Insights into the planetary dynamics of HD 206893 with ALMA

Abstract

Radial substructure in the form of rings and gaps has been shown to be ubiquitous among protoplanetary discs. This could be the case in exoKuiper belts as well, and evidence for this is emerging. In this paper we present ALMA observations of the debris/planetesimal disc surrounding HD 206893, a system that also hosts two massive companions at 2 and 11 au. Our observations reveal a disc extending from 30 to 180 au, split by a 27 au wide gap centred at 74 au, and no dust surrounding the reddened brown dwarf (BD) at 11 au. The gap width suggests the presence of a 0.9 MJup planet at 74 au, which would be the third companion in this system. Using previous astrometry of the BD, combined with our derived disc orientation as a prior, we were able to better constrain its orbit finding it is likely eccentric ($0.14^{+0.05}_{-0.04}$). For the innermost companion, we used RV, proper motion anomaly and stability considerations to show its mass and semi-major axis are likely in the range 4–100 MJup and 1.4–4.5 au. These three companions will interact on secular timescales and perturb the orbits of planetesimals, stirring the disc and potentially truncating it to its current extent via secular resonances. Finally, the presence of a gap in this system adds to the growing evidence that gaps could be common in wide exoKuiper belts. Out of 6 wide debris discs observed with ALMA with enough resolution, 4–5 show radial substructure in the form of gaps.

Characterization of HD 206893 B from Near- to Thermal-infrared

“…In Section 3.4 we show that the brown dwarf is likely responsible for stirring the planetesimals in the debris disk, lending some strength to this scenario. Delorme et al (2017) and Stolker et al (2020) suggested there may be a circumplanetary disk around HD 206893 B (similar to predictions around the red companion G 196-3 B Zakhozhay et al 2017) though recent ALMA observations of the system by Marino et al (2020) showed no dust around the brown dwarf, with a dust upper limit of 2 × 10 −4 M ⊕ .…”

“…Delorme et al (2017) and Ward-Duong et al (2021) both showed that extinction due to submicron forsterite particles in the brown dwarf's atmosphere could lead to sufficient reddening of its emission spectrum to explain the observations, though they did not take into account the feedback between the submicron particles and the atmosphere. A possible source of these submicron particles is that the brown dwarf may be accreting dust from the debris belt, populating the atmosphere with small, high-altitude grains (Marino et al 2020;Ward-Duong et al 2021). In Section 3.4 we show that the brown dwarf is likely responsible for stirring the planetesimals in the debris disk, lending some strength to this scenario.…”

“…We find that the brown dwarf has a semimajor axis of .16 . HD 206893 B is located inside the observed debris belts (Milli et al 2017;Marino et al 2020). HD 206893 B is sufficiently far away from the debris belt so that its chaotic zone does not overlap with the belt (vertical gray box in Figure 5), calculated assuming the chaotic zone's inner edge (1-1.17μ 0.28 )a pl and the outer edge (1 + 1.76μ 0.31 )a pl where μ is the mass ratio between the brown dwarf and the star, and a pl is the semimajor axis of the planet (see Morrison & Malhotra 2015, their Table 1).…”

“…The analysis from Stolker et al (2020) and Ward-Duong et al (2021) confirmed that the brown dwarf is redder than other field dwarfs with similar spectral types. Using Atacama Large Millimeter/submillimeter Array (ALMA) data, Marino et al (2020) found that the debris disk surrounding the host HD 206893, external to the brown dwarf, is comprised of two spatially separated belts of dust.…”

Debris discs in binaries: morphology and photometric signatures

“…The brightest amongst these discs have been imaged, from scattered light observations in the visible to thermal imaging up to millimetre wavelengths, revealing that most resolved discs display pronounced spatial structures, such as rings, warps, clumps, spirals, etc. (Krivov 2010;Marino et al 2020).…”