Circumstellar Gas-Disk Variability Around A-Type Stars: The Detection of Exocomets?

ABSTRACT We present medium spectral resolution (R ∼ 60,000) observations of the CaII K-line (3,933 Å) absorption profiles observed toward 21 nearby A-type stars thought to possess circumstellar gas debris disks. The stars were repeatedly observed over two observing runs on the 2.1 m Otto Struve telescope at the McDonald Observatory, Texas in 2011 May and 2012 November. Nightly changes in the absorption strength of the CaII K-line near the stellar radial velocity were observed in four of the stars (HD 21620, HD 110411, HD 145964 and HD 183324). This type of absorption variability indicates the presence of a circumstellar gas disk around these stars. We also have detected weak absorption features that sporadically appear with velocities in the range ± 100 km s-1 of the main circumstellar K-line in the spectra of HD 21620, HD 42111, HD 110411 and HD 145964. Due to the known presence of both gas and dust disks surrounding these four stars, these transient absorption features are most probably associated with the presence of Falling Evaporated Bodies (FEBs, or exocomets) that are thought to liberate gas on their grazing trajectory toward and around the central star. This now brings the total number of A-type stars in which the evaporation of CaII gas from protoplanetary bodies (i.e., exocomets) has been observed to vary on a nightly basis to 10 systems. A statistical analysis of the 10 A-stars showing FEB-activity near the CaII K-line compared to 21 A-type stars that exhibit no measurable variability reveals that FEB-activity occurs in significantly younger stellar systems that also exhibit chemical peculiarities. The presence of FEB-activity does not appear to be associated with a strong mid-IR excess. This is probably linked to the disk inclination angle, since unless the viewing angle is favorable the detection of time-variable absorption may be unlikely. Additionally, if the systems are more evolved then the evaporation of gas due to FEB activity could have ceased, whereas the circumstellar dust disk may still remain.