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A&A
Volume 646, February 2021
Article Number A51
Number of page(s) 10
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/202039004
Published online 04 February 2021

© ESO 2021

1 Introduction

Light reflected from the surface of an asteroid is polarized to varying degrees. Because light radiated from the Sun is unpolarized, we attribute this polarization degree to the optical properties of the asteroid surface (e.g., composition, roughness, and structure). By examining the degree of polarization of the asteroid, the physical properties resulting from polarization can therefore be determined.

The degree of the linear polarization of small Solar System objects such as asteroids, comets, and moons also depends on the phase angle (the angle between the Sun and the observer as seen from the object). From this phase angle-polarization degree curve (Muinonen et al. 2002), the main polarization characteristics parameterize the maximum and minimum values of the polarization, the phase angle of the negative to positive reversal point (i.e., inversion angle), and the slope at that point. Research has been conducted using these polarimetric parameters to investigate the relation between taxonomic type, albedo, and particle size (Belskaya et al. 2015 and references therein). Statistical analyses at low phase angles (typically α < 30°) that target main-belt asteroids have been performed in many previous studies (Cellino et al. 1999; Belskaya et al. 2003, 2009, 2017; Gil-Hutton 2007; Gil-Hutton et al. 2008, 2014, 2017; Gil-Hutton & Cañada-Assandri 2011, 2012; Cañada-Assandri et al. 2012; Gil-Hutton & García-Migani 2017; López-Sisterna et al. 2019). These studies reported similar trends in polarimetric slope, minima, and inversion angles for each asteroid classification type.

Observations around a phase angle of 90°–100° yield maximum polarization values (Pmax). This observational geometry can only be achieved for objects that are within the Earth orbit (i.e., the Moon, near-Earth asteroids, and comets). If limited to asteroids alone, the number of known Pmax objects is small, and there are many unknown polarimetric behaviors. Comets are roughly divided into high-Pmax, ~ 30%, and low-Pmax, ~ 10%, groups (Levasseur-Regourd et al. 1996), but Kiselev et al. (2001, 2004) and Jewitt (2004) reported that gas in the coma reduced the degree of polarization. The degrees of polarization of two comet nuclei, which can be regarded as an analogs of asteroidal surfaces, have also been obtained, which indicate possibly higher values (Jockers et al. 2005; Kuroda et al. 2015; Kwon et al. 2018). Polarimetric studies of the Moon have shown the color dependence of the phase angle–polarization curve and the well-known Umow law (Umow 1905), which correlates albedo and Pmax with particle size (Dollfus 1998; Shkuratov et al. 2011 and references therein).

New knowledge obtained in the past few years has described asteroids with large inversion angles (Cellino et al. 2006) and asteroids with extremely high Pmax (Kuroda et al. 2018; Ito et al. 2018). More than two dozen objects in the main-belt asteroids, including candidate objects, belong to the first group (Cellino et al. 2019). The group name “Barbarians” comes from the name of the first discovered asteroid (234 Barbara). Sunshine et al. (2008) suggested that some of these asteroids possessed the so-called calcium aluminum-rich inclusion (CAI) from their near-infrared spectral characteristics, and their corresponding meteorites were of CV class, such as the Allende meteorite. Barbarians have only been found within main-belt asteroids larger than 40–50 km in diameter, with the exception of the possible members of the Watsonia and Brangäne families (Cellino et al. 2019).

Only two asteroids with high Pmax have been found; unlike comets, which are composed of dust, it was unexpected that small airless objects would have such a high degree of polarization. In general, the polarization is considered to depend on the multiplescattering ratio of the surface layer, which explains the high polarization of (152679) 1998 KU2 with very low albedo. Because the other (3200) Phaethon has an intermediate albedo, it is unlikely that they have similar surface properties. In any case, the particle size of the surface layer has been proposed to be the cause of the high observed polarization (Kuroda et al. 2018; Ito et al. 2018).

Our target, the near-Earth asteroid (85989) 1999 JD6, was classified as K-type asteroid from the visible spectrum (Binzel et al. 2001; de León et al. 2010), and was later identified as L-type asteroid with the aid of the near-infrared spectrum (Binzel et al. 2019). Other physical characteristics derived from photometric and radar observations also show that it is a contact binary, with a maximum diameter of 2 km (Marshall et al. 2015), albedo 0.05–0.11 (Campins et al. 2009; Thomas et al. 2011; Reddy et al. 2012; Mainzer et al. 2014; Nugent et al. 2016), rotation period 7.6638 h ± 0.0001 h, and rotation amplitude 1.16 mag ± 0.05 mag (Szabó et al. 2001; Polishook & Brosch 2008). In consideration of these fundamental physical parameters, we aim to investigate the surface state of small Solar System objects through polarization characteristics. We here study a polarimetric observation of 1999 JD6 in the visible wavelength at a phase angle from 30°–105°.

