ASTEROID LIGHTCURVE ANALYSIS AT CS3-PALMER DIVIDE STATION: 2018 JULY-SEPTEMBER

Abstract

Lightcurves for 12 main-belt asteroids were obtained at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) from 2018 July-September. Analysis of a larger amplitude lightcurve in 2018 for 5175 Ables lead to new analysis and results for earlier apparitions. The Hungaria asteroid (37378) 2001 VU76 is possibly a super-fast rotator with a period and size that put it just above the so-called “spin barrier.”

CCD photometric observations of 12 main-belt asteroids were made at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) from 2018 July-September. The main focus at CS3-PDS is on near-Earth and Hilda asteroids. However, when there is a full moon or a dearth of NEAs sufficiently bright and placed, members of other groups or families are targeted. Any Hungaria members were observed to provide additional data for spin axis modeling. The other objects were chosen because they had no reported period, were bright enough to work despite the moon, and/or were in the same field as a planned target.

Table I lists the telescope/CCD camera combinations that were used. All the cameras use the KAF-1001E blue-enhanced CCD chip 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	STL-1001E
Borealis	0.35-m f/9.1 Schmidt-Cass	ML-1001E
Eclipticalis	0.35-m f/9.1 Schmidt-Cass	ML-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 cause a 0.1-0.3 mag 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 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.

The nightly zero points for the APASS and CMC-15 catalogs are generally consistent to about ±0.05 mag or better, but occasionally reach >0.1 mag. There is a systematic offset between the two catalogs and so, whenever possible, the same catalog was used for all observations of a given asteroid. Period analysis was done with MPO Canopus, which implements the FALC algorithm by Harris (Harris et al., 1989).

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, 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.

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 Bibcode, is also available for download. Readers are strongly encouraged, when possible, to cross-check with the original references listed in the LCDB.

4019 Klavetter.

This is a 3.4-km (Mainzer et al., 2016) member of the Flora group of asteroids. There was no previous rotation period result in the LCDB.

4483 Petofi is a Hungaria asteroid (5.85 km: Mainzer et al., 2016) that has been worked numerous times by the author (Warner 2008: 2009b: 2012: and 2015). In all cases, the period was found to be close to P = 4.333 h. A preliminary shape model shows the asteroid to be highly elongated, which is supported by the large amplitude seen at every apparition.

5175 Ables.

Mainzer et al. (2016) found D = 4.49 km and p_V = 0.042. This low albedo is not usually compatible with the type Q/Sq taxonomic type found by Lucas et al. (2017).

In previous work by the author (Warner, 2011, Warner 2014), the amplitude was always ≤ 0.1 mag. At such low amplitudes, it is not safe to assume a bimodal lightcurve (Harris et al., 2014). Even so, that assumption was made when reporting a period of about 2.798 h in the earlier works (Warner, 2011 Warner 2014).

With the lightcurve amplitude of 0.33 mag, the 2018 data removed almost all doubt about the lightcurve shape. However, they added doubt about the true period.

The data set in 2018 was not as extensive as in earlier years, but it was still sufficient to find a reliable solution of 2.6862 h, or about 0.11 h faster than the earlier results (Warner, 2011; Warner 2014). This prompted a revisit to the original data used in those works.

On the presumption that the new, shorter period was correct, the zero points on the earlier data were adjusted by very small amounts of <0.01-0.02 mag. After numerous iterations through each set, it was possible to get a reasonable fit near the shorter period. However, the average is 2.647 ± 0.005 h, or 0.04 h faster than the 2018 result. The 2018 period is adopted here, but with the realization that a definitive period has yet to be found.

5998 Sitensky, 17408 McAdams.

There were no previously reported rotation periods in the LCDB for either asteroid. The result for 5998 Sitensky is considered secure given the amplitude, even though the shape is somewhat irregular. A double-period of 13 h was considered, but the two halves of the lightcurve were essentially identical, which helped exclude the longer solution.

The solution for McAdams is not so secure. Unfortunately, there were no previous results to help guide the analysis. The period spectrum shows several “more likely than not” solutions. In the end, P = 18.42 h was adopted since it gave a bimodal lightcurve, which is almost certain given the 0.48 mag amplitude (Harris et al., 2014). The strength of the half-period was also a factor in choosing the longest period in the spectrum. Further observations are encouraged.

(25059) 1998 QA69, 26578 Cellinekim.

This appears to be the first reported rotation period for 1998 QA69, which, according to Mainzer et al. (2016), has a diameter of 3.6 km. They also found p_V = 0.286 ± 0.096, which is consistent with many other inner main-belt asteroids.

It also appears that this is the first reported period for 26578 Cellinekim. Mainzer et al. (2016) found D = 5.75 km and p_V = 0.048, the latter is expected for outer main-belt asteroids.

(37378) 2001 VU76.

