Surface Ice and Tholins on the Extreme Centaur 2012 DR₃₀

SDSS observed 2012 ${\mathrm{DR}}_{30}$ two times in the spring of 2000 (2000 March 30, April 5; Figure 1), well before the discovery of this TNO and the other HiHq objects. From these sets of images, the second run had enough S/N to measure the flux of 2012 ${\mathrm{DR}}_{30}$ (in the g, r, and i bands); the first one from March 30 was of significantly poorer quality. Due to the slow motion of 2012 ${\mathrm{DR}}_{30}$, the detections were missed by the Moving Object Catalog (SDSS MOC; e.g., Ivezić et al. 2002), but we could identify the observations with the Solar System Object Image Search tool of the Canadian Astronomy Data Center.¹² On the same website, there is a suspicious report about a Southern Astrophysical Research Telescope (SOAR) observation from 2010, but in that case, the TNO was just outside the image and had not been observed.

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We also included here the VRI images taken with the ESO MPG on 2012 June 6 (Kiss et al. 2013) in the present analysis. More ri and griz images were taken from the McDonald Observatory on 2013 March 23 and April 13. The McDonald Observatory images were obtained using a South Korean guest instrument, CQUEAN, with a 028 pixel⁻¹ resolution and with a 48 × 48 field of view. The instrument is optimized for the red wavelength, with the best and strongest image in i. We guided on stars, which resulted in 04 and 07 trails for the object. The estimated seeing was around 12 in i at zenith on both nights. However, the minimum Zenith distance of the object was larger than 63° from McDonald at the time of the observations, resulting in larger point spread functions (PSFs) in every filter.

The log of photometric observations is summarized in Table 1.

The visible spectrum of 2012 ${\mathrm{DR}}_{30}$ has been observed in service mode using the Goodman HT Spectrograph installed at the 4.1 m SOAR in Cerro Pachón, Chile. The Goodman HT Spectrograph was used in single long slit mode, with a slit of 103 in width. It was equipped with a blocking filter GG-385 and a grating of 300 line/mm, which gives a resolution R ≈ 1390. This configuration gives a spectrum ranging from 0.32 to 0.85 μm. SA102-1081 used as a telluric and solar analog, observed at an airmass of 1.18 (the airmass of 2012 ${\mathrm{DR}}_{30}$ was 1.05) and with a 5 s exposure time. Data were reduced using standard IRAF techniques, and were wavelength calibrated using HgAr lamps. The spectra of 2012 ${\mathrm{DR}}_{30}$ were then divided by the solar analog spectra.

A spectrum covering the near-infrared region between 0.8 and 2.5 μm was obtained on March 25.135 UT with the FoldedPort Infrared Echellette (FIRE) spectrograph on the 6.5 m Magellan Baade Telescope at Las Campanas Observatory, Chile. The data were taken in the low-resolution prism mode (R ≈ 500 in the J band) with the standard 0.6 arcsec slit width. An A0V-type star HD105992 was the telluric and flux standard for the NIR spectrum. The airmass for the standard was 1.03, and the airmass for the science target was 1.01; both the standard star and the science target were taken at a parallactic angle. Irradiance correction consisted of dividing by a model spectrum with solar parameters, taken from the library of Munari et al. (2005). This model spectrum is a suitable representative of the solar irradiation, making use of the widely trusted ATLAS9 models (e.g., Castelli & Kurucz 2004 and references therein), and no significant biases are expected due to this step. Additional details on the spectroscopic observation can be found in Table 2. Data reduction was performed by applying customized IDL routines.

VIS and NIR spectra do not overlap in wavelength range. In order to join both spectra we fit a straight line to the visible spectrum and extend it to cover the longer wavelengths up to 1.1 μm. Then we used this continuum model and the NIR spectrum between 1 and 1.1 μm to merge them. Being taken at different epochs, the two parts of the spectrum likely do not cover the same region on the surface. Although previous observations of 2012 ${\mathrm{DR}}_{30}$ (Kiss et al. 2013) do not detect any color variation, we cannot discard the possibility of some change in the slope due to different materials on the surface. Based on the dispersion in the R and I colors shown in Figure 3, we estimate that the difference in the slope would be of <8%.