Section 2 describes the details of our observation and analysis. Section 3 reports the maximum value of the degree of polarization and the inversion angle derived from the phase angle-polarization degree curve. Based on the results, we discuss in Sect. 4 the possibility that 1999 JD6 belongs to the Barbarians, and we report the particle size of the surface layer estimated from the comparison of meteorites.

Table 1

Observational summaries of (85989) 1999 JD6

2 Observations and analysis

2.1 Polarimetric observation

Polarimetric measurements of the near-Earth asteroid (85989) 1999 JD6 were performed using the polarization mode of the Multi-Spectral Imager (MSI) installed on the 1.6 m Pirka telescope at the Hokkaido University Observatory (MPC Observatory code: Q33; Watanabe et al. 2012). The MSI is an instrument mounted on the Cassegrain focus of the Pirka telescope. The camera of the MSI is a Hamamatsu Photonics ImagEM C9100-13 (electron multiplier CCD, 512 pixels × 512 pixels, 16 μm pixel−1), and 0.389′′ pixel−1 in normal resolution. This polarization mode derives the Stokes parameters (Q and U) by rotating a half-wave plate to 0°, 45°, 22.5°, and 67.5° using a Wollaston prism as a polarization element. Because one polarization degree was calculated from the data set obtained at these four angles, the exposure time per shot was kept short. This also reduced the effect of changes in brightness due to asteroid spins and in sky conditions. In addition, to reduce the specificity of the detector, a dithering pattern was applied at three places in the RA direction.

Our observations were made in June–July, 2015 and May–July, 2019, covering a phase angle of 30°–105°. We obtained four nights of data with the V band and nine nights with the RC band (Table 1).

2.2 Analysis

The procedure for deriving the degree of linear polarization was the same as that of Kuroda et al. (2018). The acquired images were analyzed using the Image Reduction and Analysis Facility (IRAF; Tody 1993) and SExtractor (Bertin & Arnouts 1996). In particular, we processed bias subtraction, flat-field correction, and cosmic-ray removal in dedicated scripts (MSIRED) on IRAF.

Becausethe ordinary and extraordinary lights form an image with the mask interposed in one frame, we measured both light fluxes by the aperture photometry method. The aperture size was examined within a range of one to five times at intervals of 0.1 based on the full width at half maximum (FWHM) of the Moffat function (Moffat 1969) and optimized to a size where the signal-to-noise ratio (S/N) was near the maximum value and stable. For the typical stellar image FWHM size of 1.6–4.3′′, the aperture was in the range of 2.5–4.0 times the FWHM. The background value was the mean value within a ring that was three FWHM wide, starting 2 pixels outside the aperture circle. Moreover, by premasking stars other than the target body, contamination in the aperture and the sky background was minimized. When the S/N of a single frame was lower than 100, the measurement was carried out for combined frames of each half-wave plate.

The components of the normalized Stokes parameters (Bohren & Huffman 1983; Tinbergen 1996), Q/I and U/I, were derived from the following formula: Q I =( I e,0 / I o,0 I e,45 / I o,45 −1 )/( I e,0 / I o,0 I e,45 / I o,45 +1 ), \begin{equation*} \frac{Q}{I} = \left(\sqrt{\frac{I_{\textrm{e,0}} / I_{\textrm{o,0}}}{ I_{\textrm{e,45}} / I_{\textrm{o,45}}}}-1\right) \Bigg/ \left(\sqrt{\frac{ I_{\textrm{e,0}} / I_{\textrm{o,0}}}{I_{\textrm{e,45}} / I_{\textrm{o,45}}}}&#x002B;1\right),\end{equation*}(1)

and U I =( I e,22.5 / I o,22.5 I e,67.5 / I o,67.5 −1 )/( I e,22.5 / I o,22.5 I e,67.5 / I o,67.5 +1 ), \begin{equation*} \frac{U}{I} = \left(\sqrt{\frac{ I_{\textrm{e,22.5}} / I_{\textrm{o,22.5}}}{ I_{\textrm{e,67.5}} / I_{\textrm{o,67.5}} }}-1\right) \Bigg/ \left(\sqrt{\frac{ I_{\textrm{e,22.5}} / I_{\textrm{o,22.5}} }{ I_{\textrm{e,67.5}} / I_{\textrm{o,67.5}}}}&#x002B;1\right),\end{equation*}(2)

where Io,ν and Ie,ν stand for the ordinary (o) and extraordinary (e) intensities at a given set of half-wave plate position angles in degree (ν). The degree of linear polarization (P) and the position angle of polarization (θP) are defined as P= ( Q I ) 2 + ( U I ) 2 , \begin{equation*} P = \sqrt{ \left(\frac{Q}{I}\right)^{2} &#x002B; \left(\frac{U}{I}\right)^{2} },\end{equation*}(3)

