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 β 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 Å not previously seen in spectra of β 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)=(2-4)×10¹⁶ cm^-2. Variable components with multiple velocities, which are the signatures of infalling bodies in the β Pic CS disk, are observed in the C I λλ1561 and 1657 multiplets. Also seen for the first time were two lines arising from the metastable ¹D level of carbon at 1931 Å and 1463 Å. The results of analysis of the CO A-X (0-0), (1-0), and (2-0) 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 difference 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.