EDP Sciences
Free access
Volume 422, Number 2, August I 2004
Page(s) 717 - 729
Section Planets and planetary systems
DOI http://dx.doi.org/10.1051/0004-6361:20040159

A&A 422, 717-729 (2004)
DOI: 10.1051/0004-6361:20040159

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

L.-M. Lara1, G. P. Tozzi2, H. Boehnhardt3, M. DiMartino4 and R. Schulz5

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 )

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 C 2, 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 C 3. 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  $2.43 \times 10^{26}$  $\rm s^{-1}$ with lifetimes that can be considered as representative of HCN being the parent species. In the case of C 2, the use of usual lifetimes ( $\tau_{\rm p},\tau_{\rm d}$) 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  $\rm Q_{C_2}
= 3.11 \times 10^{26}$   $\rm s^{-1}$. Other gas species as C 3 and NH 2 have been also detected from the spectrum, although with a lower S/N. The computed production rates are  $3.6 \times 10^{24}$  $\rm s^{-1}$ and $2.15 \times 10^{26}$  $\rm s^{-1}$, respectively. Determination of the CN and C 2 production rates by means of the Haser modeling (Haser 1957) with customary scalelengths indicates that C/2000 WM1 is a C 2-enriched comet at 1.178 AU heliocentric distance. The dust production rate, parameterised by  $ A(\theta) f
\rho$ , is ~ 300-400 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  $m \approx 1$ in $\log~B - \log~\rho$ 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  $\sum A f$ vs.  $\rho$ at  $700 \le \rho \le
15\,000$  km. The fit of these  $\sum A f$ profiles vs.  $\rho$ points to the existence of two different grain populations with their own scattering properties and scalelengths (i.e. lifetimes). These pronounced variations of  $\sum A f$ vs.  $\rho$ are accompanied by clear dust color gradients in the 2D maps, unlike the dust color vs.  $\rho$ 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.

Key words: comets: individual: C/2000WM1 (LINEAR) -- comets: general

© ESO 2004