A&A 378, 653-667 (2001)
H. Boehnhardt1 - G. P. Tozzi2 - K. Birkle3 - O. Hainaut1 - T. Sekiguchi1 - M. Vair1 - J. Watanabe4 - G. Rupprecht5 - The FORS Instrument Team6,7,8
1 - European Southern Observatory ESO, Alonso de Cordova 3107, Santiago de Chile, Chile
2 - Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 50125 Firenze, Italy
3 - Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
4 - National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
5 - European Southern Observatory ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
6 - Landessternwarte Heidelberg, Königstuhl, 69117 Heidelberg, Germany
(FORS Principle Investigator)
7 - Universitäts-Sternwarte Göttingen, Geismarlandstr. 11, 81679 Göttingen, Germany
8 - Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany
Received 10 January 2001 / Accepted 21 August 2001
We present visible (BVRI) and near-IR ( ) broadband photometry and visible low-dispersion spectroscopy of Transneptunian Objects (TNOs) and Centaurs. In total, 16 TNOs and 1 Centaur were observed over the past two years at ESO telescopes in La Silla and Paranal in Chile as well as at the Calar Alto Observatory in Spain. The sample consists of objects measured for the first time and those for which comparison data is available from literature. The targets were: 1992QB1, 1993RO, 1994EV3, 1995HM5, 1995SM55, 1996RQ20, 1996TL66, 1996TO66, 1996TP66, 1997CQ29, 1997CS29, 1998HK151, 1998TF35, 1998VG44, 1998WH24, 1998XY95, 1999TC36. The spectra of 5 TNOs (1995SM55, 1996TO66, 1997CQ29, 1997CS29, 1998HK151) show almost constant gradients over the visible wavelength range with only marginal indication for a flatter slope beyond 750-800 nm. The photometric colour gradients obtained quasi-simultaneously are in good agreement with the spectral data. This suggests that in general photometric colour gradients are a valuable diagnostic tool for spectral type classification of TNOs. The photometric study revealed a number of new objects with neutral and red colours. For re-measured objects the published broadband colours were - in general - confirmed, although a few remarkable exceptions exist. Two TNOs appear to be outlyers according to the available broadband colours: 1993EV3 and 1995HM5. 1995SM55 is the bluest TNO measured so far. No clear global correlation between V-I colour and absolute R filter brightness of our TNO targets is found. However, the data for the 5 brightest TNOs (brighter than 5 mag absolute magnitude) could also be interpreted with a linear increase of V-I colour by about 0.75 mag per brightness magnitude. The colour-colour diagrams show continuous reddening of the TNOs in V-R vs. B-V, R-I vs. B-V and R-I vs. V-R. The bimodality suggested from earlier measurements of Tegler & Romanishin (1998) is not confirmed. According to our colour gradient statistics (number of objects per gradient interval) most of the TNOs have surface reddening between 0 and 40%/100 nm. For the Cubewanos the major population falls between 20-40%/100 nm. The Plutinos and Centaurs show a bifold grouping, i.e. a neutral/slightly reddish group (reddening <20%/100 nm) and a red group (reddening 30-40%/100 nm). The statistical significance of the various populations found is suffering - for the Centaurs and scattered disk objects very severely - from the small number of objects measured. However, the diversity of the reddening distribution of Centaurs/Plutinos and Cubewanos, if confirmed by new observations, may indicate a different balancing of resurfacing processes for these object types: for instance, for Centaurs a possibility is that re-condensed frost from coma activity may be dominant over impact re-surfacing and high energy surface processing.
Key words: transneptunian objects - cubewanos - plutinos - scattered disk objects - Centaurs - photometry - spectroscopy
The number of discovered Transneptunian Objects (in the following TNOs) and of Centaurs has steadily - over the past 3 years even rapidly - increased. At present more than 400 TNOs and about 25 Centaurs are known, a significant part still with uncertain orbits.
