ASTEROID LIGHTCURVE ANALYSIS AT CS3-PALMER DIVIDE STATION: 2015 MARCH-JUNE

Abstract

Lightcurves for 29 main-belt asteroids were obtained at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) from 2015 March-June. All but three were members of the Hungaria orbital group or collisional family and observed as part of an ongoing program to obtain data for spin axis and shape modeling. One Hungaria, (79472) 1998 AX4 showed signs of having a satellite. Analysis indicates it is a possible binary.

CCD photometric observations of 29 main-belt asteroids were made at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) from 2015 March-June. Table I lists 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
Squirt	0.30–m f/6.3	Schmidt–Cass	ML–1001E
Borealis	0.35–m f/9.1	Schmidt–Cass	FLI–1001E
Eclipticalis	0.35–m f/9.1	Schmidt–Cass	STL–1001E
Australius	0.35–m f/9.1	Schmidt–Cass	STL–1001E
Zephyr	0.50–m f/8.1	R–C	FLI–1001E

All lightcurve observations were unfiltered since a clear filter can result in a 0.1-0.3 magnitude loss. The exposure duration varied depending on the asteroid’s brightness and sky motion. Guiding on a field star sometimes resulted in a trailed image for the asteroid.

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 found up to five comparison stars of near solar-color for differential photometry. Catalog magnitudes were 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 mag or better, but on occasion are as large as 0.1 mag. This 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 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 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”, this is the amplitude of the Fourier model curve and not necessarily the adopted amplitude for the lightcurve. The value is meant only to be a quick guide.

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., 2009a). 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. Readers are strongly encouraged to obtain, when possible, the original references listed in the LCDB for their work.

3225 Hoag.

This Hungaria member had been observed by the author at four previous apparitions (see the LCDB). Each time, a period of about 2.373 hours was found, as was the case using the 2015 data set.

3483 Svetlov.

This was the fourth apparition that this Hungaria was observed by the author. The period of 6.80 h found from the 2015 data set agrees within error bars of the previous results.

4142 Dersu-Uzala.

The 2015 apparition was a case of “too little, too late.” Observations were not started until well after opposition, when observing runs were a small fraction of the previously reported periods of 71.2 h (Warner, 2007a) or 140 h with the possibility of tumbling (Warner, 2009c). This combined with a nearly full moon lead to abandoning the asteroid after two nights. The lightcurve shows the data forced to a period of 71 h, the one found in 2007. A 2nd order Fourier period search found a period of 64 ± 21 h.

4232 Aparicio.

The only previous reported period for this Hungaria was 54.4 h (Warner, 2006b). The results from 2015 support that earlier result.

4483 Petofi.

This was the fifth apparition that the author observed this Hungaria member. In all cases, the period was found to be close to 4.33 h. A preliminary pole solution in ecliptic coordinates of (107°, +40°, 4.33299 h) was reported by Warner (2011a).

Whether or not Petofi is a member of the collisional family or just the orbital group is in debate. If using albedo, a family member should have a value of p_v ~ 0.4, or consistent with a type E (Warner et al., 2009a). Masiero et al. (2012) found p_v = 0.439 based on an absolute magnitude of H = 13.10. However, the AKARI project (Usui et al., 2011) found p_v = 0.254 using H = 13.0, which is consistent with a type S asteroid, of which there are a number in Hungaria orbital space. If a value of H = 13.57 (Pravec et al., 2012) is used with albedo from Masiero et al., and the formula by Harris and Harris (1997) is used to update the calculated albedo, this results in p_v = 0.2872, close to the value from AKARI. However, this also assumes using a value of G = 0.42, which was the value used by Pravec et al. to find a revised value for H. The result becomes p_v = 0.3033 if using G = 0.15. This is still closer to a mid-albedo type object, such as a type S, than to a high-albedo object, such as type E. The true status of Petofi waits for spectroscopic data that can firmly establish its taxonomic type.

5384 Changjiangcun.

Even though a member of Hungaria orbital space, it is almost certainly not a member of the family. IR observations by AKARI (Ususi et al., 2011) and Masiero et al. (2012) both show this to be a low-albedo object, p_v ~ 0.06, which is more consistent with a type C class. The period found from the 2015 data agrees with previous findings by the author and Behrend (2007).

