ASTEROIDS OBSERVED FROM CS3: 2018 JULY - SEPTEMBER

Abstract

CCD photometric observations of 10 main-belt asteroids were obtained from the Center for Solar System Studies from 2018 July to September.

The Center for Solar System Studies “Trojan Station” (CS3, MPC U81) has two telescopes which are normally used in program asteroid family studies such as NEAs, Jovian Trojans and Hildas. During the 3rd quarter of 2018 the targets which are normally studied were either out of season, or the Moon was too close. In these cases, targets of opportunity amongst the main-belt families were selected.

All images were made with a 0.4-m or a 0.35-m SCT using an FLI ML-Proline 1001E or FLI ML-Microline 1001E CCD camera. Images were unbinned with no filter and had master flats and darks applied. Image processing, measurement, and period analysis were done using MPO Canopus (Bdw Publishing), which incorporates the Fourier analysis algorithm (FALC) developed by Harris (Harris et al., 1989). Night-to-night calibration (generally < ±0.05 mag) was done using field stars from the CMC-15 or APASS (Henden et al., 2009) catalogs. The Comp Star Selector feature in MPO Canopus was used to limit the comparison stars to near solar color.

In the lightcurve plots, the “Reduced Magnitude” is Johnson V corrected to a unity distance by applying −5*log (rΔ) to the measured sky magnitudes with r and Δ being, respectively, the Sun-asteroid and the Earth-asteroid distances in AU. The magnitudes were normalized to the phase angle given in parentheses using G = 0.15. The X-axis rotational phase ranges from −0.05 to 1.05.

The amplitude indicated in the plots (e.g. Amp. 0.20) is the amplitude of the Fourier model curve and not necessarily the adopted amplitude of the lightcurve.

For brevity, only some of the previously reported rotational periods may be referenced. A complete list is available at the lightcurve database (LCDB; Warner et al., 2009).

1026 Ingrid.

Szekely et al. (2005) found a 5.3 h period for this Flora family member from a single lightcurve covering less than the entire period. This year’s period spectrum shows aliases at 5.4 and 10.8 h and a low amplitude. When combined with the large amplitude Szekely result, the 5.437 h as the rotational period is adopted for this paper.

1523 Pieksamaki.

This member of the Flora family has been studied several times in the past (Lagerkvist, 1979; Kryszczynska et al., 2012; Behrend, 2018) each time finding a rotational period near 5.32 h. The result found this year is consistent with those findings.

1991 Darwin.

Wisniewski et al. (1979) found a rotational period near 4.7 h with a low amplitude of 0.08 mag for this Flora family member. Due to the extremely low amplitude, a number of aliases were present in the period spectrum. These include periods near 5 h, 6 h and 8 h. The 5.92 period creates a lightcurve that is a better fit to the data, although the 4.73 h period cannot be formally ruled out.

2017 Wesson.

Behrend (2018) and Kryszczynska et al. (2012) reported periods near 3.415 h for this Flora family member. This year’s finding is in good agreement with those results.

3562 Ignatius.

Previous results found in the lightcurve database (LCDB; Warner et al., 2009) are from Falese et al. (2014) and Stephens (2016) each reporting a periods near 2.73 h. The initial observations (“No Sub” plot) showed what appeared to be a second frequency indicating a binary asteroid. The dual period analysis found a primary lightcurve of P₁ = 2.832 ± 0.001 h, A₁ = 0.09 ± 0.01 mag (“P1” plot). As suspected, subtracting this lightcurve from the data set and doing a period search found a solution that showed what appears to be an orbital period due to a satellite (“P2” plot). A number of aliases can be seen in the period spectrum. The most likely period of P₂ = 16.00 ± 0.02 h, A₂ = 0.18 mag shows a classic bimodal lightcurve. Since the asteroid was well past opposition when observations commenced, and because P₂ is so close to three quarters of an Earth day, the secondary lightcurve could not be completed and it remains a suspected binary. A good opportunity for follow up is in 2020 March.

