6 Discussion

This section compares our VLT data with those of merged sets compiled from the literature. It describes the possible group characteristics of the objects from color-color plots and outlines suggestions deduced from spectral gradient histograms of the objects.

6.1 Color-color plots

The color-color plots of the KBOs and Centaurs measured at the VLT (see Fig. 1) suggest the following:

The KBO color distributions

appear to be rather uniform over the reddening range of $B-V \sim 0.55$-1.1 mag, $V-R \sim 0.35$-0.7 mag and $R-I \sim 0.3$-0.7 mag, respectively. A bimodal distribution (Tegler & Romanishin 1998, 2000) is not at all obvious in our data. A similar conclusion on the likely absence of a bimodal B-V versus V-Rdistribution of KBOs is already mentioned by previous photometric studies (Barucci et al. 1999; Davies et al. 2000; Boehnhardt et al. 2001). The question of the bimodality is discussed in detail in Hainaut & Delsanti (2001) on the basis of the whole photometric database currently available (including those presented here) and with the help of different statistical tools. In summary, they demonstrate that, depending on the color index considered, there is either no bimodality at all, or that the limits of the bimodality are not statistically significant.

The reddening of the objects

seems to be constant over the B-V and V-R wavelength range for a given object. Some objects show a deviation in the B band; it must however be noted that these deviations are compatible with the larger error bars, caused by a lower S/N ratio for the observations in this filter. The gradient systematically drops to smaller values for the R-Icolor. This is indicated by i) the close alignment of the data points with the reddening line (see Fig. 1) in the V-Rversus B-V plot, and ii) the drop of the data points below the line of constant reddening in the R-I versus B-V plot for the red objects. The latter effect suggests that the red objects have a flatter spectral gradient towards the far red and near-IR wavelength range while the neutral ones keep the reddening slope rather constant over the whole wavelength range measured. A similar result was already reported by Boehnhardt et al. (2001).

6.2 Comparison of VLT data and merged dataset from literature

This paper's dataset has been merged with a complete compilation of the TNO colors published in the literature (totalling $\sim$100 objects); this compilation is presented and analyzed with a set of statistical tools in Hainaut & Delsanti (2001). In this section, we shall highlight some of the comparisons between the VLT observations presented here and the whole, multi-telescope, multi-observer dataset (of which this dataset represent about 25%).

Color-color diagrams and Dispersion of the data:

the general aspect of the color-color diagrams is very similar. In particular, the reddest objects in the B-V, R-I and V-R, R-I also show a general trend to be lower than the reddening line (i.e. reflectivity spectrum less red at the longest wavelengths), as described in the previous paragraph. The general location and dispersion of the data is similar, indicating that both samples are compatible, i.e. no dramatic systematic effect is biasing some groups.

Outliers:

both samples show some outliers, i.e. objects located very far from the general flock of objects group around the reddening line. In particular, 1998 SN₁₆₅, the most extreme outlier from our sample, was measured with similar colors by Gil-Hutton & Licandro (2001).

Bimodality and statistical tests:

the sample we present here, as well as the merged dataset, do not present obvious bimodality in the color distribution. Hainaut & Delsanti (2001) have performed several statistical tests on the merged dataset, showing that there is no statistical support for a bimodal distribution (i.e. it is very unlikely that the observed distribution can be extracted from a bimodal distribution). The same tests, performed on this sample, would give the same negative result, but with a smaller significance.

6.3 Spectral gradient statistics on merged dataset

In Fig. 3, we show the distribution of the spectral gradients of KBOs and Centaurs. The histograms were compiled in the same way as described in Boehnhardt et al. (2001). The dataset used is composed of the measurements presented in this paper, combined with the input of Table 5 in Boehnhardt et al. (2001). The histogram bins are 10%/100 nm wide which is the mean uncertainty of the object spectral gradients.

The bulk of KBOs falls into the reddening range of 0-40%/100 nm with the maximum objects towards the red end of the distribution. The Centaurs may have a distinctly different distribution of spectral gradients, since the distribution "peaks'' between 10-20%/100 nm. Considering the three dynamical groups among KBOs separately, it is obvious that the Cubewanos resemble best the global trend in the spectral gradient histograms of all KBOs, while the Plutinos seem to follow more the Centaurs' trend, i.e. with a peak towards smaller reddening gradients. A second "population'' of Plutinos has very red spectral slopes, while the range of the Cubewano peak is poorly occupied among the Plutinos. It is too early to draw conclusions about the group properties of scattered disk objects since only 8 objects have been measured so far. For the same reasons of low statistical significance, the suggestions concerning the Centaur and Plutino population distribution peaks have to be viewed with caution.

