A QUARTET OF NEAR-EARTH ASTEROID BINARY CANDIDATES

Abstract

Analysis of CCD photometry observations at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) made in 2014 June-October found four main-belt binary candidates. 2012 Tantalus is a probable binary that shows a strong second period but no mutual events. (68348) 2001 LO7 is a probable binary, also showing a strong second period and no confirmed mutual events, but P₂ is a close to an integral multiple to P₁, which sometimes indicates a harmonic alias. (190208) 2006 AQ appears to be probable wide binary, showing a long period lightcurve superimposed by a short period, lower amplitude component. (276049) 2002 CE26 is a known binary (Shepard et al., 2004). Pravec et al. (2006) reported P₁ = 3.293 h with no indications of the satellite. The 2014 observations at CS3 initially led to P₁ = 3.088 h. After further analysis and confirmation of the longer period (Pravec, personal communications), a period of P₁ = 3.928 h was adopted. Unlike the earlier results, there were indications of a satellite with P_orb = 16.26 h, which agrees with estimates for the orbital period based on the discovery radar observations. The lightcurve data, however, indicate a considerably larger satellite than indicated by radar.

CCD photometric observations of four main-belt asteroids made at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) in 2014 June through October showed them to be binary candidates. Table I lists the telescope/CCD camera combinations used for the observations. All the cameras use CCD chips from the KAF blue-enhanced family and so have essentially the same response. The pixel scales for the combinations range from 1.20–1.60 arcsec/pixel.

Table I.

List of CS3-PDS telescope/CCD camera combinations.

Desig	Telescope	Camera
Squirt	0.30-m f/6.3 Schmidt-Cass	ML-1001E
Borealis	0.35-m f/9.1 Schmidt-Cass	FLI-1001E
Eclipticalis	0.35-m f/9.1 Schmidt-Cass	STL-1001E
Australius	0.35-m f/9.1 Schmidt-Cass	STL-1001E
Zephyr	0.50-m f/8.1 R-C	FLI-1001E

All lightcurve observations were unfiltered since a clear filter can result in a 0.1–0.3 magnitude loss. The exposure duration varied depending on the asteroid’s brightness and sky motion. Guiding on a field star sometimes resulted in a trailed image for the asteroid.

Measurements were made using MPO Canopus. If necessary, an elliptical aperture with the long axis parallel to the asteroid’s path was used. The Comp Star Selector utility in MPO Canopus found up to five comparison stars of near solar-color for differential photometry. Catalog magnitudes were usually taken from the MPOSC3 catalog, which is based on the 2MASS catalog (http://www.ipac.caltech.edu/2mass) but with magnitudes converted from J-K to BVRI using formulae developed by Warner (2007). When possible, magnitudes are taken from the APASS catalog (Henden et al., 2009) since these are derived directly from reductions based on Landolt standard fields. Using either catalog, the nightly zero points have been found to be consistent to about ±0.05 mag or better, but on occasion are as large as 0.1 mag. This consistency is critical to analysis of long period and/or tumbling asteroids. Period analysis is also done using MPO Canopus, which implements the FALC algorithm developed by Harris (Harris et al., 1989).

In the plots below, the “Reduced Magnitude” is Johnson V as indicated in the Y-axis title. These are values that have been converted from sky magnitudes to unity distance by applying –5*log (r∆) to the measured sky magnitudes with r and ∆ being, respectively, the Sun-asteroid and Earth-asteroid distances in AU. The magnitudes were normalized to the given phase angle, e.g., alpha(6.5°), using G = 0.15, unless otherwise stated. The horizontal axis is the rotational phase, ranging from –0.05 to 1.05.

For the sake of brevity, only some of the previously reported results may be referenced in the discussions on specific asteroids. For a more complete listing, the reader is directed to the asteroid lightcurve database (LCDB; Warner et al., 2009). The on-line version at http://www.minorplanet.info/lightcurvedatabase.html allows direct queries that can be filtered a number of ways and the results saved to a text file. A set of text files of the main LCDB tables, including the references with bibcodes, is also available for download. Readers are strongly encouraged to obtain, when possible, the original references listed in the LCDB for their work.

