Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462 (HD 135344B): Implications for the Physical State of the Disk from Spiral Density Wave Theory

Abstract: We present high-resolution, H-band, imaging observations, collected with Subaru/HiCIAO, of the scattered light from the transitional disk around SAO 206462 (HD 135344B). Although previous sub-mm imagery suggested the existence of the dust-depleted cavity at r~46AU, our observations reveal the presence of scattered light components as close as 0.2" (~28AU) from the star. Moreover, we have discovered two small-scale spiral structures lying within 0.5" (~70AU). We present models for the spiral structures using the spiral density wave theory, and derive a disk aspect ratio of h~0.1, which is consistent with previous sub-mm observations. This model can potentially give estimates of the temperature and rotation profiles of the disk based on dynamical processes, independently from sub-mm observations. It also predicts the evolution of the spiral structures, which can be observable on timescales of 10-20 years, providing conclusive tests of the model. While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets. Such planets may be detected by future multi-wavelengths observations.

• The paper reveals spiral structures detected as close as 28 AU from SAO 206462, indicating active dynamical processes in its disk.
• It employs spiral density wave theory to constrain disk properties, deriving an aspect ratio of roughly 0.1 consistent with prior sub-mm observations.
• The findings imply potential planet-induced perturbations that may help locate and estimate the mass of unseen exoplanets within the transitional disk.

Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462

In this paper, Muto et al. present a high-resolution observational study of the transitional protoplanetary disk around the Herbig F star, SAO 206462, also known as HD 135344B. Utilizing the $H$-band adaptive optics imaging with the Subaru Telescope's HiCIAO instrument, they report the detection of small-scale spiral structures in the disk. The study provides substantial insights into the physical state of the disk, offering broader implications for understanding the dynamics and evolutionary processes in protoplanetary systems.

Observational Findings

The observations reveal spiral structures in the scattered light from the disk as close as $0.2$ arcseconds (approximately $28$ AU) from the star. This is notable because previous sub-mm observations indicated a dust-depleted cavity extending up to $46$ AU. Two prominent spiral arms, designated S1 and S2, were identified within $0.5$ arcseconds (about $70$ AU). The total polarized intensity (PI) within $0.42$ to $1.0$ arcseconds in the disk represents a small yet significant fraction of the star's total brightness. The spiral structures were found to be enhanced regions of the PI profile, standing approximately $30\%$ higher than the background disk emission.

Theoretical Analysis

Employing spiral density wave theory, the authors provide a framework for understanding these spiral arm structures. The proposed model, which aligns the observational characteristics with theoretical predictions, necessitates a disk aspect ratio of $h \sim 0.1$. This is consistent with previous sub-mm observations and suggests that the seen spirals could be a result of dynamical processes within the disk. The model further enables the derivation of constraints on the disk’s temperature and rotational profile.

Implications and Future Directions

The findings imply significant dynamical activity within the disk, potentially induced by embedded planetary bodies. While the study does not conclusively identify the origin of the spirals, it posits the hypothesis that planet-induced perturbations could excite such density waves. This, in turn, allows speculation regarding the location and mass of potential unseen exoplanets within the disk.

The paper's integration of observational data with theoretical modeling presents a compelling case for the use of spiral density waves as diagnostic tools for probing the physical state of protoplanetary disks. The prospect of observing the evolution of these spiral structures over short astrophysical timescales (10-20 years) could provide concrete tests for the theoretical models proposed, opening avenues for future high-resolution, multi-wavelength studies.

Conclusion

The discovery of spiral features in SAO 206462’s disk underscores the value of combining high-resolution imaging with robust theoretical models to enhance our understanding of early planetary systems’ dynamics. Future observations, particularly at varying wavelengths, could validate this study’s predictions and potentially unveil planets whose interactions might be inducing these fascinating spiral patterns. The paper by Muto et al., therefore, offers a substantial contribution to the field, suggesting paths for both observational and theoretical advancements in the study of transitional disks.