ASTEROID LIGHTCURVE ANALYSIS AT CS3-PALMER DIVIDE STATION: 2014 OCTOBER-DECEMBER

Abstract

Lightcurves for 18 main-belt asteroids were obtained at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) from 2014 October through December. All but one of the asteroids were a member of the Hungaria orbital group or collisional family, observed as follow-up to previous apparitions to check for undiscovered satellites, to improve previous binary discovery parameters, or to obtain data for spin axis and shape modeling.

CCD photometric observations of 18 main-belt asteroids were made at the Center for Solar System Studies-Palmer Divide Station (CS3-PDS) in 2014 October through December. 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.

Desiq	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 (2007c). 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.

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

For a number of the asteroids, the additional dense lightcurves allowed finding a preliminary shape and spin axis model. Those results will be presented in a future paper.

1920 Sarmiento.

The results from the most recent observations are in good agreement with previous results: Warner (2007b; 4.0501 h) and Stephens et al. (2014; 4.038 h).

3483 Svetlov.

This was the third apparition at which Svetlov was observed by the author. Previous results were 6.790 h (Warner, 2010c) and 6.811 h (Warner, 2012c), both in good agreement with the results obtained from the 2014 observations.

4125 Lew Allen.

Previous results from the author include Warner (2007b, 4.628 h; 2010a, 4.625 h; 2012a, 4.629 h). The period of 4.619 h found using the 2014 October observations is in good agreement with those earlier results.

4531 Asaro.

This was the third apparition for this Hungaria by the author. The most recent observations from 2014 December lead to a period of 4.118 h, in a good agreement with earlier results (Warner; 2013b; 2015)

4713 Steel.

Behrend (2002) found a period of 5.186 h for this Hungaria. The author found similar results at two subsequent apparitions (Warner, 2010c, 5.199 h; 2012b, 5.193 h) and from the 2014 Dec observations (5.203 h).

4765 Wasserburg.

The period for Wasserburg is well-determined, based on observations by Warner (2010b, 2013c) and Pravec et al. (2010, 2013).

No signs of a suspected satellite (Warner, 2013c) were seen in the 2014 December observations.

5841 Stone.

The period of 2.880 h from 2014 October observations is in good agreement with previous results from the author (Warner, 2007a; 2010a; 2013a; 2015).

9387 Tweedledee.

The results from 2014 October are similar to those from, e.g., Warner (2013a) and Stephens (2015).

(15786) 1993 RS.

Previous results include 13.62 h (Warner, 2007b) and 13.84 h (Warner, 2010b). The initial results from the 2014 October data favored other solutions and so the new and previous data were re-examined to see if the ambiguities could be resolved. First, a look at the 2014 results.

The period spectrum for the 2014 data is highly ambiguous, showing several periods of nearly equal probability. Presuming that the reanalysis of earlier data is correct, the preferred period from the 2014 observations is 6.82 h. However, a period of 5.97 h cannot be formally excluded. The situation is complicated by the somewhat high phase angle of 33° and the amplitude of only 0.14 mag. As discussed in Harris et al. (2014), the presumption of a bimodal lightcurve is not always correct under these circumstances. Furthermore, both periods are nearly commensurate with an Earth day, the difference between the two being 0.5 rotations over 24 hours. Despite these considerations, the longer period of about 13.6 h was ruled out because it would require a complex quadramodal lightcurve. This is not entirely impossible but considered unlikely in this circumstance.

Looking at the 2006 data, the period spectrum gives further justification for adopting a period of 6.82 h over 5.97 h. However, it does not exclude the 13.62 h period first reported in Warner (2007b). Adopting a period of 6.82 h for the 2006 requires accepting a monomodal lightcurve. Again, from Harris et al. (2014), this is not unreasonable given the phase angle and low amplitude of 0.13 mag.

The period spectrum from the 2010 data set was also ambiguous, but again seemed to reject the 5.97 h period. However, yet another pair of solutions was revealed: 6.48 h (monomodal) and 12.96 h (bimodal). The difference between 6.48 h and 6.82 h is 0.2 rotations per 24 hours, so it’s not likely the difference is a simple mismatch of halves of a symmetrical lightcurve, i.e. a rotational alias.

Zero point adjustments on the order of 0.02 mag and less were tried to see, if nothing else, the 2010 data could be forced to a period closer to 6.8 h. Those efforts proved fruitless. The period for 1993 RS must be considered uncertain although it is more likely that the period is in the range of 6.5-6.9 h than 13.6-13.9 h.

20392 Mikeshepard.

There were no previously reported periods for this outer main-belt asteroid. It was observed in honor of its namesake, a noted radar observer and frequent collaborator, who concentrates on M-type asteroids.

24654 Fossett.

The period of 6.007 h closely agrees with previous results from Pravec et al. (2005b), Warner (2010a), and Stephens (2014a).

