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. Author manuscript; available in PMC: 2020 Apr 30.
Published in final edited form as: Minor Planet Bull. 2019 Jan;46(1):73–75.

LIGHTCURVE ANALYSIS OF L4 TROJAN ASTERIODS AT THE CENTER FOR SOLAR SYSTEM STUDIES - 2018 JULY TO SEPTEMBER

Robert D Stephens 1, Brian D Warner 2
PMCID: PMC7192043  NIHMSID: NIHMS1570176  PMID: 32355917

Abstract

Lightcurves for five Jovian Trojan asteroids were obtained at the Center for Solar System Studies (CS3) from 2018 July to September.

CCD Photometric observations of five Trojan asteroids from the L4 (Greek) Lagrange point were obtained at the Center for Solar System Studies (CS3, MPC U81). For several years, CS3 has been conducting a study of Jovian Trojan asteroids. This is another in a series of papers reporting data analysis being accumulated for family pole and shape model studies. It is anticipated that for most Jovian Trojans, two to five dense lightcurves per target at oppositions well distributed in ecliptic longitudes will be needed and can be supplemented with reliable sparse data for the brighter Trojan asteroids. For all of these targets we were able to get preliminary pole positions and create shape models from sparse data and the dense lightcurves obtained to date. These preliminary models will be improved as more data are acquired at future oppositions and will be published at a later date.

Table I lists the telescopes and CCD cameras that were used to make the observations. Images were unbinned with no filter and had master flats and darks applied. The exposures depended upon various factors including magnitude of the target, sky motion, and Moon illumination.

Table I.

List of telescopes and CCD cameras used at CS3.

Telescope Camera
0.40-m F/10 Schmidt-Cass FLI Proline 1001E
0.35-m F/11 Schmidt-Cass Fli Microline 1001E
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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. 0.23) is the amplitude of the Fourier model curve and not necessarily the adopted amplitude of the lightcurve.

Targets were selected for this L4 observing campaign based upon the availability of dense lightcurves acquired in previous years. We obtained two to four lightcurves for most of these Trojans at previous oppositions, and some data were found from the Palomar Transient Factory (Waszczak et al., 2015).

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).

To evaluate the quality of the data obtained to determine how much more data might be needed, preliminary pole and shape models were created for all of these targets. Sparse data observations were obtained from the Catalina Sky Survey and USNO-Flagstaff survey using the AstDyS-3 site (http://hamilton.dm.unipi.it/asdys2/). These sparse data were combined with our dense data as well as any other dense data found in the ALCDEF asteroid photometry database (http://www.alcdef.org/) using MPO LCInvert, (Bdw Publishing). This Windows-based program incorporates the algorithms developed by Kassalainen et al (2001a, 2001b) and converted by Josef Durech from the original FORTRAN to C. A period search was made over a sufficiently wide range to assure finding a global minimum in χ2 values.

1583 Antilochus.

We observed Antilochus three times in the past. Lightcurves obtained in 2009, 2016 and 2017 (Stephens 2010), Stephens et al. (2016), Stephens and Warner (2018) all had low amplitudes between 0.05 and 0.12 mag. 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). For the 2017 observations, only a 15.89 h period resulted in a bimodal lightcurve. The 2009 and 2016 data were rephased to this period with a single modal lightcurve. The observations obtained in 2018 confirmed the 2017 period with a large amplitude ruling out the 31.5 h period found in the 2009 and 2016 data. The data collected this year, when combined with our previous data and available sparse data, were used to create a preliminary shape model with a sidereal rotational period of 15.76149 ± 0.00001 h.

graphic file with name nihms-1570176-f0001.jpg

2920 Automedon.

The synodic period found in 2018 using CS3 data agrees with previous synodic results (Molnar et al., 2008: Mottola et al., 2011 Stephens and Warner (2017) near 10.22 h. The data analysis in 2018 is in good agreement and when combined with our previous data and available sparse data, were used to create a preliminary shape model with a sidereal rotational period of 10.22368 ± 0.00001 h.

graphic file with name nihms-1570176-f0002.jpg

3709 Polypoites.

We observed this Trojan four times in the past (French et al., 2011, Stephens et al., 2016a, Stephens et al., 2016b and 2017), each time finding a period near 10.04 h. The 2018 analysis results are in good agreement and were used to create a preliminary shape model with a sidereal rotational period of 10.04745 ± 0.00001 h.

graphic file with name nihms-1570176-f0003.jpg

3793 Leonteus.

This large Trojan has been well studied in the past. Mottola et al., (2011) observed it in 1994 and 1997. We found periods in 2009, 2015 and 2016 (Stephens et al., 2016a and 2016b). Each of these periods was found to be close to 5.62 h. This year’s data analysis result is in good agreement. We were able to create a preliminary shape model with a sidereal rotational period of 5.62192 ± 0.00001 h.

graphic file with name nihms-1570176-f0004.jpg

4060 Deipylos.

Using sparse photometry from the Palomar Transient Factory, Waszczak et al. (2015) reported a period of 11.4905 h for Deipylos. We observed it three times (Stephens et al 2016a, 2016b and 2017) finding periods near 9.3 h. That period appears to be a 5:4 alias of the Waszczak period of rotation. The 2018 result of 9.38 h is in good agreement with our prior results, but the 11.5 h alias was still present as seen in the ChiSq versus Period plot. The sharper minimum in that plot as well as the synodic periods found in each of the four oppositions cause us to adopt the sidereal rotational period of 9.3316 ± 0.00001 h.

graphic file with name nihms-1570176-f0005.jpg

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Table II.

Observing circumstances and results. P in the period column indicates the period of the primary in a binary system. Pts is the number of data points. Phase is the solar phase angle for the first and last date. If there are three values, the middle value is the minimum phase angle. LPAB and BPAB are, respectively, the approximate phase angle bisector longitude and latitude at mid-date range (see Harris et al., 1984).

Number Name 2018 mm/dd Pts Phase LPAB BPAB Period(h) P.E. Amp A.E.
1583 Antilochus 07/07-07/24 386 7.9,6.7 311 32 15.759 0.002 0.31 0.02
2920 Automedon 09/24-09/30 122 10.6,10.9 296 21 10.22 0.01 0.25 0.03
3709 Polypoites 08/05-08/10 202 6.9,7.5 281 21 10.07 0.02 0.12 0.02
3793 Leonteus 07/24-08/04 202 4.4,4.3,4.4 306 22 5.621 0.001 0.31 0.02
4060 Deipylos 08/13-08/19 97 5.2,6.0 293 14 9.22 0.03 0.10 0.02
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Acknowledgements

Observations at CS3 and continued support of the asteroid lightcurve database (LCDB; Warner et al., 2009) are supported by NASA grant 80NSSC18K0851. Work on the asteroid lightcurve database (LCDB) was also partially funded by National Science Foundation grant AST-1507535. This research was made possible in part based on data from CMC15 Data Access Service at CAB (INTA-CSIC) (http://svo2.cab.inta-csic.es/vocats/cmc15/) and through the use of the AAVSO Photometric All-Sky Survey (APASS), funded by the Robert Martin Ayers Sciences Fund. The purchase of a FLI-1001E CCD cameras was made possible by a 2013 Gene Shoemaker NEO Grants from the Planetary Society.

Contributor Information

Robert D. Stephens, Center for Solar System Studies (CS3)/MoreData!, 11355 Mount Johnson Ct., Rancho Cucamonga, CA 91737 USA

Brian D. Warner, Center for Solar System Studies (CS3)/MoreData!, Eaton, CO

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