Lack of other molecules in CO-rich debris discs: is it primordial or secondary gas?

Abstract

The nature of the gas in CO-rich debris discs remains poorly understood, as it could either be a remnant from the earlier Class II phase or of secondary origin, driven by the destruction of icy planetesimals. The aim of this paper was to elucidate the origin of the gas content in the debris discs via various simple molecules that are often detected in the less-evolved Class II discs. We present millimetre molecular line observations of nine circumstellar discs around A-type stars: four CO-rich debris discs (HD 21997, HD 121617, HD 131488, HD 131835) and five old Herbig Ae protoplanetary discs (HD 139614, HD 141569, HD 142666, HD 145718, HD 100453). The sources were observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in Bands 5 and 6 with 1-2'' resolution. The Herbig Ae discs are detected in the CO isotopologues, CN, HCN, HCO${}^{+}$, C2H, and CS lines. In contrast, only CO isotopologues are detected in the debris discs, showing a similar amount of CO to that found in the Herbig Ae protoplanetary discs. Using chemical and radiative transfer modelling, we show that the abundances of molecules other than CO in debris discs are expected to be very low. We consider multiple sets of initial elemental abundances with various degrees of H2 depletion. We find that the HCO+ lines should be the second brightest after the CO lines and that their intensities strongly depend on the overall CO/H${}_2$ ratio of the gas. However, even in the ISM-like scenario, the simulated HCO+ emission remains weak as required by our non-detections.

arXiv comment15 pages, 9 figures, 7 tables Accepted for publication in MNRAS

References in corpus (58)

• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• An atomic and molecular database for analysis of submillimetre line observations
• A Hipparcos census of the nearby OB associations
• A KInetic Database for Astrochemistry (KIDA)
• The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks
• Photodissociation and photoionisation of atoms and molecules of astrophysical interest
• A self-consistent, absolute isochronal age scale for young moving groups in the solar neighbourhood
• The Disk Substructures at High Angular Resolution Project (DSHARP): II. Characteristics of Annular Substructures
• The Star-formation History and Accretion-Disk Fraction Among the K-Type Members of the Scorpius-Centaurus OB Association
• The 2014 KIDA network for interstellar chemistry
• Debris Disks: Structure, Composition, and Variability
• Molecular Gas Clumps from the Destruction of Icy Bodies in the $\beta$ Pictoris Debris Disk
• Chemistry in Disks. IV. Benchmarking gas-grain chemical models with surface reactions
• CO emission from discs around isolated HAeBe and Vega-excess stars
• UVMULTIFIT: A versatile tool for fitting astronomical radio interferometric data
• Setting the volatile composition of (exo)planet-building material. Does chemical evolution in disk midplanes matter?
• A 40 Myr Old Gaseous Circumstellar Disk at 49 Ceti: Massive CO-Rich Comet Clouds at Young A-Type Stars
• The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk
• Disk Imaging Survey of Chemistry with SMA: II. Southern Sky Protoplanetary Disk Data and Full Sample Statistics
• Exocometary gas in the HD 181327 debris ring
• Disk Imaging Survey of Chemistry with SMA (DISCS): I. Taurus Protoplanetary Disk Data
• Predictions for the secondary CO, C and O gas content of debris discs from the destruction of volatile-rich planetesimals
• ALMA observations of the molecular gas in the debris disk of the 30 Myr old star HD 21997
• Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO${}_2$ ice abundance in exocomets and Solar System comets
• Debris Disks in the Scorpius-Centaurus OB Association Resolved by ALMA
• Molecular gas in debris disks around young A-type stars
• The accretion rates and mechanisms of Herbig Ae/Be stars
• A sensitive survey for 13CO, CN, H2CO and SO in the disks of T Tauri and Herbig Ae stars
• ALMA observations of the $\eta$ Corvi debris disc: inward scattering of CO-rich exocomets by a chain of 3-30 M${}_{\oplus }$ planets?
• A survey of C2H, HCN, and C18O in protoplanetary disks
• Bright C2H emission in protoplanetary disks in Lupus: high volatile C/O>1 ratios
• High Resolution HST-STIS Spectra of CI and CO in the Beta Pictoris Circumstellar Disk
• Cometary ices in forming protoplanetary disc midplanes
• A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti
• Chemistry in Disks X: The Molecular Content of Proto-planetary Disks in Taurus
• Radial Surface Density Profiles of Gas and Dust in the Debris Disk around 49 Ceti
• Are Inner Disc Misalignments Common? ALMA Reveals an Isotropic Outer Disc Inclination Distribution for Young Dipper Stars
• Shadowing and multiple rings in the protoplanetary disk of HD 139614
• A precise asteroseismic age and metallicity for HD 139614: a pre-main-sequence star with a protoplanetary disc in Upper-Centaurus Lupus
• Imaging [CI] around HD 131835: reinterpreting young debris discs with protoplanetary disc levels of CO gas as shielded secondary discs
• Population synthesis of exocometary gas around A stars
• New millimeter CO observations of the gas-rich debris disks 49 Cet and HD 32297
• ALMA continuum observations of a 30 Myr old gaseous debris disk around HD 21997
• ALMA resolves CI emission from the $\beta$ Pictoris debris disk
• The hybrid disks: a search and study to better understand evolution of disks
• On the Ubiquity and Stellar Luminosity Dependence of Exocometary CO Gas: Detection around M Dwarf TWA 7
• Volatile-Rich Circumstellar Gas in the Unusual 49 Ceti Debris Disk
• Molecular reconnaissance of the $\beta$ Pictoris gas disk with the SMA: a low HCN/(CO+CO${}_2$) outgassing ratio and predictions for future surveys
• ALMA study of the HD 100453 AB system and the tidal interaction of the companion with the disk
• Survey of planetesimal belts with ALMA: gas detected around the Sun-like star HD 129590
• The surprisingly low carbon mass in the debris disk around HD 32297
• Photoevaporation of Grain-Depleted Protoplanetary Disks around Intermediate-Mass Stars: Investigating Possibility of Gas-Rich Debris Disks as Protoplanetary Remnants
• Unlocking the secrets of the midplane gas and dust distribution in the young hybrid disc HD 141569
• An ALMA Survey of Chemistry in Disks around M4-M5 Stars
• The Chemical Effects of Mutual Shielding in Photon Dominated Regions
• Extended Millimeter Emission in the HD 141569 Circumstellar Disk Detected with ALMA
• Using HCO${}^{+}$ isotopologues as tracers of gas depletion in protoplanetary disk gaps
• A Deep Search for Five Molecules in the 49 Ceti Debris Disk

Cited by in corpus (14)

• The vertical structure of debris disks and the impact of gas
• Primordial or Secondary? Testing models of debris disk gas with ALMA
• Vertical evolution of exocometary gas: I. How vertical diffusion shortens the CO lifetime
• The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) I: Motivation, sample, data reduction, and results overview
• Morphology of the gas-rich debris disk around HD 121617 with SPHERE observations in polarized light
• X-Shooter Survey of Young Intermediate Mass Stars -- I. Stellar Characterization and Disc Evolution
• The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) IV: CO gas imaging and overview
• Stellar winds can affect gas dynamics in debris disks and create observable belt winds
• The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) VII: Optically thick gas with broad CO gaussian local line profiles in the HD 121617 disc
• Discovery of carbon monoxide emission from five debris disks around young A-type stars
• Diversity of Exoplanets
• High CO/H2 ratios supports an exocometary origin for a CO-rich debris disk
• HD 44892: The youngest (or oldest?) gas-harbouring debris disc around an intermediate mass star
• Water vapor as a probe of the origin of gas in debris disks