NEAR-EARTH ASTEROID LIGHTCURVE ANALYSIS AT CS3-PALMER DIVIDE STATION: 2013 SEPTEMBER-DECEMBER

Abstract

Lightcurves for 36 near-Earth asteroids (NEAs) were obtained at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) from 2013 September-December.

CCD photometric observations of 36 near-Earth asteroids were made at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) in 2013 September through December. Table I gives a listing of 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 1.24–1.60 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 were made with no filter (a clear filter can result in a 0.1–0.3 magnitude loss) and were guided on a field star, resulting in some cases in a trailed image for the asteroid. The exposure varied depending on the asteroid’s brightness and sky motion.

Measurements were done 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 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 (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 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 plots below, the “Reduced Magnitude” is Johnson V (or Cousins R) 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 given in parentheses, e.g., alpha(6.5°), using G = 0.15, unless otherwise stated. The horizontal axis is the rotational phase and ranges 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. When possible, researchers are strongly to obtain the original references listed in the LCDB for their work.

Tumbling Asteroids

Some of the objects reported here are confirmed (or suspected) to be in non-principal axis rotation (NPAR), or tumbling. The analysis of the periods (frequencies) involved is beyond the capabilities of the author. In such cases, the data are provided to Petr Pravec, Astronomical Institute, Czech Republic, who has developed specialized analysis tools for such objects. Details behind the analysis as well as a thorough discussion of tumbling asteroids in general are in Pravec et al. (2005). Also included is the PAR rating system, which gives an idea on the likelihood that the asteroid is in an NPAR state. This paper should be required reading for those doing asteroid lightcurve photometry.

Very briefly, the lightcurve of a tumbling asteroid is the sum of the integral multiples of two frequencies, i.e.,

$$1.0/\left(nf1\right)+1.0/\left(mf2\right)$$

Except in those cases where f1 and f2 represent very short periods (P < 1 h) and there is an extensive data set covering many rotations of both the principal and precession axes, finding a unique solution for f1 and f2 is very difficult, if not impossible. What is often determined is a dominant period (1/f1) with one or more possible solutions for the second period. Also, it is often not possible to determine which of the two periods is due to principal axis rotation and the other due to precession.

Individual Results

4450 Pan

Pravec et al. (2008) reported a period of 60 hours and that the asteroid probably tumbling (PAR = –1). No signs of tumbling were seen in the data obtained at CS3-PDS in 2013 September, although a more definitive dominant period of 56.48 h was found.

6063 Jason

This NEA appears to be in at least a slightly excited tumbling state. While some of the deviations in zero points in the raw plot of the data (first plot below) might be accounted for by calibration errors, the number and degree of adjustments was too many to exclude the possibility of tumbling. The second plot shows the lightcurve phased to a dominant period of about 52 hours, which was confirmed by Pravec. However a solution for a second period could not be determined before the asteroid faded and moved too far south for additional observations.

(10115) 1992 SK

Pravec et al. (1999) observed this asteroid for an extended time in 1999. Data from early March led a period of 7.328 h and amplitude of 0.72 mag (α = 41.6°). By the middle of March, the period had shortened to 7.31832 h and the amplitude increased to 1.03 mag (α = 69.0°). Polishook (2012) found a period of 7.31 h and amplitude 0.70 mag (α = 73.5°). The 2013 observations at α = 14° showed, not unexpectedly, a lower amplitude of 0.50 mag as well as a period in good agreement with the earlier results.

(17511) 1992 QN

Pravec et al. (1998) reported P = 5.9902 h and A = 1.10 mag; Krugly et al. (2002) found P = 5.99 h and A = 1.1 mag. Analysis of the CS3-PDS data give a similar period with A = 0.79 mag.

(18109) 2000 NG11

Pravec et al. (2000a) reported P = 4.2534 h and A = 1.13 mag (a = 6°). The CS3-PDS data led to P = 4.255 h and A = 0.89 mag (α = 21°). Usually, a larger amplitude is expected at higher phase angles. This could be explained by the fact that the Pravec et al. observations were made when the asteroid presented a more equatorial view than during the 2013 apparition.

(52760) 1998 ML14

Hicks et al. (1998) found P = 14.98 h and A = 0.12 mag. Ostro et al. (2001) reported an effective diameter of about 1 km based on radar observations while Mueller et al. (2011) found D ~ 0.81 km based on IR data. The CS3-PDS data led to P = 14.28 h and A = 0.15. mag.

(53435) 1999 VM40

Pravec et al. (2000a) reported a period centered on 5.189 h using data from mid-January to early February 2000. The amplitude ranged from 0.26 to 0.38 mag over phase angles 1.7–21.5°. The 2013 PDS observations produced P = 5.186 h and A = 0.32 mag at α = 38.8°.

