Detection of Exocometary CO within the 440 Myr Old Fomalhaut Belt: A Similar CO+CO₂ Ice Abundance in Exocomets and Solar System Comets

Abstract

Recent ALMA observations present mounting evidence for the presence of exocometary gas released within Kuiper belt analogues around nearby main sequence stars. This represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. We here present the detection of CO J=2-1 emission co-located with dust emission from the cometary belt in the 440 Myr-old Fomalhaut system. Through spectro-spatial filtering, we achieve a 5.4σ detection and determine that the ring's sky-projected rotation axis matches that of the star. The CO mass derived (0.65 − 42 × 10 −7 M ⊕ ) is the lowest of any circumstellar disk detected to date, and must be of exocometary origin. Using a steady state model, we estimate the CO+CO 2 mass fraction of exocomets around Fomalhaut to be between 4.6-76%, consistent with Solar System comets and the two other belts known to host exocometary gas. This is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct ISM inheritance. In addition, we find tentative evidence that (49 ± 27)% of the detected flux originates from a region near the eccentric belt's pericentre. If confirmed, the latter may be explained through a recent impact event or CO pericentre glow due to exocometary release within a steady state collisional cascade. In the latter scenario, we show how the azimuthal dependence of the CO release rate leads to asymmetries in gas observations of eccentric exocometary belts.

Population synthesis of exocometary gas around A stars

“…Note that the distribution of red dots is consistent with the blue density map, and we do not expect CO masses lower than 10 −7 M ⊕ , which depending on line excitation conditions is at the limit of typical deep ALMA observations (e.g. Matrà et al 2017b), but below the sensitivity of shallow CO surveys (e.g. Moór et al 2017;Kral et al 2017).…”

“…Although such high S/N might be difficult to achieve over a single beam, azimuthal averaging with Keplerian masking can be employed (e.g. Matrà et al 2017b;Teague et al 2018) and thus obtaining high precision measurements of the intrinsic linewidth and constraints on α.…”

“…Because the CO mass in those discs only depends on the photodissociation timescale (t ph ) and the product between the mass loss rate and f CO , i.e. M CO = t ph f CO M CO , by estimating the UV field at the CO location and mass loss rate from the disc fractional luminosity it is possible to derive f CO as shown by and Matrà et al (2017b). The main obstacle for determining f CO with this method are the uncertainties on the CO masses due to NLTE effects.…”

Likely transiting exocomets detected by Kepler

“…Nor is there any specific reason to believe that there is disk activity or populations of minor bodies at orbital separations much greater than these inferred for the comets in this work. Such debris might be expected to exhibit CO emission as is seen in HD 181327 (Marino et al 2016), Eta Corvi (Marino et al 2017), and Fomalhaut (Matrà et al 2017). Nevertheless, the stars reported on herein, are sufficiently unusual among the Kepler ensemble of 2×10 5 stars, that it could be worth the gamble to use ALMA to search for CO emission around KIC 3542116 and KIC 11084727.…”

“…Radio observations, in particular detections of circumstellar CO emission around stars such as HD 181327 (Marino et al 2016), Eta Corvi (Marino et al 2017), and Fomalhaut (Matrà et al 2017) have been attributed to the presence of substantial populations of minor bodies at large orbital separations. Another sensitive method for detecting and understanding populations of extrasolar minor planets is through timeseries spectroscopy, rather than time-series photometry (e.g., as performed by Kepler).…”