New constraints on the physical properties and dynamical history of Centaur 174P/Echeclus

Abstract

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 axial 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.

Recent results of stellar occultations by (60558) Echeclus

“…We use the 3D model and pole orientation (𝜆, 𝛽 = 115.2 • , 21.5 • ) for Echeclus from ISAM service and its rotational period (P = 26.785178 ± 10 −6 hours) proposed by Rousselot et al (2021) to fit the projected limb to the stellar occultation chords. As the 2020 event has two positive chords, we propagate the rotation of Echeclus from the model's reference epoch to the epoch of the occultation.…”

“…These values indicate that Echeclus comprises ice and silicates, with a low proportion of ice and significant porosity, as rubble-pile or layered-pile (Belton et al 2007) structures. Based on Rosenberg's study of comet 67P, Rousselot et al (2021) propose that a layered structure and the internal inhomogeneity of Echeclus may be directly related to the erratic ejections of material observed by this object, with outbursts of different intensity and duration.…”

“…The rotational period error bar gives reasonable confidence in the orientation of the 3D shape at the time of occultation. The double-peak light curve exhibited amplitudes consistent with a triaxial body (𝑎 > 𝑏 > 𝑐, where 𝑐 is the rotation axis), with a semi-major axial ratio of 𝑎/𝑏 = 1.32 and 𝑏/𝑐 ∼ 1.1 based on a comparison of the projected areas of the 3D model (Rousselot et al 2021).…”

“…Subsequent studies suggest that seasonal effects can cause material ejections. Also, the variegation of the color index as a function of radial distance from the nucleus indicates that particles may have different sizes or compositions (Rousselot et al 2021).…”

“…The 2020 event allowed constraining the size and shape of Echeclus. It was also used with the 3D model and pole coordinates position proposed by Rousselot et al (2021) to obtain its size and volume. All data sets of the three stellar occultation events were used to search for signatures that could reveal the presence of secondary structures around Echeclus, whether coma, jets, arcs, or confined rings.…”