New constraints on the physical properties and dynamical history of Centaur 174P/Echeclus
Résumé
Since 2005 December, recurrent outbursts have been observed for Centaur 174/P Echeclus, confirming it is an active object. Thanks to a large number of photometric data obtained between 2001 April and 2019 December we were able to compute a shape model of this object. We obtain a sidereal rotation period P=26.785178±10 −6 h and six equally probable pole solutions, each with a large obliquity of the rotational axis (50°or more). We also find the object significantly elongated, with a semi-major axis ratio a/b = 1.32 (and b/c ∼ 1.1 but this second ratio is poorly constrained by the photometric data). Additionally, we present a detailed analysis of the dust emission from the 2016 outburst. Different colour maps are presented that reveal a change in dust colour, which becomes bluer with increasing cometocentric distance. A blue ring-like structure around the nucleus clearly visible in the images obtained on October 4 in the V-R spectral interval points out that the innermost near nucleus region is considerably redder than the surrounding coma. Different jets are also apparent, the main one being oriented southward. A detailed dynamical study is done to investigate past and future orbital elements. These elements appear stable in the period ≈ 1200 CE to ≈ 2900 CE. For a period of 12,000 years the main conclusion is that Echeclus' perihelion distance was greater than about 4 au, preventing it from following a typical cometary activity like a short-period comet. Close encounters with giant planets nevertheless prevent any study of orbital elements on longer timescale.
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