5427 Jensmartin.

Analysis by the author at three previous apparitions and by Stephens (2014) all found a period of P ~ 5.81 h for Jensmartin, a likely member of the Hungaria family based on an albedo of p_v = 0.78 found during the first stage of the WISE mission (Mainzer et al, 2011). This value is a bit excessive, mostly like due to using an incorrect value for H (13.4). Warner (2012a) found H = 14.0 and applied the formula from Harris and Harris (1997) to find p_v = 0.3487, which is still consistent with a type E asteroid, and so making this asteroid a likely member of the collisional family.

The initial period analysis of the 2015 data found a period of about 5.18 hours, considerably different from the earlier results. After forcing the period to a range between 5.7–6.0 hours and making minor adjustments to the zero points for some of the observing runs, the resulting period spectrum favored a period near 5.8 hours and had a lower RMS fit than with the previous result using the original zero points.

5475 Hanskennedy.

This appears to be the first reported lightcurve for this asteroid. The solution is far from definitive but believed to be a valid estimate.

5968 Trauger.

Based on previous results by the author (Warner, 2011a; 2012b; 2014a), the amplitude of this Hungaria never exceeds 0.20 mag, which can make the solution at least a little suspect (see Harris et al., 2014). In 2015, the error bars of the individual measurements were on order with the amplitude, adding more to the uncertainty of the period of 3.786 h. However, it does closely agree with earlier results. Mainzer et al. (2011) found an albedo more consistent with a type S or similar asteroid, of which there are a large number known to be in Hungaria orbital space.

6249 Jennifer.

The result of the 2015 analysis closely agrees with previous results by the author (e.g., Warner, 2006b; 2014). Mainzer et al. (2011) found a high albedo for the asteroid. Even if corrected for a lower value of H, the result would still likely make this a type E asteroid.

6635 Zuber.

The period of 5.541 h is slightly higher than the previous results of about 5.535 h (e.g., Warner, 2014). This may be the result of the noisy data.

8026 Johnmckav.

This Hungaria was reported to be a possible “wide binary” (Warner, 2011b), where the primary has a large amplitude lightcurve with a very long period and a short period, low amplitude component. Those earlier results were P₁ = 372 h, A₁ = 0.9 mag, P₂ = 2.2981 h, A₂ = 0.10 mag.

Analysis of the 2015 data again found a long-period, large-amplitude component of P₁ = 355 h, A₁ = 0.7 mag. Given the incomplete coverage of the long period in both cases, the two results for P₁ can be considered statistically the same. The difference is the solution for the shorter period. As the period spectrum for P₂ shows, the most favored solution is about 14.9 hours and any solution near 2.3 h is essentially non-existent. Zero-points were moved up and down and in a wide range of values to see if the earlier result could be found, all to no avail. To remove the long period entirely required changes of more than 0.5 mag in many cases, which far exceeds any likely errors in catalog magnitudes. It’s possible that future observations with well-calibrated data from multiple observers might eventually resolve the true nature of this asteroid.

10841 Ericforbes.

This appears to be the first reported lightcurve for this Hungaria. The large amplitude and four consecutive nights make the solution highly reliable.

(11058) 1991 PN10.

2015 was the third apparition at which the author observed this Hungaria (Warner, 2011a; 2012b). The period of 6.522 h is consistent with those earlier results.

(15822) 1994 TV15.

This Hungaria was observed by the author at four apparitions prior to 2015 (e.g., Warner, 2014a). As with the most recent analysis, the period was found to be P ~ 2.9599 h. The 2015 apparition showed the lowest amplitude, A = 0.18, by almost 0.1 mag, which may give an indication of its pole longitude. The phase angle bisector longitude (PAB, see Harris et al., 1984) was 188°, and so the pole spin axis longitude may be close to that value, or 8°, depending on whether the observations were favoring the north or south pole of the asteroid. Waszczak et al. (2015) using data from the Palomar Transit Factory found a similar period. They also reported H = 15.06 (assuming V-R = 0.45). This is about 0.6 mag fainter than the value in the MPCORB file.

(20996) 1986 PB.

The only previous result is 43.6 h (Warner, 2012b). The 2015 data was too sparse to find a reliable period. The plot has been forced to a solution in the range of 40-50 hours.

(21261) 1996 FF.