Due to the several sources with different filter systems of photometric data, magnitudes and color indices were converted to the widely accepted values of standard magnitudes and illumination corrected colors. A standard X magnitude of a kind X(1, 1, α) is determined as $X-5\mathrm{log}(R{\rm{\Delta }})$, where X is the measured value after standard photometric calibrations, and R and Δ are the heliocentric and geocentric distances. Here, we do not apply a correction for the α phase angle because the coefficients cannot be fitted reliably from the current data, but since the solar phase of 2012 ${\mathrm{DR}}_{30}$ was always between 24 and 39 during the observations, and the coefficients for TNOs and comets are typically found in the range of 0.05–0.08 mag deg⁻¹ around the 3° phase angle, neglecting that the phase variations will not introduce larger biases to the reduced magnitudes as 01. The observed colors were corrected to the solar illumination, leading to (g − r) − (g − r)_☉ and analogous indices. We include the resulting colors in Table 3.

Table 3.  Absolute Magnitudes and Colors of 2012 ${\mathrm{DR}}_{30}$

Date	Filter	AB mag	Color
2000 Apr 05	r(1, 1, α)	770(9)
2000 Apr 05	(g − r) − (g − r)☉		011(14)
2000 Apr 05	(r− i) − (r − i)☉		026(11)
2012 Jun 06	r(1, 1, α)syntha	740(3)
2012 Jun 06	(V − R) − (V − R)☉		015(4)
2012 Jun 06	(R − I) − (R − I)☉		008(4)
2013 Mar 23	r(1, 1, α)	722(8)
2013 Mar 23	(r− i) − (r− i)☉		030(12)
2013 Apr 13	r(1, 1, α)	763(5)
2013 Apr 13	(g− r) − (g− r)☉		005(10)
2013 Apr 13	(r− i) − (r− i)☉		053(3)

Note. Note that magnitudes are converted to the ab system, while colors are corrected to the solar values. The errors in the last digits are given in parentheses.

^aSynthesized r magnitude (r = V − 0.42(B − V) + 0.11), according to Jester et al. (2005).

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The absolute r(1, 1, α) brightnesses of 2012 ${\mathrm{DR}}_{30}$ were consistent with values between 72 and 77. However, the scatter of photometric points is definitely above the photometric errors: the suspected brightness and color variations have a standard deviation of 021 and 018, respectively. No correlation is observed with the phase angle (Pearson's r = 0.21). A similar finding can usually be straightforwardly explained by the rotation of a non-spherical body, which could cause a brightness change of several tenths of magnitudes. However, Kiss et al. (2013) gave evidence for a very little brightness variation of 2012 ${\mathrm{DR}}_{30}$, so in the case of this particular object, the scatter in the photometry is likely not of rotational origin.

The long-period light variations of 2012 ${\mathrm{DR}}_{30}$ (as it was observed in the R and r bands only) somewhat resemble that of Chiron (e.g., Duffard et al. 2002), where the amplitude of light variations can get close to 1 mag, as their data set in the mid-2000s shows. The light variation of Chiron was first detected in 1978 and suggested as a possible indicator of activity, but the coma was not imaged before the observation of Meech & Belton (1989). In the case of DR₃₀, the simplest way to explain the brightness variations would be a similar cometary activity, but due to the faintness of the object and the S/N ratio of our images, the coma has not been seen directly.

A counterargument for the suspected activity is the observed color variation. V − R and the somewhat similar g − r colors are stable between 005 and 015 for all observations, but r − i and R − I colors scatter a lot in the range of 01–05, with a median value of 028, and no correlation with the phase angle. This feature could be linked to colored spots on the surface that can evolve in the presence of irradiation. Albedo variegation due to colored surface spots is known for main-belt asteroids and also TNOs (e.g., Lacerda et al. 2008), and also found to be present on many main-belt asteroids at smaller level (Szabó et al. 2004). However, in the case of 2012 ${\mathrm{DR}}_{30}$, albedo variegation is unlikely because it would emerge in brightness variations related to the rotation, well above the upper limit derived by of Kiss et al. (2013).

From the current data, one can only propose some concepts that can explain the light variations in the r and R bands and the far-end color, but without leading to rotational light variation exceeding the limit found by Kiss et al. (2013). A temporary atmosphere can lead to frost formation on large bodies such as Hale–Bopp (Szabó et al. 2012), which can explain changes in brightness and color, but not cause colored spots on the surface. There is of course no firm proof on this or a similar scenario in the current data, and the explanation can only be expected from further photometry and imaging.

