High CO/H2 ratios supports an exocometary origin for a CO-rich debris disk

TL;DR

This work tackles whether CO-rich exocometary belts host primordial, H$_2$-rich gas or secondary gas released by exocomets. Using high-resolution CRIRES+ near-IR spectroscopy, the authors target the H$_2$ v=1-0 S(0) line at 2223.3 nm and CO v=2-0 lines in edge-on belts around HD 110058 and HD 131488, enabling direct line-of-sight constraints on $N_{ ext{H}_2}$ and $N_{ ext{CO}}$. Through radiative transfer modelling with RADIS and MCMC, they find strong CO detections but no H$_2$ detections, yielding 3σ lower limits on $rac{ ext{CO}}{ ext{H}_2}$ of $> 3.09 imes10^{-5}$ for HD 131488 and $> 1.35 imes10^{-3}$ for HD 110058; these values are inconsistent with ISM-like primordial gas and indicate H$_2$-poor, exocometary gas in at least one system. The results argue against a primordial origin for the gas and expose challenges for second-generation models to reproduce CO masses without sufficient shielding, highlighting the need for more detailed chemistry, multi-molecule constraints, and refined disc geometry in exocometary belt studies.

Abstract

Over 20 exocometary belts host detectable circumstellar gas, mostly in the form of CO. Two competing theories for its origin have emerged, positing the gas to be primordial or secondary. Primordial gas survives from the belt's parent protoplanetary disk and is therefore H$_2$-rich. Secondary gas is outgassed \textit{in-situ} by exocomets and is relatively H$_2$-poor. Discriminating between these scenarios has not been possible for belts hosting unexpectedly large quantities of CO. We aim to break this gas origin dichotomy \textit{via} direct measurement of H$_2$ column densities in two edge-on CO-rich exocometary belts around $\sim$15 Myr-old A-type stars, constraining the $\frac{\text{CO}}{\text{H}_2}$ ratio and CO gas lifetimes. Observing edge-on belts enables rovibrational absorption spectroscopy against the stellar background. We present near-IR CRIRES+ spectra of HD 110058 and HD 131488 which provide the first direct probe of H$_2$ gas in CO-rich exocometary belts. We target the H$_2$ (v=1-0 S(0)) line at 2223.3 nm and and the $^{12}$CO $v=2\rightarrow0$ rovibrational lines in the range 2333.8-2335.5 nm and derive constraints on column densities along the line-of-sight to the stars. We strongly detect $^{12}$CO but not H$_2$ in the CRIRES+ spectra. This allows us to place $3σ$ lower limits on the $\frac{\text{CO}}{\text{H}_2}$ ratios of $> 1.35 \times 10^{-3}$ and $> 3.09 \times 10^{-5}$ for HD 110058 and HD 131488 respectively. These constraints demonstrate that at least for HD 110058, the exocometary gas is compositionally distinct and significantly H$_2$-poor, compared to the $<10^{-4}$ $\frac{\text{CO}}{\text{H}_2}$ ratios typical of protoplanetary disks. We also find H$_2$ alone is unlikely to shield CO over the lifetime of the systems. Overall this suggests that the gas in CO-rich belts is most likely not primordial in origin, supporting the presence of exocometary gas.

Figures (13)

• Figure 1: Reduced and extracted 1D spectra (black lines) for a portion of the detector-order containing our line of interest for HD 131488 on 11/05/23 (top) and HD 110058 on 25/03/23 (bottom). The spectra are not flux calibrated and as such the absolute values of the y-axis in units of analogue-to-digital units (adu) are not astrophysically meaningful. The vertical green line denotes the expected location of the H$_2$ transition of interest. The spectra are in the observatory frame, but the expected H$2$ location has been shifted to the rest frame of the star.
• Figure 2: In black are the median, telluric corrected, normalised spectra of HD 131488 (top) and HD 110058 (bottom). In blue is a model of the H$_2$ v=1-0 S(0) line assuming a temperature corresponding to the CO kinetic temperature we derive from our results and an H$_2$ column density corresponding to an ISM-like, primordial $\frac{\text{CO}}{\text{H}_2}$ ratio of $10^{-4}$.
• Figure 3: The top figure shows the HD 110058 $^{12}$CO data (black solid line) overlaid with the best-fit model (blue dotted line). The bottom figure shows the $^{12}$CO residuals (black solid line) and the rolling variance (green dashed line).
• Figure 4: The top figure shows the HD 131488 $^{12}$CO data (black solid line) overlaid with the best-fit model (blue dotted line). The bottom figure shows the $^{12}$CO residuals (black solid line) and the rolling variance (green dashed line).
• Figure 5: Ratio of CO and H$_2$ abundances for protoplanetary disks, $\beta$ Pictoris, and the two exocometary belts in this study. Details of each point can be found in the references of Table \ref{['tab:co_h2_ratios']}. This plot has been adapted from and expanded upon using the similar plots in bergin_determination_2018zhang_rapid_2020.