MAIN-BELT ASTEROIDS OBSERVED FROM CS3: 2018 OCTOBER - DECEMBER

Abstract

CCD photometric observations of 18 main-belt asteroids were obtained from the Center for Solar System Studies from 2018 October to December. A pole solution was found for 4910 Kawasato of (λ, β, P_SID) = (355°, 35°, 4.66271 h). (31320) 1998 HX2 is a binary asteroid with a P₁ of 2.8149 ± 0.0001 h and P₂ of 47.06 ± 0.05 h.

The Center for Solar System Studies (CS3) has seven telescopes which are normally used in program asteroid family studies. The focus is on near-Earth asteroids, but when suitable targets are not available, Jovian Trojans and Hildas are observed. When a nearly full moon is too close to the family targets being studied, targets of opportunity amongst the main-belt families were selected.

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 1.2-1.60 arcsec/pixel. Images were unbinned with no filter and had master flats and darks applied. The exposure duration varied depending on the asteroid’s brightness and sky motion.

Table I:

List of CS3 telescope/CCD camera combinations.

Telescope	Camera
0.30-m f/6.3 Schmidt-Cass	FLI Microline 1001E
0.35-m f/9.1 Schmidt-Cass	FLI Microline 1001E
0.35-m f/9.1 Schmidt-Cass	FLI Microline 1001E
0.35-m f/9.1 Schmidt-Cass	FLI Microline 1001E
0.35-m f/11 Schmidt-Cass	FLI Microline 1001E
0.40-m f/10 Schmidt-Cass	FLI Proline 1001E
0.50-m F8.1 R-C	FLI Proline 1001E

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).

1589 Fanatica.

We previously observed this Vestoid family member (Warner, 2004; Stephens, 2015) finding rotational periods near 2.58 h. This year’s result is in good agreement with those previous results.

1600 Vyssotsky.

We studied this member of the Hungaria family many times in the past (Warner, 2014b; Stephens, 2016) each time finding a rotational period near 3.20 h. The result found this year is consistent with those findings. A previous shape and spin model had been created (Warner et al. 2008) with two possible solutions: (λ, β) = (356°, 7°) and (219°, 54°). For both solutions, the sidereal period was 3.201264 h. It is hoped that this year’s data will improve upon that model.

1756 Giacobini.

Warner (2007) found a rotational period of 3.8527 h for this inner main-belt asteroid. The rotational period found this year is in good agreement with that result.

2572 Annschnell.

Behrend (2006) and Stephens (2017) reported rotational periods for this member of the Vestoid family in the past of about 6.33 h. The period found this year, based upon two consecutive nights of data, was slightly shorter but consistent with those prior results.

3225 Hoag.

Warner (2015) observed this member of the Hungaria family five times in the past, each time reporting periods near 2.37 h. This year’s finding is in good agreement with those results.

3299 Hall.

Anzar (private communication) got two nonconsecutive nights on this member of the Flora family in 2016, reporting a period of 7.2 h on his website. The resulting plot did not cover a complete rotational cycle. It was immediately apparent from the single night slopes that a shorter period of about 7 h would not fit the 2018 data. The best fit on the period spectrum was about 37 h, (Alt. Plot) which resulted in a small amplitude lightcurve. Harris et al. (2014) shows that with a small amplitude, the lightcurve can be monomodal or have three or more extrema. Because of the Azar partial lightcurve with a high amplitude and 7 h period, we are adopting the 10.45 h period (Pref. Plot) as our preferred result, but note that the correct period could be 37 h. The next time this asteroid is observable from the Northern Hemisphere is in August 2021.

3704 Gaoshiqi.

Oey et al. (2014) reported a period of 9.7725 h for this member of the Vestoid family resulting in a lightcurve with a large scatter of observations. Our finding this year is in fair agreement with that result.

3900 Knezevic.

This Vestoid family member was observed by Hasegawa et al. (2014) who reported a period of 5.324 ± 0.001 h. That result differs from the period found this year by 0.06 h. Reviewing their plot, their data is somewhat sparse over four nights. About 40% of their lightcurve is covered by just a few datapoints.

4635 Rimbaud.

No entry was found in the lightcurve database (LCDB; Warner et al., 2009) for this Vestoid. Tumbling cannot be ruled out but there is no obvious signs of it in the data.

4910 Kawasato.

Stephens (2015), Foylan and Salvaggio (2015), and Clark (2015) all reported rotational periods near 4.66 h for this Mars-crosser. This year’s result is in good agreement. Because of our prior observations and availability of sparse data from surveys such as from the Catalina Sky Survey in Arizona (https://catalina.lpl.arizona.edu/), we attempted a pole/shape model.

The modeling processing using lightcurve inversion has been detailed previously (e.g., Warner et al., 2017, and references therein). Briefly, the idea is to find a shape and its orientation such that its modeled lightcurves closely match the original data. Main-belt asteroids usually require data from at least three oppositions at different phase angle bisector longitudes before a reliable model can be developed. We attempted the shape model because we had a wide range of phase angles for this Mars-crosser.

In the PAB longitude plot, green circles represent dense lightcurves while red squares represent sparse data from one or more of the surveys.

The green line in the period plot lies 10% above the lowest χ² value. An ideal solution has a well-defined shape with only one data point below the line. In this case, there are about 10 data points below the line, a good result.

In the pole plot, dark red represents a solution that is more than 10% above the lowest χ² value. A “perfect” solution is when there is only one dark blue region and all the others are dark red. This result shows a typical result with competing solutions 180° apart, one a prograde rotation and the other a retrograde rotation. A pole solution often has difficulty distinguishing between the two.

The solid black line in the lightcurve plots is the model lightcurve and the red dots are the original data. The model curves in are from the solution for ecliptic coordinates (355°, 35°, 4.66271 h) although the fits to the model based on (165°, 31°, 4.66271 h) are essentially identical.