TNOs are believed to populate the Edgeworth-Kuiper Belt (which was first hypothesised by Edgeworth 1949 and Kuiper 1951). TNOs are classified in three dynamical groups (Levison 2001): the Plutinos are captured in stable orbits in the 2:3 resonance with Neptune. The Cubewanos occupy the orbits between the Plutinos and the 1:2 resonance with Neptune. Scattered disk objects are found in highly elliptical orbits with semi-major axes between about 40 and several hundreds Astronomical Units. They appear to be TNOs that are scattered out of the Edgeworth-Kuiper Belt by the giant planets.
Centaurs are believed to be TNOs which migrated towards the Sun by gravitational interaction with the major planets. They may finally become short period comets as member of the Jupiter family or they may be scattered (as long-period or Oort cloud comets) into the outskirts of the Solar System (Levison 2001). A recent analysis of the comet radius distribution further supports this evolution track: Meech and collaborators (priv. comm.) showed that the observed radii are not following a power law as it would be expected if the short-period comets were directly injected in the inner Solar System by a collision process.
Since the TNOs never came really close to the Sun, they are considered to represent an old and physico-chemically unaltered population of Solar System bodies. In this respect, TNOs may be even more pristine than Centaurs and short-period comets which both show gas and dust emission acitivity and which may thus no longer have an original surface layer constitution (Meech et al. 1997). The knowledge of physical parameters of TNOs and Centaurs suffers from the faintness of the objects which calls for observations with large telescopes on the ground or for observations with the Hubble Space Telescope (HST). Size estimates exist from coarse single or two colour photometry obtained by search programs. More accurate multi-colour photometry of about 50 TNOs in the visible wavelength range is published, near-IR colours of TNOs are rarer (about 15 objects published). 6 Centaurs have visible and near-IR broadband colours measured, a few more objects of at least BVRI colours measured.
For a more recent compilation of the published photometry data of TNOs and Centaurs see Barucci et al. 1999a and 1999b (even though an update of the tables with the latest results is needed - see below).
Obviously, TNOs and Centaurs exhibit different reddening in their broadband colours which may be indicative for different physical processing of their surfaces. At present, the existence of two TNO groups with distinct colours appears to be controversial (Tegler & Romanishin 1998, 2000; Davies et al. 2000; Barucci et al. 2000; Trujillo et al. 2000). Recently, a paper by Barucci et al. (2001) outlined a new approach for the taxonomic classification of TNOs and Centaurs that follows the statistical analysis for the detection of asteroid types using photometric data.
Spectra of a handful TNOs are available in literature: while the visible spectra of TNOs (Jewitt & Luu 1998) appear featureless, absorption bands of water and hydrocarbon ices were found in one TNO in the near-IR (1996TO66; Brown et al. 1997, 1999; Noll et al. 2000).
The Centaurs have either neutral or very red colours. Spectroscopy (5 objects published, Barucci et al. 1999a) in the visible and near-IR revealed mostly reflectance spectra, but in one case also CN gas emission (Meech et al. 1997). H and K band water ice absorptions were also reported (Cruikshank et al. 1998).
Hitherto, the existing database did not yet allow to draw firm conclusions on a generally accepted taxonomic classification scheme for TNOs and Centaurs. In the following sections we present photometry (visible and near-IR) of 16 TNOs and 1 Centaur as well as visible spectra of 5 TNOs. The observations are part of a co-ordinated program for the exploration of physico- chemical properties of TNOs and Centaurs, executed by our group at observatories in Chile, Spain and USA.
