ANOTHER TRIO OF POSSIBLE VERY WIDE BINARY ASTEROIDS

Abstract

Lightcurve analysis of the near-Earth asteroids (442742) 2012 WP3, (523604) 2004 QB17, and 2018 RL indicate that they are potential members of a relatively rare class of “very wide binary asteroids.” These objects feature a primary rotational period of tens to hundreds of hours and a secondary rotational period less than 24 hours, usually less than 10 hours. These three bring to 30 the number of suspected members of the class.

CCD photometric observations of near-Earth asteroids at the Center for Solar System Studies (CS3) in 2018 October-November found three that may fit into a class of rare objects called “very wide binary asteroid.” In brief, these are objects where the primary has been spun down by YORP (Rubincam, 2000) to a very long period while the orbit of a smaller satellite has expanded to a point just short of the satellite being able to break free. Depending on the mass ratio between the secondary and primary, the system can collapse or the satellite eventually break away. The latter may result in an “asteroid pair” (Vokrouhlicky and Nesvory, 2008). In either case, the satellite period is not tidally-locked to its orbital period and so the system is “fully asynchronous.” Additional details on the formation and evolution of this class can be found in Warner (2016c and references therein).

Table I gives the equipment used for the most recent observations. All observations were unfiltered and guided at sidereal rate. The exposure time was varied to reach maximum SNR while minimizing trailing of the asteroid.

Table I.

List of telescopes and CCD cameras used for each asteroid.

Asteroid	Telescope(s)	Camera(s)
(442742) 2012 WP3	0.30-m SCT 0.35-m SCT	SBIG STL-1001E FLI ML-1001E
(523604) 2004 QB17	0.50-m R-C	FLI PL-1001E
2018 RL	0.35-m SCT	FLI ML-1001E

Measurements were made using MPO Canopus. The Comp Star Selector utility in MPO Canopus found up to five comparison stars of near solar-color for differential photometry. Catalog magnitudes were taken from the APASS (Henden et al., 2009) or CMC-15 (Munos, 2017) catalogs. Period analysis was also done with MPO Canopus, which implements the FALC algorithm by Harris (Harris et al., 1989). The dual-period feature of the software was used to find the periods of the primary and secondary.

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 distances by applying −5*log (rΔ) to the measured sky magnitudes with r and Δ being, respectively, the Sun-asteroid and Earth-asteroid distances in AU. Unless otherwise stated, the magnitudes were normalized to the phase angle in parentheses using G = 0.15. 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”, this is the amplitude of the Fourier model curve and not necessarily the adopted amplitude for the lightcurve.

For the sake of brevity, only some of the previously reported results may be referenced in the discussions on a specific asteroid. 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 bibcode, is also available for download. Readers are strongly encouraged, when possible, to cross-check with the original references listed in the LCDB.

Observing and Analysis Methodology

In recent years, the work at CS3 has concentrated on near-Earth asteroids under a grant from NASA. The three objects here were observed as part of that program. The usual method is to choose an NEA that is within reach of our equipment (V < 19.2) and follow it for as many nights as needed to find a period, if possible. In some cases, the data for a single night show only a slow increase or decrease in brightness. On rarer cases, there is a moderate to strong indication of a short period being superimposed on the down or upward trend.

After each night, the data are plotted with the raw reduced magnitudes (see above) to check if the lightcurve eventually shows some sort of periodic shape. No changes are made to nightly zero points since that may mask the true trend in the data. When sufficient data are obtained (a minimum/maximum pair seems to be defined), a low-order Fourier solution is attempted with the period search range based on the assumption of a bimodal lightcurve and so the time from one extrema to the next is about one-quarter the rotational period.

The low-order Fourier model curve is subtracted from the data and a 2^nd or 4^th order period search is made within a range that seems appropriate based on the raw data from each night. As more data become available and they continue to indicate a viable long-period solution, the dual-period search is applied after each observing run to confirm that the short period also seems viable and is not just noise on which the Fourier analysis has falsely found a period.

There are important considerations in this process. One is the possibility that the lightcurve is evolving with changing phase and phase angle bisector if the asteroid is followed for weeks or even months. This may affect the long period solution if the data are able to cover a second rotation at the presumed period. If they don’t quite fit the model and previous data at the same rotation phase, the question becomes whether the discrepancy is because of an evolving lightcurve or because the asteroid is tumbling.

If the asteroid is tumbling, then the secondary period might be a low-amplitude component of a complex lightcurve. It will depend on the amplitude of the two lightcurves and the periods. A rule of thumb is that the rotation and “wobble” frequencies cannot be separated by more than a factor of about the inverse amplitude of variation (Alan Harris, private communications). For example, if the larger amplitude is 1 magnitude, the two periods must be of the same order. If the amplitude is 0.3 mag, then the two periods might be as much a factor of 3.3 different. Of all the candidates to-date (see Table II), two or three could be tumbling or a very wide binary.

Table II.

Current list of very wide binary candidates.