and θ P = 1 2 tan −1 ( U Q ), \begin{equation*} \theta_{\textrm{P}} = \frac{1}{2} \tan^{-1} \left(\frac{U}{Q}\right),\end{equation*}(4)

respectively. The obtained degree of polarization was corrected for polarization efficiency and instrument polarization and was converted into the celestial coordinate system. The observation data from 2015 were applied to the same correction values as Ishiguro et al. (2017). Because these values are known to change over time, we used the newly derived correction terms to calibrate the 2019 data. The instrument polarization was corrected using the unpolarized standard stars (HD 14069, HD 154892, and HD 212311), and the zeropoint of the position angle of the celestial plane was determined using strongly polarized standard stars (BD+64d106, HD 155197, HD 155528, HD 161056, HD 204827, and Hiltner 960; Turnshek et al. 1990; Schmidt et al. 1992). These detailed values are given in Table A.1. According to the common approach of quantifying the polarization for Solar System objects, we represent the degree of linear polarization (Pr) and the position angle of polarization (θr) referenced to the scattering plane (Zellner & Gradie 1976) as P r =Pcos(2 θ r ), \begin{equation*} P_r = P\cos(2\theta_r),\end{equation*}(5)

and θ r = θ P −(ϕ ±  90 ° ), \begin{equation*} \theta_{\textrm{r}} = \theta_{\textrm{P}} - (\phi \,{\pm}\, 90^{\circ}),\end{equation*}(6)

where ϕ represents the position angle of the scattering plane (see Table 1). We added or subtracted 90° to ϕ so that the inside of the bracket was in the range of 0° and 180° (Chernova et al. 1993). The results of the polarization degrees (P and Pr) and position angles (θP and θr) for each night are listed in Table 2. The errors on these quantities were determined to apply the law of error propagation to the flux error and uncertainty of each correction term; see Ishiguro et al. (2017) for more information on error estimation. Because our polarization measurements were obtained on different nights, there are variations in the error bars caused by changes in brightness due to rotation and sky conditions.

Table 2

Nightly weighted mean values of our polarimetric observations.

3 Results

3.1 Degree of linear polarization for multibands

Linear polarization degrees for 1999 JD6 were derived at phase angles of 29.6°–105.3° in the RC band and at phase angles 53.9°–92.5° in the V band (Fig. 1). As described in Sect. 2, the data in the four angles of the half-wave plate are needed to determine a single linear polarization degree. We derived the weighted mean value using the continuous data (Pr and θr) taken in thedithering mode. In our observational range, the maximum value of the polarization degree was 13.38% ± 0.56% at a phase angle of 105.26°, and the minimum value was 0.39% ± 0.61% at a phase angle of 29.57°. We obtained these values in the RC band. When this isconverted into the V band at the same phase angle, we expect that the polarization degree reaches about 14%. Table 1 shows only nightly average values of the phase angle, the linear polarization degree, and the polarization position angle. The individual numerical values are listed in Table B.1.

The polarization curve of the phase angle of 1999 JD6 tends to be smaller than that of 1998 KU2 (Kuroda et al. 2018) and Phaethon (Ito et al. 2018; Devogèle et al. 2018a), which both exhibited extremely high degrees of polarization. However, it is larger than in S-complex (Kiselev et al. 1990; Ishiguro et al. 1997, 2017) and E-type (Kiselev et al. 2002) asteroids, as shown in Fig. 2. There is no counterpart to asteroids that shows similar polarimetric behavior in the positive branch to 1999 JD6. The RC -band data were obtained until a phase angle of 100°, but there appeared to be no maximum. Comparing the V - and RC-band gradients of polarization (henceforth referred to as polarimetric color) of the four nights observed in the multibands, we found that the difference in the degree of polarization tends to increase in proportion to the phase angle. We denote polarimetric bluing when the polarimetric color tends to increase with respect to the phase angle, and polarimetric reddening when it tends to decrease. This polarimetric bluing has also been observed in (25143) Itokawa (Cellino et al. 2005). In contrast, the polarimetric reddening trend with phase angle has been found for (1566) Icarus (Ishiguro et al. 2017) and Phaethon (Shinnaka et al. 2018).

