174P/Echeclus: a strange case of outburst^*

University of Franche-Comté, Observatoire de Besançon, Institut UTINAM – UMR CNRS 6213, BP 1615, 25010 Besançon Cedex, France e-mail: rousselot@obs-besancon.fr

Received: 25 June 2007
Accepted: 7 December 2007

Abstract

Context.More than ten Centaurs are now known to have cometary activity at large heliocentric distance (i.e. 5-13 AU). Among these objects, 174P/Echeclus which showed cometary activity at 13 AU from the Sun, is a unique case, because of: (i) the amplitude of its outburst, (ii) the source of cometary activity that appears distinct from Echeclus itself.

Aims.This paper aims at investigating the physical conditions that have led to this unusual outburst. The purpose is also to quantify this phenomenon and to provide observational constraints for its modeling.

Methods.We use observations from different telescopes, performed before, during, and after the outburst. We performed the main observations on March 23 and 30, 2006, with the 8.2-m ESO Very Large Telescope and FORS 1 instrument. They consist of visible images and spectra.

Results.Our main results are: (i) a cometary source distinct from Echeclus itself that presents a brightness distribution compatible with a diffuse source; (ii) a total dust production rate Q_dust 86 kg s^-1 and a parameter 10 000 cm; (iii) no emission lines (CN and C₂) can be detected in the visible range; (iv) the upper limits for the CN and C₂ production rates are 3.810²⁵ and 10²⁶ molecules s^-1 respectively; (v) we detected no Echeclus' satellite before the outburst up to M_R 26; (vi) the upper limit for the object generating the coma is about 8 km in diameter; (vi) and we detected no cometary activity one year later, in March 2007.