|Observ. Period||Observatory||Filters||Imaging Objects|
|Telelescope + Instrument||Spectral Range (nm)||Spectroscopy Objects|
|24-26 Aug. 1998||ESO Paranal||img.: BVRI (Bessell)||img.: 1993RO, 1996TL66, 1996TP66|
|VLT UT1 + TC|
|19-20 March 1999||ESO La Silla||img:||img.: 1995HM5, 1997CQ29, 1997CS29|
|3.6 m NTT + SOFI|
|13-15 May 1999||ESO Paranal||img.: VRI (Bessell)||img.: 1994EV3, 1997CQ29, 1998HK151|
|VLT UT1 + FORS1||spec.: 590-1000||spec.: 1997CQ29, 1998HK151|
|2-6 Dec. 1999||ESO Paranal||img.: BVRI (Bessell)||img.: 1992QB1, 1995SM55, 1996RQ20,|
|VLT UT1 + FORS1||1996TO66, 1997CS29|
|spec.: 350-1000||spec.: 1995SM55, 1996TO66, 1997CS29|
|3-6 Jan. 2000||Calar Alto||img.: BVRI (Johnson+special)||img.: 1997CS29, 1998TF35, 1998VG44,|
|3.5 m + MOSCA||1998WH24, 1998XY95, 1999TC36|
The observations were collected in the visible and near-infrared (near-IR) wavelength ranges during 5 runs between 1998 and early 2000 at the ESO observatories in La Silla and Paranal and at the Calar Alto Observatory in Spain (Table 1). In La Silla we used the 3.6 m New-Technology-Telescope (NTT) equipped with the near-IR instrument SOFI. At Paranal we used the 8.2 m UT1 Telescope of the ESO Very Large Telescope VLT. In 1998 UT1 was equipped with the VLT Test Camera (TC), in 1999 we used the FORS1 instrument. Both the TC and FORS1 work in the visible wavelength range. The Calar Alto data were also collected in the visible wavelength range at the 3.5 m telescope using the MOSCA focal reducer. The filters and wavelength ranges used during the various observing runs are listed in Table 1. Details on the telescopes and instruments can be found on the web pages of these observatories. The log of the observations is given in Tables 2 and 3. Note: at the ESO telescopes we used filters from the Bessell system for imaging, while at Calar Alto Johnson-type BVI filters and a special R passband filter with steep cut-off near 750 nm were installed. The objects were selected with the goal to contribute new and/or improved broadband colours in the visible and near-IR wavelength range for the taxonomic characterisation and classification of TNOs and Centaurs.
The spectroscopy part focused on a subset of the objects imaged with broadband filters. The spectra (all taken with FORS1 at the VLT) were obtained either immediately before or after the filter imaging of the respective objects. All images and spectra were taken under photometric conditions. During imaging the telescopes were used in sidereal tracking/auto-guiding mode. During spectroscopy differential tracking/auto-guiding was applied in order to keep the objects on the 1 wide slit.
Brightness variations due to rotation of the objects can happen in time intervals as short as a few hours. At the VLT all three or four colours (B)VRI of the objects were measured within a short time interval (typically within 20-25 min) at least in one night in order to minimise possible brightness variations due to rotation. The 3.5 m Calar Alto photometry had longer execution times per object. Therefore, in most cases sequences of consecutive exposures in RBRVIR filters were used for the sampling of the rotation variability.
The near-IR images were taken through filters. A similar strategy as of long lasting BVRI sequence was used during the exposure series. Random jitter offsets of the telescope were applied during the filter exposure series.
In addition to the object exposures also photometric and spectrophotometric standard stars, flatfield, arc spectra and bias exposures were taken as needed for the calibrations of the data.
In the following subsections the data reduction steps are described separately for imaging photometry in the visible and near-IR as well as for visible spectroscopy.
FORS1 and MOSCA: basic reduction steps - the usual CCD
"cosmetic'' data reduction, i.e. bias subtraction, flat field
correction and cosmic ray filtering, was at first applied
to all the object frames (science objects and standard stars). As bias
image, the median of all bias images taken at the beginning and
at the end of the nights was used, since no significant
changes were found from the frames taken at different times. As
flat field images we used the average of several twilight sky
images, normalised to 1. To reduce the cosmic ray signatures
we filtered the bias and flat field corrected object images with
a 33 pixels wide median filter. The difference between
unfiltered and filtered images showed that no other small-scale
structures, but only cosmic ray events were erased from the object images. Sky
correction was performed by subtracting a first order polynomial
sky approximation computed from the pixel areas containing no stars.