Figure 3: Spectral gradient histograms. Upper panel: all TNOs and Centaurs, lower panel: Cubewanos, Plutinos, scattered disk objects and Centaurs. The histogram shows the number of objects per spectral gradient interval. The spectral gradient is given in %/100 nm, the intervals are 10%/100 nm wide which reflects the mean uncertainty of the spectral gradient data of the object sample. The tables in the bottom of the panels list the number of objects per spectral gradient interval for the various groups considered.

  
6.4 Surface processing

The evolution of the surface of the objects is probably the result of the balance of different processes. To date, the suspected processes are:

Aging:

the surface of the objects, believed to be covered with organic-rich water ices, is irradiated by high-energy particles (cosmic rays ions, solar particles, etc.) and undergoes a progressive darkening due to the loss of hydrogen atoms, polymerization, and to aromatization of the mantle (Sagan et al.  1984; Johnson 2001; Moroz 2001). The time-scale "for a significant radiolytic evolution of the optical surface of a Kuiper Belt Object can vary from millions of years in the region of highest anomalous cosmic ray (ACR) intensity to a billion years elsewhere due to the modulated galactic cosmic ray flux'', Johnson (2001). The possible effect of radiation on reflectance depends also on the dose. An initial bright ice mixture, which displays neutral colors, high albedo and a flat visible spectrum, will evolve into a material with a red spectrum at medium dose. Subsequently, this could lead to a neutral dark material with a lower albedo and a flat spectrum at a higher dose (Johnson 2001). An interesting question appears: can a surface element of an object remain undisturbed long enough to evolve from dark-red to neutral-black?

Collisions:

impacts on the surface (Stern 1996) could modify the reflectance of the objects by excavating fresh material (i.e. non irradiated, with a neutral reflectance spectrum) from the interior. The resulting color is then the average of the surface, including the undisturbed aged ice and the freshly excavated ice (Luu & Jewitt 1996).

Cometary activity:

the active area can present fresh ice, possibly mixed with a rubble mantle. It is also possible that dust trapped by the gravity of the objects will re-cover part - or all - of the non active surface. The importance of cometary activity depends on the heliocentric distance of the object and on the thickness of the irradiated (and/or dust) mantle. However, observational evidence indicate that activity is routinely present at 15 AU (Meech 1999). Activity could also be possible beyond 20 AU, as suggested by observations reported by Hainaut et al. (2000) and Fletcher et al. (2000) (although this result is only marginally significant), and by models presented by Prialnik (2001).

These three processes effect either the objects as a whole (aging) or only a fraction of the surface (non-disruptive collisions). However, at all times of its evolution, an object surface element will present a straight reflectivity spectrum at visible wavelengths (cf. laboratory spectra, Thompson et al. 1987). As a consequence, the average reflectivity spectrum of the object is also straight, explaining why all the objects are within their error bars on the reddening line (which represents the locus in a color-color plot of objects with a linear reflectivity spectrum).

6.5 Overview of the objects

Figure 4: Reflectivity spectrum of 1994 TB during the two runs. The reflectivity is normalized to 1 for the V filter; the spectra have been arbitrarily shifted for clarity. The dotted line is the linear regression over the V, R, I range, corresponding to the gradient $\cal S$. The change of mean slope (from 44.9 to 28.1%/100 nm between the 2 runs) is the result of significant changes in the colors, indicating that this variation is real.

   

Table 5: Overview of the objects and comparison with data available in the literature.