(65733) 1993 PC

The lightcurve for 1993 PC showed significant changes over only a two-week span as the phase angle increased from 12° to 34°. In the early part of 2013 October, the amplitude was 0.73 mag. The period was 4.28 h, although this is based on data from only one night and a minimum of coverage of a second cycle. By 19–25 October, the amplitude had increased to 0.93 mag. and the period decreased to 4.184 h.

(88710) 2001 SL9

This is a known binary (Pravec et al., 2001; 2006). They reported a primary period of 2.4003 h and an orbital period of 16.40 h for the satellite. Analysis of the 2013 data from CS3-PDS showed a primary period in good agreement with the earlier finding (first plot). Only two well-defined events were observed from 25 September through 3 October, which led to 16.19 h for the orbital period. Furthermore, the secondary event, if one is to be seen at all, occurs about 0.1 cycle too soon (or late). The 2000 apparition seemed to show shadow transits as well as occultation and/or eclipses. It might be possible that the viewing geometry didn’t allow seeing the actual satellite event but did catch a shadow event. This might explain the eccentric placement of the shallow event at 0.2 phase in the second plot below.

(142781) 2002 UM11

The period of 6.8 h reported here is just one of many possible solutions (see the period spectrum). No long term trend was seen in the data, at least within the expected ±0.05 mag spread in night-to-night zero point calibrations.

(163696) 2003 EB50

If the data from three nights are excluded and some significant adjustments to zero points are made, a case can be made that this object is in (near) single axis rotation with P = 27.2 h and A = 0.73 mag, meaning that the remaining sessions have the right slope (up or down) for their locations within the modeled lightcurve and don’t “cross swords” at some point. However, included all sessions produces a lightcurve (see below) with two sessions that are essentially flat and “too high” while a third session has a slope that is decidedly different from what it should be for its location. Overall, this should be consider a likely tumbler, with PAR = –1, tending to –2, with no second period guessed at let alone determined.

(249595) 1997 GH28

No previously reported period could be found. The lightcurve is somewhat unusual for the significantly different depths of the minimums.

(25146) 2007 SJ

The period of less than three hours makes this a good candidate for being binary. However, no signs of a satellite were seen during the three nights of observations.

(262623) 2006 WY2

The low amplitude and noise on the order of at least half the amplitude made finding a unique period very difficult. Three plots are presented. The first shows a monomodal lightcurve with P = 16.3 h; the second is a bimodal lightcurve with P = 32.0 h; the final plot shows the period spectrum between 13–36 hours and that neither solution stands out from one another or the general background.

1997 WQ23

No reports of a period were found in the literature. The amplitude of 0.54 mag at the relatively low phase angle of 31° makes a bimodal solution almost certain.

2003 WX25

There were no prior entries in the LCDB (Warner et al., 2009) for this NEA. The period spectrum (not shown) showed a strong preference for the bimodal solution of a monomodal lightcurve with half the period.

2006 CT

This asteroid was observed to assist radar observations made about the same time. The lightcurve shows a very unusual shape, including an almost flat bottom at the deeper minimum. There are also some unexplained “events” (attenuations) that could not be removed with simple subtractive dual-period analysis. The radar analysis is in progress and may help provide insights into the true nature of the asteroid.

2006 HU30

This NEA presents a conundrum. The raw data (first plot) shows a definite overall trend indicating a very long period (about 800 h; third plot). If this trend is subtracted, the result is a low amplitude lightcurve with a period of either 24 or 48 hours (second plot). This is suspicious since it coincides with an integral multiple of the interval between observing sessions.

It’s possible the shorter period is due to systematic effects where the data trend up or down over the course of a night’s observations. To test this, the images were re-measured using a star near the middle of the average field. That star showed no trend over the several hours of a night’s observations. Flat fields were used in all cases and random checks of sky background levels in images with the flat field applied showed a 1% variation or less. Thus, the asteroid traveling across the field would not appear to have caused a constant trend. If anything, it would have reached a minimum or maximum when nearing field center.

For now, the “primary” period is assumed to be on the order of 800 hours and the “secondary” period is unexplained. If there is a satellite, the numbers would indicate a very long orbital period and the odds of seeing mutual events to be exceedingly small.

2010 SL13

There were no entries in the LCDB for this asteroid. The asymmetry of the lightcurve favors a bimodal solution.

2010 CL19

This appears to be the first reported period for 2010 CL19.

2010 XZ67

The data set was too sparse (too short a total time span) for Pravec to determine if this was a tumbler or not. Removing some sessions does allow finding a period of 15.05 h (second plot), which also seems to reveal itself when using all data (first plot).