The first observations of this 2.2 km Hungaria were made in 2018 late June. At that time, the best solution appeared to be a bimodal lightcurve with a period of 1.780 h. This put the asteroid above the spin barrier of about 2.2 hours. For that reason, the double period of 3.56 h was considered.

This gave a quadramodal lightcurve that was highly symmetric about the two halves, which is a suspicious result since it requires extraordinary symmetry in the asteroid shape. In the end, the 1.780 hour period was adopted.

Such extraordinary results required verification and so the asteroid was re-observed two months later. The phase angle bisector longitude and latitude (see Harris et al., 2004) did not change significantly over that time but the phase angle doubled to 36°.

Lightcurve amplitudes generally increase with larger phase angles (Zappala et al., 1990). In this case, the opposite was true when working with later data set, making it even more difficult to determine the correct period. Since the viewing aspect did not change significantly, the best guess for the unexpected behavior is deeper shadowing effects at the higher phase angle. That, in turn, lends some credibility to a “strange” shape that might result in a quadramodal lightcurve despite the larger amplitude at the lower phase angle.

The mid-August data were fit to P = 1.792 ± 0.001 h, a little longer than the period found in June. This gave a low-amplitude bimodal lightcurve. The fit to a period near 3.5 h, however, gave a lightcurve that could be only loosely interpreted to be showing alternating maximums of different heights, which was the case for the June lightcurve. The asymmetry of the mid-August 3.5-hour lightcurve may justify adopting the longer period but, for now, a period of 1.780 h is considered to be the more probable solution.

The location of the two periods is shown in the frequency-diameter plot based on the LCDB. The shorter period is above the spin barrier, but only by a small amount, i.e., the solution is not unreasonable. The longer period puts 2001 VU76 in the league of ordinary asteroids. Further observations are encouraged.

(56213) 1999 GW50.

There were no previous rotation periods found in the LCDB. The asteroid was not observed by the recent IR surveys and so its estimated size of 6.1 km is based on using an albedo of 0.1, which is assumed for middle main-belt asteroids.

(186035) 2001 RX80.

Circumstances did not allow following this 2.5 km middle main-belt asteroid long enough to get a reliable solution. A waxing moon and fading asteroid is not a good combination.

As a result, the solution is not fully secure. The period spectrum shows two more likely than not solutions. The shorter one of 10.7 hours shows a bimodal lightcurve of 0.59 mag amplitude. However, the shape is not symmetrical: the fall from the first maximum to the minimum takes only 0.35 of the period. Regardless, it was the best fit achieved without extraordinary zero point adjustments.

Given the amplitude, a bimodal solution seemed inevitable (Harris et al., 2014). However, when using a second order Fourier search a period of about 20.4 hours was found, not the double period of 21.4 hours. The data can be fit to both periods. It’s almost certain that the period is about 21 hours. It will take more data to prove that assertion.

(282505) 2004 PA102.

A good data set and large amplitude led to a very reliable solution for this inner main-belt asteroid. The estimated diameter is 1.5 km.

Table II.

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 Group column gives the orbital group to which the asteroid belongs. The definitions are those used in the LCDB (Warner et al., 2009a). FLOR: Flora, H: Hungaria, MB-I/M/O: Main-belt inner/middle/outer.

Number	Name	2018 mm/dd	Pts	Phase	LPAB	BPAB	Period(h)	P.E.	Amp	A.E.	Group
4019	Klavetter	07/23-07/23	106	11.8,11.8	322	3	4.671	0.003	0.27	0.03	FLOR
4483	Petofi	06/29-07/01	152	32.6,32.5	333	26	4.334	0.001	1.05	0.03	H
5175	Ables	06/28-07/02	108	26.4,25.6	321	21	2.6862	0.0007	0.33	0.02	H
5998	Sitensky	07/05-07/19	187	16.1,11.8	322	3	6.500	0.001	0.44	0.02	MB-O
17408	McAdams	08/22-08/28	146	32.9,32.2	27	19	18.42	0.04	0.51	0.05	H
25059	1998 QA69	08/28-09/11	356	7.8,13.6	327	9	11.759	0.004	0.24	0.02	MB-I
26578	Cellinekim	09/19-09/22	136	11.3,10.2	22	0	6.90	0.01	0.3	0.03	MB-O
37378	2001 VU76	06/28-07/01	195	18.6,20.3	261	17	1.78	0.001	0.23	0.03	H
37378	2001 VU76	08/06-08/18	157	34.5,36.7	271	24	1.792	0.001	0.1	0.02	H
56213	1999 GW50	09/16-09/18	108	18.0,17.5	36	−5	2.417	0.002	0.29	0.03	MB-M
186035	2001 RX80	09/16-09/18	87	22.2,21.6	34	−5	21.4	0.5	0.65	0.1	MB-M
282505	2004 PA102	07/23-07/23	155	15.0,15.0	320	2	7.092	0.003	0.69	0.04	MB-I