HST -GHRS observations of CO and CI in the β-Pictoris circumstellar disk

GHRS-EchA and G160M spectra of β-Pictoris have been obtained in the 1400-1600 Å range. A detailed analysis of the CO A-X (0-0, 1-0, 2-0) bands and of the CI λ1561 Å multiplet is presented. A column density of the order of 2 1015 cm−2 is found for both species. 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 13CO lines in the fit allows an estimate of the 13CO abundance: R=12CO/13CO=20. This anomalous ratio is tentatively interpreted as being due to chemical fractionation. For the CI triplet, the absorption profile is best fitted by a combination of four velocity components matching those observed independently at the same moment from ground in the CaII H and K lines. The most intense component corresponds to zero relative velocity while the others are shifted by 3 to 10 km s−1. The large turbulent width of the components (4.2 km s−1), the statistical population of the ground state fine structure levels and the similar column densities of CI and CO are interpreted as indicating that CI is formed by photodissociation of CO and destroyed by photoionization. The velocity structurewould then indicate that part of CI is formed at large distance (∼100 AU) from CO evaporating from cometary bodies while another part originates closer to the star from fast moving cometary bodies with high excentricity.