2011 CQ4

The frequent problem with observing NEAs with backyard telescopes is that the asteroid does not remain bright enough long enough for complete analysis. This is especially true if there are signs of tumbling or, in this case, a period on the order of a few hundred hours. The plot shows the data phased to P = 128 h when assuming a monomodal solution. Since the amplitude is 0.19 mag at α = 31°, it is probably safe to assume that A > 0.2 mag at α(0°). This starts to favor a bimodal solution with a period of about 255 hours.

2013 VJ2

The period spectrum (third plot) covering 2–10 hours shows that no one period dominates over any other or even the background. The first two plots show the data phased to periods of 3.8 h (monomodal) and 7.6 h (bimodal). It seems likely that these are valid solutions, i.e., not picked out of the noise, but neither can be formally excluded.

2013 YL2

The first plot shows the raw data from one of two nights (2014 Jan 1). Given the large amplitude and phase angle, there is no physical shape that can produce this lightcurve assuming single axis rotation (see Harris et al., 2014). Therefore, the object is almost certainly tumbling. Analysis by Pravec found the dominant period of about 2.97 h (second and third plots) but the second period could not be uniquely identified. One possible solution was 2.32 h, which is about 0.03 h longer that one of the minor minimums in the period spectrum.

2013 UR3

Neither a bimodal solution of P = 22.22 h nor a monomodal solution of P = 11.08 h can be formally excluded. A feature near the second minimum in the 22-hour solution does tend to favor it over the shorter period.

2013 XA4

The estimated diameter for this NEA is 150 meters. This makes it possible that it a super-fast rotator, i.e., one with P < 2 h. The period spectrum (first plot) shows that P ~ 1.8 h is possible, being only slightly less likely that the bimodal solution at 3.57 h. There appears to be enough asymmetry in the bimodal plot to make the longer period more likely, although the shorter period cannot be formally excluded.

2013 UH5

There were no previous entries in the LCDB.

2013 TE6

Given the high noise and low amplitude, the solution of P = 2.46 h may just be the result of Fourier analysis locking onto noise. A search up to 6 hours actually favors a period of 4.96 h. Neither can be formally excluded but it’s probably better to say that neither should be formally included.

2013 XY8

The two plots (first, 2013 Dec 10; second, Dec 11) show the dramatic difference that can occur in a lightcurve with changing phase angle. The plot from Dec 10 shows an amplitude of 0.13 mag. The plot from Dec 11, with the same vertical scale, has an amplitude of 0.47 mag. Short exposures (15 s) were used to minimize trailing and to avoid “smearing” the data such that the true rotation period could not be found (see the discussion in Pravec et al., 2000b).

2013 UH9

A half-period analysis led to the final result of 16.3 h.

2013 SU24

This was an unusual case where there were sufficient data to find the two periods of a tumbling asteroid after only two nights of observations. The period spectrum shows two distinct pairs, one set being the half period of the other. Of interest is that the lesser member of each pair is also well-defined versus the background and that is not harmonically related to the other member. This is not often seen but, as it turned out, the “secondary” solution in the set was near the second period found by Pravec in his analysis.

The second plot shows all data phased to the dominant bimodal solution of about 0.2334 hours (14.004 min). The third and fourth plots show the data forced to the two periods found by Pravec, 0.23335 h and 0.19894 h (14.001 m and 11.964 min). While these show the two periods fairly well, the better plots are like those shown in Pravec et al. (2005).

2013 SW24

There were no previous entries in the LCDB for this asteroid.

2013 SC25

Fortunately, the amplitude was large enough and the nights long enough to overcome the period of the asteroid being close to commensurate with an Earth day.

2013 RM43

The two plots show the data phased to the periods found by Pravec. The longer period, 32.6 h, is the more dominant. The second period, 26.1 hours, is less certain and may be an integral multiple of the true frequency (1/P). Pravec rates this as PAR = –2, with a slight tendency to –3.

2013 RH74

Hicks et al. (2013) reported P = 5.38 h and A = 0.20 based on observations around 2013 Oct 15. The CS3-PDS data obtained on Oct 2–9 lead to a solution of P = 5.346 h and A = 0.21. However, a double period of 10.692 h cannot be formally excluded.

2013 TB80

A scan of periods ranging from 1 to 10 hours found a sharp minimum at about 2.6 hours but a number of other longer periods as well, many harmonically related to 2.6 h. Assuming that the solution of 2.632 h is correct, the phase angle of 35° and low amplitude may help explain the asymmetry of the lightcurve, where the maximums are about 0.7 rotation phase apart.

Table II.

Observing circumstances.

