(12538) 1998 OH: A CONTINUING NON-RESOLUTION

Abstract

CCD photometric observations at the Center for Solar System Studies (CS3) were made of the near-Earth asteroid (12538) 1998 OH in 2018 November. The goal was to find a secure period and so resolve ambiguous solutions from previous years. Final analysis of the 2018 data found that it is anything but ordinary. One possibility is that it is a low-amplitude, fast-rotating tumbler. The other, more exotic, possibility is that it may be an asteroid pair in the making, i.e., the two fast-rotating components have not yet broken their mutual bond. Future observations may show that one of these, or yet another solution, correctly describes the asteroid.

The near-Earth asteroid (12538) 1998 OH has an estimated diameter of 2 km. It was observed by the author at two apparitions prior to 2018. The first was 2014 (Warner, 2015) where a period of 5.833 h was adopted but noting that the half-period of 2.914 h could not be formally excluded. The second time was in 2016 (2017) which also featured ambiguous periods but the two periods were 5.154 h or 5.191 h. The latter was based on reexamining the data from 2014. The 5.154 h period seemed very secure at that time. The shorter period from 2014 seemed even less likely.

Lozano et al. (2017) reported a period of 5.088 h; this was rated U = 2– in the asteroid lightcurve database (Warner et al., 2009), meaning the period was marginally useful for statistical studies. Vaduvescu et al. (2017) found a period of 2.582 h, which also seemed secure. Their publication came out after the one by Warner (2017). CCD photometric observations of 1998 OH were made at CS3 in 2018 November in hopes of finding a unique, secure period. The result was anything but that.

The observations on 2018 November 4-9 were made with a 0.35-m Schmidt-Cassegrain (SCT) and SBIG STL-1001E CCD camera. Those on November 10 and 11 were made with a 0.30-m SCT and Finger Lakes ML-1001E. Both cameras used the same CCD chip, KAF-1001E, and all exposures were 1x1 binning (1024x1024x9μ). This made the photometric characteristics nearly the same. The 240 s exposures were guided and unfiltered.

The science images were processed with master flat and dark frames. Photometry on the processed images was done using MPO Canopus. The Comp Star Selector utility in MPO Canopus found up to five comparison stars of near solar-color for ensemble differential photometry. Catalog V magnitudes were taken from the APASS catalog (Henden et al., 2009). The only zero point shift was 0.01 mag on November 10. Fourier period analysis was done with MPO Canopus, which implements the FALC algorithm by Harris (Harris et al., 1989). MPO Canopus is capable of an iterative, but not simultaneous, dual-period search. This would become important in the analysis.

In the lightcurves 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 (rΔ) to the measured sky magnitudes with r and Δ being, respectively, the Sun-asteroid and Earth-asteroid distances in AU. The magnitudes were normalized to the phase angle in the parentheses using G = 0.15. The X-axis is the rotational phase ranging from −0.05 to 1.05.

Back to the Past

A quick review of the previous results by Warner (2015; 2017) provides some background for the 2018 results. The results presented here are based on new analysis of the original data and do not necessarily repeat what was previously reported.

2014 October.

The period spectrum showed nearly equal solutions for two periods (and lesser quality ones at several other periods). The amplitude of the lightcurves was A ≤ 0.14 mag. Harris et al. (2014) have shown that a bimodal solution cannot be assumed at low amplitudes and if the data are from low phase angles, as they were in 2014. Solutions that involved tri- and higher modal lightcurves were unconvincing and so either a monomodal or bimodal solution remained.

The monomodal lightcurve had an amplitude of 0.10 mag when fit to a period of 2.592 h. The bimodal lightcurve amplitude was 0.14 mag with a period of 5.186 h. A split-halves plot (see Harris et al., 2014) was used to see if the two halves of the bimodal solution were sufficiently different to adopt the longer period with confidence. They were not and so both solutions were possible.

2016 October.

The period spectrum allowed only one of two periods. The P = 2.5780 h result corresponded to a monomodal lightcurve, although it might be considered bimodal by assuming an usual shape for the asteroid. This didn’t seem likely. The P = 5.154 h solution, which was slightly favored in the period spectrum, was a more convincing bimodal solution.

The split-halves plot for the longer period showed two, distinct parts and so the longer period was adopted. Attempts to fit the 2016 data, within 1-sigma, to either of the 2014 solutions proved fruitless.

