  Azimuthally asymmetric dust distributions observed with ALMA in transition
disks have been interpreted as dust traps. We present VLA Ka band (34 GHz or
0.9 cm) and ALMA Cycle 2 Band 9 (680 GHz or 0.45 mm) observations at 0.2"
resolution of the Oph IRS 48 disk, which suggest that larger particles could be
more azimuthally concentrated than smaller dust grains, assuming an
axisymmetric temperature field or optically thin 680 GHz emission. Fitting an
intensity model to both data demonstrates that the azimuthal extent of the
millimeter emission is 2.3 $\pm0.9$ times as wide as the centimeter emission,
marginally consistent with the particle trapping mechanism under the above
assumptions. The 34 GHz continuum image also reveals evidence for ionized gas
emission from the star. Both the morphology and the spectral index variations
are consistent with an increase of large particles in the center of the trap,
but uncertainties remain due to the continuum optical depth at 680 GHz.
Particle trapping has been proposed in planet formation models to allow dust
particles to grow beyond millimeter sizes in the outer regions of
protoplanetary disks. The new observations in the Oph IRS 48 disk provide
support for the dust trapping mechanism for centimeter-sized grains, although
additional data is required for definitive confirmation.
