2011 HM₁₀₂: DISCOVERY OF A HIGH-INCLINATION L5 NEPTUNE TROJAN IN THE SEARCH FOR A POST-PLUTO NEW HORIZONS TARGET

The New Horizons spacecraft will encounter Pluto on 2015 July 14 (Stern 2008), after which, if an extended mission is approved, the spacecraft will alter course using onboard fuel reserves to target a more distant, much smaller trans-Neptunian object (TNO). This post-Pluto encounter will likely be the only opportunity for a close flyby with any spacecraft of a member of this distant population of minor planets in the foreseeable future. No currently known object is accessible with the spacecraft's estimated post-Pluto impulse budget of approximately Δv ∼ 120 m s⁻¹, so a coordinated survey effort is ongoing in order to identify and characterize potential targets (Buie et al. 2012). This survey targets the sky position of objects which will fall into the accessible path of the New Horizons spacecraft. In 2011–2012 the search fields are very near Neptune's trailing triangular Lagrange point (L5). Sheppard & Trujillo (2010a) discovered the first L5 Neptune Trojan, 2008 LC₁₈. Our survey for a post-Pluto New Horizons target has serendipitously yielded the discovery of 2011 HM₁₀₂, a second long-term stable Neptune Trojan in the L5 cloud.

To date, eight other long-term stable Neptune Trojans are known;¹⁴ seven of these occupy the leading L4 cloud, and one occupies the trailing L5 cloud. There are other objects known to be temporarily co-orbital with Neptune; for example, the Minor Planet Center lists 2004 KV₁₈ as an L5 Neptune Trojan, but numerical integrations demonstrate that it becomes unstable on very short timescales, indicating that it is likely a scattering TNO or Centaur which is currently undergoing a transient resonant period, and does not represent a primordial Neptune Trojan (Gladman et al. 2012; Horner & Lykawka 2012; Guan et al. 2012). The eight stable Neptune Trojans occupy a broad range of inclinations (13–281) and libration amplitudes (16°–50° peak-to-peak; Lykawka et al. 2009).

The new L5 Neptune Trojan identified by our survey is not only stably resonant over Gyr timescales, it is also more highly inclined than any other known stable Neptune Trojan (i = 294) and brighter than any other known stable L5 Trojan object in the solar system (H_V ≃ 8.18). It may be a candidate for long-range imaging from the New Horizons spacecraft in late 2013, when it flies by at a minimum distance of approximately 1.2 AU. Observations from the spacecraft may constrain the phase behavior of the surface at large phase angles, and provide a validation exercise for future long-range imaging of other TNOs in the post-Pluto mission phase.

In this paper we outline the discovery circumstances of 2011 HM₁₀₂, its current state of physical and orbital characterization, and prospects for a New Horizons long-range observational campaign.

A coordinated observational campaign has been undertaken to identify a post-Pluto encounter target for New Horizons. Between 2004 and 2005, a wide survey of moderate depth was performed with the Subaru telescope and the SuprimeCam imager intended to search for bright candidates. In 2011 the survey strategy was modified to target the now much smaller core of the sky-plane distribution of accessible objects (given the current understanding of the orbital distribution of the Kuiper Belt, e.g., Petit et al. 2011) with much deeper observations in order to leverage the steep luminosity function of TNOs. Magellan MegaCam and IMACS, Subaru Suprime-Cam, and Canada–France–Hawaii Telescope (CFHT) MegaPrime were all brought to bear on the field in a coordinated effort.

The fields of interest are deep in the galactic plane, near Galactic coordinates l ∼ 12°, b ∼ −5° (Ecliptic λ ∼ 277°, β ∼ 2°; Buie et al. 2012), and are extremely crowded with background stars. Detection of faint moving sources requires high image quality observations and the application of difference imaging to reduce source confusion. Three independent difference imaging pipelines were developed to process the data in parallel, ensuring recovery of as large a fraction of moving objects in the fields as possible. All observations were referenced to a single master astrometric catalog, produced by the CFHT MegaPrime camera and tied to Two Micron All Sky Survey (2MASS) astrometry using the MegaPipe calibration software (Gwyn 2008). This catalog went much fainter than 2MASS, allowing us to utilize long exposure times while still having many non-saturated stars to generate high-precision World Coordinate System (WCS) solutions, and increased the precision and accuracy of our WCS solutions over all imagers we used compared to what would have been possible with the 2MASS stars alone.

