The Transiting Exocomets of HD 172555 C.A. Grady Eureka Scientific and Goddard Space Flight Center A. Brown, I. Kamp, A. Roberge, P. Riviere-Marichalar, and B. Welsh

What We Found Technique used: absorption spectroscoy with HST/STIS and COS – 2 datasets separated by 5 days, 23 hours What we saw: absorption reaching +160 km/s in ions of silicon and carbon in FUV spectra of the A star HD 172555. visible over 6 hours in each visit - likely not looking at transits of single bodies, but train of bodies passing between us and the star Gas absorption is opaque but does not fully cover the star Variability, especially in the carbon absorption between the two visits Emission associated with a stellar chromosphere in the A star

What we think is going on Profiles, variability are similar to what is seen in spectra of β Pictoris, and what is expected for sun-grazing comets Interpretation – we see gas clouds associated with transiting extra-solar bodies Beust 2014 Sungrazer as seen by SOHO (image credit SOHO)

And How it Differs from β Pic We find no CO absorption in the spectrum of HD 172555, and no non-variable gas at the stellar radial velocity May be due to how we view the system (i~75°), or the fact that this system seems to have no Kuiper Belt analog (Riviere-Marichalar et al. 2012)

HD 172555 Exocomets in β Pic, image credit ESO

What It Tells Us To get bodies into star-grazing orbits, need a planet exciting bodies in mean-motion resonances into high eccentricity orbits. – like Jupiter-family comets seen by SOHO in our system. Presence of activity may provide a quick(er) way to search for planetary systems, particularly in young, well-dated associations. HD 172555 is a member of the β Pic Moving Group: 24±1 Myr: 5 A stars, 3 early F stars: of these 2 have imaged planets, 2 systems with transiting exocomets; 37.5% (3/8) are likely to have planets. Presence of a Kuiper Belt may not be required.

A possible way to locate systems with Jovian-mass planets?