1. Eureka Scientific, 2. European Space Agency, 3. Kapteyn Institute, 4. NASA's GSFC, 5. University of Colorado
The Earth is thought to have formed dry, in a part of the Solar Nebula deficient in organic material, and to have acquired its water and organics through bombardment by minor bodies. Observations of this process in well-dated systems can provide insight into the probable origin and composition of the bombarding parent bodies. Transiting cometary activity has previously been reported in Ca II for the late-A member of the ~23 Myr old Beta Pictoris Moving Group member, HD 172555. We present HST STIS and COS spectra of HD 172555 which demonstrate that the star has chromospheric emission and variable infalling gas features in transitions of silicon and carbon ions at times when no Fe II absorption is seen in the UV data, and no Ca II absorption is seen in contemporary optical spectra. We discuss apparent optical depths for the infall features. The lack of CO absorption and stable gas absorption at the system velocity is consistent with the absence of a cold Kuiper belt analog in this system. The presence of infall in some species at one epoch and others at different epochs suggests that, like beta Pictoris, there may be more than one family of exocomets. If perturbed into star-grazing orbits by the same mechanism as for beta Pic, these data suggest that the wide planet frequency among A-early F stars in the BPMG is at least 37.5%, well above the frequency estimated for young moving groups independent of host star spectral type.
Support for this work was provided by NASA through grant Number HST-GO-13798 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.