Spiral Arms in Disks: Planets or Gravitational Instability?

TL;DR

Spiral arms in protoplanetary disks may indicate planetary presence or gravitational instability, with recent findings suggesting disks around Herbig stars often have more massive disks than previously thought, impacting planet formation theories.

Contribution

This study analyzes the occurrence and origin of spiral arms in Herbig star disks, proposing gravitational instability as a plausible cause for multi-arm spirals and challenging existing disk mass estimates.

Findings

Two-arm spirals are more common in Herbig stars than giant planets in mature systems.

Disks may be more massive than submillimeter observations suggest, supporting gravitational instability.

High accretion rates imply larger disk masses, influencing planet formation models.

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 well-studied Herbig intermediate mass stars (i.e., Herbig stars $> 1.5M_\odot$). 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 multi-arm spirals. Future observations can lend insights into the issues raised here.