Because the multiple independent pipelines were applied in some cases to the same data, not all astrometric measurements are entirely independent. See Table 1 for a table of astrometry and Appendix B for an extended explanation of our approach to treating astrometric records from multiple pipelines.

In 2011, the target search area was approximately 2 deg², though this area was co-moving with the typical rates of TNOs accessible to New Horizons so the actual sky area is somewhat larger. CFHT, Subaru, and Magellan observations covered subsets of this region to varying depth, typically at least r ∼ 25 and at the deepest reaching a 50% completeness depth of r ∼ 26.1 in the central 0.18 deg² field (this depth was measured by implanting synthetic moving sources of known brightness into our data, then blindly recovering them with our pipelines simultaneously with real TNOs). At the current state of reduction, 25 probable new TNOs have been discovered in the 2011 data (as of the submission of this paper).

2011 HM₁₀₂ was first detected in Magellan IMACS observations from 2011 July 1–2, while at a heliocentric distance of ∼27.8 AU. Recoveries in observations from Subaru, Magellan, and CFHT spanning 2011 April 29 to 2011 August 30 indicated that 2011 HM₁₀₂'s semi-major axis was consistent with that of Neptune, but with large uncertainty. During the first re-observation of the New Horizons fields in 2012, early-evening observations were made of 2011 HM₁₀₂ (which is now significantly outside the nominal New Horizons survey fields) and the one-year arc confirmed that the object was in a low-eccentricity orbit with a semi-major axis within 0.1% of Neptune's, consistent with the orbit of an object in 1:1 mean-motion resonance. All astrometry is available in Table 1.

2011 HM₁₀₂ was recovered in a parallel effort by the citizen scientists involved in Ice Hunters¹⁵ in the data acquired at Magellan over 2011 April 29–30. The Ice Hunters users provided a list of transients for one of our pipelines by identifying sources in difference images by visual inspection. Users that contributed to the recovery are listed in Appendix A.

2011 HM₁₀₂ is roughly a magnitude brighter (r ∼ 22.55) than the next brightest TNO discovered. To date, no other object has been found in any of the New Horizons fields which is consistent with a Neptune Trojan, even though our survey has reached a depth at least 2.5 mag fainter (from r ∼ 25 up to 26.1) over the rest of the survey field. For populations of minor planets with power-law luminosity functions of moderate slope (α ∼ 0.4–1.2) which are continuous over the dynamic range of a given survey, the peak detection rate is generally found near the survey's 50% completeness magnitude. Adopting a Heaviside-function efficiency curve truncating at r = 25, for any continuous power-law luminosity function with slope α steeper than ∼0.52, we would expect >95% of any Neptune Trojans detected by our survey to be fainter than 2011 HM₁₀₂. However, if the luminosity function truncates at r ∼ 23.5, as indicated by Sheppard & Trujillo (2010b), then much steeper bright-object slopes would be consistent with the detection of a single object with the luminosity of 2011 HM₁₀₂ in our survey. We therefore interpret the detection of 2011 HM₁₀₂ along with the lack of any fainter detections as further evidence for a lack of intermediate-size planetesimals as suggested by Sheppard & Trujillo (2010b).

The current barycentric orbital elements for 2011 HM₁₀₂ are listed in Table 2. Of note is its very high inclination of 294, making it the highest inclination Neptune Trojan known. The only other known long-term stable L5 Neptune Trojan, 2008 LC₁₈, also has a high inclination at 276. The New Horizons survey fields fall approximately 2° from the ecliptic, and so are significantly more sensitive to lower inclination objects. 2011 HM₁₀₂ was discovered at an ecliptic latitude of ∼246, and given its inclination it spends less than 9% of its time at lower latitudes.