Object identification - the TNOs/Centaurs were then identified by blinking
two images recorded at different times. As we observed only objects
whose orbit is well known, this ensures proper identification.
|1992QB1||1999 Dec. 05.022||300+400||V||23.69||0.08|
|1993RO||1998 Aug. 24.350||600||V||24.37||0.08|
|1994EV3||1999 May 13.989||300||V||23.91||0.13|
|1995HM5||1999 Mar. 20.300||3120||J||22.39||0.29|
|1995SM55||1999 Dec. 02.026||300||V||20.54||0.03|
|02.045||1200||590-1000 nm||Fig. 1||red only|
|1996RQ20||1999 Dec. 04.031||200+400||R||22.92||0.05|
|1996TL66||1998 Aug. 24.250||600||B||21.81||0.07|
|1996TO66||1999 Dec. 03.020||200||V||21.62||0.03|
|03.035||2x1200||350-1000 nm||Fig. 1||blue+red (1)|
|1996TP66||1998 Aug. 24.332||600||B||22.89||0.08|
|1997CQ29||1999 Mar. 20.142||2400||J||21.68||0.18|
|UT1+FORS1||1999 May 13.975||300||V||23.53||0.11|
|15.007||3600||590-1000 nm||Fig. 1||red only|
|1997CS29||1999 Mar. 20.060||720||J||20.27||0.10|
|UT1+FORS1||1999 Dec. 05.300||2x200||R||21.27||0.04|
|05.325||1246||350-1000 nm||Fig. 1||blue+red|
|1998HK151||1999 May 14.302||300||V||22.12||0.03|
|15.258||3600||590-1000 nm||Fig. 1||red only|
Photometry - to measure the magnitude of the TNOs in the reduced FORS1 images, several procedures were applied. The first one was the measurement of the total counts in an annulus of thickness equal to 1 pixel versus the annulus radius. This function should start at 0 for an annulus radius equal to zero, then reach a maximum and decrease to zero again at a certain distance depending on the seeing. It allows at the same time to check the possible presence of residual sky flux in the data (i.e. if skylight is present, the flux does not tend to zero for larger annuli), and to measure the point-spread-function (PSF) of the objects. The integral of this function over the object PSF gives the total counts recorded for each TNO. The second method used the classical magnitude measurements in a synthetic aperture (with the sky measured in a surrounding annulus removed). Third, the fitting of the objects with a two-dimensional Gaussian and the successive calculation of the counts from the measured Gaussian parameters was applied. Usually, the results of all three methods agreed within small margins, much smaller than the photon noise errors.
|1997CS29||2000 Jan. 04.057||600||V||22.17||0.05|
|1998TF35||2000 Jan. 04.978||600||V||22.26||0.05|
|1998VG44||2000 Jan. 05.940||600||V||21.75||0.03|
|1998WH24||2000 Jan. 03.967||1200||B||22.39||0.04|
|1998XY95||2000 Jan. 05.061||900||V||23.40||0.12|
|1999TC36||2000 Jan. 04.799||600||B||21.71||0.03|
From the measured magnitude of the
standard stars, after the application of the instrumental colour and air
mass corrections (for detailed information see the ESO VLT Quality Control web
the zero-points for the nights were computed.
With these zero-points and with first order measurements of
objects' colours, the magnitudes of the TNO (see Tables 2 and 3) were obtained applying the quoted colour and
corrections. The measurement errors given in these tables are the
quadratic combination of the photometric and calibration errors.
The resulting magnitudes are in the Bessell filter system as
defined in Landolt (1992).
Generally, apart from the relatively bright objects, the
calibration errors are negligible in comparison with the
photometric ones. Regarding the photometric errors the largest
source is produced by the sky noise, even in the V band where
the sky intensity is at minimum. For that reason, to avoid any
possible source of systematic errors, it is important to
subtract the sky background correctly and to measure the total
counts in the smallest possible aperture. In our case the
aperture radius varied from 6 to 10 pixels, depending on the
seeing present at the time of the observation (note: the
diameter for the photometry was larger - by a factor of about 3 - than the full-width-at-half-maximum FWHM of the object images
which is typically taken as a measure for the image quality).