Individual Results

2102 Tantalus.

Pravec et al. (1997) reported a weighted average period of 2.391 h, although one set of observations found a synodic period of 2.380 h, which is close to the period given here.

The plot labeled “No Sub” shows the results before using the dual period search feature of MPO Canopus. The larger than usual scatter prompted a search for a second period on the off-chance that it was due to something other than systematic or random reasons. The “P1” plot shows the eventual result for the purported primary of a binary system and shows considerably less scatter. The “P2” plot shows the lightcurve for the second period. This can be interpreted as an elongated satellite with a rotation period of 16.49 hours with the viewing geometry not favorable for seeing mutual events (occultations and/or eclipses). A half-period solution is also possible, but that would be too short if assuming typical densities for the two bodies. Since there are no mutual events seen, this is listed as a probable binary.

(68348) 2001 LO7.

Skiff (2011) reported a period of 3.324 h based on two nights of observations in 2011. He did not report any indications of a satellite based on about 12 hours of observations.

The “No Sub” plot shows some significant deviations, too much in most cases to be due to systematic or random effects. This prompted a dual period search which led to the results shown in the “P1” and “P2” plots. The P1 plot shows considerably less scatter. The period is also with the typical range for a small binary asteroid (Pravec et al., 2010). The P2 plot has a period of 17.54 h, also within range of typical values given the primary period. The lightcurve is not complete but does show the typical bowing due to the rotation of a tidally-locked, elongated satellite. There are no obvious signs of mutual events, and so this must be listed as a probable binary candidate.

There is a caveat to the results for 2001 LO7. P2 happens to be very close to an integral ratio of 9:2 with P1. When the two periods have an integral ratio, there is a possibility that they are harmonically related and, therefore, due to the Fourier analysis locking onto noise of the shorter period to find a longer period. This is sometimes called a harmonic alias. Such aliasing is not considered likely in this case given the significant improvement in the P1 lightcurve when subtracting P2 or, put another way, the amplitude of P2 is larger than might be expected due to random or systematic effects.

(190208) 2006 AQ.

There are three basic types of small binary asteroids. One is the fully synchronous binary, where both bodies have the same rotation period that is also the orbital period. The fully asynchronous binary has both the primary and satellite rotating with different periods and the satellite’s period is different from its orbital period. The two asteroids before are examples of the third type, the semi-synchronous binary, which is characterized by the primary and secondary having different rotation periods but the satellite’s rotation period is also its orbital period. Jacobson and Scheeres (2011) discuss the different types in detail, how they form, and how they evolve. It is highly recommended reading.

2006 AQ is believed to be a member of the second type, i.e., a fully-asynchronous type and, more specifically, part of the wide binary subset. These objects show a long period lightcurve with an amplitude of at least 0.2 mag, often much larger, and periods usually in the hundreds of hours. On top of this is a short period, low amplitude lightcurve that is sometimes hard to distinguish from noise. Due to the conservation of rotational energy, the primary is responsible for the long period lightcurve. The fully-asynchronous satellite produces the short period lightcurve. The orbital period is very long and will likely never be known by photometric means since the odds of seeing mutual events are extremely remote.

The “Raw” plot shows the data for 2006 AQ before any period analysis. A long period component seems very apparent. There are sometimes hints of the short period on this scale but usually the evidence that prompts a dual period search is found when looking at the individual nights and seeing what appears to be short term periodic behavior beyond random noise. The dual period search in MPO Canopus found the results shown in “P1” and “P2”. The plot in P1 is one of the clearer cases of a short period component seen to-date. The curve in P2 is seen to have “cleaned up” somewhat from the raw plot after removing the P1 component.