The position of 2012 ${\mathrm{DR}}_{30}$ in the slope–albedo diagram is shown in Figure 2, in comparison with other TNOs in the literature. 2012 ${\mathrm{DR}}_{30}$ is on the edge of the dark neutral objects close to the large group of bright red objects as defined by Lacerda et al. (2014). This can either signify a transient object between the two classes, e.g., it was bright and red, and now the surface is under darkening and losing the color. In this case, the surface composition reflects the processes of this transition between the two main classes. The other possibility is that 2012 ${\mathrm{DR}}_{30}$ stably exhibits the observed spectral features, and it is a special example of the dark neutral group. Interestingly, in addition to some Centaurs and scattered disk objects in the transition region between the two groups, some irregular satellites of the giant planets also exhibit similar color–albedo properties (see Farkas-Takács et al. 2017 and references therein) with similar heliocentric distances to the perihelion distance of 2012 ${\mathrm{DR}}_{30}$.

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The normalized VIS+NIR spectrum is shown in Figure 3. The spectrum of 2012 ${\mathrm{DR}}_{30}$ is red and featureless in the visible and up to 1.9 μm. The slope is consistent with color indices of (g − r) − (g − r)_☉ = 018 and (r − i) − (r − i)_☉ = 014. Here, g − r is redder than any of the photometric measurements and r − i close to the bluest extremities, but the general slope is consistent with the photometric color indices. There is a clear absorption at  2.0 μm. This absorption is typically attributed to the presence of water ice on the surface of the objects. Water ice also causes another absorption band near 1.52 μm; however, we do not detect any other absorption in our spectrum above the noise. This is not new; there are other TNOs or Centaurs that show only the strongest absorption of water ice in the NIR, for example, Lorenzi et al. (2014) estimate 25% of water ice on the surface of Varuna and they do not detect an absorption at 1.5 μm.

To compare our spectrum with other red spectra in the literature we compute the slope parameter (Luu 1993) of the continuum between 0.4 and 1.7 μm. The normalized reflectivity gradient is S' = 14.5%/1000 Å. The precision, i.e., the standard deviation of the slope parameter is 0.05%, but considering the usual systematics, the derived slope is accurate to ≈1%. We also estimate the depth of the water-ice band at ≈2.0 μm as D: = (1 − R_b)/R_c = 0.12 ± 0.01, where R_b is the reflectance in the center of the band and R_c is the reflectance of the continuum in the same wavelength calculated with a linear fit to the right and left sides of the band.

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In Figure 4, we compare the spectrum of 2012 ${\mathrm{DR}}_{30}$ with the spectrum of other objects representative of different dynamical populations. This figure shows that the optical slope parameter of 2012 ${\mathrm{DR}}_{30}$ is redder than most of the comparison objects, but not as red as the red Centaurs (which have a slope parameter between 27% and 48%; Duffard et al. 2014), represented in this figure by Pholus. The spectrum also differs from that of a blue or red Trojan. This comparison and the shape of the spectrum of 2012 ${\mathrm{DR}}_{30}$ suggest that this is an irradiated surface composed of complex organic material with some amount of water ice. Other similar surfaces can also contain a certain amount of amorphous silicates. When we compare the spectrum with other similar objects (Figure 4), we find that it is more similar to the gray Centaurs (e.g., Thereus, a gray Centaur with water ice, or the retrograde Centaur 2008 YB₃) than to the red Centaurs such as Pholus (gray Centaurs have a slope ranging from −1% to 17%; Duffard et al. 2014). In the next section we model the spectrum of this object to better understand its surface composition.

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Fitting reflectance spectra of TNOs and Centaurs with synthetic spectra is an effective tool to study the surface composition of these objects (Hapke 1993 and references therein; Shkuratov et al. 1999). These methods have been widely used, leading to important discoveries in the understanding of the surfaces of minor bodies, e.g., the first discovery of water ice and organics on the surface of asteroid 24 Themis (Campins et al. 2010), the characterization of the first family of TNOs (Pinilla-Alonso et al. 2007, 2009) or, more recently, the study of the surface of Pluto through modeling of New Horizons spectra (Protopapa et al. 2017).

In the modeling efforts, the grain sizes and relative abundances of candidate materials are adjusted iteratively to minimize the differences between the resulting synthetic albedos and the observations. In this work, we have used the Shkuratov method to obtain the reflectance of a surface composed of a maximum of five ingredients mixed intimately (also known as "salt and pepper" mixture). We follow a similar approach that was used previously by our group for other minor icy objects (Duffard et al. 2014; Lorenzi et al. 2014). In a first step, we ran models using different materials that have been previously proposed as part of the surface of other TNOs. We included amorphous and crystalline water ice in pure form and with tholin inclusions (Mastrapa et al. 2009), amorphous pyroxenes and olivine with different relative ratios of Mg/Fe (Dorschner et al. 1995), titan tholin (Khare et al. 1984), triton tholin (McDonald et al. 1994), amorphous carbon (AC; Rouleau & Martin 1991), kerogen (Khare et al. 1990), and methanol (J. Emery 2018, personal communication).

