The gas and dust coma of Comet C/1999 H1 (Lee)^*

¹ Instituto de Astrofísica de Andalucía, CSIC, PO Box 3004, 18080 Granada, Spain e-mail: rodrigo@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 Isaac Newton Group of Telescopes, PO Box 321, 38700 Santa Cruz de La Palma, Tenerife, Spain e-mail: pleisy@ing.iac.es

Corresponding author: L. M. Lara, lara@iaa.es

Received: 20 August 2003
Accepted: 19 February 2004

Abstract

Comet Lee (C/1999 H1) was observed on June 6, 1999 when it was at r_h = 0.98 AU and  AU. The spectrophotometric observations, between 0.6 and 1 μm, were aimed at the detection of the CI(¹D) doublet 9823/9850 Å. The non-detection of these lines, with a flux upper limit of the order of , confirms the CO depletion already noted by other authors. Several CN and NH₂ emission bands lie in that spectral range, making it possible to derive production rates for both species as ~  and  , respectively. The oxygen forbidden line at 6300 Å was used to obtain . Assuming that CN and NH₂ are directly produced by HCN and NH₃, Comet Lee has a and at r_h = 0.98 AU, in reasonable agreement with what has been found in other long-period comets. The structural analysis carried out on cometary images acquired with broad band R Bessel filter clearly displays two pairs of ion rays likely produced by the H₂O⁺ doublet at 6198 and 6200 Å, wavelengths covered by the bandpass filter. Identical features are found in the images acquired with the Gunn i filter.
The dust brightness profiles in the east-west direction do not deviate from a  law (with ) as expected for a steady state model coma with a constant dust production rate and expanding at constant velocity. The dust production rate, as obtained from the parameter, is ~500 cm, which compared with the gas production rate classifies this comet as a dust poor one with relatively high (6.5–11.7) gas-to-dust mass ratio. Analysis of the normalized reflectivity gradient (i.e. continuum color) as a function of ρ indicates a slight reddening of the solid component in the coma at large cometocentric distances, whereas the average dust color within an aperture of 20 000 km, centered at the nucleus, is ~ per 1000 Å. Mie scattering computations applied to an ensemble of particles indicate that the dust coma is characterised by a relative broad size distribution with a typical mean size of 1 μm. These grains might be composed of a mixture of silicates and icy material.