Table 2. Orbit and Color Properties

2011 HM102 J2000 osculating barycentric orbital elements at epoch 2455680.8
Semi-major Axis	Eccentricity	Inclination	Lon. of Asc. Node
30.109 ± 0.002 AU	0.0803 ± 0.0002	293780 ± 00005	1009870 ± 00002
Argument of Periastron	JD of Periastron	Libration Amplitudea	Libration Periodb
ω = 1522 ± 03	2452464 ± 46	194 ± 08	9545 ± 4 years
Observed and derived magnitudes
2011 HM102:	r = 22.55 ± 0.03c	V = 22.75 ± 0.04c	HV(1, 1, 0) = 8.18d
2007 VL305:	r = 22.76 ± 0.03	V = 23.00 ± 0.03	HV(1, 1, 0) = 8.5d
2006 RJ103:	r = 22.03 ± 0.02	V = 22.27 ± 0.04	HV(1, 1, 0) = 7.4d
Observed and derived color indices
2011 HM102:	g − r = 0.51 ± 0.04c	r − i = 0.31 ± 0.04c	g − i = 0.82 ± 0.04c
B − V = 0.72 ± 0.04c	V − R = 0.41 ± 0.04c	R − I = 0.52 ± 0.04c	B − I = 1.66 ± 0.04c
2007 VL305:	g − r = 0.62 ± 0.05	r − i = 0.27 ± 0.05	g − i = 0.89 ± 0.05
B − V = 0.83 ± 0.05	V − R = 0.47 ± 0.05	R − I = 0.48 ± 0.05	B − I = 1.78 ± 0.05
2006 RJ103:	g − r = 0.61 ± 0.03	r − i = 0.06 ± 0.04	g − i = 0.67 ± 0.04
B − V = 0.82 ± 0.03	V − R = 0.47 ± 0.03	R − I = 0.27 ± 0.04	B − I = 1.56 ± 0.04

Notes. ^aMean peak-to-peak libration amplitude over 10⁷ year numerical integration. ^bMean libration period over 10⁷ year numerical integration. ^cUncertainties only represent estimate of shot noise, as sample was too small to empirically estimate scatter around the mean. ^dAbsolute magnitude at zero phase derived assuming phase slope of G = 0.14 mag deg⁻¹.

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The discovery of such a high inclination Trojan prior to the discovery of any low inclination Trojans in near-ecliptic fields lends support to the L5 Neptune Trojans being a highly excited population, as suggested by Sheppard & Trujillo (2010a). For comparison, ∼10% of all Jupiter Trojans with H < 10 have higher inclinations than 2011 HM₁₀₂.

3.1. Resonant Behavior

3.2. Colors of 2011 HM₁₀₂, 2007 VL₃₀₅, and 2006 RJ₁₀₃

On the night of UT 2012 May 23–24, observations with Magellan IMACS (f4 camera, 2 × 4 array of 2k × 4k E2V CCDs, 1 × 1 binning, in fast read mode) measured the gri colors of 2011 HM₁₀₂. Observations were made in photometric conditions, and the Landolt standard 107 351 (Landolt 1992: V − R = 0.351, slightly bluer than known Neptune Trojans 〈V − R〉 ≃ 0.46; Sheppard & Trujillo 2006) was observed over the same air mass range (z ∼ 1.2–2.2) to determine photometric zero-points and extinction coefficients (no color term corrections were calculated or applied). Exposure times for 2011 HM₁₀₂ were 300 s in each filter, and filters were cycled four times throughout the observations to ensure that any rotational light curve (though no evidence was seen for one) would not affect the color measurements. By choosing a standard star with similar color indices to our target population, we minimize potential differences between the Sloan Digital Sky Survey (SDSS) photometric system and the native system of the IMACS camera and filters due to uncharacterized color terms.