It is important to emphasize that there is not a binary hiding within every long period lightcurve. Considerable care and review must be used to make sure that the short period component is not just the result of the Fourier analysis locking onto random noise. One way to avoid this is to use low-order analysis, e.g., no more than 4^th order. Often, because of the relatively sparse coverage of the long period, a 2^nd order search is the highest used for that period.

(276049) 2002 CE26.

This asteroid was discovered to be a binary by radar observations (Shepard et al., 2004). The estimates at the time were for a primary of 3 km and satellite of 0.2 km effective diameters. This gives Ds/Dp ~ 0.07. As such, it would not seem likely that ground-based photometry with small telescopes could detect the satellite since the mutual events would be on the order of 0.01 mag (Pravec et al., 2006). They observed the asteroid in 2004 August and found a period of 3.293 h but no evidence for the satellite, which radar also indicated had an orbital period of about 16 hours. These numbers and results set the background for the analysis of the 2014 data from CS3-PDS.

The “No Sub” plot shows the 2014 data after a period search from 2 to 5 hours without trying to subtract any effects of a satellite. The scatter does not appear to random, which would lead to a dual period search even if a satellite was not known to exist. The period spectrum shows the results after finding a second period of P = 16.26 h, which is in agreement with the estimated period from radar observations, and is shown in the “P2” plot.

The period spectrum shows that two periods are favored, the one from Pravec et al. being the lesser of the two over one of 3.088 hours found from the PDS data. The results are labelled as “Warner” and “Pravec” in the lightcurves. It should be noted that using either short period in the dual period search produced the same value for P2 and an identical lightcurve.

The difference between the two short periods is one-half cycle over 24 hours, or what is sometimes called a rotational alias, which is where the number of rotations over a given period is ambiguous. The data set for PDS and Pravec et al. spanned 6 days, which should have been enough to resolve the period. However, the Pravec data were obtained from several stations from different longitudes, which favors their result since rotational aliases are often difficult to break when using data from a single station. In fact, Pravec (personal communications) confirmed that the 3.293 hour period that Pravec et al. reported is secure. Their period spectrum shows no indication of a solution near 3.088 hours. Therefore, the longer period found using the PDS data, i.e., 3.298 h, is adopted for this paper.

Just as much of a mystery is the lightcurve for P2. It does not seem to show signs of mutual events but, in general, does resemble a tidally-locked, elongated satellite. If the “dip” at about 0.5 rotation phase is due to an event, then this would indicate a Ds/Dp far greater than 0.07, i.e., it would be more on the order of 0.25. This contradicts the radar evidence. Even if not due to an event, the feature is too deep and wide to fit with the estimated size of 0.2 km for the satellite.

Follow-up observations would usually be encouraged. However, except for the 2015 January apparition when the asteroid is V ~ 18.8 at –43° declination, it does not get brighter than V ~ 19.7 until 2024 September. At which time it will range from V ~ 15.6 and +22° at the first of the month to V ~ 16.9 and –70° at the end of the month. Unfortunately, it will be moving through the rich star fields of Cygnus, Aquila, and points south, making it a difficult target. Even so, the next generation of photometrists should make note.

Table II.

Observing circumstances. The period and amplitude are for the primary of the binary system. The phase angle (ɑ) is given at the start and end of each date range, unless it reached an extremum, which is then the second of three values. If a single value is given, the phase angle did not change significantly and the average value is given. L_PAB and B_PAB are each the average phase angle bisector longitude and latitude, unless two values are given (first/last date in range).

Number	Name	2014 mm/dd	Pts	Phase	LPAB	BPAB	Period	P.E.	Amp	A.E.
2102	Tantalus	06/19–06/22	490	55.1,52.8	255	46	2.384	0.001	0.12	0.01
68348	2001 LO7	06/22–06/30	176	11.8,14.9	265	20	3.869	0.001	0.1	0.01
190208	2006 AQ	09/18–09/25	264	8.4,15.2	349	4	2.621	0.001	0.08	0.01
276049	2002 CE26	08/18–08/24	245	42.0,39.0	337	31	3.298	0.002	0.07	0.01