|Dyn. Type||Radius||B-V||V-R||R-I||J-H||grad(V-I)||Spectral Type|
|[mag]/[km]||[mag]||[mag]||[mag]||[mag]||[mag]||[%/100 nm]||[%/100 nm]|
|Plutino||(1)||--||--||--||0.47||--||--||outlyer?, peculiar H-K|
|Scattered||0.09||0.08||0.09||--||--||neutral to slightly red|
|Cubewano||0.07||0.04||0.05||--||--||neutral to slightly red|
|as above||0.08||0.06||0.06||--||--||as above|
|Plutino||--||0.04||0.04||--||--||neutral to slightly red|
The MOSCA data was processed in the same way as the FORS1 ones except for the following steps: (1) for each colour the extinction values of the site were determined from standard star images taken every night in the same filter at different air masses. (2) the colour coefficients were obtained from the same standard star data using a linear correction term for R-I. This colour transformation took care of the transition of the instrumental colours in the Johnson/special filter set used for the observations into the Bessell filter system needed for the scientific analysis of the results. (3) we applied aperture photometry measurements only as described in the second method of the FORS1 reduction process. For each object the aperture radius of the measurement series varied between 1 and 4 sigma of the PSF.
Photometry - the measurement of the magnitude of the observed objects was made according to the same procedures used for the FORS1 data. Actually the procedures work much better here, because the background stars were dimmed by the median average. The measurements of the standard stars followed the same procedure apart that the realignment was done to the stars themselves. By applying colour and air mass correction for La Silla the NTT/SOFI zero points were obtained. Since the emission of the sky in the infrared region is much larger than that in the visible, the greatest error contribution in the measured magnitude was the sky noise.
The filter photometry of the observed TNOs and Centaur as obtained from our images is given in Tables 2 and 3. Table 4 lists the absolute brightness MR, the equivalent radius and the colours of the objects as derived from the data in Tables 2 and 3. It also lists the spectral gradients of the TNOs as obtained from the colours (B-I and V-I) and from spectra (red, blue+red) and provides a type indication from both the orbit (Dyn. Type) and the photometric (Spectral Type) data.
The reflectivity spectra of the observed TNOs are shown in Fig. 1. Figure 2 shows their V-I colours versus the absolute brightness in the R passband. Colour-colour diagrams of the TNOs and Centaurs are given in Fig. 3. The reddening gradient statistics of the TNOs and Centaurs is displayed in Fig. 4. Figures 1 and 2 show results of our own observations only. Figures 3 and 4 show the results from the merged set of our data and the ones found in literature (see also Table 5).
The absolute magnitude MR of the object is derived from its R filter
brightness using the formula for asteroids (adopted by IAU Commission 20; Meeus 1998).
For the phase correction factor G a
standardised value of 0.15 is applied. The error of MR is identical with the
measurement uncertainty of the R filter brightness. MR can be considered as a
measure of the product "geometric cross-section
albedo of the
object''. The equivalent radius of the object is obtained applying the classical
formula by Wyckhoff (1982) and assuming an albedo of 0.04. With one exception
(1995HM5 for which we have near-IR data only) our absolute magnitude and
equivalent radii refer to R filter brightnesses of the objects.
The colour gradients grad(C1-C2) indicate the reddening of the objects. For our
data presented in Table 4 grad(C1-C2) is
derived from B-I and V-I colours. They are given in "percent per 100 nm'' and
can be calculated via formula (1):
, : the brightness of the TNO/Centaur in filters C1 and C2,
, : the filter brightness of the Sun,
: the central wavelength of the filters C1 and C2. The corresponding spectral gradients can be measured directly from the reflectance spectra of the objects (using the "reference wavelength ranges'' of 600-900 nm for the red spectra and 400-900 nm if blue and red spectra are available).
|Figure 1: Reflectance spectra of 5 TNOs. The spectra were taken with two different settings for the wavelength coverage, see Table 1. The scatter beyond 750 nm is due to incomplete subtraction of strong skylines. The reflectance spectra of the individual objects are offset by an arbitrary value for better display. For 1996TO66 the "red+blue'' and the "red only'' spectra (see note (1) in Table 2) are shown: the good overlap of both spectra proves that first order contamination in the red is neligible.|
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The spectral gradients are very constant over the measured wavelength ranges. However, marginal changes of the spectral slopes exist towards the red end of some spectra possibly indicating less steep reddening of the objects towards the near-IR region. Overall, the spectral gradients agree very well with the photometric ones for all measured objects except 1995SM55 for which the spectrum shows a negative gradient while the photometry gives neutral colours; see also Sect. 4.2.
