THREE ADDITIONAL CANDIDATES FOR THE GROUP OF VERY WIDE BINARY ASTEROIDS

Abstract

The very wide binary asteroids (VWBA) are a subgroup of binary asteroids that exhibit very long primary periods and, mostly, short secondary periods that are similar to those of the primary of “normal” small binary asteroids. It is unlikely that confirming mutual events will be seen by photometric observations, mostly because the orbital periods of the assumed satellites will be on the order of days. This paper introduces three additional candidates for this subgroup: (215442) 2002 MQ3, 2009 EC, and 2016 BU13. All three are considered to be among the more convincing examples that such systems exist.

CCD photometric observations of near-Earth (NEA) and main-belt asteroids (MBA) were made at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) from 2016 April to July. For details on the equipment and general processing and analysis procedures see Warner (2016). Three of the asteroids, all NEAs, were found to be suspected binaries that appear to belong to a subgroup that will be called the “very wide binary asteroids,” or VWBA for lack of a more appealing acronym.

Jacobson and Scheeres (2011) postulated that these systems might exist through a complex series of steps that involves fission, binary-YORP (BYORP), and tidal forces. BYORP is a thermal effect that acts on the primary and satellite of a binary system that can lead to the satellite’s orbit expanding to where it escapes from the primary or collapsing until the satellite and primary collide and possibly merge.

In the very wide binaries, the presumed primary has a large amplitude and very long period. In most cases, the period is in the hundreds of hours. The satellite, on the other hand, has a short period and low amplitude (P₂ = 2-4 hours, A₂ < 0.2 mag) that often resembles the attributes of the primary in a “normal” small binary asteroid system. Table I gives the primary and secondary periods and amplitudes for the 14 suspected members of this group.

Table I.

The very wide binary asteroid candidates. Periods are given in hours and amplitudes in magnitudes.

Num	Name	P1	A1	P2	A2	Ref
	2009 EC	48.6	0.44	3.261	0.13	This paper
	2014 PL51	205	0.43	5.384	0.09	Warner et al. (2015; MPB 42, 31-34)
	2016 BU13	39.5	0.24	2.4499	0.11	This paper
1876	Napolitania	45.6	0.39	2.825	0.08	Warner (2016; MPB 43, 57-65)
8026	Johnmckay	372	1.0	2.298	0.10	Warner (2011; MPB 38, 33-36)
15778	1993 NH	113	0.61	3.320	0.04	Warner (2015; MPB 42, 60-66)
19204	Joshuatree	480	0.25	21.25	0.08	Stephens et al. (2016; MPB 43, 220-222)
23615	1996 FK12	368	0.23	3.646	0.09	Warner (2015; MPB 42, 183-186)
67175	2000 BA19	275	0.25	2.716	0.07	Warner (2013; MPB 40, 36-42)
119744	2001 YN42	625	0.52	7.24	0.07	Warner (2014; MPB 41, 102-112)
190208	2006 AQ	182	0.25	2.621	0.08	Warner (2015; MPB 42, 79-83)
215442	2002 MQ3	473	0.38	2.6491	0.31	This paper
218144	2002 RL66	587	0.32	2.49	0.04	Warner et al. (2010; MPB 37, 109-111)
463380	2013 BY45	425	0.49	15.63	0.09	Warner (2016; MPB 43, 240-250)

The primary (larger body) is assumed to have the long period and amplitude, otherwise the dilution of amplitude in the combined lightcurve would require that the smaller body be unreasonably elongated. According to Pravec et al. (2010), the limiting size ratio for binaries is about 0.6, or a difference of about 1.0 mag. For a secondary 1.0 mag fainter than the primary to produce a combined lightcurve amplitude of about 0.4 mag would require the secondary’s undiluted amplitude to be several magnitudes, or have near-infinite elongation, as well have a near-equatorial viewing aspect. Furthermore, for such a long period for the primary, the orbital period would be unlikely to synchronize to the spin period because the tidal locking force would be too weak (Alan Harris, private communications).

At the recent Binaries IV workshop in Prague, Czech Republic, (http://www.boulder.swri.edu/binaries4-mtg/) in 2016 June, some of the discussions concentrated on the long period primary. The general thought has been that after fission, the primary would usually spin up again. If the circumstances are right, it can also slow down. If the initial fission event created a satellite with almost but not quite enough energy to escape, this would lead to a very wide binary with a slowly-rotating primary body and a very long orbital period for the satellite. The very wide binaries appear to be evidence for this particular formation mechanism, but additional studies, theoretical and observational, are needed. For example, it’s possible that the systems are not as rare as thought and that the systems found so far are where the primary has yet to spin up again.

In the plots below, the Y-axis gives the Johnson V “reduced magnitude.” These are sky magnitudes converted to unity distances by applying −5*log (rΔ), with r and Δ 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 X-axis is the rotational phase, ranging from −0.05 to 1.05. If the plot includes an amplitude, e.g., “Amp: 0.65”, it is the amplitude of the Fourier model curve and not necessarily the adopted amplitude for the lightcurve. The value is provided as a matter of convenience.