(25076) 1998 QM98.

The raw plot of the 2014 data covering almost three weeks in 2014 shows not only a long period but signs of being in non-principal axis rotation (NPAR). See Pravec et al. (2005a, 2014) for a detailed discussion of “tumbling” asteroids.

The period spectrum from MPO Canopus shows a two solutions near 30 and 60 hours. A lightcurve was generated that forced the solution to a range near 60 hours. This clearly demonstrates the probability of tumbling action. The data were sent to Petr Pravec (private communications) who agreed that the asteroid was tumbling but he could not find a reliable dominant period.

(40229) 1998 TO3.

There were no previous entries in the LCDB for 1998 TO. The short period, amplitude, and lightcurve shape make it a good candidate for being a binary. No signs of a satellite were seen, i.e., attenuations due to occultations and/or eclipses or a second period. Observations at future apparitions are encouraged. The next chance is 2016 May when the asteroid is at V ~ 17.5 and −49° declination.

(54234) 2000 JD16.

Warner (2012a) found a period of 6.059 h for 2000 JD16, a period that is wholly inconsistent with data from 2014 October-November. The data set from 2011 was sparse in comparison and did not cover as wide a range of dates as the 2014 data set. Therefore, the period of 2.664 h reported here is considered correct and the longer period should be rejected. Here, too, the period, amplitude, and lightcurve shape make this a potential binary. The next opportunity is 2016 June at V ~ 17.7 and −9° declination.

(68553) 2001 XF68.

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

70030 Margaretmiller.

This is a suspected binary (Warner, 2012a). No signs of a satellite were found in the 2014 October data set.

(96518) 1998 RO3.

The period spectrum strongly favored the solution of 6.62 h, which was adopted despite the unusual shape of the lightcurve. This reasonable based on Harris et al. (2014) in that lightcurves with amplitudes of only 0.10 mag or so cannot be assumed to be simple or bimodal, even at low phase angles.

(99395) 2002 AB19.

There were no previous entries in the LCDB for 2002 AB19.

Table II.

Observing circumstances.

Number	Name	2014 mm/dd	Pts	Phase	LPAB	BPAB	Period	P.E.	Amp	A.E.	Group
1920	Sarmiento	10/03–10/06	210	27.6,26.5	45	−18	4.048	0.02	0.35	0.02	H
3483	Svetlov	12/27–12/30	175	26.3,25.8	149	23	6.795	0.005	0.24	0.03	H
4125	Lew Allen	10/12–10/14	138	26.5,25.9	60	21	4.619	0.005	0.23	0.02	H
4531	Asaro	12/26–12/30	132	27.4,28.3	54	26	4.118	0.002	0.18	0.02	H
4713	Steel	12/27–12/30	186	21.0,20.0	131	11	5.203	0.002	0.38	0.02	H
4765	Wasserburg	12/26–12/30	183	24.7,25.7	49	−8	3.664	0.003	0.1	0.01	H
5841	Stone	10/16–10/20	151	23.8,25.5	352	11	2.88	0.001	0.11	0.01	H
9387	Tweedledee	10/16–10/20	183	27.1,28.4	343	13	3.531	0.001	0.13	0.02	H
15786	1993 RS	10/06–10/12	200	33.3,0.5,32.9	63	18	6.82	0.02	0.14	0.02	H
15786	1993 RS	200610/24–10/31	241	16.0,18.3	24	21	6.82	0.01	0.13	0.01	H
15786	1993 RS	201001/21–02/13	201	17.6,14.7	134	21	6.48	0.01	0.08	0.01	H
20392	Mikeshepard	11/20–11/25	186	8.1,6.2	79	−1	29.2	0.5	0.75	0.05	MB–O
24654	Fossett	11/04–11/06	185	12.5,11.6	56	−16	6.007	0.003	0.52	0.04	H
25076	1998 QM98	09/25–10/12	724	15.4,8.3	22	−7	58.3T	0.5	0.35	0.1	H
40229	1998 TO3	10/02–10/04	178	9.1,8.1	19	−9	2.664	0.002	0.12	0.02	H
54234	2000 JD16	10/29–11/04	169	10.4,6.3	47	6	2.664	0.002	0.13	0.02	H
68553	2001 XF68	10/08–10/10	191	23.6,23.2	36	25	3.13	0.01	0.22	0.03	H
70030	Margaretmiller	10/26–10/29	248	15.3,15.4	34	24	4.331	0.002	0.42	0.03	H
96518	1998 RO3	10/01–10/03	141	7.6,8.8	1	7	6.62	0.05	0.11	0.02	H
99395	2002 AB19	10/09–10/12	150	20.9,20.1	36	22	7.08	0.05	0.29	0.03	H

^Tindicates a possible period for a tumbling asteroid.

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;I MB-O = outer main-belt.