THE 2007 OBSERVING CAMPAIGN OF THE MUTUAL EVENTS WITHIN (617) PATROCLUS

Introduction

State of knowledge

Predictions

Conclusion

References

Contacts

Updated June 24, 2007

Introduction

In 2006-2007, the binary Trojan system Patroclus-Menoetius is reaching one of its annual equinoxes. As a consequence, the system will undergo a season of mutual eclipses and occultations very similar to the one that the Galilean satellites undergo every 6 years.

This page is aimed at organizing a campaign of observations of these mutual events along the first semester of 2007. Such opportunity occurs only under favorable geometric conditions when the Sun and/or the Earth are close to the orbital plane of the system. We took advantage of a reliable orbit solution of the similary-sized binary system to predict a series of mutual eclipses and occultations observable from January to July 2007 (cf. Predictions) During the campaign, the magnitude of Patroclus system will varies from 15.8 to 16.6 and its solar phase will varies from 9°.5 to 2°.7 at opposition (end of March). The amplitude of the events should range between 0.2 and 0.3 magnitude. With such favorable circumstances, photometric observations of the events will provide tight constraints regarding physical properties of the system such as sizes, shapes, sidereal spin period and surface composition. No complete lightcurve of the system has been published so far, but Mottola et al. (personal communication) lightcurve recorded in 1991 suggest that the system should be synchronized.

That's why we greatly encourage observers to observe as much as possible the mutual events within Patroclus system and to send us (asterOA@imcce.fr) your observed lightcurves. Your data will be highly useful to improve the dynamical and physical knowledge of this unique binary Trojan system.

617 Patroclus: State of knowledge

Following several observations of Patroclus that we made thanks to the LGS adaptive optics system available on the Keck II (10-m telescope at the summit of Mauna Kea in Hawaii, USA), our team has proposed a dynamical and physical modelization of Patroclus system (Marchis et al., Nature, 439, 2006). We reported that the components, separated by 680 km, move around the system's centre of mass, describing a roughly circular orbit. Using this orbital information, combined with thermal measurements to estimate the size of the components, we derive a very low density of 0.8 g/cm^3 (±0.2). The components of 617 Patroclus are therefore very porous or composed mostly of water ice, suggesting that they could have been formed in the outer part of the Solar System.

At that time, the orbital parameters of the system were estimated independently using two algorithms: a Monte Carlo technique and a generalized least-squares method. Both methods have been successfully applied to several mainbelt binary systems in the past (for example 121 Hermione and the moonlets of 87 Sylvia). The orbital solution we obtained is quite accurate considering 2004-2005 data with a root-mean-square residual error of 9 mas, corresponding to ~35 km. It is slightly degraded when the lower angular resolution 2001-2002 data are included (17 mas).

The two components, separated by 680±20 km, revolve around their centre of mass in 4.283±0.004 days in a roughly circular orbit (e~0.02±0.02). Because we could obtain a purely keplerian solution matching four years of observations, we can deduce that the pole orientation of the orbit seems not to precess, and is thus probably aligned with the angular momentum pole. The low eccentricity of the orbit, compared for instance with the extremely high eccentricity (~0.8) of the binary Kuiper Belt Object15 1998WW31, indicates that dissipation effects, such as tides, must be considered to circularize the orbit of this double Trojan system. The angular resolution provided by the Keck LGS adaptive optics data (0.06 arcsec) is above the angular size of the components. Their size can be estimated only using near-simultaneous mid-infrared and visible observations. Assuming the same albedo for the components (a realistic assumption because the brightness ratio in H and K bands are constant and the objects are spherical) and a size ratio of R1/R2=1.082, we derive a radius of R1=60.9 km and R2=56.3 km (with an error of 1.6 km and an albedo in the visible spectrum of Av=0.04) and a beaming parameter equal to 0.94. Recent Spitzer spectra taken between 5 and 40 micro-meter of Trojan asteroids confirm that the effect of the beaming factor is negligible.

Prediction of events

The prediction of events is based on our last orbital model of Patroclus system, taking into account the 2007-01-13 Gemini-8m North LGS observation of the system. First successful observations of mutal events performed on Jan. 26 and 29, 2007 at Pic du Midi observatory (France) with the 1-m telescope by F. Colas and J. Lecacheux and with the 0.25-inch Ironwood North telescope by V. Reddy (U. of North Dakota) and F. Pino (Ironwood North Observatory, Mesa, AZ) confirms that our model is in aggrement with the observations (see CBET 836 and click on the green symbol in the 'Status' column of the table to see the observed minus predicted lightcurve).Thus we encourage observers to choose most of the observable events for which they can record lightcurve during the longest period.

The following table lists the events to be observed from January 2007 to the end of the campaign. More events will be added day to day. The columns display the following predicted parameters:

• Date: date of the event
• Event: type of event: 'O' = occultation, 'E' = eclipse, '1' = Patroclus, '2' = Menoetius
• Start: starting UTC date of the event
• End: ending UTC date of the event
• View: the icons allow one to download an MPEG animation of the event, a GIF animated view of the orbital motion of the planetary system, and the predicted lightcurve of the event
• Status: information about the event

The timing accuracy of the events is of the order of few minutes. For the period of the events, the geocentric distance of Patroclus varies from 4.9 AU (in April) to 6.8 AU (in October) and its visual magnitude varies from 15.8 to 16.6.

Note (June 2007): The last observed events in May have shown that the predictions are shifted of -3h to -5h with the predictions. Thus we have quickly fitted a new orbital solution taking into account the observations of this campaign. The June predictions have been re-computed with this new model and should now show an offset between 30 minutes and 2 hours depending on the event. When it will be confirmed, the last July events will also be re-computed. The events concerned are written in blue in the table.

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Conclusion

These upcoming mutual events within the Patroclus system represent an unprecedented opportunity to conduct very precise astrometry of the two components with modest aperture telescopes equipped with CCD cameras while also providing access to the physical properties of the system.

We encourage amateur and professional astronomers with access to CCD camera even on modest aperture telescopes to dedicate part of their program to observe these events.

References

• Marchis et al., 2007. CBET 836
• Marchis et al., 2006. Nature, 439, 565-567 (article)
• Merline et al., 2001. IAU, 7741
• Morbidelli et al, 2005. Nature 435, 462-465
• Tsiganis et al., 2003. Nature 435, 459-461
• Walsh and Richardson, 2003. Icarus 180, 1, 201-219

Contacts:

• J. Berthier, IMCCE, Paris Observatory, France
• P. Descamps, IMCCE, Paris Observatory, France
• F. Marchis, Univ. of Berkeley, California, USA

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