(215442) 2002 MQ3.

This is probably the best candidate to-date for the VWBA group. Appropriately, it was first observed remotely from Prague during the Binaries IV workshop using telescopes at the Center for Solar System Studies in California.

The “Raw” plot shows the data covering 2016 June 12 thru July 2. The long-period component of the lightcurve is easily seen. The raw plots of the individual nights (e.g., July 1) clearly showed a short period component. Such obvious evidence for the secondary period is not usually seen.

When data from individual nights seem to be nearly flat with some minor amplitude “wiggles,” the temptation is to attribute a slowly rising or falling trend from night-to-night to poor zero point calibrations in the photometry or an incorrect value for the phase slope parameter (G) used to account for changing phase angle and viewing geometry. If temptation wins, the individual sessions will be forced to align vertically by adjusting the zero points so that a single-period solution is found. If this temptation can be overcome and the data are left to fall where they may, a long period component may be revealed, as was the case for the three asteroids presented here.

For 2002 MQ3, the long-period component became apparent after a few nights. Initially, because of large gaps in the full-period lightcurve, a half-period solution using only second-order harmonics in the Fourier analysis was found as part of the dual-period search in MPO Canopus. With each night, the long period became more certain and the short-period solution stabilized at P₂ = 2.6491 ± 0.0001 h and A₂ = 0.31 ± 0.04 mag. Eventually, a full-period solution could be found (P₁ = 473 ± 5 h, A₁ = 0.38 ± 0.03 mag), although a second-order fit was still used to produce a smoother lightcurve.

To get the final solution for P₁ required adjusting a few of the nightly zero points by up to 0.1 mag, which is the most usually expected when using the MPOSC3 catalog in MPO Canopus (see Warner, 2016). However, these were much smaller than required to get a single-period solution by forcing zero points such that the long-period component was arbitrarily removed.

2009 EC.

This is one of the unusual (“dark horse”) candidates for the VWBA group, mostly because the long period is only 48.7 hours. Since this is almost commensurate with an Earth day, it was not possible to get complete coverage of P₁ from CS3 alone. The period spectrum shows that a half-period of about 24 hours could be reasonably eliminated. As with 2002 MQ3, the half-period solution based on a second-order fit was used for the initial stages of dual-period analysis. Otherwise, the Fourier model lightcurve had very large and physically impossible gyrations. Eventually a full-period, second-order fit was found and used in the final analysis to find the two periods.

The period spectrum for P₂ shows a possible solution at about 3.2 hours as well as a half-period solution near 1.6 hours. The lack of a half-period solution near 2.5 hours and a trimodal lightcurve at a full-period near 5 hours helped confirm that P₂ = 3.261 h was most likely correct.

Despite the noisy data, the solution for both periods is considered sufficiently secure to list this NEA as another member of the very wide binary asteroid group.

2016 BU13.

This is another unusual very wide binary asteroid candidate because the “short” primary period, i.e., P₁ << 100 hours. The secondary period and amplitude (P₂, A₂), however, are in line with most of the other secondary members in the group.

After finding P₁ and P₂, a period search was done that subtracted both Fourier model curves. This was done to check that the result is a nearly flat line (no third period) or to spot obvious outliers. As with the other two candidates, there was no reasonable evidence for a third period.

Looking Back for Confirmation

Only two of the fourteen VWBA candidates have been observed at CS3 more than once. Duplication of results is an important element of good scientific method. The first object is 8026 Johnmckay. Observations in 2011 (Warner, 2011) found P₁ = 372 h based on a lightcurve with about 2/3 coverage of the full period. P₂ was 2.2980 h. In 2015 (Warner, 2015), a lightcurve with about 90% coverage but some large gaps fit P₁ = 363 h, or essentially the same result given the data sets and error bars. On the other hand, the P₂ lightcurve was almost flat and a period near the earlier one had to be forced. That gave P₂ = 2.2942 h but it would not have been accepted as a stand-alone solution, i.e., without prior knowledge of the “correct” answer.

The other case was 19204 Joshuatree. Stephens and Warner (2016) found P₁ = 480 and P₂ = 21.25 h, the latter having small deviations from a bimodal lightcurve that made it a little suspect. These results prompted a second look at data from 2013 (Warner, 2013). There, the temptation to play with zero points had taken over and a single-period solution of 19.55 h was found. The original data were revised to use the original zero points. This led to a lightcurve covering only 25% of a lightcurve with a period of ~480 hours, but it had the approximately correct shape and amplitude. This emphasizes the need for having the patience to follow an asteroid long enough to assure a good solution and to trust the star catalogs – until there is good reason not to.

It will be important to do follow-up on all the known VWBA candidates and any others that may be found from here on. The design of the observing program at CS3 (and having up to nine telescopes) allows concentrating on these difficult targets, and they are difficult for a number of reasons. However, doing so in the past few years, looking carefully but with a healthy skepticism – not every long-period asteroid is binary, has allowed finding what may the first known members of a somewhat rare and highly interesting group of binary asteroids that will occupy the theorists for some time to come.