In the end, we chose (355°, 35°, 4.66271 h) because it had the lowest χ² value. In both cases, the estimated error for the pole is a circle of about 10° radius and 0.00001 h for the period.

(9873) 1992 GH.

Warner (2014a) observed this Hungaria four times in the past, each time finding a rotational period near 2.93 h. This year’s result is in good agreement.

(13665) 1997 GK17.

No entry was found in the LCDB for this member of the Flora family.

(18842) 1999 RB22.

No entry was found in the LCDB for this outer main-belt asteroid. Only a single night of coverage could be obtained. The 9.6 h period was estimated based on the half-period solution. A bimodal solution is assumed because of the ~0.45 mag amplitude and low phase angle (Harris et al., 2014).

(28650) 2000 GE8.

Waszczak et al. (2015) found a period of 9.591 h, rated U = 3. Our observations, when phased to 9.6 h resulted in a single modal lightcurve. Phasing the observations to 19 h created a classic bimodal lightcurve. One reason for rejecting the double period is the split halves plot which shows the bimodal lightcurve to be completely symmetrical. Also, both the Waszczak result and our data have low amplitudes. Harris et al. (2014) show that with a small amplitude, the lightcurve can be monomodal or have three or more extrema. The next favorable opposition to observe this asteroid is in 2021 March.

(31320) 1998 HX2.

There are no entries in the LCDB for this member of the Hungaria family. The initial observations showed what appeared to be a second frequency indicating a binary asteroid. The dual period analysis found a primary lightcurve of P₁ = 2.8149 ± 0.0001 h, A₁ = 0.16 ± 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). Two aliases can be seen in the period spectrum. The most likely period of P₂ = 47.06 ± 0.05 h, A₂ = 0.13 mag shows a classic bimodal lightcurve. A half period (“P2 - Half” plot) is also presented showing a monomodal lightcurve plot. Since the asteroid was past opposition when observations commenced, and because P₂ is so close to twice that of an Earth day, the secondary lightcurve could not be completed and it remains a suspected binary. The next opportunity for follow up is in 2022 January when it will only be observable from the Southern Hemisphere. The next opportunity for Northern Hemisphere observers will be in 2023 July when it will just be brighter than V = 18.

(65715) 1992 WV1.

Chang et al. (2016) found a period of 5.89 h, but it was rated U = 1+ in the LCDB. A complete lightcurve was not obtained in the single night it was observed, but a search for the half-period helped find the bimodal solution which, given the amplitude, is probable.

(135690) 2002 OO18.

No entry was found in the LCDB for this inner main-belt asteroid. Despite the noise in the observations, the solution is solid since both nights covered the entire curve at the adopted period.

(298808) 2004 RV33.

No entry was found in the LCDB for this member of the Flora family. Despite the error bars being larger than the amplitude, the solution is still reasonable, but it needs confirmation at a later date. The next time this asteroid is V < 19.0 is 2025 Nov 2. Most of the time it's V > 20 at brightest in other apparitions.

Table II.

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): H, Hungaria; MB-I/O, main belt inner/outer; MC, Mars-crosser; FLOR, Flora; V, Vestoid.

Number	Name	mm\dd	Pts	Phase	LPAB	BPAB	Period	P.E.	Amp	A.E.	Grp
1589	Fanatica	12/23-12/24	85	20.0,20.3	50	−3	2.578	0.002	0.22	0.02	V
1600	Vyssotsky	12/27-12/30	304	32.9,32.7	158	26	3.199	0.002	0.20	0.02	H
1756	Giacobini	12/23-12/26	148	25.8,26.4	43	5	3.854	0.002	0.29	0.02	MB-I
2572	Annschnell	09/29-09/30	114	7.9,7.5	23	0	6.27	0.01	0.70	0.03	V
3225	Hoag	10/28-10/31	182	32.7,32.9	336	9	2.372	0.001	0.23	0.02	H
3299	Hall	10/18-10/21	618	4.2,3.2	30	5	10.45	0.02	0.08	0.02	V
							36.72	0.07	0.14	0.02	V
3704	Gaoshiqi	10/19-10/20	439	3.4,3.8	20	4	9.699	0.001	0.26	0.02	V
3900	Knezevic	11/23-11/26	225	19.9,21.0	27	8	5.260	0.005	0.22	0.02	V
4635	Rimbaud	12/18-01/02	1152	7.7,4.0,4.2	99	7	117.91	0.03	1.08	0.03	V
4910	Kawasato	11/15-11/19	358	4.3,5.9	49	−6	4.659	0.002	0.40	0.02	MC
9873	1992 GH	10/24-10/30	523	25.0,27.3	358	9	2.925	0.001	0.39	0.02	H
13665	1997 GK17	11/15-12/28	519	24.0,3.1	99	3	37.631	0.004	0.92	0.03	FLOR
18842	1999 RB22	11/02-11/02	49	4.4,4.4	48	6	9.6	0.2	0.49	0.03	MB-O
28650	2000 GE8	10/05-10/19	380	11.3,5.3	115	−5	9.625	0.004	0.16	0.02	EOS
31320	1998 HX2	10/03-11/02	549	5.6,3.1,17.5	16	3	2.8149	0.0001	0.16	0.01	H
							47.06	0.05	0.13	0.02	H
65715	1992 WV1	12/05-12/05	50	16.3,16.3	98	3	8.75	0.5	0.36	0.03	MB-I
135690	2002 OO18	10/08-10/09	111	4.4,3.9	23	0	7.61	0.05	0.56	0.05	MB-I
298808	2004 RV33	10/08-10/11	138	5.4,3.5	22	0	3.64	0.01	0.17	0.03	FLOR