This appears to be the first reported lightcurve for this Hungaria. The period makes it a possible candidate for being a binary. Future observations are planned and encouraged.

(24465) 2000 SX155.

The two previous results from the author (Warner, 2009b, 9.156 h; 2012c, 21.46 h) could not have been much more different. The 2012 results also included an alternate solution of 19 hours. Analysis of the 2015 data did little to resolve matters. Forcing the data to the previous results were unsatisfactory, the closest being a trimodal solution near 9.69 h. Given the reasonably good, but not perfect, fit to a period of 5.71 h, it should be adopted for the asteroid until more definite proof is available.

(30856) 1991 XE.

The results from four previous apparitions (e.g., Warner, 2012c) ranged from 5.353 to 5.361 h, or statistically the same. The 2015 result is consistent with those.

32890 Schwob.

This appears to be the first lightcurve for this Hungaria asteroid. The period solution is considered secure despite the single coverage of the lightcurve from rotation phase 0.80-0.95.

(37568) 1989 TP and (43331) 2000 PS6.

No previous results were found in the LCDB for these two Hungarias.

(79472) 1998 AX4.

Warner (2012c) found a period of 2.28802 h and amplitude of 0.21 mag. Both of these made the asteroid a good candidate for being a binary. However, no evidence of such was found in the initial analysis of those earlier data.

The 2015 data, however, showed what appeared to be strong indications of a satellite based on deviations from the average lightcurve of a single period solution. A dual period search was run in MPO Canopus, with the results being P₁ = 2.8796 h and P₂ = 16.14 h. The shape of the secondary lightcurve does not appear to show signs of mutual events, i.e., occultations and/or eclipses, but instead favors the rotation of an elongated satellite that is tidally locked to its orbital period. The case is not conclusive given the noise in the data and the some irregularities in the secondary lightcurve.

The results of the analysis on the 2015 data prompted another look at the data from 2012. The dual period search found a very reliable solution of P₁ = 2.8800 h, in good agreement with the 2015 period. However, the secondary period of P₂ = 15.52 h is significantly different. Moreover, the shape of the lightcurve is asymmetric and would not favor the same interpretation of a tidally-locked satellite.

Many attempts were made to force the data from one apparition to fit the solution other, with the constraint that the primary period (P₁) was near 2.880 hours. This included zero point shifts and searching an extended range of possible periods. In the end, the results were the same and the two solution sets cannot be fully reconciled. For now, the asteroid is considered a possible binary.

(82066) 2000 XG15.

Analysis of the 2015 data favored a period of 11.23 hours. However, the asymmetry of the lightcurve and the single coverage near rotation phase 0.9 prompted a look at a period in the 7-8 hour range, or about 2/3 the favored solution.

While a fit could be obtained, there were significant deviations from the Fourier curves, especially for the data from April 17. If those data were removed from the solution, the result was a single peak at about 0.2 rotation phase, a minimum at 0.3, and steady, flat-line rise from 0.3 to 1.0. This seemed an improbable solution and so the period of 11.23 hours is adopted for this paper when using the 2015 data.

The author worked the asteroid in 2012 (Warner, 2012c) and a period of 19.07 hours reported. The 2015 data could not be fit to that solution and a new search using the 2012 data was run. The best fit, still not convincing, is to a period of 15.6 hours, which is not commensurate with the period from 2015. The true rotation period for 2000 XG15 remains a mystery.

(105155) 2000 NG26.

Warner (2012c) worked this asteroid in 2012 February. Analysis found many possible periods with the one of 5.08 h adopted. Analysis of the 2015 data produced a pronounced solution at 5.62 hours. Despite the somewhat unusual shape, not unexpected due to the phase angle and low amplitude, the new result is considered reliable, though not absolutely definitive.

(140381) 2001 TR46 and (183581) 2003 SY84.

No entries were found in the LCDB for either Hungaria. The solution for 2001 TR46 is considered secure. 2003 SY84 may be in non-principal axis rotation (NPAR, or tumbling). The slopes of some of the sessions are in conflict with the slope of the Fourier model curve at the time.

(185854) 2000 EU106.

Skiff et al. (2012) reported a period of 3.50 hours based on observations in 2008. Analysis of the PDS data from 2015 gives a period consistent with that earlier result.

Table II.

Observing circumstances.