|[mag]||[mag ]||[mag]||[mag]||[mag]||[mag]||[mag]||[%/100 nm]|
In summary: in our ESO and Calar Alto observations from 1998 to 2000 we could identify
Table 5 lists the merged new table of mean broadband colours and spectral gradients per TNO (Plutinos, Cubewanos and scattered disk objects) and Centaur. The list contains in total 69 objects: 35 Cubewanos, 19 Plutinos, 4 scattered disk objects and 11 Centaurs. From the table 63 objects are used for the colour-colour plots shown in Fig. 3, and 62 for the colour gradient statistics shown in Fig. 4.
The low number of scattered disk objects and Centaurs with photometry measured, makes statistical conclusions on group properties very difficult. At best first trends can be derived. Also, for all dynamical classes the near-IR range is not yet covered by a large enough set of measured objects such that for the time being we will not include it in our discussion below. Colour-magnitude diagram: this diagram (see Fig. 2) plots the V-I colour versus the absolute magnitude MR of the objects in the R band filter (see Table 4). Over the whole range of absolute magnitudes measured our data show no conclusive correlation between MR and the V-I colours of the objects. At best - however based on only 6 data points - one could hypothesise a linear trend between MR and V-I for the brightest TNOs (brighter than MR = 5 mag). The absence of a clear relationship between MR and V-I in Fig. 2 is in contrast with the results published by Jewitt & Luu (1998) and Davies et al. (2000). Both groups found a strong correlation between MV and V-J using a sample of only 5 selected TNOs with high-quality measurements. Davies et al., however, cannot confirm this correlation using a larger sample of 14 TNOs. Our sample also suggests that the colour dispersion does not depend on MR, i.e. on the product between cross-section (or size) and albedo of the object. Such a correlation is predicted from theoretical simulations of collision resurfacing of the objects (Luu & Jewitt 1996). However, a larger statistical sample needs to be analysed to confirm or disprove this suggestion.
|Figure 2: Colour-absolute magnitude diagram of TNOs and Centaurs. The figure plots the V-I colour of our own TNO & Centaur measurements versus the absolute magnitude as derived from the R filter brightness of the objects. The plot includes 16 TNO and 1 Centaur measurements.|
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|Figure 3: Colour-colour diagrams of TNOs and Centaurs. Top: V-R versus B-V. Middle: R-I versus B-V. Bottom: R-I versus R-V. The symbols are: filled circles = Cubewanos, filled triangles = Plutinos, open circles = scattered disk objects, open triangles = Centaurs. The solar colours are indicated by the open star symbol. For the plots the data from Table 5 are used as input. The lines indicates the direction of increased reddening from -10 to 90%/100 nm.|
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Colour-colour diagrams: three colour-colour plots are presented using the merged data set of Table 5: V-R versus B-V, R-I versus B-V, and R-I versus V-R (Fig. 3).
General trends: the well known reddening trends of the TNO and Centaur populations are clearly visible in the plots. The colours range from about solar ones to about 1.2 mag in B-V and 0.8 mag in V-R and R-I. The colour ranges are about the same for the 4 dynamical object classes (Cubewanos, Plutinos, scattered disk objects, Centaurs). In the V-R vs. B-V plot the colour distribution of the objects follows nicely the line of increased reddening. However, there seem to be a trend of smaller reddening in the R-I vs. V-R plot since the measured objects "fall'' below the line of constant reddening towards the redder end of the colour distribution. This effect indicates slope changes towards the near-IR as noticed in the visible spectra of some of the objects (see Sect. 4.1). The deviations seems to be pronounced for the red population, but is widely absent for the neutral objects.
It is noteworthy that the TNOs of solar-type B-V colours have also neutral V-R and R-I colours. The red B-V cloud objects are also red in V-R and R-I. This confirms the conclusion from our spectroscopy measurements that usually no sudden and significant changes of the spectral slope are seen in optical spectra of TNOs (see also Sect. 4.1).