1999 JD6 has an elongated shape (Marshall et al. 2015) and a wide range of literature values for the albedo, from 0.05 to 0.11 (Campins et al. 2009; Thomas et al. 2011; Reddy et al. 2012; Mainzer et al. 2014; Nugent et al. 2016). Because polarization and albedo are closely related, we investigated the change in polarization with the rotation phase. The data collected on July 16, 19 and 20, 2015, which consist of many measurement points, show slight fluctuations around the rotationphase 0.0–0.2 and 0.35–0.65 (see Fig. 3). The plotted data were subtracted from the phase angle-dependent polarization changes. As the data specify the rotation phase, we used the light-curve data acquired eight days before our observation (Vaduvescu et al. 2017). Several large asteroids and the 5.8 km sized asteroid Phaethon have reported rotational variations of the degree of polarization smaller than 0.3% (Degewij et al. 1979; Cellino et al. 2016b; Nakayama et al. 2000; Takahashi et al. 2004, 2009; Belskaya et al. 2010; Borisov et al. 2018). However, our measurements are not accurate enough to assess this threshold. No variation in phase with the object rotation is detectable, and the scatter of the data appears to be compatible with the measurement uncertainties.

thumbnail Fig. 1

Phase angle–polarization degree curve of 1999 JD6. The data arethe nightly weight means (see Table 2). The symbols in the legend indicate each of the nightly data, with the V band in green and the RC band in red. The June 14, 2015, data, which had a noticeably larger error bar, had faint magnitude and poor sky conditions, but were adopted because no other data were available around this phase angle.
thumbnail Fig. 2

Comparison of the linear polarization degrees of 1999 JD6 and other near-Earth asteroids in the red region (about 650 ± 50 nm). The open circles indicate data corresponding to 1999 JD6. Other symbols denote data for (152679) 1998 KU2 (open black crosses; Kuroda et al. 2018), (3200) Phaethon (cyan triangles; Ito et al. 2018; Shinnaka et al. 2018), (1566) Icarus (blue squares; Ishiguro et al. 2017), (1685) Toro (green pluses; Kiselev et al. 1990), (4179) Toutatis (orange crosses; Ishiguro et al. 1997; Mukai et al. 1997), and (33342) 1998 WT24 (dark green diamonds; Kiselev et al. 2002). 1999 JD6 exhibits a different polarimetric behavior than was reported in previous studies.
thumbnail Fig. 3

Relation between rotational phase and polarization degree of 1999 JD6. The open squares, open circles, and crosses indicate individual data obtained on July 16, 19, and 20, 2016, respectively (see Table B.1). The plotted data are the deviations after subtracting the increasing portion of the polarization degree from the phase angle. Both bands (V band in green and RC band in red) show small but similar fluctuations in the rotation phase (light curve from Vaduvescu et al. 2017). Our data provide insufficient coverage of the rotational phase to confirm its repeatability.

3.2 Polarimetric parameters

Polarimetricparameters in the RC band were derived using the Lumme and Muinonen function (Lumme & Muinonen 1993; Goidet-Devel et al. 1995; Penttilä et al. 2005), as shown in Fig. 4. This function is defined by the following expression: P(α)=b ( sinα ) c 1 ( cos( 0.5α ) ) c 2 sin( α− α 0 ), \begin{equation*} P(\alpha)=b\left(\sin \alpha\right)^{c_1} \left(\cos\left(0.5\alpha\right)\right)^{c_2} \sin\left(\alpha-\alpha_0\right),\end{equation*}(7)

where b, c1, c2, and α0 denote positive constants. By definition, the derivative of P(α) at α0 represents the polarimetric slope (i.e., h =  dP dα | α =  α 0 $ h\,{=}\,\left. \frac{\mathrm{d}P}{\mathrm{d}\alpha} \right|_{\alpha\,{=}\,\alpha_{\mathrm{0}}}$). Equation (7) is an empirical equation and may not apply to the polarization phase angle change of any asteroid, but no other function can cover small to large phase angles. Because we lack small-angle data in our observations, we attempted to perform a function fit by fixing the minimum polarization value (Pmin at αmin). Specifically, from the spectral types of 1999 JD6, we adopted −0.846% at 9.75° and −0.794% at 8.18°, the mean values of Pmin for K and L types, and the Barbarianmean value of −1.410% at 12.21° (López-Sisterna et al. 2019). This is a parameter estimation by this function, therefore we tried to fit it with two different methods. When fitting with the nonlinear least-squares method (Levenberg–Marquardt algorithm), the chi-square values (χ2) of fitting with Pmin of K and L types and Barbarians were 114.2, 127.0, and 42.2, respectively (left panel of Fig. 4). In the Markov chain Monte Carlo (MCMC) method (Salvatier et al. 2016), the difference in the chi-square values of each case is small (54.3, 54.7, and 47.2, respectively) because the three fitting results converge to close models (see the right panel of Fig. 4). Employing the Pmin of Barbarians, both methods yielded similar convergence solutions. We therefore conclude that this model (i.e., Barbarians) is the best solution for calculating the parameters of 1999 JD6. In fitting the Levenberg–Marquardt method, the maximum polarization degree obtained is 13.0% ± 0.8% at a phase angle of 107.0° ± 3.1°, the inversion angle is 26.9° ± 0.7°, and the slope is 0.150%/° ± 0.001%/°. According to the MCMC method, the maximum is 12.9% ± 0.1% at a phase angle of 106.4° ± 0.5°, the inversion angle is 27.0° ± 0.4°, and the slope is 0.150%/° ± 0.001%/°. Table 3 shows the polarimetric parameters derived above, and for comparison, the values of the major Barbarians members and the average values of K and L types and Barbarians.

