Elsevier

Icarus

Volume 219, Issue 2, June 2012, Pages 741-742
Icarus

Note
Photometric lightcurve and rotation period of Himalia (Jupiter VI)

https://doi.org/10.1016/j.icarus.2012.03.021Get rights and content

Abstract

Due to their great distance from their primaries and rapid solar perturbations of their orbits, the outer irregular satellites of the giant planets are not in tidal rotational lock. They are generally thought to be asteroid-like bodies with irregular shapes and rotation periods much shorter than the orbital ones, whose rotational properties can be found by the methods used to study asteroids. We report here the results of two sets of independent photometric CCD observations overlapping in time that were obtained using 0.35- and 1.23-m telescopes. The combined data set yields for Himalia a unique synodic rotation period of 7.7819 ± 0.0005 h, with a lightcurve amplitude of 0.20 ± 0.01 magnitudes. This determination corrects the erroneous period of ∼9.5 h published by Degewij et al. (Degewij, J., Andersson, L.E., Zellner, B. [1980a]. Icarus 44, 520–540).

Introduction

Although the brightest and largest of Jupiter’s outer satellites, Himalia (Jupiter VI), is at reach of small-sized telescopes, to our knowledge only one rotational lightcurve has been published to date. Degewij et al. (1980a) obtained lightcurves on 1976 November 28 and 29 which they used to determine a period of 9.5 ± 0.3 h and an amplitude of 0.12 magnitudes. Luu (1991) reported photometric observations of several of Jupiter’s fainter irregular satellites, but not including Himalia. These measurements were sufficient to find short-period magnitude variations in an interval of a few hours. However, they did not produce definitive rotation periods. Although no other attempts to find a rotation period for Himalia have been published, there have been several investigations of the reflectance spectrum. Degewij et al. (1980a) measured magnitudes in the standard U, B, V, R, I bands. In a second publication in the same year, Degewij et al. (1980b) added J, H, K infrared magnitudes and associated color indices. High resolution reflectance spectra between 0.4 and 0.72 μm have been obtained by Luu (1991) and between 0.4 and 0.95 μm by Jarvis et al. (2000).

Here, we report independent observations near the time of Jupiter’s 2010 opposition made by F.P. at the Organ Mesa Observatory (OM, Pilcher, 2010) and by S.M. and T.D. at Calar Alto Observatory (CA, Denk et al., 2010) which have been combined herein after learning of each other’s work (see Table 1 for the observational circumstances). The goal of both groups was to obtain a very accurate determination of Himalia’s rotational lightcurve by use of the method of differential photometry. Equipment at the Organ Mesa Observatory consists of a 35.6 cm Meade LX200 GPS Schmidt–Cassegrain, operated at the Cassegrain focus of 355.6 cm effective focal length, SBIG STL-1001E CCD, 60 s exposures, clear filter, unguided. The CCD has an array of 1024 × 1024 pixels of 24 × 24 μm pixel size, providing a field of 25 × 25 arcmin and 1.46 arcsec per pixel. Adding readout time, 54 data pairs could be obtained each hour for 6–8 h on each of seven nights 2010 August 23–October 14. This enables a very high time resolution of the rotational variation. After some clearly discordant data affected by appulses to stars, hot pixels, and cosmic ray hits, were deleted, a total of 1908 data pairs were used in subsequent analysis. The 1.23 m reflector at Calar Alto Observatory was operated remotely using a CCD camera 2b 17 with SITe 2048 × 2048 pixel detector, 0.46 arcsec/pixel, 15.8 × 15.8 arcmin field of view, and Johnson V filter. A total of 115 data pairs were obtained on six consecutive nights, from 2010 September 8 to 13.

Section snippets

Methods and results

In differential photometry the target magnitude is compared with the average of several comparison stars in the same field of the CCD to obtain an instrumental magnitude with no attempt to reduce to real magnitudes. The time variation of the magnitude is preserved, and in addition changes in sky transparency affect equally all objects in the field and do not affect the instrumental magnitude. On different nights different comparison stars are available, and the instrumental magnitude of the

Acknowledgment

The first author (F.P.) thanks Richard Binzel for helpful suggestions in the preparation of this manuscript.

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