3890 Bunin.

No entry was found in the lightcurve database (LCDB; Warner et al., 2009) for this Vestoid.

3893 DeLaeter.

This Phocaea family member has been studied twice in past with differing results. A previous rotational period of 13.84 h was found in 2003 (Stephens 2004). Warner (2014) found a period of 5.633 h. The Warner result had an asymmetric lightcurve with an amplitude of only 0.13 mag. The analysis of this year’s data does not support either of those periods. The period spectrum for 2018 shows several aliases at 6 h, 10 h and 16 h.

The period spectrums of the 2003 and 2018 data formally ruled out the possibility of a 5.6 h period. Because the asteroid was well past opposition when observing started, a complete lightcurve could not be obtained. However, the period spectrums for all three oppositions allow for the possibility of a 9.6 h period. Plotting the 2018 data to that period shows a classic bimodal lightcurve with an amplitude of 0.19 mag. The 2003 data results in a single modal lightcurve with a 9.73 h period. Finally, the 2014 data results in a 9.61 h period with a trimodal lightcurve. With such low amplitudes, it is possible that a lightcurve could have a single minimum/maximum pair, or three or more pairs (Harris et al., 2014). Presently, the 9.61 h period is the only one that seems to fit all three datasets and is adopted for this paper.

3894 Williamcooke.

Macias (2015) reported this member of the Eunomia family as having a low amplitude asymmetrical curve with three nights of coverage. Behrend (2018) reported a period of 8.33 h with eight extrema with an amplitude of 0.15 mag. At the time the asteroid was in a dense star field. These observations started two weeks after the Behrend result and were carefully controlled to remove background stars from the measurements. The period spectrum showed possible periods near 2, 4, 7, and 8.5 h. Because of the bimodal shape of the lightcurve and 0.20 mag amplitude, the 4.16 h period is adopted for this paper.

31098 Frankhill.

No entry was found in the lightcurve database (LCDB; Warner et al., 2009) for this Mars-crosser.

(44588) 1999 JF124.

No entry was found in the lightcurve database (LCDB; Warner et al., 2009) for this Hungaria.

Table I.

Observing circumstances and results. Pts is the number of data points. The phase angle values are for the first and last date. LPAB and BPAB are the approximate phase angle bisector longitude and latitude at mid-date range (see Harris et al., 1984). Grp is the asteroid family/group (Warner et al., 2009).

Number	Name	mm\dd	Pts	Phase	LPAB	BPAB	Period	P.E.	Amp	A.E.	Grp
1026	Ingrid	08/26-09/09	185	27.4,23.1	23	−7	5.437	0.001	0.12	0.01	FLOR
1523	Pieksamaki	08/26-08/28	90	24.2,24.0	35	5	5.326	0.003	0.45	0.02	FLOR
1991	Darwin	08/21-08/25	197	20.1,18.1	357	3	5.92	0.01	0.09	0.02	FLOR
2017	Wesson	08/22-08/25	181	23.2,22.0	6	0	3.4153	0.0002	0.59	0.01	FLOR
3562	Ignatius	09/13-09/19	271	8.5,6.2	7	−7	2.832	0.001	0.09	0.01	FLOR
3562	Ignatius						16.00	0.02	0.18	0.02	FLOR
3890	Bunin	09/25-09/30	224	19.7,21.4	331	7	13.39	0.03	0.1	0.01	V
3893	DeLaeter	07/25-08/07	162	31.1,32.8	256	29	9.61	0.01	0.19	0.04	PHO
3894	Williamcooke	07/28-08/01	131	11.1,12.4	289	14	4.16	0.01	0.2	0.03	EUN
31098	Frankhill	07/04-07/24	242	14.9,22.1	282	22	50.04	0.03	0.44	0.10	MC
44588	1999 JF124	07/02-07/03	36	11.4,11.0	287	13	4.88	0.002	0.18	0.03	H