2020ApJ...905..120B

Millimeter observations of disks around young stars reveal substructures indicative of gas pressure traps that may aid grain growth and planet formation. We present Submillimeter Array observations of HD 34700: two Herbig Ae stars in a close binary system (Aa/Ab, ∼0.25 au), surrounded by a disk presenting a large cavity and spiral arms seen in scattered light, and two distant, lower-mass companions. These observations include 1.3 mm continuum emission and the ¹²CO 2-1 line at ∼0.''5 (178 au) resolution. They resolve a prominent azimuthal asymmetry in the continuum and Keplerian rotation of a circumbinary disk in the ¹²CO line. The asymmetry is located at a radius of 155⁺¹¹_–7 au, consistent with the edge of the scattered-light cavity, being resolved in both radius (72 ⁺¹⁴_–15 au) and azimuth (FWHM = 64^°+8_–7). The strong asymmetry in millimeter continuum emission could be evidence for a dust trap, together with the more symmetric morphology of ¹²CO emission and small grains. We hypothesize an unseen circumbinary companion responsible for the cavity in scattered light and creating a vortex at the cavity edge that manifests in dust trapping. The disk mass has limitations imposed by the detection of ¹²CO and nondetection of ¹³CO. We discuss its consequences for the potential past gravitational instability of this system, likely accounting for the rapid formation of a circumbinary companion. We also report the discovery of resolved continuum emission associated with HD 34700B (projected separation ∼1850 au), which we explain through a circumstellar disk.