First attempt at interpreting millimetric observations of CO in comet C/1995 O1 (Hale-Bopp) using 3D+t hydrodynamical coma simulations ^*

¹ Institut de Radioastronomie Millimétrique, 300 rue de la piscine, Domaine universitaire, 38406 Saint-Martin-d'Hères, France e-mail: boissier@iram.fr
² LESIA, Observatoire de Paris, 5 place Janssen, 92195 Meudon Cedex, France
³ TsNIIMASCh, Central Research Institute of Machine Building, Pionyerskaya St. 4, Korolev, Moscow region 141070, Russia
⁴ CNRS and UVSQ, LATMOS, BP 3, 91371 Verrières-le-Buisson Cedex, France

Received: 1 July 2009
Accepted: 3 October 2009

Abstract

Context. Millimetre line observations of comet C/1995 O1 (Hale-Bopp) close to perihelion, completed using the IRAM Plateau de Bure Interferometer, have detected temporal variations in the CO J(2–1) 230 GHz line shape, and in the position of its maximum emission brightness within the field-of-view, whose heuristic analysis has suggested the presence in the coma of a slowly rotating spiral-shaped enhancement of the CO density.

Aims. Here, we reanalyse these data using a physically consistent model of the coma.

Methods. We consider a large, rotating, icy nucleus with an arbitrarily aspherical shape and an adhoc rotation mode, and compute its tridimensional, time-dependent (”3D+t”) mixed CO + H₂O coma, using a previously developed tridimensional hydrodynamical code (HDC). The line emission of CO is then computed using a molecular excitation and radiation transfer code (ERC). In the present, pioneering phase, the HDC and ERC both contain crude, and not thoroughly mutually consistent approximations. Several alternative CO surface flux distributions are considered, and the resulting CO 230 GHz line spectra and brightness maps are compared with observations.

Results. We find that when an uniform surface flux of CO is assumed, the spiral structures created by the nucleus asphericity in the CO coma are too faint to account for the observational data, whereas we confirm earlier conclusions based on a heuristic approach that the assumption of an area of suitable dimensions and localization with increased CO flux leads to results in agreement with a large subset of (but not all) the data. This suggests that the true CO coma production map may be more complex than the presently assumed rather simple-minded one. Refined and mutually consistent HDC and ERC are needed for a more satisfactory interpretation of the present and any similar future data.