BINARY ASTEROID LIGHTCURVE ANALYSIS AT THE CS3-PALMER DIVIDE STATION: 2013 JUNE-SEPTEMBER

Abstract

Lightcurves from CCD photometry observations for four suspected or confirmed binary asteroids were obtained at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) from 2013 June through September. All four objects are members of the Hungaria family/group. 5968 Trauger showed signs of a satellite based on a single possible event. (11217) 1999 JC4 is a more likely candidate based on a strong secondary period; no mutual events were observed, however. (15822) 1999 TV15 is the most likely new binary discovery by the author, showing mutual events in its lightcurve, albeit they were close to the limit of detection. (76818) 2000 RG79 was a known binary asteroid (Warner et al., 2005). The 2013 observations provided additional data for modeling the system.

CCD photometric observations of four Hungaria asteroids were made at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) in 2013 June through September. Table I gives a listing of the telescope/CCD camera combinations used for observations at the facility. 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 1.24–1.60 arcsec/pixel.

Table I.

List of CS3-PDS telescope/CCD camera combinations.

Desig	Telescope	Camera
PDS-1–12N	0.30-m f/6.3 Schmidt-Cass	ST-9XE
PDS-1–14S	0.35-m f/9.1 Schmidt-Cass	FLI-1001E
PDS-2–14N	0.35-m f/9.1 Schmidt-Cass	STL-1001E
PDS-2–14S	0.35-m f/9.1 Schmidt-Cass	STL-1001E
PDS-20	0.50-m f/8.1 Ritchey-Chretien	FLI-1001E

All lightcurve observations are made with no filter (a clear filter can result in a 0.1–0.3 magnitude loss) with comparison stars for differential photometry limited to near solar-color in order to minimize errors due to color differences. The exposures are guided. The duration varies depending on the asteroid’s brightness and sky motion. In most cases, however, it is 240 seconds.

Measurements are done using MPO Canopus and its Comp Star Selector utility that finds up to five comparison stars of near solar-color to be used in differential photometry. Catalog magnitudes are 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 (2007b). 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 magnitude or better, but on occasion are as large as 0.1 mag. This reasonably good 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 primary lightcurves below, the magnitudes in the Y-axis are sky magnitudes in Johnson V (or Cousins R). If the Y-axis title includes “Reduced Magnitudes”, the values have been converted to unity distance by applying –5*log (rΔ) to the measured magnitude with r and Δ being, respectively, the Sun-asteroid and Earthasteroid distances in AU. The magnitudes were normalized to the phase angle given in parentheses, e.g., alpha(6.5°), using G = 0.15, unless otherwise stated. The horizontal axis is the rotational phase, ranging from 0.0 to 1.0. For the secondary lightcurves, the magnitudes are in reference to the average value of the data after subtracting the primary lightcurve.

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 referred to the asteroid lightcurve database (LCDB, Warner et al., 2009a). The on-line version allows direct queries that can be filtered a number of ways and the results saved to a text file. A set of text files, including the references with bibcodes, is also available for download at http://www.minorplanet.info/lightcurvedatabase.html. Readers are strongly encouraged to obtain, when possible, the original references listed in the LCDB for their work.

5968 Trauger

This was the fourth apparition at which the asteroid had been observed by the author (Warner, 2006; 2011a; 2012). Careful re-examination of the data sets prior to 2013 did not find any traces of a satellite, be it a second period or apparent mutual events due to occultations and/or eclipses involving a satellite.

In 2013, one such event appears to have been observed on Sep 5 but, unfortunately, there were no confirming detections on any other night. The supposed event, seen as a deviation between 0.0 and 0.3 rotation phase in the “No Subtraction” plot seemed to have the correct shape and duration as judged from the lightcurves of known binary asteroids. When the primary rotation (“Primary Period” plot) of 3.786 ± 0.001 h is removed, the alleged event is very apparent. The two plots showing the lightcurve after removing the rotation of the primary are phased to two possible solutions. Without a second event, however, these are only first-order estimates.

If nothing else, the 2013 observations helped confirmed the rotation period of about 3.78 h. Given the lack of evidence from past apparitions (not always the best test) and only the one night showing a deviation, the most that can be said is that this is an asteroid of interest and observations are encouraged when the asteroid is again near the same phase angle bisector longitude or its 180° opposite, i.e., 10° or 190°.

(11217) 1999 JC4

No previously published period could be found for 1999 JC4.