Objects	(1)	Our data				Other published
		cf	Gradient	(2)	Comparison with other data(3)	information
		text	Color
1993 SB	Plut		Neutral		V-R compatible with Gil01	Dav00: V-J color
					B-V, V-R compatible with Teg00
					V-R, R-I comp. with Lag00 (1$\sigma$)
					R mag compatible with Wil95
1994 TB	Plut		Red		BVRI comp. with Luu96 & Teg97	Dav00: V-J color
				v	B-V, V-R: good agreem. with Bar99
					V-R, R-I comp. with Lag00 (1$\sigma$)
					V-R compatible with Teg98
1995 SM55	QB1		Neutral		V-R compatible with Gil01	Boe01: visible spectrum
			Bluish	v	BVRI colors compatible with Boe01	Hil01: J light-curve
					(Run 2 at 1.5$\sigma$ level)
1995 TL8	Scat		Red		First published
1996 RQ20	QB1		Red	v	BVRI colors compatible with Boe01
					V-R 0.17 mag> Teg98 (1.5$\sigma$)
1997 QH4	QB1		Neutral		Teg00: B-V equal, V-R compatible (1$\sigma$)
					R-I first published
1997 QJ4	Plut		Neutral		V-R in agreement with Gil01	Dav00: V-J color
			Bluish		B-V, R-I first published
1998 BU48	Scat		Red		First published	She01: light-curve
1998 SG35	Cent		Neutral		Dor01: V-R, R-I compatible; Our
					B-V<0.2 mag (margin. signif.)
1998 SM165	QB1		Red		B-V, V-R margin. comp. with Teg00
1998 SN165	QB1		Neutral		V-R compatible with Gil01
			Bluish		Dor01: V-R, R-I margin. compatible;
					Our B-V<0.25 mag (margin. signif.)
1998 TF35	Cent		Very red		B-V, V-R compatible with Boe01
1998 UR43	Plut		Neutral	v	V-R <0.13 mag than Gil01
			Bluish
1998 WH24	QB1		Red		B-V, V-R compatible with Teg00	Bro01: "Pholus-like''(4)
					V-R marginally comp. with Boe01	near-IR spectrum
1998 WV31	Plut		Neutral		First published
1998 WX31	QB1		Red		First published
1999 CC158	Scat		Neutral		First published
1999 CD158	QB1		Neutral		First published
1999 CF119	Scat		Neutral	v	First published
1999 DE9	Scat		Neutral		First published	She01: light-curve
						Bro01: "Pholus-like''(4) spec.
1999 OX3	Scat		Neutral		R-I first published	Dor01: B-V, V-R
1999 RZ253	QB1		Red		VRI first published	Dor01: B-V
						Bro00: near-IR spectrum
1999 TC36	Plut		Red		BVRI colors compatible with Boe01	Bro00: near-IR spectrum
					Dor01: B-V, V-R compatible;
					Our R-I <0.1 mag (1$\sigma$)
1999 TD10	Scat		Neutral		V-R compatible with Con00	Bro01: "Pholus-like''(4) spec.
1999 UG5	Cent		Red		In agreement with Gut01	Gut01: R light-curve
					and marginally with Pei01
2000 OK67	QB1		Neutral	v	First published
2000 QC243	QB1		Neutral		First published
			Bluish

(1) Class: QB1 = Cubewano, Plut = Plutino, Scat = Scaterred Disk Object, Cent = Centaur.
(2) Object displaying a variability during the VLT observations.
(3) Abbreviation code for the references: Bar99 = Barucci et al. (1999), Boe01 = Boehnhardt et al. (2001), Bro00 = Brown (2000), Bro01 = Brown (2001), Con00 = Consolmagno et al. (2000), Dav00 = Davies et al. (2000), Dor01 = Doressoundiram et al. (2001), Gil01 = Gil-Hutton & Licandro (2001), Gut01 = Gutiérrez et al. (2001), Hil01 = Hillier et al. (2001), Luu96 = Luu & Jewitt (1996), Lag00 = Lagerkvist et al. (2000), Pei01 = Peixinho et al. (2001), She01 = Sheppard & Jewitt (2001), Teg97 = Tegler & Romanishin (1997), Teg98 = Tegler & Romanishin (1998), Teg00 = Tegler & Romanishin (2000), Wil95 = Williams et al. (1995).
(4) ``Pholus-like'' spectrum : characterized by 2 features of water ice and a feature of methanol, Brown (2001).

In this section, we compare our color values with those reported in the literature. We also mention the other information available for these objects. This study is presented in Table 5; some additional comments can be found below for some of the objects.