Back to the Future

Analysis of the 2018 data showed two possible solutions near the shorter periods previously reported, i.e., P ~ 2.5 h.

The dual-period search in MPO Canopus found two fast periods that were close to one another but not harmonically related, meaning that the two could not be represented by an integral ratio.

There are previous examples (see Harris et al., 2014) where the two periods of a tumbling asteroid produced a beat frequency. Fourier analysis that led to finding the beat frequency period produced an improbable, if not impossible, lightcurve shape. With this is mind, the CS3 data were sent to Petr Pravec (Astronomical Institute, Czech Republic) who has the necessary software to analyze tumbling asteroids properly.

The next part of this discussion quotes extensively from Pravec’s findings (private communication). They are used with his permission and are enclosed in quotes. Comments or paraphrases are enclosed in square brackets if within a quote or in a standalone paragraph without quotes.

“We cannot resolve between [1998 OH] being a binary with two short rotational periods or a tumbler. Both models fit the data equally well (RMS residual 0.026-0.027 mag). If a binary, the lightcurve consists of two additive components with periods 2.4680 ± 0.0005 h and 2.5262 ± 0.0004 h.”

[The CS3 dual-period lightcurves are shown above and have slightly different periods from those found by Pravec. His periods are adopted for this paper and appear in Table I.]

Table I.

Observing circumstances and results. The phase angle (α) is given at the start and end of each date range. L_PAB and B_PAB are, respectively, the average phase angle bisector longitude and latitude (see Harris et al., 1984). The first line gives the results for the primary in the binary system. For 2018, the second line gives the second period with both periods being those found by Pravec (see text).

Number	Name	20xx mm/dd	Pts	Phase	LPAB	BPAB	Period (h)	P.E.	Amp	A.E.
12538	1998 OH	2018/10/22–10/25	176	5.1,3.8	33	6	2.4680 2.5262	0.0005 0.0004	0.11 0.09	0.01 0.01
12538	1998 OH	2016/10/01–10/06	196	22.7,20.8	46	3	2.5780	0.0007	0.18	0.02
12538	1998 OH	2014/10/22–10/25	176	5.1,3.8	33	6	2.592	0.002	0.10	0.01

“We might be tempted to consider it unlikely that it could be a binary with both bodies rotating fast and showing similar amplitudes in the combined lightcurve. However, I note that we observed three near-equal mass ratio asteroid pairs (q = 0.6-0.8) with both the primary and secondary rotating fast (periods 2.7 to 4.7 h) and having amplitudes < 0.3 mag and sizes 1-2 km (see Section 6.1, paragraph 3 in our new paper “Asteroid pairs: a complex picture…” [(Pravec et al., 2019)]. “What if (12538) is a similar system, just with the components bound, not separated into an asteroid pair (yet)?”

“[If a tumbler], the NPA rotation frequencies are two of the following three: 1/2.4664 h, 1/2.5257 h, 1/105 h. Note that 1/2.4664 – 1/2.5257 = 1/105.” Note that in a plot of a tumbling asteroid, there are no “simple” curves since the lightcurve is never the same over multiple rotations.

“[The asteroid] may be somewhat similar to 99942 Apophis (Pravec et al. 2014). It could be a SAM [Short Axis Mode] with I₂/I₃ close to 1, P_ψ = 105 h and Pφ = 2.4664 h. [However,] it would require physical modeling (probably using data from more than one viewing geometry) to confirm.

Conclusions

Without mutual occultation/eclipse events or other definitive observations such as a stellar occultation, the binary status of 1998 OH cannot be confirmed. With the data fitting two significantly different solutions, binary or tumbling, the ultimate resolution will have to wait on further observing campaigns that are planned to allow for either solution. For those looking ahead, Table II shows data for the next five apparitions.

Table II.

The opposition and brightest dates through 2023 are given for 1008 OH. Opposition is judged by the time the asteroid’s RA is 180° from the Sun’s. It is not unusual for an NEA to fail to meet this requirement in one or more years.

	Opposition		Brightest
Year	Date	V Mag	Date	V Mag	Dec
2019	None	–	May 23.3	15.2	+35
2020	None	–	Dec 27.5	16.4	−49
2021	Jan 02.1	16.4	Apr 17.7	16.3	−09
2022	None	–	Dec 31.9	17.4	+03
2023	Jul 31.0	18.1	Jan 02.1	17.4	−47