Exocometary gas in the HD 181327 debris ring

Abstract

An increasing number of observations have shown that gaseous debris discs are not an exception. However, until now, we only knew of cases around A stars. Here we present the first detection of 12 CO (2-1) disc emission around an F star, HD 181327, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with radiative transfer calculations, we study the dust and CO mass distribution. We find the dust is distributed in a ring with a radius of 86.0 ± 0.4 au and a radial width of 23.2 ± 1.0 au. At this frequency, the ring radius is smaller than in the optical, revealing grain size segregation expected due to radiation pressure. We also report on the detection of low-level continuum emission beyond the main ring out to ∼200 au. We model the CO emission in the non-local thermodynamic equilibrium regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging between 1.2 × 10 −6 M ⊕ and 2.9 × 10 −6 M ⊕ , depending on the gas kinetic temperature and collisional partners densities. The CO densities and location suggest a secondary origin, i.e. released from icy planetesimals in the ring. We derive a CO+CO 2 cometary composition that is consistent with Solar system comets. Due to the low gas densities, it is unlikely that the gas is shaping the dust distribution.

Molecular Gas in Debris Disks around Young A-type Stars

“…From the FG stars, two objects were found to harbor CO gas (HD 181327 and HD 146897). However, with its 12 CO 2-1 line luminosity of ∼10 3 Jy km s −1 pc 2 (based on Marino et al 2016), HD 181327 is at least an order of magnitude fainter than any of the other known gaseous disks from the studied samples. Its detection was only possible because of its proximity and the long exposure time.…”

“…2) and their ALMA measurements achieved the same detection threshold (∼1.4×10 4 Jy km s −1 pc 2 ) as that of our A-type sample. We found 16 such systems: 14 belonging to the Sco-Cen association (Lieman-Sifry et al 2016), and two young moving group members, HD 61005 and HD 181327 (Olofsson et al 2016;Marino et al 2016). From the FG stars, two objects were found to harbor CO gas (HD 181327 and HD 146897).…”

“…While the gas in the disk of β Pic, and around the two F-type hosts is secondary (Kral et al 2017;Marino et al 2016;Matrà et al 2017a), the presence of significantly larger amounts of CO gas at HD 21997 and HD 131835 indicates that these systems may have hybrid disks, where the gas is leftover from the primordial phase (Kóspál et al 2013;Kral et al 2017, Moór et al in prep.). The likely existence of hybrid disks would be a strong indication that in some systems the evolution of dust and gas, at least in the outer disk, is decoupled.…”

Detecting Weak Spectral Lines in Interferometric Data through Matched Filtering

“…Recently Matrà et al (2015) and Marino et al (2016) introduced an image-plane line detection technique (also independently introduced and formalized by Yen et al 2016) that provides some similar benefits to the matched filter technique. In their approaches, pixels from a dirty image are adjusted for an assumed velocity offset (from a source model), and the velocity-corrected spectra are then stacked.…”

First Scattered-light Images of the Gas-rich Debris Disk around 49 Ceti

“…Yet, a handful of young systems exhibit a substantial amount of CO gas while having ages and dust properties of debris disks (Zuckerman et al 1995;Moór et al 2011Moór et al , 2015aDent et al 2014;Greaves et al 2016;Lieman-Sifry et al 2016;Marino et al 2016). They all harbor large quantities of dust, indicated by their fractional infrared luminosity L IR /L å ∼1×10 −3 and have ages between 15 and 50 Myr (Greaves et al 2016).…”