar system of this abundant meteorite type. The origin of the OCs is one of the
most important and perplexing problems in planetary science; several solutions to this dilemma have been suggested:
-- the analysis of asteroidal spectral data is systematically wrong and some unknown regolith process is modifying
an ordinary chondrite­type substrate to produce the S spectra (Gaffey et al., 1989). The most carefully studied
S asteroids are differentiated surfaces assemblages. Although the data does not exclude the possibility of a small
component of undifferentiated bodies within the S population, there is no observational evidence to support the
interpretation that S--type asteroids should be the parent bodies for the OC, irrespective of plausible calibration
uncertainties or regolith processes.
Send offprint requests to: M. Di Martino
? Based on observations carried out at the European Southern Observatory, La Silla, Chile

2 M. Di Martino et al.: 1993 VW: an ordinary chondrite­like near­Earth asteroid
-- In a model proposed by Bell (Bell, 1986; Bell et al., 1989) the OC parent bodies would be small primordial inner
main belt objects below the current observation limits and therefore till now scarcely studied (Gaffey et al.,1989;
McFadden et al., 1985), in a favorable location, perhaps adjacent to the 3:1 Kirkwood gap or in Earth­crossing
orbits. These objects escaped the heating episode that produced abundant igneous assemblages among the larger S
asteroids; the post­accretionary heating mechanism had a lower size cut­off, below which objects escaped heating.
Some researchers (Bell et al., 1989) suppose that, once opportunely observed, the majority of small asteroids will
show characteristics similar to those of the OCs.
-- Gaffey (1984) suggested that OC assemblages were expelled from the inner solar system by very early dynamical
processes to an external reservoir and that now they might be absent from the main belt at any size The process
is similar for comets, which are now returning from such an external reservoir, the Oort cloud. Returning rocky
bodies could be captured by Jupiter into short­period orbits just as cometary nuclei are, or be involved in collisions
with main belt objects, injecting some fraction of their fragments in short­period orbits.
-- OCs may have been generated by a small number of big, resonance--bordering asteroids, such as S--type asteroid (6)
Hebe, (Farinella et al., 1993; Morbidelli et al., 1994; Broglia et al., 1994); the physical properties of their surfaces
may have been changed during billions of years of exposure to solar radiation, cosmic rays and interplanetary
dust. In this case the main belt asteroid (3628) BoŸznŸemcov'a, which shows a spectral reflectance (Binzel, 1993)
very similar to that of the OCs, may be simply a fresh fragment coming from the inside of a big object following
a catastrophic collision (its young surface would not yet be optically changed). This small (about 7 km) body is
located close to the 3:1 Kirkwood gap and ejecta from this asteroid could readily be transfered into the chaotic
zone associated with the resonance and thence into Earth­crossing orbits. This hypothesis may provide a solution
for the spectrophotometric paradox.
On the basis of a systematic spectral analysis recently carried out on a large subset of the S--type asteroid population,
Gaffey et al. (1993) suggest a possible answer to the identification of the OC parent bodies. Resuming the previous
conclusions, they argue that the most large main belt S asteroids cannot be the parent bodies of OC; only the S(IV)
subtype, among the large S asteroids which have the lowest spectral slope, are the most likely candidates for main
belt OC parent bodies. Among these there are (3) Juno (slope = 0.053), (6) Hebe (0.17) and (7) Iris (0.22). The
OC parent bodies could constitute only about 10 % of the larger members of the S asteroid population. Gaffey et al.
(1993), combining the results by Bell (1993) and Binzel et al. (1993), support the model of Bell et al. (1989), which
proposed that OC originate from small primordial main belt asteroids rather than from fragments coming from the
larger bodies.
With these motivations and following the suggestion by Binzel & Bus (1994), which observed 1993 VW in the range
0.4--1.0 ¯m, we carried out spectroscopic observations of the asteroid 1993 VW at the European Southern Observatory
(ESO, La Silla, Chile).
Last year (The Hague, 1994), in a preprint presented in the poster session of the XXII General Assembly of the
International Astronomical Union, appeared that also the Amor asteroid 1992 LR is a probable Q class object (Williams
et al., 1994).
2. Observations and data reduction
Observations were performed at ESO, when 1993 VW was in a favorable apparition, on 17 April 1994, using the 1.52--m
telescope equipped with a Boller & Chivens spectrograph and a CCD 2048 \Theta 2048 (windowed at about 300 \Theta 2048)
as a detector. The grating used was a 225 gr/mm with a dispersion of 330 š A/mm in the first order. The CCD has a
15 ¯m square pixel, yielding a dispersion of 4.9 š A/pixel in the wavelength direction. The useful spectral range is from
about 4500 š A to 9200 š A with an instrumental FWHM of 9.8 š A. A single exposure of 35 minutes was obtained at an
airmass of 1.01.
Table 1. Aspect data of the observed asteroid
Date R.A. Decl. Long. Lat. r Delta Phase V
[0 UT] [2000.0] [2000.0] [AU] [AU] [deg] [mag]
1994 04 17 18 48.9 ­37 05.6 279.4 ­14.1 1.02 0.177 64.3 15.35