Number	Name	F/G	P1	P2	Reference
1876	Napolitania	H	45	2.825	WBD 2016a
2759	Idomeneus	TJ	479	32.17	SRD 2018a
5626	1991 FE	NEA	134	2.735	WBD 2017c
6063	Jason1	NEA	682	48.6	WBD 2017d
8026	Johnmckay	H	355	14.93	WBD 2011
15778	1993 NH	MC	113	3.320	WBD 2015b
19204	Joshuatree	PHO	480	21.25	SRD 2016
23615	1996 FK12	H	367	3.646	BDW 2015d
24495	2001 AV1	MC	24	2.737	SRD 2017
52750	1998 KK17	NEA	26	3.131	WBD 2017a
67175	2000 BA19	H	275	2.716	WBD 2013
119744	2001 YN42	MBI	625	7.24	WBD 2014
139345	2001 KA67	NEA	44	6.011	SRD 2018b
190208	2006 AQ	NEA	182	2.621	WBD 2015c
215442	2002 MQ3	NEA	473	2.649	WBD 2016c
218144	2002 RL66	MC	587	2.49	WBD 2010
252793	2002 FW5	NEA	61	8.33	WBD 2017b
442742	2012 WP3	NEA	221	8.19	This work
463380	2013 BY45	NEA	428	15.63	WBD 2016b
464797	2004 FZ1	NEA	45	12.49	WBD 2017a
523604	2004 QB17	NEA	75	2.351	This work
	2009 EC	NEA	48	3.261	WBD 2016c
	2009 ES	NEA	28	2.988	WBD 2017a
	2013 US3	NEA	450	2.405	WBD 2018b
	2014 PL51	NEA	205	5.384	WBD 2015a
	2015 KN1202	NEA	46	9.107	WBD 2018a
	2016 BU13	NEA	39	2.450	WBD 2016c
	2016 EV27	NEA	61	18.0	WBD 2016b
	2018 RL	NEA	75	7.782	This work
	2018 KE3	NEA	47	4.168	WBD 2019

¹Radar could not confirm long period.

²Might be tumbling instead binary. F/G is the family or orbital group using definitions from Warner et al., (2009): H, Hungaria; MC, Mars-crosser; MBI, inner main-belt; NEA, near-Earth asteroid, PHO, Phocaea; TJ, Jupiter Trojan. WBD: Warner (et al.); SRD: Stephens (et al.).

This brings up the most important consideration: lack of definitive proof. When using lightcurve data alone, an asteroid is usually not considered to be binary unless mutual eclipse/occultation events are captured. These are seen as attentions in the primary lightcurve as the satellite passes in front of the primary (primary event) and behind the primary (secondary event). Because the orbital periods of very wide binaries are very long, the chances of capturing one event, let alone two or more for confirmation, are exceedingly small. For this reason, all candidates for the very wide binary group at this time are, at best, only suspected and not confirmed binaries.

The Nominees Are…

(442742) 2012 WP3.

The observations for 2012 WP3 were made on nine nights 2018 November. At that time, the LCDB did not have any rotational periods listed. After several nights, a long period component was seen and the dual-period analysis described above was implemented. The solution for the long period is mostly secure. One important feature that helps support this is that the slope of data for each night’s run fits the slope of the Fourier curve. If this were not the case, then tumbling action might have been the better interpretation.

The period spectrum for a secondary period shows several weak possibilities. We adopted the one that gave a bimodal lightcurve, i.e., P = 8.19 h. Given that the noise in the data rivals the amplitude of the secondary lightcurve, 2012 WP3 is among the weaker candidates for belonging to the very wide binaries group.

(523604) 2004 QB7.

The secondary solution with a bimodal lightcurve is slightly better than for 2012 WP3, even though the noise again rivals the amplitude of the lightcurve and the overall data set was noisy because of interference from the moon.

2018 RL.

The presence of a long period component for 2018 RL was well-established but the period solution was not because of the large gaps in the lightcurve. Once a short period seem sufficiently defined, attention was turned to other targets because the data for 2018 RL were too noisy. After a point, additional data did not improve the solution and only added more noise.

Conclusions

Table II shows the current list of candidates for the very wide binary asteroid class. To be certain, not all have equal status. About a dozen are considered “convincing” that there are two periods and they are not the result of tumbling. Others, such as two of the three here, are “marginal,” if that. The rest fall somewhere in-between.

Properly working the very wide binaries requires either an extended campaign from one station or one involving several stations that are well-separated in longitude. In either case, careful night-to-night zero point calibrations are required. If the situation arises where the zero point adjustments become excessive, especially if they follow a trend up or down, the data should be examined anew after forcing the zero points back to 0.0 (or other fixed value) and seeing if a plot of the raw data starts to show something like a long period lightcurve.

Since the secondary periods are often less than 10 hours, it becomes important to get sufficiently dense data sets each observing run such that a single period could be found if forcing the zero points to get the data sets to match. This generally precludes working more than one target a night or getting only a few data points, even if at the start and end of the night. If nothing else, observers should be aware of the possibilities and, if a short period is seen superimposed on a steady trend of data in a given night, they should consider altering their observing program to see what results might come of concentrating on the single asteroid.

Table III.

Observing circumstances. The first line for each asteroid gives the period of the primary; the second line gives the period of the presumed satellite. The phase angle (α) is given at the start and end of each date range. If there are three values, the middle one is the minimum phase angle reached during the period. L_PAB and B_PAB are, respectively the average phase angle bisector longitude and latitude (Harris et al., 1984). Grp is the family or group (Warner et al., 2009).

Number	Name	2018 mm/dd	Pts	Phase	LPAB	BPAB	Period(h)	P.E.	Amp	A.E.	Grp
442742	2012 WP3	11/01-11/10	669	10.6,0.5	47	2	221	3	0.30	0.02	NEA
							8.19	0.01	0.03	0.01	NEA
523604	2004 QB17	10/11-11/05	675	18.2,17.2,19.2	35	13	74.97	0.06	0.15	0.02	NEA
							2.3509	0.0003	0.03	0.01	NEA
	2018 RL	11/02-11/08	552	43.8,38.2	19	14	155	5	1.00	0.10	NEA
							7.782	0.005	0.13	0.03	NEA