We show the comparison of the reflectance of some individual components tested in this step and the observed comet spectrum in Figure 5. The components that we are going to use in the next step are drawn with solid lines, while the ones that were omitted from the final fit are drawn with dashed lines (see the caption for the list of the components). The model spectra show different kind of homogeneous surfaces, assuming that pure material is present in the form of 20 μm sized particles. In the case of methanol, we plot the reflectance spectrum based on the laboratory measurement of a 13.7 μm thick layer, published in Cruikshank et al. (1998). Of course these curves are not considered as template patterns during the fitting procedure, where the spectrum of the mixture with variable compositions and grain sizes were calculated exactly. But this comparison gives a hint about the prominent spectrum shape and specific features due to the different components, and for example, nicely illustrates the presence of the water band (around 2 μm) in the spectrum. For example, carbon and kerogen behave similarly, they show a featureless curve with moderate red slope and low albedo, both are discarded for the final step. Methanol ice has been used before to model Pholus and there are hints of its presence on a small number of TNOs in the RR class, the most diagnostic feature when it comes to TNOs surface composition is the band at 2.27 μm (Brunetto et al. 2006; Barucci et al. 2011). Methanol is also discarded in the first step of our modeling. This is consistent with our observations as we do not detect this absorption band (see Figure 3). Also, the slope of 2012 ${\mathrm{DR}}_{30}$ corresponds to a TNO in the BR class, not the RR class.

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After running many combinations of materials, relative abundances, and particle sizes, our modeling selects the components that provide the best fits based on the value of the reduced χ². We found that the best fits come from a mixture of five ingredients (see Table 4 for the details of the optical constants): amorphous water ice; amorphous pyroxene (with a low amount of iron, 30%); Triton tholin; Titan tholin and amorphous water ice with fine inclusions of Triton tholins (the size of the inclusions is much smaller than the size of the water ice particles) accounting for the 12% of the volume of water ice (hereafter "water with inclusions").

As a second step, we calculated a grid of models varying the relative abundances and the particle sizes of the accepted materials (see Table 5 for the range of variation of these variables covered in this step). Our modeling considers the possibility of 0% of abundance of one or more materials, so the final number of components in the surface will be determined by the modeling. We selected from these models those which were consistent with 7.5%–9.2% visible albedo (Kiss et al. 2013), and we ranked them using the reduced χ² value.

From the grid of models we selected the models that have at least 90% confidence, a total of 62 models. This statistically selected sample provides constraints for the values of the relative abundances and the particle sizes. In the last step, we proceed with the visual confirmation of the collection of 62 models. Fortunately, the shape of the spectrum of 2012 ${\mathrm{DR}}_{30}$ is peculiar and its characteristics (i.e., red slope in the visible, clear absorption at 2.0 m, and lack of absorption at 1.52 m) resulted in good sensitivity in defining the acceptable model compositions of the quite limited number of components we involved in the final modeling step.

In Figures 6 and 7, we show the collection of 18 synthetic models that best reproduce the spectrum of 2012 ${\mathrm{DR}}_{30}$ together with the one that we prefer as "best fit." In the plot we normalized each synthetic albedo and the spectrum to unite at 0.7 μm. This is better to show the variation allowed for the albedo at 0.55 μm, based on the estimated optical albedo by Kiss et al. (2013). The space of parameters covered by these models are presented in Figure 7. The characteristics of the best fit are detailed in Table 6.

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We examined the degenerations in the parameter space, which is tolerable and supports the stability of the best-fit solution (Table 7). The most important degenerations emerge between the two water-ice components (pure ice and ice with inclusions), and also pyroxene and Triton tholin. The two water-ice components together sum up to 30% in the best fits, hence the total water content is well determined. The identified correlation between pyroxene and Triton tholin is most likely due to the similar spectral shape of these components (Figure 5), which may make them mathematically similar, but their difference in albedo enables us to conclude on a lower amount of pyroxene and larger amount of Triton tholin in the fitting procedure: in all of the best models, the modeled amount of Triton tholin well exceeded the modeled amount of pyroxene. In summary, the results are based on our initial assumptions including a limited set of different chemical compounds, but we could get a rough picture on the composition.