Gas and dust in Comet C/2000 WM1 during its closest approach to Earth: Optical imaging and long-slit spectroscopy

¹ Instituto de Astrofísica de Andalucía, CSIC, PO Box 3004, 18080 Granada, Spain e-mail: lara@iaa.es
² INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: tozzi@arcetri.astro.it
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: hboehnha@mpia-hd.mpg.de
⁴ INAF, Osservatorio Astronomico di Torino, 10025 Pino Torinese (TO), Italy e-mail: dimartino@to.astro.it
⁵ Research and Scientific Support Department of ESA, ESTEC, Keplerlaan 2, 2200AG Noordwijk, The Netherlands e-mail: Rita.Schulz@esa.int

Received: 2 June 2003
Accepted: 17 April 2004

Abstract

We have investigated the dust and gas coma of the comet C/2000 WM1 (LINEAR) during Dec. 2-4, 2001, its closest approach to Earth. The gaseous coma is slightly asymmetric in CN and C₂, indicating the presence of some structures which are detected either by applying enhancement techniques or studying the azimuthal distribution of material. This asymmetry is due to the existence of a double jet in a direction almost perpendicular to the Sun–comet direction. These structures are not clearly detected in C₃. The morphological analysis of the dust coma shows a faint, short and narrow sunward structure, besides the dust tail in a position angle of ~60°.
CN is produced at a rate of   with lifetimes that can be considered as representative of HCN being the parent species. In the case of C₂, the use of usual lifetimes () requires a modified two-steps photolytic model to account for the pronounced flatness of the column density profile in the inner coma. Usual steady-state model gives rise to a  . Other gas species as C₃ and NH₂ have been also detected from the spectrum, although with a lower S/N. The computed production rates are   and  , respectively. Determination of the CN and C₂ production rates by means of the Haser modeling (Haser [CITE]) with customary scalelengths indicates that C/2000 WM1 is a C₂-enriched comet at 1.178 AU heliocentric distance. The dust production rate, parameterised by , is ~ cm when measured in two continuum regions, centered at 4845 and 6840 Å, whilst this value is slightly lower in Bessel R and Gunn i (~230 cm). Thus, the gas-to-dust mass ratio is in the order of 6.3, a relatively gassy comet.
The surface brightness profiles of the continuum, either azimuthally averaged profiles from the broadband images or in east-west direction from the long-slit spectrum, can be well fitted with  in representation. On the other hand, the study of the dust coma as imaged with the broadband and narrowband filters reveals that variations in the size and/or composition of the grains might be occurring while traveling outward. This fact is manifested as a sharp decrease of  vs. ρ at  km. The fit of these  profiles vs. ρ points to the existence of two different grain populations with their own scattering properties and scalelengths (i.e. lifetimes). These pronounced variations of  vs. ρ are accompanied by clear dust color gradients in the 2D maps, unlike the dust color vs. ρ as derived from the spectrum. Since spectroscopic measurements are only obtained in east-west direction and with a very long exposure time, the effect of color variations inside the coma due to disintegrating grains is difficult to detect.