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

Comet Lee (C/1999 H1) was observed On June 6, 1999 when it was at r(h), = 0.98 AU and Lambda = 1.195 AU. The spectrophotometric observations. between 0.6 and 1 mum, were aimed at the detection of the CI(D-1) doublet lambdalambda 9823/9850 Angstrom. The non-detection of these lines. with a 3sigma flux upper limit of the order of 4.6 x 10(-17) erg cm(-2) s(-1), 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 similar to3.1 x 10(26) s(-1) and 1.2 x 10(27) s(-1), respectively. The oxygen forbidden line at 6300 A was used to obtain Q(H2O) = (1.22 +/- 0.7) x 10(29) s(-1). Assuming that CN and NH2 are directly produced by HCN and NH3. Comet Lee has a HCN/H2O approximate to 0.25% and NH3/HO approximate to 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 H2O+ doublet at 6198 and 6200 Angstrom, 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 cast-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 Af&rho; parameter, is &SIM;500 cm, which compared with the gas production rate classifies this Comet as a dust pool one with relatively high (6.5-11.7) gas-to-dust mass ratio. Analysis of the normalized rellectivity gradient (i.e. continuum color) as a function of &rho; indicates a slight reddening of the solid component in the coma at large cometocentric distances. whereas the average dust color within all aperture of 20000 km, centered at the nucleus, is &SIM;9% per 1000 &ANGS;. Mic 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 &mu;m. These grains might be composed of a mixture of silicates and icy material.