UV Spectroscopy of Star-Grazing Comets within the 49 Ceti Debris Disk

TL;DR

This study analyzes variable UV absorption features in the 49 Ceti debris disk, revealing star-grazing comet activity with gas velocities and possible super-solar C/O ratios, similar to the well-known β Pictoris system.

Contribution

First detection and analysis of variable UV absorption features in 49 Ceti indicating star-grazing comet activity, expanding understanding of gas dynamics in debris disks.

Findings

Variable absorption features are seen in C II and C IV lines.

Infalling gas velocities range from tens to hundreds of km/s.

The infalling gas may have a super-solar C/O ratio.

Abstract

We present analysis of time-variable, shifted absorption features in far-UV spectra of the unusual 49 Ceti debris disk. This nearly edge-on disk is one of the brightest known, and is one of the very few containing detectable amounts of circumstellar gas as well as dust. In our two visits of Hubble Space Telescope STIS spectra, variable absorption features are seen on the wings of lines arising from C II and C IV, but not for any of the other circumstellar absorption lines. Similar variable features have long been seen in spectra of the well-studied $\beta$ Pictoris debris disk and attributed to the transits of star-grazing comets. We calculate the velocity ranges and apparent column densities of the 49 Cet variable gas, which appears to be moving at velocities of tens to hundreds of km s$^{-1}$ relative to the central star. The velocities of the gas in the redshifted variable event in Visit 2 show that the maximum distances of the infalling gas at the time of transit are about 0.05 to 0.2 AU from the central star. A preliminary attempt at a composition analysis of the redshifted event suggests that the C/O ratio in the infalling gas may be super-solar, as it is in the bulk of the stable disk gas.