Low CI/CO Abundance Ratio Revealed by HST UV Spectroscopy of CO-rich Debris Disks

TL;DR

This study uses HST UV spectroscopy to analyze CO-rich debris disks, revealing high CO densities, low CI/CO ratios, and challenging the secondary gas origin model, suggesting complex gas evolution processes.

Contribution

It provides the first detailed UV spectroscopic analysis of CI and CO in debris disks, highlighting discrepancies with secondary origin models and proposing alternative explanations.

Findings

High CO column densities (3-4 orders of magnitude above Beta Pictoris)

Low CI/CO ratios consistent with recent ALMA results

Secondary release models cannot fully explain observed gas properties

Abstract

The origin and evolution of CO gas in debris disks has been debated since its initial detection. The gas could have a primordial origin, as a remnant of the protoplanetary disk or a secondary exocometary origin. This paper investigates the origin of gas in two debris disks, HD110058 and HD131488, using HST observations of CI and CO, which play critical roles in the gas evolution. We fitted several electronic transitions of CI and CO rovibronic bands to derive column densities and temperatures for each system, revealing high CO column densities ($\sim$3-4 orders of magnitude higher than $\beta$ Pictoris), and low CI/CO ratios in both. Using the exogas model, we simulated the radial evolution of the gas in the debris disk assuming a secondary gas origin. We explored a wide range of CO exocometary release rates and $\alpha$ viscosities, which are the key parameters of the model. Additionally, we incorporated photodissociation due to stellar UV to the exogas model and found that it is negligible for typical CO-rich disks and host stars, even at a few au due to the high radial optical depths in the EUV. We find that the current steady-state secondary release model cannot simultaneously reproduce the CO and CI HST-derived column densities, as it predicts larger CI/CO ratios than observed. Our direct UV measurement of low CI/CO ratios agrees with results derived from recent ALMA findings and may point to vertical layering of CI, additional CI removal, CO shielding processes, or different gas origin scenarios.