Number	Name	2015 mm/dd	Pts	Phase	LPAB	BPAB	Period	P.E.	Amp	A.E.	Group
3225	Hoag	03/26–03/28	123	15.4,14.5	207	8	2.373	0.002	0.12	0.01	H
3483	Svetlov	03/29–04/03	349	22.1,23.1	156	30	6.80	0.01	0.28	0.04	H
4142	Dersu-Uzala	04/04–04/05	65	36.6,36.6	134	34	71	3	0.65	0.10	H
4232	Aparicio	04/29–05/19	1084	12.3,18.2	216	20	54.32	0.02	0.6	0.03	H
4483	Petofi	04/04–04/08	104	27.9,27.6	248	26	4.33	0.01	0.87	0.05	H
5384	Changjiangcun	03/29–04/03	456	24.5,24.8	174	32	12.517	0.003	0.76	0.03	H
5427	Jensmartin	04/05–04/12	135	27.7,28.6	142	−22	5.819	0.002	0.50	0.03	H
5475	Hanskennedy	04/19–05/03	320	19.2,17.4	225	31	66.9	0.5	0.63	0.05	H
5968	Trauger	03/29–04/05	197	24.9,22.8	226	15	3.786	0.005	0.09	0.02	H
6249	Jennifer	05/20–05/25	179	12.4,15.1	218	2	4.957	0.001	0.33	0.02	H
6635	Zuber	04/30–05/05	143	23.3,22.3	254	27	5.548	0.005	0.67	0.05	H
8026	Johnmckay	05/26–06/15	277	14.0,0.0,23.1	207	7	355P	5	0.66	0.03	H
10841	Ericforbes	04/13–04/16	153	23.3,24.0	178	27	4.908	0.002	0.95	0.05	H
11058	1991 PN10	04/04–04/07	241	12.0,12.4	196	17	6.522	0.003	0.78	0.05	H
15498	1999 EQ4	1412/31–12/31	25	12.0,0.0,12.4	203	1	?				EOS
15822	1994 TV15	04/07–04/12	189	5.6,8.7	188	0	2.962	0.002	0.18	0.02	H
20996	1986 PB	06/11–06/17	156	29.5,28.3	303	26	46	1	0.4	0.1	H
21261	1996 FF	05/23–05/26	145	12.6,13.8	230	16	3.260	0.001	0.31	0.02	H
24465	2000 SX155	04/07–04/12	173	9.2,10.5	190	13	5.71	0.02	0.21	0.03	H
30856	1991 XE	04/19–04/21	150	22.4,21.7	239	20	5.356	0.002	0.80	0.03	H
32890	Schwob	04/19–05/07	277	30.2,25.5	238	35	7.930	0.001	0.67	0.04	H
37568	1989 TP	05/26–06/11	240	19.1,27.1	220	6	51.85	0.05	0.47	0.03	H
43331	2000 PS6	04/09–04/12	184	18.1,18.7	194	26	7.338	0.005	0.53	0.04	H
79472	1998 AX4	04/29–05/16	356	16.4,15.3,15.5	226	26	2.8796P	0.0002	0.19	0.03	H
79472	1998 AX4	1203/22–04/10	457	20.3,22.2	178	34	2.8800P	0.0001	0.19	0.03	H
82066	2000 XG15	04/13–04/17	255	14.4,13.0	213	18	11.23	0.02	0.17	0.02	H
82066	2000 XG15	1203/10–03/14	132	23.6,243	152	33	15.6	0.1	0.15	0.03	H
105155	2000 NG26	06/21–06/24	164	26.1,26.1	262	37	5.62	0.02	0.15	0.03	H
140381	2001 TR46	05/28–06/08	214	7.6,13.3	239	5	15.46	0.01	0.22	0.02	H
183581	2003 SY84	06/12–06/26	508	32.1,34.7	262	42	260	5	0.87	0.05	MC
185854	2000 EU106	06/01–06/03	165	28.8,28.6	262	32	3.505	0.002	0.46	0.03	MC

^XXBefore a date: observations in 20xx.

^Apreferred period of an ambiguous solution.

^Pperiod of the primary in a 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). The Group column gives the orbital group to which the asteroid belongs. The definitions and values are those used in the LCDB (Warner et al., 2009). H = Hungaria; MC = Mars-crosser.