HST-GHRS OBSERVATIONS OF CO AND CI IN THE BETA-PICTORIS CIRCUMSTELLARDISK

GHRS-EchA and G160M spectra of beta-Pictoris have been obtained in the 1400-1600 Angstrom range. A detailed analysis of the CO A-X (0-0, 1-0 , 2-0) bands and of the CI lambda 1561 Angstrom multiplet is presented . A column density of the order of 2 10(15) cm(-2) is found for both s pecies. For CO, the rotational temperature is found close to 20 K and the turbulent Doppler width is b = 0.8 km.s(-1). Including the (CO)-C- 13 lines in the fit allows an estimate of the (CO)-C-13 abundance: R = (CO)-C-12/(CO)-C-13 = 20. This anomalous ratio is tentatively interpr eted as being due to chemical fractionation. For the CI triplet, the a bsorption profile is best fitted by a combination of four velocity com ponents matching those observed independently at the same moment from ground in the CaII H and K lines. The most intense component correspon ds to zero relative velocity while the others are shifted by 3 to 10 k m s(-1) The large turbulent width of the components (4.2 km s(-1)), th e statistical population of the ground state fine structure levels and the similar column densities of CI and CO are interpreted as indicati ng that CI is formed by photodissociation of CO and destroyed by photo ionization. The velocity structure would then indicate that part of CI is formed at large distance (similar to 100 AU) from CO evaporating f rom cometary bodies while another part originates closer to the star f rom fast moving cometary bodies with high excentricity.