1. 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

2. 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
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

3. 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)

4. And How it Differs from β Pic
We find no CO absorption in the spectrum of HD ,
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)

5. HD
Exocomets in β Pic, image credit ESO

6. 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 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.

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