Multiple Rings in the Transitional Disk of GM Aurigae Revealed by VLA and ALMA

Abstract

Our understanding of protoplanetary disks is rapidly departing from the classical view of a smooth, axisymmetric disk. This is in part thanks to the high angular resolution that (sub)mm observations can provide. Here we present the combined results of ALMA (0.9 mm) and VLA (7 mm) dust continuum observations toward the protoplanetary disk around the solar analogue GM Aur. Both images clearly resolve the ∼35 au inner cavity. The ALMA observations also reveal a fainter disk that extends up to ∼ 250 au. We model our observations using two approaches: an analytical fit to the observed deprojected visibilities, and a physical disk model that fits the SED as well as the VLA and ALMA observations. Despite not being evident in the deconvolved images, the VLA and ALMA visibilities can only be fitted with two bright rings of radii ∼40 and ∼80 au. Our physical model indicates that this morphology is the result of an accumulation or trapping of large dust grains, probably due to the presence of two pressure bumps in the disk. Even though alternative mechanisms cannot be discarded, the multiple rings suggest that forming planets may have cleared at least two gaps in the disk. Finally, our analysis suggests that the inner cavity might display different sizes at 0.9 mm and 7 mm. This discrepancy could be caused by the presence of free-free emission close to the star at 7 mm, or by a more compact accumulation of the large dust grains at the edge of the cavity.

A Multifrequency ALMA Characterization of Substructures in the GM Aur Protoplanetary Disk

“…The southwest side is brighter than the northeast side, with a 0.073±0.014 mJy beam −1 (∼ 8%) difference in peak intensities. Macías et al (2018) report a similar brightness asymmetry at the 5σ level in lowerresolution 930 µm data and at the 2σ level in 7 mm data. In our 2.1 mm data, the peak intensity of the northeast side is actually ∼ 5% brighter than the southwest side, but the difference is not statistically significant.…”

“…B40, D67, and B84, as well as the diffuse outer emission, were previously inferred from 930 µm observations at a resolution of ∼ 0. 3 in Macías et al (2018). D15 corresponds to the GM Aur disk's well-known central cavity (e.g., Hughes et al 2009), although the cavity might be more precisely described as an annular gap given the detection of interior emission.…”

“…This surface brightness prescription is partly motivated by Macías et al (2018), which used three concentric Gaussian rings to model lower resolution observations of GM Aur at 930 µm and 7 mm. The additional free parameters are the disk's P. A., cos i, R. A. offset from the phase center (δ α ), and Decl.…”

“…Gaussian priors are selected for the parameters governing the disk orientation and phase offset. The priors for the P. A. and cos(i) are set to the best-fit values derived in Macías et al (2018), while the standard deviations are set to be a few times wider than the posteriors from Macías et al (2018) because additional substructures are being modeled and can affect the disk orientation measurements. The priors for δ α and δ δ are centered at 0, and each has a standard deviation of 0.…”

“…Based on apparent deviations from Keplerian rotation in low resolution CO observations, Dutrey et al (2008) and Hughes et al (2009) hypothesized that the GM Aur disk is warped by a planet. Millimeter interferometric observations have resolved a dust cavity with a radius of ∼ 40 au and revealed additional annular dust substructures (e.g., Hughes et al 2009;Macías et al 2018).…”