M. Di Martino et al.: 1993 VW: an ordinary chondrite­like near­Earth asteroid 3
The aspect data of 1993 VW referred to the date of observation are listed in Table 1. In addition to 1993 VW, the
solar analog stars (HD 144585, HD 76151, HD 44594, HR 6060), the spectral standard L970­30, flat fields and bias
images, and lamp spectra (He, Ar) were all observed as simultaneously as possible using the identical instrument
configuration. The spectrum was reduced using the software packages IRAF (Image Reduction Analysis Facility)
installed on ULTRIX workstation according to the standard procedures described by Luu & Jewitt (1990) and Vilas
et al. (1993). The procedures include:
1. subtraction of bias level: five biases were weighted in order to increase the signal­to­noise ratio and to remove
cosmic rays on the final bias. After it the mean of the resulting images was computed and subtracted to each
images (flat fields too);
2. flattening of data: ten flat fields have been used. On the weighted image a spline function (of about 35 order) was
interpolated and these two frames were divided each other. The final frame was divided with all spectra to remove
spatial inhomogeneity of the CCD;
3. to eliminate the cosmic rays a procedure which detects and replaces many cosmic rays with the mean of neighbouring
pixels was used. The cosmic rays located near the spectrum and near the sky lines were each singularly dealt with
the IMEDIT package;
4. subtraction of sky: this step was done in interactive way, using a spline function;
5. wavelength calibration: we used the frame of the calibration lamp and the RMS was about 0.20 yielding to an error
of about 1 š A in the wavelength direction;
6. collapsing the two dimensional spectra: this was done interactively to control that the signal was extracted in a
proper manner;
7. extinction correction: this is an automatic step. The package needs the place, the extinction file with the coefficient
and automatically works;
8. asteroid spectrum: the resulting spectrum of the asteroid was divided for different solar analogs (Hardorp 1978,
1980a, 1980b, 1981, 1982).
3. Results
Figure 1 shows the reflectance spectrum of 1993 VW (bottom), and for comparison the laboratory spectrum of the
powdered OC LL4 Soko--Banja meteorite (top), taken from Gaffey (1976). A strong absorption feature, characteristic
of a mixture of olivine and piroxene (Cloutis et al., 1986; Gaffey & McCord 1978), appears in the red part of the
spectra, while a weak absorption band, centered at about 0.65 ¯m, is also visible in both the spectra. The nature of
this feature is controversial. Burns et al. (1973) attribute it to spin forbidden Fe 2+ transition, while Mao & Bell (1972)
to an Fe 3+ --Fe 2+ charge transfer. The ratios between the spectrum of the analog stars HR 6060 and that of HD 44594,
HD 76151 and HD 144585, respectively, differ of about 10% (Fig. 2).
Fig. 1. Optical reflectance spectrum of 1993 VW obtained on 1994 April 17.4027 divided by HR 6060 solar analog (bottom)
compared with the spectrum of the LL4 OC Soko­Banja meteorite (top).
The spectrum of 1993 VW, as well as that of the Q--type asteroid 1862 Apollo, is practically identical to the spectrum
of OCs, and in particular to LL4 Soko­Banja meteorite (see also Lipschutz et al., 1989). Both spectra, shown in Fig. 1,
are characterized by an evident absorption feature longward 0.6 ¯m and shortward 0.7 ¯m (see also McFadden et al.,
1989). The absence of spectral coverage besides 9200 š A limits the mineralogical interpretation. In fact, for example,
at 2 ¯m the piroxene shows a strong absorption band.

4 M. Di Martino et al.: 1993 VW: an ordinary chondrite­like near­Earth asteroid
Fig. 2. The ratios between the spectrum of the analog stars HR 6060 and that of HD 44594 (top), HD 76151 (center) and HD
144585 (bottom). Differences are about 10% in the red part of the spectrum.
Moreover, the spectrum of another near--Earth asteroid recently observed, the Amor 1992 LR, is very similar to that of
the supposed Q--type asteroid 1862 Apollo (Williams et al., 1994). The orbits of 1993 VW and 1992 LR are currently
outside the main resonances, which provide the transport routes from the main belt to near­Earth orbits, that is the
3:1 and 4:1 mean motion Jovian resonances and the š 6 secular one. Both of them probably evolve in a random­walk
way, owing to encounters with the inner planets. However, the evolution of 1993 VW is probably much faster, because
at perihelion it can approach the Earth, whereas 1992 LR can encounter only Mars, which is much less effective due to
its smaller mass. The strongly chaotic nature of this type of orbits unfortunately erases quickly any memory of their
past evolution, and thus it is impossible to say, on the basis of a purely dynamical ground, whether these NEAs (plus
Apollo) come from the fragmentation of a common parent asteroid in the main belt, or have completely independent
origins (Farinella, 1994). Hopefully, more physical data will help in clarifying this important issue.
Acknowledgements. We are grateful for valuable comments and suggestions to the reviewer, G.W. Wetherill, and to C. Barbieri
for useful discussions. This work has been partially supported by grant ASI--94--RS--69 of Italian Space Agency (ASI).
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