Due to field crowding, photometry was performed on difference images; thus the reduction of the target was unavoidably different from the reduction of the (stationary) standard star. The images in each band were point-spread function (PSF) matched (using the ISIS package; Alard & Lupton 1998) to the image in that band taken at lowest air mass (to limit the propagation of extinction correction error from our photometric standard calibration). A template image was constructed by taking the minimum pixel value across the stack of the four PSF-matched images in each band, which was then subtracted off of each PSF-matched image. The motion of the target was sufficient that in the four visits in each filter, the PSF of the source did not overlap in all images in any pixel—but the motion was not so great as to cause any significant elongation of the target's PSF. At this point, since all images are photometrically scaled to the image taken at lowest air mass, we estimated the zero-points and extinction coefficients for all difference images based on the lowest observed air mass in that band. Photometry was extracted with a fixed aperture radius of ∼1 FWHM, and while aperture corrections were determined for each image independently (from the PSF-matched yet unsubtracted images), all were effectively identical (as expected from the PSF-matching).

The Solar System Object Search service (Gwyn et al. 2012) provided by the Canadian Astronomy Data Centre also turned up unpublished color measurements of two bright L4 Neptune Trojans—2007 VL₃₀₅ and 2006 RJ10. These data were collected by the CFHT MegaPrime camera in photometric conditions in November of 2010 through gri filters. The MegaPipe software (Gwyn 2008) was used to perform photometric calibration for these data, and photometry for the two Trojans was extracted from the calibrated images. The measured color indices were then transformed into the SDSS system.¹⁶

Table 2 lists the gri colors of all three objects, as well as these colors translated into the BVRI system using the transformations from Smith et al. (2002). These transformations were verified as accurate for similar TNO surfaces by Sheppard & Trujillo (2006).

We find that the colors of 2011 HM₁₀₂ are moderately red; entirely consistent with the L4 Neptune Trojans measured by Sheppard & Trujillo (2006) as well as those measured here. In general the colors of the Neptune Trojans are consistent with the neutral Centaurs or the Jupiter Trojans (〈V − R〉 ≃ 0.445; Fornasier et al. 2007); Figure 1 demonstrates the color of 2011 HM₁₀₂, the L4 Neptune Trojans, and their relation to other populations. The similar colors of the L4 Trojans and 2011 HM₁₀₂ supports both the L4 and L5 Neptune Trojan clouds sharing a common composition and environmental history, suggestive of a common origin. However, as demonstrated by the Jupiter Trojans' near-IR spectral dichotomy, optical colors alone are not sufficient to rule out compositional variation in an otherwise apparently homogeneous population (Emery et al. 2011).

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3.3. Size and Population Comparisons

The New Horizons spacecraft will make its closest approach to 2011 HM₁₀₂ in late 2013, at a minimum distance of approximately 1.2 AU. Assuming an (H,G) phase curve parameterization with phase slope parameter G = 0.14 (similar to asteroid surfaces), 2011 HM₁₀₂ will reach peak brightness as viewed from the spacecraft several months prior to closest approach. Figure 2 illustrates the separation of New Horizons and 2011 HM₁₀₂ over 2012–2015, and the predicted apparent V-band magnitude of 2011 HM₁₀₂ as seen from New Horizons, accounting for the changing viewing geometry and estimated phase curve. At peak, we estimate that 2011 HM₁₀₂ will appear approximately V ∼ 18 from New Horizons. A single 10 s exposure from the LOng-Range Reconnaissance Imager (LORRI; Cheng et al. 2008) in 4 × 4 binning mode would expect to detect a V ∼ 18 source with S/N ∼ 4; multiple such exposures can be combined to provide useful signal-to-noise ratio (S/N). No observation of a d ∼ 100 km object at this range would be resolved by the LORRI PSF, though binary companions may be resolvable. Such an observation may provide the opportunity to verify the procedures for imaging TNOs during long-range flybys, as there will be several such encounters in the Kuiper Belt. In addition, a detection from the unique vantage point of the New Horizons spacecraft would verify the large phase-angle behavior of TNO surfaces (un-observable from the Earth), which would provide useful information for predicting the outcome of other long-range encounters, and for planning the optimal navigation strategy for targeting these future flybys.