Number	Name	2013 mm/dd	Pts	Phase	LPAB	BPAB	Period	P.E.	Amp	A.E.
4450	Pan	09/25–10/01	580	16.0,10.9	12	10	56.48	0.02	0.64	0.02
6063	Jason	10/03–11/03	1887	27.5,25.2,36.5	46	−10	51.7	0.5	0.11	0.02
10115	1992 SK	11/03–11/06	168	12.9,14.2	36	14	7.323	0.005	0.50	0.02
17511	1992 QN	11/01–11/03	202	12.4,13.8	30	11	5.985	0.005	0.79	0.03
18109	2000 NG11	12/24–12/26	209	21.6,20.2	119	1	4.255	0.005	0.89	0.03
52760	1998 ML14	12/05–12/15	210	23.0,12.4	95	5	14.28	0.01	0.15	0.02
53435	1999 VM40	12/09–12/13	240	39.7,37.7	115	20	5.186	0.002	0.32	0.02
65733	1993 PC	10/06–10/06	49	12.7,12.7	3	5	4.28	0.03	0.73	0.02
65733	1993 PC	10/19–10/25	126	30.8,39.0	120	6	4.184	0.001	0.97	0.03
88710	2001 SL9	09/25–10/03	368	21.7,14.5	13	14	2.4013	0.0004	0.12	0.02
142781	2002 UM11	10/26–10/30	294	23.1,19.8	42	19	6.81	0.03	0.03	0.01
163696	2003 EB50	12/21–12/31	1088	62.4,54.5	44	8	27.2	0.1	0.73	0.03
249595	1997 GH28	12/27–01/01	290	37.1,35.0	126	15	19.74	0.05	1.20	0.03
251346	2007 SJ	11/03–11/06	268	21.6,23.3	38	19	2.718	0.002	0.14	0.02
262623	2006 WY2	12/23–12/26	329	16.7,11.8	104	−3	16.3	0.2	0.14	0.02
	1997 WQ23	11/28–11/29	303	29.9,29.3	47	1	14.6	0.1	0.54	0.03
	2003 WX25	10/28–10/31	257	9.4,6.8	43	−1	5.118	0.005	0.22	0.02
	2006 CT	12/27–01/02	457	41.7,26.4	118	1	16.69	0.02	0.80	0.03
	2006 HU30	11/05–11/17	428	16.5,11.7,15.3	53	7	47.6	0.3	0.20	0.02
	2010 SL13	12/11–12/15	130	31.1,30.5	102	−9	7.79	0.02	0.20	0.02
	2010 CL19	12/05–12/08	489	47.0,42.1	45	−2	3.1597	0.0005	0.39	0.03
	2010 XZ67	12/26–01/01	949	26.7,8.1	91	6	15.05	0.02	0.24	0.05
	2011 CQ4	12/27–01/02	304	30.7,38.6	68	−1	128.	3.	0.19	0.02
	2013 VJ2	11/07–11/07	208	11.7,11.7	52	−1	3.8	0.2	0.22	0.03
	2013 YL2	01/01–01/01	962	30.0,30.0	114	−8	2.97	0.01	0.96	0.05
	2013 UR3	11/18–12/08	565	27.9,38.4	40	4	11.08	0.01	0.08	0.01
	2013 XA4	12/21–12/23	206	7.0,8.5	87	4	3.57	0.05	0.15	0.03
	2013 UH5	10/31–11/02	264	8.6,6.3,8.4	42	0	2.69	0.02	0.18	0.03
	2013 TE6	10/25–10/30	239	9.1,6.2	38	4	2.46	0.02	0.12	0.02
	2013 XY8	12/10–12/10	553	12.4,12.4	75	−6	0.06055	0.00001	0.13	0.01
	2013 XY8	12/11–12/11	188	31.2,31.2	94	−5	0.06054	0.00003	0.47	0.03
	2013 UH9	11/07–11/14	153	67.4,69.1	93	5	16.3	0.5	0.80	0.05
	2013 SU24	10/01–10/02	876	9.0,2.3	7	−2	0.23335	0.0001	0.63	0.10
	2013 SW24	10/01–10/02	234	19.9,19.3	3	11	7.09	0.02	0.18	0.02
	2013 SC25	10/31–11/02	182	30.0,31.6	42	23	12.96	0.02	0.72	0.03
	2013 RM43	10/26–11/03	1275	11.1,31.1	43	9	32.6	0.1	0.75	0.05
	2013 RH74	10/02–10/09	570	16.2,23.5	23	4	5.346	0.005	0.21	0.02
	2013 TB80	12/05–12/09	220	36.7,38.4	40	0	2.63	0.01	0.09	0.01

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).