Shocking detections: Characterising exocometary shock fronts by tracking star-grazing comets using the UV AlIII line

Exocomets are small icy bodies, which orbit stars other than the Sun and sublimate when close to their star, producing a tail of dust and gas (like solar system comets). During transit, the gas causes spectroscopic absorption signatures that are routinely detected in several systems. The most prolific case is beta Pic, which continuously displays narrow, variable, non-photospheric absorption features superimposed on the much broader stellar lines. The features vary on short timescales (hours) and are seen at optical (e.g. CaII) and UV (e.g. FeII, SiIV) wavelengths.

Previous HST observations of beta Pic in the UV showed the presence of AlIII lines in the exocomet gas. The mere existence of AlIII is puzzling as beta Pic is unable to twice photoionise Al. Hydrodynamical models suggest that these highly-ionised species are formed in a shock where compression and thus collisions within the shock surface are sufficient to generate AlIII. This is theorised to occur when the exocomets get sufficiently close to the star (a few stellar radii).

We propose to monitor beta Pic to characterise the doppler movement of the AlIII lines and from this directly measure the exocomets' acceleration. This we use to calculate the comet-star distances. The highly ionised species are thought only to be present in the shock fronts of exocomets close to the star. Measuring the distance between the star and the exocomets will allow us to directly test this hypothesis. The observations cover a wavelength range with other species thought to only emerge in shocks. Measuring the transit ingress/egress times of these species will give us the composition of the shock as a function of radial distance.