As mentioned in Sect. 3.1, the albedo is an essential factor in considering the degree of polarization. According to Gil-Hutton et al. (2008) and Lupishko et al. (2020), in (172) Baucis, (234) Barbara, and (599) Luisa, known as Barbarian asteroids, their albedos using the polarimetric slope were reported to be smaller than the literature values from thermal infrared observations. Therefore we tried to derive the geometric albedo of 1999 JD6 using the well-known slope-albedo law. Based on the study of scattering characteristics (Zellner et al. 1974; Dollfus et al. 1989), the following equation holds between the geometric albedo (pV) and the polarimetricslope (h): log \mbox$ p V (h)$ =  C 1  log h+ C 2 , \begin{equation*} \textrm{log}\ \mbox{$p_{V}(h)$}\,{=}\,C_1\ \textrm{log}\ h &#x002B; C_2,\end{equation*}(8)

where C1 and C2 represent constants. We applied C1 = –0.989 ± 0.047 and C2 = –1.719 ± 0.040 (Lupishko 2018), C1 = –1.111 ± 0.031 and C2 = –1.781 ± 0.025 (Cellino et al. 2015), C1 = –1.207 ± 0.067 and C2 = –1.892 ± 0.141 (Masiero et al. 2012) as these constants and calculated the geometric albedos of 1999 JD6 for our polarimetric slope (h ~ 0.150°/%) as pV(h) = 0.125 ± 0.016, pV (h) = 0.136 ± 0.011, and pV (h) = 0.127 ± 0.044, respectively.

The slope parameters applied to this equation were calculated from the Rc band, but the V band was assumed to have similar values of slope parameters. The geometric albedo obtained from previous thermal infrared studies (Campins et al. 2009; Thomas et al. 2011; Reddy et al. 2012; Mainzer et al. 2014; Nugent et al. 2016) is in the range of 0.05–0.11, and our calculated values are above the upper limit. Unexpectedly, we obtained values that are higher than in the literature values, but 1999 JD6 was found to deviate from the relationship that satisfies the majority of asteroids, as well as other Barbarians.

Table 3

Estimates of polarimetric parameters.

thumbnail Fig. 4

Comparison of phase-polarization curves under two fitting methods. The left and right panels demonstrate curve fitting using the nonlinear least-squares method (the Levenberg-Marquardt algorithm) and the Markov chain Monte Carlo method, respectively, using Eq. (7). The open red circles indicate the RC band data of 1999 JD6 in this work. The solid blue, dashed purple, and dotted cyan lines correspond to the Barbarians, L-type, and K-type asteroids, that is, the values of Pmin we used for the fit. Both methods yielded similar convergent solutions in the Babarian model.
Table 4

Comparison of polarimetric parameters for meteorites.

4 Discussion

4.1 New member of Barbarian asteroids

We found that 1999 JD6 has an uncommonly large inversion angle, α0 ~ 27.0°. Considering that primary feature of Barbarian asteroids is a large inversion angle, we propose that 1999 JD6 is the first of the Barbarians to be detected in the near-Earth asteroids. Although our data do not reach the negative branch, we assumed Pmin in our limited data set and derived the reasonable solution from fitting the empirical equation. It is unlikely that the polarimetric behavior has no negative branch, and considering the previous researches, the slope parameter, h ~ 0.150°/%, becomes more loose. Such a large inversion angle is a known characteristic of the Barbarians, of which (234) Barbara was the first example (Cellino et al. 2006). A select group of the Barbarians were previously classified as S, K, L, and A type through visible spectroscopy (SMASS; Bus & Binzel 2002). Compared with the Bus classification, the polarimetricparameters of the Barbarians indicate significant differences from other taxonomic types (López-Sisterna et al. 2019). With a focus on inversion angles and slope parameters, their values with error bars for 1999 JD6 are within the typical range of the Barbarians. Moreover, 1999 JD6 exhibits different polarimetric characteristics from those of other K- and L-type asteroids, except for the Barbarians (see Table 3). According to Devogèle et al. (2018b), these were reported to be L type in the Bus-DeMeo classification (Demeo et al. 2009) extended to the near-infrared wavelength. The spectrum of 1999 JD6 has been reported in several studies, including Bus & Binzel (2002), Lazzarin et al. (2005), de León et al. (2010), Reddy et al. (2012), and Binzel et al. (2019). Evaluated together with the Bus-DeMeo classification by Binzel et al. (2019), 1999 JD6 has been classified as an L type. However, it appears to have characteristics that are intermediate between K and L types (top panel in Fig. 5), and it is not remarkable at an absorption band around 2 μm, which is associated with CAI minerals (bottom panel in Fig. 5). The obvious difference between the spectrum of 1999 JD6 and that of other Barbarians may be caused by the mineral composition ratio and the degree of subsequent alterations, considering spectral fitting models (Devogèle et al. 2018b).