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2011 HM₁₀₂ represents a surprising discovery in a survey not directly designed for the detection of Neptune Trojans. Its brightness compared to the depth of the survey supports the evidence for a break in the Neptune Trojan luminosity function, and its colors suggest that the L4 and L5 clouds share similar physical properties and history. Because of its highly excited orbit, it spends very little time near to the libration center of the L5 Trojans or to the ecliptic (where the survey was directed) suggesting a large unseen population of similar objects. It is likely larger than any L5 Jupiter Trojan, and from its detection we infer that d ≳ 100 km L5 Neptune Trojans are at least an order of magnitude more populous than Jupiter's L5 Trojans of similar size.

Numerical integration suggests that 2011 HM₁₀₂ is stably resonant over the age of the solar system with libration amplitudes comparable to other stable Neptune Trojans.

During mid- to late-2013, 2011 HM₁₀₂ may be detectable by the LORRI instrument on the New Horizons spacecraft, potentially allowing measurements of the phase curve of a TNO surface at far greater solar elongation than has ever been possible. Such a measurement would be valuable for predicting the success of detecting other TNOs at long range in the post-Pluto mission.

This paper includes data gathered with the 6.5 m Magellan Telescopes located at the Las Campanas Observatory, Chile, as well as data collected at Subaru telescope, which is operated by the National Astronomical Observatory of Japan, and on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. Magellan time was acquired through the CfA, MIT, Carnegie, and Arizona TACS, and CFHT time was acquired through the Canada, France, and Hawaii TACs.

This work was supported in part by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. Some of the data presented herein, obtained at the Subaru telescope, was made possible through a time swap from the W. M. Keck Observatory allocated to the National Aeronautics and Space Administration through the agency's scientific partnership with the California Institute of Technology and the University of California. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

A list of the users who contributed to the recovery of 2011 HM₁₀₂ through the Ice Hunters citizen science project:

A. Agbedor, A. Assioli, E. Baeten, T. D. Beer, P. Bel, M. C. Blanaru, M. Bovingdon, P. Brayshaw, T. Brydon, D. Cameron, J. Campos, E. Conseil, M. Cotton, C. Cripps, A. Crouthamel, J. Dadesky, J. M. Dawey, T. Demko, L. Dinsdale, G. Dungworth, A. Duvall, A. Erena, R. Evans, P. Fitch, R. Frasier, R. Gagliano, B. Gilbert, A. Gillis, V. Gonano, F. Helk, F. Henriquez, M. Herrenbruck, J. Herridge, D. Herron, T. Hodge, S. Ivanchenko, M. Kelp, C. Kindel, J. Koopmans, H. Krawczyk, A. Lamperti, D. V. Lessen, S. Li, N. Macklem, M. H. Massuda, A. Maya, M. T. Mazzucato, K. McCoy, P. A. McDonald, R. Mideke, G. Mitchell, V. Mottino, D. O'Connor, M. Olga, N. N. Paklin, A. Pandey, C. Panek, E. R. Pearsall, K. Pidgley, S. Pogrebenko, B. Replogle, J. Riley, K. Roovers, C. Schlesinger, T. Sieben, P. D. Stewart, S. R. Taylor, J. Thebarge, H. Turner, R. H.B. Velthuis, P. Verdelis, E. Walravens, B. Way, B. Wyatt, A. Zane, M. Zehner, and D. R. Zeigler.

As the data acquired for this project was reduced by multiple pipelines, our astrometric record includes overlapping measurements. While these measurements are not wholly independent, they are sufficiently distinct that we include all measurements here. Distinctions include reductions via different astrometric basis functions, difference imaging pipelines, and duration of bins in time. Future submissions to the Minor Planet Center of other discoveries by our team will also include all measurements. In Table 1 in the online journal, measurements made by each independent pipeline are labeled as "SWRI," "CFA," or "HIA." Similarly, the data submitted to the Minor Planet Center is distinguished by unique headers for each pipeline. Treating the non-independence of overlapping measurements should be done at the time of any new orbit fitting; relative weighting of observations may be changed by the addition of new data.