| Abstract
| - Comet Lee (C/1999 H1) was observed on June 6, 1999 when it was at r h = 0.98 AU and $\Delta = 1.195$ AU. The spectrophotometric observations, between 0.6 and 1 μm, were aimed at the detection of the CI( 1D) doublet $\lambda \lambda$ 9823/9850 Å. The non-detection of these lines, with a $3 \sigma$ flux upper limit of the order of $4.6 \times 10^{-17}$$\rm erg\,cm^{-2}\,s^{-1}$, confirms the CO depletion already noted by other authors. Several CN and NH 2 emission bands lie in that spectral range, making it possible to derive production rates for both species as ~ $ 3.1 \times 10^{26}$ $\rm s^{-1}$ and $1.2 \times 10^{27}$ $\rm s^{-1}$, respectively. The oxygen forbidden line at 6300 Å was used to obtain $Q_{\rm H_2O} = (1.22 \pm 0.7) \times 10^{29}$$\rm s^{-1}$. Assuming that CN and NH 2 are directly produced by HCN and NH 3, Comet Lee has a $\rm HCN/H_2O \approx 0.25\%$ and $\rm NH_3/H_2O \approx 1\%$ 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 H 2O + 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 $\rho^{-m}$ law (with $0.7 < m \le 1.2$) 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 $A f \rho$ 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 ~ $9\%$ 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.
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