Sunshine et al. (2008) and Devogèle et al. (2018b) argued that the polarimetric behaviors of the Barbarians are the result of the high content of spinel-bearing minerals on their surfaces. Devogèle et al. (2018b) also indicated that based on the spectroscopic properties of the Barbarians, their composition fits with the mixed minerals of the matrix, MgO-rich olivine fluffy CAI, in CV3 meteorites. On the other hand, the CV3 meteorites belong to the primitive meteorites with low albedos. There are discrepancies with the fact that the Barbarians have intermediate albedos between about 0.15 and 0.20, as reported in Cellino et al. (2016a). Gil-Hutton et al. (2008) mentioned the possibility that asteroids with large inverse angles have coarse-grained regoliths of dark particles mixed with fine-grained white inclusions. In their scenario, a mixture of two components with different geometric albedos affects the polarization parameters, so that the albedo-slope relationship equation as in Eq. (8) has been interpreted as inapplicable. In general, the near-Earth asteroids are assumed to have been the source or common parent body of the meteorites. The presence of 1999 JD6 in low albedo (Pv ~ 0.08) is of interest in this respect, although this study is unable to link any specific CV3 meteorite to it. In addition, the CAI abundance mismatch between meteorites and asteroids may also be resolved.

In the range of phase angles we measured, the polarization degrees of 1999 JD6 were always higher in the V band than in the Rc band. The Umow law (Umow 1905), which states that the polarization degree is inversely proportional to reflectance, is well known. This law also corresponds to the case of 1999 JD6. The spectropolarimetry of (236) Honoria at α = 9.1° and (599) Louisa at α = 26.9°, identified as Barbarian, displayed a positive gradient of both polarization and reflectance concerning wavelength in the case of Honoria (Bagnulo et al. 2015). Considering these results at the negative branch and the polarimetric color of this study, we suggest that the polarimetric wavelength gradient varies from positive to negative and the difference increases with the phase angle, as partially mentioned in Bagnulo et al. (2015). Even within the same Barbarians, there are objects whose spectroscopic and polarimetric features do not coincide with each other. Thus, we propose that other surface structures (particle size, porosity, etc.) can be derived by evaluating polarimetirc features in addition to surface composition (e.g., spectroscopic features).

The geometry of 1999 JD6 indicates that it is a contact binary, with an estimated diameter of approximately 0.7 × 2 km (Marshall et al. 2015). This object would qualify as the smallest of the Barbarian group; as previously observed, Barbarian members are tens of kilometers in size within the main-belt asteroids. If the size of the surface regolith depends on the size of the asteroid (Gundlach & Blum 2013), the polarimetric behavior of Barbarians may be attributable to a specific grain size. We elaborate on the surface particle size of 1999 JD6 in Sect. 4.2.

thumbnail Fig. 5

Spectroscopic similarities of 1999 JD6 and others. Top panel: spectra of 1999 JD6 (red circles) and the Barbarians (172) Baucis (green squares), (234) Baraba (cyan diamonds), (679) Pax (blue crosses), and the range of K type (purple triangles) and L type (black pluses) according to the Bus-DeMeo classification (Demeo et al. 2009; Binzel et al. 2019). The spectra refer to textual data obtained from MITHNEOS1. Bottom panel: compares the spectra of the CV3-type Allende (cyan crosses) and CO3-type Lance (green pluses) meteorites with those of 1999 JD6 (red and blue circles). The 1999 JD6 spectra are multiplied by the albedo within the literature values that best match the respective meteorite data. At the side where the wavelength is longer than 0.6 μm, it matches Allende well, while at the blue side, Lance is matched well. The scaled spectra of the meteorites were obtained from the RELAB2 database.
thumbnail Fig. 6

Phase angle dependence of polarimetric color for 1999 JD6 and size-dependent meteorites. The open red circles and solid lines represent data corresponding to 1999 JD6. The polarimetric data of two meteorites, CO3-class NWA 4868 and CV3-class Allende, have been measured under deposited layer conditions in the laboratory (Hadamcik et al. 2011). The blue squares and cyan triangles adjacent to the dotted lines show a grain size of <50 μm and <200 μm for the NWA4868 meteorite, respectively. The orange crosses and dark green diamonds adjacent to the dashed lines denote <50 μm and <500 μm Allende meteorite particles, respectively.

