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TwitterA&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. Schulz51 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 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°.
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
(
) 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 C
3 and NH
2
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 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
, 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
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.
at
km. The fit of these
profiles vs.
points to
the existence of two different grain populations with their own
scattering properties and scalelengths (i.e. lifetimes). These
pronounced variations of
vs.
are accompanied by
clear dust color gradients in the 2D maps, unlike the dust color vs.
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