1994 TB: there is a significant decrease of brightness by 0.25-0.30 mag between September and November (in R and I band), which is about 2 times the error bars. This could be due to rotational effects. To date, no light-curve has been published. We present the reflectivity spectra at the two different epochs in Fig. 4. It is also interesting to note that the colors and spectral gradient of the object changed significantly between the 2 epochs, suggesting a variegated surface. This object deserves a detailed multi-color light-curve.

1995 SM₅₅: the absolute magnitude is marginally brightening by 0.07 magnitude (including a solar phase effect correction of $-\beta\alpha$ of 0.008 mag/ $\hbox{$^\circ$ }$). The gradients are compatible between the two periods (cf. Table 3).

It is interesting to note that Hillier et al. (2001) observed 1995 SM₅₅ several times in J band over the Oct. 1999-Jan. 2001 period. The object's J light-curve is compatible with a constant, except for rapid variations (about 0.4 mag in a few hours) on 8 October 1999 and during January 2000, and for a brightening of almost 1 mag on 22 October, 1999. While the brightening could possibly be explained by mediocre observing conditions, the authors affirm that the rapid changes seen over the January 2000 period are real and that they must correspond to a physical process occurring on the object.

Our observations, corrected for V-J, are compatible with the "quiescent'' phase of the object. Considering the neutral-bluish colors of the object, the short time-scale of the variations observed and their seemingly random behavior, 1995 SM₅₅ could possibly host some cometary activity, and deserves a detailed study.

1996 RQ₂₀: displays a marginally significant variability in R over half an hour.

1998 BU₄₈: no variability within the same night. Sheppard & Jewitt (2001) report a light-curve with an amplitude of 0.4 mag and a period larger than 4 hours.

1998 UR₄₃: significant variation in R band of $\sim$0.3 mag over $\sim$1 hour suggesting a light-curve effect. Our V-R value is 0.13 lower than that of Gil-Hutton & Licandro (2001). As our measurements is right on the reddening line, it is possible that their color was affected by the rotation.

1999 CF₁₁₉: shows a variability of 0.1 mag in the Rband over half an hour, suggesting a rotational effect.

1999 DE₉: Sheppard & Jewitt (2001) report a light-curve constant within 0.05 mag over few hours. Jewitt et al. (2000) reported a spectrum showing water ice features; however, this result has not been published yet.

1999 UG₅: a light-curve in R band has been published by Gutiérrez et al. (2001). The authors reported noticeable changes in brightness (0.2 mag) over 5 hours, which may be interpreted as the rotational light-curve (they cannot discriminate whether the variations are caused by the shape or by an albedo feature). Other causes are still possible, although cometary activity does not seem very likely.

2000 OK₆₇: the repeated observations in V band indicate a significant variation of 0.2 mag between the two runs.

6.6 Discussion

As explained in Sect. 5.4, the locus of objects with a linear spectrum (but with different spectral slopes) is the "reddening line'' shown in the color-color plots (Fig. 1) for a slope $\cal S$ ranging from -10 to 70$\%$/100 nm. The objects presented in this paper are located along this line (within their error bars, see Fig. 1), indicating that they display linear reflectivity spectra over the visible (VRI) wavelengths.

We could expect that an object undergoing an aging process caused by irradiation (i.e. reddening of its surface, cf. Sect. 6.4) may move along this line toward redder colors. At even higher irradiation dose, objects display a neutral but darker reflectivity spectrum (Thompson et al. 1987; Johnson 2001), which means that they should move back down toward more neutral colors along this reddening line on the color-color plots. It is however unclear if a TNO can remain undisturbed long enough to evolve into a dark neutral colored object.

More generally, the possible path of a generic Kuiper Belt object in a color-color plot during its life has still to be determined, possibly by the modeling of the contribution of the different evolution processes involved (i.e. not only by the aging process). We can also consider that bluish objects are likely to be covered by fresh ice (in the case they are not covered by high-irradiated neutral-dark material Thompson et al. 1987), and are therefore more likely to host some cometary activity. As neutral objects could therefore have very young surfaces, with possible cometary activity, or very old surfaces with a very low albedo, we recommend observers to devote telescope time to these objects, especially with instruments that could directly measure their albedo (e.g. the thermal emission) such as SIRTF, or detect a cometary activity (e.g. very high spatial resolution with a large collecting area.)