Outlyers: there are a few exceptions from the general trends: 1994ES2 and 1994EV3, two Cubewanos that appear to be outlyers in their colours. According to their colours they should exhibit interesting spectra in the visible range; however, up to now no spectra of these TNOs are published. The 5145 Pholus is at the very red end of the colour distribution which makes it to the reddest Centaur known so far. For more details on these objects see Sect. 4.2.
TNOs (Cubewanos, Plutinos and scattered disk objects): in the presented version the colour-colour plots of Fig. 3 do not show a clear bimodal distribution of TNOs (separated mostly in B-V with a gap between B-V of 0.7 to 0.9 mag) as described Tegler & Romanishin (1998). At best a certain clustering of the Cubewanos towards the red end of the colour distribution may be noteworthy while the two other dynamical classes - Plutinos and scattered disk objects - do not (yet) show any clear trends in the plots of Fig. 3. However, for both classes the number of objects measured is still small.
The bimodal distributions in the V-R vs. B-V plots reported earlier may be a selection effect due to the small number of objects measured. However, even with the much larger data set available now a two-dimensional Kolmogorov-Smirnow statistics analysis of the available colour data (Hainaut, priv. communication) concludes that the observed colour distribution (this includes also B-V vs. V-R) is compatible with both a single continuous as well as with a double peak colour population of TNOs.
Centaurs: the measured Centaurs - although low in number - seem to populate the neutral and the red end of the colour distributions with only a single object (1999UG5) inbetween. 3 red Centaurs (5145 Pholus, 7066 Nessus and 1994TA) are placed at the upper end and beyond the red TNO cloud in the B-V vs. V-R diagram (see Fig. 3).
|Figure 4: Reddening gradient statistics of TNOs and Centaurs. Upper panel: all TNOs and Centaurs. Lower panel: dynamical classes: Cubewanos, Plutinos, scattered disk objects, Centaurs. The reddening of the objects (in %/100 nm) as compared to the Sun is calculated according to Eq. (1) and the number of objects (Y axis) per reddening interval (X axis) is counted.|
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Colour gradient statistics: Table 5 lists also the colour gradients (or reddening) of the TNO and Centaurs as calculated from the colours of the same table. For the colour gradient statistics the number of objects per gradient interval is counted. The gradient intervals are set in steps of 10%/100 nm over the total gradient interval measured (which was -10 to 90%/100 nm). The chosen step width is approximately twice the average standard deviation of the majority of the individual gradient values. Although of arbitrary nature, this selection grid may somehow represent groups of objects with distinct differences in surface reddening. The gradient statistics obtained are presented in Fig. 4. One plot shows the statistics for the 4 dynamical classes Cubewanos, Plutinos, scattered disk objects and Centaurs individually, the other one compares the distributions of all TNOs (Cubewanos + Plutinos + scatterd disk objects) and Centaurs.
For TNOs - Cubewanos, Plutinos, scattered disk objects - the gradient statistics suggests the following conclusions:
The gradient statistics suggests a diversity in the pre-dominant reddening between the 4 dynamical classes: the Cubewanos have a single peak distribution with a typical reddening of 20-40%/100 nm, while the 3 other classes resemble more a double peak distribution with a deficiency of objects in the reddening range of 20-30%/100 nm. At least for the Centaurs that are considered to be sunward scattered TNOs (Levison 2001) - but maybe also for the Plutinos and scattered disk objects - one can speculate that they may have undergone a colour change as compared to the TNOs in the classical Edgeworth-Kuiper-Belt.
The colour palette seen in objects of the outer solar system is usually attributed to the combined action of three effects:
In other words: a "blue-shifted'' Centaur population (ignoring extreme cases like Pholus and Nessus) as compared to the Cubewanos suggests that activity resurfacing plays a much larger role for the Centaurs than for the latter objects. Since Centaurs are believed to be sunward migrating TNOs, this scenario also implies that TNOs change colours when becoming Centaurs (trend to neutral colours).