4.2 Estimation of the regolith grain size

We derived the surface particle size of 1999 JD6 by comparing the polarization characteristics of 1999 JD6 and two meteorites. Spectra of well-known Barbarian asteroids, (387) Aquitania and (980) Anacostia, present a strong 2 μm absorption feature (Burbine et al. 1992). Such spinel-enriched meteorites have similar features to CO and CV chondrite meteorites, which possess a high abundance of CAI (Sunshine et al. 2008). Against this background, we compared the polarization properties of 1999 JD6 with those of Allende (CV3) and NWA 4768 (CO3) meteorites.

Hadamcik et al. (2011) reported the results of measuring these meteorites with multiple particle sizes of polarized light in green (543.4 nm) and red (632.8 nm). The polarimetric parameters (Pmax, αmax, and h, and α0) are similar to the 25–50 μm of Allende or 50–200 μm of NWA 4768, as shown in Table 4. αmax is about 10 degrees larger for both meteorites than for 1999 JD6. However, this may be due to the fitting gap around Pmax of 1999 JD6. As a further constraint, we introduced the phase angle change of the polarimetric color, shown in Fig. 6. The color variation in the phase angle is polarimetric reddening for Allende and polarimetric bluing for NWA 4768. The latter seems to match the slight phase bluing for 1999 JD6. However, while the slope of NWA 4768 varies little with particle size, that of Allende is closer to flat with smaller particle size. Assuming that the composition of 1999 JD6 is similar to that of the Allende meteorite, the grain sizes are probably between 25 and 50 μm because the polarimetric parameters and polarimetric color trends are similar.

The shape model constructed from the radar and light curves by Marshall et al. (2017) shows that 1999 JD6 is a contact binary, with a peanut-like overall shape. Because the amplitude of the light curve is large (approximately 1 mag.), their maximum and minimum points correspond to bipartite and single faces, respectively. We acquired the polarization degrees around two maxima and minima, and the polarimetric variations were consistent within the observational errors, as shown in Fig. 3. This fact indicates that there is almost no difference in the surface states between the two objects that form the contact binary. In other words, we conclude that the overall surface of 1999 JD6 is uniform, with no significant differences in albedo or particle size.

5 Conclusions

We have performed multiband polarimetric observations of the near-Earth asteroid (85989) 1999 JD6 over a wide range of phase angles. A summary of our findings is listed below.

  • 1.

    The linear polarization degree of more than 13% in the RC band, which is close to the maximum, exhibited a new trend compared to previously observed near-Earth asteroids.

  • 2.

    The inversion angles obtained from the function fitting are as large as 27° and are consistent with those of popular Barbarians. It is the first Barbarian identified by polarimetry among the near-Earth asteroids. It is noteworthy that this asteroid is the smallest Barbarian identified so far.

  • 3.

    Based on comparisons with laboratory measurements of the meteorites, we can interpret that the surface of 1999 JD6 is covered with 25–50 μm grains that are rougher than those on the Moon.

In future work, additional examples of polarimetric observations of asteroids at high phase angles and measurements of meteorites under various conditions (e.g., particle size and porosity) may advance our understanding of the surface of asteroids.

Acknowledgements

We thank the anonymous referee for the helpful suggestions. We would like to thank Mr. Yoonsoo P. Bach for providing the source code that was used as a reference when developing the MCMC code used in this study. M.I. was supported by the NRF funded by the Korean Government (MEST) grant No. 2018R1D1A1A09084105 and the Seoul National University Research Grant in 2018. The work of S.H. was supported by JSPS KAKENHI (grant nos. JP18K03723, JP19H00719, and JP20K04055) and the Hypervelocity Impact Facility (former facility name: the Space Plasma Laboratory), ISAS, JAXA. This research was partially supported by the Optical & Near-Infrared Astronomy Inter-University Cooperation Program, MEXT, of Japan. Part of the data used in this publication was obtained and made available by The MIT-UH-IRTF Joint Campaign for NEO Reconnaissance. The IRTF is operated by the University of Hawaii under Cooperative Agreement no. NCC 5-538 with the National Aeronautics and Space Administration, Office of Space Science, Planetary Astronomy Program. The MIT component of this work is supported by NASA grant 09-NEOO009-0001, and by the National Science Foundation under Grants Nos. 0506716 and 0907766. This research utilizes spectra acquired by Fraser P. Fanale (MH-FPF-057-A/CAMH57) and Michael J. Gaffey (MR-MJG-114-P2/MGP108) with the NASA RELAB facility at Brown University.

