Spiral Arms in Disks: Planets or Gravitational Instability?

Abstract

Spiral arm structures seen in scattered-light observations of protoplanetary disks can potentially serve as signposts of planetary companions. They can also lend unique insights into disk masses, which are critical in setting the mass budget for planet formation but are difficult to determine directly. A surprisingly high fraction of disks that have been well studied in scattered light have spiral arms of some kind (8/29), as do a high fraction (6/11) of wellstudied Herbig intermediate-mass stars (i.e., Herbig stars >1.5 M e ). Here we explore the origin of spiral arms in Herbig systems by studying their occurrence rates, disk properties, and stellar accretion rates. We find that two-arm spirals are more common in disks surrounding Herbig intermediate-mass stars than are directly imaged giant planet companions to mature A and B stars. If two-arm spirals are produced by such giant planets, this discrepancy suggests that giant planets are much fainter than predicted by hot-start models. In addition, the high stellar accretion rates of Herbig stars, if sustained over a reasonable fraction of their lifetimes, suggest that disk masses are much larger than inferred from their submillimeter continuum emission. As a result, gravitational instability is a possible explanation for multiarm spirals. Future observations can lend insights into the issues raised here.

Dust evolution in the circumstellar disc of the unclassified B[e] star HD 50138

“…However, they could not directly link the asymmetry to the presence of a companion, rather they hypothesized that it may be a spiral arm or other disc asymmetry. We note that spiral arms in circumstellar discs can be induced by companions or planets (e.g., Bae & Zhu 2018;Boehler et al 2018;Dong et al 2018;Müller et al 2018;Wagner et al 2018). Such a scenario can be compatible with the observed features of the disc around HD 50138.…”

The observational impact of dust trapping in self-gravitating discs

“…The DSHARP collaboration report no companion detections in any of these 18 discs despite many of the observed features being suggestive of massive companions which ought to be observable at such high angular resolution. It may then be the case that either massive planets are fainter than previously thought (Dong et al 2018), or that the observed rings are driven by lower mass, fainter planets which remain invisible to the DSHARP survey. If the latter, then these lower mass companions may not be capable of driving the observed spiral structure in Elias 27, WaOph 6 and IM Lup alone, but a combination of both GI and planet-disc interactions may be a plausible scenario (e.g.…”

Are the spiral arms in the MWC 758 protoplanetary disc driven by a companion inside the cavity?

“…Where is the companion driving the spiral arms in the MWC 758 circumstellar disc? While spiral arms have long been expected from the theory of planet-disc interaction (Goldreich & Tremaine 1979, 1980, recent high resolution images of circumstellar discs have shown a remarkable array of spiral arm morphologies (Garufi et al 2017;Dong et al 2018b). The disc around MWC 758 is one of the most spectacular -with two prominent spiral arms seen in scattered light (Grady et al 2013;Benisty et al 2015) and tentative evidence for a third spiral arm and a point-like source located roughly 20 AU from the central star (Reggiani et al 2018).…”