The single period analysis lightcurve (“No Subtraction”) appears to have large amounts of scatter. The dual period search feature of MPO Canopus improved things considerably by finding a primary lightcurve with a period of P = 4.8219 ± 0.0004 h and amplitude of A = 0.11 ± 0.01 mag along with a secondary period of 19.17 ± 0.01 h. The secondary lightcurve shows a trait common to binary asteroids, i.e., an upward bowing. This is thought to be the result of the rotation of an elongated satellite that is tidally locked to its orbital period. There are very faint hints of mutual events at about 0.3 and 0.8 rotation phase in the secondary curve, but they are barely above the noise level, if at all.

Generally speaking, when using only photometry, an asteroid is not considered to be a confirmed binary unless mutual events are seen, even though there is strong evidence for a second period and the shape of the secondary lightcurve is similar to the one for 1999 JC4. Therefore, this must be considered only a probable, possibly likely, binary until additional supporting evidence is produced.

(15822) 1994 TV15

The evidence for a satellite in an unsubtracted lightcurve is not always obvious, especially when the amplitude of the lightcurve is on the order of 0.3 mag and the deviations due to mutual events are only 0.05 mag. Such was the case for 1994 TV15. However, the unsubtracted lightcurve showed deviations from about 0.7 to 1.0 rotation phase on several nights and so the dual period search feature in MPO Canopus was applied.

This resulted in finding a primary period identical to the unsubtracted period of P = 2.95998 ± 0.00006 h and a significantly lower RMS value for the fit to the Fourier curve. The purported events are seen after subtracting the primary period from the data and appear at about 0.4 and 0.9 rotation phase. The depth of the shallower event can be used to estimate the effective size ratio of the secondary to the primary. In this case, the drop was 0.05 mag, which gives Ds/Dp = 0.19 ± 0.02. Since the deeper event does not appear to be total, i.e., it is not flat, this ratio is a minimum, meaning that the purported satellite could be larger.

This Hungaria had been observed at two previous apparitions (Warner, 2007a; Warner and Pray, 2011b). There were no indications of a satellite in 2007. However, the observations from 2010 reported by Warner and Pray did suggest the existence of a satellite with an orbital period of about 37 hours. Suspected events were seen on three nights in mid-June but none in late June or early July. Assuming there is a satellite, those observations may have been out of “eclipse season” due to changing geometry.

The 2010 data set was re-visited to see if it could be fit to the 2013 results. Two observing sessions used in the initial analysis were discarded for being too sparse and/or noisy. The revised analysis found good evidence for mutual events with an orbital period very similar to the one based on the 2013 data set. The size ratio estimate from this data set is also about Ds/Dp = 0.19. This asteroid should be considered a likely binary, if not confirmed, although future observations are strongly encouraged to refine and confirm the results reported here.

(76818) 2000 RG79

Since this was a known binary (Warner et al., 2005), the 2013 observations were intended to provide additional data for modeling the system. The unsubtracted lightcurve left no doubt for there being evidence of a satellite, if one could safely assume no random or systematic errors in the data. The primary period of 3.1669 ± 0.0002 h agrees well with previous results (Warner et al., 2005, 2009b; Pravec et al., 2012), as does the orbital period of 14.134 ± 0.002 h. The mutual events seemed to have evolved a little over the 10-day span of the observations, most notably in the secondary (shallower) event. Assuming a depth of 0.12 mag, this gives an estimated size ratio of Ds/Dp = 0.32 ± 0.02. This is a minimum since the events do not appear to be total. Pravec et al. (2012) gave Ds/Dp≥ 0.35.

Table II.

Observing circumstances. Rows in bold italics text indicate members of the Hungaria group/family. Period is that of the primary in a suspected or known binary system. The phase angle (α) is given at the start and end of each date range, unless it reached a minimum, 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	2013 (mm/dd)	Pts	Phase	LPAB	BPAB	Period	P.E.	Amp	A.E.
5968	Trauger	09/02–09/11	349	18.9 14.1	9	5	3.786	0.001	0.11	0.01
11217	1999 JC4	07/07–08/01	420	28.3 23.1	323	28	4.8219	0.0004	0.11	0.01
15822	1994 TV15	08/15–08/25	549	23.1 18.5	350	17	2.95998	0.00006	0.27	0.01
76818	2000 RG79	08/03–08/13	774	13.7 11.2	326	14	3.1669	0.0002	0.15	0.02