A multi-frequency ALMA characterization of substructures in the GM Aur
  protoplanetary disk
  The protoplanetary disk around the T Tauri star GM Aur was one of the first
hypothesized to be in the midst of being cleared out by a forming planet. As a
result, GM Aur has had an outsized influence on our understanding of disk
structure and evolution. We present 1.1 and 2.1 mm ALMA continuum observations
of the GM Aur disk at a resolution of ~50 mas (~8 au), as well as HCO$^+$
$J=3-2$ observations at a resolution of ~100 mas. The dust continuum shows at
least three rings atop faint, extended emission. Unresolved emission is
detected at the center of the disk cavity at both wavelengths, likely due to a
combination of dust and free-free emission. Compared to the 1.1 mm image, the
2.1 mm image shows a more pronounced "shoulder" near R~40 au, highlighting the
utility of longer-wavelength observations for characterizing disk
substructures. The spectral index $\alpha$ features strong radial variations,
with minima near the emission peaks and maxima near the gaps. While low
spectral indices have often been ascribed to grain growth and dust trapping,
the optical depth of GM Aur's inner two emission rings renders their dust
properties ambiguous. The gaps and outer disk ($R>100$ au) are optically thin
at both wavelengths. Meanwhile, the HCO$^+$ emission indicates that the gas
cavity is more compact than the dust cavity traced by the millimeter continuum,
similar to other disks traditionally classified as "transitional."