Plutinos show a similar gradient statistics as Centaurs. Whether this implies that the bluish colours are produced mostly through the same process as proposed for Centaurs (i.e. cometary activity), is uncertain. At least in one Plutino, Pluto itself, a thin atmosphere is known to form during its perihelion period.
The situation is completely unclear for the scattered disk objects. From the few objects observed no firm conclusion should be drawn and further observations are urgently needed.
In summary: the scenario for the colour evolution path of TNOs towards Centaurs is built on very weak trends in the gradient and colour distributions of these objects. Due to the low number statistics in both cases they are to be considered as uncertain and very speculative for the time being. In particular, more Centaurs need to be measured to put the colour statistics of these objects on safer grounds. The same applies for Plutinos and scattered disk objects.
Spectroscopy: with our spectra the number of objects with visible reflectance spectra published is more than doubled. The spectroscopy delivered featureless spectra with spectral gradients which confirm - in principle - the correctness of spectral gradient estimates from broadband filter photometry. The flattening of the object reddening towards the near-IR starts beyond 750-800 nm. Because of S/N limitations no spectral feature specific for a particular surface chemistry was found. Much longer integration times with 8-10 m class telescopes are needed for a search of emission and absorption features in the visible spectra of TNOs and Centaurs. However, because of the assumed surface chemistry (mostly clean and/or radiation processed ices) chances for detections of spectral features are much higher in the near-IR wavelength range.
The colour gradients obtained from spectroscopy are considered to be more reliable than the photometric gradients since they typically fit many wavelength points (not just 4 as for the broadband BVRI photometry). Colour corrections due the rotation light curve are not important. Spectroscopy has longer exposure times (factor of 2-4) than photometry which makes the use of the largest telescopes a nearly indispensable requirement. Spectral slope changes with rotation phase, however, may be difficult to detect with spectroscopy because of the long integration time for good S/N spectra (in particular for fainter objects). Spectroscopy should certainly be applied for objects which show unusual colour gradients or photometric slope changes (like 1994ES2 and 1994EV3) in order to get wavelength resolved information on these features.
Photometry: the overall colour range of TNOs (Cubewanos, Plutinos, scattered disk objects) and Centaurs in the visible wavelength range seems to be well established. However, the statistical significance of different populations among TNOs and Centaurs is not yet proven based upon the existing set of broadband colours. The spectral gradient population of TNOs and Centaurs may be different, although the number of Centaurs, scattered disk objects and - to a less extend - Plutinos with accurate photometry measured is low. That makes such conclusions somewhat uncertain. However, if confirmed by colour data of a larger sample, the colour gradient diversities between TNOs and Centaurs and among the Centaurs can be indicative for the relative importance of the various resurfacing processes proposed for both object classes.
Obviously, there is a lack of near-IR colour data for all object classes. At present, the existing broadband photometry in the visible is of little use for guidance in the selection of objects that may have interesting surface chemistry.
The existing data set of Cubewanos has already revealed outlyers, i.e. objects which clearly deviate from the typical colour range of TNOs. These TNOs may be interesting targets for spectroscopic follow-up observations which should allow to get a broader and more detailed characterisation of the peculiarity of these objects.
The observations in August 1998 were part of the VLT Science Verification (SV) program and were executed in service mode by the VLT SV team. We thank the VLT SV team (R. Gilmozzi, B. Leibundgut, J. Spyromilio, K. Virenstrand, A. Walander) for their great work and support in getting our proposal done. The observations in May and December 1999 were part of the FORS1 Guaranteed Time Observing (GTO) program. We thank the FORS principle investigator, Prof. I. Appenzeller (Landessternwarte Heidelberg) and his instrument team with scientists, engineers and workshop people from the Landessternwarte Heidelberg, the Universitäts-Sternwarte Göttingen and the Universitäts-Sternwarte München, for our participation and the friendly help in/during the FORS GTO program.
The support of the telescope and instrument operators N. Hurtado, F. Lecaros, A. Lopez, H. Nunez and G. Martin at the ESO VLT and NTT as well as of U. Thiele, J. Aceituno and S. Pedraz at Calar Alto is also greatly acknowledged.