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

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This article has an erratum: [erratum]

Issue		A&A Volume 420, Number 1, June II 2004


Page(s)		371 - 382
Section		Planets and planetary systems
DOI		http://dx.doi.org/10.1051/0004-6361:20034214

A&A 420, 371-382 (2004)
DOI: 10.1051/0004-6361:20034214

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

L.-M. Lara¹, R. Rodrigo¹, G. P. Tozzi², H. Boehnhardt³ and P. Leisy⁴

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

(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 $\Delta = 1.195$ AU. The spectrophotometric observations, between 0.6 and 1 $\mu$ m, 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 $\rho$ 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 $\mu$ m. These grains might be composed of a mixture of silicates and icy material.

Key words: comets: individual: Comet C/1999 H1 (Lee) -- comets: general

Offprint request: L. M. Lara, lara@iaa.es

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