Appendix A Correction terms of MSI in 2019 and 2015

The polarimetric observation data of MSI need to correct polarization efficiency, instrument polarization and the zeropoint of the position angle of the celestial plane. Table A.1 presents the correction terms, which were calculated based on data obtained in 2019 June and September. See also Ishiguro et al. (2017) for the 2015 correction terms.

Table A.1

Correction terms of MSI in 2019 and 2015.

Appendix B Individual measurement data of 1999 JD6

Table B.1 shows the linear polarization degree, the polarization position angle, the phase angle, and the Modified Julian Date.

Table B.1

Measurement results of our polarimetric observations.

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All Tables

Table 1

Observational summaries of (85989) 1999 JD6

In the text
Table 2

Nightly weighted mean values of our polarimetric observations.

In the text
Table 3

Estimates of polarimetric parameters.

In the text
Table 4

Comparison of polarimetric parameters for meteorites.

In the text
Table A.1

Correction terms of MSI in 2019 and 2015.

In the text
Table B.1

Measurement results of our polarimetric observations.

In the text

All Figures

thumbnail Fig. 1

Phase angle–polarization degree curve of 1999 JD6. The data arethe nightly weight means (see Table 2). The symbols in the legend indicate each of the nightly data, with the V band in green and the RC band in red. The June 14, 2015, data, which had a noticeably larger error bar, had faint magnitude and poor sky conditions, but were adopted because no other data were available around this phase angle.
In the text
thumbnail Fig. 2

Comparison of the linear polarization degrees of 1999 JD6 and other near-Earth asteroids in the red region (about 650 ± 50 nm). The open circles indicate data corresponding to 1999 JD6. Other symbols denote data for (152679) 1998 KU2 (open black crosses; Kuroda et al. 2018), (3200) Phaethon (cyan triangles; Ito et al. 2018; Shinnaka et al. 2018), (1566) Icarus (blue squares; Ishiguro et al. 2017), (1685) Toro (green pluses; Kiselev et al. 1990), (4179) Toutatis (orange crosses; Ishiguro et al. 1997; Mukai et al. 1997), and (33342) 1998 WT24 (dark green diamonds; Kiselev et al. 2002). 1999 JD6 exhibits a different polarimetric behavior than was reported in previous studies.
In the text
thumbnail Fig. 3

Relation between rotational phase and polarization degree of 1999 JD6. The open squares, open circles, and crosses indicate individual data obtained on July 16, 19, and 20, 2016, respectively (see Table B.1). The plotted data are the deviations after subtracting the increasing portion of the polarization degree from the phase angle. Both bands (V band in green and RC band in red) show small but similar fluctuations in the rotation phase (light curve from Vaduvescu et al. 2017). Our data provide insufficient coverage of the rotational phase to confirm its repeatability.
In the text
thumbnail Fig. 4

Comparison of phase-polarization curves under two fitting methods. The left and right panels demonstrate curve fitting using the nonlinear least-squares method (the Levenberg-Marquardt algorithm) and the Markov chain Monte Carlo method, respectively, using Eq. (7). The open red circles indicate the RC band data of 1999 JD6 in this work. The solid blue, dashed purple, and dotted cyan lines correspond to the Barbarians, L-type, and K-type asteroids, that is, the values of Pmin we used for the fit. Both methods yielded similar convergent solutions in the Babarian model.
In the text
thumbnail Fig. 5

Spectroscopic similarities of 1999 JD6 and others. Top panel: spectra of 1999 JD6 (red circles) and the Barbarians (172) Baucis (green squares), (234) Baraba (cyan diamonds), (679) Pax (blue crosses), and the range of K type (purple triangles) and L type (black pluses) according to the Bus-DeMeo classification (Demeo et al. 2009; Binzel et al. 2019). The spectra refer to textual data obtained from MITHNEOS1. Bottom panel: compares the spectra of the CV3-type Allende (cyan crosses) and CO3-type Lance (green pluses) meteorites with those of 1999 JD6 (red and blue circles). The 1999 JD6 spectra are multiplied by the albedo within the literature values that best match the respective meteorite data. At the side where the wavelength is longer than 0.6 μm, it matches Allende well, while at the blue side, Lance is matched well. The scaled spectra of the meteorites were obtained from the RELAB2 database.
In the text
thumbnail Fig. 6

Phase angle dependence of polarimetric color for 1999 JD6 and size-dependent meteorites. The open red circles and solid lines represent data corresponding to 1999 JD6. The polarimetric data of two meteorites, CO3-class NWA 4868 and CV3-class Allende, have been measured under deposited layer conditions in the laboratory (Hadamcik et al. 2011). The blue squares and cyan triangles adjacent to the dotted lines show a grain size of <50 μm and <200 μm for the NWA4868 meteorite, respectively. The orange crosses and dark green diamonds adjacent to the dashed lines denote <50 μm and <500 μm Allende meteorite particles, respectively.
In the text

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