High‐Resolution HUBBLE SPACE TELESCOPE STIS Spectra of C i and CO in theβ Pictoris Circumstellar Disk

Abstract

High-resolution far-ultraviolet (FUV) echelle spectra showing absorption features arising from C I and CO gas in the b Pic circumstellar (CS) disk were obtained on 1997 December 6 and 19 using the Space Telescope Imaging Spectrograph (STIS). An unsaturated spin-forbidden line of C I at 1613.376 not A Ž previously seen in spectra of b Pic was detected, allowing for an improved determination of the column density of C I at zero velocity relative to the star (the stable component), N(3P) \ (2È4) ] 1016 cm~2. Variable components with multiple velocities, which are the signatures of infalling bodies in the b Pic CS disk, are observed in the C I jj1561 and 1657 multiplets. Also seen for the Ðrst time were two lines arising from the metastable 1D level of carbon at 1931 and 1463 The results of analysis of the CO A Ž A Ž. AÈX (0È0), (1È0), and (2È0) bands are presented, including the bands arising from 13CO, with much better precision than has previously been possible, as a result of the very high resolution provided by the STIS echelle gratings. Only stable CO gas is observed, with a column density N(CO) \ (6.3 ^ 0.3) ] 1014 cm~2. An unusual ratio of the column densities of 12CO to 13CO is found (R \ 15 ^ 2). The large di †er-ence between the column densities of C I and CO indicates that photodissociation of CO is not the primary source of C I gas in the disk, contrary to previous suggestion.